Guide to Optimizing Injection Molding Tolerances
Injection molding is the ideal process for manufacturing many plastic products. It is mainly due to its low unit cost and high production rates. The feature of tolerance plays an important role in part production, just like any other manufacturing process. When incorrectly controlled or specified, the final parts end up no fitting well during assembly. These errors are problematic since it molds cost upfront is very significant. The factors influencing injection molding tolerances include tool design, material selection, and part design. Here is further insight into these factors and why tolerancing is so critical.
Below Main Points To Key Factors to Affect The Product Tolerance
Material Selection
Injection molding relies on a wide range of plastic resins to make parts. The process of material selection depends mainly on the use of the final product. However, each resin material has varying shrink rates. While designing, there is a need to take into account the shrinkage rate of each resin used. At Creatingway, we recommend oversizing the dimensions of the tool by the shrinkage percentage of the material. It is important to design tools after finalizing the material selection. In cases with multiple material resin use, design for each different shrinkage rate. The consequence of failing to account for this shrinkage results in parts that do not fit well together. This is usually a costly mistake in injection molding. Thus, the tool design is very dependent on the shrinkage rate of the chosen material.
Part Design
The design for manufacturing principles is critical when designing parts. They help limit excessive shrinkage, warping, and part misalignment during injection molding. Working with a trusted injection molding partner, such as Creatingway, guarantees adherence to these principles. Here is a simple breakdown of the principles to ensure accurate injection molding tolerances.
- Wall Thickness– variable wall thickness contributes greatly to uneven shrink rates seen in parts. We recommend using coring if it a must to use thick walls in the design. However, thick walls are not always the best solution to ensuring strength in a part. The risk of getting uneven walls that cause part deformation is too costly. It even interferes with part assembly. A great alternative is using gussets and ribs. These feature greatly improve part strength while also achieving the desired part tolerance.
- Draft Angles– the feature allows easy ejection of a finished part once cooled from the tool. Parts get stuck and get defects with sub-optimal draft angles. The common defects are warping and scraping of the finished product. At Creatingway, we vary the draft angle from 50 to 30 depending on the requested surface finish and part design.
- Boss Features– we use this feature to attach fasteners during the assembly of multiple parts. Very thick bosses can make sink marks on the part surface. Also, poor connection to the part’s sidewalls through ribs may result in significant distortion. The mentioned defects may make the whole process of part assembly near impossible.
Process Control
After considering the part design and material selection, there is still a chance the part may be out of tolerance. The next phase remaining is tweaking the injection molding process. This is after noticing imperfect tolerances with the first sample of parts made. The factors controlled include holding time, temperature, and pressure. Good control of these factors increases the chances of getting quality parts. The first is getting the ideal set of conditions needed. Then, it is simply creating parts with little dimensional variation consistently.
Products with multi-feature parts can benefit from pressure and temperature embed sensors within the tool. This allows for a more accurate real-time measurement of the controlled parameters. Keeping these parameters consistent ensures maintenance of injection molding tolerances.
Tool Design
The tool design follows after material selection. We use an oversized tool to factor in material shrinkage. However, part shrinkage is not the same for all dimensions. For instance, there is a different cooling rate for thicker parts when compared to thinner parts. Thus, parts with a mix of both may present a challenge due to the varying cooling rates. This results in warping and sinking of the parts affecting the injection molding tolerances. At Creatingway, we take into account the following factors while designing tools for injection molding.
- Gate Site– the gate allows the inflow of the liquid resin during injection molding. When placed in the wrong site, it results in a poor cosmetic finish. It may also result in uneven fill rates during the process. This causes shrinkage and warping of the parts. Thus, it important to put into place many gates to allow equal fill rates, and subsequently resolving the problems.
- Ejector Pin Site– the ejector pin pushes away the molded part out once the tool opens. This step needs to take place quickly to reduce the cycle time. Improperly placed ejector pins result in part damage. Non-rigid materials get affected while leaving the tool and become warped.
- Cooling Channels– to maintain equal shrinkage rates, it is critical to control cooling. Poor control of cooling cause uneven shrinkage rates and drastic deviation of tolerance needs. thus, proper placement of cooling channels within tools greatly improves part consistency. Contact Creatingway now for your injection molding services.
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