Uniform Walls

EFUNDA PLASTIC DESIGN RULES

Uniform Walls Parts should be designed with a minimum wall thickness consistent with part function and mold filling considerations. The thinner the wall the faster the part cools, and the cycle times are short, resulting in the lowest possible part costs. Also, thinner parts weight less, which results in smaller amounts of the plastic used per part which also results in lower part costs.

The wall thicknesses of an injection-molded part generally range from 2 mm to 4 mm (0.080 inch to 0.160 inch). Thin wall injection molding can produce walls as thin as 0.5 mm (0.020 inch).

The need for uniform walls

Thick sections cool slower than thin sections. The thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section.

This builds stresses near the boundary of the thin section to thick section. Since the thin section does not yield because it is solid, the thick section (which is still liquid) must yield. Often this leads to warping or twisting. If this is severe enough, the part could even crack.

Uniform wall thicknesses reduce/eliminate this problem.

Uniform walled parts are easier to fill in the mold cavity, since the molten plastic does not face varying restrictions as it fills.

What if you cannot have uniform walls, (due to design limitations) ?

When uniform walls are not possible, then the change in section should be as gradual as possible.

Coring can help in making the wall sections uniform, and eliminate the problems associated with non-uniform walls.

Warping problems can be reduced by building supporting features such as gussets.

Radius

Sharp corners greatly increase the stress concentration. This high amount of stress concentration can often lead to failure of plastic parts.

Sharp corners can come about in non-obvious places. Examples of this are a boss attached to a surface, or a strengthening rib. These corners need to be radiused just like all other corners. The stress concentration factor varies with radius, for a given thickness.

As can be seen from the above chart, the stress concentration factor is quite high for R/T values less than 0.5. For values of R/T over 0.5 the stress concentration factor gets lower.

The stress concentration factor is a multiplier factor, it increases the stress.

Actual Stress = Stress Concentration Factor K x Stress Calculated

This is why it is recommended that inside radiuses be a minimum of 1 x thickness.

In addition to reducing stresses, fillet radiuses provide streamlined flow paths for the molten plastic resulting in easier fills.

Typically, at corners, the inside radius is 0.5 x material thickness and the outside radius is 1.5 x material thickness. A bigger radius should be used if part design will allow it.

Voids and Shrinkage

Shrinkage is caused by intersecting walls of non-uniform wall thickness. Examples of these are ribs, bosses, and other projections of the nominal wall. If these projections have greater wall thicknesses, they will solidify slower. The region where they are attached to the nominal wall will shrink along with the projection, resulting in a sink in the nominal wall.

Shrink can be minimized by maintaining rib thicknesses to 50 to 60% of the walls they are attached to.

Bosses located at corners can result in very thick walls causing sinks. Bosses can be isolated using the techniques illustrated.

Warpage

Thick sections cool slower than thin sections. The thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the un-solidified areas, which are connected, to the already solidified thin section. This builds stresses near the boundary of the thin section to thick section. Since the thin section does not yield because it is solid, the thick section (which is still liquid) must yield. Often this leads to warping or twisting. If this is severe enough, the part could even crack.

Other causes

Warping can also be caused due to non-uniform mold temperatures or cooling rates.

Non-uniform packing or pressure in the mold.

Alignment of polymer molecules and fiber reinforcing strands during the mold fill results in preferential properties in the part.

Molding process conditions--too high a injection pressure or temperature or improper temperature and cooling of the mold cavity. Generally, it is best to follow the resin manufacturer's guidelines on process conditions and only vary conditions within the limits of the guidelines.

It is not good practice to go beyond the pressure and temperature recommendations to compensate for other defects in the mold. If runners need to be sized differently to allow for a proper fill, or gate sizes that need to be changed, then those changes need to happen.

Otherwise the finished parts will have too much built in stresses, could crack in service or warp-leading to more severe problems such as customer returns or field service issues.

The reason for draft Drafts (or taper) in a mold, facilitates part removal from the mold. The amount of draft angle depends on the depth of the part in the mold, and its required end use function.

The draft is in the offset angle in a direction parallel to the mold opening and closing.

It is best to allow for as much draft as possible for easy release from the mold. As a nominal recommendation, it is best to allow 1 to 2 degrees of draft, with an additional 1.5 min. per 0.025 mm (0.001 inch) depth of texture. See below.

The mold parting line can be relocated to split the draft in order to minimize it. If no draft is acceptable due to design considerations, then a side-action mold (cam-actuated) may be required at a greater expense in tooling.

The reason for texture Textures and Lettering can be molded on the surfaces, as an aesthetic aid or for incorporating identifying information, either for end users or factory. Texturing also helps hide surface defects such as knit lines, and other surface imperfections. The depth of texture or letters is somewhat limited, and extra draft needs to be provided to allow for mold withdrawal without marring the surface.

Draft for texturing is somewhat dependant on the mold design and the specific mold texture. Guidelines are readily available from the mold texture suppliers or mold builders.

As a general guideline, 1.5 min. per 0.025mm (0.001 inch) depth of texture needs to be allowed for in addition to the normal draft. Usually for general office equipment such as lap-top computers a texture depth of 0.025 mm (0.001 inch) is used and the min. draft recommended is 1.5 . More may be needed for heavier textures surfaces such as leather texture (with a depth of 0.125 mm/0.005 inch) that requires a min. draft of 7.5.

The use of ribs Ribs increase the bending stiffness of a part. Without ribs, the thickness has to be increased to increase the bending stiffness. Adding ribs increases the moment of inertia, which increases the bending stiffness. Bending stiffness = E (Young's Modulus) x I (Moment of Inertia)

The rib thickness should be less than the wall thickness-to keep sinking to a minimum. The thickness ranges from 40 to 60 % of the material thickness. In addition, the rib should be attached to the base with generous radiusing at the corners.

