Tuesday, 27 January 2015

Designing for CNC milling for casting and injection moulding

Having got to grips (sort of) with Inkscape and gcodetools we've managed to design a few different patterns for CNC milling. Our original plan was to mill some masters to make silicone moulds (which in turn would be used to case 15mm terrain in dental plaster) so we've got some designs ready to do just that:

We've found a slightly different approach to making our CNC patterns than in earlier attempts (which we'll detail further in a future blog post) and using a bit of manual intervention, even managed to improve the paths for pocket milling. The end results in OpenSCAM look quite encouraging...

So we've got some designs to make blanks for silicone moulds. When we've got the CNC router up and running, we've already got the materials to try out, so we'll hopefully see some new rubber moulds in the next few days.

Re-drawing the patterns to leave the required shapes in relief (almost like making a negative of the final design) and manually adjusting the images in Inkscape to allow for different milling bits has meant drawing "oversized" shapes. Just out of interest, we had a play creating an "opposite" mould - one that acts like a cavity, suitable for resin casting, for example.

Drawing this type of mould required a very different approach and took some thinking about, just to get all the layers in the right order, and the different cutting depths the right way around! But after a few false starts, we managed to come up with something that looks like it might just work

This time, we had to make all our deepest  cut lines follow the inside of the shapes (rather than the outsides, as before) so that they defined the actual shapes required (whereas when making shapes for the silicone moulds, the cut lines could be as wide as we liked, so long as they were outside of the material we wanted to leave behind).

It also meant that the bits we wanted "intended" on the final piece had to be milled to stand slightly proud of the bottom of the milled pocket (rather than cut into the shape we defined). It took about 30 attempts to get this one right!

And then things went a bit crazy.
So we could make a mould that, when we fixed to a flat back, we could pour resin (or similar casting compound) into, provided we allowed a little hole somewhere, to pour it into. And then we got to thinking that perhaps, instead of a flat back, we could duplicate the pattern, to allow "proper" 3d shapes to be cast.

By adding a few locating holes, and a small channel to feed the casting compound into the mould, we could simply copy-and-paste to create a mirror image of the shape to be cast. These two halves could be placed back-to-back, to create one, single, double-sided shape!

Although the Inkscape drawings look a little sketching, with overlapping lines and multiple layers that don't quite look right, the resulting gcode looked pretty promising, in OpenSCAM.

So now we've a pattern to CNC rout a mould directly from the milling material (rather than covering a "negative blank" with silicone rubber to create the mould indirectly). Resin is an ideal substance for direct casting (it is possible to create a two-part mould with silicone rubber and use it to cast resin, but resin does cause the mould to degrade quite quickly - in anywhere between 10 and 20 casts) but already we're considering alternatives!

Resin is a relatively expensive material for casting - it's probably why most "homebrew" miniature casting set-ups use white metal (a mix of lead and pewter usually) though some companies do offer "resin masters" - at anywhere between five and ten times the price of a white metal miniature. But we've got one eye on another, far cheaper, material altogether...

Plastic injection moulding is a relatively simple process, providing you have the materials and equipment to do it. It's not as popular in homebrew/diy circles than resin, because it does require some equipment up-front. Firstly, the moulds need to be made from tough, strong, durable, heat-resistant materials, not the relatively soft, heat-damaged, short-lived silicone rubber that are suitable things like plaster and resin casting. But if we had the equipment, the actual plastic material can be very cheap indeed.

Obviously, we need to prove that we can CNC mill our sample materials - if we can mill directly from blocks of aluminium, that would be perfect. But even if not, we're already aware that there's a multitude of materials on the market, suitable for making moulds from. So if we can make the mould, we'd need some kind of injection moulding machine.

Just a few minutes on eBay and we've already ordered a couple of cartridge heaters (http://www.ebay.co.uk/itm/291048781200)

and a thermocoupler module, for controlling the temperature of the heater elements

The idea being that we've already got a hydraulic bench press over at BuildBrighton - so there's no need to recreate a frame and lever arrangement so often found in homebrew/diy injection moulding machines. We could create a heater block, into which we place plastic pellets (or recycle some PET2 drinks bottles by cutting them into strips by hand) then plunge the press onto a piston in the melting chamber, to squeeze the hot plastic out into our (aluminium, CNC milled) moulds.

The heating block could be as simple as some 2" or 2.5" square aluminium solid bar, cut to about 70mm-80mm, drilled with a couple of 10mm holes (for the heater elements) and a larger - possibly up to 30mm - hole for the melting chamber.

At the bottom of the melting chamber, we'll probably need a smaller hole (the larger one does not go all the way through the block, but stops short of the bottom of the aluminium bar) into which we'll tap a thread, so that a cone-shaped nozzle can be fitted to the bottom of the heater block. The purpose for this is twofold. Firstly, it should allow for easier cleaning of the machine (should the nozzle get blocked with hardened plaster) but, more importantly, it provides something that can easily be placed over the opening in the mould - to ensure that the plastic gets forced into the inside of the mould and doesn't simply get squirted all over the exterior!

As ever, the eBay stuff will probably take a few days to arrive, so it's unlikely to be here in time for the next BuildBrighton meeting. But in the meantime, there's no reason why we can't prepare some aluminium block and spend a bit of time trying out the CNC router. If we can't make the moulds in the first place, all this fancy talk of building an injection moulding device will be for nothing anyway!