Soft TPU sticking in sprue bushing

[PlasticMan]

We recently built a few tools that use 75A TPU. All of them have the same issue: the sprue sticks hard in the sprue bushing. Has anyone ever had experience with acid etching or similar process to the sprue bushing to give it texture so it doesn’t stick so hard?

[JayDub]

Have you done a gate-freeze study to be sure you aren't over-packing the sprue & runners? For small urethane parts we use very short (typically 2 -4 seconds) pack times and the sprues generally have a lot of sink/voids so they don't grip the bushing.

[PlasticMan]

Yes, we have done a gate freeze study and the sprue still sticks in the sprue bushing while it is molten...then at 1-1.5 second pack it becomes so hard to get out you have to use pliers. Back down on pressure and you get sink.

[JGentry]

What size is the orifice of the nozzle tip versus the orifice of the sprue bushing on the mold?

[rickbatey]

Have you had the sprue bushing vapor honed? Is there a good cold slug under the bushing and an ejector pin to help pull it out of the mold?
I ran a PUR job once and it took 4-5x the pack/hold time as it did for cooling/recovery. Without it you had sticking sprue and sinks in and the parts.
Rick.