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Plastic Purging Compounds, Mechanical, Chemical And
Today's plastic technician has a wide range of plastic purging compounds to choose
from; this article can help to decide which is best for each requirement. Whether it is for injection molding,
extrusion or a hot runner system, there are products that use mechanical, chemical and a combination of the two to
Plastic processors around the world all have the same problem: What is the best way
to purge the barrel, screw and hot runner system? Obviously, there is no one size fits all product that works
perfectly for every machine and material; there are just too many variables to account for.
This type of compound scours the machine to clean it of all
residue. Often manufacturers use a base resin and add various abrasive agents, such as calcium carbonate, ground
cast acrylic or some other material to scrub the machine.
The mechanical purging compound is generally highly effective
and is quite popular. However, not all products are suitable for hot runner systems because the scouring agent does
not fully melt.
In the past chemical agents had a bad reputation due to the very
strong, caustic odor produced during the purging process. Ammonia was a favorite chemical and was very effective,
but became obsolete due to the objectionable odor and health concerns.
Today's chemical compounds are very effective and do not suffer
from the same odor problems. In fact, some even have pleasant scents added to the formula to make the process
somewhat more pleasant.
Basically, the compounds break the polymers down into smaller
chemical chains and use a foaming type agent to purge the broken down plastic from every part of the machine.
Depending on individual requirements, it is possible to buy FDA
approved chemical purging agents. This alone demonstrates just how far these compounds have come since the harsh
chemicals of the not-so-distant past.
Just as the name implies, these are a combination of the two
methods. This process can be highly effective on the most stubborn or neglected injection molding or extruding
See how purging works
JIT, Just-In-Time, Lean
One of the tenets of a lean manufacturing initiative is keeping
little or no inventory as a way of reducing costs. Another concept is a quick response to change and adapting to
the changing needs of the customer.
This is a great way to win clients and keep them happy, but it
can make life quite challenging for the molder, who must keep changing materials in the molding machine. Some runs
are very short and change-overs can happen on an almost daily basis.
In a perfect world, each machine would be dedicated to a
specific type of plastic, making the need for purging almost non-existent. The real world experience is very
different, as any molding technician can attest to.
This constant purging of material can create a great deal of
scrap, especially if inferior methods of purging are used. Some molding companies report that their single largest
scrap expense is due to the material lost during the purging process.
Individual cases require unique solutions for purging plastic
material from injection mold and extrusion machines. Variables such as material type, machine type, use of hot
runner systems, budget, and melt temperature all contribute to make the decision making process more
Choosing the correct plastic purging compound can greatly affect
the profitability of a project, making an intelligent decision imperative.