Why not share! Embed Size px. Start on. Show related SlideShares at end. WordPress Shortcode. The mold designer should also know the technical contact and project manager at the molder to verify molding preferences, machine specifications, and project coordination.
As indicated near the top right of Fig. These dates are frequently negotiated since they are related to technical feasibility, market success, and also payment terms. In particular, the application lifetime and total production quantity is related to the determination of the type of tooling Class , etc. This production capability can then be compared to the target production quantity to determine the number of molds to be made, or otherwise to guide the mold design with respect to number of cavities and cycle time requirements.
It should be noted that the cycle time and other mold design data in Fig. However, some customers will provide these details as specifications that the mold designer must satisfy. If these items are not specified by the customer, then the mold designer should perform a preliminary analysis and design with cost analyses to provide the customer with a reasonably efficient mold design proposal.
Part cost estimation is crucial in many plastic part design applications. Product designers, mold designers, and molders are all conscious of trade-offs between the cost of the mold, material, and processing. Their documentation near the top center of Fig. A few common end-use part requirements are provided at the left of the center of Fig. Some of these requirements drive the geometry, material selection, and other design details about which the mold design engineer may seem to have little control.
Even so, the mold designer should be aware of these requirements, as they can influence selection of the mold materials, surface finish and treatments, mold design, and performance evaluation of the finished mold. In many segments of the plastics industry, such as medical devices, regulatory agencies have developed extensive standards governing the design, manufacturing, and testing of plastic products.
A detailed discussion of regulatory compliance is beyond the scope of this book. However, the mold designer should be aware of any regulatory compliance issues that may affect the mold engineering.
A few common regulatory agencies and their compliance programs are provided at the center of Fig. However, the mold designer should inquire about any governing regulations that may affect the mold design.
Ideally, the customer should provide a copy of any such regulations and highlight the specific requirements related to the molded product design. The specification of dimensions and tolerances is of critical importance to both the mold designer and injection molder. Dimensions in product designs are typically specified with absolute tolerances.
The most common method is the general tolerance, typically specified in the signature block, which is applied to any dimension without an explicitly specified tolerance.
The height offset of The two decimals of precision do not reflect any additional precision on the tolerance but rather the absolute range of acceptable dimensions for the hole diameter. Finally, the shaft diameter is specified with a unilateral tolerance, meaning that the diameter of the shaft must be between 18 and The product and mold designer would both understand from these tolerance specifications that the shaft is meant to nominally fit into the hole with an interference fit such that there is no diametral clearance between the two.
While product designers will usually consider tolerances in absolute terms as described with respect to Fig. The achievement of very tight tolerances requires careful mold design, process engineering, and consistent material properties. For this reason, product designers are encouraged to specify a single general tolerance governing most dimensions along with only a few tighter tolerances on specific dimensions that are critical to product.
Just because a tolerance is specified does not mean that it is achievable. In fact, it is not uncommon for product designers to over-specify the tolerances on many dimensions . Mold designers should discuss tight tolerance specifications with the product development team, and communicate that such specifications may require prototype molding to characterize the shrinkage behavior, nonuniform profiling of shrinkage rates in different areas of the mold, and mold modifications during mold commissioning.
Product designers will often provide specifications on the aesthetics, including requirements on color, color matching across multiple components, and gloss levels. Also, the mold design engineer should be made aware of critical aesthetic surfaces in which aesthetic defects such as from knit-lines, gate blemish, sink, witness marks, etc. Such involvement often provides for significantly improved plastic part designs that are more functional and efficient. Rather than assume that the product design is finished and unchangeable, the mold designer should check that the part has been specifically designed for injection molding.
Some of the most common guidelines with respect to design for manufacturing are provided at the bottom left of Fig. With regard to design for assembly guidelines [13, 14], the mold designer should inspect the part design s that have been provided and check that the design for assembly is reasonable. There are two goals for this task. First, it may be possible to improve the overall design of the product by consolidating multiple components, facilitating top-down assembly, etc.
Second, the mold designer can reduce the number of late design changes that can cost money and time by verifying that such design considerations have been performed. Some mold designers may be aware of product design issues, yet choose to directly implement molds for the provided part designs.
While such a strategy may result in additional work and profit for the mold designer in the short term, it is a losing long term strategy. However, the mold designer needs to know some specific 2. Sample data for some generic grades of plastic are provided in Appendix A.
