3 Benefits of Two-Step Purging for Difficult Temperature Changeovers

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Use a two step purge to avoid resin degradation.

Over the past 25 years, the plastics industry introduced countless new resins and additives that dramatically improve the performance of products and allow plastics to replace more existing building materials.  Current plastics processors’ applications change in order to grow rapidly. This has lead to more challenging variables for plants to need to stay on top of.

For example:

  • Resins: low temperature, high temperature, transparent, colors, special additives...
  • Machines: blow molding, extrusion, injection, high-cavitational hot runners, different dies...
  • Clients: automotive, medical, pharmaceutical, food, consumer goods, aerospace, toys...
  • Quality criteria: cosmetic, functional, safety, durability, strength…

These lists could go on and on.

As part production changed, the level of engineering and sophistication of purging compounds has changed as well.  

It is therefore advisable to have different purging compounds depending on your particular processing in the plant. If you have a machine and tooling for each product, it is logical that you may require a specially optimized purge for certain products as well.

The cornerstone of most purge programs is a workhorse high-quality general purpose grade like our Asaclean U Grade.  It excels in most cases on its own and could be very valuable in a two-step purge.

Although we all would prefer one-size-fits-all solutions, as some applications and changeovers become more complicated, you need to purge accordingly.

Consider this customer example:

Injection Molder

  • Machine: High-cavitational Hot runner molds
  • Resins: polypropylene with fiberglass, ULTEM, & acetal (POM)
  • Possible scenarios: color change, change from POM to PP and change from ULTEM to POM

Purging Challenge

The use of a single purge does not allow us to change the temperature from normal process ranges to that of a super engineering resin like ULTEM to something low temp like POM.  Attempting this change would cause serious degradation of the low temperature resins or freezing-up of the ULTEM. In the long run, both cases cause black spots on the pieces and would likely necessitate a total teardown and screw pull.

Solution

Use a general purpose purging compound like U Grade that allows working in the temperature range of most resins. Chase it with a small amount of a purging compound like PF Grade that was specially engineered to work with super-engineering resins. This allows you to make the transition from a high temperature range to other conventional engineering resins quickly and painlessly.  Reverse the order or purges as you drop back down for POM.

Benefits

There are serious benefits of using two purging compounds on these kinds of changes.

In the short term, you don't have scrap from start-up.

In the medium term, you don't have to tear down or pull screws. This saves you from  3-16 hours depending on the size of the machine.

In the long term, these time and cost savings add up to more sellable parts and reduce overall operating costs.

Recommendation

Your commercial purge supplier can write you a recipe for success. With the right tools and the right process, you can overcome any processing challenges associated with difficult changeovers. Get your free consultation today and make sure you're getting the most out of your process.

Ready to reduce your production downtime to protect your profits? Learn more about how purging compounds and process efficiency work in tandem.

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Topics: Color Changes, Plastics Processing, Extrusion, Plastics Production, reduce downtime, plastics manufacturing, black specs, material changes, Cleaning Injection Molding Machine, changeovers, faster changeovers, Reduce Scrap-rate, preventative purging, Preventative Maintenance, two-step purge

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