What happened
It was a Wednesday in March 2024, and I was in my office when a client called at 9:00 a.m. They make industrial humidity sensors. The enclosures were molded from ABS, and after about six months in the field, the parts were cracking around screw bosses and vent openings. The production line needed to restart on Friday. We had 36 hours to find a replacement material, qualify it, ship it, and get parts molded.
I'm a materials engineer at a specialty polymer distribution company. I've handled more than 200 rush orders in my career, including same-day turnarounds for medical device clients. In my role, I triage every emergency by time, feasibility, and risk. That morning, all three pointed the same way. We needed to replace ABS with polycarbonate.
The ABS failure was not the real problem
An ABS failure is rarely just a crack. It's a combination of material, process, and environment. Most buyers focus on impact strength and gloss, and completely miss what happens under long-term humidity and residual stress. The common question is 'What is the notched Izod?' The better question is 'How does this grade behave at 85 degrees C and 85 percent relative humidity?' I've seen ABS parts survive a drop test and then fail quietly in the field.
When I say polycarbonate amorphous thermoplastic, I'm referring to the random arrangement of the molecular chains. Unlike semi-crystalline plastics, PC does not have crystalline melting regions. That gives it tight dimensional stability and optical clarity. It also means it can be sensitive to certain chemicals and, in some grades, to hydrolysis. Standard PC might be fine for one application and completely wrong for another.
The first plan made sense, until it didn't
My first plan was to find a standard PC grade, ship it overnight, and let the client run a quick mold trial. We had a material on the shelf. The datasheet looked fine. I almost sent it out.
Then I noticed the hydrolysis testing note. That material had not been optimized for continuous moisture exposure. It probably would have molded fine. It might have even looked great in a 24-hour test. Then nine months later, it could have failed in the field. The line restart penalty was $50,000, but a silent field failure would have been worse.
I pulled up the product list from the Covestro official website while standing in the supplier's warehouse. According to the Covestro official website, Makrolon is the brand name for its polycarbonate products. The specific grade we needed was a hydrolysis-resistant polycarbonate, and it was available at a regional distribution center. I double-checked the technical datasheet and saw a much lower loss of tensile elongation after accelerated humid aging compared with the generic grade. That sealed it.
The Covestro vs Teijin question
While the resin was moving, the client's purchasing manager asked me about the market share polycarbonate Covestro vs Teijin. Should we, he said, be buying from the bigger supplier?
I do not know the exact market share numbers by heart, and I'd be skeptical of any single number that didn't include a region and an application. Covestro is one of the largest polycarbonate resin producers globally, and its Makrolon line is used widely across medical, mobility, and electronics. Teijin is also a major producer, especially in Asia, with a strong portfolio in automotive and electronic parts. Both are legitimate suppliers. Here's the thing: global market share does not tell you which grade will survive humidity in your particular part design.
Is silicone a type of plastic?
Later that day, an engineer on the client's side asked: 'Is silicone a type of plastic?'
No. Silicone is not a plastic. It is a synthetic polymer, but its backbone is based on silicon and oxygen atoms, not carbon. Most engineered plastics have carbon-based backbones. Silicone behaves like an elastomer. It handles heat well and feels rubbery, but it does not have the rigidity needed for a thin-walled enclosure. In this case, switching to silicone would have solved one problem and created another.
The delivery and the aftermath
The resin arrived at 4:30 p.m. on Thursday. The client's molder ran a trial that night with adjusted packing pressure. By noon Friday, they had approved parts and started boxing them. The production line restarted Monday.
The material cost was $2,100, and the rush freight was $380. Normally this order would have been about $1,300 with standard shipping. The extra cost was annoying. The alternative, missing the deadline, would have triggered a $50,000 penalty clause and probably lost a long-term customer.
What I learned
Three things. First, time pressure makes you want to skip the exact line on the datasheet that matters most. Second, a successful mold trial is not a successful material qualification. Third, market share is a proxy, not a technical specification.
Those specific grades and logistics were accurate as of March 2024. Resin manufacturers change product lines, and freight prices move. Verify current datasheets and lead times before you make a sourcing decision.
This approach worked for us because our client had the molds, a molding partner, and an internal quality lab. If your part does not have an established mold, or if the failure mechanism is something other than moisture-related stress cracking, the calculus might be different. I can only speak to the context I lived through.
Would I recommend hydrolysis-resistant polycarbonate for every ABS failure? No. If the ABS part is cracking because of a poorly designed snap fit, changing to polycarbonate might just move the failure point. If a specific solvent is attacking ABS, that solvent might attack PC even faster. The right move is to identify the failure mechanism first, then choose a material, and only then worry about the supplier.
In my experience, the most useful material recommendation starts with an honest sentence: this works for this situation, and I do not know if it works for yours. That is the approach I bring to every rush order. It sounds less confident, but it is how you avoid expensive surprises. And it is why, when I told the client that the hydrolysis-resistant Covestro polycarbonate was the right call, they actually believed me.
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