Injection Molding: Crush Rib Design Guidelines

Crush Rib Design Guidelines

Crush Rib Design Guidelines

We all know injection molding involves injecting molten plastic into a mold cavity. It requires pressure and heat 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 in such a way that it can withstand the pressure and heat of the injection process, as well as be able to 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 crush ribs. 

The paramount concern when adding crush ribs is that they do not cause any adverse effects on the function of your part. This blog post will make an informed discussion on how to design proper crush ribs for your injection molded parts.

What is a crush rib?

Figure 2 Crush Rib

A crush rib is a raised area on a molded part that helps to distribute the stress of injection molding evenly across the surface of the part. 

The rib is typically located on the underside of the part, where it will contact the mold during the injection process. The crush 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

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.
  • To reduce sink marks and warp, the rib should be as short as possible while providing the required stiffness.
  • 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 drafting angles on the rib 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.
  • Use a thicker wall thickness for the rib at the root than at the top to prevent deformation during assembly.

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:

The plastic resin is forced into the mold cavity under high pressure during molding. 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, it’s possible to leave the hole as-is and still get 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 ribs 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. But with some guidance, you can create a successful design to avoid potential problems during the manufacturing process. 

Go4 mould is a team of the best engineers. We live and breathe injection molding, And we’re passionate about helping our clients overcome any challenge they may face during the product development process. 

Apart from this, we also provide different kinds of moulds such as pipe fitting mould, dustbin mould, and many 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 to calculate rib thickness?

To calculate rib thickness, the designer should consider the bending moment and deflection of the rib under load and the finish part’s allowable deflection.

Scroll to Top