Designing and building the LPM compression barrel heater

The key limitation to the design of the CBH was the installation location. LPM facilities run in remote places where access to technical parts suppliers is limited. Everything on the machine had to be simple, robust, and repairable. As LPM was still young and developing at the time, budget constraints also favoured a simple approach. Achieving tight tolerances or highly technical performance was not the main priority. Our goal was to engineer a machine that runs consistently and can be fixed by the people standing next to it.

Driving an auger of that size properly to reach the required pressures was the next challenge. The electric motors and gearboxes didn't give enough torque: 150 to 175 Nm after efficiency losses. I expected to need over double that for reliable, continuous extrusion. On a friend's suggestion, I looked at a hydraulic motor and drive system. Research showed potential to not only fix the torque but also cut electrical power draw at the same time, which would be a huge advantage considering the remote install locations. One issue: I'd never built a hydraulic system, and neither had anyone at work. With a mixture of YouTube, AI, and a very helpful sales guy at an online hydraulic parts supplier, I learned enough to design and build a functioning hydraulic power unit and drive. To my surprise, it worked on the first try, tripling our torque output and extruding plastic at an ideal rate.

On the other end of the CBH was a die head to extrude the profile we wanted. This became a very tricky part of the machine, and we have now been through multiple shapes and versions, fine-tuning the extrusion rate. We also needed to design a cooling system as hot plastic coming straight out of a die doesn't hold its shape. The cooling head comprises an aluminium tube and cooling system which has evolved significantly through the development process. What began as a very simple system which relied on spraying water onto the aluminium and blowing a fan over it, became a custom-built shower head and fan system, before we landed on a custom-built, fully enclosed, water-cooled die head. That final version has the profile tube internally, a 10 mm aluminium bar bent in a spiral around the tube and welded in place, and an aluminium tube slid over the top as a casing. Water pumps into the casing at one end, spirals its way around the die head, and comes out the other side. This worked so well. We did originally have the water running in a closed loop, and it did begin to heat up over long runs, but I plan to solve that by running the water through a car radiator.

The design of the third CBH version also needed to address structural issues that had become apparent in the initial two versions. Version one generated so much pressure through the screw drive that it flexed the frame. Version two did something similar, even when bolted to a table. I had vastly underestimated the forces and incorrectly balanced the structure, which I was able to rectify in version three. By adding structural supports to the top of the machine and cross-bracing to minimise twisting, I had a frame design I was confident could survive the forces applied to it. I did all of the cutting, welding and fabrication of the frame and the other components in-house. I designed it specifically so that I could remove, repair and upgrade any part of the CBH without rebuilding the machine. It took careful planning, and it worked a treat: I have fully stripped and reassembled this version of the machine three times now.

There is so much more that I have done with this machine than what I have described. Putting four years of development work, fabrication and design decisions on one page would fill a small book. However, there are a few more highlights that I am incredibly proud of:

• I designed and built the electrics: the control box, programming the PID control units, installing the heating bands and K-probes, and all grounding and safety requirements, with a couple of electricians looking it over to confirm.
• I manufactured a split muff coupling from a solid steel bar through to an on-machine functioning part. This involved lathe and mill machining of steel, tapping, and accurate cutting for the split.
• I machined and welded the aluminium barrel comprising two stock aluminium tubes (3” and 4”), joined by a custom machined tapered reducer. All parts were completed to extremely tight tolerances. The barrel is the only part of the initial machine that has remained across all three versions.