Injection Mold Design Engineering

David O. Kazmer

Injection Mold Design Engineering ISBN-10: 3-446-41266-2 ISBN-13: 978-3-446-41266-8 Vorwort Weitere Informationen oder Bestellungen unter http://www.hanser.de/978-3-446-41266-8 sowie im Buchhandel

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 develop sophisticated molds that often exceed customer expectations. However, the lack of fundamental engineering analysis during mold design frequently results in 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 on average 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 on the table”. The word“engineering”in the title of this book implies a methodical and analytical approach to mold design. The engineer who understands the causality between design decisions and mold performance has the ability to make better and more informed decisions on an application by application basis. Such decision making competence is a competitive enabler by supporting the development of custom mold designs that outperform molds developed according to standard practices. The proficient engineer also avoids the cost and time needed to delegate decision to other parties, who are not necessarily more competent. The book has been written as a teaching text, but is geared towards professionals working in a tightly integrated supply chain including product designers, mold designers, and injection molders. Compared to most handbooks, this textbook provides worked examples with rigorous analysis and detailed discussion of vital mold engineering concepts. It should be understood that this textbook purposefully investigates the prevalent and fundamental aspects of injection mold engineering. I hope that Injection Mold Design Engineering is accessible and useful to all who read it. I welcome your feedback and partnership for future improvements. Best wishes, David Kazmer, P. E., Ph. D. Lowell, Massachusetts June 1, 2007

David O. Kazmer

Injection Mold Design Engineering ISBN-10: 3-446-41266-2 ISBN-13: 978-3-446-41266-8 Leseprobe Weitere Informationen oder Bestellungen unter http://www.hanser.de/978-3-446-41266-8 sowie im Buchhandel

3

Mold Cost Estimation

3.1

The Mold Quoting Process

The quoting process for plastic parts can be difficult for both the mold customer and supplier. Consider the view of the mold customer. The procurement specialist for the product development team sends out requests for quotes (RFQs) to several mold makers. After waiting days or weeks, the quotes come back and the customer discovers that the development time and cost of the mold may vary by a factor of 3 or more. In such a case, prospective mold purchasers should ask about the details of the provided quotes, and check if the costs can be reduced through product redesign. To reduce uncertainty related to pricing and capability, many prospective customers maintain a list of qualified suppliers, who tend to provide faster turn-around, more uniform quality, and better pricing across multiple projects. Long-term, trusting partnerships can provide for rapid application and mold development by avoiding the quoting process altogether and invoicing on a labor cost plus materials cost (referred to as “cost plus”) basis. Now consider the view of the mold supplier. The mold designer must invest significant time developing 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 to provide the quote. While the quote may seem high to the prospective customer, the design may correspond to a mold of higher quality materials and workmanship that can provide a higher production rate and longer working life than some other lower cost mold. This more expensive mold may quickly recoup its added costs during production. From time to time, mold-makers and molders will adjust their quote based on whether or not they want the business. If the supplier is extremely busy or idle, then the estimated number of hours and/or hourly rate may be adjusted to either entice or to discourage the potential customer from accepting the quote. Such adjustments should be avoided since the provided quote does not represent the true costs of the supplier, which would become the basis in a long term and mutually beneficial partnership between the mold supplier and the customer. The provided quote typically provides payment and delivery terms for the mold(s) and perhaps even the molded part(s). A typical mold purchase agreement may specify that the cost of a mold is paid in three installments: •

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

•

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

•

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

38

3 Mold Cost Estimation

20000

Payment amounts ($)

Maintenance Process Material

15000

Mold

10000

5000

0 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18

Time (months)

Figure 3.1: Schedule of mold and molding expenses

After the mold is purchased, molds are typically shipped to the specified molder or the customer’s facility where the parts are molded and marginal costs are incurred on a per part basis. 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 and improve the mold design; a hundred or so pre-production parts may be sampled at this time for marketing and testing purposes. Later, monthly costs are incurred related to production. Maintenance costs may appear intermittently throughout production to maintain the quality of the mold and moldings. There has been a trend in the industry towards large, vertically integrated molders with tightly integrated supply chains who can supply molded parts (and even complete product assemblies). As such, the structure of the quote can vary substantially with the structure of the business. With a vertically integrated supplier, there is typically an up-front fee for the costs 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 production quantities 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 prospective buyer of plastic parts should solicit quotes from multiple vendors and select the quote from the supplier that provides the most preferable combination of molded part quality, payment terms, delivery terms, and service.

