Applied Plastics Engineering Handbook || Compression Molding

17 Compression Molding

Applied Plast

Copyright � 2

Robert A. TataraDepartment of Technology, Northern Illinois University, DeKalb, IL 60115, USA

17.1 Basics of Processing byCompression Molding

Compression molding is among the oldest materials pro-cessing techniques. For plastics, it was one of the firstindustrial methods, with equipment dating back 100 years,and is also known as matched die molding. The basic processconsists of heating a thermoset resin, under severe pressure,within a closed mold cavity until the resin cures througha chemical reaction of crosslinking polymeric chains. Underpressure, the resin liquefies and flows, taking the shape of themold cavity, and then hardens into the desired part orproduct. Once sufficiently cooled and strong, the part isremoved from the mold and the cycle is complete althoughthe curing reaction continues while cooling to ambient(room) conditions. Common resins include phenolic (phenol-formaldehyde), urea-formaldehyde, melamine-formaldehyde,epoxy, polyester, vinyl ester, silicone, alkyds, diallyl phtha-late (allyls), and various rubbers and elastomers.

This process is suitable for a wide range of industrial,commercial, and consumer parts and products ranging fromvery small to large automobile body panels. Plastics productareas are lighting and electrical devices, closures, trans-portation, and appliances. Specific items are electrical wallswitch plates and receptacles, circuit breakers, bottle caps,buttons, packaging, containers, covers, protective helmets,pump components, gears, brake parts, frames, pulleys,vehicle panels, dishware, and appliance housings, bases,handles, and knobs. It is especially useful for a wide varietyof fiber-reinforced products. Possibilities range from simplegeometries to complex three-dimensional shapes;Figure 17.1 shows an automobile electronic-throttle-controlhousing made from a polyester bulk molding compound withglass reinforcement.

Fundamentally, a compression molding device is a vice-like press with heat; hence, it is also known as a heated press,and a schematic view of the basic machine is found inFigure 17.2. The apparatus is compact but heavy and oftenhas its own support structure; or it may be placed on a sturdytable or platform. The heavy-duty metal base plate of thepress supports a lower platen and four guide (slide) rodswhich enables the up and down motion of the hydraulics. Thelower platen is heated in a variety of ways; common areelectric cartridge heaters, hot oil, or with steam. Electric

ics Engineering Handbook

011 Elsevier Inc. All rights reserved.

heaters are easily controlled in the common molding range of300e400 �F (150e200 �C), but hot oil is the preferred modewhen higher temperatures are called for. In addition toproviding a platform for any mold, this platen directly heatsthe lower half of the mold. Set directly above the lower platenis the upper platen, similarly heated to transfer thermalenergy to the top surface of an upper mold half. A part ejectorsystem consisting of ejector (knockout) pins connected to anejector (knockout) plate may be integral to the platen systemor part of the mold. The upper portion of the machine ofFigure 17.2 houses the hydraulic unit which basicallyconsists of a hydraulic-powered piston, or ram. Operation ofthe ram may be manual (pumped by hand), semiautomatic(opening a valve that supplies pressure), or automatic (timer-controlled). The hydraulic ram forces the upper platen downon the upper half of any mold placed within the machine, andthis type of compression press is termed a downstrokemachine. If the hydraulic force is upward, it is an upstrokemachine. Press forces may also be mechanical, as in a togglearrangement, or pneumatic that utilize facility-compressed-air for lower-force moldings. The daylight opening is thedistance between the main plates and represents the availablespace for the platens, mold halves, and handling of the mold.If the platens are integral to the machine, then the daylightopening would be the space between the platens and limitsthe thickness of the mold. At a minimum, the daylight mustaccommodate a stroke of twice the part’s depth. Molds maybe bolted to the platens or simply placed upon the lower one.In the configuration of Figure 17.2, the platens transfer heatonly through the flat surfaces in direct contact with the moldhalves. To improve thermal transfer, an option is to recess theplatens for mold insertion; this provides excellent mold bodysupport as well as more uniform heating, due to the fact thatthe mold body is surrounded by heated surface area.

Once the compression stage is completed, the part is readyto be removed from the mold cavity. A small mold is simplytaken from the daylight opening and cooled by drippingwater until sufficiently cool for safe handling. Or it is cooledby circulating water through the upper and lower platenswhile still contacting the closed mold. Thermosets are nearlyfully cured and can be taken out even when still somewhathot and require minimal cooling. For larger and morecomplex molds, cooling channels are machined into the molditself and water is pumped through them. Thermoplastic parts

289

Figure 17.2 Schematic view of the major components ofa typical compression press.

Figure 17.1 Complex automobile electronic-throttle-control housing compression molded with a glass-reinforced polyester bulkmolding compound. (Bulk Molding Compounds, Inc., West Chicago, IL)

290 APPLIED PLASTICS ENGINEERING HANDBOOK

must be completely cooled so they are not deformed whenejected from the mold.

17.2 Molding Force and Pressure

A compression press is rated by its capacity or maximumforce. The force is determined by the hydraulic fluid-pressureand ram area:

Press capacity ðtonsÞ

¼ ram area�in2

�, hydraulic pressure ðpsiÞ

2000 lbf=ton

or

Press capacity ðkNÞ

¼ ram area�cm2

�, hydraulic pressure ðMPaÞ

10

Selecting a machine’s capacity is a function of the forcenecessary to mold parts; a proper machine selection shouldinclude excess capacity and a 25% safety factor beyond theforce needed is reasonable:

Press capacity ¼ 1:25 ,F

where F ¼ force needed to mold the part(s). The requiredmold force is dependent on many factors including thespecific resin, resin form (granules or preforms), resinviscosity, fillers, additives, reinforcements, molding temper-ature, part thickness, and complexity of part design. Due tothe fact that the reaction kinetics and polymer rheology arecomplex in plastics processing, the actual molding force isbest set through experience or experimentation. However,there is some guidance for this force expressed as compres-sion molding pressure, P, computed from F/A. Here A is thepart’s projected area. The projected area is based on overallmaximum width and length dimensions of the cavity. Forregular, simple shapes this equals the actual surface area. Inmost cases the part is irregular and the projected area exceedsthe actual surface area which results in a conservative pres-sure requirement (more pressure than necessary). Therequired molding force is computed by:

ROBERT A. TATARA 291

FðtonsÞ ¼ PðpsiÞ ,A�in2�

2000 lbf=ton

or

FðkNÞ ¼ PðMPaÞ ,A�cm2�

10

The molding force is also directly related to the hydraulicfluid-pressure. If this pressure is available, then the moldingrequirement can be expressed in terms of the hydraulicpressure as:

Hydraulic pressure ðpsiÞ ¼ PðpsiÞ , A�in2

�

ram area�in2

�

or

Hydraulic pressure ðMPaÞ¼ PðMPaÞ , A�cm2

�

ram area ðcm2Þ

Various general recommendations are available for themolding pressure. Most span 2000e10,000 psi (14e69 MPa)with 2000e6000 psi (14e41 MPa) suitable for most appli-cations and molding granules and powders; 3000 psi (21 MPa)constitutes a reasonable first approach.

To improve upon the accuracy of predicting molding forcewhen accounting for very thick parts, a better model is calledfor. Strong [1] defines an excess depth factor, r, that adds tomolding force on parts exceeding some base thickness, d, andhas recommended values for parts of thickness, t:

FðtonsÞ ¼ A�in2

�,PðpsiÞ þ rðpsi=inÞ , ðt � dðinÞÞ

2000 lbf=ton

The usual base thickness is 100 (2.5 cm) so that thisequation becomes:

FðtonsÞ ¼ A�in2

�,PðpsiÞ þ rðpsi=inÞ , ðt � 1:000Þ

2000 lbf=ton

Figure 17.3 Photographic view of a 25-ton (220 kN) manual-hydraulic compression press. (Plastics Technology Labora-tory, Northern Illinois University, DeKalb, IL)

or

FðkNÞ ¼ A�cm2

�

,PðMPaÞ þ rðMPa=cmÞ , ðt � 2:5 cmÞ

10

where

� P ¼ 1500e8000 psi (10e55 MPa) and r ¼ 500e750psi/in (1.4e2.0 MPa/cm) for t > 100 (2.5 cm), or

� P ¼ 1500e8000 psi (10e55 MPa) and r ¼ 0 for t �100 (2.5 cm).

Specifically, Berins [2] recommends for phenolic moldingresins:

� P¼ 3000 psi (21 MPa) and r¼ 700 psi/in (1.9 MPa/cm)for t > 100 (2.5 cm), or

� P ¼ 3000 psi (21 MPa) and r ¼ 0 for t � 100 (2.5 cm);

or, when the charge is preheated the required pressure maydrop to 60 by 70%. In any case, all force equations mayunder-predict when molding extra thick sections orcomplex parts requiring excellent part definition.

