Time to read: 4 min

Injection molding tolerances are acceptable variations in size for part features and overall dimensions. They are expressed as plus or minus (±) values in millimeters or inches and vary by the type of material that’s used and the overall size of the part. Designers add tolerances to CAD files to tell an injection molder the amount of variation that’s allowed. In turn, injection molders use these tolerances during design for manufacturability (DFM) reviews and to make tooling and processing decisions.

Parts with tighter tolerances are more expensive because they have stricter tooling and processing requirements. Mold quality is important, but so are cavity fill rates and consistent cooling temperatures. Even with the best injection molding machines, process repeatability varies. The shrink rates for different plastics can be calculated, but a designer’s decisions also affect shrinkage and, ultimately, which tolerances are achievable. If a plastic has a higher shrink rate, tolerances are harder to control.

injection molding tolerances

The right injection molding partner performs careful DFM reviews, provides quality tooling, and expertly controls processing. When tighter part tolerances are required, such as with some medical injection molding projects, a product design could require modifications. Otherwise, parts may not work properly or fit together in assemblies. Conversely, designers may need to balance tolerances against costs. Fictiv can help you to optimize injection molding tolerances as you consider these and other questions.

  1. Which injection molding tolerances must be specified?
  2. What are standard injection molding tolerances for parts?
  3. How does overall size and wall thickness affect part tolerance?
  4. Why is material selection so important for calculating shrinkage?
  5. How does tooling affect plastic injection molding tolerances?
  6. What’s the relationship between T1 tests and part tolerances?7.      
  7. Where can you get help with plastic injection molding tolerances?
tolerance of injection molded parts

#1 Which injection molding tolerances do you need to specify?

There isn’t just one type of tolerance. Designers of injection molded parts may need to consider some or all of the following tolerance types, which are presented in injection molding tolerance guidelines.

  • Dimensional
  • Straightness/flatness
  • Hole diameter
  • Blind hole depth
  • Concentricity/ovality

Each tolerance chart or table contains less expensive commercial tolerances and more expensive precision tolerances for various materials in ranges of sizes. In addition to dimensional tolerances, a designer must specify tolerances for critical features, such as for holes that must accommodate fasteners. Depending on the part, such as the base for a medical device, a designer may need to specify a general tolerance and a flatness callout instead.

#2 What are some standard injection molding tolerances for parts?

For non-critical applications, such as for some consumer products, the tolerance for injection molded parts is typically ±0.1 mm. For applications that require tighter tolerances, such as injection molded medical parts, ±0.025 mm is typical. With assemblies of injection molded components, designers must also consider how all of the parts fit together. This tolerance analysis, or tolerance stack up, needs to account for any interference, such as from a fastener head, and the required amount of clearance.

part tolerance

#3 How does overall size and wall thickness affect part tolerance? 

Because larger injection molded parts experience more shrinkage, their dimensions are harder to control. Therefore, it’s more difficult to hold tighter tolerances both in terms of overall size and part features. For injection molded components of any overall size, maintaining a uniform wall thickness helps to prevent uneven shrinkage. When wall sizes are non-uniform, the walls cool and shrink at different rates, which increases warping.

#4 Why is material selection so important for calculating shrinkage?

Injection molding supports the use of many different plastic materials, some of which are strengthened or modified with additives such as glass fillers and UV stabilizers. Designers like this broad material selection, but different plastics have different shrinkage rates, and these rates differ not only with the size of the overall part, but also with the inclusion of additives. Injection molds are oversized to account for predicted part shrinkage, but there’s a lot to consider when it comes to tooling and tolerances.

plastic injection molding tolerances

#5 How does tooling affect plastic injection molding tolerances? 

Injection molds need to provide consistent, repeatable heating and cooling between shots so that tolerances are maintained. Avoiding mistakes such as the improper placement of cooling channels can improve part consistency, but more cooling support is needed for multi-cavity molds. When the cavities of a mold are made from steel, such as with the tooling that Fictiv offers, mold dimensions are fixed and injection molders have a stable baseline for plastic shrinkage calculations.

Out-of-tolerance molds result in out-of-tolerance parts. That’s why during the CNC machining of tools, mold makers must control dimensions carefully. Typically, injection molds are machined to tolerances of ±0.127 mm. However, tight tolerances may require molds that are machined to  ±0.0508 mm, while very tight molds are machined to ± 0.0254 mm. Precision machining adds costs, so it’s important to consider not only which tolerances are achievable, but also which ones are essential to your design.  

#6 What’s the relationship between T1 tests and part tolerances?

T1 tests are the first part samples produced from the tool that will be used during production. These samples, which are presented to you for review and approval, demonstrate an injection molder’s ability to produce the end-use parts. T1 samples also provide an opportunity to check all drawing dimensions, especially critical dimensions in medical injection molding. Fictiv can deliver T1 samples (even for low-volume production) in as fast as 10 days.

injection molded parts

#7 Where can you get help with plastic injection molding tolerances?

Fictiv’s manufacturing platform makes it easy for designers to get a quote in about 24 hours. Plus, every part that’s uploaded to our platform is reviewed by experts and receives next-day DFM feedback. For some manufacturing processes, customized recommendations and instant, automated warnings are also provided. Fictiv’s manufacturing partners can support tight tolerances, and you’ll have real-time visibility of your injection molding project. 

Learn more about our injection molding services and create an account to get a quote today.