ABS PC cracking after few days assembly

[cindy518]

Dear All,

We are current have the problem with the plastic cracking issue after few days assembling, we don't know the cause of this issue, attached the some picture, hope some experts can answer my question.

- Plastic we are using is PC/ABS C2950, Brand: Sabic
- Drying material time: 3.5 hrs
- Dry material temperature: 95 degree
- injection machine temperate: 220-240 degree
- Injection pressure: 86, 60,85,70
- Injection speed(mm/s): 28,15,45,25
- injection position (mm): 88, 86.5, 65,0
- Packing pressure: 85
- Packing time: 2
- Back pressure : 15%
- Cooling time (s): 18

If you know any cause and solution, please email me: cwu0518@gmail.com, Thanks for your help in advance!!


Cheers
Cindy

 

[Paulkuklych]

There will be a weld line next to the screw hole where the 2 flow fronts meet during filling. This is normal but you will need to improve the strength of it by increasing injection speed or adding more pack pressure or study the screw fit - maybe too much interference with the part.

Paul Kuklych

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

 

[cindy518]

Dear Paul,

Thanks for your quick response.

This is a water proof product, seal with silicon o-ring insert and screw, screw size is M2*4.5， screw head diameter=5.0mm, silicon insert outer diameter 4.5mm, inter diameter 2mm. thickness 1.5mm. We found the plastic got a lot of stress from the screw areas. We are thinking if the silicon insert size is improper with this screw as well.

Attached picture for your reference.

Many thanks for your help in advance.

Cheers
Cindy

 

[Demon3]

Cracking is probably due to stresses locked in the plastic plus exposure to chemicals like oil, detergents, cleaning agents.

Chris DeArmitt PhD FRSC
President

Plastic materials consultant to the Fortune 100

http://www.phantomplastics.com

 

[rickfischer51]

Are those flat-head screws? If so, the tapered underside of the head will try and spread the hole open like a wedge. Over torquing will put a hell of stress there, that may result in creep rupture in a short period of time. Note that you are getting two cracks roughly opposite of each other. This may not be a weld line problem, although a weld would certainly contribute to it. Find your weld line by visual inspection, with a short-shot series, and/or with mold filling simulation.

If those are pan head screws with a flat underside, is there a proper o-ring gland that contains the o-ring and controls crush on the o-ring? If not, the o-ring might be contacting the inside of the hole and applying what is essentially an internal pressure. To a first order approximation, the o-ring can be treated like an incompressible liquid.

Are you controlling torque during assembly?

Are the cracks consistantly in the same places? Are they at an internal feature like a web or gusset?

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[Pud]

Rick, I guess you meant are they countersunk screws? If so, then you are quite correct in your observations re virtually a line contact stress. (The rather poor picture looks like they are countersunk head).

I would suspect it's not a weld line either. Position seems wrong for that - I would expect one crack.

I get fed up with getting designs which show countersunk screw holes! And these are from "professional" design houses!

H



www.tynevalleyplastics.co.uk

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

 

[CoryPad]

Rick and Pud are both half right, the head shape in question is referred to as "flat countersunk" in standards such as ASME B18.6.3, ASTM F879, etc.

 

[chicopee]

I am in line with over-torquing the screw and probably lack of reinforcement around the hole under the panel. Poorly designed if there is no reinforcement on that corner.