17.3 Typical Presses

Compression press capacities range to 100 tons (890 kN)for smaller parts or parts with a short production run; largerand more automated machines extend to 5000 tons (44,500kN). As a minimum, a 250-ton (2200 kN) press is needed toprocess some reasonable range of products. Figure 17.3presents a photographic view of a 25-ton (220 kN) manual-hydraulic compression press. The press sits on its own large

Figure 17.4 Photographic view of a 30-ton (270 kN) electric-hydraulic compression press. (Plastics Technology Labora-tory, Northern Illinois University, DeKalb, IL)

Figure 17.5 Photographic view of a 500-ton (4450 kN)hydraulic press to process desk parts. (Lawton MachineryGroup, De Pere, WI)


metal base. The hand lever to pump and pressurize is on themachine’s left-hand side while the pressure release valve ison the opposite side. It is an upstroke unit and is shown nearlyfully closed, with a mold. In this position, visible is theextended hydraulic piston underneath the lower platen. At thetop of the machine is the control panel with a force gauge,toggle switches for the lower and upper platen heaters, andlight indicators that display when the heaters are cycling on.The controls to set each platen temperature are on thebackside of the unit. The cooling water-control valve is onthe machine’s left-hand side just below the control panel;the water lines are also visible in the figure. Slots are in theplatens that house bulb-type temperature readouts. For themold shown, sensors for mold cavity pressure and tempera-ture readings exit the front side of the rectangular mold.Figure 17.4 is a 30-ton (270 kN) electric-hydraulic upstrokeunit where an electric-driven pump moves the hydraulicpiston. This figure also includes the main control box witha power switch, heater switch, two press closing switches,press opening stroke, and upper and lower platen temperaturereadouts. To close this press, two buttons must be depressedsimultaneously; this ensures that the operator does not havea hand inside the machine’s clamping region. Figures17.2e17.4 are examples of small units. Presses larger thanabout 150 tons (1330 MPa) become quite heavy and largenecessitating solid, stable ground support. Figure 17.5

presents a 500-ton (4450 kN) upstroke press. It was used tomold school desk parts from a melamine compound; note theheavy-duty hydraulic features. A 2000-ton (17,800 kN)downstroke machine is available in Figure 17.6; this press iscapable of high closing speeds and utilized for compressingthermoplastic sheets of fiber-reinforced mats. The high forceis generated with three pistons. Rather than standard guiderods, this unit has 45� gib-guided columns for betterpositioning.

Electric molders, where an electric motor closes andopens the press and replaces the hydraulic pump, offeradvantages. In addition to eliminating the need to maintaina clean hydraulic system, an electric motor allows forsmoother, quieter, and more accurate platen movement andpart ejection. There are fewer energy demands as onceclamped, further energy is not expended in holding the moldclosed, and hydraulic fluid cooling is not needed. Butalthough becoming popular for injection molding applica-tions, all-electric compression presses are not yet plentiful.Generally, their electric components are more costly thanequivalent hydraulic ones but the cost differential is expectedto decrease in the future, especially for smaller presses.

The basic mechanics of the compression molding processhave not changed much and at the present time modificationstend to be driven by the need to accommodate new molding

Figure 17.6 Photographic view of a 2000-ton (17,800 kN)hydraulic press to process thermoplastic fiber-reinforcedsheets. (Lawton Machinery Group, De Pere, WI)

Figure 17.7 Automated resin charge delivery system. (HullIndustries, Inc., New Britain, PA)


materials. Typically, the molding sequence involves a loaderplacing the charge into the open mold; the closing andopening strokes are completed; and the ejector pins push outthe finished part which is picked or stripped from the cavity.A mold can have multiple cavities in order to manufactureseveral like, or different, items in the same cycle. Of course,as with all industrial processes, there is a preference tominimize cycle time and production costs. This is accom-plished by introducing automation and controls, keeping inmind that not all parts lend themselves to automation. Themost automated machine will provide a metered resin chargeinto the mold cavity, mold the part(s), eject the part(s), andtrim flash. A series of metering cups and resin delivery tubesis shown in Figure 17.7; a measured amount of resin isautomatically loaded into the cups and dispensed into themold cavities. A new set of charges is supplied to the cupswhile the part cures. Another automated machine type is therotary press. Here multiple molds move in a circular patternbut the unit is limited to smaller molding forces. Newerpresses have faster closing speeds, both to charge contact andthe compression stage. Structurally, large fiber-reinforced,molded parts require a very level molding platform andexcellent parallelism. Improved controllers allow forcomplicated molding recipes, multiple degassings, and bettermonitoring of the curing. For instance, rather than measuring

only time, the mold cavity is instrumented with sensorsmonitoring a specific resin property that correlates withcuring; the mold opens when an electrical or thermal prop-erty criterion is reached. Platen heaters may be subdividedinto several zones, each with an independent set-point andtemperature controller; this will smooth out the moldtemperature profile.

Here, for illustrative purposes, is a sample procedure withmanual manipulation of a small mold. This procedure is hotmolding of thermosetting granules, where both the mold andthe press platens are preheated before the resin is chargedinto the cavity of the mold. Of course, precautions must beobserved when molding, including wearing safety glasses,goggles, or a face shield; using heat-resistant gloves whenhandling the hot mold, molded products, and in the vicinityof the hot platens; and ensuring that operators be familiarwith the features and operation of the compression moldingpress prior to its use.

17.3.1 Sample Compression MoldingProcedure

1. Power on the machine by engaging the switch.

2. Choose either manual or automatic running mode.

3. Set desired process parameters including platen heat-ing temperatures, molding force, mold closure speeds,and cycle times.

Figure 17.8 Sample digital programming keypad of acompression molding machine. (Plastics Technology Labora-tory, Northern Illinois University, DeKalb, IL)


4. Close the mold without resin.

5. Open the safety door on the compression press andposition the mold in the middle of the lower platen;then close the safety door.

6. Turn on the upper and lower platen heaters at thetemperature set-point(s).

7. Close the press, contacting the lower and uppersurfaces of the mold with the corresponding platens,and hold until the mold is heated to the moldingtemperature.

8. Release the press and remove the heated mold.

9. Open the mold and separate its halves.

10. Apply a mold-release wax to the inside cavitysurfaces of both mold halves.

11. Pour a measured amount of resin (charge) into thelower half cavity, tilting it from side-to-side to spreadthe charge evenly.

12. Match the upper mold half with the lower and onceagain place the closed mold centered on the lowerplaten of the compression press.

13. With the safety door shut, stroke the press to pres-surize the mold cavity to an intermediate level fora specified amount of time, and then release thisclamping force to degas the mold cavity.

14. Pressurize to the desired maximum level and hold forthe required compression time to complete the curing.

15. Open the press, remove the mold, and turn off theplaten heaters.

16. Cool the mold, open it, remove the part, and deflash.

17. Reduce the lower and upper platen temperatures byengaging the platen cooling circuit to move waterthrough internal cooling channels.

18. Once the platens are cooled to near room temperature,stop the flow of water.

19. To force out residual water in the cooling system,gradually open the air valve for several seconds offlushing.

20. Power down the machine.

All process control parameters must be set beforea molding operation. Figure 17.8 is an example of an inputkeypad. The keypad is used in conjunction with an LCDdisplay showing set-points and processing messages. It isused to establish any process parameter such as moldingtemperature, molding pressure, molding time, manual versusautomatic control, and closing and opening speeds. Addi-tionally, other parameters such as machine alarms, molddegassing, heating rates, and complex processing schedulesmay be set, and saved for future repetition, by scrollingthrough the various menu options. Once the operatorbecomes familiar with the operation of the press and the

molding conditions are entered with the keypad, the machineis ready to produce a part. Of course, any procedure isdependent on equipment, tooling, charge, and partcomplexity.

17.4 Compression Moldsand Associated Tooling

Compression molds must withstand large shear andcompressive forces. Common materials for these molds arestainless and tool steels such as P20 mold steel. Steel moldsmay be chrome plated for extended life, improved wear, andsurface hardness. H13, alloyed with 0.35% carbon, 5%chromium, 1.5% molybdenum, and 1% vanadium, is adesirable tool steel having good hardness at elevatedtemperature, wear resistance, and impact resistance. Beryl-lium copper offers hardness, high strength, and good heattransfer to accelerate mold heating and cooling. Polishing ofinterior mold cavity surfaces ensures part quality; due to thehigh compressive forces, any cavity imperfections will bemirrored in the part itself. Other possible materials for shorterruns and lighter duties are 6061 and 7075 aluminums. Moldsare made by hobbing, EDM, or machining.

For smaller parts and limited production runs, molds aremanually manipulated; they are charged, placed between theplatens, and removed for cooling and part ejection. When thisis no longer practical, mold halves are bolted directly to theplatens and the ejector system is integral to the movement ofthe press. The charging of resin and taking the ejected partfrom the press is still a manual procedure. A fully automatedsystem does not require manual charging or part removal.

