.

Tech Tip March 1997

Purging, The Process:


When we go from one material to another in the molding or extrusion process we typically clean out the barrel of the first material and than proceed with the second material. This can be for colors, types of materials or safety consideration. When doing this, it is typically referred to as purging.

The methods used, can be broken down to two. Chemical, which uses an agent that causes a reaction, this than scours the barrel. Mechanical where we would use a lower melting material to clean the previous material out, this actually mechanically pushes out the previous material.

Whichever method you use, there are a few things to remember;

1) What material is in the barrel.

2) What material are we going too.

3) What is the recommended purging material to use.

4) What temp differences are there.

In cleaning out the screw and barrel, safety is a concern. Some will pull the nozzle off the barrel so they do not build up undo pressure, especially if using a chemical cleaning agent. This prevents the possibility of the nozzle freezing off, and building unsafe pressure in the barrel. Further, it usually works better if you can run the barrel dry of existing material and than follow with the purging agent.

Clear materials, posse the biggest problem, especially with going from a colored material to clear. Some people have a standard policy of just pulling the screw and physically cleaning the entire operation. Others will try and purge out the barrel and go from there. The real concern is the reject rate on the product due to contamination. Not only well you loose the product you are molding but also the material, time and further time to mold for someone else. Please note that there are some screw that well purge out better than others due to design, and material selection / hardness of the screws.



Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers