Injection Mold Design Engineering

David O. Kazmer

Injection Mold DesignEngineering

ISBN-10: 3-446-41266-2ISBN-13: 978-3-446-41266-8

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Preface

Mold design has been more of a technical trade than an engineering process. Traditionally,practitioners have shared standard practices and learned tricks of the trade to developsophisticated molds that often exceed customer expectations.

However, the lack of fundamental engineering analysis during mold design frequently resultsin molds that may fail and require extensive rework, produce moldings of inferior quality,or are less cost effective than may have been possible. Indeed, it has been estimated that onaverage 49 out of 50 molds require some modifications during the mold start-up process.Many times, mold designers and end-users may not know how much money was “left onthe table”.

The word“engineering”in the title of this book implies a methodical and analytical approach tomold design. The engineer who understands the causality between design decisions and moldperformance has the ability to make better and more informed decisions on an application byapplication basis. Such decision making competence is a competitive enabler by supportingthe development of custom mold designs that outperform molds developed according tostandard practices. The proficient engineer also avoids the cost and time needed to delegatedecision to other parties, who are not necessarily more competent.

The book has been written as a teaching text, but is geared towards professionals working ina tightly integrated supply chain including product designers, mold designers, and injectionmolders. Compared to most handbooks, this textbook provides worked examples withrigorous analysis and detailed discussion of vital mold engineering concepts. It should beunderstood that this textbook purposefully investigates the prevalent and fundamental aspectsof injection mold engineering.

I hope that Injection Mold Design Engineering is accessible and useful to all who read it. Iwelcome your feedback and partnership for future improvements.

Best wishes,

David Kazmer, P. E., Ph. D.

Lowell, MassachusettsJune 1, 2007

David O. Kazmer


ISBN-10: 3-446-41266-2ISBN-13: 978-3-446-41266-8

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3 Mold Cost Estimation

3.1 The Mold Quoting Process

The quoting process for plastic parts can be difficult for both the mold customer andsupplier. Consider the view of the mold customer. The procurement specialist for the productdevelopment team sends out requests for quotes (RFQs) to several mold makers.After waitingdays or weeks, the quotes come back and the customer discovers that the development timeand cost of the mold may vary by a factor of 3 or more. In such a case, prospective moldpurchasers should ask about the details of the provided quotes, and check if the costs can bereduced through product redesign. To reduce uncertainty related to pricing and capability,many prospective customers maintain a list of qualified suppliers, who tend to provide fasterturn-around, more uniform quality, and better pricing across multiple projects. Long-term,trusting partnerships can provide for rapid application and mold development by avoidingthe quoting process altogether and invoicing on a labor cost plus materials cost (referred toas “cost plus”) basis.

Now consider the view of the mold supplier. The mold designer must invest significant timedeveloping a quote that may have a relatively small chance of being accepted. Sometimes,the mold designer may have to redesign the product and perform extensive analysis toprovide the quote. While the quote may seem high to the prospective customer, the designmay correspond to a mold of higher quality materials and workmanship that can provide ahigher production rate and longer working life than some other lower cost mold. This moreexpensive mold may quickly recoup its added costs during production.

From time to time, mold-makers and molders will adjust their quote based on whetheror not they want the business. If the supplier is extremely busy or idle, then the estimatednumber of hours and/or hourly rate may be adjusted to either entice or to discourage thepotential customer from accepting the quote. Such adjustments should be avoided since theprovided quote does not represent the true costs of the supplier, which would become thebasis in a long term and mutually beneficial partnership between the mold supplier and thecustomer.

The provided quote typically provides payment and delivery terms for the mold(s) andperhaps even the molded part(s). A typical mold purchase agreement may specify that thecost of a mold is paid in three installments:

the first third: on acceptance of the quote (after which the mold base and key materialsare typically purchased);

the second third: half-way through the mold making project (often when cavity insertshave been machined); and

the final third: upon acceptance of the quality of the molded parts.

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38 3 Mold Cost Estimation

After the mold is purchased, molds are typically shipped to the specified molder or thecustomer’s facility where the parts are molded and marginal costs are incurred on a per partbasis. The cash outlays for a typical project are plotted in Figure 3.1 on a monthly basis.The material and processing costs in month 3 are related to molding trials to validate andimprove the mold design; a hundred or so pre-production parts may be sampled at this timefor marketing and testing purposes. Later, monthly costs are incurred related to production.Maintenance costs may appear intermittently throughout production to maintain the qualityof the mold and moldings.

