Wear Symptoms

What are the symptoms of wear in barrels, screws, and non-return valves?

In an injection molding environment, the symptoms of wear and typical causes are:

Symptom Typical Cause
Screw rotating during injection Worn or damaged non-return valve and/or barrel allows screw to move forward without pushing the plastic causing the screw to rotate
Inability of screw to hold a cushion Worn or damaged non-return valve and/or barrel permits screw to move forward without pushing and packing the plastic in the mold
Longer than normal recovery time Reduced plasticizing efficiency resulting from worn barrel and/or screw
Increase in overall cycle time Caused by all of the above. Worn barrel, screw and non-return valve result in slower and inefficient plasticizing
Defective, streaked or non-uniform appearing parts Poor melt quality resulting from worn barrel and/or screw
Metal particles in parts Flaking or peeling of screw plating or barrel lining
Dark or burned specks in parts Barrel and/or screw worn or pitted, allowing plastic to gather in wear spots and degrade
Difficulty in achieving a color change Plastic hangs up in worn areas of barrel or screw requiring repeated cycles to clear out

An extruder operation may experience many of the injection molding symptoms plus those listed below:

Symptom Typical Cause
Increased screw speed (RPM) to maintain normal output Worn barrel and/or screw reduce plasticizing efficiency requiring higher screw speed to maintain output
Above normal melt temperature As clearance between the screw and barrel increases, plastic flows through the clearance causing excessive shear
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