39

3.2 Cost Drivers for Molded Parts

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 components. It is important to note that these costs do not include indirect costs such as overhead or profits. However, such indirect costs may be accounted through the adjustment of hourly rates and other costs.

Molded part cost

Material cost

Amortized mold cost

Mold base cost

Part weight

Cost per kilogram

Machining

Yield

Processing cost

Finishing

Rework

Production quantity

Processing time

Regrind

Hourly rate

Finishing

Figure 3.2: Cost drivers for a commodity and specialty part

Commodity part, $0.01

Specialty part, $0.65

Material, $0.05 Processing, $0.20

Processing, $0.0033 Material, $0.0050

Mold, $0.0017

Figure 3.3: Cost drivers for a commodity and specialty part

Mold, $0.40

40


Even though most molded products have the same cost drivers, the proportion of costs varies widely 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). While these two products are approximately the same weight, it is observed that the magnitude and proportion 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 and molding 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 savings related to the processing and material costs per part. Consider the data provided in Table 3.1 for a molding application with production quantities of 50,000 and 5,000,000 pieces. As indicated, the lower production quantity may be satisfied with a two cavity, cold runner mold. By comparison, the mold design for the higher production quantity utilizes a hot runner system allowing the simultaneous molding of 32 cavities with a lower cycle time and reduced 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 quantities are not precisely known, so the molder and customer must carefully consider the possible result of using molds that are over or under designed. For this reason, break-even analysis should be utilized to consider the sensitivity of different mold designs to the total molded part 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

3.2 Cost Drivers for Molded Parts

3.2.2

41

Break-Even Analysis

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

(3.1)

where Cfixed is the total cost of the mold and its maintenance, ntotal is the total production quantity 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 will remain fairly constant across the life of the application (though there may be cost decreases related to elimination of defects, reductions in cycle times, etc. as well as cost increases due to 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, and the total production costs estimated and compared via break-even analysis. Example: Consider the cost data provided in Table 3.1. Calculate the production volume where 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_runner = Cfixed + ntotal ⋅ Cmarginal Ctotal hot_runner hot_runner hot_runner = Cfixed + ntotal ⋅ Cmarginal Ctotal

Equating these two costs and solving for the production volume provides the break-even quantity: breakeven ntotal =

hot_runner cold_runner Cfixed − Cfixed cold_runner hot_runner Cmarginal − Cmarginal

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

$250,000 − $10,000 $240,000 = = 615,000 parts $0.55/part − $0.16/part $0.39/part

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

42


10,000,000 2 cavity cold runner 32 cavity hot runner

Total cost for ntotal pieces

1,000,000

100,000

10,000

1,000 1,000

10,000

100,000

1,000,000

10,000,000

Total production quantity, n total

Figure 3.4: Break-even analysis

The cost analysis will typically indicate the need for different mold designs at extremely low and extremely high production quantities. In the previous example, the upfront cost of the 32 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 maximizing profitability since the marginal costs of operating the hot runner mold are significantly less than those of the cold runner mold. While the breakeven analysis supports clear design decisions at very low and very high production quantities, the mold design can be less certain at intermediate production volumes. If the production quantity is on the order of 500,000 parts, then the best mold design may utilize neither 2 nor 32 cavities for this application, but rather 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 achieved between higher upfront investment and lower marginal costs. If necessary, the customer can be 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 examine the total cost curve to accept the use of a hot runner system with high cavitation only if a desirably short payback period can be achieved. Sometimes, however, mold design decisions are 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 may not 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 does not have the experience or auxiliaries required to utilize a hot runner system, then a cold runner mold may best be utilized.

3.3 Mold Cost Estimation

•

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 size of mold to maximize production flexibility and reduce setup times.

