How We Solved Cracking Around Molded Holes in an EPDM Rubber Component
Introduction
For precision rubber components, a small crack can become a much bigger problem after assembly or under pressure. In one recent project, LT-RUBBER was asked to address fine cracks found around the holes and edges of an EPDM rubber sealing component.
The cracks were difficult to identify in some parts and were located close to the molding parting line. Instead of treating the issue as a simple trimming or appearance problem, we reviewed the mold structure, hole-forming process, material properties, and production stability together.
This case shows how the right molding method and material verification can help eliminate a recurring rubber cracking problem.
Defect Analysis: Cracking Around Mold Formed Holes
The main defect was fine cracking along the parting line, especially around the molded holes and component edges. The cracks were not caused by external tearing. They were generated during the compression molding process and could potentially grow after assembly or under pressure.
The hole location was a key factor. The circular hole was positioned close to the mold parting line, while sharp areas around the hole created local stress concentration. This increased the tendency of the EPDM to tear during molding.
Another concern was small internal cracks that were difficult to detect by quick visual inspection, creating a risk of hidden defects reaching the customer.
There were also signs of local material shortage. Possible process factors included insufficient venting, undersized runners or gates, contamination or rust on the mold surface, and unsuitable vulcanization time.
Our Approach: Two Hole-Forming Solutions
After reviewing the defect mechanism, LT-RUBBER developed two practical solutions.
Solution 1: Precision Pin Molded Hole
The first option was to add a precision pull-pin mechanism inside the mold.
During hot-pressure vulcanization, the pin directly forms the complete inner hole. This allows the hole to avoid the product parting line, removing the root cause associated with parting-line mismatch, flash and local stress concentration.
The biggest advantage is that the hole becomes an integrated molded feature, with no secondary machining required.
Refer to proven examples of similar products shown in the reference image：
Solution 2: Secondary Punching
The second option was to leave the hole area solid during molding.
After the rubber was fully vulcanized and cooled, a dedicated CNC punching process was used to cut the required round or irregular holes accurately.
This solution required less mold modification and offered a lower initial tooling cost. However, it introduced an additional production step, with an estimated 10% increase in process cost, and its consistency could be affected by cutting tools and rubber rebound.
Refer to proven examples of similar products shown in the reference image：
Solution Comparison
| Item | Precision Pin Molding | Secondary Punching |
|---|---|---|
| Crack improvement | Eliminates parting-line cracking at the root | Avoids the parting line but may create secondary defects |
| Mold modification | Larger; requires pull-pin mechanism | Smaller |
| Initial tooling cost | Higher | Lower |
| Production cost | Product unit price unchanged | Approx. +10% process cost |
| Hole consistency | High | Medium |
| Best suited for | Stable long-term mass production | Flexible or lower-volume production |
For this project, the molded-hole solution provided the stronger long-term approach because it addressed the defect at its source rather than adding a corrective operation afterward. The project also included rapid sampling, with LT-RUBBER covering the agreed mold modification, sampling, and shipping costs for both proposed solutions.
EPDM Material Verification
Mold optimization alone was not enough. We also verified the EPDM material used for the sealing component.
The material, EPDM compound WER-S260802, completed a full physical-property evaluation. The test results showed stable performance in hardness, tensile strength, tear strength, compression set and aging resistance.
| Test Item | Specification | Test Result | Test Method |
|---|---|---|---|
| Typical Properties Test | |||
| Hardness (SHA) | 71°–75° | 73° | ASTM D2240 |
| Tensile strength (MPa) | / | 17.84 | ASTM D412 |
| Elongation at break (%) | / | 198.95 | ASTM D412 |
| Tear strength (kg/cm) | / | 24.75 | ASTM D624 |
| Proportion | / | 1.117 | ASTM D792 |
| Compressive Deformation (%) | ≤12 | 6.4 | ASTM D395 22Hr/150°C |
| Compressive Deformation (%) | ≤10 | 5.7 | ASTM D395 22Hr/70°C |
| Compressive Deformation (%) | ≤6 | 5.4 | ASTM D395 22Hr/23°C |
The overall material evaluation was qualified for engineering application, with the tested compression set well below the specified limit.
For mass production, LT-RUBBER also planned third-party verification through qualified laboratories such as SGS or CTI to confirm batch-to-batch material consistency and compliance.
Common Mistakes When Dealing With Rubber Cracks
Several approaches can make the problem harder to solve:
1. Only trimming the visible crack
If the crack originates from the parting line, trimming does not remove the underlying molding problem.
2. Focusing only on the rubber compound
A suitable EPDM compound cannot compensate for poor hole positioning or excessive stress concentration in the mold.
3. Ignoring small hole cracks
Fine cracks may be difficult to identify during visual inspection and can become larger during assembly or service.
4. Adding secondary machining without comparing long-term costs
Punching can reduce initial tooling investment, but the additional process and tool wear need to be considered for mass production.
FAQ
Q1: Why do cracks often appear around molded holes?
Hole edges can create local stress concentration. When the hole is also close to the mold parting line, the risk increases.
Q2: Is molded-hole forming always better than punching?
Not necessarily. Pin molding is more suitable when consistent hole geometry and long-term production stability are priorities. Punching can be attractive when mold modification needs to be minimized.
Q3: Can LT-RUBBER help with mold optimization?
Yes. We can evaluate the relationship between rubber material, mold structure, hole formation and production requirements to develop a suitable manufacturing solution.
Conclusion
This project demonstrated that rubber cracking is not always a material problem. In this case, the location of the hole, parting-line structure and molding process were closely connected to the defect.
By combining mold optimization, precision hole forming, EPDM material testing and production quality control, LT-RUBBER developed a practical path toward stable mass production.
From precision rubber components and EPDM sealing parts to customized mold development and mass production, LT-RUBBER supports OEM customers from engineering evaluation through production.
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