At rib intersections, the resulting thickness will be more than the thickness of each individual rib. Coring or some other means of removing material should be used to thin down the walls to avoid excessive sinking on the opposite side.

The height of the rib should be limited to less than 3 x thickness. It is better to have multiple ribs to increase the bending stiffness than one high rib.

The rib orientation is based on providing maximum bending stiffness. Depending on orientation of the bending load, with respect to the part geometry, ribs oriented one way increase stiffness. If oriented the wrong way there is no increase in stiffness.

Draft angles for ribs should be minimum of 0.25 to 0.5 degree of draft per side.

If the surface is textured, additional 1.0 degree draft per 0.025 mm (0.001 inch) depth of texture should be provided.

Boss Design

Bosses are used for the purpose of registration of mating parts or for attaching fasteners such as screws or accepting threaded inserts (molded-in, press-fitted, ultrasonically or thermally inserted).

The wall thicknesses should be less than 60 % of nominal wall to minimize sinking. However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws.

The base radius should be a minimum of 0.25 x thickness

The boss can be strengthened by gussets at the base, and by attaching it to nearby walls with connecting ribs.

Hoop stresses are imposed on the boss walls by press fitting or otherwise inserting inserts.

The maximum insertion (or withdraw) force Fmax and the maximum hoop stress, occurring at the inner diameter of the boss, smax is given by

Failures of a boss are usually attributable to:

High hoop stresses caused because of too much interference of the internal diameter with the insert (or screw).

Knit lines -these are cold lines of flow meeting at the boss from opposite sides, causing weak bonds. These can split easily when stress is applied.

Knit lines should be relocated away from the boss, if possible. If not possible, then a supporting gusset should be added near the knit line.

DSM ENGINEERING PLASTICS

http://www.dsm.com/en_US/html/dep/generaldesignguidelines.htmWall thickness

Just as metals have normal working thickness ranges based upon their processing method, so do plastics. Typically, for injection molded parts, the wall thickness will be in the range 0.5 mm to 4 mm (0.02 - 0.16 in). Dependent on the part design and size, parts with either thinner or thicker sections can be molded.While observing functional requirements, keep wall thicknesses as thin and uniform as possible. In this way even filling of the mold and anticipated shrinkage throughout the molding can be obtained in the best way. Internal stresses can be reduced.Wall thickness should be minimized to shorten the molding cycle, obtain low part weight, and optimize material usage. The minimum wall thickness that can be used in injection molding depends on the structural requirements, the size and geometry of the molding, and the flow behavior of the material. As a starting point the designer can often refer to spiral flow curves which give a relative measure of the maximum achievable flow length for a given wall thickness and injection pressure. See figure below.Spiral flow length of Akulon Ultraflow at 260 and 1400 bar.

INCLUDEPICTURE "http://www.dsm.com/en_US/images/empty.gif" \* MERGEFORMATINET If parts are subjected to any significant loading the part should be analyzed for stress and deflection. If the calculated stress or deflection value is not acceptable a number of options could be considered including the following:

Increase wall (if not already too thick)

Use an alternative material with higher strength and/or modulus

Incorporate ribs or contours in the design to increase the sectional modulus

Other aspects that may need to be considered include: Insulation characteristics Generally speaking insulating ability (whether for electrical or heat energy) is related to the thickness of the polymer.Impact characteristics Impact resistance is directly related to the ability of a part to absorb mechanical energy without fracturing. This in turn is related to the part design and polymer properties. Increasing the wall section will generally help with impact resistance but too thick (stiff) a section may make a design unable to deflect and distribute an impact load therefore increasing stresses to an unacceptable level.Agency approval When a part design must meet agency requirements for flammability, heat resistance, electrical properties etc, it may be necessary to design with thicker sections than would be required just to meet the mechanical requirements.Where varying wall thicknesses are unavoidable for reasons of design, there should be a gradual transition (3 to 1) as indicated in the figure below.

Gradual transition of wall thicknesses.

Generally, the maximum wall thickness used should not exceed 4 mm. Thicker walls increase material consumption, lengthen cycle time considerably, and cause high internal stresses, sink marks and voids (see figure below).

Sink marks due to large wall thicknesses.

Voids due to large wall thicknesses.

Care should be applied to avoid a "race tracking" effect, which occurs because melt preferentially flows faster along thick sections. This could result in air traps and welds lines, which would appear as surface defects. Modifying or incorporating ribs in the design can often improve thick sections.

Influence of rib design on flow behavior of the melt.

Example of design study of multi-connector.

Ribs and profiled structures

If the load carrying ability or the stiffness of a plastic structure needs to be improved it is necessary to either increase the sectional properties of the structure or change the material. Changing the material or grade of material, e.g. higher glass fiber content, may be adequate sometimes but is often not practical (different shrinkage value) or economical. Increasing the sectional properties, namely the moment of inertia, is often the preferred option. As discussed in other sections, just increasing the wall section although the most practical option will be self-defeating.

Increase in part weight and costs are proportional to the increase in thickness. Increase in cooling time is proportional to the square of the increase in thickness.

If the load on a structural part requires sections exceeding 4 mm thickness, reinforcement by means of ribs or box sections is advisable in order to obtain the required strength at an acceptable wall thickness. The efficiency of a ribbed structure can be illustrated with the following example:

Solid plate vs. ribbed plate in terms of weight and stiffness. Although ribs offer structural advantages they can give rise to warpage and appearance problems, for this reason certain guidelines should be followed: The thickness of a rib should not exceed half the thickness of the nominal wall as indicated in the figure below. In areas where structure is more important than appearance, or with very low shrinkage materials, ribs with a thickness larger than half the wall thickness can be used. These will cause sink marks on the surface of the wall opposite the ribs. In addition, thick ribs may act as flow leaders causing preferential flows during injection. This results in weld lines and air entrapment.Maximum rib height should not exceed 3 times the nominal wall thickness as deep ribs become difficult to fill and may stick in the mold during ejection. Typical draft is 1 to 1.5 deg per side with a minimum of 0.5 deg per side. Generally draft and thickness requirements will limit the rib height.At the intersection of the rib base and the nominal wall a radius of 25 to 50% of the nominal wall section should be included. Minimum value 0.4 mm. This radius will eliminate a potential stress concentration and improve flow and cooling characteristics around the rib. Larger radii will give only marginal improvement and increase the risk of sink marks on the opposite side of the wall.

