November 2005 Design Tip

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 moulding, in which each part is perfectly formed in a meticulously machined mould! That, at least, is the theory.

In reality, achieving the desired cosmetic appearance of a moulded part requires careful attention to five separate factors: gating, ejection, mould polish, resin choice, and part geometry. First, until someone devises a way to teleport resin into a mould, 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 mould. As the part is ejected, the gate is sheared off, leaving a small round 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 due to wear they can cause to the mould.

Finally, there is the hot tip gate, which allows resin to be injected directly into the mould through a heated nozzle tip. It leaves a small bump and an area of gate blush (discolouration) 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 mould polish. The selected mould 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 mould surface during ejection. Similarly, textured surfaces must be drafted so the part can release without dragging. For more information on surface finish, go to /designguidelines/surfacefinish/

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

For more information on part ejection, go to /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 colourants 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 /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.