Flow issues in beam molds

I have been working on an extruder build, and have been having issues with the beam molds. With a 1/2″ (13mm) copper pipe as a mold My extruder seems to have a maximum casting length of around 20″ (500mm).

It appears that the issues  comes from the way the plastic is flowing through the mold. I had assumed when i was planning and building it that the molten plastic would harden and be pushed through the mold with the pressure of the plastic behind it.

What actually seems to be happening is the molten plastic is ballooning into the mold by pushing itself through the hot core of the beam. Once it gets to a certain length,  the plastic hardens and my screw doesn’t have enough pressure to push.

I have yet to experiment with larger diameter molds, i think this will help by having a larger core for the plastic to flow through, or pre heating or chilling the mold.

Has anyone else had this problem?

Interesting, I tried something similar with an injection machine and got something similar. A description and some section views here http://onearmy.world/community/forums/topic/portable-desktop-injection-machine/page/4/  . I think the V4 beam production efforts made similar observations http://onearmy.world/community/forums/topic/beam-production-v4/ . I’m not sure if you can get the plastic to slide along the wall as opposed to coating it and then shearing internally. If it is shearing internally, then it is a battle between the push force (a function of the pressure and area) and the shear force ( a function of the viscosity, perimeter, and length). For small diameters like you and I tried, the shear force wins pretty quickly, for the V4 sized beams it takes longer. One option might be to really heat the mold and raise the plastic temperature to keep the viscosity low.

Thanks for posting, it is interesting that we got to the same results using completely different machines.

Thanks, I think I see a winter project forming.

You may be able to put a larger pipe on the outside and blow a heatgun through the gap.

S2019, It is indeed the 21″ bit. I didn’t have to dress the bit or ream the pipe I think it had around a 1/32″ difference from the OD to ID. I did have multiple problems with warpage of my barrel, BIG TIP DON’T cut out your hopper opening before you weld the support legs on. My first attempt looked like a banana, my only stainless welding rods were 1/8″ stick rods, with my 300 amp Lincoln it was tricky.

thanks long, I have been using pp mostly from milk jugs. I will definitely try preheating the mold. I need to get an old toaster anyway to heat up my welding rods.

@snyder-devin the HF concrete drill bit approach is exactly what I’ve been considering. Did you use the 21 inch kit https://www.harborfreight.com/21-in-sds-max-type-masonry-drill-bit-set-3-pc-62793.html ? Once you take off the bits at the end the fluted portion fits the 3/4 inch pipe or did you have to drill or ream the pipe?


If you need a cheap mold warmer. Get a toaster oven, cut out the ends, bypass the switch so top and bottom heating elements stay on, and slide that along the mold.

Keep constant heat on the mold; 300°F – 350°F works really well. I suggest using Polypropylene; it flows really well, and it has really nice shrinkage when cooling making it easier to remove from the mold. Cap the end of the mold, but drill a couple holes for gas vents. When the mold fills, let the extruder continue to run for awhile longer to pressuruze. You should know your motor well enough to know when it’s struggling due to pressure. When the motor shows sign of pressure, immediatly remove the mold and cool both ends as quickly as possible in a bucket of water to retain the internal pressure. What happens is when the plastic is cooled, it will have a wrinkle free, very smooth surface.

also the shaft is made for a SDS max drill chuck, which has a 18mm diameter i think. I had to use a 3/4″ split coupling with a sheet metal shim all the way around.

The Screw is made from a 1″ concrete drill bit i found at Harbor Freight Tools, it was only $40 for a set of 3 bits. It was the closest off the shelf item I could find that matched the designed screw from the blueprints.

I had to un-braze the carbide teeth off the end and grind down the shaft on the hopper end of the screw to allow better feeding. It fit surprisingly well inside a 3/4″ Schedule 40 seamless pipe.

Unfortunately I didn’t think to take pictures of it before I ran a batch of plastic through it, Its about impossible to clean it off at this point.

Nice, I like the simplicity of the wood frame build. What are you using for the extrusion screw?

Please note the the end of the second largest casting. It was made with the shorter mold and it developed a mushroomed head at the end when i filled up the mold.

here are some photos of my machine