Recommendations for rib dimensions. Parallel ribs should be spaced at a minimum distance of twice the nominal wall thickness; this helps prevent cooling problems and the use thin blades in the mould construction.

Ribs are preferably designed parallel to the melt flow as flow across ribs can result in a branched flow leading to trapped gas or hesitation. Hesitation can increase internal stresses and short shots.

Ribs.

Ribs should be orientated along the axis of bending in order to provide maximum stiffness. Consider the example in the figure above where a long thin plate is simply supported at the ends. If ribs are added in the length direction the plate is significantly stiffened. However, if ribs are added across the width of the plate little improvement is found.

Ribbing is typically applied for:1. Increasing bending stiffness or strength of large flat areas 2. Increasing torsional stiffness of open sectionsAdding corrugations to the design can stiffen flat surfaces in the direction of the corrugations (see figure below). They are very efficient and do not add large amounts of extra material or lengthen the cooling time. The extra stiffness is a result of increasing the average distance of the material from the neutral axis of the part, i.e. increasing the second moment of inertia.Corraguations.

Flat and open areas.

Ribs and box sections increase stiffness, thus improving the load bearing capability of the molding. These reinforcing methods permit a decrease in wall thickness but impart the same strength to the section as a greater wall thickness.

Dimension image with chart of case 1-6.

INCLUDEPICTURE "http://www.dsm.com/en_US/images/empty.gif" \* MERGEFORMATINET Comparison of profiles in terms of torsional rigidity and bending.

The results demonstrate that the use of diagonal ribs have the greatest effect on the torsional rigidity of the section. The change from an I section to a C section helps in terms of horizontal bending terms but not in torsional terms. As double cross ribs (option 6) can give tooling (cooling) problems option 8 is the recommended solution for the best torsional performance. Depending on the requirements of the part the acceptability of possible sink marks at the intersection of the ribs and profile wall need special consideration. For maximum performance and function the neutral lines of the ribs and profile wall should meet at the same point. Deviation from this rule will result in a weaker geometry. If, due to aesthetic requirements, the diagonal ribs are moved slightly apart then the rigidity is reduced 35%. If a short vertical rib is added to the design then the torsional rigidity is reduced an additional 5%. See figure below.

Torsional rigidity and resistance to torsional stress as a function of the way in which the ribs are connected to the profile.

Gussets

Gussets can be considered as a subset of ribs and the guidelines that apply to ribs are also valid for gussets. This type of support is used to reinforce corners, side walls, and bosses.

Guidelines for gussets.

The height of the gusset can be up to 95% of the height of the boss or rib it is attached to. Depending on the height of the rib being supported gussets may be more than 4 times the nominal wall thickness. Gusset base length is typically twice the nominal wall thickness. These values optimize the effectiveness of the gusset and the ease of molding and ejecting the part.

Height of the gusset.

Bosses

Bosses often serve as mounting or fastening points and therefore, for good design, a compromise may have to be reached to achieve good appearance and adequate strength. Thick sections need to be avoided to minimize aesthetic problems such as sink marks. If the boss is to be used to accommodate self tapping screws or inserts the wall section must be controlled to avoid excessive build up of hoop stresses in the boss.

General recommendations include the following

Nominal boss wall thickness less than 75% nominal wall thickness, note above 50% there is an increased risk of sink marks. Greater wall sections for increased strength will increase molded-in stresses and result in sink marks.A minimum radius of 25% the nominal wall thickness or 0.4 mm at the base of the boss is recommended to reduce stresses.Increasing the length of the core pin so that it penetrates the nominal wall section can reduce the risk of sink marks. The core pin should be radiused (min 0.25 mm) to reduce material turbulence during filling and to help keep stresses to a minimum. This option does increase the risk of other surface defects on the opposite surface.A minimum draft of 0.5 degrees is required on the outside dimension of the boss to ensure release from the mold on ejection.A minimum draft of 0.25 degrees is required on the internal dimension for ejection and or proper engagement with a fastener.

Proper boss design. Further strength can be achieved with gusset ribs or by attaching the boss to a sidewall.Bosses adjacent to external walls should be positioned a minimum of 3 mm (.12 in) from the outside of the boss to avoid creating a material mass that could result in sink marks and extended cycle times.

Correct positioning of bosses. A minimum distance of twice the nominal wall thickness should be used for determining the spacing between bosses. If placed too close together thin areas that are hard to cool will be created. These will in turn affect quality and productivity.

Holes

Holes are easily produced in molded parts by core pins. Through holes are easier to produce than blind holes because the core pin can be supported at both ends.

Blind holes

Core pins supported by just one side of the mold tool create blind holes. The length of the pins, and therefore the depth of the holes, are limited by the ability of the core pin to withstand any deflection imposed on it by the melt during the injection phase. See information on bosses & cores.

As a general rule the depth of a blind hole should not exceed 3 times the diameter. For diameters less than 5 mm this ratio should be reduced to 2.

Blind cores.

Through holes

With through holes the cores can be longer as the opposite side of the mold cavity can support them. An alternative is to use a split core fixed in both halves of the mold that interlock when the mold is closed. For through holes the length of a given size core can be twice that of a blind hole. In cases where even longer cores are required, careful tool design is necessary to ensure balanced pressure distribution on the core during filling to limit deflection.

Through cores.

Holes with an axis that runs perpendicular to the mold opening direction require the use of retractable pins or split tools. In some designs placing steps or extreme taper in the wall can avoid this. See section on draft. Core pins should be draw polished and include draft to help with ejection.The mold design should direct the melt flow along the length of slots or depressions to locate weld lines in thicker or less critical sections. If weld lines are not permissible due to strength or appearance requirements, holes may be partially cored to facilitate drilling as a post molding operation.The distance between two holes or one hole and the parts edge should be at least 2 times the part thickness or 2 times the hole diameter whichever is the largest.

Minimum hole spacing dimensions.

For blind holes the thickness of the bottom should be greater than 20% of the hole diameter in order to eliminate surface defects on the opposite surface. A better design is to ensure the wall thickness remains uniform and there are no sharp corners where stress concentrations can occur.

Blind hole design recommendations.

Radii & corners

In the design of injection molded parts sharp corners should always be avoided; generous radii should be included in the design to reduce stress concentrations. Fillet radii should be between 25 and 60% of the nominal wall thickness. If the part has a load bearing function then the upper end is recommended. A minimum radius of 0.5mm is suggested and all sharp corners should be broken with at least a 0.125 mm radius.

Sharp corners, particularly internal corners introduce: High molded in stressesPoor flow characteristicsReduced mechanical propertiesIncreased tool wearSurface appearance problems, (especially with blends).

The inclusion of a radius will give: Uniform coolingLess warpageLess flow resistanceEasier fillingLower stress concentrationLess notch sensitivity.

The outside corner radius should be equal to the inside radius plus the wall thickness as this will keep a uniform wall thickness and reduce stress concentrations.

Corner radius.

For a part with an internal radius half the nominal wall thickness a stress concentration factor of 1.5 is a reasonable assumption. For smaller radii, e.g. 10% of the nominal wall, this factor will increase to 3. Standard tables for stress concentration factors are available and should be consulted for critical applications.

Stress concentration as a function of wall thickness and corner radius.

In addition, from a molding view point, it is important is to avoid sharp internal corners. Due to the difference in area/volume-ratio of the polymer at the outside and the inside of the corner, the cooling at the outside is better than the cooling at the inside. As a result the material at the inside shows more shrinkage and so the corner tends to deflect (see figure below).

Sharp corners.

Tolerances

Establishing the correct tolerances with respect to the product function is of economic importance. The designer should be aware that dimensions with tight tolerances have a big influence on the costs of both product and mold.Even slightly over specifying tolerances may adversely influence tool costs, injection molding conditions, and cycle time. It is recommended to indicate only critical dimensions with tolerances on a drawing.

Depending on the application, a division into three tolerance classes can be made:

normal; price index 100

accurate; technical injection molding; price index 170

precise; precision injection molding; price index 300

The most important characteristics of the tolerance classes are shown in the figure below.

Characteristics of tolerance classes.

Factors affecting parts tolerance.

Mold design, mold cavity dimensions, product shape, injection-molding conditions and material properties determine the tolerances that can be obtained. The figure below provides a summary of the factors that play a major role in establishing dimensional accuracy.

Coring

Coring refers to the elimination of plastic material in oversized dimensioned areas by adding steel to the mold tool that usually results in a pocket or opening in the part. For simplicity and economical reasons cores should ideally be placed parallel to the line of draw.Coring designs.

INCLUDEPICTURE "http://www.dsm.com/en_US/images/dep/coring2.gif" \* MERGEFORMATINET Cores in other directions require the use of some form of side action (cam operated or hydraulic) to be actuated thus increasing tooling costs (see figure below).

Constuction B could cost as much as 60% less than A.

Undercuts

Undercuts should be avoided if possible through redesign of the part. Ideally, the mold tool should open in a direction parallel to the movement of the machine platen.

Undercuts.

In the figure above, hanging the form of the hole reduces initial costs and also maintenance cost during production.

For some complex parts the ideal situation will not exist and mechanical movement of one sort or another will be required. A description of possible movements is given below:

Deflection Dependent on the material and amount of undercut it may be possible to deflect the part out of the mold.

Inserts The use of removable inserts that eject with the part is an option, certainly for prototype tooling. The disadvantages are the inserts must be removed from the ejected part and repositioned in the mould thus possibly extending the cycle time.

Cams Cams or hydraulic/pneumatic cylinders move part of the mold out of the way to permit part ejection. These increase the complexity of the mold making it more expensive and also mean a controller is required to operate them during the molding cycle. Cycle times will also be affected.

Slides By means of angled pins and rods mounted in the mold it may be possible to move the part of the mold forming the undercut in the direction of the angled pin during the opening sequence of the mold. This then allows ejection of the part.

Stepped parting line By repositioning the parting line it may be possible to eliminate undercut features; although this may add to the complexity of the tool it is the most recommended solution.

The figure below shows an example of a sliding cam. The cam pins that operate the cams are mounted under a maximum angle of 20 - 25 in the injection side. The angle is limited because of the enormous force that is exerted on these pins during mold opening and closing.

Cammed mold for part with undercut cams move in vertical direction when mold is opened.

Draft angle

Part features cut into the surface of the mold perpendicular to the parting line require taper or draft to permit proper ejection. This draft allows the part to break free by creating a clearance as soon as the mold starts to open. Since thermoplastics shrink as they cool they grip to cores or male forms in the mold making normal ejection difficult if draft is not included in the design. If careful consideration is given to the amount of draft and shutoff in the mold it is often possible to eliminate side actions and save on tool and maintenance costs.For untextured surfaces generally a minimum of 0.5 deg draft per side is recommended although there are exceptions when less may be acceptable. Polishing in draw line or using special surface treatments can help achieve this.For textured sidewalls use an additional 0.4 deg draft per 0.1mm depth of texture.

Draft (A) in mm for various draft angles (B) as a function of molding depth (C).