There are three main types of mold closures. But thesetypes may always be redesigned, modified, and altered tobetter serve the process. The flash type is the simplest andleast expensive and suitable for relatively thin, shallow parts.The cavity is slightly overcharged with resin so that excessflash material, a few thousandths of an inch (hundredths ofa millimeter) thick, must be trimmed off in a post-processingoperation. This design relies on flash formation to ensure thatthe part is fully formed. If extensive flash removal is required,the initial advantage of inexpensive mold construction isoffset by post-processing costs. Since there is an outlet for

Figure 17.9 Schematic view of semipositive and flash moldclosure designs.


excess material, higher compression forces produce moreflow ending as flash, rather than maximizing the cavitypressure. Thus, products having only medium part definitionare produced; such parts may have inconsistent density andcorrespondingly mediocre physical and mechanical proper-ties. The semipositive type of mold closure allows for a smallamount of excess resin that is easily pinched off and sepa-rated from the part; separation may occur in conjunction withthe mold opening and part removal steps. With no flash ora very thin layer of attached flash, the part requires little post-processing. With this closure type, resulting parts have a highdegree of compaction yielding very good performanceproperties. Figure 17.9 illustrates the key differencesbetween the flash and semipositive designs. For the flashtype, a flash ridge creates an overflow path to a flash cavity orpocket, where excess material accumulates. (Rather thana contained, internal pocket, an alternate design can direct theflashing to outside the mold’s boundary, where it may expandwithout limitation.) The material freezing at the flash ridge isthinner than the part and processed to separate it from thepart. This figure also highlights the use of mold guide pins toensure more accurate clamping alignment. Figure 17.10 isa photographic view of a sample part with the associatedflash when molded with each type of mold closure.

(a) Excess flash from semipositive mold closure

Figure 17.10 Pictures of a sample part with flash made from two

Figure 17.10.a illustrates the thin, uneven flash layer that istypical with the semipositive closure. Here the flash thicknessis approximately 0.002500 (0.06 mm) and easily stripped off.In Figure 17.10.b, a portion of the heavier flash is broken offto show the boundary between the part and the waste materialfrom the flash ridge and pocket in the flash design.

Lastly, the positive type utilizes an exact metering ofcharge, and the two mold halves form a complete seal; ofcourse, the amount of charge must be carefully controlled asany excess or deficiency of resin will affect part dimensions.But severe overcharging will prevent complete closure of themold halves while a significant deficiency in charge willproduce an incomplete product. Small clearances for flash canstill be provided for, especially if there is a chance of excesscharge, even though very little or no flashing occurs duringthe molding. This form of tool is well-suited when processingfiber-reinforced molding compounds since fiber-rich layersdo not extrude easily into flash. Figure 17.11 displays a thin,300 (76 mm) square plate with its tooling. The plate is madewithout any flash as the mold is of the positive closure type.

In any case, due to the high molding pressures, as withinjection-molded products, it is difficult to remove a finishedpart from the mold cavity, despite the fact that the part shrinksupon cooling. Even with a mold taper and coating a releaseagent on the inside surfaces of the cavity, great surface tensionforces and mechanical compression can adhere the part toeither mold half. The force to remove a part depends on theresin as well as the cavity construction, and a well-designedejector system is needed. Since at this point the part is not fullyhardened, small-diameter ejector pins may penetrate or scarthe part’s surface as the pins push it out. Using additional pinsor larger-diameter pins better distributes the ejection force.When the part tends to stick to the lower mold half, a solutionis to fabricate a thin metallic plate in the shape of the bottomsurface of the part and put it at the bottom of the mold half; the

(b) Excess material from flash mold closure

different mold closures.

Figure 17.11 Square plate with its positive closure compression mold.


charge is placed on this plate and the part is molded as usual.Now the ejector pins force out the plate with the part, evenly,without pin-to-part contact. Figure 17.12 is a photographicview of such a design. Once removed successfully, any partmay be supported in a fixture until completely cooled; this willminimize dimensional distortion and warpage from thermalcontraction and stress relief.

17.4.1 Mold Instrumentation

It must be remembered that any specified moldingtemperature is that of the mold body, not the platens. But,since presses have programmable platen temperatures, inmany cases it’s the platen temperatures that are tracked.Therefore, it is imperative to minimize the differencebetween platen and tool (mold) values. This difference existsdue to two factors. Firstly, because the tool material has

Figure 17.12 Sample bottom mold half with part cavity plate and

a thermal capacity, there is a time lag between the platen set-point value and the actual mold temperature. Secondly,unless the mold is well-insulated, radiative and convectiveheat losses from the exposed surfaces of the closed mold toits environment occur.

Figure 17.13 shows an illustrative temperature profile.Here, a thermosetting resin is compressed while still at roomtemperature (cold formed), and then the platen heaters areactivated until the mold is brought up to a curing temperatureof 340e350 �F (171e177 �C). The mold’s mass (empty) was30.5 lbm (13.8 kg) and it was fabricated from 4140 steel; itsrectangular dimensions when closed were 8.87500 (225 mm)long, 2.31300 (59 mm) wide, and 3.50000 (89 mm) high.Throughout the cycle, there is relatively close agreementbetween the upper and lower mold sections’ temperatures,within 10 �F (6 �C). But note that each platen is 20e40 �F(11e22 �C) warmer than its corresponding mold half. Thus,

ejector pins.

3020100Compression time, min

50

100

150

200

250

300

350

400T

em

peratu

re, °F

40

80

120

160

200

°C

upper platen

lower platen

upper mold half

lower mold half

Figure 17.13 Temperature profile for upper and lower platensand steel mold halves during a compression molding cycle,starting with the mold at room temperature.


any platen set-point would not accurately represent the tooltemperature, and it is beneficial to be able to directly monitormold temperatures in addition to platen values.

The heat transfer path from the surface of each platen intothe mold is conduction while heat is lost simultaneously fromthe sides of the closed mold to the surrounding air (ambient).Kuczmarski and Johnston [3] have demonstrated that theconvective losses constitute a more significant factor thanmold body conduction when minimizing the platen and tooltemperature differential. Their numerical simulation indi-cated that reducing the exposed surface area by redesigning

Figure 17.14 Common probes for monitoring mold body and mLaboratory, Northern Illinois University, DeKalb, IL)

the mold is a reasonable way to decrease the temperaturevariation, and more practical than using a mold materialhaving a higher thermal conductivity.

A mold is easily instrumented with thermocouples, at leastone in each half of the mold. For the expected temperatures,type J (iron-constantan) probes are suitable and have a stan-dard measurement uncertainty of �4.0 �F (2.2 �C), whilea corresponding digital monitor or readout will add another�0.7 �F (0.4 �C). Additionally, a measure of the cavitypressure can be obtained with a force transducer. Althoughthe cavity is not pressurized as in the case of a true fluid,cavity pressure readings may be used to verify that themolding force is completely transmitted to the resin charge.In some mold designs, there is the potential of metal-to-metalcontact between the upper and lower mold halves prior tocomplete closure; in such a case, the machine’s force wouldnot be equal to the force on the resin. A typical surface,diaphragm-tipped transducer is quite accurate and hasa range to 10,000 psi (69 MPa); it is also available witha thermocouple at its tip. However, the pressure must besensed by contacting the part’s surface and will create a smallblemish. Thus, such a probe must be located where a highsurface finish is not a requirement. Figure 17.14 showsa thermocouple probe with a threaded pipe fitting that isscrewed into a mold body. Also seen is a pressure andtemperature combination probe, whose tip is fitted flush withthe cavity wall of a mold to monitor the melting charge.

17.5 Commonly Used Resins

Phenolic resins are produced by chemically combiningphenol and formaldehyde through a condensation reaction

old cavity temperatures and pressure. (Plastics Technology

Figure 17.15 Granules of a melamine-phenolic moldingcompound.


under alkaline conditions. Depending on the specific reactionmechanism employed, either a resole resin or a novolak isformed. Resoles are referred to as single-stage polymerssince they do not require any curing agent. Novolaks, on theother hand, are designed to incorporate a curing agent, suchas hexamethylene tetramine (usually called hexa), and arereferred to as two-stage; fillers and additives are then addedto produce standard phenolic molding compounds. Appli-cations include castings from molding powders, coatings,adhesives, and binders. Phenolic resins can tolerate hightemperatures and bear large mechanical loads with minimalcreep and are also bonding agents, since they mix well withboth inorganic as well as organic fillers and reinforcements.They have very good solvent resistance and product life.Additionally, the phenolic molecules crosslink through thefiller and reinforcement, which helps to provide excellentfinal properties.

Closely related to phenolics, amino plastics are the resultof aldehydes reacting with amines. Examples include urea-formaldehyde and melamine-formaldehyde. Urea-formalde-hyde crosslinks with water as a byproduct that is vented outthe mold. Its molding compounds, similar to those ofphenolics, contain filler, catalyst, colorant, plasticizer, andlubricant. A stabilizer can be included to increase the storagelife by retarding chemical activity. Parts made from urea-formaldehyde have very good electrical properties along withresistance to various chemicals, including solvents and weakacids and bases. For better property performance, melamine-formaldehyde is harder and even more resistant to chemicals,moisture, and heat. Melamines offer many moldingcompounds in a wide range of colors. Melamine-phenolicresins are mixed compounds known for brighter coloring andless part shrinkage than melamine-only molding compounds.