There has been a trend in the industry towards large, vertically integrated molders withtightly integrated supply chains who can supply molded parts (and even complete productassemblies). As such, the structure of the quote can vary substantially with the structure ofthe business. With a vertically integrated supplier, there is typically an up-front fee for thecosts associated with the development of the mold, followed by a fee for each molded part.To protect the supplier, contracts are typically developed that specify minimum productionquantities with discounts and/or fees related to changes in the production schedule.

Since the structure and magnitude of quotes will vary substantially by supplier(s), a prospectivebuyer of plastic parts should solicit quotes from multiple vendors and select the quote fromthe supplier that provides the most preferable combination of molded part quality, paymentterms, delivery terms, and service.

0

5000

10000

15000

20000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time (months)

Paym

ent

am

ounts

($)

MaintenanceProcessMaterialMold

Figure 3.1: Schedule of mold and molding expenses

39

3.2 Cost Drivers for Molded Parts

There are three main drivers of the cost of a molded part:

the cost of the mold and its maintenance,

the materials cost, and

the processing cost.

Figure 3.2 provides a breakdown of these primary cost drivers and their underlying compo-nents. It is important to note that these costs do not include indirect costs such as overheador profits. However, such indirect costs may be accounted through the adjustment of hourlyrates and other costs.

•

•

•

Molded part

cost

FinishingMachiningMold base

cost

YieldCost per

kilogramPart weight

Processing

cost

Amortized

mold cost

Material

cost

RegrindProcessing

time

Hourly

rateFinishing

ReworkProduction

quantity

Molded part

cost

FinishingMachiningMold base

cost

YieldCost per

kilogramPart weight

Processing

cost

Amortized

mold cost

Material

cost

RegrindProcessing

time

Hourly

rateFinishing

ReworkProduction

quantity

Figure 3.2: Cost drivers for a commodity and specialty part

Commodity part, $0.01

Material,

$0.0050

Mold,

$0.0017

Processing,

$0.0033

Specialty part, $0.65

Material,

$0.05

Mold, $0.40

Processing,

$0.20

Figure 3.3: Cost drivers for a commodity and specialty part



Even though most molded products have the same cost drivers, the proportion of costs varieswidely by application. Figure 3.3 shows the cost breakdown for a commodity application(such as a cable tie with a production volume of 10 million pieces) and a specialty application(such as a custom electrical connector with a production volume of 100,000 pieces). Whilethese two products are approximately the same weight, it is observed that the magnitude andproportion of costs are vastly different.

3.2.1 Effect of Production Quantity

Minimization of the total molded part cost is not a simple task since injection molds andmolding processes are optimally designed for different target production quantities. Typically,there is a trade off between the upfront investment in the mold and later potential savingsrelated to the processing and material costs per part. Consider the data provided in Table 3.1for a molding application with production quantities of 50,000 and 5,000,000 pieces. Asindicated, the lower production quantity may be satisfied with a two cavity, cold runnermold. By comparison, the mold design for the higher production quantity utilizes a hotrunner system allowing the simultaneous molding of 32 cavities with a lower cycle time andreduced material consumption.

In theory, the production quantities should be known beforehand and used to design an“optimal” mold for the specified quantity. In reality, the production schedules and quantitiesare not precisely known, so the molder and customer must carefully consider the possibleresult of using molds that are over or under designed. For this reason, break-even analysisshould be utilized to consider the sensitivity of different mold designs to the total moldedpart cost.

Table 3.1: Part cost data for low and high production quantities

Production quantity 50,000 5,000,000

Number of mold cavities 2 32

Runner system Cold runner Hot runner

Mold cost $10,000 $250,000

Cycle time 30 s 20 s

Effective cycle time/part 15 s 0.6 s

Processing cost/part $0.40 $0.04

Mold cost/part $0.20 $0.05

Material cost/part $0.15 $0.12

Total cost/part $0.75 $0.21

41

3.2.2 Break-Even Analysis

Break-even analysis should be applied to ensure the design an appropriate mold. Consider theprevious case for the two molds described in Table 3.1. It is useful to consider the total costsincurred to produce a given quantity. The total costs, Ctotal, may be computed as:

= + ⋅total fixed total marginalC C n C (3.1)

where Cfixed is the total cost of the mold and its maintenance, ntotal is the total productionquantity across the life of the mold, and Cmarginal is the total marginal cost of the resin, machine,labor, and energy on a per part basis. For a given mold design, the marginal cost per piece willremain fairly constant across the life of the application (though there may be cost decreasesrelated to elimination of defects, reductions in cycle times, etc. as well as cost increases dueto material pricing or shipping costs). To provide the best possible mold design and quote,multiple mold designs should be developed for different target production quantities, andthe total production costs estimated and compared via break-even analysis.

Example: Consider the cost data provided in Table 3.1. Calculate the production volumewhere a hot runner mold becomes more economical than a cold runner mold.

