In today’s issue of Cliff’s Chronicles:
It’s axiomatic in injection molding, that time is money and cycle time is critical to making money. A “few” years ago, I was collaborating with an OEM and molder who was developing a high pressure/high heat gear pump to rapidly brew a single cup of coffee in seconds. Gathering the critical performance requirements for the injection molded gears, we targeted these ctq’s: moldable to tight dimensional tolerances, resistance to hot water exposure, elevated temperature creep resistance, retention of
mechanical properties at elevated temperature, very low moisture absorption, NSF compliance/compatibility with potable water and long-term wear resistance. A prime candidate pointed to a performance lubricated, glass fiber reinforced PPS. Well, the look on my molder customer’s face when he heard this said it all, and it wasn’t a smile ear to ear. PPS requires very high mold temperatures.
In my 40+ years in the plastics business, I have yet to meet anyone in the injection molding profession who is excited about molding engineering resins that require mold temperatures greater than 100 oC. Working around high temperature molds almost always leads to a 1st degree burn to exposed skin (or worse). Older mold heating technology relied on either high temperature oils instead of water, or electric cartridge heaters. Oil-heated molds suffer from leaks at hose connections and worse yet, from hoses not rated for the use temperature. Mismatched hoses lead to serious injuries from super hot fluid spraying anyone standing too close. Fortunately, technology has advanced, bringing operational safety to the same reliable level as hot-water-heated molds. Cartridge heaters sound great but are generally a miserable failure because uniform heating of the entire mold cavity is a fool’s dream. But I digress…
So, we collaborated on the design details for the prototype mold, including material shrinkage and required mold surface temperature of at least 140 oC. The OEM and I both were emphatic on these points. Prototype molds were completed. The molder then rushed sampling and shipped parts to the OEM. The first sign of trouble was the parts were on the high side for size (too big). Initial trials proceeded anyway, that’s when the stuff hit the fan. The gear pair failed catastrophically in less than 1000 cycles, at 110 oC water temperature.
We received molded samples as well as samples that had failed. We then proceeded to make various analytical tests to check for contamination, confirm reinforcement loading and define percent crystallinity. We compared crystallinity in the as molded gears and the failed parts and measured < 25% in both samples. For comparison, the crystallinity of our QC retains was measured at > 50%. Digging further, the molder acknowledged that they used their “best thermolator” and recorded an outlet temperature of 105 oC, which they thought was plenty hot enough!
The low-percent crystallinity and the nearly out of spec large size were due entirely to molding with too low a mold temperature. When the same lot of material was molded again, but with thermocouple confirmed steel temperatures at 141 oC, the test rig performed perfectly for 50,000 cycles, meeting all of the design criteria. Processing guidelines are really processing requirements, and here at PolySource, you’ll always know the ‘must do’ when molding our resins and compounds, ensuring no one has egg on their face while they enjoy their freshly brewed single cup of coffee.
Contact The Fixers at PolySource today to learn how we can help you with our superior technical support: https://polysource.net/contact-us/