In rubber molding, early design decisions can have the most significant impact on your end product quality, cost, and overall manufacturability. Whether you’re developing a custom seal, grommet, or complex vibration isolator, optimizing your designs for the rubber molding process is just as critical as selecting the right elastomer.
Design your products with manufacturing in mind:
Tolerances: Expect Variation, Design Accordingly
Unlike metal or plastic parts, molded rubber components typically don’t hold tight dimensional tolerances. The unpredictable rate of elastomer shrinkage becomes a much more significant factor in the curing of rubber materials versus plastics (more on that below). Other factors like mold wear and complex part geometries will introduce additional variability.
Design Tips for Optimizing Rubber Molding Tolerances:
- Use the ARPM Tolerance Guide (formerly RMA).
- Avoid unnecessarily tight tolerances, as they increase tooling cost.
- Call out only functional dimensions and let the rest float.
Typical Tolerances Chart (per RMA/ARPM standards):
Rubber Shrink Rates: What Engineers Need to Know
Rubber materials naturally shrink as they cool and cure in the mold. While Panova takes full responsibility for tool design and shrinkage compensation, it’s important for engineers to understand how shrinkage can affect critical dimensions – especially when working on precision rubber components – and thus set realistic tolerances.
Design Tips to Address Shrinkage:
- Not all rubber parts shrink uniformly – complex features like varying wall thicknesses can introduce additional distortion.
- Collaborate early with your molder so shrinkage is accounted for before tooling is cut.
- Communicate which dimensions are functionally critical so that your molder prioritizes accuracy in those areas.
- Identify and flag critical-to-function dimensions on your drawings or models.
- Know that while rubber isn’t a “tight tolerance” material like metal or plastic, thoughtful DFM can still yield highly consistent results.
At Panova, we handle all elastomer shrinkage calculations and tool offsets as part of our design-for-manufacturing process. The more insight we have into your part’s end use, the better we can engineer the tooling to ensure the functional performance, fit, and repeatability of your end products.
Parting Lines: They’re Inevitable, So Plan for Them
A parting line is where the two halves of a mold meet. The tiny gap creates a small flash (extra material) on the molded part that’s typically trimmed post-molding. However, flash can’t be eliminated entirely, so you have to plan around it in your product designs.
How to Properly Place Parting Lines:
- Flash can interfere with fitment or sealing surfaces, so design such features carefully.
- In addition to assembly headaches, poorly placed parting lines can create cosmetic issues.
- Collaborate with your molder early to determine optimal placement.
- Avoid placing parting lines on sealing surfaces or high-wear areas.
- Communicate whether aesthetics or function is the priority.
Draft Angles: Not Just for Plastics
While not as critical as in plastic injection molding, optimal draft angles (tapered surfaces) nevertheless help improve the release of rubber parts from the mold. Maintaining proper draft angle is especially beneficial for deeper cavities and/or complex geometries.
When Is Draft Angle Critical?
- Deep internal features.
- Tall walls or thin ribs.
- Parts with undercuts (which may require side actions or slides).
Design Tips for Optimizing Draft Angle:
- Include 1–3 degrees of draft where possible.
- Use radii and chamfers to ease ejection and reduce stress concentrations.
- Ask your molder to review your 3D model before DFM freeze.
Further Reading: A Guide to Rubber Molding Materials
In addition to understanding how to optimize your product designs, it’s equally as important to choose the right elastomer for your application. Different rubbers come with vastly different physical, mechanical, and performance properties. Learn all about the industry’s most common rubber compounds in Panova’s Guide to Rubber Materials.
Engineer Early, Save Later with Optimized Rubber Molding Designs
Refining your product designs while keeping in mind the inherent challenges of rubber molding reduces the risk of manufacturing errors, lowers your tooling costs, and accelerates your time to market. Involving your contract manufacturer early ensures your design is not only functional but cost-effective and manufacturable at scale.
Ready to optimize your rubber part design?
At Panova, we partner with engineers early in the design cycle to ensure your parts are not only functional—but also manufacturable at scale. Reach out for a complimentary design-for-manufacturing review.
Download the Panova Rubber Molding Tolerances Chart to help guide your design decisions and ensure alignment with industry standards.