Inj. Molded Nylon 6-6.....Poor/ Inconsistent 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!

 

[MikeHalloran]

The pellets have to be thoroughly dry before loading them into the machine.
That means not merely keeping them out of the rain, but actually having them pass through some kind of drying apparatus. ... and actually having it switched on and working properly.

A 'get aquainted' visit to the supplier while they are running your parts might be sufficient incentive to keep them honest for a while. Be open to input also; you should learn a little from such a visit.






Mike Halloran
Pembroke Pines, FL, USA

 

[bngneer1138]

Thanks Mike,

He has been to the supplier multiple times. This has been a problem for over 2 years. The plant is capable of making a good part they ARE NOT capable of doing it consistently. He has mentioned that he does not think his dryer is sufficient. My guess, if i had to choose, is that the pellets are too damp going in nylon is so sensitive to moisture pre and post mold....We have accepted alot of his input, much of which has helped. He initially had the molding machines parameters wrong and that was causing a different issue. He was very helpful, but this is a different problem. I believe most of the work he does is cosmetic plastic with limited structural applications. His "set it and forget it" attitude may be OK for parts like that. On the other hand, we are dealing with a structural part that needs better observation of machine variables. If it was up to me i would look into other avenues...but there are other issues at play as well as the fact it is not my product..I am just giving my input to help.

Thank you,

 

[bngneer1138]

It is hard ...the whole point of using an outside vendor is to not do the work yourself...he has other things going on and needs a reliable supplier that doesn't need his hand held.

 

[MikeHalloran]

I hate to say this, but it's probably time to change molders.

I've had lots of "Supply Chain Managers" who yanked molds from long time reliable suppliers and sent them to, well, radiator shops who were branching out into molding because business was off, and offered a better price because they didn't know crap about what they were doing with a molding machine.

Every damn one of them produced crap on the first try, insisted it was the mold's fault, and extorted huge sums to rebuild the molds. ... after which they mostly still produced crap. ... until they had gained enough experience, at our expense, to have a clue what they were doing. ... at which time the price went up.

In this case, I wouldn't try to salvage the mold. I'd send out the print for quotes and buy a whole new mold and set up production with a new vendor. ... and just let the current vendor starve until he learns a little more, at someone else's expense.



Mike Halloran
Pembroke Pines, FL, USA

 

[bngneer1138]

Thank you mike. This forum has already proved itself very useful.I am glad that you have a similar opinion as I do. Unfortunately, I think think my friend is having the same thoughts, but it is hard considering the time and money he has already invested.

Much appreciated.

 

[MikeHalloran]

Note to all: bngneer has started another discussion in a more appropriate forum:

nylon 6-6 poor impact strength !
thread712-352030

Please make further contributions there.


Mike Halloran
Pembroke Pines, FL, USA