Crush Rib Design Guidelines
We all know injection molding involves injecting molten plastic into a mold cavity. Pressure and heat are required to ensure the plastic fills the mold. One of the most critical aspects of injection molding is the design of the mold itself.
The mold needs to be designed to withstand the pressure and heat of the injection process and eject the finished product.
But how do you design a part that will function in its intended environment and withstand the forces of plastic injection molding? This is where the crush rib comes in. To improve the stability of your part during injection molding, you can add crushed ribs.
The paramount concern when adding crushed ribs is that they do not adversely affect the function of your part. This blog post will provide an informed discussion on how to design proper crush ribs for your injection-molded parts.
What is a crush rib?
A crush rib is a raised area on a molded part that helps distribute the stress of injection molding evenly across the part’s surface.
The rib is typically located on the underside of the part, which will contact the mold during the injection process. The crushed rib helps to prevent the part from warping or cracking during injection molding.
Crush ribs should be used when the material you are injection molding cannot evenly distribute the stress of injection molding across the entire surface of the part. This can be due to the material being too thin or the part being too large.
The crush rib helps to evenly distribute the stress by redistributing the force of the injection process across a larger area. But do you know how to design a proper crush rib for your injection molded parts? Let’s discuss this further.
Injection molding: Crush rib design guidelines
Figure 3: Test Print Crush Ribs
Typically ribs are more or less thin-walled areas added to a part to stiffen it. Press fits for injection molded parts can be challenging.
A well-designed injection molded part will usually have a draft, but the same draft that helps eject a part from the mold may also keep a press-fit part from staying firmly engaged.
Plastic rib design is paramount to the success or failure of the overall product. Some crucial guidelines for designing crush ribs are as follows:
- The fabrication process of the rib must be carefully considered during design.
- The rib should be as short as possible while providing the required stiffness to reduce sink marks and warp.
- The root radius of the rib should be as large as possible to avoid stress concentration and cracking.
- Ribs should be designed to distribute the plastic flow evenly around the rib cross-section. The uneven flow will result in an uneven wall thickness and an inferior quality.
- Increase spacing between ribs to reduce stress concentrations and the risk of cracking.
- Avoid notches or steps in the rib design, as these will also concentrate stress and lead to cracking.
- The rib’s drafting angles should be carefully considered. Too much draft may make it difficult to eject the part from the mold, while too little draft may make it difficult to insert it into the mold.
- Stack the ribs toward the highest draft angle to reduce the risk of warping.
- To prevent deformation during assembly, the rib at the root should have a thicker wall thickness than at the top.
Designing crush ribs is a complex task that must consider many factors. By following these guidelines, you can ensure that your ribs are appropriately designed for manufacturability and function.
Options for drafts and tight fit besides adding crush ribs
Well-structured crush ribs are essential to successful plastic injection molding, providing uniformity and strength to your molded part.
But what if your design doesn’t lend itself well to ribs, or you’re looking for an alternative to the traditional rib design? Below are the options for drafts and tight fits besides adding crush ribs:
Leave the hole as-is:
During molding, the plastic resin is forced into the mold cavity under high pressure. If your design has a hole or other feature too small for the resin to flow easily, the pressure will cause the plastic to deform.
In many cases, leaving the hole as-is is possible and still getting a usable part. The key is to ensure that the hole is as large as possible without affecting the aesthetics or functionality of the final product.
The options require a high degree of design knowledge and experience to execute correctly.
Add draft to the hole:
Drafts permit easier removal of the molded part from the mold and can improve the dimensional accuracy of the final product. The taper of the draft should be consistent with the type of material being molded and the depth of the feature.
For example, a shallower hole will require less draft than a deep hole. The added gussets should be carefully designed to avoid entrapment of air or material. In rib injection molding, the draft angle should never exceed 2° so that the rib remains visible after molding.
Crush Rib Design Guidelines: Conclusion
Designing a part for injection molding can be tricky, especially when incorporating features like ribs. However, with some guidance, you can create a successful design to avoid problems during manufacturing.
Go4mould is a team of the best engineers. We live and breathe injection molding, and we’re passionate about helping our clients overcome any challenges they may face during product development.
We also provide various kinds of moulds, such as pipe fitting moulds, dustbin moulds, and more.
If you have questions about designing a part for injection molding or need help with any other aspect of the product development process, we encourage you to contact us today. We’ll be happy to help you get the results you need.
Crush Rib Design Guidelines-FAQs
What is the allowable interference for a crush rib?
The minimum interference should be 0.01 inches, which allows the rib to support the loads on the part.
How do you calculate rib thickness?
To calculate rib thickness, the designer should consider the rib’s bending moment and deflection under load and the finish part’s allowable deflection.