Equation (3.1) is used to calculate the costs with the cold runner and hot runner as:

= + ⋅cold_runner cold_runner cold_runnertotal fixed total marginalC C n C

= + ⋅hot_runner hot_runner hot_runnertotal fixed total marginalC C n C

Equating these two costs and solving for the production volume provides the break-evenquantity:

−=−

hot_runner cold_runnerbreakeven fixed fixedtotal cold_runner hot_runner

marginal marginal

C Cn

C C

The analysis assumes that the marginal cost per molded part consists primarily of theprocessing and material costs. Then, the marginal costs for the cold and hot runners are$0.55 and $0.16, respectively. Substituting these values provides:

−= = =−

breakeventotal

$250,000 $10,000 $240,000615,000 parts

$0.55/part $0.16/part $0.39/partn

The costs for the cold and hot runner mold designs are provided in Figure 3.4. While thecost function of Eq. (3.1) is linear, a log-log scale has been used in the figure to providebetter resolution of the cost across a wide range of production volumes. In this example,the total cost for the 2 cavity cold runner mold and the 32 cavity hot runner mold areplotted as a function of the “realized” production quantity, Q. For this example, the 2cavity cold runner mold has a lower total cost up to the 615,000 part quantity, after whichthe 32 cavity hot runner mold provides a lower total cost.



1,000

10,000

100,000

1,000,000

10,000,000

1,000 10,000 100,000 1,000,000 10,000,000

Total production quantity, n total

Totalcostforn t

otalpieces

2 cavity cold runner

32 cavity hot runner

Figure 3.4: Break-even analysis

The cost analysis will typically indicate the need for different mold designs at extremely lowand extremely high production quantities. In the previous example, the upfront cost of the32 cavity hot runner system can not be justified at low or moderate production quantities.At very high production quantities, however, a hot runner system is essential to maximizingprofitability since the marginal costs of operating the hot runner mold are significantly lessthan those of the cold runner mold. While the breakeven analysis supports clear design deci-sions at very low and very high production quantities, the mold design can be less certainat intermediate production volumes. If the production quantity is on the order of 500,000parts, then the best mold design may utilize neither 2 nor 32 cavities for this application, butrather an intermediate quantity of 4, 8, or 16 cavities with or without a hot runner. As such,multiple designs and cost estimates should be developed until a good balance is achievedbetween higher upfront investment and lower marginal costs. If necessary, the customer canbe given more than one design to select the design that they think will ultimately be best.

Many molders and customers require a quick return on investment, and so will examinethe total cost curve to accept the use of a hot runner system with high cavitation only if adesirably short payback period can be achieved. Sometimes, however, mold design decisionsare not based solely on economics but rather by other concerns such as:

The need for a mold to permit rapid color changes, for which a hot runner feed system maynot be desirable. The color change issue in hot runners will be revisited in Section 6.4.8.

The capability and preference of the molder that will use the mold. If the molder doesnot have the experience or auxiliaries required to utilize a hot runner system, then a coldrunner mold may best be utilized.

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43

The lean manufacturing strategies of the molders to reduce costs and improve quality.For instance, it is not uncommon for molders to standardize on a specific type and sizeof mold to maximize production flexibility and reduce setup times.

As a general practice, the mold should be designed to maximize the molder’s capability unlessthe application requirements and cost constraints dictate otherwise. When an advancedmolding application has special requirements, it may be critical to select a molder with aspecialized set of molding capabilities and standard operating procedures. Chapter 13 providesa survey of mold technologies, many of which require special molder capabilities.

3.3 Mold Cost Estimation

Many cost estimation methods have been developed for molded plastic parts with varyingdegrees of causality and accuracy [10–21]. The following cost estimation method wasdeveloped to include the main effects of the part design and molding process while beingrelatively simple to use. To use the developed method, the practitioner can refer to the cost dataprovided in Appendices A, B, and D, or provide more application specific data as available.

The total mold cost, Ctotal_mold, is the sum of the cost of all cavities, Ccavities, and the cost ofthe mold base, Cmold_base, and the cost of the mold customization, Ccustomization:

= + +total_mold cavities mold_base customizationC C C C (3.2)

Mold maintenance costs are included as a portion of the mold amortization, and are calculatedwith the part cost. To demonstrate the cost estimation method, each of these cost drivers isanalyzed for the laptop bezel shown in Figure 3.5. The example analysis assumes that 1,000,000parts are to be molded of ABS from a single cavity, hot runner mold. The relevant applicationdata required to perform the cost estimation is provided in Table 3.2.

