Basics of Injection Molding Design

A detailed review of the plastic part design should be conducted prior to the design and manufacture of the injection mold. The design review should consider the fundamentals of plastic part design, as well as other concerns related specifi cally to mold design. Some of the most basic part design considerations are next discussed.

Uniform Wall Thickness

Parts of varying wall thickness should be avoided due to reasons related to both cost and quality. 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.

Image below provides several part designs with different thicknesses. The worst part design, shown at top left , has the melt gated into a thin section and then fl owing to a thick section with a sharp transition in the thickness. This design may lead to moldings with poor surface finish due to nonuniform fl ow of the melt as well as poor surface replication and dimensional control in the thick section related to premature solidification of the plastic molded in the thin section. The quality of the molded product would be greatly improved as shown at the top center. Just by gating into the thicker section, the molded product would have much better aes-thetics and dimensional stability since the thicker section would allow the packing of the thinner section prior to its own solidification. 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.

Worst thin to thick
Worst: thin to thick
Very bad thick to thin
Very bad: thick to thin
bad thick to thin
Bad: thick to thin
Standard uniform thickness
Standard: uniform thickness
Best thin with ribs
Best: thin with ribs
Option one-sided grid
Option: one-sided grid

A standard approach is to increase the nominal thickness of the molded part so as to eliminate the need for thick sections in local areas as shown at the center left in Fig. The decision to increase the wall thickness will eliminate many issues related to part quality, but can lead to excessive material consumption and extended cooling times. For these reasons, the best design may be to use a thinner wall thickness together with vertical ribs in those areas requiring greater stiff ness and strength as shown at the center right in Fig. The height and/or density of the ribs may be altered to change the relative stiff ness throughout the part .

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 show alternative strategies that are increasingly common. At the bottom left is a thinner wall section with a matrix of thin, short ribs. At the bottom right is the same thicker wall section that has been dimpled on both sides to reduce the eff ective wall thickness. Both strategies are useful reducing the wall thickness while still increasing the amount of material away from the part’s neutral axis in bending, thereby contributing to a signifi cant increase in stiff ness without an increase in the material consumption. Furthermore, both strategies provide a signifi cant increase in surface area, which will result in improved mold cooling and molding productivity.

Rib Design

Consider the ribbed part design shown at the center of Fig relative to a part having a greater uniform thickness shown at left . In this ribbed part design, the base thickness of the rib is 70% of the wall thickness of the part, H, and the height of the rib is four times the wall thickness of the part. The two ribs are spaced at ten times the wall thickness of the part. Analysis of this design indicates that this design has a stiff ness equivalent to the part that is 30 % thicker but does not have ribs. However, the 30 % thicker part will consume approximately 15 % more mate-rial and have a 70 % longer cycle time than the thinner part with ribs. As such, the addition of ribs can provide signifi cant performance and economic advantages.

Uniform Thickness Part
Uniform Thickness Part
Thinner Ribbed Part
Thinner Ribbed Part
Rib Detail
Rib Detail

Product designers will tend to maximize the stiff ness of the ribs by making the thickness of the rib equal to the nominal wall thickness of the part, H, while also minimizing the draft angle. Ribs thicker than 70 % of the wall thickness will tend to draw material away from the center of the opposite wall when the rib cools. 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 ncreased. Other-wise, a rib thickness less than 70 % of the nominal thickness should be used in molding applications with unfi lled materials . Similarly, draft of 1° per side is oft en used to facilitate ejection of the molded ribs. This amount of draft may be in -suffi cient for deep ribs or for parts molded with heavily fi lled resins. Conversely, parts with very short ribs (such as shown at the bottom left of Fig) can often be made with zero draft.

Boss Design

Bosses are typically used to secure multiple components together with the use of self-threading screws. Some diff erent boss designs are provided in Fig. 2.6. The left -most design provides a boss near a corner with two ribs and a gusset placed at 120°. The center design shows a boss on a rib with two gussets at 90°. The right-most design shows a free-standing boss with gusseted ribs that provide for an elevated assembly surface. All these boss designs utilize a boss, rib, and gusset thickness of 70 % times the nominal wall thickness. Similar to the guidelines for rib design, the wall thickness and draft angle for gussets can be modifi ed 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. 2.6, no draft was utilized on the bosses and gussets. 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 stiff ness and strength of the molding without signifi cantly 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.

Effective Boss Design1

Corner Design

Sharp corners are oft en specified in product design to maximize the interior volume of a component, to facilitate mating between components, or to improve the aesthetics. However, sharp corners in molded products should be avoided for many reasons related to product performance, mold design, and injection molding:

  • Relative to product performance, sharp corners will result in a stress concentration that may cause many (and especially brittle) materials to fail under load. Furthermore, a box with sharp corners and tall sides may not have the torsional stiffness of a rounded box with shorter sides.
  • Relative to mold making, sharp corners can be very difficult to produce, requiring the use of electrical discharge machining or the use of multiple cutting passes with tools of decreasing size.
  • Some common guidelines for filleting and chamfering corners are provided in Fig. 2.7. As shown, the fillet radius on an external corner should be 150 % of the wall thickness. To maintain the same thickness around the corner, the fillet on the internal corner is set to 50 % of the wall thickness. In most modern sol-ids-based CAD systems, these fillets can be readily achieved by filleting the outside edges prior to shelling of the part. 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.
No fillet
Proper fillet
Improper fillet

Chamfers are oft en used to break sharp corners with a single beveled surface connecting the outer surfaces, oft en at a 45-degree angle. As shown in Fig. 2.8, a shallow chamfer of less than one-half the wall thickness is oft en utilized on exter-nal corners to provide for adequate relief while avoiding potential negative issues related to melt fl ow and part strength. Similar to fi llets, larger chamfers (such as the one shown at right in Fig. 2.8) may be applied prior to shelling to provide im-proved part stiff ness and heat transfer near the corners.

