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What is a Purging Compound

by | Blow Moulding, Injection, Pet Preform

purging compound

What is a compound and why you should using it in material changes

A purging compound is a product most thermoplastic producers use to clean screws, nozzles, mold, hot runners or die of injection molding machines and extruders. These compounds were developed to reduce machine downtimes and scrap rates during material or colour changes as an alternative to purging with virgin resin.

Types of purging compound

Mechanical Purging Compound

It’s a common purging compound that relies on sheer force to push contamination out the barrel. Mechanical purges come in pellets where common resins with a low Melt Flow Index are mixed with some additives. Typically comes in different grades.  

Liquid Purging Compound

Liquid purging compounds behave differently from the conventional purges in pellets. They rely on chemical reactions that will increase the flow and create turbulence inside the barrel. This enables to speed up the removal of contamination and to reach negative flow sites where residues stay longer. They usually are used at small percentages due to their concentrated formula (1-5%).

Chemical Purging Compounds

This type of purging compound also uses chemical agents to clean. They usually require a soaking time of 5 – 30 minutes. These purging compounds also come in different grades. These formulas are the most expensive type.  

Why should you be using a purging compound? Because it’s faster and more effective. 

Purging compounds contain components as emulsifiers, abrasives or foam agents that speed up removing contamination out the barrel. They are specially designed for this purpose. It’s simply faster and more efficient than using resins and regrings that were designed to be moulded into part not to push impurities out of the system.

[Poner en You could probably wash your hands using only water but, does it make sense?]

7 Reasons why you should use a purging compound

  1. Reduces Scrap

Clear out last production resins and colours residues effectively reducing the amount of waste generated. This improves your factory productivity and efficiency.  

  1. Reduces downtime

Reduce the time wasted in resin changeovers. Should we mention the value of time in a production facility?

  1. Eliminate contamination

Their chemical components help you to remove carbon residues, colour streaks and gels during production changeovers. This improves your factory rejection rates and avoids annoying machine stops during productions. 

  1. Prevent Screw Pulls

The contamination needs to be removed somehow. If you don’t clean the machine properly, you will have to pull out the screw to do a manual cleaning at some point. 

  1. Remove contamination from the Hot runners

Some injection moulding changeovers might take you longer during stubborn colour residues inside the hot runners. Now, you can find purging compounds that can be used safely through the mould. Liquid purging compounds are especially useful in this scenario. 

6. Reduce rejections rates

By cleaning the machine properly after a material/colour change or long shutdown, you will prevent product rejections by your quality team or awkward orders returned by your customer. 

7. Cost effective

Purging compounds help you to decrease cost by reducing downtime and scrap during colour and material changes. When evaluating this solution, all factors must be considered: material, machine, labour, scrap, customer satisfaction, lost production and purging compound price. 

Discover the most cost effective purging compounds in this purging compound price guide.

Types of contaminants

  • Black Specks (carbonized material) lodged in extruder & barrel dead zones.
  • Colour Streaks: Remaining Color Pigments lodged in dead zones.
  • Resins blocked inside the barrel.
  • Gels.
  • Unmelted resins. 

When should you be using purging compounds to prevent black specks?

  • Between two production batches
  • Colour or material change overs. 
  • Machine shutdowns/startups

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