Realistic Injection Molding Tolerances: What You Need to Know

Tight tolerance injection molded parts are critical for the fit and function of many product components. Take Lego brand build blocks, for example. They are built to exact precision with molds accurate to five microns to achieve what they call clutch power. Legos must be able to attach securely to Lego blocks ever produced, including those built in 1958, and be easily dismantled by a child. Certainly, if a child’s toy must be made with such tight tolerance, medical, industrial, and automotive must need tight tolerances. Well, not exactly. It depends on several factors. 

Many of these industries have products with a legitimate need for tight tolerance components. Parts that require pressing, sliding, fitting, or snapping with another component are prime candidates for tight tolerance. Tight tolerance can have a different meaning depending on the molder, but it is generally recognized as ± 0.002 inches, and very tight tolerance is ± 0.001 inches.  

What is Tolerance?

To understand tolerance, you must first understand precision and accuracy. While the terms are often casually used, there is a fundamental distinction between them. Precision is the repeatability of producing an injection molded part without error, including introduced and random errors. Precision will tell you how identical a product will be from run to run. With high-precision injection molding, assuming all parameters remain the same, the part produced today will be nearly identical to one made last month. However, that also means if a part made last month was unacceptable, without any changes, the part today would be equally unacceptable. Improper mold maintenance can result in declining precision over time. 

Since high precision doesn’t mean your part will be acceptable, you need accuracy, too. Accuracy tells you how close you are to a desired dimension. The more accurate it is, the closer it is to that dimension. Accuracy without precision would mean you aren’t consistently molding acceptable parts. 

Molding a part to an exact dimension is impossible because of material and process variations. That level of accuracy and precision is not achievable. How close the part needs to be to the dimension is called tolerance. Tolerance or dimensional accuracy is the acceptable variance, and it is expressed as a minimum at maximum (±). For example, a tolerance of ±0.002 inches means that the dimensions of a part with a target value of 2.542 inches can have an acceptable range of 2.540 inches to 2.544 inches. 

The Tighter the Injection Molding Tolerance, the Better? 

Dimensional tolerances on your drawing tell the manufacturer how critical a specific dimension is. Tight tolerances are often required in products requiring reliability, safety, or fit. In these products, maintaining tight tolerances is important because critical failures can occur if the dimensions are off by the smallest amount. Some of the features that may need defined tolerance include:

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

Consider tolerance stack up as well, which is the cumulative variation permitted, because it will impact how the parts fit together during assembly. For example, multiple components are assembled with holes that must line up, even if they are all within tolerance individually, but on both the plus and minus sides, they may not have the clearance needed for the bolt. 

Injection molds are typically machined to ± 0.005 inches when tighter tolerance is not indicated. Requiring tighter tolerance can add cost to your project for non-critical dimensions and those that can function fine with a lower tolerance. Parts with high tolerances have higher costs for tooling and processing. It is essential to balance the need for tight tolerance against cost. 

Considerations for Material and Design

Beyond the mold, the material and design play a role in achieving the dimensional accuracy desired. Not all plastic materials can produce products with the same tolerances. Plastic has different shrink, thermal expansion, and moisture absorption rates, which can impact the part’s tolerance. Use must also be considered. If the component will be used in a wet or humid environment and its material absorbs moisture, the part can be in tolerance when manufactured but go out of tolerance in use. 

Part design also needs to be considered. Following design for manufacturability (DFM) rules will help prevent issues impacting tolerance. For example, uneven wall thicknesses can cause sinks and divots because thicker areas will cool slower than thinner areas. This can cause issues with flatness tolerance. 

Expert Advice For Plastic Injection Molding Tolerances

Helping you find the balance between acceptable tolerances and managing costs may mean consulting with experts. At LMC Industries, we excel in designing parts that meet your requirements. Following DFM principles, we can help with material choice and part design to ensure the tolerances you need can be met. Plus, we have a 14,000 sq ft. toolroom for fabricating, repairing, and maintaining injection molding tooling, so we have more control over the quality of your components.  

Let’s get started on your project. Call us today for a quote.