Nylon part presenting undesired marks and spots

[vmarc]

I am injection Zytel ST801 BK to make 3 inches long insulators and I am getting parts with random marks and spots in some areas of the part while the rest of the part appears shiny and ok. These marks and spots tend to apear in the same areas of the parts but not always. These parts are molded in a 8 cavities mold. The material is brand new and had been dryed at the correct temperatura and time. The Machine has the correct temperature parameters and I have tried everything to solve this problem. From changing temperatures to injection speeds and presures, but nothing seems to be related to the bad apearence I got. I have been doing these parts for a while and some times the parts are produced just fine and then the problem starts just to eventually decrease to a level that we can handle.

I will really appreciate all help I can get.

Thank you

Victor

 

[Demon3]

Hi Victor,

Even though you have dried the material, I still suspect that the problem is due to water in the nylon. If the drier is a desiccant drier, then you may need to change the desiccant may need to be renewed for it to dry properly.

I would suggest checking the water content after drying, for example using a standard water analyzer that works by heating the nylon and measuring the loss in weight due to the water given off.

http://www.northshorecare.com/pdf/moisture-methods.pdf

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

I agree with Chris.

Your material is not properly dry and is out gassing.

Further to that you have jetting from the gate and quite likely poor vents and maybe the mould is to cold in some places.

Truly dry material, bigger gates, good vents, faster injection speed, increased and uniform mould temperature and plenty of hold up pressure might all help.

Also you might have to much melt decompress.

Change the easy things and the things that can be undone first.

Check the condition of your dryers first no matter what.

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

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for site rules

 

[Pud]

I have always found this stuff difficult to dry. It takes 40 - 50 minutes in our vacuum driers, which the same dryers will dry PC/PA6 in 25 mins. no problem

A poor finish is usually not dried material.

Does it mould ok out of the bag? Sometimes Du Pont supply material which is dry enough to use out of the bag.

And as Pat suggested - your gates are in the wrong place (as best I can see from picture) - you should be feeding 90 deg into the core - not squirting it up the cavity. You will never get a decent finish if this is the case.

Have just re looked at pics and definitely jetting. This causes the wavy surface lines, and probably the dull patches too.

Rgds


Harry

http://www.tynevalleyplastics.co.uk

 

[ornerynorsk]

Pat said it, but it bears repeating ~ Venting! Many other factors, as well, will cause a multitude of issues, but so many tools are improperly vented.