Chamfer designs

Surface Finish and Textures

Surface fi nish and texture are commonly specifi ed by the part designer, yet have a signifi cant impact on the mold design and cost. Most mold-making companies are capable of providing high-quality surface fi nishes, though polishing can be out-sourced to lower-cost companies and countries due to its high labor content. Sur-face texturing requires a higher level of skill and technology, with a relatively small subset of companies providing a signifi cant portion of mold texturing sur-faces.

SPI Surface Finishes and Roughness

SPI FinishFinishing methodMicrofinish (µm)Surface roughness (µm)
A1#3 diamond polish~1~0.01
A3#15 diamond polish~2~0.04
B3#320 grit cloth~6~0.12
C3#320 stone~12~0.3
D2#240 oxide blast~30~0.8
D3#24 oxide blast~160~4

Surface finishes are commonly evaluated according to standards of the Society of the Plastics Industry (http://www.plasticsindustry.org). These finishes range from the D3, which has a sand-blasted appearance, to A1, which has a mirror finish. Table 2.1 provides some common SPI finishes, the finishing method, and the measurable surface roughness.
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 #320 stone. For this reason, higher levels of surface finish cost significantly more than lower levels. 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.

As an alternative to smooth surface finishes, many product designs specify a textured finish. One common reason is that textures may be used to impart the appearance of wood, leather, or other materials as shown in Table 2.2. 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. To apply a texture, mold surfaces must first be finished typically to SPI class B for shallow textures (in which the texture depth is on the order of a few microns) or class C for rough textures. Otherwise, the underlying poor surface finish may be visible aft er the applied texture. Aft er 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 and money for the mold texturing.

SPI Surface Finishes and Roughness

TextureImageTexture depthSPI fi nish required
Sand50 µmB
Leather125 µmC
Netting150 µmC
Wood grain250 µmD

Draft

Draft refers to the angle of incline placed between the vertical surfaces of the plastic moldings and the mold opening direction. Draft is normally applied to facilitate ejection of the moldings from the mold. Product designers frequently avoid the application of signifi cant draft , since it alters the aesthetic form of the design and reduces the molding’s internal volume. 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 specifi ed. In the previous rib design shown in Fig. 2.5, for instance, a 2° draft angle was applied to facilitate the ejection of the molded part from the mold. In terms of product functionality, a lesser draft angle may be desired since this allows for taller and thicker ribs with greater stiff ness.
Unfortunately, lower draft angles (such as ½ or 1°) may cause the part to excessively stick in the mold. This issue of sticking upon part ejection can be compounded when molding with mica- and/or glass-fi lled materials that have low shrinkage and high surface roughness. As such, the allowable draft angle is a complex function of the material behavior, processing conditions, and surface finish.

Draft Examples

Surface finishResinRoughness (µm)Draft
Class A1Acrylic0.010.5°
Class B3ABS121.5°
Sand texture20% GF PC12
Leather textureSoft PVC125
Leather textureABS1257.5°

A minimum draft angle of 0.5° is normally used, with 1 to 2° commonly applied according to material supplier recommendations. Rough and textured surfaces typically require additional draft , with an additional 1° of draft commonly applied per 20 μm of surface roughness or texture depth. Table 2.3 provides some recommended draft angles for a few diff erent surface fi nishes and materials; the recom -mended draft angle increases with the surface roughness. With respect to the material properties, the draft angle should increase for glass-fi lled and/or low shrinkage materials but may be decreased for highly fl exible materials such as soft PVC.

Undercuts

An undercut is a feature in the product design that interferes with the ejection of the molding from the mold. Four typical design features that require undercuts are shown in Fig. 2.9. These design features include, for example, a window in a side wall, an overhang above the bottom wall of the part, a horizontal boss, and a snap beam or “finger.”

common features with undercuts

To provide some insight into why these features are commonly avoided, consider three diff erent but common mold design strategies for molding a snap beam as shown in Fig. 2.10 with the mold closed prior to ejection and Fig.2.11 with the moving side retracted and the ejectors extended forward. 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 aft er the mold opens. Please note that the provided designs are not in -tended 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.

The design at the center is also a very common design in which a sliding, angled pin or “lift er” is used to core out the volume of the mold trapped beneath the undercut. Aft er the mold is opened, the forward movement of the lift er acting on the inclined surface of the mold causes the lift er to move laterally, thereby clearing the undercut upon ejection. There are three issues associated with lift ers 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. Sec-ond, 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 lift er requires adequate clearance between features within the molded part. As indicated in Fig. 2.11, this particular design likely does not provide suffi cient clearance, such that the lift er will interfere with the rightmost snap beam if the lift er is further extended.

Three mold designs for producing a snap beam, with mold closed
Three mold designs for producing a snap beam, with mold closed
Three mold designs for producing a snap beam, with mold opened
Three mold designs for producing a snap beam, with mold opened

It may seem that such complexities in mold design would dictate the avoidance of snap beams, but in practice, such designs are not usually problematic for experienced mold designers. More challenging than such undercutting features is the horizontal boss of Fig. 2.9 that is oriented internal to the part and designed with a molded internal thread. Together, these design features would require a complicated mold design with timed actuation of various cores prior to ejection of the product. Alternatively, a “lost core” could be used in which the mold core that forms the internal features of the part is melted away aft er the part is molded as described in Section 13.4.3.

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.

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