As a general practice, the mold should be designed to maximize the molder’s capability unless the application requirements and cost constraints dictate otherwise. When an advanced molding application has special requirements, it may be critical to select a molder with a specialized set of molding capabilities and standard operating procedures. Chapter 13 provides a 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 varying degrees of causality and accuracy [10–21]. The following cost estimation method was developed to include the main effects of the part design and molding process while being relatively simple to use. To use the developed method, the practitioner can refer to the cost data provided 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 of the mold base, Cmold_base, and the cost of the mold customization, Ccustomization: Ctotal_mold = Ccavities + Cmold_base + Ccustomization

(3.2)

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

Figure 3.5: Isometric view of laptop bezel

44


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 the laptop 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, the cost of the mold base is $3,700, and the cost of the customizations including the purchase of all associated components is $43,200. As such, the estimated total cost the mold is: Ctotal_mold = Ccavities + Cmold_base + Ccustomization = $27,900 + $3,700 + $43,200 ≈ $74,800

3.3.1

Cavity Cost Estimation

The cost of the core and cavity inserts is typically the single largest driver of the total mold cost. The reason for their expense is that they need to contain every geometric detail of the molded part, be made of very hard materials, and be finished to a high degree of accuracy and 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: Ccavities = (Ccavity ⋅ ncavities ) ⋅ f cavity_discount

(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: Ccavities = ($27,900 ⋅ 1) ⋅ 1 = $27,900


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, the insert machining costs, Ccavity_machining, and the insert finishing costs, Ccavity_finishing: Ccavity = Ccavity_material + Ccavity_machining + Ccavity_finishing

(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 cavity machining is $25,800, and the cost of the cavity finishing is $1,700. As such, the total cost for one core and cavity set is: Ccavity = $435 + $25,800 + $1,700 ≈ $27,900 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, multiplied by the density, ρcavity_material, and the cost of the material per kilogram, κcavity_material: Ccavity_material = Vcavity_material ⋅ ρcavity_material ⋅ κcavity_material

(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: Vcavity_material = Lcavity ⋅ Wcavity ⋅ H cavity

(3.6)

The size of the cavity set is finalized during the mold layout design process as discussed in Chapter 4. From generalization of the later analysis, these dimensions can be roughly estimated as a function of the part size as follows: Lcavity = Lpart + max [0.1 ⋅ Lpart , H part ] Wcavity = Wpart + max [0.1 ⋅ Wpart , H part ]

(3.7)

H cavity = max [0.057,2 H part ]

It should be noted that for the formula to work with the data provided in the Appendices, all dimensions must be stated in meters or converted with the data to another consistent set of units. As previously suggested, the analysis should be conducted using application specific data for the material properties, part geometry, mold geometry, or manufacturing processes when such data is available.

46


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 dimensions provided in Table 3.2, the preliminary dimensions of the inserts are: Lcavity = 0.24 m + max [0.1 ⋅ 0.24 m, 0.01 m] = 0.268 m Wcavity = 0.16 m + max [0.1 ⋅ 0.16 m, 0.01 m] = 0.176 m H cavity = max [0.057,2 ⋅ 0.01 m] = 0.057 m

which provides a volume of: Vcavity_material = 0.264 m ⋅ 0.176 m ⋅ 0.057 m = 2.65 ⋅ 10−3 m 3 To calculate the cost of the core and cavity insert materials, the type of material must be known. Since this is a tight tolerance part with a high production quantity, tool steel D2 is 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: Ccavity_material = 2.65 ⋅ 10−3 m 3 ⋅ 7670

kg $ ⋅ 21.4 = $435 3 kg m

3.3.1.3 Cavity Machining Cost The cavity machining cost, Ccavity_machining, is the single most significant driver of the total mold 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 machining time, tcavity_machining, with the machining labor rate, Rmachining_rate: Ccavity_machining = t cavity_machining ⋅ Rmachining_rate

(3.8)

The machining labor rate, Rmachining_rate, varies substantially with the cost of living in the location where the mold is manufactured. A mold maker in a high cost of living area (such as Germany) 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, and plant utilization of the mold maker. For example, a mold maker using a 5 axis numerically controlled milling machine will tend to have more capability and charge more than a mold maker using manually operated 3 axis milling machines. Some approximate cost and efficiency


