Composition of Purge Compound

[TechQA]

Hi all,

Anyone have any idea what's the composition of purging compounds used to purge extrusion and injection machines that can effectively remove the carbon and dirt within the screw ? As you know, these purge compounds are quite expensive and i'm trying to find out the concept of this compound. From various MSDS, i only found out it's made of styrenic polymers and additives. Any idea what's the additive ?

 

[patprimmer]

Maybe surfactants.

Maybe mild abrasives.

Reground cast acrylic scrap makes a good purging compound for most polymers. I have seen laundry detergent and/or water added. I would recommend extreme caution however as the water converts to steam, and depending on quantities and temperatures, it may cause some minor explosions as the high pressure steam escapes.

Any chemical added should be known to not degrade into toxic compounds at the temperatures that might be reached in the worst case scenario. I have never seen such data for laundry detergents.

Regards

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[Demon3]

Highly filled polyethylene or polypropylene is good. 40-60 weight % of CaCO3 (as a very mild abrasive) in one of those polymers should be effective and really cheap. Omya and Imerys both sell concentrates of CaCO3 in polymers or talk to Borealis.

 

[TechQA]

Unexpectedly, I asked my technicans to try using 10%glass filled crastin(PBT) which i have some KG on hand to try removing the black spot contaminations. It turned out quite well, we see big improvement using after about 1kg.

I was thinking instead of having 40-60% of CaCO3 which is likely to cause pigments flying around, why not adding 3-5% of glass fibers with Polypropylene and use it for purging instead ? Usually, we tend to use PC for purging when we encounter such black contamination, however it is quite time consuming to increase and drop temperature and the amount of PC used it quite hefty. Thought of using materials like SAN/AS/PMMA but it also seems quite expensive for the long run.

 

[patprimmer]

As cast acrylic is sold in sheet form and fabricated by cutting, shaping and thermoforming, and is used in very large volume for signs and automotive exterior clear plastic accessories, quite a lot of waste is generated. Regrinding this waste is relatively cheap. As it is cast sheet, the PMMA is cross linked and only melts to a rubber like state. This high viscosity has quite an effective scouring effect.

PC can also be quite effective, but it in itself can be difficult to purge, and it tends to build up a glassy layer on the screw.

CaCO3 filled PP is dirt cheap and moderately effective, and normally good enough for most purges.

Glass fibre is better, but 3-5% won't have much effect.

50% glass filled high viscosity nylon 6.6 will effectively scour the screw and barrel, especially if allowed to saturate with water, but may degrade other polymers and generate black specks to replace those removed.

It really is horses for courses, and depends on the material being purged, the new material to be used and the condition of the screw and barrel.

The purging material must be compatible with the temperature and chemical stability of both the materials being purged and to be used next.

Scored barrels need a purging agent that will pull the old and degraded material from the score in the barrel. This is where abrasive or fibrous fillers help. Some steam generated from water also helps to dislodge material stuck in dead spots and scores in the barrel.

Regards

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[TechQA]

Hi Patprimmer,

Can you explain further on the concept behind on the purging material compatability with temperature and chemical stability of both material being purged and to be used next. You mentioned something about steam generated from water helping to dislodge material stuck in dead spots and barrel/screw, to my understanding usually materials especially PC without drying will produce excessive silver streakings. I believe this streaks will contribute in removing the black spots as the molded part surface is rough which probably explains some scouring effect.

Water in materials usually converts to steam during plasticizing, the steam do not usually escape which explains silver streaks. The steam which is actually trapped air, may cause oxidation within the barrel and will cause more black spots in a way or rather. This is my understanding, correct me if i'm wrong.

 

[patprimmer]

Some materials react violently when mixed with some other materials at melt temperatures eg PVC and Acetal. They react so violently they can blow an injection moulding machine barrel and cause injury or death.

When overheated, Polyurethane can cross link and form a non meltable rubber, so if you are to mould a material after it that has a substantially higher melting temp, it must first be purged at it's temperature, then the temperature turned up for the next material. The purging compound must be melted and stable at both temperatures.

Steam is most definitely not trapped air, and contains no free oxygen, so it does not cause oxidation. It can react with some materials to cause hydrolysis, which degrades physical properties. It but does not cause black specks, but it dislodges some that have been trapped inside the barrel for some time.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[unclesyd]

There is a type of purging material that doesn't depend o
entirely on the mechanical displacement of the target polymer. This material, Rapid Purge, depolymerizes the polymer which lowers the molecular weight coupled with a lowering of viscosity. This change in physical properties results in a material that is easily purged from a machine.

http://www.rapidpurge.com/2.html