The materials used in mold construction are usually specified by the mold designer. The molder will often have substantial input to the mold materials given their prior experience with molding the target plastic resin. Mold materials selection will be discussed in Chapter 4; sample data for some commonly used metals are provided in Appendix B. The mold designer should document the mold material properties listed at the bottom right of Fig.
The design review should consider the fundamentals of plastic part design, as well as other concerns related specifically to mold design. Some of the most basic part design considerations are next discussed. The fundamental issue is that thick and thin wall sections will cool at different rates: thicker sections will take longer to cool than thinner sections.
When ejected, parts with varying wall thickness will exhibit higher temperatures near the thick sections and lower temperatures near the thin sections. These temperature differences and the associated differential shrinkage can result in significant geometric distortion of the part given the high coefficient of thermal expansion for plastics.
Extreme differences in wall thicknesses should generally be avoided if at all possible since internal voids may be formed internal to the part due to excessive shrinkage in the thick sections even with extended packing and cooling times. The worst part design, shown at top left, has the melt gated into a thin section and then flowing to a thick section with a sharp transition in the thickness.
The quality of the molded product would be greatly improved as shown at the top center. The design would be further improved by gradually transitioning the thick section to the thin section. Even so, any product design with significant variations in wall thickness will exhibit extended cooling times and different shrinkage rates in the thick and thin sections. For these reasons, the best design may be to use a thinner wall thickness together with vertical ribs in those areas requiring greater stiffness and strength as shown at the center right in Fig.
The injection molding process is unique compared to other molding process in its ability to economically provide very complex structures. The bottom two part designs in Fig. At the bottom left is a thinner wall section with a matrix of thin, short ribs. The two ribs are spaced at ten times the wall thickness of the part. As such, the addition of ribs can provide significant performance and economic advantages. The volumetric shrinkage in this region can cause internal voids or sink to appear on the side of the part opposite the rib.
In nonaesthetic applications that use highly filled materials with lower shrinkage, the rib thickness can be increased. This amount of draft may be insufficient for deep ribs or for parts molded with heavily filled resins. Conversely, parts with very short ribs such as shown at the bottom left of Fig. Some different boss designs are provided in Fig. Similar to the guidelines for rib design, the wall thickness and draft angle for gussets can be modified in view of their height and the material being molded.
Designed bosses must be able to withstand the torque applied during insertion of the self-threading screws as well as the potential tensile pull-out forces applied during end-use. At the same time, however, bosses should not be designed with overly thick sections that may require extended cycle times or cause aesthetic problems. In the designs of Fig. These design features are vital to the structural integrity of the part, yet are small relative to the size of the entire part.
As such, using less draft on these features can aid in increasing the stiffness and strength of the molding without significantly increasing the ejection forces. Still, ejection of bosses can be an issue in injection molding, so draft and ejector sleeves can be used to assist in ejection of tall bosses that will require large ejection forces.
However, sharp corners in molded products should be avoided for many reasons related to product performance, mold design, and injection molding: 2. Furthermore, a box with sharp corners and tall sides may not have the torsional stiffness of a rounded box with shorter sides.
These fillet recommendations are only guidelines. In fact, even larger fillets can be used to encourage more uniform mold cooling. In all cases, the mold designer should suggest a fillet radius that corresponds to readily available tooling geometry so that custom tools need not be custom-made. Similar to fillets, larger chamfers such as the one shown at right in Fig. Most mold-making companies are capable of providing high-quality surface finishes, though polishing can be outsourced to lower-cost companies and countries due to its high labor content.
Surface texturing requires a higher level of skill and technology, with a relatively small subset of companies providing a significant portion of mold texturing surfaces. Table 2. These finishes range from the D3, which has a sand-blasted appearance, to A1, which has a mirror finish. The cost of molded parts can increase dramatically with higher levels of surface finish. The reason is that the application of a given surface finishing requires the mold maker to successively apply all the lower-level surface finishing methods.
For example, to obtain an SPI C3 finish, the mold would first be treated with coarse and fine bead blasts followed by polishing with a stone. For this reason, higher 2. Furthermore, molds with high levels of finish can produce moldings in which defects are highly visible, thus adding cost to the injection molding process and mold maintenance requirements. One common reason is that textures may be used to impart the appearance of wood, leather, or other materials as shown in Table 2.
As a result, textures may increase the perceived value of the plastic molding by the end-user . Another reason is that textured surfaces provide an uneven depth which may be used to hide defects such as knit-lines, blemishes, or other flaws. In addition, textures may be used to improve the function of the product, for instance, by providing a surface that is easy to grip or hiding scratches during end-use. Texturing does add significantly to the cost of the mold.
