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Author
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Topic: Colour Separation
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Ozducati
Senior Member Posts: 6
From:Brisbane, QLD, Australia
Registered: Apr 2003
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posted April 10, 2003 06:36 AM
   
We are experiencing a colour separation problem with white injection moulded HIPS components.
The problem manifests itself as a small dark line which usually, but not always, occurs on the same region of the moulding. Closer examination of the dark line reveals that it is actually an abscence of white pigment.
Increasing screw speed and backpressure, to increase the shear & mixing only make the problem worse. Reducing these two parameters eliminates the problem but the increased charging time and subsequent increased cycle time is a penalty.
The IMM is fitted with a barrier-type screw (this is the IMM manufacturer's standard kit). I suspect that excessive shear over the barrier flights could be causing a separation of the masterbatch from the natural HIPS, possibly due to differences in the MFI of the masterbatch and the HIPS.
Does anyone have any thoughts on this or perhaps experience with a similar problem?
Any help here would be greatly appreciated.Rod. IP: Logged |
zabielski
Senior Member Posts: 128
From:McHenry, IL USA
Registered: Nov 2002
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posted April 10, 2003 07:24 AM
The 1st thing I'd check is to ask what the base carrier is for the M/B being used. In most colorants, a high MI LDPE is generally used.I'd also do a trial with liquid colorants as well. IP: Logged |
Tom C
Moderator Posts: 115
From:Wharton, NJ, USA
Registered: Jun 2001
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posted April 10, 2003 10:03 AM
I think I recall reading about flow fronts containing less pigment, and therefore weld lines can show up sometimes. Kind of like the surface of the molded part being resin rich which gives it a glossy appearance. I would think that weld line cures, like hotter mold surface, would help.I doubt it is related to the screw design. Tom C
IP: Logged |
Ozducati
Senior Member Posts: 6
From:Brisbane, QLD, Australia
Registered: Apr 2003
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posted April 10, 2003 08:37 PM
Thanks for your reply, Tom & Zabielski.I have just checked out some samples with the mark on them and in most cases the dark line is in close proximity to the weld lines. I have asked the guys to raise the mould temperatures. This shouldn't affect the cycle time since the shot weight is about 2.8kg (6.16lbs) but the component only has a 2mm (0.079")wall section, therefore the overall cycle time is dependent on the charging time rather than cooling time.
We have been in discussion with the masterbatch supplier for quite a few months now regarding this problem, and have trialed a number of different masterbatch formulations, including different carriers. These have all been to no avail. I will keep you posted on the results from the increased mould temperature. Regards, Rod. IP: Logged |
Ozducati
Senior Member Posts: 6
From:Brisbane, QLD, Australia
Registered: Apr 2003
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posted April 10, 2003 11:10 PM
Thanks for the adviceIncreased mould temperatures have significantly reduced the appearance of the dark lines!! We are now running with a reduced cycle time as we have been able to increase the screw RPM and still keep the problem at bay. Thanks again!! Regards,
Rod 
[This message has been edited by Ozducati (edited April 11, 2003).] IP: Logged |
zabielski
Senior Member Posts: 128
From:McHenry, IL USA
Registered: Nov 2002
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posted April 11, 2003 07:15 AM
Ozducati: Thanks for the follow up o your results, as that is key to this forum, and will most likely help others as well.IP: Logged |
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