ABS / PC components cracking - Please help me :)

[p0etter]

Hi guys, thanks for taking the time to read this...

I have a problem with my parts cracking.

The parts are fine before they have been assembled but after assembly the cracks appear. Sometimes this can happen in a couple of hours but sometimes it can take days. The part in question is kind of a rectangular tube with a 1.6mm wall thickness which then has an end cap pressed into the end. The cracks develop right in the center of the shortest edge of the rectangle exactly where the end cap is fitted.

On first inspection of the parts it seems it has been designed with approximately 0.1mm of interference on all sides so I was assuming that this is a major factor, however, I have since read that the ABS PC blends tensile strength should cope with this. Am I correct?

Also the inside of the moulding is exposed to a slightly increased level of humidity. Can ABS PC be affected by hydrolysis or is that just a PC problem? And could this be another factor?

Any other ideas at all?

Many thanks.

 

[Demon3]

Hello,

ABS is not affected by humidity. The PC will not hydrolyze either under normal conditions.

How is the tube made? It is injection molded or extruded? If injection molded it may be that the injection point is where the failure occurs because stresses get locked in there when the part cools. You could perhaps solve it by changing molding conditions. Or, change from injection molding to extrusion as that avoids the injection point altogether.

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500:

http://www.phantomplastics.com

Webinars on plastics, fillers & impact modification:

http://www.plastictraining.com

 

[p0etter]

Hi,

Thanks for your response. It is injection moulded and there are a few other features on the part which means extrusion is not an option.
I will have to check where the injection points are. Thinking about it though there are 2 rough looking rectangles in that area which are perhaps 2mm x 3mm in size and look like they have had tabs broken away from them. Do they sound like injection points?

 

[MikeHalloran]

The part of the mold where the liquid plastic enters the cavity is called a 'gate'. The same word is used to refer to the corresponding scar on the part where the part is broken off the 'tree' after the solidified part and tree are ejected.

Pressing anything into a plastic tube is likely to split the tube, eventually, and the interference dimension you've reported is excessive for parts of modest size. Rectangles absent rounded corners are worse.

Have you considered solvent bonding of the end caps?
Mechanical retention, e.g. with molded cantilever latches?
Having the cap fit over the tube, as opposed to inside it?






Mike Halloran
Pembroke Pines, FL, USA

 

[p0etter]

Hi Mike,

I thought the interference seemed excessive. The method of retention previously used was in the form of molded clips which snapped into recesses in the tube, however, when the product was drop tested the end cap shot out. That constituted a failure so the interference was added. So now it passes the drop test but will crack all by its self while sitting on the shelf! I'm not sure solvent bonding is possible because the products internals are very sensitive to solvents and would be damaged due to exposure. Oh and I forgot to mention that the rectangle has indeed got rads on the corners. The rectangle is approximately 40mm x 35mm with 8mm rads.

It seems like a combination of too much interference and an "unfortunately placed" gate.

 

[Pud]

The tool surface temperature should be from 70°C to 90°C - the higher the better to reduce residual stresses. Unfortunately, it's pretty easy to mould PC/ABS at a much lower temperature. Drying prior to moulding will also be critical. Around 100°C, depending on grade. Moisture much less than 0.02% required - less than would produce visual cosmetic defects.

Is the crack developing at a weld line position? (opposite the gate/gates?)

H

www.tynevalleyplastics.co.uk

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

 

[p0etter]

Hi Pud,

Ok thanks. I'll have to find out more about the process and see if there are any variants from what you are saying.

I'll have to have another look at the part tomorrow but i'm pretty sure the crack develops through the center of where the gate is located.

 

[Demon3]

Can you make the cap of a plastic that's soft so it grips on the end without exerting much pressure on the tube? That would help a lot. Also adjust the molding conditions as described. Mike's idea to have the cap fit over the tube is good as then you would have a compressive force that would not break the tube.

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500:

http://www.phantomplastics.com

Webinars on plastics, fillers & impact modification:

http://www.plastictraining.com

 

[MikeHalloran]

I wouldn't give up on the molded clips just yet.
If, instead of snapping into recesses in the tube, they snapped right through holes in the tube, you could make the assembly virtually inseparable. ... especially if the transversely oriented faces of the holes and the clips were, uh, transverse, with essentially no draft, so that once the tabs snapped through the holes, no amount of axial force on the end plugs would produce any radial force component on the tab/pin part of the clip.

Something like that may, or may not, be a minor modification to a mold that already exists.



Mike Halloran
Pembroke Pines, FL, USA

 

[btrueblood]

Mike's last idea was my thought too - like the caps on tubes that some integrated circuit components ("chips") come packaged in.

 

[Dougt115]

Check your radii at the corners. If the outside radius of the cap is larger than the inside radius of the square tube your interference may be greater than you thought.