47

data for machining and labor rates are provided in Appendix D, though application specific data 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 be machined, as well as the speed of the machining processes used. In theory, the exact order and timing of the manufacturing processes can be planned to provide a precise time estimate. In practice, however, this approach is fairly difficult unless the entire job can be automatically processed, 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 consider geometric complexity as well as a machining factor, fmachining, then divided by an efficiency factor, fmachining_efficiency: ⎛ t cavity_volume + t cavity_area ⎞ t cavity_machining = ⎜ ⎟ ⋅ f cavity_complexity ⋅ f machining f machining_efficiency ⎝ ⎠

(3.9)

The cavity volume machining time is a function of the volume of material to be removed and the material removal rate. To provide an approximate but conservative estimate, the assumption is made that the removal volume is equal to the entire volume of the core and cavity inserts. This may seem an overly conservative estimate, but in fact much of the volume must 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 tolerances required, as well as the properties of the mold core and cavity insert materials. To simplify the analysis, a geometric complexity factor will later be used to capture the effect of different machining processes and tolerances needed to produce the required cavity details. As such, the volume machining time captures only the time to require the material removal as follows: t cavity_volume =

Vcavity_material Rmaterial_volume

(3.10)

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

1

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

David O. Kazmer

Injection Mold Design Engineering ISBN-10: 3-446-41266-2 ISBN-13: 978-3-446-41266-8 Leseprobe Weitere Informationen oder Bestellungen unter http://www.hanser.de/978-3-446-41266-8 sowie im Buchhandel

4

Mold Layout Design

During the mold layout stage, the mold designer commits to the type of mold and selects the dimensions and materials for the cavity inserts, core inserts, and mold base. Mold bases are only available in discrete sizes, so iteration between the inserts’ sizing and mold base selection is normal. The goal of the mold layout design stage is to develop the physical dimensions of the inserts and mold so as to enable procurement of these materials. Mold material selection is also an important decision, since the material properties largely determine the mold making time 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 been determined. To develop the mold layout, the mold opening direction and the location of the parting plane are first determined. Then, the length, width, and height of the core and cavity inserts are chosen. Afterwards, a mold base is selected and the inserts are placed in as simple and compact a layout as possible. It is important to develop a good mold layout design since later analysis assumes this layout design and these dimensions are quite expensive to change once 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 prevent melt leakage. This seal is maintained through the application of literally tons of force (hence the 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-plane features. The mold designer must first determine the mold opening direction to design the parting 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 that the mold opening direction is normal to the parting plane. In fact, the mold usually opens in a direction normal to the parting plane since the moving platen of the molding machine is guided by tie bars or rails to open in a direction normal to the platen. Accordingly, guide bushings and/or mold interlocks are almost always located on the parting plane to guide the mold opening in a direction normal to the parting plane. It may appear that there is nothing about the mold opening direction to determine since the 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 mold opening direction: 1. First, the mold cavity should be positioned such that it does not exert undue stress on the injection mold. The mold cavity is typically placed with its largest area parallel to the parting plane. This arrangement allows the mold plates, already being held in compression under the clamp tonnage, to resist the force exerted by the plastic on the surfaces of the mold cavity. 2. Second, the mold cavity should be positioned such that the molded part can be ejected from the mold. A typical molded part is shaped like a five-sided open box with the side walls, ribs, bosses, and other features normal to its largest area. If so, then the part ejection requirement again supports the mold opening direction to be normal to the part’s largest projected area. Consider the cup and lid shown in Figure 4.1. A section of the core and cavity inserts used to mold these parts was previously shown in Figure 1.6. There are only two potential mold opening directions relative to the part. One mold opening direction is in the axial direction of the cup, while the second direction is in the radial direction of the cup.

