Injection molding is one of the most popular processes used to manufacture plastic products in high volumes for its high production rates and very low unit cost. The tolerances are of vital importance. If critical tolerances aren’t met, the final parts won’t fit or perform correctly during assembly. These types of errors are problematic because of the cost of the mold. Therefore, control injection molding tolerances is significant. This article will introduce how to control injection molding tolerances with DFM (design for manufacturing) principles, material selection, tool design, and process control.
Why are Tolerances Important for Injection Molded Parts?
Though injection molding is fairly precise, there is always slight variations when manufacturing parts. Therefore, it is significant to define a permissible range of deviations, which allows the part to function effectively as needed after assembly. The assembling will fail when the tolerances are not correctly specified or controlled.
As such, controlling and optimizing injection molding tolerances is critical when multiple parts are being assembled. For instance, if you want to bolt two flat parts together, all kinds of possibilities need to be taken into consideration for the locational tolerances of the holes on each part. Any error in the location and size of the holes can lead to loss of function when multiple components need to fit together during assembly. Injection molding tolerances of multi-part assemblies can be optimized with the use of tolerance analyses. These include worst-case method, tolerance stack, and statistical analysis, which can assume the “worst-case scenario.”
How to Optimize Injection Molding Tolerances
There are a host of measures can be taken to optimize injection molding tolerances, including design for manufacturing, the right injection materials selection, tool design, and process control.
During the Design phase
There are many problems that affect the tolerance of injection molded parts during the manufacturing process, such as warping, excessive shrinking of parts, and part misalignment. It is an efficient way to use the DFM principles when designing a part, which is helpful to minimize the occurrence of these issues.

  • Wall thickness

Uniform wall thickness contributes to ensuring dimensional stability and even shrinkage, which can reduce defects like warping, sinking, cracking and twisting. An even wall thickness can be maintained with coring when thick areas are unavoidable. Uneven wall thickness can lead to part divots and imperfections. Having consistent wall thickness throughout the part is critical to stay within tighter tolerance ranges.
A uniform wall thickness can be achieved by doing the following:

  • Avoid parts geometries such as long unsupported spans, sharp internal corners, and poorly designed bosses

  • Placing radius on inside corners alleviates warping

  • Use ribs and gussets to strengthen walls if essential

  • Use the right materials with wall thickness in mind

  • Avoid using thick walls as this reduces the cooling rate

  • Draft Angles

Draft angles are critical to easily eject a part from an injection mold, which can reduce the damage of friction. With draft angles, wear and tear can be minimized when a smooth finish can be ensured. Changes in draft angle can affect part dimensions, thereby changing expected tolerances. The part can get stuck during ejection without incorporating draft angle, and that, in turn, scrap and warp the finished product.
The followings are some rules of thumb.

  • A draft angle of 10 to 20 work well in most situations.

  • Add 10 for 1-inch depth.

  • Use 0.50 on all vertical surfaces.

  • Use 30 for light texture and >50 for heavy texture.

  • Boss Features

Bosses are generally designed to accommodate fasteners during part assembling. However, too thick boss can result in the creation of the sink marks and voids, expand the cycle time, and split the plastic while fastening. Two points should be taken into consideration in the process of designing a boss.

  • The wall of the boss must not be thick.

  • Core bosses to improve the part frigidity and material flow with additional load distribution

Right Material Selection For Tight Injection Tolerance
The selection of materials is crucial. Determining the right material plays an important part in the final product. Each material has a different shrink rate due to the different characteristics. Shrinkage is closely connected with wall thickness,temperature, parts, and the type of material. The following factors are what you should take into account when it comes to material selection.

  • Plastic Composition:

    Compared with semi-crystalline plastic, amorphous plastics have lower shrinkage due to their less-compact structure.

  • Molecular Weight:

    With high viscosity and a high-pressure drop, high molecular weight resins boast a higher shrink rate.

  • Additives:

    The shrink rate can be reduced with the addition of fillers with low thermal expansion.

Tool Design
Mold designers often oversize the tool to account for the relevant material shrinkage when selecting the material. Different materials have different shrinkage rates. A complex part with a mixture of thin and thick walls will have variable cooling rates. This can result in warping or sink, which can affect tolerances. Therefore, the following factors should be taken into consideration by toolmakers.

  • Tool Cooling:

    Controlling cooling is a crucial step in maintaining even shrinkage rates. Poor cooling will lead to shrinkage, sink marks, jetting, warping, etc.

Cooling should be uniform, which can severely affect the quality of the final product. Part consistency can be improved a lot with intelligent placement of cooling channels.
The following parameters should be monitored in order to achieve uniform cooling.

  • Injection pressure

  • Resin viscosity

  • Fill time

  • Tool Tolerances:

    Tool tolerances can be tightly controlled and monitored due to the use of CNC machining. This is helpful to maintain accuracy throughout the cycle

of heating and cooling of the process. Out of tolerance tooling will lead to inaccuracies and severe defects like shrinkage, warping and sinks.

  • Ejector Pin Location:

    Ejector pins push the final product from the mold, which should happen fast enough to minimize the cycle time. Once the ejector pins are

placed in a wrong location, the finished product can be damaged from warping and dimensional inconsistency.

  • Gate Location:

    The gate refers to the part of the mold where the resin flows. Failure to place the gate in ideal positions will cause poor cosmetic finishes. If the fill

rates are uneven, warping and erratic shrinkage will occur. Multiple gates are required in complex parts, which can achieve even fill and reduce these defects.
The following factors should be considered in the process of deciding the location of the gate.

  • Do not place gates next to obstructions such as pins and cores.

  • Do not let the gate position affect the deformation of the runner and the aesthetics of the user.

  • Place the gate in the deepest cross-section.

  • Place the gate in thick-walled areas.

Process Control
Process control is a necessary step to improve tolerance compliance. It is often utilized by manufactures to calibrate variables that have an impact on the quality of the part. These variables include temperature, pressure, and holding time. There are several ways of process control.

  • Embed pressure and temperature sensors into the tool to measure these parameters , enabling real-time feedback and process control.

  • Ensure consistent tolerances by consistently maintaining pressure and temperature in the tool. 

Get Started with Echofast Injection Molding

Echofast is able to produce consistent and durable plastic parts due to its rich experience in offering custom injection molding services. We can ensure successful products through part optimization in spite of considerable complexity. Tight injection molding tolerances also can be made with proper planning to minimize cost overruns.

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