Even with the most diligent design and material selection efforts a rubber compound that is not processed correctly can lead to failure. The two most important aspects in processing of a thermoset rubber compound are mixing and curing. This article takes a look at one case study were processing inadequacies led to premature failures of a rubber component.
Case Study – Rubber Exhaust Tube
In this case a rubber exhaust tube was exhibiting failure during assembly to the sheet metal housing of a blower motor unit. As the bands were being stretched over the housing they were breaking. This breakage included a snap back of the band which was resulting in the injury of assembly line workers. These failures not only resulted in product fallout but also personal injury and a shutdown assembly line.
Investigation revealed that the rubber exhaust tube supplier recently switched from manufacturing these parts in the U.S. to China for a cost savings. The supplier stated that in the manufacturing switch that “nothing had changed.” This is a red flag because it is likely that the compounding ingredients were sourced through China rather than the U.S. Ingredient differences can have a significant impact on the performance of a rubber compound.
Test data was provided that demonstrated the rubber compound used in China met the ASTM D2000 material callout on the exhaust tube drawing. It is important to note that:
- Rubber compound performance is often validated on rubber slabs produced in a laboratory or small-scale environment, not production scale.
- No part performance requirements were available to validate processing aside from hardness.
Microscopic examination of the tube fracture surfaces within the band revealed distinct spherical rubber particles embedded within the rubber matrix. Fourier transform infrared spectroscopy (FTIR) further confirmed that the rubber particles were of the same composition as the surrounding rubber matrix and therefore were not consistent with contamination. The totality of these results were indicative of the rubber curing prematurely during mixing. These particles acted as defects within the rubber that essentially reduced the cross-sectional area over which stress could be applied thus substantially decreasing the band’s elongation at break.
During further discussions with the supplier, it came to light that the U.S. facility had utilized a two-stage mixing process while the China facility used a single-stage mixing process. A two-stage mixing process adds the cure system at the end of mixing in order to prevent premature vulcanization of the compound, which was what was being experienced in this case.
The hardness requirement that was included on the part drawing for the final part would address issues of under cure however, it would not address the issue incurred here. It was recommended that a minimum elongation at break of tube band be added to the part drawing since this was a critical requirement for assembly.
In summary, proper processing of rubber compounds is critical to the performance of the end part, beyond cure state. Rubber compound performance is typically validated on specimens made in a laboratory or on small-scale production equipment which means that the process is likely to be different during production. Adding a critical performance requirement of the part on the part drawing can help to ensure part processing was appropriate in production. For the rubber exhaust tube, a minimum elongation requirement on the drawing for the band would have found the processing issue before it hit the assembly line.