Issue about position of flow tab with respect to plastic flow and location of weld line

[akshayashok]

Hi All,
This is my first post to the forum. I am a mechanical engineer designing plastic enclosures for electronic security systems. In a recent project for a plastic enclosure for a remote controller we are seeing cracks starting at specific locations of weld lines. The failure varies from batch to batch but we are trying to find a way to minimize that. Any guidance will greatly help.

I have uploaded one picture with the post and detailed information is below. We have tried and investigated some solutions. preliminary results can be provided by separate email.

The material used: PC-ABS - we can switch to ABS to reduce chances of brittleness of material.

The number of gates used: 1 at the far end south of the big hole. bottom of the remote plastic component in picture.

The gating is on the inside,

There are cams on the inside to make snaps

I have plastic flow contact angle, fill time and weld line location simulations done from our molder in asia.

We tried to use flow tabs to reduce the effect of weld lines as per suggestions on this webpage:

http://steinwall.le8.getliveedit.com/pages/WeldLines

and in another document (can be provided by email). But the results vary based on where we connect the flow tab to the main plastic. we need somebody to help us make the decision. The flow tab is being placed inside the large hole.

Please let me know if somebody can help.

With Best regards
Akshay

 

[Pud]

I think your problem lies with the countersunk screw. Countersinks are a very, very bad idea in plastics - the "wedging" action, normally off centre is not good. Counterbores should be used.

A hot tool will assist with weld lines (70° - 90°C), but generally, they are designed in, not moulded in.

H

www.tynevalleyplastics.co.uk

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

 

[akshayashok]

Hi H,

What you see in the picture is not a counter sink screw hole but an opening for the LED for the remote. We can't remove the chamfer as it is part of the industrial design on the product. I need to work around that hole.

With Best Regards
Akshay

 

[Paulkuklych]

Since the quality varies from batch to batch does that mean there are batches that do not crack?

You might have too much variation in the process such as inadequate material drying before processing in the moulding machine. Different fill times and temperature variation in the material can also cause failures if the change is too large.

I suggest the first thing to do is moinitor the injection molding process in order to identify any variation and then eliminate as much as possible.

Additionally, I agree that a flow tab be added just be sure to vent it so plastic flow is not hindered by too much air entrapment.

Also, as already mentioned by Pud, a warmer tool will assist with weld line strength.

I would be interested to see how your weld line strength changes with the position of the tab.

Paul Kuklych

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

 

[GilPolymers]

Hi,

Have you consideres to change the gate location? Probably just by offsetting the gate you will manage to move the weld line to a less critical area.

I agree with the comments before, if the issue is not constant then probably has something to do with processing or resin variations.

GP