Understanding Injection Molding Tolerances

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Understanding Injection Molding Tolerances


While injection molding is a relatively precise process, there may be very slight variations between your design and your finished part. Tolerance is the range of deviation in specifications that still allows your part to function as needed. This is especially important when your project involves multiple components that need to fit together during assembly. If critical tolerances aren’t met, your parts won’t fit or perform correctly. 

In injection molding, there are two different types of tolerances to consider: machining and resin. Our machining tolerance of +/- 0.003 in. (0.076mm) refers to the tolerance built into the mold tool used. The resin tolerance refers to the finished molded part and can be greater than but no less than +/- 0.002 in./in. (0.051mm/mm).  

Let’s get into some key design, material, and process considerations that’ll help ensure key tolerances are met and that your molded parts perform perfectly. 

Designing for Moldability: Part Dimensions

Designing with moldability in mind is important. Our injection molding design for manufacturing (DFM) analysis looks at a few key design advisories intended to improve the manufacturability of your part, should there be any design issues right off the bat. Here are a couple design best practices to ensure accurate dimensions in your finished injection-molded part.

Wall Thickness

The thickness of part walls is one of the biggest contributors ensuring dimensional stability, which is critical to stay within tighter tolerance ranges. During the injection molding process, there is a distinct time in which the resin is unstable as it cools and hardens into a completed part.  

Thick parts tend to sink. Thick areas take longer to cool than the thinner areas. This can lead to divots and imperfections on the part exterior as the molten core shrinks inward and pulls the exterior walls with it. Warp is common in parts that don’t have consistent wall thickness, which means it may not meet the desired tolerance range. The best way to prevent these issues is to have consistent wall thickness throughout your part. 

Proper wall thickness will reduce the risk of cosmetic defects in plastic parts.


Adhering to proper design principles is important when thinking about manufacturing constraints throughout the injection molding process. For example, molding requires draft for milling and ejection from the mold. If a designed part has assigned tolerances based on models that have too little draft, it will require changes to part geometry to be molded effectively. Changes to draft angles affect part dimensions, and that, in turn, changes expected tolerances. As a rule of thumb, including 1 to 2 degrees of draft works well in most situations. 

An undrafted (left) versus drafted (right) cube.


How materials affect moulding tolerances

The choice of resin is critical. Each material has different properties and melts, flows and cools at different rates as it hardens into its final form. Determining the correct material for your application can make the difference between success and failure of your part. 



All materials shrink as they cool in the mould. Arguably one of the most important characteristics of material selection is its shrinkage rate, and it is important to consider how the mould will accommodate it.  

For example, suppose you have designed a mould for a part that will be made from ABS plastic, which has a shrinkage rate of 0.003 in/in. (0.076 mm/mm). You need to design the mould to accommodate the shrinkage of ABS plastic so that the final part will match the tolerances required by the nominal design. If you decide to switch to a polypropylene material with a shrinkage of 0.018 in/in. (0.457 mm/mm), the resulting variance is 0.015″ in/in. (0.381 mm/mm). Polypropylene shrinks faster than ABS, so parts will end up about 0.015″ in/in. (0.381 mm/mm), which can be a big problem. 

Finally, you need to consider part design, material selection and tool design together to ensure that your preferred tolerances are achieved.If you have injection moulded parts for the semiconductor, medical and optical sectors, please send us a drawing enquiry for further evaluation.Please contact sales820@xy-global.com