Otherwise, the underlying poor surface finish may be visible after the applied texture. After surface finishing, the texture is imbued to the mold surfaces using chemical etching or laser machining processes. Since dedicated processing equipment is required, the mold development process must provide adequate time 2.
Draft is normally applied to facilitate ejection of the moldings from the mold. Even so, draft is commonly applied to plastic moldings to avoid ejection issues and extremely complex mold designs. Draft angles on ribs must be carefully specified. In the previous rib design shown in Fig. In terms of product functionality, a lesser draft angle may be desired since this allows for taller and thicker ribs with greater stiffness.
As such, the allowable draft angle is a complex function of the material behavior, processing conditions, and surface finish. Four typical design features that require undercuts are shown in Fig. Because the snap beam is narrower at its neck than at its tip, there is an undercut in the mold that the mold designer must be aware of and make the design such that the part can be ejected after the mold opens.
Please note that the provided designs are not intended to suggest the use of all three strategies in a single mold, but only provide a basis for discussion. The design at the left is the simplest of the three, in which an opening or window at the base of the snap beam allows a protrusion from the stationary side to core out the area cavity beneath the undercut.
This is a reliable technique, but leaves a hole in the part that alters its function and aesthetics. At design at the right is also very common, which uses an ejector pin with a profiled or contoured surface on its side adjacent the snap beam.
This contoured profile on the ejector pin provides a miniature cavity to allow the molding of the head of the pin. When the mold is opened and the ejectors are extended, the pin and part will move together until the part fully clears the mold cavity and can clear the height of the undercut.
When using contoured ejectors, a dowel pin or some other design feature must be used to maintain proper orientation of the contoured surface with respect to the mold cavity. Otherwise, the cavity surfaces would not align and defective parts would occur. After the mold is opened, the forward movement of the lifter acting on the inclined surface of the mold causes the lifter to move laterally, thereby clearing the undercut upon ejection.
There are three issues associated with lifters that the plastic part and mold designer should consider when adopting their use. First, there is the added design time and complexity used to implement the design.
Second, there is the potential for wear, sticking, and increased maintenance associated with the sliding surfaces on the slider itself, on the mold surfaces, and within the ejector assembly. Third, the use of the lifter requires adequate clearance between features within the molded part. As indicated in Fig.
More challenging than such undercutting features is the horizontal boss of Fig. When possible, these types of product design features should be avoided since complex mold mechanisms must be designed and machined for forming and ejecting the molded part. These additional mold components can make the mold more difficult to use and even damage the mold if used improperly.
For these reasons, the mold design engineer should identify problematic features, alert the customer, and work with the product design engineer to remove the undercuts. However, such undercuts should not be designed out of the product if the function provided by the feature s with the undercut is vital to the product or the removal of the undercut would necessitate additional post-molding operations or the redesign of a single part into multiple pieces .
The results of this analysis will be used to design the layout of the mold. In later chapters, the design and analysis of various mold subsystems are conducted. Eppinger, and D. Former Series 4 4 : pp. Walleigh, Defining next-generation products: an inside look, Harvard Business Review 75 6 : pp.
Aided Des. Rosato, and M. Peikert, and J. 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. To demonstrate the cost estimation method, each of these cost drivers is analyzed for the laptop bezel shown in Figure 3. The example analysis assumes that 1,, 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. This example corresponds to the mold design shown in Figure 1. 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. 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.
First, the dimensions of the core and cavity inserts are estimated. From the dimensions provided in Table 3. Since this is a tight tolerance part with a high production quantity, tool steel D2 is selected for its wear and abrasion resistance. 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. 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. 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.
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 .
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. 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. 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. The primary purpose of the parting plane is to tightly seal the cavity of the mold and prevent melt leakage.
The mold designer must first determine the mold opening direction to design 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.
It may appear that there is nothing about the mold opening direction to determine since the mold opens normal to the parting plane. Best wishes, David Kazmer, P. Contents Preface. V Injection Mold Design Engineering downloaded from www. XV 1 Introduction. Heat Transfer. Contents XIII Mold engineering requires analysis, and so an extensive nomenclature has been developed. L, W, and H refer to the length, width, and height dimensions as shown in Figure 1. Peter Burkholder. Wynn DVM.
Kazmer Ph. He performs research and teaches courses related to plastics product and process development. He is a licensed professional manufacturing engineer, and is a fellow of the American Society of Mechanical Engineers and the Society of Plastics Engineers.