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AIM Processing Small Plastic Parts Blog

Precision Matters: Mastering Small Part Injection Molding for High-Quality Components

[fa icon="calendar"] May 2, 2024 12:01:00 AM / by Jon Gelston


Maintaining precision and repeatability is crucial to any custom injection molding project, especially when it comes to small parts with potentially complex designs. With small part injection molding, even the tiniest variance in a dimension outside of a tolerance range can ruin a whole run of products.

Essentially, the smaller the molded part and the closer the tolerances, the more careful we have to be to ensure accuracy and precision.

How AIM Ensures Precision for Your Injection Molded Small Parts:

Choosing the Right Materials

As with any custom injection molding project, you have to start with the right materials to achieve the desired quality and precision of the finished product.

During the cooling process and even post-cooling, plastic resins shrink at different rates depending on their morphology and whether they are filled or unfilled. This isn’t necessarily a problem from run-to-run if the same material is used and a sound process is developed, but it can be if you later decide you need to change your material or you want to start offering it in different base materials.

While some materials shrink more than others, they still do so somewhat reliably and predictably. So, the more important decision is determining which plastic resin is right for the application before tool construction takes place. AIM has specialized in small part injection molding for more than 30 years and counting, giving us ample hands-on experience in choosing the best materials for virtually every application.

Building the Tool Steel-Safe and Adjusting

Once the material is determined, we can identify the shrink rate and plan to build the tool slightly larger so that dimensions land in or near the tolerance range. In the final purpose of most parts, there is a limited set of critical-to-function (CTF) dimensions. For those, a prudent way to build the tool is such that the part is generally just outside tolerance for those dimensions so that removing steel will later bring the dimension into size. In an example of a hole, one would make a removable pin that is slightly oversized, producing a slightly oversized hole. Upon molding first samples and observing the real-life shrink of the material in the final part, that pin could be adjusted by machining the steel smaller to produce a smaller hole for instance. This is a practice called “building the tool steel-safe”, since it is comparatively easier to remove steel than it is to weld and recut.

There are other aspects in the tool design review that we pay attention to so that warping & other distortions do not occur.  Some distortions occur from variations in wall thickness, while others occur as a natural reaction as the stresses of injection molding are relieved during cooling. Other aspects such as proper gate location and setting the proper draft angle are also crucial to ensuring precision molded small parts.

Setting up the Right Process

Once we have selected the injection molding material and built the tool, it’s time to set up an optimized process for your unique small parts. That means choosing the optimum press, setting the ideal injection speed and temperature for the material, along with many other precise machine parameters. There are a variety of ways to make a part, but there are disciplined approaches to ensure a consistent outcome from run-to-run that accommodates variation in the variables that are more difficult to control.

Following Through with Quality Control

With a sound discipline for material selection, tool build, and developing a stable & accommodating process, producing a plastic part in a tolerance range is generally … well, straight forward even +/- 0.001”. Whether the dimension is always high in the tolerance or low in the tolerance range is a more fundamental reflection on the tool and how many times the customer, molder, and toolmaker wanted to iterate through adjustments. How much the dimension varies is a function of whether a stable process was developed or whether a non-ideal process was developed to accommodate a shortcoming in the tool or a material change. Really. When a customer contacts us with the question “what tolerance can you hold”, they’re often just hoping to hear “yes”.  What they should want to hear is the answer to “what is your discipline to holding a tolerance”.

At AIM, we scrutinize all finished small parts with a refined and careful eye to ensure the precision of the final product. After all, your end product is only as good as the quality control process behind it. Our robust, hands-on quality control process ensures ongoing accuracy and consistency for injection molded small parts, backed by more than three decades of experience in the industry.

Custom Injection Molding Small Plastic Parts and Miniature Components

AIM Processing specializes in the manufacturing of quality injection molded small parts. Our experts apply specific engineering and design considerations to ensure a reliable production process of miniature components. As an ISO 9001:2015 registered American manufacturer, we offer superior quality and consistency with every molded small part.


Topics: Plastic Injection Molding Defects, Plastic Types, Quality, Texture, Choosing the right plastic injection molder

Jon Gelston

Written by Jon Gelston