Typically 1 to 3 deg draft is recommended. As the draft increases ejection becomes easier but it increases the risk that some sections may become too heavy.

Try to keep features in the parting line or plane. When a stepped parting line is required allow 7 deg for shutoff. 5 deg should be considered as a minimum. Drag at the shutoff will cause wear over time with the risk that flash will form during molding. More frequent maintenance will be required for this type of tooling if flash free parts are to be produced.Parting line.

Mechanical connectionshttp://www.dsm.com/en_US/html/dep/mechanical_conn.htm

Mechanical connections are generally used where connections can be easily assembled and disassembled.

The main mechanical connections used for plastic assembly are:

Bolted assemblies Bolted connections are used for frequent assembly and disassembly of components. Bolted connections can be used when frequent assembly and disassembly of components are required. Bolted connections are expensive and aesthetically are not the most elegant of connections due to the bolt head and nut being exposed. Since the bolts need tightening, which necessitates the use of tightening tools, the space available may be a constraint and has to be considered prior to designing.

Self-tapping screws Self tapping screws are directly fixed on the plastic parts to be assembled. Self-tapping screws are used for assembling parts. Fewer parts need to be assembled compared to bolt-and-nut connections, with lower fastener and equipment costs. No nuts are required, so one smooth surface is obtained. Also the aesthetics of self-tapping screws are better than if bolts are used. Recyclability is good. In self-tapping screw assemblies, mating plastic threads are formed directly on the part when the screw is tightened into the assembly. They have limited durability and repeated assembly is possible to a certain extent.

A self tapping screw assembly typically includes a through clearance boss and a boss with a pilot hole. The holes can be molded-in or drilled through. The diameter of the pilot hole is greater than the root diameter of the screw, but smaller than the outside diameter of the screw.

Self tapping screws can be distinguished as either thread cutting screws or thread forming screws.

Thread cutting screws cut the thread during assembly. That means that every time the screw is assembled some material will be cut away. For that reason this type of screw is not recommended for repeated assembly and disassembly. In general, thread cutting screws are used for polymers with a low elongation at break and no ability to deform plastically. The relatively low hoop stress level associated with their use makes them suitable for use with glassy amorphous materials subject to crazing. As this type of screw generates small chips during the cutting process, space for the chips must be provided when blind holes are used. The chips can be a nuisance when through pilot holes are used.

Thread forming screws do not cut but deform the thermoplastic. Close to the screw the stresses can be high. Thread forming screws are generally used with lower modulus plastics, since ductility or cold flow is a prerequisite for their use. Thread forming screws can be used for repeated assembly and disassembly. In general, thread forming screws have higher drive and strip torque values than thread cutting screws.

Snap fits A snap fit is an effective method to design the fastening system into the product design itself. A snap-fit allows parts to be either permanently fastened (or pre-determined to be broken off) or to have frequent assembly and disassembly. A snap fit is an effective method to design the fastening system into the product design itself. A snap-fit can be designed to offer parts to be either permanently fastened (or pre-determined to be broken off) or for frequent assembly and disassembly.

In combination with O-rings or proper seals, even gas-tight and fluid-tight connections can be made.

Designing a snap-fit is rather complex due to a combination of factors:- the functional requirements of the product- the requirements for the assembly- the mechanical properties of the thermoplastic- the design of the mold and notably part ejection.

Snap-fits can be found in a wide variety of shapes.

Cantilever beam snap fit

Cylindrical type snap fit

Spherical type snap fit

Threaded metal inserts A threaded insert is a way to create a connection that can be assembled and disassembled repeatedly without problems. A metal part is inserted in the product and the connection made using a standard screw or bolt. Methods for inserting threaded metal inserts* Ultrasonic insertion The insert is pressed into a hole in the plastic by a horn that vibrates at ultrasonic frequency. The ultrasonic energy melts the plastic around the insert. Once the insert is pressed in, the plastic freezes off evenly around the insert.

Inserts as shown below specially developed for ultrasonic insertion, are commercially available in various types and sizes. Ultrasonic insertion gives a shorter molding cycle than parts with molded-in inserts. However, it also represents an additional manufacturing process. Care should always be taken to ensure the insert is solidly embedded in the substrate.

* Inserts for ultrasonic insertion

* Heated inserts

A special press that pre-heats the insert is used for hot pressing-in. The inserts are pressed into the hole when they have reached the desired temperature. The heat-transfer melts the polymer and the molten polymer flows into the undercuts and secures the insert after cooling down. The advantages of this method are:- strong connection, high loads can be absorbed- low internal stresses in the plastic if well-designed and executed- low equipment costs compared to ultrasonic insertion.

A longer insertion time is needed for cooling-down the plastic.

* Cold pressed in inserts

Inserts can also be pressed directly into a hole. This can be done cold as well as hot. Pressing-in cold is done with a small press. The inserts are provided with knurls under an angle at the outside. It is the fastest and easiest way of insertion, but high stresses will be present in the material round the insert making the connection weak.

* Molded in inserts

The insert is put into the mold (cavity) during the injection molding cycle. It is important to heat the inserts to a temperature close to the mold temperature before molding to remove differences in thermal expansion that can lead to a stress build up at the metal/plastic interface. It is also essential that the inserts are clean and free of any process lubricants. Molded-in inserts have the following advantages:- strong connection, high loads can be absorbed- low internal stresses in the plastic if well-designed and executed.

There are some disadvantages in this process:- a longer injection cycle is required to position the inserts in the mold- heating of the inserts is necessary- thermal stresses in the plastic if not well executed- severe damage to the mold is possible if inserts move.

* Coil inserts

Coil inserts offer a better wear resistance and strength than the surrounding polymer, but high stresses may be introduced into the boss. Coil inserts offer only limited connection strength.

* Thread cutting inserts

Thread-cutting inserts are comparable to self-tapping screws. The insert is screwed into a drilled or injection molded hole without internal thread. A cutting edge at the outside of the insert cuts a thread in the plastic.

