Injection does not press HDEPE into mold – help!

Hey guys!

We stumbled upon a problem with our precious plastic injector:

We worked with HDPE plastic and shredded the plastic is small pieces of 1 to 4 mm.

We tried :
1. Heating the injector till warm according sensor on 180 degrees. Then added mold and plastic. Waited for 20 min to melt plastic. Then applied pressure. Plastic was still too thick.

2. Tried higher temperature of 220 up and 240 down and filled it into the injector. Again the plastic was too thick and didn’t go into the mold.

3. The third time we heated the plastic even more up to 260degrees down 240 down celcius and it still was not liquid enough to inject into the mold.

What we already did:
The old was off to see if plastic drops down, once it came a little bit out ,we added the mold.
We used more plastic to fill the whole tube up.
We waited longer.
Also we made the arm longer to have more leverage.

What we want to do now:
– Thermic isolation – any suggestions?
– Second bar
– We want to reinforce the machine so that we can apply more pressure

We are incredibly thankful for any help!
As we want to present the machines at a school and on a festival next week… Time is running… Thank you TRIBE <3

Lots of love from Costa Rica!

@lila we run ours at 263 barrel and 265 nozzle temp, preheat the mold w/ heat gun or LP torch and it still requires a lot of pressure.  Even then, I dont like to press out anything more than 6 in volume.  I’m taking some time this week to mod ours so that it’s a little easier to operate at public demonstrations.  would love to know if you made progress with your design mods

My desktop machine also uses a 4 mm nozzle and works with HDPE.

If your control thremocouple is close to the heaters, you may be controlling more on local heater temperature than on the plastic volume and the HDPE is actually cooler. It may be worth getting a separate thermocouple and readout that you can use for diagnostic measurements. They are very low cost.

Are you compacting the plastic while it is heating up? I do that (with the nozzle blocked) a number of times during the heatup to get the air out of the plastic and to make sure it is uniformly heated.

If your mold design has a narrow channel and lots of aluminum mass near the entrance, cartridge mold heaters are very low cost, a heat gun can also help.

For some molds, make sure the injection motion is quick and continuous so you can get a plug solidifying near the entrance.

Good luck

@andyn, could you add some pictures of your nozzle/mold connectors ? this stuff should be definitively in the PP v3.1 manuals 🙂

Are you sure the plastic is actually reaching the temperatures you are measuring? 260+ °C sounds too hot. I notice discolouration/burning much above 230°C. Do you have the heater as close to the nozzle as possible? Maybe add some insulation over the top. Have you tried heating the mould, this can make a big difference?


HDPE is a lot thicker than eg. polypropylene but I have no problem injecting it through a 4mm nozzle with a small manual machine.

here the ‘plug’ we use (extrusion,injection), it keeps the distance from heated barrel to the mold to a minimum, M20. Needs a lathe…  If you have such equipment you can also make the mold connector perfectly fitting in a heatband, this section seems pretty sensitive, a major bottleneck so to say but having a M20 Nut on the mold works fine too.

a problem you can check is the mold connector not being on temperature. in this case you experience exactly this problem, nothing moves, that was the reason why I switched from standard outer thread (done with that heavy plumping thread cutters, for nearly 80+ crap Euro) on the barrel to a M20 thread plugged onto the barrel (2.5 cm stickout, press-heat-fit); that way you can have a heatband partly covering the mold connection.

would be happy to know other solutions too 🙂

but yeah, a foto would definitely help !

What does the mold look like? What are you trying to make?