How to Achieve Tight Tolerances in Injection Molding

When it comes to designing and launching a new product or component, there are three things design engineers can always count on:

a. The design is the driving factor behind the success of the product or part and its performance
b. If something goes wrong, the design should be the first place you look
c. The importance of design is magnified considerably with tight tolerances



No pressure, right? As you can see, there’s a lot riding on designers to develop complex plastic parts and products with tight tolerances, especially when they’re used in medical, automotive, or defense applications. There’s no room for error when designing these critical-use parts, especially when the margins are as slim as +/- .001 inches.

In order to ensure those tolerances are met, it’s crucial that you get control of them early on in the design phase, and to help you do that – as well as take some pressure off designers – here are some tips for achieving tight tolerances in injection molding.



It Starts with Design

Identifying tight tolerances early in the design phase is key, because design engineers must factor in requirements for part geometry, overall size, and wall thickness – all of which have an influence on tolerance control.

If your design has thick walls, for 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. Likewise, when it comes to part size, the larger the dimension; the harder it is to hold tight tolerances. A larger dimension also equates to larger shrinkage, which makes it more challenging to maintain and control it.


Pay Attention to Complexity

Another major factor in the design phase is the complexity of the part or product. When done right, that complex design can help aid in the control of tight tolerances. However, common issues with complex designs include shrinking and warping. If the part has too much shrinkage or warpage, the molding process may not be 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 – which all revolve around tooling design and material flow. Moldflow analysis is critical here, as it can accurately predict mold heating and cooling, as well as shrinkage and warpage. By taking this Design for Manufacturability (DfM) approach, designers can do what’s needed for optimal control.


Environment Impacts Tolerance

When designing your part or product, it’s critical that your designers have a clear understanding of the environment where the part or product will be used. Why is this so important? Because the environment influences the behavior of plastic, which in turn, affects tolerance. To better 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.

For example, if the part or product will be exposed to temperature extremes during normal operation, it will expand and contract. Knowing this beforehand might mean exploring alternative options to a tight-tolerance part or product, and save designers a lot of headaches. This is why it’s so important to consider temperature in the design phase.



Only the Tip of the Iceberg

While the tips above provide a good start, there’s much more to the success of tight tolerance parts or products than the design alone. While it’s the obvious place to start since design plays a major role in the overall success of a project, it’s always a good idea to get up to speed on other factors involved in working with tight tolerances, ranging from material selection to tooling, and even process design and control.