* Expansion inserts

Expansion inserts can simply be pushed into a hole in the product. They are provided with a saw cut at the bottom, thus forming separate segments. The two segments are spread after putting the insert in place, either before or during insertion of the screw.

Expansion inserts offer limited connection strength.

Hook and loop type fasteners Hook-and-loop fasteners are used for connecting parts together which need to be frequently removed. They are available in a variety of shapes, sizes and colors. They are very durable. Hook-and-loop fasteners are available in a variety of shapes, sizes and colors. They can be opened and closed hundreds of times and can be used for many applications such as attaching doors and panels and other frequently removed parts, but also to attach electric cables, optical fiber cables, or hoses. Hook-and-loop fasteners are available either plain-backed or adhesive-backed. Plain-backed tape can be attached to a plastic part by rivets.

Hook and loop tape

Molded-in threads Molded-in threads are used where assemblies are seldom disassembled. Internal as well as external threads are common. Boss cap Boss caps are cup-shaped rings stamped from metal sheet which are pressed onto the top of hollow plastic bosses by hand or tools. A boss cap is used to reinforce a mechanical connection. Boss caps are cup-shaped rings stamped from metal sheet which are pressed onto the top of hollow plastic bosses by hand, with a pneumatic device, or with a light press. The caps reduce the tendency of the bosses to crack by reinforcing the boss against the expansion force exerted by screws. The caps are used with thread-forming screws and include a single thread for additional strength.

Sheet metal boss cap pressed onto the top of the boss to reinforce the boss and provide additional assembled strength

Push-on / Turn-on fasteners Push-on and turn-on fasteners are self locking, self threading fasteners that replace standard nuts. These fasteners are pushed-on or screwed-on to studs molded into a part for capturing the mating component. They are easy to use, inexpensive and vibration resistant. These fasteners are effective to provide a light to medium duty assembly. Push-on and turn-on fasteners are self-locking and self-threading fasteners respectively that replace standard nuts. These fasteners are pushed or screwed onto studs molded into a part, capturing the mating component. They are easy to use, inexpensive and vibration resistant, and provide a light to medium duty assembly, where clamp load requirements are minimal.

Push-on fastener for a permanent assembly

The type of fastener shown below produces a permanent assembly, whereas a removable assembly can be made with a Tinnerman clip in combination with a stud with a D-shape cross-section.

Tinnerman clips for a removable or permanent assembly

The self-threading nut in the figure below also produces a removable assembly and allows for some control over the clamp force. Figure A is a push on nut and Figure B a self threading nut.

Rivets Rivets provide a simple and an economical method to produce strong and permanent mechanical joints. They are generally used to assemble thin sections. Rivets provide a simple and economic assembly, and produce a strong permanent mechanical joint. They are used to join thin sections of plastics, plastic to metal sheet or plastics to fabric. The process can easily be automated. Different types of rivet heads are available. The diameter of the head must preferably be three times the shank diameter to reduce the stresses in the parts by distributing the clamp force over a larger area. A conical head should not be used, as it produces high tensile stresses.

Rivets can be produced from metal or plastic. Aluminum and plastic rivets produce smaller compressive stresses in the parts. A clearance between the rivet and the molded hole of 0.25 mm (0.010 inch) is recommended to account for tolerance variations and the coefficient of thermal expansion mismatch. A head is formed at the shank by plastic deformation of the material. The head can be made with a hand vice or a press. The figure below shows a typical example of a heading tool. Load control devices should preferably be used during rivet installation to ensure correct clinching pressure and consistent assembly thickness.

Heading tool

A reinforcing washer under the head of the rivet helps to minimize the stresses in the parts, just like a shouldered rivet.

Shouldered rivet

Press fits Press-fits are a simple and cost effective means to connect two parts. A press-fit is usually applied to connect a hub to a solid or hollow shaft. Mechanical forces between the two parts supply the required pre-stress to enable the connection to transmit an axial force or a torque.

Press-fits are a simple and cost effective means to connect two parts. A press-fit is usually applied to connect a hub to a solid or hollow shaft, or to fix a bush in its housing. Interference between the two parts supplies the required pre-stress to enable the connection to transmit an axial force or a torque. The hub and the shaft may both be of plastic, but a combination of plastic and metal is also possible. If different materials are used, attention must be paid to differences in thermal expansion, which may cause loosening of the connection or too high stresses.

Plastic Part Design in SolidWorksWriting a set of rules that applies perfectly in all situations for design with plastic parts in SolidWorks is impossible. Below is a set of guidelines that I try to use when I'm working on plastic parts. Sometimes you have to bend the rules, and sometimes you have to throw them away altogether, but for most situations, these rules of thumb for plastic part design should serve you well.

* Fillets cause many of the conflicts with other types of plastics features.

* Fillet order, with respect to the other features, is critical.

* Small convex fillets cause problems with shells, and any fillets between faces prevent multi-thickness shells.

* Fillets running perpendicular to the direction of pull cause problems with draft.

* Large fillets should be applied first.

* Vertical fillets that can be tapered can go before the draft.

* Fillets to be applied between the draft and shell: Large, non-tapered fillets between faces of equal shell thickness.

* Fillets to go after the shell:- Fillets with a radius smaller than the shell thickness.

- Fillets between faces with different shell thicknesses.

- Fillets that should not be transferred to the inside of the shell.

* The most common cause for the shell command to fail is that there are fillets on the model that have a smaller radius than the thickness of the shell.

* Use Tools, Check to find minimum face curvature, geometry errors or short edges.

* Severely sharp, pointy geometry may also frustrate the shell.

* Cut away sections of the model that look suspicious and retry the shell, then narrow in on the problem area.

* Existing bad geometry may cause a shell to fail for no apparent reason. Use Tools, Check to find bad faces or edges.

