Injection molding- cooling time vs. melt cushion
Injection molding- cooling time vs. melt cushion
(OP)
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,
thanks,






RE: Injection molding- cooling time vs. melt cushion
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
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.
RE: Injection molding- cooling time vs. melt cushion
Regards
RE: Injection molding- cooling time vs. melt cushion