Figure 3.5: Isometric view of laptop bezel

•



Table 3.2: Laptop design data

Parameter Laptop bezel

Material ABS

Production quantity 1,000,000

Lpart 240 mm

Wpart 160 mm

Hpart 10 mm

Apart_surface 45,700 mm2

Vpart 27,500 mm3

Hwall 1.5 mm

Example: Estimate the total cost of a single cavity, hot runner mold for producing thelaptop bezel. This example corresponds to the mold design shown in Figure 1.8.

Subsequent analysis will show that the cost of the core and cavity inserts are $27,900, thecost of the mold base is $3,700, and the cost of the customizations including the purchaseof all associated components is $43,200. As such, the estimated total cost the mold is:

= + += + + ≈

total_mold cavities mold_base customization

$27,900 $3,700 $43,200 $74,800

C C C C

3.3.1 Cavity Cost Estimation

The cost of the core and cavity inserts is typically the single largest driver of the total moldcost. The reason for their expense is that they need to contain every geometric detail of themolded part, be made of very hard materials, and be finished to a high degree of accuracyand quality.

The total cost of all the cavity and core inserts is driven by the cost of each set of inserts, Ccavity,multiplied by the number of cavity sets, ncavities, and a discount factor, fcavity_discount:

= ⋅ ⋅cavities cavity cavities cavity_discount( )C C n f (3.3)

Example: Estimate the total cost of all core and cavity insert sets for the laptop bezel.

Since there is only one cavity and no cavity discount, the cost of all inserts sets is:

= ⋅ ⋅ =cavities ($27,900 1) 1 $27,900C

45

3.3.1.1 Cavity Set Cost

The cost of each cavity set is estimated as the sum of the materials costs, Ccavity_material, theinsert machining costs, Ccavity_machining, and the insert finishing costs, Ccavity_finishing:

= + +cavity cavity_material cavity_machining cavity_finishingC C C C (3.4)

Example: Estimate the cost of one set of core and cavity inserts for the laptop bezel.

Subsequent analysis will show that the cost of the materials is $435, the cost of the cavitymachining is $25,800, and the cost of the cavity finishing is $1,700. As such, the total costfor one core and cavity set is:

= + + ≈cavity $435 $25,800 $1,700 $27,900C

3.3.1.2 Cavity Materials Cost

The cost of the cavity insert materials is the simplest and least significant term to evaluate.Specifically, the cavity materials cost is the volume of the cavity set, Vcavity_material, multipliedby the density, ρcavity_material, and the cost of the material per kilogram, κcavity_material:

ρ κ= ⋅ ⋅cavity_material cavity_material cavity_material cavity_materialC V (3.5)

Cost data for some common metals are provided in Appendix B.

The cavity insert volume is the product of the cavity length, Lcavity, the cavity width, Wcavity,and the cavity height, Hcavity:

= ⋅ ⋅cavity_material cavity cavity cavityV L W H (3.6)

The size of the cavity set is finalized during the mold layout design process as discussed inChapter 4. From generalization of the later analysis, these dimensions can be roughly estimatedas a function of the part size as follows:

= + ⋅

= + ⋅

=

cavity part part part

cavity part part part

cavity part

max [0.1 , ]

max [0.1 , ]

max [0.057,2 ]

L L L H

W W W H

H H

(3.7)

It should be noted that for the formula to work with the data provided in the Appendices, alldimensions must be stated in meters or converted with the data to another consistent set ofunits. As previously suggested, the analysis should be conducted using application specificdata for the material properties, part geometry, mold geometry, or manufacturing processeswhen such data is available.



Example: Estimate the cost of the core and cavity insert materials for the laptop bezel.

First, the dimensions of the core and cavity inserts are estimated. From the dimensionsprovided in Table 3.2, the preliminary dimensions of the inserts are:

= + ⋅ =

= + ⋅ =

= ⋅ =

cavity

cavity

cavity

0.24 m max [0.1 0.24 m, 0.01 m] 0.268 m

0.16 m max [0.1 0.16 m, 0.01 m] 0.176 m

max [0.057,2 0.01 m] 0.057 m

L

W

H

which provides a volume of:

−= ⋅ ⋅ = ⋅ 3 3cavity_material 0.264 m 0.176 m 0.057 m 2.65 10 mV

To calculate the cost of the core and cavity insert materials, the type of material must beknown. Since this is a tight tolerance part with a high production quantity, tool steel D2is selected for its wear and abrasion resistance. This material has a density of 7670 kg/m3

and a cost of 21.4 $/kg, which leads to a cost for the core and cavity insert materials of:

−= ⋅ ⋅ ⋅ =3 3cavity_material 3

kg $2.65 10 m 7670 21.4 $435

kgmC

3.3.1.3 Cavity Machining Cost

The cavity machining cost, Ccavity_machining, is the single most significant driver of the totalmold cost, and is a function of many variables including

the volume and geometric complexity of the part to be molded,

the core and cavity inserts’ material properties,

the machining processes,

the labor cost, and

the quality of the inserts required.

