Injection molding- cooling time vs. melt cushion 1

[henks580]

My company uses a reciprocating screw set-up for injection molding of nylon 66. I have noticed that varying the cooling time parameter on the injection unit affects my melt cushion. What is the relationship between these two, if any? I thought the cooling time allows time for the injection unit to fill up for the next shot. Obviously, the cooling time also affects the properties of the molded part. However, I cannot see the relationship between the former two. Can anyone shed some light?

thanks,

 

[patprimmer]

The cooling time is exactly what it says.

It is the time taken for the molten material injected into the mould to freeze prior to ejection.

If you are loosing a very small amount of cushion, this might be due to material packing the mould as the material contracts as it cools and solidifies.

You can check this by weighing the parts. If a loss of cushion with time matches an increase in part weight with time, this is pack. If the loss of cushion is larger than the increase in weight, you have a leak somewhere. If no material is obviously leaking from a spot such as between the nozzle and the back of the mould, you probably have a leaky check valve at the front of the screw.

Nylon 6.6 is especially sensitive to leaky check valves, and glass filled or flame retardant materials quickly wear the check valve.

A loss of pack by the screw bottoming out or using up the entire cushion will cause more mould shrinkage and more distorted and weaker parts than a fully packed moulding.

This gets a little more complicated when you also consider gate freeze off time

Regards

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[injectionmolder]

Sure there is a relationship between cooling and the cushion. On semi crystal resins, we can increase melt temperature by decreasing screw rpm. As for amorphous polymers, melt temperature increases with higher rpm. Nylon is a semi crystal. With that said, will pretend a recovery time of nine seconds into a cooling of ten seconds. Nevertheless, we see an opportunity to run faster, and now we end with five seconds cooling. Well along with that change, the recovery time must be shorted lets say, down to four seconds. (Pretending that parts are dimensionally fine and there are no other changes to the process.) What we are doing by now, is not only running fast. Nevertheless, we are decreasing melt temperature by recovering faster in order have a shorter cycle. At this time we are increasing viscosity in the molted plastic. There are two things in play: 1) A colder melt will not be packed into the cavity as easy as hot melt will be, at this time pressure might not be enough, to pack the molded part due to the grater viscosity. That could drive the cushion higher. 2) If the check valve is slightly by passing upon injection, now that viscosity is higher therefore it will by pass less, then the time when the polymer was hotter, “cushion will be higher.”

Regards

 

[henks580]

thanks, guys for being so helpful. I am now more comfortable with this issue. FYI the root cause of my problems was not in the material, but rather on a leaky screw tip and calibration of parameter tolerances on the controls. It seems to be working fine now.