Mold active cooling

I posted in the discord, but will repost observations here.


I am making grain elevator buckets from recycled HDPE.


I shred the plastic and extrude in a v2 extrusion machine with the compression screw upgrade.


Melted plastic at 205c exits the machine and into a air turbo cooker where it is kept hot until enough plastic is there to make one item.


That piece of plastic int he trade is called a “slug”  If a piece of plastic is going into a injection mold, that is called a “shot”


Plastic is removed from the heated pot with 2 silicone spatulas and transferred to the compression molder.


The compression mold is hollow and has water pumped thru it via a aquarium pump.


Excess heat is transferred to a blue Plastic HDPE barrel full of water.  That water is not actively cooled, rather it uses thermal mass and convection to release the excess heat which seems to work well.


Initially we had a lot of problems, with bubbles and shrinkage.


I had assumed that the plastic viscosity was too low.  But then Peter, our guest engineer from the Netherlands suggested that we lower the plastic temp, and found the sweet spot between 200-205c


Previously the plastic was looking similar to the texture on the Precious Plastic building blocks, wrinkled.


Now the texture is super smooth.


second video turn down the volume.


I apologize for the build quality.  It was hacked together last year from scrap bits and I had not yet bought a good MIG welder to handle the thin tube.


The molder is made from a used 6 ton hydraulic log ssplitter.


However problems arose as the log splitter is unable to easily hold the pressed plastic under pressure.  There is no check valve and the cylinder moves backward.  Also the cylinder is offset from the mold and it has damaged the alignment pins.


Next version will be back to the pneumatic bag jack, and using a oiless air compressor (sound issue)

Also I will be disassembling this extrusion machine and reassembling into a more proper design with a better method of holding the molten slug of plastic.  Likely similar to a pizza oven


We have however been on virus extreme lockdown since March 15 and unable to do anything as we are banned from leaving the house.




Those look really great. I really like your “engineered with what’s cost effective locally” approach to building your machines.
Would a motor (drill?) driven scissor jack work as a replacement for the log splitter?

Thanks for the update.



So at present we are getting a maximum of 14 pieces per hour.  Mostly limited to the slow speed of the extrusion machine at 3.1 kilos per hour.  I will be speeding up the extrusion machine RPM and hopefully get the production level up more.




210 gram parts (with flashing attached) take about 1 min to cool enough to remove from the mold.  However the parts could be in the mold longer.


Temperature was reduced to 200c last stage and first stage up to 195c.    I only have 2 stages on heat on my machine, and the barrel has been extended an additional 100 mm past the end of the screw to give it more melt time and to dump more heat into the plastic before it comes out.


My extrusion machine appears to be running too slow.  Based on calculations from a engineer who buuilds these things for a living, he told me that a 1 meter screw with 25mm diameter he runs up to 600rpm.  He said that since half the length of the v3 Precious plastic design, I should look at 300 rpm max speed.


The heated pot design needs to be modified.  Probably something with a pizza oven sort of tray  to remove the slug, and re-use the turbo cooker heater top in a new box.  On the priority list it is a bit low.


New extrusion machine is under construction as budget and time allow.  Frame was mig welded up yesterday partially.


Extrusion machine barrel is up in size from the PP design on the next unit.  Starting out with 1.75 inch round bar stock 24 inches long, turning down to 42mm OD and drilling out the inside to 1 inch ID  so that gives a 0.4mm clearance on the screw.   Also drilling and tapping the barrel to directly screw the thermocouple into the barrel.  this gives me a 9.5mm thick wall on the barrel to work with as the portions where there is no heater band will remain the original 44mm 1.75 inch size.


The screw is custom made with a 1 inch shaft and key way.  To better work with chain flex couplings.


The setup is strong enough that if the machine would be started cold the belt would slip before the shaft would break.


The new pneumatic bag jack powered molder and molds is on order in Laser along with more high speed shredder parts.


I believe you. For general purpose, large area compression you need lots of force. The thought was for the relatively small, pyramid shaped mold that a 2 ton jack  might work. 3 broken jacks say otherwise.

the scissor jacks, I just do not see them as useful.


Have built several machines for compression molding and broke 3 brand new jacks already.


As power for a injection molder yes, fine, but for the compression mold, not even close.