The approach used here is to estimate the cavity machining cost by multiplying the machiningtime, tcavity_machining, with the machining labor rate, Rmachining_rate:

= ⋅cavity_machining cavity_machining machining_rateC t R (3.8)

The machining labor rate, Rmachining_rate, varies substantially with the cost of living in thelocation where the mold is manufactured. A mold maker in a high cost of living area (such asGermany) will tend to have a higher labor cost than a mold maker in a low cost of living area(such as Taiwan). Furthermore, the labor rate will also vary with the toolset, capability, andplant utilization of the mold maker. For example, a mold maker using a 5 axis numericallycontrolled milling machine will tend to have more capability and charge more than a moldmaker using manually operated 3 axis milling machines. Some approximate cost and efficiency

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47

data for machining and labor rates are provided in Appendix D, though application specificdata with the negotiated machinist’s rate should be used if this data is available.

The cavity machining time is driven by the size and complexity of the cavity details to bemachined, as well as the speed of the machining processes used. In theory, the exact order andtiming of the manufacturing processes can be planned to provide a precise time estimate. Inpractice, however, this approach is fairly difficult unless the entire job can be automaticallyprocessed, for instance, on a numerically controlled mill.1

The cavity machining time is estimated as the sum of the volume machining time, tcavity_volume,and the area machining time, tcavity_area. To take application specific requirements into account,the cavity machining time is then multiplied by a complexity factor, fcavity_complexity, to considergeometric complexity as well as a machining factor, fmachining, then divided by an efficiencyfactor, fmachining_efficiency:

⎛ ⎞+= ⋅ ⋅⎜ ⎟

⎝ ⎠cavity_volume cavity_area

cavity_machining cavity_complexity machiningmachining_efficiency

t tt f f

f(3.9)

The cavity volume machining time is a function of the volume of material to be removedand the material removal rate. To provide an approximate but conservative estimate, theassumption is made that the removal volume is equal to the entire volume of the core andcavity inserts. This may seem an overly conservative estimate, but in fact much of the volumemust be removed around the outside of the core insert and the inside of the cavity insert.

The material removal rate is a function of the processes that are used, the finish and tolerancesrequired, as well as the properties of the mold core and cavity insert materials. To simplifythe analysis, a geometric complexity factor will later be used to capture the effect of differentmachining processes and tolerances needed to produce the required cavity details.As such, thevolume machining time captures only the time to require the material removal as follows:

= cavity_materialcavity_volume

material_volume

Vt

R(3.10)

where Rmaterial_volume is the volumetric mold material removal rate measured in cubic meters perhour. Machining data for different materials are provided in Appendix B, though applicationspecific material removal rates can be substituted if the depth of cut, speed, and feed ratesare known [22].

1 Prototype molds are, in fact, increasingly being produced in a nearly fully automatic mode on high speednumerically controlled milling machines. Due to limitations in the process, the core and cavity insertsare typically machined from aluminum with very small end-mills used to provide reasonably detailedfeatures. While this mold-making approach does provide very precise cost estimates and low costs, theresulting molds are comparatively soft and often not appropriate for molding high quantities. Higherstrength and wear resistant aluminum alloys, however, have recently been and continue to be developedthat are increasingly cannibalizing conventionally manufactured steel molds.


David O. Kazmer


ISBN-10: 3-446-41266-2ISBN-13: 978-3-446-41266-8

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4 Mold Layout Design

During the mold layout stage, the mold designer commits to the type of mold and selects thedimensions and materials for the cavity inserts, core inserts, and mold base. Mold bases areonly available in discrete sizes, so iteration between the inserts’ sizing and mold base selectionis normal. The goal of the mold layout design stage is to develop the physical dimensions ofthe inserts and mold so as to enable procurement of these materials. Mold material selectionis also an important decision, since the material properties largely determine the mold makingtime and cost as well as the mold’s structural and thermal performance.

The mold layout design assumes that the number of mold cavities and type of mold has beendetermined. To develop the mold layout, the mold opening direction and the location of theparting plane are first determined. Then, the length, width, and height of the core and cavityinserts are chosen. Afterwards, a mold base is selected and the inserts are placed in as simpleand compact a layout as possible. It is important to develop a good mold layout design sincelater analysis assumes this layout design and these dimensions are quite expensive to changeonce the mold making process has begun.

