Mirror, Mirror on the Wall...

They say that beauty is in the eye of the beholder. But when the beholder is your customer, you naturally want to deliver a product that is as cosmetically pleasing as possible. And what could produce a more cosmetically perfect product than injection molding, in which each part is perfectly formed in a meticulously machined mold! That, at least, is the theory.

In reality, achieving the desired cosmetic appearance of a molded part requires careful attention to five separate factors: gating, ejection, mold polish, resin choice, and part geometry. First, until someone devises a way to teleport resin into a mold, it will still have to be injected through a gate, and gates inevitably leave a blemish on the finished part. This is called a vestige.

At Protomold, we use four types of gates for resin injection. The most common is an edge gate, which leaves a rectangular vestige at the parting line.

Figure 1: Edge gate

A post gate allows resin to be injected through an ejector-pin hole. When the part is ejected, a small “post” of plastic is left on the part where the ejector pin is located.

Figure 2: Post gate

Usually, this post can be easily trimmed off, and since ejector pins are rarely on the cosmetic side of a part, the resulting vestige may not be of concern.

A tunnel gate injects resin below the surface of the mold. As the part is ejected, the gate is sheared off, leaving a small elliptical vestige that is typically less noticeable than the vestige from an edge gate.

Figure 3: Tunnel gate

Tunnel gates are generally not suitable for use with filled resins as the filler may cause the gates to wear.

Finally, there is the hot tip gate, which allows resin to be injected directly into the mold through a heated nozzle tip. It leaves a small bump and an area of gate blush (discoloration) at the point of injection. If this area is on the cosmetic side of the part, it could be a problem.

Figure 4: Hot tip gate

The second area of consideration is mold polish. The selected mold polish should be appropriate to the application, and the part design should include draft to support the chosen polish. For example a part with vertical sides and an A2 finish needs to be drafted. If the sides are not appropriately drafted, the surface will be marred as the part is dragged along the mold surface during ejection. Similarly, textured surfaces must be drafted so the part can release without dragging. For more information on surface finish, go to http://www.protomold.com/designguidelines/surfacefinish/

The third consideration is the marks left by ejector pins. These push the cooled part out of the mold, leaving flat circular marks on the part. The good news is that, on most parts, the pins can be placed on non-cosmetic surfaces.

For more information on part ejection, go to http://www.protomold.com/designguidelines/partejection/

Figure 5: Consider marks left by ejector pins

The fourth consideration is resin choice. Glass filled resins may be a poor choice for highly cosmetic parts as the glass filler can show at the surface. Resins with added colorants can show flow lines and weld lines.

Figure 6: Consider resin choice

Finally, part geometry can affect cosmetics. Sink,warp, flow lines, and weld lines can result from flaws in design. Careful attention to design is your best defense. For more information on design considerations, go to http://www.protomold.com/designguidelines/.

Figure 7: Consider part geometry

When you submit a model to Protomold for quote or production, you need not identify gate or ejector locations. Based on your design and resin choice, we will return a gate/ejector layout for your approval. In reviewing that layout, however, you should keep in mind where, and to what degree, you can tolerate cosmetic blemishing. When submitting your model, you can, if you wish, provide that information in the field called “additional information” on the part upload page. We will work with you to achieve both the functional and cosmetic results you want.

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Design Guidelines

As always, you can visit the Protomold Design Guide for helpful Rapid Injection Molding design information.

Don't try this at home!

In filming the 1974 movie Phantom of the Paradise, actor William Finley nearly had his head crushed by an injection molding press borrowed from the Ideal Toy Company. The film crew had carefully rigged the machine with foam and chocks to merely appear to disfigure Finley's character. But the machine proved too powerful for the protective devices and the actor barely escaped serious harm. His scream was scripted, but the one heard in the film was apparently quite real.

The response to last month's question: Over 200 people responded to our question “Polypropylene (PP) can be recycled into which of the following?”

• Car battery cases
• Egg cartons
• Fleece clothing
• Garden hose
• All of the above

Their answers are represented in the following chart:

Correct answer:
The correct answer, according to the American Plastics Coucil, is A. Car battery cases.

Please email suggested topics for future Design Tips, questions for future Designer Surveys, and obscure bits of Plastics Trivia to marketing@protomold.com.