Another popular resin is unsaturated polyester, especiallycommon in fiber-reinforced parts. Unsaturated polyesters arecured through heating and styrene monomers crosslink thepolymer chains. Polyesters have strength and resilience whilevinyl esters offer better chemical and moisture resistance.Chemically within the polyester family are alkyds. Theirmolding compounds are standard formulations of filler,catalyst, colorant, and lubricant and quickly cure at relativelylow pressure and temperature. Molded products are dimen-sionally stable, especially when glass fibers are present, andhave very good electrical properties.

For higher temperature applications, epoxies can be castinto thick sections and easily reinforced becoming dimen-sionally stable; molded parts resist attack by moisture andchemicals. Epoxies polymerize when rings of epoxy groupatoms open and link other monomers. Diallyl phthalate (DAP)is a common allyl molding dough consisting of filler and shortfibers, and its curing does not create corrosive byproducts.Parts are easily compression molded and tend to have goodstrength and excellent electrical, thermal, moisture, andchemical resistance. Silicones are primarily elastomericmaterials but also are filled or glass-reinforced and molded

into stiffer, solid, structural parts. These lightweight productsexhibit excellent electrical and thermal performance.

Less used are various thermoplastics including nylons,polypropylene, and polyethylene terephthalate. Although notas strong, these have more impact resistance than thermosetsand also may be reinforced.

17.6 Resin Charge Characteristics

Unlike many conventional thermoplastic processingmethods which utilize resin as pellets, in compressionmolding the resin charge is available in several forms.Common are granules, a coarse powder, that when heated andpressurized liquefy and cure to harden. Figure 17.15 isa photographic view of a charge of melamine-phenolicmolding compound in standard granular form. Pastes arecombinations of a liquid thermoset with its filler. Doughmolding compounds are viscous pastes that are mixes of theresin, filler, and reinforcement as in a BMC (bulk moldingcompound). Other engineered compounds, SMCs (sheetmolding compounds) and GMTs (glass mat-reinforcedcompounds), are manufactured in sheets. Preforms are gran-ules that have been already compacted into a simple shape,such as a disk, for easy handling and loading into the moldcavity and overall convenience. Preforms introduce a degreeof automation to the charging step and they may be made atthe molder’s facility or at a remote site; preform-use greatlyreduces cycle time and increases part production rates.

17.6.1 Thermoset Resin Pricing

Thermosetting resins are the primary materials for thecompression molding process. As with all resins, pricing canchange rapidly and significantly, trending with petro-chemicalfeedstocks. Pricing can also vary greatly from moldingcompound to compound even for the same base resin; thepresence and type of reinforcement as well as certain

Table 17.1 Relative pricing of common thermosetting resins[4]

Thermoset Resin Relative Price)

Alkyds 0.9

Phenolic moldingcompounds

1.0

Melamine-phenoliccompounds

1.1

Melamines 1.2

Epoxies 1.6

Polyester 2.1

Phenolics, reinforcedgrades

1.4e3.6

Vinyl ester 3.1

Silicone epoxy 4.5

Diallyl phthalate (DAP) 3.3e6.6

Silicone molding compounds 8.1

)Price of the resin divided by price of standard phenolic molding

compound


additives will substantially alter the molding compound cost.Table 17.1 lists common resins with relative costing, based onstandard phenolic molding compound. The phenolics andmelamines are common, inexpensive molding resins as areureas. Alkyds also are priced attractively. Epoxies and poly-esters are slightly higher in price but still cost-effective fora wide range of products. The high cost of allylics, such asDAP, is an obstacle and somewhat restricts their use. Like-wise, silicones are limited to applications where their propertyperformance is needed to justify the higher cost.

Common thermoplastic molding pellets are derived fromfossil fuels and suffer the price fluctuations present in thismarket. However, thermoset molding compounds containa high percentage of filler; the fillers are usually inorganicand made of inexpensive, plentiful materials. Thus, thethermoset molding compounds are not as sensitive to marketcrude oil and natural gas pricing. Currently, in response tosustainability issues, molding compounders are lookingtoward recycled materials for fillers, including waste streamsfrom fiberglass manufacturers. Also there are moldingcompounds where bio-based, renewable polymers havereplaced some portion of the standard, active polymer;“Green BMC” incorporates a soy-based resin and has prop-erties comparable to standard formulations.

17.7 Processing Parameters forGranules, Powders, and Preforms

Processing by compression molding ranges fromcompletely manual to a high degree of automation. The

charge can be automatically weighed and poured into themold cavity where the press cycles through heating,compression, degassing, and cooling. Programmable roboticarms can open, close, and move a mold and handle thefinished part(s). As with other plastics processing techniques,there are several important parameters that call for closecontrol.

17.7.1 Curing and SolidificationReaction

Most thermosetting resins are heat-activated and theirpolymerization reaction ultimately crosslinks the chains asthe molecular weight of the material approaches infinity. Anypolymerization via condensation reaction produces smallamounts of gas or vapor (such as water), that must be ventedout of the mold cavity. Venting ports may be provided withinthe mold or the mold may be slightly opened during thecompression molding cycle to release any gas; such a step isknown as bumping, breathing, or degassing the mold cavity.A minimum 1/16” (1.6 mm) opening of the mold cavity, heldfor a few seconds, would provide sufficient venting. Ventingis especially important for thick phenolics and any otherformaldehyde-based resins.

Polymerization, by addition or condensation, is usuallyexothermic adding to the external heat from the platens. Dueto the fact that the resin in its raw state, and also whenfinished, is a thermal insulator, excess heat can be generatedand trapped within thick parts. (Only the layers near thesurface of the curing part will have high heat transfer rates tothe conductive mold surface.) The excess heat generationwill accelerate the crosslinking and can be great enough sothat a thicker part can cure faster than a thinner one that reliesmainly on the heat transferred from the mold cavity surfaces.In the extreme, thick parts or part sections may suffer thermaldegradation of the polymer chains, reducing part appearanceand performance properties.

In the case of thermoplastics which are already fullypolymerized, the curing of a heated liquid or semi-solid toa complete solid is a phase change accompanied with heatremoval. The heat sink consists of the upper and lower moldsurfaces, so that the resin layers in the vicinity of thesesurfaces freeze first. The process is complete when the coresolidifies.

17.7.2 Resin Charge Control

Compression molding is charge-dependent; so over-charging prevents the mold halves from seating properly andcreates added flash. For optimal physical and mechanicalperformance, any preform or resin granules should bedistributed to match the part’s shape; this promotes uniformcured density which yields maximum toughness andcompressive strength. Thus, a circular preform would not beappropriate for a rectangular cavity. Any flash removalimparts additional cost in preparing the part. In some cases,


especially with tougher resins, the flash may be difficult toremove. Methods such as cryogenic flash removal involvecooling the part and the additional brittleness allows the flashto be broken off. If the charged mass is deficient, the part willbe dimensionally incorrect. Or the plastic will havevoids from gases evolved and trapped where not enoughresin was available to displace the gas as the mold waspressurized.

When starting with a room-temperature charge, thethermal energy is transmitted by conductive heat transferfrom the platen surface, through the mold body, and into thecharge layer in contact with the mold cavity. For very thickparts and/or when utilizing preforms, preheating the chargeoffers substantial molding benefits primarily in shorteningcycle time, since the preheating is done external to themolding machine. A 70 or 100 MHz high-frequency elec-tronic heater can bring a preform’s temperature up to 212 �F(100 �C) in 10 s and because the heating is internal, similar tothat from microwaves, the preform temperature is moreuniform than would be from platen-generated heat [5]. Afterpreheating, the preform is transferred to the mold cavity andless time is required for the platen heaters to bring the chargeup to the standard 300e400 �F (150e200 �C) moldingtemperature. Furthermore, the softened charge will floweasier upon initial contact by the mold halves. But preheatinga thermoset will initiate the curing so that transfer time mustbe kept to a minimum.

17.7.3 Mold Closure Control

As with most non-Newtonian fluids, liquid polymericmaterials are sensitive to shear and temperature. The appli-cation of the molding clamping force must be smooth andeven. If the application is too rapid, the resin will shear,overheating the molecular chains. On the other hand, if themolding pressure is ramped up too slowly, the resin will cureand harden before maximum compaction occurs. One usefulfeature of many molding machines is two-stage clampingwhich allows for a relatively rapid mold closure up to thepoint at which the resin begins to compress; then theclamping rate is reduced as the closure continues tomaximum cavity pressure. Hull [5] recommends 200e800in/min (85e339 mm/s) and up to a maximum of 80 in/min(34 mm/s) for the first and second closing stages, respec-tively. Alkyds and polyesters polymerize rapidly, thus thecompression speed must be quicker than usual. Although therate to open the mold and reset the compression stroke shouldbe as rapid as possible, the initial release of the closed moldshould be gradual to prevent part damage. This is especiallycritical for thin parts which may have sections pulled outalong with the opening mold half, while the rest of the partremains inside the other mold cavity. Along with speedconsiderations, the platens and guide rods must ensure thatthe compression force is applied exactly perpendicular to themold body for uniform pressure upon the charge.