4.1 Parting Plane Design

The parting plane is the contact surface between the stationary and moving sides of the mold.The primary purpose of the parting plane is to tightly seal the cavity of the mold and preventmelt leakage. This seal is maintained through the application of literally tons of force (hencethe term “clamp tonnage”) that are applied normal to the parting plane. While the term“parting plane” implies a flat or planar surface, the parting plane may contain out-of-planefeatures. The mold designer must first determine the mold opening direction to design theparting plane.

4.1.1 Determine Mold Opening Direction

Examination of any of the previous mold designs (e.g., Figure 1.4 to Figure 1.8) indicates thatthe mold opening direction is normal to the parting plane. In fact, the mold usually opensin a direction normal to the parting plane since the moving platen of the molding machineis guided by tie bars or rails to open in a direction normal to the platen. Accordingly, guidebushings and/or mold interlocks are almost always located on the parting plane to guide themold opening in a direction normal to the parting plane.

It may appear that there is nothing about the mold opening direction to determine sincethe mold opens normal to the parting plane. However, it is necessary to determine the mold

68 4 Mold Layout Design

opening direction relative to the mold cavity. There are two factors that govern the moldopening direction:

1. First, the mold cavity should be positioned such that it does not exert undue stress onthe injection mold. The mold cavity is typically placed with its largest area parallel to theparting plane. This arrangement allows the mold plates, already being held in compressionunder the clamp tonnage, to resist the force exerted by the plastic on the surfaces of themold cavity.

2. Second, the mold cavity should be positioned such that the molded part can be ejectedfrom the mold. A typical molded part is shaped like a five-sided open box with the sidewalls, ribs, bosses, and other features normal to its largest area. If so, then the part ejectionrequirement again supports the mold opening direction to be normal to the part’s largestprojected area.

Consider the cup and lid shown in Figure 4.1. A section of the core and cavity inserts usedto mold these parts was previously shown in Figure 1.6. There are only two potential moldopening directions relative to the part. One mold opening direction is in the axial directionof the cup, while the second direction is in the radial direction of the cup.

Figure 4.1: Sectioned isometric view of cup assembly

69

A section of a cavity block with an axial mold opening direction is shown in Figure 4.2. Thetwo bold horizontal lines indicates the location of the parting plane where the two halves ofthe insert are split to form the cavity insert (top) and the core insert (bottom).

Consider next the same cavity block but with a radial mold opening direction for a portionof the cavity insert as shown in Figure 4.3. For this design, four bold lines separate the sidesfrom the top and bottom. Since the metal core is located inside the molded part, there is noway to remove the core other than in the part’s axial direction. The cavity insert, however,can be separated into three pieces that move along two different axes in order to remove themolded part.

Of these two designs, the axial mold opening direction shown in Figure 4.2 is the simplestdesign and is usually preferred. However, the second design is sometimes used in practicesince it allows for a more complex part design as well as more options in locating the partingline. For instance, the second design might be required if a handle were added to the cup, orif it was necessary to move the parting line to a location away from the top lip. This seconddesign is known as a “split cavity mold” and is discussed in more detail in Section 13.9.1.

As another example, consider the laptop bezel shown in Figure 3.5. There are again twopotential mold opening directions. The first opening direction is in the screen’s viewingdirection, as indicated by the section view shown in Figure 4.4. In this case, the mold sectionis split by two horizontal lines into a cavity insert forming the outside surface of the bezeland a core insert that forms the inner surface and ribs of the bezel. When the core and cavityinserts are separated as indicated by the arrows, the molded bezel can be readily removed.

Figure 4.2: Axial mold opening directionfor cup

Figure 4.3: Radial mold opening directionfor cup

Figure 4.4: Normal mold opening direction for bezel



Alternatively, the cavity block for the PC bezel can be split as indicated with the three verticallines shown in Figure 4.5. In this case, the former cavity insert is split into two pieces, resultingagain in a split cavity mold design. The two halves of the former cavity insert must nowbe removed in oblique directions in order to remove the molded part; the mold openingdirection is inclined in order to allow the mold surfaces to separate from the molded partwithout excessive surface friction or shearing of features on the molded part. This movementrequires several additional mold components to control the moving cavity inserts, which addsignificantly to the cost of mold design, manufacture, and operation.

4.1.2 Determine Parting Line

The term “parting line” refers to the location at which the cavity insert, the core insert, andthe plastic molding meet. Since the core and cavity insert meet at this location, any significantdeflection of the cavity insert away from the core insert will result in a gap into which theplastic will flow and form a thin film of plastic known as“flash”. Imperfections in the core andcavity inserts at this location, for instance due to wear or improper handling, will also creategaps into which the plastic will flow. Even with new and well-crafted molds, the location ofthe parting line usually results in a very slight “witness line” along its length.

