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Injection molding resin selection of proactivity in testing

As a custom injection molder we face a broad range of customer requirements, and a wide variety selection of resins. To give the fuzziest of answers regarding proactivity in testing, the answer is: "It depends."

The level of testing we choose to perform is custom tailored to the resin family, the molded product applications, our internal molding needs, and the customer's expectations, and certainly the presence of customer warranty claims where, this testing can add a bit of insight or forewarning of trouble in the absence of other testing that is far less obscure in nature.

As an example, if faced with a problem that appears to be isolated to a particular component it is probably cheaper and the test results certainly less obscure, to develop a destructive test with built in and appropriate safety margins for the part. Everyone can quickly grasp and accept the concept and result of a broken v. not broken part. Better still is a destructive test that can incrementally describe the force required to fail the part, you test to destruction and record the highest force attained.

Some resins are very wide ranging in melt flow rate, primarily the olefins and the styrenes. Generally, we would not do routine "proactive" melt flow index testing on these unless some problem, identifiable to variations in flow rate as received, or variations in flow rate as an indicator of degradation (in my experience, virtually unheard of in the olefins) That is, the part may be so poorly designed for the resin selected that you run up against short shots or other visual flaws and in this case it becomes a business survival cost balancing issue. Cost of test, ability to deal with the inevitable out of specification lots received. (Specification in this sentence means a UPG imposed specification) Vs. the desirability of the business. Custom injection molders know that it is very difficult to cherry pick the projects coming from your largest customers, so you take the trouble children along with the gravy.

There are other resins such as PBT, PET that give you no visual clues that the resin is degraded and for these we have mandated proactively, testing upon receipt and in process to avoid molding large quantities of bad product that looks good, but will fail mechanically if put out in the world.

Lastly, a little clarification about compounders. I have no complaints about compounders. You generally get what you pay for, and I've found them to be honorable and honest within their corporate business models, as long as you are equally honorable and honest.

If you need very tight control over a compounded resin, you should be willing to pay the price to get what you need.

Some compounders have built their business upon supplying cheap resin compounds, and that's ok, just recognize (and this is for the benefit of any OEM designers) that cheap resins have a large variability component that must be compensated for in the part/assembly/end use design.

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