Shop Air Injection Molder

Automated injection molder using shop air for most operations

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The Heater

The heater design is based on this machine.




The heater unit will be 6" long and 1/2" OD. It will be heated by Nichrome wire ~95" long to provide approx 10 turns of wire per inch of heater. Using a calculator I have come up with 38 gauge wire.

The pipe will be black pipe from home depot. It will be wrapped in kapton tape then NiChrome wire, then insulation (suggestions?), then a wood block housing.

I plan to use a thermometer to check the temperature at the pipe's tip.

The power is controlled with a lamp dimmer (suggestions?), the Nichrome will be attached to the lamp dimmer terminals with a screw and washers or with screw terminals if available.

The Ram

The pneumatic ram which injects the molten plastic is based on this design on thingiverse.

The injection molder mentioned earlier appears to use a 6:1 ratio and have an ID of 1". Judging by the struggle the user puts up with I'd guess he is applying about 150 lbs of force. 6*150= 900 lbs force. Approx 900 PSI inside the injector/ram.

Shop air is about 90 PSI. The burst strength of PVC is below that so picking a diameter is just about choosing force. I chose 2" schedule 40 diameter pipe. 3.14*2^2= 12.56 in^2. 12.56*90psi = 1130 lbs force.

I do not know if the pvc pipe will buckle under 900lbs of force. I plan to build a lexan shield for if it buckles and bursts explosively. Intuition in working with PVC pipe tells me it won't buckle but better safe than sorry. I'd be appreciative of anyone smarter than I who can do the calculations.

I have sourced an electronic 3 way valve that costs less than a manual valve. It uses a 1/4" threaded fitting, which coincided nicely with standard shop air fitting sizes.

The injector

At the end of the ram will be a PTFE bushing screwed into the ram's shaft. It should be slightly large that the heater pipe.

The Frame

The test frame will be made of 2x4 lumber. The PVC pipe will be supported by lumber on all sides. The heater will also be supported. The base of the frame will include a bench vice.

The Plan (Subject to change)

So it's been a year and the injection molder isn't complete!

The new plan is to...

1. Mill out aluminium blocks to make the mold halves and heating block.

2. Beg someone to lathe the plunger since I can't use a lathe (red tape).

3. Test it! Hopefully something cool will happen.

  • Melted Plastic and Printed Parts

    UkiyoWeee04/19/2015 at 15:32 0 comments

    I got around to heating up the extruder and putting some plastic in it. Results were poor. The extruder did not have even heating due to the uneven winding of the wire. The whole mess had to be pushed out with a metal rod and it came out like clay despite being well over 230 F.

    According to the source below, ABS has a molten viscosity at the lowest of 155 Pa*s. Corn syrup is between 1-2. Vegetable shortening is 250 so molten ABS is about half as viscous as vegetable shortening. No wonder common injection pressure is 15000 PSI. Other sources say peanut butter has a viscosity of about 150-250 Pa*s. So I'll probably use that for testing.

    I also found this cool write up on ABS plastic.

    Lots of research to be done on translating shear rates and viscosity into molding pressure.

    Printed Parts:

    I finished printing all the parts I needed to make a PVC hydraulic cylinder. Only to discover the parts don't fit any PVC pipe available at Home Depot. Luckily the parts are simple enough to design myself in Autocad with the correct dimensions. It's a real bummer though, 6 hours of printing wasted.

  • Taxes!

    UkiyoWeee04/14/2015 at 04:58 0 comments

    The government has found a way to delay my project. Gathering info for taxes will eat up a few days.

    Thanks Obama! </sarcasm>

  • Successful test of heat element and Personal Injury

    UkiyoWeee04/11/2015 at 22:08 0 comments

    I discovered I had some Nichrome wire, 24 gauge, lying around from a hair dryer. I wrapped Kapton tape around black pipe and wrapped the nichrome around it. I bought a $6 dimmer from home depot, wired it up and plugged it all in.

    Success! The makeshift wire reach 200 F before the tape I used to hold it together melted.

    I then proceeded to slice my finger open while building an enclosure. Never used dull drill bits and always clamp down the workpiece folks! The drill quickly snagged the workpiece and split me open pretty good. No stitches will be needed.

    Also, I printed most of the parts for the PVC pneumatic cylinder.

    I am also working on a laser cut tumbler to finish the 3d printed parts.

    Next steps:

    I need a reliable way of winding Nichrome around the pipe evenly. I plan to make a jig out of threaded rod and power drill to wind the Nichrome. I will probably use a gloved hand to apply tension as it winds.

    I need to figure out how to secure the heater while pressure is applied by the ram. I think the answer is a coupler for black pipe...

  • After Some Reflection

    UkiyoWeee04/09/2015 at 13:31 0 comments

    At 90 PSI the 3d printed plastic parts are undergoing 1300 lbs of force. They'd pop right off the pipe at probably around 300 lbs. Yet, PVC pipes are used for dangerous (but working) rocket engines and the nozzles don't shear away from the pipe (every time).

    A 12 ton pneumatic jack will only cost me $80 at home depot though, so I may do the testing with the jack while I figure out the 3d printed cylinder. I may forego the 3d printed cylinder in favor of a $180 metal cylinder, but that would set future build costs at a $360 minimum before fittings, framing, etc.

    Priority #1 is making a working heater. I will order 38 gauge NiChrome and a lamp dimmer tonight. The shipping time will give me time to research insulation materials.

  • Need Help on a Few Things

    UkiyoWeee04/09/2015 at 04:29 2 comments

    There's a few things I need help on.

    1. I've never used a hobby injection molder. If you have one, there's many assumptions I'd like clarified!
    2. Feel free to check my assumptions and math on how the heater will work.
    3. Please suggest sourcing!
    4. I'd like to use a metal version of the 3d printed parts. Accepting donations.

  • Similar Idea

    UkiyoWeee04/09/2015 at 04:23 0 comments

    Here is a similar idea

    Here is another similar machine but closer to my aim of full automation.

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