Today’s plastic injection molders are pushing the envelope — or the mold, if you will — to produce parts which are lighter and more complex than ever before. And while that effort is laudable, pushing the limits can a be a risky proposition from a design and production stand-point.
Why? Because the complex plastic parts being produced from these molds — many of which are customer-facing “appearance” parts — require molds which don’t “stress them out”; i.e. cause cracking, warpage, premature failure, sink marks and other problems.
How to avoid this kind of stress? Here are two of the most important factors to consider:
No Sharp Turns
Many appearance parts don’t follow straight lines; they curve, make right angles and are frequently asymmetrical. That’s great for them, but would be disastrous in a mold, where sharp turns and other angles can cause plastic resin molecules to distort as they attempt to make these turns. When the resin cools, these imperfections are locked into place and can cause all the problems detailed above.
At AdvanTech Plastics, a plastic injection molding company based in Woodstock, Illinois, we design molds which incorporate smooth transitions, utilizing rounds and fillets whenever possible, especially in transition areas where plastic resin has to flow from one feature to another. We’ve had years of experience designing these kinds of “low stress” molds, which result in plastic parts which are as durable as they are beautiful.
Good Gates
There are two types of gates for plastic injection molded parts — manually-trimmed and automatically-trimmed. The four most popular types are:
- Edge Gate
As their name implies, an edge gate is located on the edge of the part and is typically used for flat parts. Edge gates leave a scar at the parting line - Sub Gate
The Sub Gate is automatically trimmed, requires ejector pins and leaves a pin-sized scar on the part - Hot Tip Gate
These kinds of gates are often located at the top of the part and are utilized for round or conical shapes where uniform flow is mandatory. Hot Tip Gates leave a small raised nub on the part surface - Sprue Gate
Perhaps the easiest to design, the sprue gate has low cost and maintenance requirements but leaves a large scar on the part at the point of contact
There are many more factors to consider when designing a plastic part and its mold but that is the subject of future posts.