Moisture is a potential problem in all phases of the compounding process. In the primary mixer, water vapor can condense in the feed pipes causing hang-up and blind-off in the dust sock and can promote the formation of scale on the mixer lid. In the cooling mixer, moisture can condense on the cold mixer walls causing the fine ingredients in the blend to build up and form an insulating layer. In the extruder, moisture can inhibit smooth feeding of the blend, produce poor quality extrudate, and negatively affect the compound’s physical properties.
Water can be introduced into the material-handling system via leaks around unloading hoses, dust collectors, etc. Just as when compressed air depressurizes allowing cool water droplets to form, such can be the case with conveying air and the compressed air used for blasters and other aeration devices. All possible sources of unwanted water must be eliminated.
Moisture is driven out of the compound during the primary mixing process when the batch temperature rises above 100 degrees centigrade. This moisture needs to be removed from the processing environment. This calls for adequate venting in conjunction with a good dust-collection system. Equipment manufacturers now offer air purge or air sweep systems for this specific purpose.
Another problem associated with moisture is the formation of scale on the cooling mixer which results from condensation on the cooling mixer walls. To minimize the chances of this, mixer walls should be prevented from getting too cold in the period between batches by stopping the flow of coolant flow when the correct mixer temperature is reached.
The vents on the extruder should be unobstructed and the vacuum systems operational. These vents will further remove water and other volatiles that come off the batch at the higher processing temperatures.
This section of Specialty Minerals Inc.’s web site contains a number of pages that deal with the technology of preblending and melt compounding rigid PVC. Click on one of these topics to go to that page:
- Introduction to PVC Compounding
- How to Compound - discusses some of the basic aspects of melt compounding
- Flow and Handling – provides some information to help solve problems in this area
- Problem Solving – a general discussion about methods of investigating problems
- Running a Trial – covers procedures that will make a trial more productive
Learn more:
- SMI minerals for plastics
- SMI minerals for rigid PVC
- SMI minerals for PVC window and door profiles
- SMI minerals for PVC fence and decking
- SMI minerals for PVC siding
- SMI minerals for calendared and thermoformed rigid PVC
- SMI minerals for injected-molded PVC
- SMI minerals in automotive products. See the parts of your car that are made with Specialty Minerals products.
- SMI minerals in housing. See the places Specialty Minerals products are used around your house.
- “Fillers in PVC – A Review of the Basics” - download brochure
- Download brochure on Specialty Minerals Plastics Applications Laboratory
- About Specialty Minerals Inc.
- Quality and ISO at Specialty Minerals
