Melt Pressure before Transfer

[The Rookie]

Hi all!

We had an applications engineer in last week for a new mold and machine we are working on. Upon viewing our current process, he recommended we adjust our injection velocity in order to get a more stable melt pressure before transferring.

We are seeing slight waviness at the open end of our tubes, and he thinks this could be a solution. So I tested it out, and I do believe that the tubes look less wavy, but others in my office disagree that we should be transferring with the melt pressure leveling off. Any thoughts?

I am also still working on the pack stage as well. This is a valve gated hot runner mold. I am seeing the cushion spike at the end of packing, due to plastic pressure pushing the screw back. My current back pressure is 650 psi but should I go higher to prevent this? (molding PET)


Below are links to view pictures, I know they are hard to see on the phone or in the post.

Injection: https://drive.google.com/open?id=1ecQ2HhmwEojvV7pkKASwt0DaKiA_2JQ9
Pack: https://drive.google.com/open?id=1RyQwVDJI8EriyMNgxdFCn0sl0yOaL0zr

[Jendalf]

Hi,

a very nice article about speed before switchover and also about packing / holding phase has Suhas at his pages at Fimmtech. Check that ist worth it.
BR
Jendalf

[JayDub]

My work computer won’t let me open your links, so I’m just going off what you wrote.

“We are seeing slight waviness at the open end of our tubes” – parts with a high ratio of flow length to wall thickness are difficult to fill and you do need a high injection speed. You may need to profile the injection velocity to avoid gate blush.

“others in my office disagree that we should be transferring with the melt pressure leveling off.” If the pressure graph is flat-lining, then you are pressure-limited and you aren’t getting the speed you want anyway. Can you raise the high pressure limit?

“I am seeing the cushion spike at the end of packing” – If the screw isn’t moving during pack, then the pack pressure applied to the back of the screw is balanced by the plastic pressure ahead of the screw. When the pack pressure cuts off, there is no pressure at the back of the screw but there is still plastic pressure ahead of it. Given that pressure divided by area is force, you now have a force on the front of the screw that will push it backwards. This is just physics. Other things being equal, increasing the pack pressure will make the screw bounce further, whether or not it does anything for the parts.

[The Rookie]

Thanks Jay! The images show the profile for the injection phase and then the pack that might be useful for you to view at a later time.

High Injection Speeds - The faster the injection speed, the more waviness I am seeing in the tubes.

Pressure limited - We are not pressure limited, but I am using an injection profile to slow the screw down before transfer thus the melt pressure curve begins to flatten out. I will say though, I’ve Pressure limited the process with melt pressure monitoring and the tubes actually looked great. If I let the machine transfer at 20mm at 16,000 psi they look wavy. But if I set the limit to 15,000 and force the machine to transfer earlier, say 25mm, they look great

Screw Bounce Back - So I found a mistake in my packing profile. I had the pack pressure taper down to zero instead of holding constant at the end. Once I set the machine to pack at 10,000 psi for 1.5 seconds, that bounce back went away. Initially it was set at 10,000 for like 1.25 seconds and then going to 0 for the rest of the pack. I was following instructions in their manual for setting up the pack profile and I misunderstood the directions.