Injection moulding with polycarbonate

[dogbural]

Hi,

Lately, we had an issue with the polycarbonate part being moulded.
There is a crack after tapping the material. My understanding is that polycarbonate is very unlikely to be cracked.
So I would not think this is machining issue.

My question is that how I can check quality of moulded part.
By looks of it, it seems okay as in no flow mark or sink mark.

Is there any way to section them and view the section in microscope?
Or would we see any joint line inside using x-ray machine?

This issue happened after making a small modification to the die. During our first off-tool sample, we only checked visually and did fitment check.

Any advice is much appreciated.

Thanks.

 

[cowski]

Are you drilling the hole and then tapping it or is the hole molded into the part?
If the hole is molded, there will be a knit line roughly opposite the gate. This could be a point of weakness.

Also, PC doesn't have good chemical resistance. Are you using a cutting fluid during the tapping process or other chemical to clean the surfaces after the operation? If so, make sure that it is compatible with PC.

 

[3DDave]

Use a polarized light source on one side of the material and examine the part through a polarized filter from the other side.

The strain in plastics from molding leaves molecules aligned and that alignment will interact with the polarized light.

 

[dogbural]

@ Cowski,

We are drilling a hole and tapping it.

We use HD fluid, it was confirmed with manufacturer that it should not be an issue.

@ 3DDave

That means I need to test with parts moulded with clear material ??

 

[3DDave]

Ah, my mistake. All the parts I have ever seen of polycarbonate have been transparent.

If not, then you are using filled or dyed material which may alter the properties.

 

[Pud]

Likely causes of brittle PC mouldings:

1. Material not dried correctly.

PC should have 0.02% max water.
Surface marks only show when the content is well over the stage at which hydrolysis has occurred.
In my experience over the years, either a dessicant or vacuum dryer is required. Hot air dryers don't work very well at all with PC.

2. Tool surface temperature too low.

Tool surface temp needs to be at least 80°C. Hotter better. 90-100°C ensures minimising internal moulded-in stresses.

3. Pigments

Is it black PC, bought as black grade?

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[dogbural]

@Pud,

1. We are using air hot dryer (Arburg Thermolift 100kg).
Can you please elaborate a bit more as to why this dryer does not work at all with PC?

2. We are monitoring surface temperature of die.

3. Pigment
Is not black.

 

[Pud]

Dogbural,
Hot air dryers (even Arburg,;which are good hot air dryers), are not too good on PC. You need to calculate the machine throughput to check if the PC is actually at +120°C for at least 4hrs. Have used several hot air dryers on critical parts and eventually ended up with vacuum dryers which would dry 15kg every 35-40mins.

Good you're monitoring surface temp.

If you're using a universal masterbatch for colour, get a polymer specific version made. Or dry colour for trials.

Don't forget with PC you also need minimum holding time to reduce residual stresses.

I would also suspect the coolant. Can't quite see why you need any tbh. Ignore coolant supplier. Treat anything they say with caution. They want sales!

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[dogbural]

Thanks Pud,

Just update

1. We did some sample tests on machining moulded parts with using 2 different lubricant agents - HD Cutting Fluid and just air

2. Right after cutting them, there was no difference (meaning no crack).
However, to accelerate againg time, put them in oven at 80 deg for 4 hours. Then found out that the material machined with fluid experienced crack.
According fluid composition, it has

Glycerols
Distillates (petroleum), Hydrotreated light maphthenic
Methyl Salicylate
Chlorinated Paraffin

Any advice on this?

 

[Pud]

Polycarbonate suffers badly from environmental stress cracking. (ESC)
Oils, fats, solvents etc. You name it, it'll crack it.
Do you need coolant? Tried water?


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[dogbural]

No, we haven't, now we are just machining with air. Did not want to take risk.