Form Loves Function

Continuing the discussion of geometric variation on mechanical components, it’s time to talk about plastic. Before getting into details of injection molded plastic part tolerances, you might want to have a look at these two posts that provide a foundation for the following discussion:
Injection Molded Plastic Parts Checklist
EDM Tolerances

Let’s have a look at the major factors that affect plastic part tolerances.

• Part Design
  As a general rule the achievable tolerance on any given dimension is a function of feature size; roughly ±0.2% to ±0.5% depending on some other factors we will discuss below. It’s easier to hold tighter absolute tolerances on smaller parts and features than it is on larger parts.
• Material
  All engineering thermoplastics shrink as they cool, providing a challenge for the tool designer who must create a mold at the size the part will be at the time of molding. The adjustment can be made via non-uniform scaling of design geometry based on the material specified. Since each material has a unique shrink rate it can be difficult or impossible to run a different material without significant tool modifications.
• Tool Design and Construction
  Injection mold tools can be incredibly complex and sensitive and designing them is an art. The design must compensate for the material shrink as mentioned above, and that’s just getting started.  Tools can require mechanisms to provide transverse motion, water lines and heaters for thermal control, and geometry to accommodate the molding press. Tools with multiple cavities for the same part are quite common. Depending on how each cavity is cut and the tolerances required there can be significant variation between cavities.
  A complete description of tool design concerns is far beyond the scope of this article. Understanding the complexity and importance of the tool design and construction is the point. So give your tooling engineers a cookie next time you see them.
• Processing
  A mold press can be quite complicated with many moving parts, sensors, heaters, fluid channels, and control systems. The process of getting all these parameters dialed in to produce a good part can be tricky. Once dialed, any deviation may cause a change in part geometry after the part cools. Like tool design, a complete description of processing is far beyond the scope of this article and simply understanding the complexity and importance of processing is the point.
• Equipment
  Just as variations in processing result in part geometry variation, variations in equipment will also result in more variation in geometry. Expect wider tolerances on parts coming off of older, more worn equipment. The same goes for older tools; variation increases with tool-wear.
• Cost
  Tight processes typically require longer cycle times which results in less capacity (parts-per-hour per tool) so you can expect to pay more. On the other hand, if you run the process fast you get more capacity but with higher geometric variation. Finding the sweet spot between cost and tolerance range takes time and effort. So give your tooling engineers another cookie next time you see them.
  You now know that part geometry drifts as tools wear out. Newer tools produce more consistent parts but new tools are expensive and the expense of tooling usually gets rolled into part cost.

All of these aspects will play a part in the overall geometric variation. You won’t know the actual limits of your part size until tools have been cut and T0 parts have been run and inspected. So where do you begin when you’re in the early design phase? The Form Loves Function Plastic Part Design Checklist is a good place to start.

There are several good resources available:

• Protomold Design Guidelines
• IDES Document Search
• Engineer’s Edge
• BASF Design Guides
• DSM Engineering Design Guidelines (PDF Download)
• Bayer Material Science
• Ticona
• Plastics1 Plastic Part Design Guide

{image: Roberto Bouza}

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