Injection mold piece of Lexan breaks 2

[Teunis]

I am trying to gather as much information as I can. Therefore this mail :

I 2 have kinds of injection molded pieces of Lexan 143 R.
The color is black.
From both parts I have 40 samples from the injection molder.
2 out of 40 samples tend to break very easilly (britle break), without the normal extreme deformation the material should have first.

What could cause this breaking of the material?
Is it permitted for the material Lexan to break like that, or is it that the injection molding parameters are not right ?

Any help appreciated !

 

[Pud]

Hi Teunis,

in order of probability:

1. Material not dried prior to moulding (4hrs minimum in dessicant dryer at 120 deg C)

2. If a masterbatch has been used to colour them black, the carrier may not be compatible.

3. Tool temperature too low (min 80, max 120, ideal 90 - 100 deg C) resulting in moulded-in stress.

4. Parts post moulded contaminated with oil/grease causing environmental stress cracking prior to testing (item 3 will exacerbate this)

5. Material degraded when moulding. Not easy to see characteristic 'yellowing' of parts when moulding black parts.


There are other variables too numerous to mention, but the above are the 'classic' causes of PC moulding failures.


Rgds


Harry

 

[Soupcat]

You should inquire if the moulder is using regrind material as this can affect mechanical properties. Also find out if there's a filler in the material such as glass or a mineral such as talc which cheapens the material and is probably uncall for. I'd be careful when you broach the moulding parameters subject with the moulder as a lot of companies don't like customers questioning their ability to injection mould!

Kieran.

 

[Pud]

hi Both,

Lexan 143R is unfilled UV stabilised, easy release and has no fillers (or shouldn't!!). Comments re regrind very appropriate - forgot to mention as we do not reprocess PC as parts are clear and require good aesthetics and water whiteness clarity.

Soupcat - would be quite happy to have customer's input - as long as it was useful, that is!!!

PS Talc (or any filler) makes material more expensive!!


Rgds both,

Harry

 

[Teunis]

thanks all for your replies.

I've done some research myself, and now the case is the following :

The part made of Lexan, should be fixed in an aluminium part. in fixed position, the lexan part is being elastically bent permanently. When I removed one of those parts the lexan part broke (brittle break). Now the purchaser told me it is caused by creep rupture. (The chains of lexan get oriented because of the bent position and then the part breaks, when bent any further.) I have some doubts.

My question is now :
1. Is the Lexan, or polycarbonate very sensitive for creep rupture ? Or should the part be bent very much then (plastically ?)

 

[Pud]

Hi Teunis,

PC does (as most polymers) does not like sharp corners - is it breaking due to this?

Also stress combined with oils/greases causes failure in PC - is the al. part clean?

I would expect PC to be able to bend 180 deg easily before breaking and show no signs of brittleness - so much depends on part design though.

One question I would ask is why did most parts NOT break??

imho the reply from your supplier is very unsatisfactory - bovine droppings I suspect even!


Rgds

Harry

 

[Teunis]

Hallo Harry, and others....

I am going to order transparant samples, which I should be able to check with a polarization filter, whether there are some unusual stress concentrations. I also should put a part in the oven at 220 degrees, which should even out the stress concentrations. (That's what another expert on the issue could tell me)

I am sorry to admit that the part has some sharp edges, that I know are not helping the performance. However the part does not always break at those sharp edges. (Consider it a rectangular cantilever beam bent by a force at the end. The edges on that beam would be sharp in my design).
Also I have some fairly identical parts from other companies that have the same sharp edges, and don't break either.

About the grease : From the old samples, I cannot tell anymore. But I will check with the next samples. Any tip on how to check this ?

I have absolutely no idea why the other parts did NOT break.
My guess is, that the process of injection molding is not done under constant conditions. (Hard to tell, cause the company is in China. sigh..)

I will keep all interested informed....
Thanks for your help so far !

 

[TheAnswerGuy]

Teunis,

In reference to your first post and noticing that you only have 40 pieces as samples and two of them fractured. My immediate guess is that the tool in which they were molded may not have been up to proper molding temperature for all of the parts when molded or another molding anomaly. Your molder may not have discarded enough of the first parts to come off the tool.
40 parts, is not a big run.

