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the tool will be coated with a blue stain whereas there will not be a trace when compounded in ABS.
Also not all polymers will drag out contamination all the time, the mfi has an effect as does residence time, injection speed, screw speed and also which additives and colourants are used;
TiO2 is a good example as it is very abrasive and is found in a lot of colours either as a colourant, a brightener or an opacifi er but the quantity varies as does the particle size. So it will not always drag out contaminants.
Another area of contamination source is with materials that are processed at their upper end of their processing window where even very minor variations can cause degradation and subsequent contamination.
Types of purging compounds There are many types of purging compounds available ranging from very specifi c applications based around polymer type, heat range and mode of operation to universal products. These can be roughly divided into three main categories.
Mechanical
These act by using abrasion to “scrub” the metal surfaces clear of polymer, colourants and carbonised materials. These are often based on semi-abrasive materials such as cast acrylic and can also incorporate other abrasive ingredients such as calcium carbonate, glass fi bres etc. They have proved themselves to be very effective as purging compounds but by their very
nature mechanical type purging compounds are viewed with some reservation as there are concerns over damage and wear to processing equipment. Clearly the purging compound manufacturer understands and where there is risk of abrasive damage to processing equipment provides appropriate warnings. Mechanical purging agents are also not well suited to hot runner systems but development is currently underway to rectify this.
Chemical
These employ chemical reactions to break down residual polymers and carbonised material from screws and barrels. They can act to reduce polymer chain length and incorporate surfactants to loosen contaminants which are then fl ushed away by the purge. Blowing agents are also sometimes incorporated into the formulation to ensure diffi cult to reach nooks and crannies are not missed where materials can hold up and carbonise. This type of purging agent can be suitable to hot runner systems
Mechanical and Chemical These are a combination of both of the above and should, in theory, offer best of both worlds
Why use purging
compounds? Not only do purging agents they should save the polymer processor time, money and decrease the occurrence of rejects due to contaminants, but also improve machine availability when the pressure is on to supply contamination free componenets.
Time is money and long drawn out purging with ineffective materials is a costly business. The aim is to clean through as quickly and effi ciently as possible. Not only does this save time but also reduces the amount of expensive virgin polymer used to purge which is ultimately thrown away.
As good practise machines should be purged on start up and on shut down as well as when changing from one material/colour to another. Regular use of a good purging agent will keep the levels of carbonised material to a minimum and should be incorporated into the preventative maintenance programme of each machine.
The use of a quality purging compound when the black specks start appearing this can save pulling screws, to remove carbon, which is a time intensive process that can also require additional skill and requires suitable risk assessment to avoid injury to personnel.
Plastribution stock the UK’s most effective and proven range of mechanical/chemical range of purging compounds to suit all polymer and process types.
If you are unsure of what type would be most suitable or are having issues with colour changes, black specks or other purging related issues please contact us and we will be happy to help.
Plastribution is the UK’s leading distributor of plastics raw materials, delivering reliability, know-how and true business partnership to the world’s best polymer suppliers and our UK customers.
In the event that you require further information about the shrinkage of polymers, please do not hesitate to contact a member of the Plastribution team for further information by emailing
sales@plastribution.co.uk or phone 0845 3454560
© Plastribution Limited. All rights reserved. 2012
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