Injection machine problems and my solutions

1. Plunger sticking in the top of the tube.
2. Plastic running out of the nozzle between injections.
3. Very rough finish and excessive sinking/shrinking in finished parts.

1. Moved one of the heaters to the top of the tube and moved the temp sensor under the hopper.
2. installed ball valve on nozzle end.
3. Changed the the geometry of the handle to double the injection pressure, added a stirrup to stand in to apply pressure for 1 minute after injection.

My parts now have a smooth, shiny finish and the accuracy and strength is greatly improved.  Attached are pictures of my hacks.

Here are some pictures of my parts. They are bearing holders for linear slides for a cnc machine. They are 20mm thick and about 50cc of HDPE.

Hi Mark,


Any chance you could release the linear guide as a 3D printable STL?  Really desperate here to build a CNC router.  The MPCNC design is not sturdy enough in my opinion.  I built a couple and lots of flex issues.





Hi, @tathata, thank you for the kind words. The plastic is HDPE and I’m using a Precious Plastic injection machine built exactly to the plans and then modifying it slightly as my previous posts show. The nozzle temp is 210C and the barrel is 190C.

I also love cnc machines. I’m now building a cnc router with the parts I’ve made. I think it will be rigid enough to cut aluminum using a 1.5 KW spindle. I’ll post my results under “Share you creations” here in the forums.

I hope this helps.

markriem  – your injected parts look great! I have a lot of catching up to do! Would you please tell me what plastic material you’re using? My beginner-experiments right now are focused on discovering the melting characteristics of the various plastics I can pick-up in my locality. I’m reading a lot too, but hands-on is the real proof! 🙂

Hi, guys! This desing of CNC runner is of interest to me too.
I have been thinking along similar lines for some time. My variant on the idea has been to cast the bearing-holder bracket (looking much the same as your drawing) from molten scrap aluminium. If plastic is stiff enough do the job, that’s even easier! The rest of my plan is to make a large, level table in the garden – with straight *scaffolding poles* (about 9 feet long) running along the table top on each side. These brackets & bearings would then be suitably-sized to run on the scaffolding poles. The moving bridge 90 degrees across would carry a small table with a universal fittings for interchangeable tools. Even without CNC control to begin with, I could mount a swivelling circular saw, and cut up full sheets of ply in both the x & y direction… or change the tool to a router, and mill a rough piece of wood flat by moving the router about. Maybe larger versions of this fitting *would* be sturdier, if cast in aluminium. The process is not really harder: Plenty of youtube videos show people melting scrap aluminium in steel cans in their back garden. I apologise for not contributing much to the *main* issues in this thread. I’ll do better next time! I guess this CNC table idea excites me more, on this occasion, because I really miss not having one.

very nice! Love the pin idea

Here is the updated design, a lot stronger, and how how I get the holes in the right place. The pins are easy to remove with a drill or a small arbor press or even a drill press.

I couldn’t upload a dxf file. Here is a image of my current mold design.

Hi, I have changed the design a little to add a little more rigidity, I’ll upload my current dxf file. The bearings are 625 (5mm bore, 5mm wide, 16mm diameter. I’m going to machine a new mold tomorrow. Allowing for some shrinking is a little tricky.
To get all the holes in the right place I milled grooves in each half of the mold and drilled the holes for the aluminum framing. I placed threaded rods in the grooves and bolts in the holes and removed them after injecting the part. I then drilled the holes to size. I’m going to try 5mm rods in my new mold and try to press them out with a small arbor press. If that doesn’t work well I’ll go back to threaded rods, they are easy to unscrew.

Oh that is so beautiful!!!


One question about the bolt holes… Do you drill the holes with a jig afterwards?


I absolutely love the design and am drawing up something now.  Printer will be running test parts ASAP.

Yes, I drew some inspiration from MPCNC. I’ve built some machines using maker slide with the plastic rollers and I think this is more rigid, that was my intent. I designed the slides around larger tubing, 1.5″(38mm) tubing for more rigidity as well. Do you know if 38mm is a common size in Europe? 1.5″ tubing is easy to get and pretty cheap here in the US. I’ll share my results here when I finish a machine with these parts. An inexpensive machine to make molds is my intent.

Nice work, reminds me of  the “mostly printed cnc”. How rigid is it?