For this reason, the parting line should be located along a bottom edge of the part, or someother non-visual, non-functional edge. Consider the previous cup shown in Figure 4.1.Placing the parting line very close to the lip as indicated by the dashed line in the left drawingof Figure 4.6 would result in a witness line and possible flash that might make the moldedcup unusable. Alternatively, a better location for the parting line is at the bottom of the rimas indicated in Figure 4.2, corresponding to the parting line shown in the right drawing ofFigure 4.6.

Figure 4.5: Complex mold opening directions for bezel

Figure 4.6: Two parting line locations for cup

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For the laptop bezel, the parting line will be located around the bottom edge of the part asshown in Figure 4.7. It is observed that, unlike the cup, the parting line for the bezel is not ina single plane. Rather, the parting line follows the profile of the features on the side walls. Thisnon-planar parting line is required to fit the core insert which hollows out the mold cavityto form the holes required for the various connectors. As will be seen in the next section, thiscomplex parting line shape will cause a more complex parting plane.

4.1.3 Parting Plane

Once the parting line is identified, the parting plane is projected outwards from the part, soas to separate the core insert from the cavity insert. The preferred parting plane for the cup isshown in Figure 4.8. The cavity insert will form the outer and top surfaces of the part, whilethe core insert will form the rim and inner surfaces.

Figure 4.7: Parting line location for bezel

Figure 4.8: Parting plane for cup



For the laptop bezel, the parting line in Figure 4.7 can be radiated outward to form the partingsurface shown in Figure 4.9. It can be observed that all of the out of plane features along theparting line now become complex surfaces on the parting plane. These surfaces pose twosignificant issues during mold operation. First, any misalignment between the sharp featureson core and cavity inserts will cause wear between the sliding surfaces if not an outright impactbetween the leading edge of the core and the mating cavity surface. Second, the clamp tonnageexerted on the core and cavity inserts can cause the surfaces to lock together with extremeforce, causing excessive stress and potential mold deformation during mold operation.

To avoid excessive stress, interlocking features on the parting plane should be inclined at leastfive degrees relative to the mold opening direction. The parting surface is now typically createdvia three dimensional computer aided design (“3D CAD”) using lofted surfaces. Each lofted

Figure 4.9: Parting plane for bezel

Figure 4.10: Modified parting surface for bezel

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surface blends a curved feature along the parting line to a line of corresponding width onthe parting plane. The result is a surface with the needed profile at the parting line and thenecessary draft down to the parting plane. The lofted surfaces are then knit together with theparting plane to provide a parting surface, as shown for the bezel in Figure 4.10.

4.1.4 Shut-Offs

Shut-offs are contact areas between the core insert and the cavity insert that separate portionsof the cavity formed between the core and cavity inserts. A shut-off will need to be definedfor each window or opening in the molded part. Conversely, if a part has no windows, like thecup, then no shut-offs are defined. Each shut-off is defined by a parting line, which shouldbe located in a non-visual area where a witness line or slight flashing would not reduce thevalue of the molded part.

For example, the laptop bezel has one large opening above the parting plane for the display.A shut-off is necessary across the entire area of the opening. As indicated in Figure 4.11, thereare essentially two possible locations for the shut-off ’s parting line, corresponding to the topand bottom of the shelf that supports the display.

Either location (or even any location in between) would likely be acceptable since the entireshelf is hidden from view. If the parting line is placed at the top of the shelf as indicated atthe right of Figure 4.11, then a shut-off surface as shown in Figure 4.12 will result.

Figure 4.12: Shut-off surface for bezel

Figure 4.11: Shut-off surface for bezel



4.2 Cavity and Core Insert Creation

With the definition of the parting plane and all necessary shut-offs, the core insert and thecavity insert have been completely separated. To create the cavity and core inserts, the length,width, and height of the inserts must be defined.

The length and width of the cavity and core inserts must be large enough to:

enclose the cavity where the part is formed,

withstand the forces resulting from the melt pressure exerted upon the area of thecavity,

contain the cooling lines for removing heat from the hot polymer melt, and

contain other components such as retaining screws, ejector pins, and others.

All of these requirements suggest making the core and cavity inserts as large as possible. Forsmaller molded parts, increasing the sizing the core and cavity inserts may have little addedcost. However, the cost of larger core and cavity inserts can become excessive with increasesin the number of cavities or molded part size.

4.2.1 Height Dimension

The height dimension is often determined by two requirements. First, the core and cavity insertshould have enough height above and below the molded part to safely pass a cooling line.Cooling line diameters typically range from 4.76 mm (3/16″) for smaller molds to 15.88 mm(5/8″) for large molds. Generally, large inserts with larger cooling lines will provide fasterand more uniform cooling as will be analyzed in Chapter 9. While cooling line design willbe later discussed, the minimum height dimension between the molded part and the top or

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Figure 4.13: Insert height allowance

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bottom surface of the insert is typically three times the diameter of the cooling line to avoidexcessive stress as analyzed in Chapter 12. The initial height dimensions for the core andcavity inserts are shown in Figure 4.13.

