Friday, 30 January 2015

Heating chamber for injection moulding

Last night was BuildBrighton once again, and this time I took our CNC machine along, to have a go at milling some of the material samples that arrived the other day.

Whenever anyone mentions CNC milling, CNCPaul bangs on about getting a decent spindle and things like runout and tolerances of 0.01mm and all that. Of course, when you're building a CNC machine yourself, the most important thing is getting the frame right, and getting it to move accurately. For any homebrew application, sticking a Dremel drill onto the z-axis is probably good enough.

This may be the case - but when you've got all that working, and you're cutting shapes with a 1mm bit, aiming for sub-millimetre accuracy, then it turns out that CNCPaul is absolutely right - the spindle is a pretty critical part of the machine! There's no point being able to place the cutting head to within 0.1mm only for it to gouge out a 1mm trench, 3mm wide!

If the milling bit is perfectly centred, and only rotates perfectly vertical, a 1mm bit creates a 1mm wide cut. But if the bit is even slightly off-centre, or slightly off-vertical, the cutting end of the bit follows a circular path on the end of a cone, and creates a much wider cut than required.

Unfortunately, our Dremel clone that is clamped to the CNC has a serious problem with runout. When the spindle turns, you can visibly see that the end of the cutting bit is drawing a rather wide arc, rather than simply spinning. We tried swapping out the collets, using different bits (in case either of these was bent) and packing the non-cutting end of the bit so that it couldn't wobble about (and cause the cutting end to move off-centre) but nothing worked. 

It seems that the Dremel clone in our CNC is simply not up to the job. We'll have to find an alternative - or maybe even follow CNCPaul's advice and invest in a half-decent cutting spindle!

However, just before setting off for the BuildBrighton Open Night, a parcel arrived, containing two cartridge heaters. Talk about fast service! So I took those along too, and CNCPaul brought along a lump of aluminium block he had knocking around going spare. A perfect opportunity to start on a heater block for an injection moulding machine.

The first thing to do was to cut off a section of aluminium block. We stuck with our original design and went for a 2" square block.

The bandsaw took forever to cut through the block, as the blade had about a million teeth per inch. Plenty of cooling lubricant didn't seem to do much. And after the blade got about half-way through, it started to veer outwards, producing a rather peculiar profile shape to our block. It wasn't exactly perfect, but it would have to do!

Then Paul got the block onto the Myford lathe and drilled out the centre hole for the chamber. This involved drilling with a 7mm bit to create the initial cavity, then expanding this using a 13mm bit - the largest metal cutting bit available at the workshop. Paul then used a boring tool, to open this cavity to about an inch in diameter.

Having discussed how important getting a drill/cutting bit perfectly vertical for any kind of drilling or milling application, it was ironic that the pillar drill in the workshop was so reluctant to drill - even a small pilot hole - exactly where we wanted it. With Jake's help and a lot of piling blocks under the piece to get it to drill far enough into the aluminium, we eventually managed to get the heating element holes drilled too.

The heater chamber on the left is noticeably further away from the main chamber than the one on the right. It's just something we'll have to learn to live with for now. Note how the top of the block is not square, and how the sides taper towards a much more square base - all thanks to the wonky bandsaw!

The cartridge heaters are exactly 10mm and are an extremely tight fit into a 10mm hole. So tight (and, perhaps, because they may not have been manufactured to the most exacting of tolerances) we had to make the holes 10.5mm to get them in. Obviously an air gap between the heater and the block is undesirable, so we'll coat them liberally with some heat-sink paste to ensure a good strong contact.

The biggest job of the evening was clearing up. There was swarf everywhere! After clearing the workshop, we had to be very careful emptying the swarf from inside the main chamber.

This chamber will have moulten plastic in it, and we'll use a snug-fitting ram to push it out of a nozzle on the bottom. So we don't really want the walls of the chamber to have marks and scratches in them, or to provide any little nooks and crannies that hot plastic can settle in - as this will make the ram difficult to operate in future uses.

At the end of a long night, we ended up with a half-decent (ignoring the dodgy cutting and off-centre drilling) heating chamber. We're waiting on the thermistor now, then we can build our temperature controller.