I want to ask questions regarding MFI value and degrading of pc/abs.

Virgin PC/ABS's MFI value is 7 and PC/ABS molded part's value is 10. My customer insists on that increasing from 7 to 10 shows that there is a problem with my process and material was decomposed.
In my opinion,  MFI testing is not enough by oneself to judge if the material is degraded or not.
Therefore i have it made some testing at the university polymer labaratory.
Performed testings are as follows;
***GPC (Gel Permation Chromatography)
***DSC ( Differential Scanning Calorimetry)

According to the results of these GPC and DSC testings there is no degradation.


I kindly want to ask, if MFI increasing from 7 to 10 absolutely indicates that decomposition has occured?

And MFI testing is enough by oneself for describing the decompositon?

If the MFI increases substantially the molecular weight has been decreased. I was of the impression that for moulding grades of polymers, DSC was mainly used to determine crystallinity and crystalline melting points. I don't see where this is any more useful than a very cheap and simple MFI test to determine degradation in the range that would be expected during moulding.

Even the best processing will cause some degradation.

PC/ABS is prone to decomposition if it is overheated, kept to long at melt temperatures, is not dried properly prior to moulding, is overheated or kept at temperature to long during drying, or is subject to to much shear during moulding.

How much loss of molecular weight is acceptable is arguable, but 7 to 10 seems a lot if processed under ideal conditions and for a flow path that is not excessively tortuous. Degrading from MFI 7 to MFI 10 might be unavoidable if the flowpath is difficul.

This is complex. First, DSC won't tell you anything. The MFI increasing points to loss of molecular weight but the change you report is not enough to worry about in my opinion.

My best guess as to why it's brittle would be that the polybutadiene rubber in the ABS has degraded. That is by far the most heat sensitive component in there. You won't see any change in the rubber by GPC because the rubber is cross-linked and insoluble.

I've worked quite a bit with ABS and it does change properties even due to one short extrusion.

8 min is a long time in the barrel.

0.6mm is very thin unless it is a small part with short flow path. This may well induce stress in the moulded part.

40 deg C is a fairly cold mould.

I would try a mould temperature of 60 deg C and maybe reduced melt temperature.

I would also seriously consider trying to use a machine with a considerably smaller shot size.

I would try adjusting temperature profile to minimise degradation, but I cannot comment without seeing screw size, current profile, shot size, screw profile, screw speed, back pressure and if any temperature controllers are currently overriding. To be honest, one really needs to be there to assess that aspect.

Also is it holding a cushion and is the cushion small but consistent, like about 5mm

Are you adding anything, like a masterbatch or lubricant or mould release.

How do you know you are drying it thoroughly. One of the most common problems when moulding PC or its alloys is inadequate or defective drying despite the moulder being genuinely convinced that he dried it properly.

Excessive shear is a very common problem when moulding PC/ABS.

Overheating by excessive shear in the compression zone of the screw as indicated by the temperature controller over riding is a common cause of degradation.

A leaky ring check valve can also cause excessive shear and local thermal degradation.

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