Figure 4.1: Sectioned isometric view of cup assembly

4.1 Parting Plane Design

Figure 4.2: Axial mold opening direction for cup

69

Figure 4.3: Radial mold opening direction for cup

A section of a cavity block with an axial mold opening direction is shown in Figure 4.2. The two bold horizontal lines indicates the location of the parting plane where the two halves of the 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 portion of the cavity insert as shown in Figure 4.3. For this design, four bold lines separate the sides from the top and bottom. Since the metal core is located inside the molded part, there is no way 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 the molded part. Of these two designs, the axial mold opening direction shown in Figure 4.2 is the simplest design and is usually preferred. However, the second design is sometimes used in practice since it allows for a more complex part design as well as more options in locating the parting line. For instance, the second design might be required if a handle were added to the cup, or if it was necessary to move the parting line to a location away from the top lip. This second design 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 two potential mold opening directions. The first opening direction is in the screen’s viewing direction, as indicated by the section view shown in Figure 4.4. In this case, the mold section is split by two horizontal lines into a cavity insert forming the outside surface of the bezel and a core insert that forms the inner surface and ribs of the bezel. When the core and cavity inserts are separated as indicated by the arrows, the molded bezel can be readily removed.

Figure 4.4: Normal mold opening direction for bezel

70


Figure 4.5: Complex mold opening directions for bezel

Alternatively, the cavity block for the PC bezel can be split as indicated with the three vertical lines shown in Figure 4.5. In this case, the former cavity insert is split into two pieces, resulting again in a split cavity mold design. The two halves of the former cavity insert must now be removed in oblique directions in order to remove the molded part; the mold opening direction is inclined in order to allow the mold surfaces to separate from the molded part without excessive surface friction or shearing of features on the molded part. This movement requires several additional mold components to control the moving cavity inserts, which add significantly 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, and the plastic molding meet. Since the core and cavity insert meet at this location, any significant deflection of the cavity insert away from the core insert will result in a gap into which the plastic will flow and form a thin film of plastic known as “flash”. Imperfections in the core and cavity inserts at this location, for instance due to wear or improper handling, will also create gaps into which the plastic will flow. Even with new and well-crafted molds, the location of the 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 some other 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 drawing of Figure 4.6 would result in a witness line and possible flash that might make the molded cup unusable. Alternatively, a better location for the parting line is at the bottom of the rim as indicated in Figure 4.2, corresponding to the parting line shown in the right drawing of Figure 4.6.

Figure 4.6: Two parting line locations for cup


71

Figure 4.7: Parting line location for bezel

For the laptop bezel, the parting line will be located around the bottom edge of the part as shown in Figure 4.7. It is observed that, unlike the cup, the parting line for the bezel is not in a single plane. Rather, the parting line follows the profile of the features on the side walls. This non-planar parting line is required to fit the core insert which hollows out the mold cavity to form the holes required for the various connectors. As will be seen in the next section, this complex 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, so as to separate the core insert from the cavity insert. The preferred parting plane for the cup is shown in Figure 4.8. The cavity insert will form the outer and top surfaces of the part, while the core insert will form the rim and inner surfaces.

Figure 4.8: Parting plane for cup

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Figure 4.9: Parting plane for bezel

For the laptop bezel, the parting line in Figure 4.7 can be radiated outward to form the parting surface shown in Figure 4.9. It can be observed that all of the out of plane features along the parting line now become complex surfaces on the parting plane. These surfaces pose two significant issues during mold operation. First, any misalignment between the sharp features on core and cavity inserts will cause wear between the sliding surfaces if not an outright impact between the leading edge of the core and the mating cavity surface. Second, the clamp tonnage exerted on the core and cavity inserts can cause the surfaces to lock together with extreme force, causing excessive stress and potential mold deformation during mold operation. To avoid excessive stress, interlocking features on the parting plane should be inclined at least five degrees relative to the mold opening direction. The parting surface is now typically created via three dimensional computer aided design (“3D CAD”) using lofted surfaces. Each lofted

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 on the parting plane. The result is a surface with the needed profile at the parting line and the necessary draft down to the parting plane. The lofted surfaces are then knit together with the parting 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 portions of the cavity formed between the core and cavity inserts. A shut-off will need to be defined for each window or opening in the molded part. Conversely, if a part has no windows, like the cup, then no shut-offs are defined. Each shut-off is defined by a parting line, which should be located in a non-visual area where a witness line or slight flashing would not reduce the value 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, there are essentially two possible locations for the shut-off ’s parting line, corresponding to the top and bottom of the shelf that supports the display. Either location (or even any location in between) would likely be acceptable since the entire shelf is hidden from view. If the parting line is placed at the top of the shelf as indicated at the right of Figure 4.11, then a shut-off surface as shown in Figure 4.12 will result.

