Home » Injection Molding FAQ » Prevent silver streaks of injection molding

Prevent silver streaks of injection molding

With respect to suck back/ decompression, other than decreasing to rule out air introduction into the nozzle, also check that the final screw position before each shot is repeatable. A variable gain control on the valve (hydraulic system) can cause a large variation that can be seen by the screw position repeatability after suck back. If at times the suck back is not enough then drooling can cause material solidification at the nozzle interface. You may see this as silver streak as the initial injection pressure may spike slowing screw speed initially until the pressure overcomes the cold slug and blasts the material thru the gate causing silver streaks and burning as you have occasionally noted.

I am not sure what the manufacturer specification is for PC + ABS, but for Polycarbonate the minimum specification is 0.01% moisture to prevent silver streaks and molecular weight loss. Depending on the initial moisture which can be a max of 0.2%, the typical recommendation is 120C @ 4 hours and a -40F Dew point using a desiccant drier. If you are pushing hot humid air thru the material, it will not dry properly.

Nozzle misalignment or interface/ seating wear can also have the same effect as well as a poorly designed, screw, compression section or faulty check ring. For the check ring, I would check to see the screw final position at hold stage. You should have a sufficient cushion to allow repeatable shot size and hold pressure change, but not so much to have a high residence time of material in the screw, depending on your injection molding process conditions.

Too many times instead of getting to the root of the problem, folks just increase the decompression. This is also done for a more positive check of the check ring for a more consistent cushion, instead of specifying the correct stroke on the tip assembly or replacing the worn pieces. There are usually two parameters, which are the stroke of the decompression and the speed of the decompression that can be adjusted. In most cases we find that root cause for most things is operator error, whether it was the process or the equipment that wasn't properly addressed. Next we look at the material. The bottom line on decompression is that we should always use the minimum required and only when necessary.

Post a Comment:

    
Plz Calculate (8 - 9) =
(Your comment will show after approved.)

You may also like:

Featured Articles
Plastic Injection Molding Process ... Plastic Injection Molding Process SimulationThe propositions behind injection molding process simulation are very basic. First, that although our plastic materials and ...
Visit injection molding factory in ... Visit injection molding factory in China before closing the dealNot being in the injection molding industry, I was looking for tips on how to judge a company or factory on their capability or ...
Injection Mold Cooling Design Injection Mold Cooling DesignThe design of the injection mold cooling system is very important. The cooling time takes up 70% to 80% of injection molding ...
Plastic injection molding hot runner ... Plastic injection molding hot runner systemHot runner is a set of temperature control heating-device, installed in the plastic injection mold, it is part of the mold. A Hot ...
How to choose plastic injection mold ... How to choose plastic injection mold steel?Hardness and wear resistance Surface roughness, dimensional accuracy of plastic parts and injection mold life heavily rely on the ...