That Day, the Plastic Pipe in the Machine Suddenly Failed
Do you recall two years ago? A newly introduced wet process machine in our factory had been running for over half a year when the production line began experiencing intermittent shutdowns. Initially, everyone suspected software bugs, and programs were repeatedly modified, yet the problem persisted intermittently. Later, Ah-Hao from the night shift, with his keen eye, discovered that every shutdown was related to an issue with the chemical supply pipeline. Upon closer inspection, to our surprise, the plastic pipe, which should have been perfectly straight, looked somewhat like it had been scalded by hot water, having "shrunk" slightly at a certain bend.
"Peter, has this pipe gone 'soft'?" Ah-Hao asked, looking puzzled. I walked over and touched it; it was indeed somewhat deformed, and noticeably harder and more brittle than the adjacent pipes. Well, now it's not just soft, but also brittle. This wasn't ordinary plastic fatigue; this was clearly the "creep" effect in plastics. Simply put, it's when plastic parts slowly deform under long-term stress, eventually losing functionality.
Where the Problem Lies: The "Midlife Crisis" of Plastic Parts
Plastic parts are ubiquitous in semiconductor manufacturing processes, from chemical fluid pipelines and connectors to various structural components; there are likely more hidden ones than visible ones. However, unlike metals, plastics do not have an "elastic limit." You pull it, it stretches; you release it, it returns. But if you continue to pull it, it will slowly and quietly continue to stretch in unseen areas until it can no longer withstand the force. This is "Creep."
Imagine filling a plastic water bottle and leaving it for half a year. You'll notice the plastic at the bottom of the bottle will bulge slightly; this is creep. It slowly deforms under constant stress.
So what is "Stress Relaxation"? This is essentially the reverse of creep. If you clamp a plastic part, maintaining a fixed deformation. Initially, it will generate a large reaction force, but over time, this force will gradually decrease. This is like using a plastic clip to hold documents; it clips tightly at first, but after a long time, the clip loses some of its grip. This is stress relaxation, where stress gradually decreases under constant deformation.
In essence, creep is "deformation increasing over time," while stress relaxation is "clamping force decreasing over time." Both phenomena are unavoidable "midlife crises" for plastic parts subjected to long-term stress or constraint.
What to Do in Practice: Don't Just Look at Short-Term Performance
When encountering such problems, we cannot solely rely on the initial strength of the newly installed parts. You must consider time.
- Material Selection: First and foremost, the most fundamental step is material selection. The creep and stress relaxation characteristics of plastic materials vary significantly. For example, PEEK and PVDF demonstrate excellent corrosion and temperature resistance, yet their creep rates still differ under long-term stress. Our deformed pipe, for instance, was later found to be made of a lower-cost PVC material. In the high-temperature corrosive chemical environment, coupled with internal pipeline pressure, creep was accelerated.
- Design Margin: Next comes design. During the design phase, the long-term deformation of plastic parts must be considered. If you design a clip that needs to maintain a 30N clamping force after five years, then during design, you cannot just look at its initial 40N clamping force when newly manufactured. You must use the material's stress relaxation curve to extrapolate how much initial clamping force is needed to ensure 30N after five years. This is similar to knowing a battery will degrade to 80% capacity after five years, so you provision it with 120% of the required capacity from the start.
- Environmental Factors: Temperature and chemicals are two major accelerators of plastic creep. For every 10-degree Celsius increase in temperature, the creep rate can accelerate by 2-3 times. Chemical corrosion further degrades the material structure, making creep uncontrollable. Our pipe's creep problem was exacerbated by the high-temperature chemical environment.
Therefore, the key is that when evaluating plastic parts, you must ask suppliers for "long-term performance data," especially creep curves and stress relaxation curves at different temperatures. If they only provide short-term tensile strength, frankly, they are playing word games with you.
The Most Common Pitfalls: You Get What You Pay For, and Incomplete Data
The most absurd case I've encountered was selecting a plastic pipe "claimed" to be high-temperature resistant, just to save a small amount on procurement costs. The result? Costs were saved, but the machine had to be shut down twice a month to replace the pipe. Factoring in the loss from downtime, plus labor, it ended up costing many times more than if we had bought the slightly more expensive pipe from the start.
Another major pitfall is "incomplete" data provided by suppliers. They only provide tensile strength at room temperature, or short-term creep test results. If you ask for high-temperature creep curves or stress relaxation curves, they start to prevaricate. Honestly, that's when you should be wary, because it means they themselves might not be confident about the material's long-term performance. Without this data, you simply cannot accurately assess the reliability of the component in its actual application environment.
Finally, there's a minor pitfall: the "pre-stress" issue of parts. Some plastic parts are pre-stressed during assembly, such as gaskets tightened by screws. If this pre-stress is too high, or the material itself has poor stress relaxation characteristics, then over time, this pre-stress will decay, leading to loose connections or even leaks.
One Thing You Can Do Today
Check the plastic parts in your machinery and ask your suppliers if they provide long-term performance data.