Tolerance For Screw Wear

An frequent e-mailer recently asked this question…

What is the downside to a worn screw?  Why should I measure it, and why should I replace it if it is worn beyond tolerance?
My Response
Each process and company has a different tolerance for wear. A company molding medical devices needs much more control over melting and residence time than a company molding commodities.
The benefits to regularly measuring your screw and barrel is that you can determine what level of wear tends to indicate inconsistent processes. Ultimately, an excessively worn screw will increase residence time, decrease melt consistency, and reduce melting efficiency.
Additional Thoguhts
Many companies will have a couple ‘junk’ machines which are not worth replacing the screw and barrel… but can run a handful of jobs well enough. Other companies keep every machine in perfect shape. Remember, you don’t need a race car to get your groceries… but you also don’t want to enter a race with a Yugo.

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