Overmolding or insert molding?

[klondike17]

I will appreciate someone's advice about the right method. I tried to overmold a steel part with hard plastic (ABS/PC blend). A few months later cracks start appearing. I guess plastic dries and there is no room for shrinkage. Can anybody offer advice? Maybe insert molding is the right method when one side of metal is visible...Thank you.

[JayDub]

Need a lot more information for a constructive response. In general, I wouldn't expect PC/ABS to shrink significantly after, say, 24 hours post-molding if it was just sitting on a shelf at room temperature, so there must be something in the part design, processing, or use environment that's causing problems.

How big is the metal? How thick is the plastic? Plastic doesn’t “dry”, but it does have molded-in stresses. Where is/are the gate(s) on the part? How big? Bigger gates mean less injection/packing pressure and lower stresses. Sharp corners are stress risers. Where are the cracks in relation the gates? In relation to knit lines? Do the parts have one or two cracks in consistent locations, or generalized crazing? What environmental conditions is the part seeing in the months until cracks appear? Heat/cold? Chemical exposure (and “chemical” could be as innocuous as grease, alcohol or even warm, soapy water)? Mechanical stress (bending)?

[Suhas]

Excellent response and Questions by JayDub.
The only other thing I would add is to see if annealing would help.
-Suhas

[klondike17]

The injection gate is on the part's bottom. The gate's size is small. In relation to gates, the cracks are on the opposite side. 70% of cracks are in the parting line (opposite to gate), 29% are next to opposite parting line, 1% is on the gate's side, on or close to the parting line. There is neither special condition, nor stress - room temperature, sitting on the shelve. The metal part is much bigger than the plastic around it. The wall thickness varies and it is 2mm minimum.

[Jendalf]

Hi,
the issue is in the material used.
Try to think about some filler - like glass fiber for example.
There is no need to make any other correction in the tool or process.
Let us know.
J.

[klondike17]

Thank you for your input. Will glass fiber worsen a nice, glossy finish?

[rickbatey]

I ask; did you tell preheat metal inserts? Differential shrinkage can be the root cause. But I also ask did you clean/degrease inserts prior to molding? Cutting oils normally have sulphur in them that cracks PC something terrible. If you washed them, did you rinse and dry inserts? I do think your gate is too small for this process. If you add glass, you’ll need even larger gates and YES, the surface will be gray/matte unless you run resin and tool hotter to bury it in the part. You didn’t talk about barrel usage, cycle time or melt and mold temperatures. All possible root causes.
Rick.

[Jendalf]

Thank you for your input. Will glass fiber worsen a nice, glossy finish?
yes
it depends on the % of content.

you can make some improovements by process and setup, like tool temperature for better filler orientation.
try 5-10 % and test for cracking. This content is not usually apparent on the part and it will stay glossy at the surface.
j

[klondike17]

I think there are multiple issues with this project. Having a small gate is one of them. Since most cracking occurs on the opposie side of the gate, instad of making the existing gate bigger, we can add another gate on the side where cracking is. Is having two small gates better than one large?

[ijchan223]

try to slow the cooling rate of the finish part by putting in an isolated box with.

[Mark H.]

try to slow the cooling rate of the finish part by putting in an isolated box with.

Well I've just spent about 45minutes writing up a reply to help with this only for the system to say I am not allowed and don't have permission. Oh and I'm not logged in.

really - how am I posting?

[klondike17]

Hello Mark,

Please reply via a private message if you cannot post. Your input is very important, thank you.

[JayDub]

Well I've just spent about 45minutes writing up a reply to help with this only for the system to say I am not allowed and don't have permission. Oh and I'm not logged in.

really - how am I posting?

I've had issues where if I take too long to compose a reply the system times out and logs me out. If I have more than a couple of sentences to type, I'll do it in a Word doc, then log in and cut'n'paste into the thread.