* It is recommended to add small fillets after the shell.

* Draft can come before or after shell. Remember that if it comes after, you will also have to draft all the interior faces.

* Drafting a face that is adjacent to a fillet that runs perpendicular to the direction of pull will not usually give acceptable results, even if the feature does not fail.

* All tangent faces should be drafted the same amount.

* Be aware of how draft changes the wall thickness of parts.

* Draft is sometimes not required on faces that will pull away from the mold due to shrink. It is always best to consult the tooling engineer before doing this.

* Drafting a fillet running parallel to direction of pull results in a tapered or variable radius fillet. The alternative is to apply the draft before the fillet.

* Draft features can be applied to faces which already have draft on them (draft is not cumulative).

* Draft can even be applied to straighten out curved faces when axis of curvature is perpendicular to direction of pull.

* Scale should be added to the engineered part as a separate configuration.

* Scale does not change the size or position of sketches or planes.

* In assemblies, avoid making in-context references or mates to unfinished plastic parts, unless you are sure that the face or edge being referenced is not going to be consumed later by a draft, fillet or split line feature.

* Mates to drafted plastic parts may be best handled with planes.

Why Plastic Products FailThe development of plastics and their associated processing techniques has been a phenomenal episode in the history of materials science. With large scale development taking place only within the last 60 years, the use of plastics in product design and manufacture has spiraled at a rate unrivaled by conventional materials. Due to the wide spectrum of properties available, plastics have become one of the most sought after materials in the world today.

More plastics are now available to the designer and engineer than at any previous stage in the history of industry. Today there are over 90 generic plastics and around 1000 sub-generic modifications with 50 thousand commercial grades available from over 500 manufacturers.

The short history of plastic development and proven usage has meant for the designer and engineer that for critical engineering applications there has never been enough time to fully explore service life and problems that might occur during the use of plastics. There has always been the question of vulnerability to failure and the ramifications of potential litigation. To some degree this situation has improved, as the portfolio of successful plastic designs has grown in demanding engineering applications. However, for new innovative applications pushing the boundaries of material performance the problem remains.

Designing to ensure plastic product reliability is critical due to the increasing importance of:

* Product liability claims * Environmental concerns * Certification in order to become an approved supplier * An awareness of quality costs * Product liability can be the most damaging with settlements and penalties in the order of thousands or even millions of pounds, particularly when failure has resulted in personal injury or death. In addition to litigation financial costs, there is the distraction of key employees from normal duties, loss in product perception, brand credibility and manufacturer reputation.

Considering that approximately 70% of plastic products fail prematurely, failures have been poorly reported since the owners of failed products are naturally generally reluctant to publicize the fact. Failure investigations of such cases tend not to be disseminated due to client confidentiality agreements and for this reason the activity is predominately covert. As a consequence the potential benefits such as learning from the mistakes and misfortunes of others, and identifying priorities for research and critical issues in product development are far from being fully exploited.

It is clear from the extent of plastic and rubber failure investigations conducted by Smithers Rapra that limited dissemination of plastic and rubber failure knowledge within the public domain has resulted in a continual cycle of plastic and rubber failure incidents from all industrial sectors. The lessons of good plastic and rubber product design are not being learnt even in light of the enormous growth in product liability cases that have imposed an entirely new dimension on the consumer product environment. It is now well established in law that manufacturers are liable for injuries resulting from defective product; for injuries from a hazard associated with a product against which the user should have been warned; or for damages caused by misapplication of a product which could have been foreseen by the manufacturer.

It is a practical necessity to understand why plastics fail in order to minimize the failure scenario. Smithers Rapra has acquired this knowledge due to 50 years dealing with a diverse clientele providing technical services aimed at problem solving and in particular failure diagnosis.

Failure is a practical problem with a product and implies that the component no longer fulfils its function. Frequently, the ability to withstand mechanical stress or strain (and thereby store or absorb mechanical energy) is the most important criterion in service and consequently mechanical failure is usually a primary concern. However failure may also be attributed to loss of attractive appearance or shrinkage.

In order to avert product failure it is critical that at all stages of the design process there must be a concurrent engineering approach to product development. This system ensures that from inception of the project until final high volume manufacture all parties involved (marketing, industrial design, product engineers, plastic expert, tooling designers/engineers and processors) continually communicate in order to take advantage of the valuable knowledge and experience of all. Key to successful design is that all aspects of the performance, production, assembly and ultimate use of the part are considered. Furthermore all parties promote building reliability and safety into the product.

In order to reduce the likelihood of product failure all parties within the design process must have the ability to imagine how their designed plastic part could fail. This can only be achieved if the product design team has a good appreciation of plastics material selection, product design, processing and specific material weaknesses and fault/ failure modes and avoidance.

Plastic product failure is commonly associated with human error or weakness.Human Causes of Failure (%)

In an attempt to reduce the incidence of plastic product failure we must react to the fact that they are typically due to human error, misunderstanding and ignorance of plastic materials and associated processes and that the material or process is usually not at fault.

It is hoped that the following information will provide some insight into complexity of plastics design and plastic failure modes.

Poor Material Selection / Substitution Failures arising from incorrect material selection and grade selection are perennial problems in the plastics industry. In order to perform plastic material selection successfully a complete understanding of plastic material characteristics, specific material limitations and failure modes is required. Good material selection requires a judicious approach and careful consideration of application requirements in terms of mechanical, thermal, environmental, chemical, electrical and optical properties. Production factors such as feasible and efficient method of manufacture in relation part size and geometry need to be assessed. In terms of economics the material cost, cycle times and part price need to be considered.