17.7.4 Temperature Control

Although thermosets tolerate a range of temperatures,excessive heat degrades the resin resulting in brittleness,other reduced mechanical strength properties, and poor partsurface quality. A liquefying resin’s viscosity is inverselyrelated to and strongly dependent on temperature. Therefore,if the mold temperature is set too low, the resin’s viscosity isgreatly increased and the material may not flow enough,especially to constricted-volume regions, for optimal partdefinition. As in the case of molding pressure, the optimaltemperature will be a function of the specific resin, resin form(granules or preforms), resin viscosity, fillers, additives,reinforcements, molding pressure, part thickness, andcomplexity of part design. This temperature is best deter-mined by experience and testing, although the consensus is300e400 �F (150e200 �C) for most resins, includingphenolics. Slightly lower molding temperatures are recom-mended for amino-based resins [6]: 260e340 �F (127e171 �C)for urea-formaldehyde and 260e360 �F (127e182 �C) formelamine-formaldehyde, and likewise 275e350 �F (135e177 �C) for allyls [7]. Of course, preheating the charge priorto insertion into the mold cavity may lead to better partquality and shorter cycle time.

17.7.5 Cycle and Molding Times

For economic efficiency, overall cycle time must beminimized. Note that the molding time is the time under fullpressure and does not include other steps such as loading,mold closure, degassing, and part ejection, all of whichcontribute to the overall cycle time. Compression moldingtime must be adequate for complete curing, but maintainingthe resin too long at compression temperature and pressurewill degrade the material. Clearly, part thickness is a crucialfactor as a thin part will harden in seconds while a relativelythick one requires many minutes to complete the curingprocess. Recommended compression (curing) times are ½to 5 min, with 1½ min at 340 �F (171 �C) as typical [2].Although the part can be removed from the mold onceminimal stiffness is achieved (hot rigidity), the polymeriza-tion reaction continues and may require hours for completecure. Thus, there exists the potential for part deformation inthe form of warping if the part is unsupported after removalfrom its mold. Increasing molding temperature at least 9 �F(5 �C) and/or the compression time 5e10 s will acceleratemolecular chain crosslinking and reduce warpage [5].

17.8 Resin Matrix Modifiers

A modifier may be classified as an additive, filler, orreinforcement. Additives are blended into a resin at lowconcentration, a few percent by weight. An additive isemployed to significantly affect a physical, chemical,thermal, optical, electrical, environmental, or processing


property and can be relatively costly. On the other hand, up to60e70% filler may be mixed into the resin. Loadings of even90% are possible, but in that case the resin serves as anadhesive or binder and the part made is not a true plastic.Fillers are inert and are used to add bulk volume or to replacethe more costly resin. Reinforcing agents are longish fibersmolded with the resin matrix to improve mechanical strength.Tensile, compressive, impact, and bending strengths are allgreatly increased. Reinforcing materials range from commonand inexpensive glass or paper mats to exotic polymer orcarbon ones.

17.8.1 Additives

To improve mechanical, physical, chemical, electrical,optical, thermal, or molding performance, additives may becombined with the resin matrix. Reaction catalysts includehexamethylene tetramine for phenolics. Mica, alumina, andsilica provide electrical arcing resistance. Halogenatedcompounds are a fire retardant, while zinc stearate and wax-like materials improve part release from the mold cavity. Themolding process is improved through solvent-like additivesthat reduce the charge’s viscosity. Wetting and adhesionpromoters improve bonding between a resin matrix and anyreinforcement fibers. Viscosity thickeners minimize theseparation of fiber from the resin matrix. Reaction inhibitorsprovide longer storage life for bulk and sheet moldingcompounds by delaying the onset of crosslinking. For SMCs,low profile additives reduce the shrinkage, important foroptimal surface finish. Examples of physical propertymodifiers are colorants such as dyes and inorganic or organicpigments. Antioxidants, heat stabilizers, impact modifiers,and ultraviolet stabilizers are needed for thermoplastics,which lack the durability of thermosetting materials.

Any additive may be mixed into a resin charge or isalready present as with standard molding compounds. Ifphysically mixing the additive, care must be taken to thor-oughly combine it with the resin granules. Unlike extrusionand injection molding, where extensive mixing takes placeautomatically via the action of the screw or auger, little flowoccurs in compression molding and uneven additive distri-bution in the charge will display itself in the finished part.Additives, typically, are added in concentrations under 5%; atlevels above this they could be considered fillers.

17.8.2 Fillers

Currently, many plastic products utilize low-cost materialsas fillers. Ideally, the filler is added in a concentration thatallows the product to retain sufficient mechanical strength,physical properties, and final quality. Certain fillers mayimprove a plastic’s shrinkage, thermal stability, color, andopacity. If the filler is added in a reasonable quantity, anydegradation in strength from that of unfilled resin can beoffset by the cost savings. Common fillers include clay, talc,ground limestone, carbon black, marble dust, glass, paper,

wood flour, and metals, and are added in concentrationsranging from 10 to 50% (by weight). To enhance biode-gradability, bio-based fillers are receiving increased atten-tion. Examples include wood flour, sugar cane, lignin, flax,grasses, bamboo, starch, chicken feathers, soy protein, andcellulose. This trend is consistent with recent United Statesgovernmental policies that prioritize the procurement ofmaterials having significant bio-based content; this programis targeted to increase to a 50% bio-based level over the nextseveral decades. Natural and bio-based materials are suitableonly for low to moderate operating conditions as they havelow thermal resistance and tend to absorb moisture; theprinciple advantage is low cost.

For phenolics, biofillers are added in two ways: chemicalreaction and simple mechanical mixing. Chemically,combining a biofiller with phenolic and using sodiumhydroxide as a catalyst, then molding, is a more complexactivity but imparts added strength to molded specimens [8].But any such pretreatments (including any drying) make thefiller more costly. Simple mechanical mixing can lead tosatisfactory parts. A study on compression molding corn-based DDGS (distillers dried grains with solubles) withphenolic, generated specimens that were tested for tensileyield strength, Young’s modulus, and percent elongation atyield [9]. Results demonstrated that at 25% DDGS, byweight, the tensile yield strength is still approximately halfthat of the pure phenolic case. Higher levels of biofillerfurther reduce the strength to nearly one-quarter (at 50%DDGS), and under one-sixth (at 75% DDGS) the baseline.Young’s modulus is also reduced from that of 100% resin, butless severely. At 25% DDGS, a 10e15% stiffness reductionis noted, while 50% and 75% filler levels cause 50% and 70%decreases in Young’s modulus, respectively. The ductility ofthe blends, as measured through percent elongation, isrestricted about 50% by inclusion of biofiller, althougha clear trend is not seen. In this study, bio-based filler contentwas also trended with hardness (Figure 17.16); data includedthe DDGS as well as Alpha grass lignin and wood flour fromother researchers. Figure 17.16 indicates a general softeningas filler level increases. The Shore D values are about10e30% lower for 30e90% filler; data all correlate reason-ably well which indicates that performance is mostlydependent on filler concentration rather than the actualbiomaterial.

17.8.3 Reinforcements

To directly improve tensile strength, stiffness, compres-sive strength, and impact resistance, fibers are added toa thermoset or thermoplastic. Often, the fibers are glass butcarbon, metallic, inorganic, and other polymeric ones arepossible. Of course, these are the key component in a BMC,SMC, GMT, or a long fiber thermoplastic (LFT). Althoughcompression molding is well-suited to making parts andproducts with high strength-to-weight ratios, in some

Figure 17.17 Stack of bulk molding compounds. (Bulk MoldingCompounds, Inc., West Chicago, IL)0 20 40 60 80 100

Biofiller concentration by weight, %

0.0

0.2

0.4

0.6

0.8

1.0

1.2R

atio

o

f s

urfa

ce

h

ard

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ss

w

ith

b

io

fille

r to

ha

rd

ne

ss

a

t 0

% b

io

fille

r

BIOFILLER

DDGS

Alpha grass lignin

wood flour

best fit of data

Figure 17.16 Relative Shore D surface hardness responsefor phenolic resin with varying percentages of bio-based fillers.


instances their mechanical properties are affected by theprocessing. Additionally, part surface quality, mold design,and cycle efficiency are issues.

