{"id":10865,"date":"2022-12-19T00:44:38","date_gmt":"2022-12-19T08:44:38","guid":{"rendered":"https:\/\/www.fictiv.com\/?post_type=cpt_blog&p=10865"},"modified":"2024-01-12T05:22:30","modified_gmt":"2024-01-12T13:22:30","slug":"injection-molding-tolerances-an-in-depth-look","status":"publish","type":"cpt_blog","link":"https:\/\/www.fictiv.com\/articles\/injection-molding-tolerances-an-in-depth-look","title":{"rendered":"The Ultimate Guide to Injection Molding Tolerances"},"content":{"rendered":"\n

Injection molding tolerances are acceptable variations in size for part features and overall dimensions. They are expressed as plus or minus (\u00b1) values in millimeters or inches and vary by the type of material that\u2019s used and the overall size of the part. <\/p>\n\n\n\n

Part designers add tolerances to CAD files to tell an injection molder the amount of variation that\u2019s allowed. In turn, injection molders use tolerances during design for manufacturability (DFM) reviews and to make tooling and processing decisions.<\/p>\n\n\n\n

As a rule, parts with tighter tolerances are more expensive because they have stricter tooling and processing requirements. Mold quality is critical, as are proper cavity fill rates and consistent cooling temperatures. <\/p>\n\n\n\n

Even with the best injection molding equipment, process repeatability varies. A skilled molder can calculate shrinkage rates for various plastics, but your design decisions affect part shrinkage and, ultimately, which tolerances are achievable. If an injection molding material has a higher shrink rate, tolerances are harder to control.<\/p>\n\n\n\n

This article examines the importance of injection molding tolerances and their relationship to tolerance stack-ups. It also explains how part tolerances are specified and describes the factors that affect them. In addition, there\u2019s a section for designers with frequently asked questions (FAQs).\u00a0<\/p>\n\n\n\n

\"Injection<\/figure>\n\n\n\n

Tolerances and Tolerance Stackups<\/h2>\n\n\n\n

Many injection molded parts are used in larger assemblies. A tolerance stack-up<\/a> refers to how all assembled parts must fit together. Because each component has its own tolerances, designers need to account for the cumulative effect of dimensional deviations. <\/p>\n\n\n\n

Consider the example of three injection molded parts that are bolted together. Each part has a hole with tolerances within the dimensions you specify. That\u2019s great, but all three holes still need to align so that a fastener can pass through them.<\/p>\n\n\n\n

Download Fictiv\u2019s Tolerance Analysis Calculator<\/a><\/p>\n\n\n\n

\"tolerance<\/figure>\n\n\n\n

Types of Injection Molding Tolerances<\/h2>\n\n\n\n

There isn\u2019t just one kind of tolerance for an injection molded part. Rather, there are tolerances for specific part features. Here are some of the most common.<\/p>\n\n\n\n

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  1. Dimensional:<\/strong> Overall sizing of the part. Increased part size results in larger shrinkage during cooling. This is why injection molding tolerances are listed for specific size ranges.<\/li>\n<\/ol>\n\n\n\n
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    1. Straightness or Flatness:<\/strong> Covers the general warping of large, flat areas. Mold design features like gate location and uniform cooling can reduce warpage.<\/li>\n<\/ol>\n\n\n\n
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      1. Hole Diameter:<\/strong> Larger holes need a larger tolerance range due to increased shrinkage. <\/li>\n<\/ol>\n\n\n\n
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        1. Blind Hole Depth:<\/strong> Blind holes require cantilever-type features in the mold. If the plastic is injected at high pressure, the pin inserts can deflect, and the deeper the hole, the higher the likelihood of deflection.<\/li>\n<\/ol>\n\n\n\n
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          1. Concentricity\/Ovality:<\/strong> A large cylindrical part with a thin wall can shrink unevenly so that the part loses its circularity. <\/li>\n<\/ol>\n\n\n\n

            Injection molding tolerances aren\u2019t just about part features. They can be divided into commercial and fine tolerances depending on the amount of precision that you need.<\/p>\n\n\n\n