Corrosive Wear Causes
1. Improper Component Materials
Corrosion-resistant materials are available for barrel linings and screws. Stainless steels in a hardened condition, nickel alloys, special flight hardsurfacing materials and iron-free barrel linings all help avoid corrosive wear. Give us a call and we will be glad to discuss the selction of materials with you.
2. Incorrect Shut-down and Start-up
Shut-down and start-up procedures that permit the “soaking” of the resin at operating temperatures for an extended time (longer than the recommended residence time for the resin) will ultimately cause corrosive wear. This is especially true if any moisture is allowed to permeate the resin.
Shutting down without cleaning the barrel of plastic and without leaving the screw in the forward position not only encourages corrosion but can also contribute to cold start-up breakage. Excessive heat-soak at start-up time can also cause corrosive wear.
3. Inadequate Screw Design
Screw designs that cause excessive shear heat also contribute to corrosive wear. Some resins that do not conduct heat readily can be easily burned or degraded, allowing corrosion to occur.
4. Inadequate Moisture Removal
Several resins absorb moisture readily (hygroscopic) and must be thoroughly dried before processing. They include: ABS, PMMA, FEP, PA, PBT, PC, PET, PPO, PVC, SAN, PSU and PEI. If moisture is allowed to remain in the resin, it can unite with other elements to produce corrosives at operating temperatures.
5. Incorrect Heat Profile
The same conditions that can result from improper screw design (excessive shear heat) can also result from an improper heat profile or runaway heater bands. These conditions contribute to corrosive wear.
6. Excessive Residence Time
If the shot size is very small relative to the shot capacity of the machine, lengthy residence times will result. The over-soaking of the resin at high temperatures can encourage corrosion in some resins.