17.9 Engineered Fiber-ReinforcedMolding Compounds

Commonly used to make strong, stiff, and lightweightproducts are bulk molding compounds (BMCs) and sheetmolding compounds (SMCs), often unsaturated polyesterresin with glass fiber reinforcement. Non-thermosettingalternatives, introduced around 1980, are the GMT (glass matthermoplastic) and LFT (long fiber thermoplastic). Theautomotive and truck industries make great use of these fourresin forms for bumpers, panels, lids, hoods, and pumpimpellers and housing. Other products are boat hulls, doors,roofing, satellite dishes, snowmobile panels, appliancehousings and control panels, lighting fixtures, sinks, andfurniture. Compression processing of these compounds hasspecial requirements and outcomes.

A typical BMC, also termed DMC (dough moldingcompound) or premix, is composed of a resin, filler, rein-forcement, and catalyst. The filler loading may be quite high.The components are simply mixed together, forminga viscous paste; mixing intensity is limited to avoid shearheating the thermosetting resin and preventing damage to anyglass fibers. Figure 17.17 presents a stack of BMCs; thesemay be cut and used directly in a mold or pressed into pre-formed shapes. Fibers can be up to 100 (25 mm) long, but 1/800to 1/200 (3e13 mm) is more typical. A standard formulation isunsaturated polyester resin, calcium carbonate filler, mold

release, colorant, inhibitor, other additives, and 10e25%, byvolume, glass fibers [10]. Due to the fact that the material ispartially polymerized, it is tacky with a consistency similar todough. In the compression process, the resin charge iscentered in the lower mold half and the material will flow tofill the cavity. But there may be separation of the BMCcomponents if the flow length is too great; this would result inparts having non-uniform mechanical performance. This isthe restrictive factor so other combinations of resin andreinforcement are sought. Additionally, if more concentratedand/or longer fibers could be utilized, many parts wouldexhibit better mechanical strength.

One option is the sheet molding compound which is verysuited to larger parts. Here, the reinforcement fibers arechopped, randomly oriented, and commonly 0.7500e200(19e51 mm). A reasonable mix, by weight, of resin/inor-ganic filler/100 (25 mm) chopped glass reinforcement is25/45/30 [11]. The fibers are securely encased betweenpolyethylene films that have been coated with resin (poly-ester, vinyl ester, or epoxy) and filler. Rather than mixing ofcomponents as in BMCs, fibers are carefully placed by pro-cessing equipment and contained by resin layers. This alsoallows for the placement of longish glass fibers in adjustableconcentration and direction, thus increasing the strength,compared to the randomly directed material, of any finishedpart. Like a BMC, a sheet molding compound is tacky and thepolyethylene films separate individual resin/fiber layers.SMCs are available as large rolls, and individual sheetsmatching the desired product’s size are cut from the roll. (Thethermoplastic cover layers are discarded.) Recently, attentionis being devoted to reduce the weight of SMC composites.Calcium carbonate, the common inorganic filler, contributesto the total material’s specific gravity of 1.9; substitution byglass beads reduces this by 1/3 with only minor property loss,and nanocomposite fillers and carbon fibers have even greaterpotential [12].


Glass mat-reinforced sheets are composed of thermo-plastic polypropylene (polyamides, polycarbonate, and PVCare occasional options) with glass fibers 100 (25 mm) or morein length. A variety of fiber layer types are available such asdirectional, random, shorter, longer, mats, or weaves. Whena natural fiber such as flax is used, the composite is termed anNMT (natural fiber mat-reinforced thermoplastic). Usinga polypropylene matrix, along with typical glass reinforce-ment, provides stiffness and superior low-temperature impactstrength. Since with thermoplastics polymerization iscomplete, GMTs must be preheated before compressionmolding. Although not as mechanically strong, stiff, andheat-resistant as an SMC, a GMT panel is tougher and morelightweight.

Having a fiber length of at least 0.400 (10 mm) is the longfiber thermoplastic, LFT. Here, polypropylene is combinedwith fibers, typically glass but aramid, carbon, and stainlesssteel are also available, and often molded into automotiveparts and panels. Other resins are polyamides, polycarbonate,and thermoplastic polyesters. Fiber content ranges from 20 to60% by weight with 40% standard. There are two resindelivery methods to a compression press. One way is to meltpre-compounded pellets that contain fibers spanning thelength of the pellet, and then place the charge within themold; pellets up to 1½00 (38 mm) are available fromcompounders. The melting is done with some type of plas-ticizer, such as an extruder. An alternative is to compoundfibers with plain pellets in an extruder on site, and direct theformulation to the mold. Here, the fibers are introduced in theextruder as late as possible to minimize fiber damage. Due tosimpler processing, LFT material is less expensive than GMTwhile capable of lightweight, impact resistant, and strongparts.

17.9.1 BMC, SMC, GMT, and LFTProcessing

A BMC resin is centered in the lower mold cavity; uponpressurization, it will flow. The flow will be predominatelydirected in-plane and radially from the center point of thecharge.

For SMCs, material can be centered or distributedthroughout the entire mold, and part thickness is built upwithin the mold cavity by adding more sheets. Thus, thiscompression produces far less flow, compared to the BMC

Table 17.2 General processing conditions for engineered fiber-re

Fiber-Reinforced MoldingCompound

Molding Pressure, psi(MPa)

BMC [15] 350e2500 (2.4e17)

SMC [15] 500e2500 (3.4e17)

GMT [17] 1500e2000 (10e14)

process, and the integrity of the reinforcement is maintained.Generally, 1 to 4 sheet(s), each about 1/800 (3 mm) thick, areused and occupy about half the mold’s surface [13]; ofcourse, extra sheets can be added where additionalmechanical strength in a part is sought. The compressionprocess creates some flow, orientating the fibers; the chargelayers contacting the hot mold cavity surfaces will flow andharden before the center layers. Thus, properties havedirectional dependency in the completed parts. The place-ment and shape of the charge are even more significant thanthe molding conditions, in spite of the fact that the resin flowsa rather short distance in the mold’s cavity. Due to fiberorientation in the direction of flow, tensile strength andtensile modulus can be over 2½ and 1½ times, respectively,the value perpendicular to flow for a sample part witha charge initially covering 38% of the mold [14]. The degreeof anisotropy also affects residual stresses, shrinkage, andwarping and diminishes as the mold coverage area isincreased. A reasonable area is 60e80% and fiber orientationis always considered while placing the charge.

A glass mat thermoplastic charge spans about 80% of thepart’s surface, but layers ought not to be placed side by side[15]. Here, two separate flow fronts will fuse together at thejunction between the layers. This creates a classical weld(knit) line and produces a latent, or obvious, part defect. Thedefect may be in part appearance or mechanical strength. Partthickness control is through stacking of individual sheets.Molding problems include part blistering, shrinkage, andwarpage.

Although not too different from processing powders orgranules, the compression molding conditions for BMC andSMC resins include a wide range in pressure due to variationsin the shape, size, and molding compound formulation.Forces tend to be less while molding temperature and curingtime also vary depending on catalyst, part thickness, andmold closing speed. The curing reaction is temperature-sensitive and, approximately, each 18 �F (10 �C) decrease intemperature increases curing time by 50% for SMC partsthinner than 1/400 (6 mm) [16]. Representative conditions arepresented in Table 17.2. Davis et al. [10] propose somewhathigher pressures, 2000e6000 psi (14e41 MPa) with a cycletime of 55e95 s for SMCs, and 3000 psi (21 MPa) along with120e140 �F (50e60 �C) for GMTs and LFTs with at least60e90 s just to preheat a glass mat thermoplastic prior totransfer to its mold. Note that Table 17.2 lists cycle time

inforced molding compounds

Molding Temperature,�F (�C)

Cycle Time (Exclusive ofAny Preheating)

250e350 (121e177) 30 s to many minutes

250e350 (121e177) 30 s to many minutes

77e160 (25e70) 30e60 s


minus any preheating, but it does include platen travel,unloading, loading, and curing. The reaction curing time maybe half of the cycle time for an SMC. But care must beexercised when attempting to minimize the overall cycletime, especially in the mold closure rate. Closing too rapidlymay prevent adequate venting of air, water vapor, or othervolatile gases that will be trapped within the resin matrix.(Using a vacuum to increase the venting potential is oneprocessing option.) This detracts from product appearance,creating surface blisters and internal voidage, and diminishesperformance properties. Of course, if the rate is too slowwhen molding thermosets, the material will become overlyviscous as it solidifies.

In any case, a uniform mold cavity temperature isimportant when manufacturing SMC parts. For instance,Barone and Caulk [18] have shown that a 45 �F (25 �C)temperature variation in the surface temperature of the moldexists in larger SMC-molded parts and is proportional to(part’s length)2/(thickness , cycle time). This temperatureinconsistency is magnified when trending to thinner partsmade with shorter cycle times. Such variance leads toinconsistent surface quality and is mitigated throughimproved design of platen heating. A non-uniform moldingtemperature may also lead to blistering. When moldingSMCs, the required pressure of Table 17.2 is also a functionof charge viscosity, preheating, and composition; fillers aswell as denser and longer fibers require more force. Pressspeeds, to first close the mold and then to apply pressure onthe charge, have greatly increased as equipment technologyhas progressed. For thermoset-based reinforced material(BMC and SMC), speeds to fully pressurize the cavity’scharge have accelerated from about 12e35 in/min(5e15 mm/s) up to 80 in/min (34 mm/s).