Second, the core and cavity insert should have a height that is matched with the heightof available cavity and core insert retainer plates (the “A” and “B” plates). These plates arecommonly available in ½″ increments in English units, and in 10 mm increments in metricunits. As such, the insert heights should be adjusted up such that the faces of the cavity andcore inserts are flush or slightly proud with respect to the “A” and “B” plates on the partingplane. It should be noted that the height of the core insert as indicated in Figure 4.13 is notits total height but rather the height dimension from the rear surface to the parting plane.For materials procurement and cost estimation, the total height of the core insert should alsoinclude the height of the core above the parting plane.

4.2.2 Length and Width Dimensions

The length and width dimensions are similarly determined by two requirements. First, if acooling line is needed around the exterior of the mold cavity, then the inserts should be sizedlarge enough to accommodate such a cooling line. As for the height allowance, length andwidth allowances of three cooling line diameters per side are typical. Second, the width andlength dimensions of the inserts should provide side walls, also known as “cheek”, that arethick enough to withstand the lateral loading of the melt pressure exerted on the side wallsof the mold cavity. This requirement will become dominating for deep parts with large sidewalls. While the structural design will be discussed in detail in Section 12.2.4, a safe guidelineis that the thickness of the side wall in the length and width dimension should equal thedepth of the mold cavity.

Figure 4.14 demonstrates an allowance that should be added to the length and width of themold cavity to derive the length and width of the core and cavity inserts. It can be observedthat for the laptop bezel, the requirement of fitting a cooling line will exceed the structuralrequirement. For the molded cup, however, the insert length and width dimension are drivenby the structural requirement.

Figure 4.14: Insert length and width allowance

4.2 Cavity and Core Insert Creation


4.2.3 Adjustments

The core and cavity inserts can now be created with the prescribed dimensions. However, it issometimes desirable to adjust the cavity insert dimensions to provide a more efficient molddesign. In general, the length and width dimensions of the inserts are more critical than theheight dimension, since these dimensions will

drive the size of the mold base in multi-cavity applications, and

contribute more to the material and machining costs.

As such, these dimensions may be decreased somewhat by effective cooling and structuraldesigns, which will be supported by later engineering analysis.

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Figure 4.15: Core and cavity inserts for cup

Figure 4.16: Core and cavity inserts for bezel

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Figure 4.15 provides the core and cavity inserts for the cup. Since the molded part is round,the design of the core and cavity insert may also be round. This shape provides a benefit withrespect to ease of manufacturing, since both the core and cavity inserts can be turned ona lathe. While the allowances in the axial and radial dimensions are sufficient to fit coolinglines, the allowance in the radial dimension may not be sufficient to withstand the pressuresexerted on the side wall by the melt.

There is no fundamental requirement on the external shape of the core and cavity inserts.While the insert design in Figure 4.15 showed round inserts, the mold design for the cupshown previously in Figure 1.4 used square inserts. Rectangular inserts with or without filletedcorners are also quite common. The design of the insert should be dictated by the shape ofthe molded part, the efficiency of the mold design, and the ease of manufacture.

The core and cavity inserts for the laptop bezel are shown in Figure 4.16. In this case,rectangular inserts are designed. The length and width dimensions of the inserts havebeen designed quite aggressively. While the bezel is quite shallow and the inserts arestructurally adequate, the thickness of the surrounding cheek may not allow for sufficientcooling around the periphery of the mold cavity while also providing space for other moldcomponents.

4.3 Mold Base Selection

After the core and cavity inserts have been initially sized, the mold layout can be furtherdeveloped and the mold base selected. It is critical to order a mold base with appropriately sizedplates and materials, since any mistakes in the mold base selection can consume significanttime and expense. To determine the appropriate size, the mold designer must first arrangethe mold cavities and provide allowances for the cooling and feed systems. Afterwards, themold designer should select a standard size from available suppliers and verify suitabilitywith the molder’s molding machine.

4.3.1 Cavity Layouts

The goal of cavity layout design is to produce a mold design that is compact, easy tomanufacture, and provides molding productivity. If a single cavity mold is being designed,then the cavity is typically located in the center of the mold, though gating requirements maynecessitate placing the mold cavity off center. For multi-cavity molds, there are essentiallythree fundamental cavity layouts:

cavities are placed along one line

cavities are placed in a grid, or

cavities are placed around a circle.

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4.3 Mold Base Selection

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Injection Mold Design Engineering