nylon 6-6 poor impact strength !

[bngneer1138]

Hello,

I am posting this question on here for someone i have been trying to help sort out a plastics problem for sometime now. The following is a brief description of the part and problem.

I have no actual experience in the plastics industry, but i am an engineer and know what i know from reading and classes; by no means am i a plastic expert of any sorts.

The part itself is, essentially, a small removable bracket that ,in the past, had been made from mild steel and is used to support a beam for horse jumping. The industry/demand has changed from metal to plastics. To stay competitive, my friend decided to get into the plastics market. The parts are nylon 6-6 (black), spend their life outside, see regular impact and are essentially disposable. However, the parts he is receiving from the molder are horribly inconsistent some are very good some are glass-like brittle. To try and determine good vrs. bad parts, we developed a fairly crude impact tester as a crude "incoming inspection". Upon breaking a few parts I noticed a few things.

1- Parts that failed prematurely always had clean brittle failures. Good parts that were tested to failure always deformed (plasticly) before breaking.
2- Some parts (good and bad) have small air bubbles within the part exposed upon breaking
3- Visible flow lines on part(some not all) surface that are NOT detectable by feel, but clearly visible.

Possible causes and tests we have narrowed it down to

soaked parts in water for over a week and showed little to no difference in impact strength as opposed to the untreated control. This was done from batches that were known to be good as well as bad batches. Definite change in feel as Post soak parts had much more solid sound when hit together as opposed to pre-soak which have a distinct hollow sound.

Moisture in granules pre molding?

supplier claims no regrind is used....regrind issue?

Cold mold cavity?

machine itself is too small

possible mold gate issue ( wrong location and/or diameter)

I think we have pointed out the obvious problem areas. I am posting on here to get other opinions/view points. The parts are disposable, but still need to have a somewhat realistic life span. Any tips or suggestions would be greatly appreciated. We have tested other commercially available parts of similar design and material and they far outlast our parts. I can post pictures as well if needed.

Thanks!

 

[ornerynorsk]

You mentioned gas pocketing/porosity. Does the part have a consistent wall thickness? Any unusually thick sections? A picture would be of great help.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[bngneer1138]

This is not the exact part...however the part is very similar in design to this....in the way of internal voids and approx wall thicknesses. I, nor my friend, have any association with the part in the picture.

http://www.google.com/imgres?sa=X&r...bnh=181&tbnw=248&start=0&ndsp=38&tx=102&ty=78

 

[bngneer1138]

I will post pics of actual part failures when they are available.

Thanks,

 

[dgallup]

Tell us more about the grade of nylon you are using: Filler content, is it lubricated, UV stabilized, toughened, etc. There are millions of different grades of plastic out there.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[bngneer1138]

Well I can tell you it is Nylon 6-6 (Dupont) ....No additives that i know of besides black coloring im assuming some type of carbon black....the nylon is bought by the molder black he does not change the color himself. I can tell you with certanty that there is no glass filler or any other type of reinforcement. The filler, from what i understand, would be counter productive as it seems to decrease toughness and impact resistance it just improves overall strength. Some parts/batches come out good from the same lot of plastic. This trend has held true through many lots of plastic. This would lead me to believe the plastic is not the issue, but something in the molding process, pre, during or post....I just do not know the industry well enough to know every specific parameter and how that could possibly effect toughness. To help, environmental degradation isnt a factor because the parts have been failing before seeing any sunlight.

 

[Pud]

Sounds like excess moisture prior to moulding.

Check your supplier is drying the material prior to moulding. It's not reliably dry even out of sealed bags. Your description of visible flow lines suggests quite a lot of moisture. (Silvery streaks?)

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[bngneer1138]

just further confirming my suspicions...well there are some silver streaks, but the flow line i am talking about could best be described as the snaking pattern carved out in sand by a small stream.

 

[cowski]

Something like this?

http://www.plastictroubleshooter.com/ThePlasticTroubleshooter/jetting.htm

 

[Paulkuklych]

As already mentioned your quality issues could be related to so many different things: too much moisture in nylon before processing, wrong processing conditions and incorrect mould design.

That is the easy part now comes the hard part: trying to find the solution. start with the easy things first such as decreasing melt temp in order to try to eliminate the flow lines, then try increasing injection pressure & then reduce injection time. Change one parameter at a time. Also check the moisture level is below what the manufacturer recommends - dont rely upon drying times and temps check the material with a moisture analyzer before processing to be 100% sure. Excessive moisture will also reduce toughness.

Best off luck!

Paul Kuklych

http://www.improve-your-injection-molding.com

 

[Demon3]

I recently heard that Cray Valley sell an additive for nylon that makes it process fine even when there's moisture in there. That or good drying are likely solutions.

Chris DeArmitt - PhD FRSC

Plastics & Materials Consulting

http://www.phantomplastics.com

Plastic Training Seminars

http://www.plastictraining.com

 

[dgallup]

Nylon 6-12 can be used without drying (within limits) because it does not absorb as much moisture as Nylon 6-6.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[HokieFan]

Processing issue swill be difficult to fix without addressing the problem, like insufficient drying. You could also look at another DuPont product like ST801AW BK (super tough and weatherable) This might help with durability.