Generally, for thermoplastic sheets, closing andcompression speeds are much faster than for thermosetcharges. For GMTs, the material must be quickly transferredfrom the preheating stage into the mold and processed.Recommended speeds are 1000 in/min (425 mm/s) and24e70 in/min (10e30 mm/s) for the closing and compress-ing stages, respectively [17]. GMT and LFT parts are oftenquite thin and cool rapidly, and 2100 in/min (900 mm/s) tocontact and then 70e165 in/min (30e70 mm/s) forcompression [15] are better. The preheating time is thelargest contributor to the processing rate, and this heating canadd one to several minutes to an overall cycle. Moldingpressure is dependent on other conditions such as preheating,mold temperature, and fiber loading that factor into the cycletime. For instance, a higher processing temperature reducesthe molding pressure requirement but also will add morecooling time to the overall part cycle. Likewise, higher fibercontent in the GMT charge requires significantly morepressure. For LFTs, the charge is already heated from theplasticizing delivery system. Generally, molding conditionsfor LFTs are similar to GMT processing, and certainlydependent on fiber type, size, and content.

Viscosity-control additives can improve the flow charac-teristics of the fiber-reinforced resin matrix during the pres-surization stage. Specifically, thickeners minimize fiberseparation from the resin itself. If the viscosity of the chargeis too low, the resin tends to flow farther and faster, and iteasily fills tight cavities and sections compared to the flow ofthe fibers. The result is that the part can have regions ofnearly pure resin along with other regions having highly-concentrated fiber. To minimize separation between thecompound components, improved coupling agents promotestronger bonding between fibers and resin.

17.10 Comparisons with TransferMolding and Injection Molding

The choice of optimal plastics processing method is basedon many engineering and economic factors. Any reasonablecomparisonmust bemadewith other processes that also utilizehigh pressure with heat to mold a part within a closed cavity.Two such commonmethods are transfer molding and injectionmolding. Compression and transfer molding routinely usethermosetting resins. Injection molding of thermosets ischallenging and requires close control. Any premature poly-merization will freeze the injection screw and require a costlyoverhaul of the injection molder; raising the barrel heaters willnot re-melt the material. However, injection molding allowsfor much more mold and part complexity, including undercutsand hollow parts, than compression. Transfer molding falls inbetween, closer to injection molding as undercuts and hollowfeatures are possible in the mold design.

Large parts are sometimes cost-prohibitive in injection ortransfer molding. And low mold construction costs, inconjunction with the simple operation of a heated press, givea great advantage to the compression molding process as partsize increases to several pounds and up. Likewise, operation ofa compression molding machine is simple with fewer temper-ature zones, and less movement of machine and mold compo-nents; this leads to lower maintenance and operating costs. Ofcourse injection molding provides the shortest cycle time,usually measured in seconds, with transfer molding measuredin minutes to allow sufficient time for curing. Compressionmolding cycle times are on the order of minutes also, generallyeven longer than transfermolding. Flash removal is exclusive tocompression molding but this represents the least amount ofscrap material among these competing processes.

Due to the fact that in compression molding the resinliquefies and flows a very short distance, there is insufficienttime for much molecular orientation of the polymer chains.This generally provides for uniform part properties and densityin all three dimensions as well as fewer residual stresses. Ininjection and transfer molding processes, the molecular chainswill tend to elongate along the flow streamlines and thisorientation bias will freeze in place upon part cooling,increasing tensile strength in that direction. This orientation


control is useful in many products. On the other hand, a part’smold shrinkage is less and more uniform with compressionmolding, and many times better than with injection-moldedparts.

Another important advantage of compression molding iswith the use of reinforcements in longish fiber form. Althoughthe resin does flow, the distances are quite short so that fibersare not stressed and retain their integrity. Thus, bio-based andnatural fibers are mostly processed by compression presses.Transfer and injection molders are limited to short fibers, sincelong ones would be damaged, especially from passing throughnarrow runners and gates as well as from the churning actionof an injection molding screw. Thus, longish-fiber cloths ormats are successfully employed, even in high concentration,with compression molding resulting in composites havingsuperior mechanical properties. Additionally, with compres-sion molding, fiber orientation is controllable with carefulpositioning of the reinforcements within the mold so that partdimensional strength is adjustable.

17.11 Similar Processes

There are a few variations on the conventional compres-sion molding process. These include cold forming (coldmolding) and sintering, and are useful when processingcertain resin formulations, including thermoplastics, but mayneed post-processing actions such as additional pressuriza-tion, annealing, and/or machining to finalize the shape andproperties. Items for the electrical industry are often made bythese methods.

For molding common thermoplastics such as ABS, PVC,and polycarbonate, cold forming compacts by a factor of 3e6a granular charge at 3000e5000 psi (21e34 MPa), butwithout heat into a preform [1]. This preform is notmechanically strong with inferior dimensional control andsurface quality, but any air has been forced out and it is readyto be further processed. Phenolic thermosetting material iscold molded, and then cured in an oven.

Preforms may be strengthened by sintering. This may takeplace outside or within the compression mold and the heat iscontrolled just below the melting temperature, so the finalpart is fused, as is the case with rotomolded products. Thefusing does harden the part but not to the degree achieved byany process where melting occurs. Plastics in the fluoropoly-mer family are difficult to process by traditional means but aresuccessfully molded through sintering.

17.12 Modeling the Fluid Dynamicsand Heat Transfer of Mold Filling

The closing of the mold creates a perpendicular (normal)force upon the charge, compacting it. Once enough compactionis obtained, the charge resists further normal deformation and

will be squeezed to move tangentially to the molding force. Asthe resin charge liquefies under pressure, a flow front is formedthat eventually extends to fill the entiremold cavity. This lengthof travel may be relatively long, especially when using mats ofSMCs andGMTswhichmay occupy only a portion of themoldsurface. It is advantageous to be able to predict this mold-fillingprocess, but to do so an appropriate mathematical treatment isneeded. Physically, the flow is quite complex. The polymermelt is a viscous fluid described in one, two, or three dimen-sions. Furthermore, the fluid is not isothermal and experiencesheat transfer between the melt and inside mold surfaces.Additionally, there is an internal heat generation with ther-moset curing. Finally, the fluid undergoes a transient phasechange when solidifying, and physical properties must beadjusted with changing temperatures. Thus, to properlyaccount for the hydraulic and thermal effects, differentialcontinuity and momentum equations must be solved inconjunction with the differential energy equation. The non-Newtonian nature of polymer melts requires viscosity to bea function of the temperature and flow conditions. Any rein-forcing fibers are reoriented, or even damaged during flow.

Since it is unlikely that a simple, overall analytical solu-tion would be obtained for any part, the approach must be toanalyze the flow on a differential and local basis with finiteelement techniques. Such an effort relies on extensivecomputational power for adequate detail and resolution.Finite element techniques require subdividing, or meshing,the part geometry into minute, individual volume elementswhere the mathematical relationships are applied. Twosample parts subdivided into individual finite elements arepresent in Figure 17.18; meshing can be made more detailed(or coarser) depending on desired solution accuracy. Moredetailed meshing implies more elements and increasesrequired computational capacity and program running time;so its selection is a compromise between numerical accuracyand computer hardware and software limitations. Propervolume connectivity ensures that the equations propagatethroughout all the interior volumes of the part and to the moldsurface interface, where boundary conditions are stated. Themost accurate solution requires thousands of elements. Finiteelement methods are commonly applied to a variety of fluidflows but the complex rheology of compression moldingrequires significant simplifications in the modeling or canonly be utilized for very simple part shapes.

For thermosets, the velocity profile of the flow front hasbeen established to be relatively flat and plug-shaped. A thinmelt region exists between the charge core and moldsurfaces, which ensures that core fluid layers move togetherin the direction of flow. In the case of thermoplastics, the flowfront assumes a more parabolic shape, characteristic oflaminar flow. However, due to the fact that solidificationinitiates in the material in contact with the cooler mold, resinat the core migrates toward the cavity surfaces creating asecondary flow pattern that is known as the fountain floweffect.