Figure 4.11: Shut-off surface for bezel

Figure 4.12: Shut-off surface for bezel

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4.2

Cavity and Core Insert Creation

With the definition of the parting plane and all necessary shut-offs, the core insert and the cavity 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 the cavity, • 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. For smaller molded parts, increasing the sizing the core and cavity inserts may have little added cost. However, the cost of larger core and cavity inserts can become excessive with increases in 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 insert should 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 faster and more uniform cooling as will be analyzed in Chapter 9. While cooling line design will be later discussed, the minimum height dimension between the molded part and the top or

Figure 4.13: Insert height allowance

4.2 Cavity and Core Insert Creation

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bottom surface of the insert is typically three times the diameter of the cooling line to avoid excessive stress as analyzed in Chapter 12. The initial height dimensions for the core and cavity inserts are shown in Figure 4.13. Second, the core and cavity insert should have a height that is matched with the height of available cavity and core insert retainer plates (the “A” and “B” plates). These plates are commonly available in ½″ increments in English units, and in 10 mm increments in metric units. As such, the insert heights should be adjusted up such that the faces of the cavity and core inserts are flush or slightly proud with respect to the “A” and “B” plates on the parting plane. It should be noted that the height of the core insert as indicated in Figure 4.13 is not its 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 also include 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 a cooling line is needed around the exterior of the mold cavity, then the inserts should be sized large enough to accommodate such a cooling line. As for the height allowance, length and width allowances of three cooling line diameters per side are typical. Second, the width and length dimensions of the inserts should provide side walls, also known as “cheek”, that are thick enough to withstand the lateral loading of the melt pressure exerted on the side walls of the mold cavity. This requirement will become dominating for deep parts with large side walls. While the structural design will be discussed in detail in Section 12.2.4, a safe guideline is that the thickness of the side wall in the length and width dimension should equal the depth of the mold cavity. Figure 4.14 demonstrates an allowance that should be added to the length and width of the mold cavity to derive the length and width of the core and cavity inserts. It can be observed that for the laptop bezel, the requirement of fitting a cooling line will exceed the structural requirement. For the molded cup, however, the insert length and width dimension are driven by the structural requirement.

Figure 4.14: Insert length and width allowance

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4.2.3

Adjustments

The core and cavity inserts can now be created with the prescribed dimensions. However, it is sometimes desirable to adjust the cavity insert dimensions to provide a more efficient mold design. In general, the length and width dimensions of the inserts are more critical than the height 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 structural designs, which will be supported by later engineering analysis.

Figure 4.15: Core and cavity inserts for cup

Figure 4.16: Core and cavity inserts for bezel

4.3 Mold Base Selection

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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 with respect to ease of manufacturing, since both the core and cavity inserts can be turned on a lathe. While the allowances in the axial and radial dimensions are sufficient to fit cooling lines, the allowance in the radial dimension may not be sufficient to withstand the pressures exerted 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 cup shown previously in Figure 1.4 used square inserts. Rectangular inserts with or without filleted corners are also quite common. The design of the insert should be dictated by the shape of the 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 have been designed quite aggressively. While the bezel is quite shallow and the inserts are structurally adequate, the thickness of the surrounding cheek may not allow for sufficient cooling around the periphery of the mold cavity while also providing space for other mold components.

4.3

Mold Base Selection

After the core and cavity inserts have been initially sized, the mold layout can be further developed and the mold base selected. It is critical to order a mold base with appropriately sized plates and materials, since any mistakes in the mold base selection can consume significant time and expense. To determine the appropriate size, the mold designer must first arrange the mold cavities and provide allowances for the cooling and feed systems. Afterwards, the mold designer should select a standard size from available suppliers and verify suitability with 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 to manufacture, 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 may necessitate placing the mold cavity off center. For multi-cavity molds, there are essentially three fundamental cavity layouts: • • •

cavities are placed along one line cavities are placed in a grid, or cavities are placed around a circle.

Injection Mold Design Engineering

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