When we prototype components for our cutomers we supply them with 24 components or shots(from a muti cavity tool). It was common practice of mine when I was sampling a prototype tool, to set the parts in order as they came out of the tool. In that way I could visually tell that if earlier molded parts looked a little different from the later. That was usually my first tip-off that more components needed to be molded to make the 24 samples which I submitted to the client consistantly molded.

theanswerguy@tr-usa.com

 

[gambro]

I have a few points which might be of some help :

1. Black masterbatch
- if you are using a masterbatch with a Polycarbonate carrier, be sure to dry it along with the base material.
- if you are using a "universal" Masterbatch with a non-PC carrier, the problem might be due to incompatibility of the masterbatch carrier with Polycarbonate

2. Drying
As already pointed out, pre-drying is extremely important (120°C, 4 hours). Also, Polycarbonate is hygroscopic, so make sure that the time span between the PC leaves the dryer and enters the injection molding machine is not too long (no more than 15 minutes). Don't use a big hopper for small parts or else the PC will sit there too long and take up moisture.

3. Start-up
Discard the first 10-20 shots after start-up.

4. Annealing
To minimize internal stresses due to the injection molding, place the parts in an oven at appx 130-135°C for appx 30-40 minutes. Do not anneal at 220°C.

5. Stresses due to assembly
If the bending angle of the assembled parts is too high, stresses are built up which may lead to a sudden brittle fracture of the parts. This effect is called physical aging (as opposed to chemical ageing, which is usually due to oxidation). If you want, you can call this effect "creep failure" although this is not the correct technical term. If the parts that fractured were not fitted properly into the aluminum housing, bending stress maxima might habe been induced locally which to the observed fracture.

 

[Teunis]

thanks all.

De part is under constant stress, so physical aging can be an issue.
However, some (!) of the parts do break brittle at initial bending. I cannot find a logical solution to that. To my logic, if it would be a design flaw, every part should break and it should break at the same weak point (which it doesn't).
(I wish I could post a photo some parts, but I guess this site doesn't support that.)
On the other hand I have some serious doubts the injection moulder knows what he is doing.
For example in earlier samples the parts showed non-dried polycarbonate by the "flamed" surface (I don't know the real term.)
Now the new samples parts don't show the flames.
But does that directly mean that the pc granulate is dried properly ?

TIA.

 

[gambro]

Probably what you mean by a "flamed" surface is what is generally called surface splay. There are several causes of splay, all of which may lead to a weakening of the part and possible brittle fracture. Please note that even of you do not see any visible splay, the underlying effects might still be there. This is especially true with black parts, where it is difficult to see anything at all.

1. Splay (silver marks) due to local burning (burn marks) :
High shear or high mass temperature, especially just behind narrow cross-sections or sharp-edged corners. Also, tools with a hot runner system or shut-off nozzles or narrow runner channels can show this type of behavior. Reduce mass temperature, reduce injection speed, reduce screw speed. Reduce residence time of the melt by using an injection molding machine which is in proportion to the shot weight.

2. Splay due to moisture
This can be the case if the residual moisture in the material is too high or when there is moisture on tool surface. To find out if the residual moisture in the material is too high, inject the material into the air. If you see water vapor, hear a distinct "popping" noise when the melt comes out of the nozzle or the melt is bubbley or frothy the reason is improper drying.

3. Splay due to air
This occurs when air is drawn into the nozzle during decompression (screw retraction phase), especially if the injection speed was too high.

Another reason for embrittlement but not splay is a too cold die temperature. The ideal die temperature for PC is 80-110°C.

I'm pretty sure your problem is due to improper drying or high residual moisture, which can lead to a massive embrittlement in Polycarbonate, especially with "easy-flow" grades. Even if you don't see any splay this might still be the reason.

 

[patprimmer]

Due to the inconsistency, the failure is almost certainly due to moulded in stress and possibly amplified by solvent stress cracking.

This is normally due to a cold mould, and if it is only a few parts so affected, it is probably as the mould is cold at start up at start up.

It is also possible that all parts have borderline moulded in stress levels, but only a few are exposed to a solvent that will cause stress cracking.

There are a lot of common chemicals that will solvent stress crack polycarbonate.

Regards
pat pprimmer@acay.com.au
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