Figure 17.18 Geometrical meshing of some sample parts prior to finite element analysis. (TECHNALYSIS, INC., Indianapolis, IN)

Figure 17.19 Theoretical representation of the behavior ofthe charge during the compression molding process. (TECH-NALYSIS, INC., Indianapolis, IN)


Because many SMC parts have a thickness much less thanthe length in the direction of flow, an adequate model may beconstructed based on classical Hele-Shaw flow; the flow fieldis essentially a flat sheet of a constant thickness. Tucker andFolgar [19] proposed such an approach, using furthersimplifications of an isothermal and Newtonian melt. Theirmodel allowed for variable mold closure speed leading to flowin a two-dimensional plane with the thickness specified in thethird coordinate. The energy equation neglected conductionwithin the plane but included viscous heating and a heatgeneration term to simulate the exothermic curing reaction.This methodology was further refined where, among otherimprovements, a frictional boundary condition replaced theconventional no-slip at the inside mold surface [20]. In a sheetmolding compound, the fibers will reorient themselves withflow. A computerized predictive technique for this event isavailable [21]. Addressing the fountain flow effect charac-teristic of thermoplastic rheology, the methodology of Mav-ridis, Hrymak, and Vlachopoulos [22] clearly predictsstreamlines originating from the flow’s core and radiating insemi-circular patterns backward and to the upper and lowermold surfaces. Although this study focused on injectionmolding, its findings can be extended to other flows in narrowchannels such as during compression molding.

Commercial software is available to simulate thecompression molding process. One such product isPASSAGE/COMPRESSION. Its capabilities include New-tonian and non-Newtonian fluids, non-isothermal flow, fiberorientation prediction, shrinkage and warpage analysis, andstress analysis under external static and dynamic loads forthin-walled BMC and SMC parts. The first stage of thesimulation represents the pressurization of the charge by themolder’s force accounting for the speed of mold closure.Next, the flow front is established as the charge flows outwardwithin the thin gap formed by the upper and lower moldsurfaces. Once the mold cavity fills, the problem is a transientheat transfer analysis. Figure 17.19 displays these stages ofmolding. In the top view, the press begins, at time equals zero(t¼ t0), exerting force on the resin charge; h(t) is the thickness

of the charge and is a function of time. Next, the flow frontvelocity profile takes shape. Lastly, the part is formed, flow iszero, and heat transfer takes place at the interface between thepart and mold cavity surfaces. The software package relies onthe Barone and Caulk and Hele-Shaw methodologies.Figure 17.20 is a sample analysis of the filling of a truck hood.Taking advantage of symmetry, only half of the hood needs tobe modeled, reducing the computational demand andimproving accuracy. The finite element grid is shown and themoving flow front is represented by the darker shading. Thecharge moves from the centerline of the hood in two separateflows which merge near 1/4 s (Figures 17.20.b and c),establishing a weld line. The remaining cavity volume fillstoward one corner (Figure 17.20.e), identifying a regionpotentially needing venting. The simulation shows theprogression of the melt front until it fills the part after 0.60 s.

Figure 17.20 Computerized simulation of the compression molding of a truck hood. (TECHNALYSIS, INC., Indianapolis, IN)


Table 17.3 Generalized part defect categories

Strength Dimensional Surface Material

Cracking Chippededge

Dieseling Contamination

Weld line Excessiveflash

Blister Porosity

Fiberseparation

Sink mark Dragmarks

Under-cure

Shrinkage Dullfinish

Voids

Warpage Ripple


Although predictive methods d computerized or not dare useful, it should be remembered that the actualcompression molding process is quite complex. Largeuncertainty arises from modeling assumptions and propertydata. Optimally, once a mathematical model is formulated, itought to be benchmarked and verified using a known, simplersolution or case. Only then should other trials be attempted.Theoretical models are best used to eliminate competingdesigns and for useful relative comparisons. For instance, fora specific part, the difference in filling times under twodifferent molding conditions may be calculated as 20%; thismay be more accurate than the absolute filling time (calcu-lated in seconds) for either run. Thus, for absolute data, priorexperience and prototyping are better. In all cases, it is vitalto experimentally verify the final design.

17.13 Ensuring Part Quality

Of course, there are many factors affecting the quality ofa part or product. Quality problems may be readily apparentfrom visual inspection. Other defects may be internal and notascertained until mechanical or other property testing.

Figure 17.21 Rectangular block with severe edge chipping.

Processing conditions, mold design, and the resin itself canbe the source of imperfections. Direct effects betweenproduction parameters and quality issues are impossible toexactly quantify and are best solved through experience andtrial-and-error testing. However, compression moldingdefects have been characterized and guidelines for mini-mizing problems are available [10]; Table 17.3 lists some ofthese defects arranged in general categories.

Figure 17.21 presents a rectangular block measuring 300 by200 (76 by 51 mm) and displays severe chipping; the damageis especially pronounced at the block’s right-hand side andlower left corner. Also, the entire lower edge shows chipping.An example of porosity is in Figure 17.22, where voidagedefects extend to the part’s surface; the part is a 500 (127 mm)remnant of a tensile bar. Extensive pitting is seen along thebar’s surface. The porosity was caused by incomplete dryingof the filler used in the molding compound; during the heatedcompression, the water evaporated leaving voids. This is aninstance where an internal, material defect also negativelyaffects the surface of the item. Both figures are examples ofquality problems that must be addressed through moldingconditions, part ejection, or tool construction.

In a more general sense, for over a 100 years the basicconcepts behind the compression molding technique havegone unchanged. But with the world looking for products thatare more flexible, durable, aesthetically pleasing, and at lowcost and weight, lately there have been technologicaladvances to the equipment and resins used. Additionally,maintaining the highest part quality must coincide with fasterprocessing and more efficient power consumption andmaterial usage. Resins have been improved in terms ofrecyclability, cost, and property performance. Progress with

Figure 17.22 Tensile bar with voidage defects extending tothe part’s surface.


computers has allowed for simulations to make improve-ments in the equipment such as more uniform platen heatdistribution. Molds are produced to a higher degree ofprecision. Computers have integrated touch screen interfacesand menus with measurement, imaging, and statistical anal-ysis. In this type of control, sensor data are relayed back tomicroprocessors and various types of actuators throughoutthe system. Compression molding automation allows forsmall, but important changes in pressure, temperature, speed,time, and material handling. To maintain the highest quality,consistency is vital and automation includes the use ofrobotics which increases reliability of loading charges intothe mold, part ejection, and post-processing steps. Thus,machine operators are free to focus on process and qualitymonitoring. Overall, recent technological advances have ledto more reliable and cost-effective parts and products withthe same basic hot press.

Acknowledgments

The author would like to thank James E. Blanch forfigures and data collection. For commercially availableproducts, Robert N. Boland of Lawton Machinery Group, DePere, WI; Sinan Ecer of TECHNALYSIS, INC., Indianapolis,IN; Len Nunnery of Bulk Molding Compounds, Inc., WestChicago, IL; and Scott A. Trail of Hull Industries, Inc., NewBritain, PA, provided valuable insights and illustrations.

References

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[3] M.A. Kuczmarski, J.C. Johnston, Improved thermaldesign of a compression mold, Adv. Polym. Tech. 26(2007) 86e99.

[4] Resin Pricing, Plastics Technology, Gardner Publica-tions, Inc., June 2009. www.PTOnline.com.

[5] J.L. Hull, Compression and Transfer Molding, in:C.A. Harper (Ed.), Handbook of Plastic Processes, JohnWiley & Sons, Inc., Hoboken, NJ, 2006.

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[10] B.A. Davis, P.J. Gramann, T.A. Osswald, A.C. Rios,Compression Molding, Hanser Gardner Publications,Inc., Cincinnati, OH, 2003.

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[12] E. Zenk, Composites in the trucking industry, Societyof Plastics Engineers ANTEC 2007, Cincinnati, OH,2007, pp. 1377e1382.

[13] T.A. Osswald, Polymer Processing Fundamentals(Chapter 8),HanserGardnerPublications, Inc.,Cincinnati,OH, 1998.

[14] D.L. Denton, Effects of processing variables on themechanical properties of structural SMC-R composites,36th Annual Conference, Reinforced Plastics/Composites Institute, Session 16-A, 1981, pp. 1e8.

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[16] M.R. Barone, D.A. Caulk, The effect of deformation andthermoset cure on heat conduction in a chopped-fiberreinforced polyester during compression molding, Int. J.Heat Mass Transfer. 22 (1979) 1021e1032.

[17] E. Haque, P. Bristow, H. Giles, Processing of glass fibermat reinforced thermoplastic composites, Society ofPlastics Engineers ANTEC 2001, Dallas, TX, 2001,pp. 2079e2083.

[18] M.R. Barone, D.A. Caulk, Compression molding:nonuniform cavity surface temperature and cycletime, Proceedings of the First International Conferenceon Reactive Processing of Polymers, Pittsburgh, PA,1980.

[19] C.L. Tucker, F. Folgar, A model of compression moldfilling, Polym. Eng. Sci. 23 (1983) 69e73.

[20] M.R. Barone, D.A. Caulk, A model for the flow ofa chopped fiber reinforced polymer compound incompression molding, J. Appl. Mech. 53 (1986)361e371.

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[22] H. Mavridis, A.N. Hrymak, J. Vlachopoulos, Finiteelement simulation of fountain flow in injectionmolding, Polym. Eng. Sci. 26 (1986) 449e454.

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Applied Plastics Engineering Handbook || Compression Molding