DIY bench moulder

[Robin Hewitt]

It's coming along, thought I'd try and make the moulder frame and plates from one length or 5"x1" steel bar... still have over 3 feet left

I have 1.5" diameter band heaters to go around the melting pot, but the BIG question is, how large can I make the hole inside where the plastic melts

I have plenty of driving force on the ram that comes down from above to do the injecting, but there must be an upper limit where the heat just doesn't make it to the plastic in the middle.

Commercial units look to be about 3/4" diameter inside, but are they limited by shop air pressure or because of that heat transfer problem?

Could I go nearer the inch or would that cause problems melting the middle?

[Robin Hewitt]

Hmmm, no replies, looks like I'm breaking new ground here

Bought some stainless tubing that will ream out to 27/32", sounds reasonable, hope it is brazeable.

I've fitted the moveable plate that holds the injector, next comes the melting pot and the ejector plate.

Will post more pics when it's moved on a bit.

[zephyr9900]

Robin, have you discussed your design elsewhere? I'm interested, but this is the first I've seen of your machine (which is looking fine so far!)

Best regards,

Randy

[Robin Hewitt]

I did ask about whether I could use brass for the melting pot, was told it was not a good idea

There are two parts I'm not sure about in the design that still bother me, holding the melting pot firmly without transfering too much heat and the measure that adds plastic between shots.

Current thoughts for the melting pot fixing are a triangular shaped hole in the fixing plate for alignment with minimal contact and a flange so I can bolt it to the fixing plate using a non-asbestos bunsen burner mat as insulating gasket.

For the plastic measure, a sort of loose fitting rotary vane pump? The injection ram has die springs at the top and a hydraulic bleed valve so I can pre-set the injection pressure. Theory is, if I time the injection to when that valve cracks I know how empty the pot is and can guestimate how long I need to turn on the refill motor at the end of the cycle. Problem is designing a vane pump that doesn't leak and doesn't jam.

[iswidcat]

If you are talking about the chamber inside which the plastic granules are melted within a piston type injection moulding machine, my experience is that you may need a torpedo shaped insert inside the chamber to force the granules against the heated surface. The torpedo will need some stand off pins or fins to keep it centralised within the bore and you will need to adjust the stroke of the piston so that it doesn't hit the torpedo.

I know this because I obtained a small piston type moulding machine with this part missing and it took me a while to figure out why the granules weren't melting consistently.

HTH

[Robin Hewitt]

Originally Posted by iswidcat you may need a torpedo shaped insert inside the chamber to force the granules against the heated surface.

That's interesting, wouldn't happen to have a pic of that would you?

I was planning on 27/32" bore melting pot, 5.6" tall with the lower 3.6" heated. This is much too large so the plastic won't reach the botttom before several shots have been taken. The injection force is adjustable up to about 1000 lbs.

Just cutting the ejector plate, pic.

[iswidcat]

Sadly no longer have the machine, it "disappeared" from storage, so no pics. I'm not too bothered as it was very old when I got it. It was originally used for moulding clear acrylic telephone dials in the days when the verb "dial" meant putting your finger in a hole in a disc and turning it rather than clicking a mouse on a link.

Really, it all depends on what shot size (volume) you are trying to achieve, which is obviously a function of the volume swept by the piston less a compaction factor as the granules melt.

You need a cone inside the cylinder because that's easier to make than a tapered cylinder. The cone will direct the granules into more intimate contact with the heated cylinder wall. So, the molten volume is more or less equivalent to the dia x height of the outer cylinder less the dia x height of the cylindrical portion of the torpedo, less an allowance for compaction (the diminishing space between the granules as they melt) and plus an allowance for expansion of the molten plastic (depending on what it is). Then you need a shaped lower part to suit your nozzle, essentially an offset, hence the torpedo shape.

Obviously you don't want the ram (piston) hitting the torpedo but you'll want to get a reasonable volume of granules into the hot area. If this means your stroke is now too short you may have to machine a conical hole out of the end of the ram to suit.

The main problem is that plastics are insulating materials. You might like to investigate ways of heating the torpedo as well, possibly a high wattage soldering iron element?

The machine I had was so old that it had a dial type thermometer sticking out of the cylinder, similar to what you might use for checking the temperature inside your xmas turkey. Its original "electronics" consisted of a wire wound variable resistor. I upgraded that with what was effectively a light dimmer circuit (actually a motor speed controller off an industrial power tool with a feedback loop adapted to a thermocouple sensor). I used it to mould cable sleeves for power tools anyway so there was a certain irony in the cannibalisation.

The ram on mine was driven by an air cylinder about 125mm in dia with about a 200mm stroke. There was a slot cut in the top end of the main cylinder to allow the granules to fall through a chute from a hopper, a simple gravity feed. The machine was similar in appearance to the one now offered by Travin in the UK.

HTH

[iswidcat]

BTW, I'm impressed by your casual disregard for personal safety in the wiring on your mill. I "thought" I was the only one who could get away with stuff like that. I hope it's just a 24V control circuit.

[iswidcat]

Something like this rather unclear quick sketch...

[iswidcat]

I neglected to mention that your machine work is absolutely beautiful.

Further trip down memory lane: the granules came into the cylinder through a port in the side near the top, this was so that the cylinder was guided by the ram keeping everything in alignment i.e. the ram couldn't enter the cylinder cock eyed and burr the top edge. The cylinder below the heater band(s) was stepped down in diameter and sat on a big coil spring, presumably partly to act as a buffer against an overstroke but also to give clearance so that the die could be removed easily. Thus the whole cylinder assembly was floating. I think that the bigger more gutsy machines have a secondary clamping mechanism to force the nozzle against the die before the injection stroke happens.

Autocad tip: Try typing "jpgout" into the command line of autocad, in most versions this gives you a jpeg of the drawing area itself without all the buttons, bells and whistles that show up in a screenshot. Also works for "pdfout" and a few other formats. There's a fuller description in the help menu.

[iswidcat]

Maybe this is a little clearer

[Robin Hewitt]

Just did some sums, don't reckon I will ever need to take more than 1/3 of the melting pot at a time and as there is no particular hurry it has plenty of time to melt. I have a 400W heater but if I extend the big brass heat reserve down around the nozzle don't I risk sprue stringing problems

I will modify my injection nozzle design, but think I will chance it without that torpedo, I can always make a new pot if it has trouble. There will be much cussing if it comes to that but at least I have a solution

Don't think Travin uses this part and his seem to work okay. They are not far from here and many's the time I've watched their 'Eagle' pot style moulder at work while waiting for Brian to explain the latest problem making my moulds.

I agree, the spring on your machine was probably to lift the melting pot clear when the injection ram retracted. Those big cylinders use a lot of air so there is a big saving in cubic feet if it doesn't have to work too hard for the return stroke. My shop air compressor is noisy and tends to get complained about which is why I opted for the silent hydraulic route. I have no problems using the up stroke to lift the pot.

The wires are 42 volt steppers. I think the record for killing yourself with low voltages was 12 volts, but he had to get a very good contact and lots of amps across his chest to achieve it. Apparently he was climbing to the top of an electro plating tank, reached up and grabbed the huge copper bus bars, one in each hand, then pulled himself up.

Ejector plate done and waiting for me to finish fitting it and make all it's little ancilliary doo-dads. It will eventually stand 1" clear of the platen so that's my max ejector stroke. I have 7" of daylight between the platens fully open, looked about right.