Melt Temperature ?

[Teunis]

What is the effect of a (too) low melt temperature of PolyCarbonate ?

I have structural molded part. It doesn't break (short term) and its dimensions are ok.
But I have found that the melt temperature used of the PC is around 245 degrees celcius. It should be (if I am correct) around 300 degrees.
Based on my tests and dimensions I should just approve the part, but I think I should make a point of the melt temperature. Do I have a point ?

 

[patprimmer]

How are you measuring the melt temperature?

Appropriate temperatures are very dependant on the grade of material, the mould and the machine being used.

245 deg C is low but not impossible.

If the temperature is to low you might get moulding faults like:-
Flash near the gate, but short shot at the extremities.
Moulded in stress, leading to stress relieving at a later date, by warping or by stress cracking.
Flow marks, especially ones that look like the groves in a vinyl record, or like horizontal folds in a curtain.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

Hi Tuenis,

I suspect from your post you may be confusing processing temperature with melt temperature.

PC, in common with all amorphous plastics does not have a defined melting point. It starts to go "rubbery" around 140 deg C, and as the temperature rises it gradually gets more liquid, until at around 320/340 deg C it starts to decompose and turn yellow/brown.

PC should, for the reasons that Pat has indicated, be processed around 290 - 300 deg C melt temperature.

Like Pat, I would be interested in how you arrived at your figure for the melt temperature.

Rgds


Harry

 

[Teunis]

Hi Pud,

I have the moulding data sheet from the Chinese moulder.
It has temperatures measured on three points :

Temperature zone 1 250 deg(celcius)
Temperature zone 2 245
Temperature zone 3 240

It doesn't say where these temperatures are measured at.
I cannot find processing temperatures anywhere on the sheet.

Anyway :
The "specialist" at our purchasing department told me the above specified temperatures were on the low side. According to him these should be around 300 deg C. That's why I assumed the temperatures were melt temperatures. (I am not a specialist)

As Pat has allready mentioned and what I wanted to know, what the results of a too low temperature were.
Especially the moulded in Stress are of importance to me.

Is there a test to see if there is any moulded in stress in my part ?

 

[patprimmer]

The 3 points will be the barrel temperatures from the feed zone to the nozzle. They may be the set points or the actual measured value, which may be different to each other due to overriding from shear.

Barrel temperatures are not all that accurate an indication of melt temperature due to variations in heat transfer due to variations in shot size and cycle times, and due to shear heat generated in the material from the physical action of the screw.

Having said that 240 to 250 is a bit low. 270 to 280 is more typical, but I have seen good mouldings produced with a set point as low as 240 in some cases, and as high as 320 in others.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mysundance2000]

There is no way to know what the melt temp was for the above reasons. To complicate things more, Ive measured actual barrel temps up to 100 degress F higher than what the machine (bad thermocouple) said it was.

Also I think molded in stresses are as much related to mold cooling as they are to injection temperature.

Options: A large engineering plastics MFG. tech services might analyze a small random sampling of parts. Or, mold some parts at ideal conditions then compare mechanical testing with your suspect parts.

Mike