Two common reasons for improper material selection are that the material selector has limited plastics knowledge and expertise and is unfamiliar with the material selection process. Alternatively, a suitable material has been specified but not used. Materials substitutions most commonly occur when the customer is unable to enforce quality procurement specifications, particularly if manufacturing site is remotely based. Common problems encountered include:

Processor simply substituting with a cheaper material. Use of the wrong grade of material (incorrect MFI). Use of general purpose PS rather than HIPS. Homopolymer used instead of copolymer Incorrect pigments, fillers, lubricants or plasticizers used. Poor Design

There are no absolute rules pertaining to plastic product design. However, some general principles and guidelines are well established particularly between amorphous and semi-crystalline thermoplastics and thermosets and the various processing techniques. These are readily available from material suppliers.

The basic rules apply to fillets, radii, wall thickness, ribs, bosses, taper, holes, draft, use of metal inserts, undercuts, holes, threads, shrinkage, dimensional tolerance. Design rules which apply to secondary joining and assembly processes (welding, mechanical fastening and adhesive/solvent welding) need to be carefully evaluated too.

The designer and engineer should be aware that due to the diverse range of plastic materials and properties the design criteria will change form material to material as well as application to application.

Common design errors are related to abrupt geometrical changes excessive wall thickness, sharp corners and lack of radii, lack of understanding of the creep mechanism due to plastic visco-elasticity, environmental compatibility, draft, placement of ribs and injection gates.

A significant number of plastic parts fail due to sharp corners / insufficient radius. Sharp corners create stress concentrations resulting in locally high stresses and strains. Since plastics are notch sensitive the stress concentration will promote crack initiation and ultimately fracture. They also impede material flow and ejection form the tool.

A significant number of failures can be attributed to excessive wall thickness and abrupt geometrical change. A pre-requisite is that uniform wall thickness is maintained since this keeps sink marks, voids, warpage, and moulded-in stress to a minimum.

Designers and engineers must be fully conversant with the visco-elastic nature of plastics and their creep, creep rupture, stress relaxation and fatigue mechanisms.

Visco-plastic materials respond to stress as if they were a combination of elastic solids and viscous fluids. Consequently they exhibit a non-linear stress-strain relationship and their properties depend on the time under load, temperature, environment and the stress or strain level applied. An example of viscoelasticity can be seen with Silly Putty. If this material is pulled apart quickly it breaks in a brittle manner. If, however, pulled slowly apart the material behaves in a ductile manner and can be stretched almost indefinitely. Decreasing the temperature of Silly Putty, decreases the stretching rate at which it becomes brittle. Key is that the designer and engineer understand that:

Plastics will deform under load When subjected to static low stress / strain a ductile / brittle transition will occur at some point in time resulting in brittle failure Cyclic stressing will result in a ductile / brittle transition resulting in brittle failure at low stress level Premature initiation of cracking and embrittlement of a plastic can occur due to the simultaneous action of stress and strain and contact with specific chemical environments (liquid or vapor)

Design failure may also be attributed to reduced safety factors due to cost pressures and the use of plastics is demanding applications taking them to their design limits where on occasion they are exceeded.

Poor Processing

Poor processing, accounts for many in-service failures. Often the problem can be traced to a blatant disregard for established processing procedures and guidelines provided by material manufacturers. The driving force behind this is often economic - the need to achieve reduced cycle times and higher production yield.

Typical processing faults are given in Table 1.0. Many of these faults can generally be overcome by attention to processing variables such as temperature, shear rates, cooling times and pressure.

Use of inappropriate process equipment Non-uniform wall thickness Short shots Bubbles Sink marks Post-molding shrinkage Warping / distortion Foreign body contamination Voids Cosmetic - discoloration, splay marks Degradation(insufficient drying of material, process temperature too high, residence time in the barrel too long, shear heating, too much regrind Self-contamination (e.g., part-melted granules). Self-contamination (e.g., part-melted granules). Poor material homogeneity Poor weld lines and spider lines Residual stress Molecular orientation Development of low or excessive crystallinity Abnormal crystalline texture Insufficient packing Scorching Jetting Flashing Abnormal spatial and size distribution of phases in composites

Mis-use / Abuse

Plastic product failure due to misuse may result from a disregard for manufacturer installation instructions and failure to heed warnings. Failure may also occur due to simply using a product beyond its recommended service life, for function it was not intended or simply due to malicious attack.

Plastic Failure Modes

The main failure modes of plastics can be classed as mechanical, thermal, radiation, chemical and electrical. Classification of failure mode by mechanism shows that mechanical failure is the predominant mechanism although it is often the end result of many other failure modes.

From Smithers Rapra's experience we have found that the vast majority of plastic product failures are due to the cumulative effects of synergies between creep, fatigue, temperature, chemical species, UV and other environmental factors.

Mechanical Modes Deformation and distortion due to creep & stress relaxation, Yielding, , Crazing Brittle Fracture due to Creep rupture (static fatigue), Notched creep rupture, Fatigue (slow crack growth from cyclic loading), High energy impact Wear & abrasion,

Thermal Modes Thermal fatigue Degradation - thermo-oxidation Dimensional instability Shrinkage Combustion Additive extraction

Chemical Modes Solvation, Swelling, dimensional instability and additive extraction Oxidation Acid induced stress corrosion cracking (SCC) Hydrolysis (water, acid or alkali) Halogenations Environmental stress cracking (ESC) Biodegradation

Radiation Modes Photo-oxidative degradation (UV Light) Ionizing radiation ( gamma radiation, X rays)

Electrical Modes Electrostatic build-up, Arcing, tracking, Electrical and water treeing

Synergistic Modes Weathering - effects due to photo and thermo-oxidation, temperature cycling, erosion by rain and wind-borne particles and chemical elements in the environment

Smithers Rapra have undertaken over 5000 failure investigations of which a significant number can be attributed to embitterment and / or brittle fracture resulting from slow degradation or deterioration processes. From Figure 2.0 it can be seen that ESC, fatigue, notched static rupture, thermal degradation, UV degradation and chemical attack fall into this category, even when the material was reported to be ductile.