Vidrios

Surface finish and texture are commonly specified by the part designer, yet have a significant impact on the mold design and cost. Most mold making companies are capable of providing

Figure 2.5: Comparison of fillets

Figure 2.6: Comparison of chamfers

2.3 Design for Injection Molding

32 2 Plastic Part Design

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 the mold texturing.

Surface finishes are commonly evaluated according to standards of the Society of the Plastics Industry (http://www.plasticsindustry. org/). These finishes range from the D-3, which has a sand-blasted appearance, to A-1, which has a mirror finish. Table 2.12 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 to effective apply a given surface finishing method, the mold maker must successively apply all lower level finishing methods. For example, to obtain an SPI C-3 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 2.13. As a result, textures may increase the perceived value of the plastic molding by the end-user [9]. 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 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 and money for the mold texturing.

Table 2.12: SPI surface finishes and roughness

SPI Finish Finishing method Microfinish (μm) Surface roughness (μm)

A-1 #3 diamond polish ~1 ~0.01

A-3 #15 diamond polish ~2 ~0.04

B-3 #320 grit cloth ~6 ~0.12

C-3 #320 stone ~12 ~0.3

D-2 #240 oxide blast ~30 ~0.8

D-3 #24 oxide blast ~160 ~4

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2.3.6 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 significant 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 specified. In the previous rib design shown in Figure 2.3, 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 taller and thicker ribs with greater stiffness. 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

Table 2.13: Texture examples

Texture Image Texture depth SPI finish required

Sand 50 μm B

Leather 125 μm C

Netting 150 μm C

Wood grain 250 μm D

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34 2 Plastic Part Design

ejection can be compounded when molding with mica and/or glass filled 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.

A minimum draft angle of 0.5° is usually necessary, 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 2.14 provides some recommended draft angles for a few different surface finishes and materials; the recommended draft angle increases with the surface roughness. With respect to the material properties, the draft angle should increase for glass filled and/or low shrinkage materials but may be decreased for highly flexible materials such as soft PVC.

2.3.7 Undercuts

An undercut is a feature in the product design that that interferes with the ejection of the molding from the mold. Four typical design features that require undercuts are shown in Figure 2.7. 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 finger. Much of the time, the product designer is unaware of the difficulties associated with these undercuts.

Table 2.14: Draft examples

Surface finish Resin Roughness (μm) Draft

Class A-1 Acrylic 0.01 0.5°

Class B-3 ABS 12 1.5°

Sand texture 20% GF PC 12 2°

Leather texture Soft PVC 125 4°

Leather texture ABS 125 7.5°

Figure 2.7: Some common features with undercuts

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When possible, undercuts should be avoided since complex mold mechanisms must be designed and machined for the forming and ejection of the 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 undercuts, alert the customer, and work with the product design engineer to remove the undercuts. However, undercuts should not be designed out of the product if the function provided by the feature 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.

2.4 Chapter Review

After reading this chapter, you should understand:

The basic stages of the molded part development process including the role of management reviews,

What information is needed to begin the mold design process, The common specifications on a molded product,

Where to find additional information relevant to product and mold design, and Basic part design guidelines for injection molding.

In the next chapter, the mold cost and the part cost will be analyzed with respect to critical mold design decisions. 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.

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2.4 Chapter Review