Toughened 6 vs toughened 6/6 ? 3

[ornerynorsk]

Hi Everyone,

Spec'ing polyamides is not my strong point. From a practical perspective, what are the key differences between 6 an 6/6, injection molding grades. No, this is not homework, they'd never take anyone as old and ornery as me back into school!

I'm interested chiefly in mechanical props, effects of different mold releases, and impact resistance.

Thanks all.

 

[Demon3]

You can compare the mechanical properties in detail for free at

http://www.matweb.com

or download the CAMPUS database.

Of course, take the conditioned values where the polymer has been allowed to take up moisture from the atmosphere. That makes a huge difference for nylons. Moisture uptake halves the modulus and increases the impact resistance dramatically because the water acts as a plasticizer. The as-molded (dry) mechanicals are close to meaningless.

Generally, PA6 and 6,6 are very similar. I know that Pat and Pud have given replies stating some differences though.

Chris DeArmitt

http://www.phantomplastics.com

Consulting to the plastics industry

 

[Pud]

The "classic" Du Pont Zytel ST801 (PA6.6 based) is the one the others try to beat or even match. Most compounders offer their version of the Zytel. Few match the properties - in fact I know of only one and that is EMS Chemie. Their grade ref escapes me at the moment.

If you compare data sheets, it is rare for "others" to specify impact test values at -30°C. We have just completed trials of a component which has severe impact requirements as it's part of a track-laying vehicle track link. The only material which has (historically, on similar parts)worked successfully was Zytel ST801. The only other which we have sampled is the EMS Chemie version. (I don't like this "only one material on drawing" idea - no leverage) Parts currently on vehicle test.

This low temp figure is achieved by co-polymerisation rather than compounding. Afaik, the above are the only two to do it.

The key differences are:

PA6 has a lower melt and operating temp than PA6.6
PA6.6 has somewhat better chemical resistance (Patprimmer advises that it's much better with Zinc salts than 6 - cheers Pat)

I do not know of a copolymerised impact modded PA6 - but then I don't know much anyway...

Mould release should not be needed for nylon - it is after all a bearing material. If parts are sticking something somewhere is wrong.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Virtually all injection moulding grades of nylon have a small amount of mould release built in by the manufacturer. Some do it at the reaction vestle stage to avoid a compounding process. They traditionally used things like about 0.1 or 0.2% of stearates and monoethanolamine, however there are more efficient chemicals available now. I am not at liberty to disclose them.

The main reasons nylons stick on a mould are shrinking onto a core as they are high shrinkage materials or flashing down a pin and creating a suction cap effect on the end of the pin. This flash can be so fine so as to be invisible to the naked eye.

Mould release can help the first but can actually make the second more severe.

In 30 years of close involvement with moulding nylons and detailed knowledge of the practices of hundreds of moulders who use nylon, I have only ever seen two or three occasions where extra mould release over the standard compound content was required, and that was over 20 years ago. These days there is always a grade that will release unless as Harry says, there is another problem like undercuts or flash.

There are grades of impact modified nylon 6 that can match most of the properties of ST801. They may exceed a few in some areas and fall short a bit on others, but overall pretty close if not identical. You need to examine your needs in detail then compare the properties compared to your requirements.

Of course you need to do your comparisons moisture conditioned to the environment of use.

re 6 vs 6.6 comparisons, once the impact modifiers are added, this has less relevance as the more effective modifiers alter all properties substantially and you need different amounts of the modifiers to modify 6 or 6.6 to the same impact, but essentially, 6 has a substantially lower melting point, is a little less crystaline, is a little tougher, has lower tensile compressive strengths and lower creep resistance. It absorbs a little more moisture and absorbs it a bit quicker and also absorbs other chemicals easier and faster, thus lowering it's resistance to them in some cases. As I say, impact modifier have an effect on this and mostly have a far greater influence on properties that most of the differences seen between straight 6 and 6.6, except for maybe temperature effects.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[ornerynorsk]

Thanks everyone, you've answered my questions.