Controlling Tight Tolerances for Injection Molded Parts

Posted by Kaysun Corporation on Aug 4, 2015 4:35:00 PM

blank.pngGet Control of Tight Tolerances in the Design Phase and Come out Ahead

Here are three things design engineers fully understand:

  1. Design has almost everything to do with the success of product or part performance.
  2. The importance of design is magnified considerably with tight tolerances.
  3. If something goes wrong, check to see if the design is the root cause.

No pressure right? Wrong. There is a lot riding on the designer when developing complex plastic parts and products with tight tolerances, especially when they’re used in critical applications like medicine, auto manufacturing, and military among many others. Here’s why getting control of it in the design phase is a good thing, along with advice that should help take some of the pressure off.

Improvements in design matter

Whether it’s a typical tight tolerance for injection molding in the neighborhood of +/- .002 inches, or a very tight tolerance of +/- .001 inches, there are numerous benefits to making improvements in the design phase.

For starters, controlling them almost always boils down to quality. In other words, customers like it when their part or product performs as expected. When the part or product doesn’t perform, it leads to headaches and financial pain in the form of a tooling and/or process overhaul.

Another benefit has to do with cost savings, which everyone likes. Designed correctly, a part or product with tight tolerances can eliminate secondary operations like machining. It also means it’s easier to procure mating parts, and it opens the door for potential to transition from metal parts to plastic parts. Add to that the ability to ensure repeatability and manufacturability.

It starts with design

In the design phase, design engineers must factor in requirements for part geometry, overall size, and wall thickness since all have an influence on tolerance control.

Thick walls are an excellent example. They may have differential shrink rates within the thick sections, making it difficult to hold tight tolerances since the variable shrink can “move“ within the section.

Part size is another example. Simply put, the larger the dimension the harder to hold tight tolerances. A larger dimension also equates to larger shrinkage, which makes it more challenging to maintain and control it.

Attention to complexity

Another major factor in the design phase is the complexity of the part or product. Done right, it aids in control of tight tolerances.

A main issue is shrinking and warping. If the complexity of the part or product has too much shrinkage or warpage, it’s not repeatable. That's why it’s absolutely essential for the product design and manufacturing teams to be on the same page.

Also driving the success of managing tight tolerances is the ability to fill cavities quickly, maintain proper cooling temperature, and manage the overall cooling process – all of which revolves around tooling design and material flow. Critical to the process is a moldflow analysis, which is commonly done as part of a design for manufacturability approach, since it can accurately predict mold heating and cooling, as well as shrinkage, and warpage. Armed with this information, designers can do what’s needed for optimal control.

Environment impacts tolerance

What also helps is having a clear understanding of the environment where the part or product is used. And the reason is clear: it influences the behavior of plastic, which in turn, affects tolerance.

To further illustrate, consider that plastics typically have large thermal expansion coefficients. That means parts may have to be measured at a consistent temperature to ensure accuracy in determining the part’s ability to maintain a tight tolerance.

It’s also important to consider temperature in the design phase. For example, if the part or product will be exposed to temperature extremes during normal operation it will expand and contract. Knowing this might even mean the need to explore an alternative to a tight-tolerance part or product.

Tip of the iceberg

There’s clearly much more to the success of tight tolerance parts or products than the design alone. But it’s definitely the place to start since design does in fact play a major role in the overall success of a job.

All that aside, it’s always a good idea to get up to speed on other factors involved in working with tight tolerances, which range from material selection, to tooling, to process design and control. But it’s a lot to digest with one blog. We’ll cover those topics another day.

Stay tuned – or download our tight tolerances white paper now to learn more.

Topics: Overmolding