Many of you won’t be wondering what I do for a living, but I’m going to tell you anyway so that I’ll get out of this nasty habit of only putting up one post a week. To that end, I’m going to walk you through a typical lab process that I’ve had to carry out about nine times in the last week: processing an acetal resin clasp.

Stage 1. The framework fitted down to a duplicate model with the clasps patterned in casting wax

Stage 1. The framework fitted down to a duplicate model with the clasps patterned in casting wax

We start with a duplicate model, on which we pattern the clasps in casting wax. This model is then trimmed to within an inch of its life, leaving us with only those parts of it that are supporting the denture and the wax clasps. Then we get out an injection moulding flask.

A flask, already greased with petroleum jelly to aid in devesting.

A flask, already greased with petroleum jelly to aid in devesting.

The bottom half of the flask is filled with plaster, into which the prepared model is sunk, taking care not to allow the plaster to cover the wax patterns. The surface is smoothed out and undercut areas are eliminated. This leaves us with something like this.

The bottom half of the mould, ready for spruing.

The bottom half of the mould, ready for spruing.

The resin will need to get into the mould somehow, so the next stage is to add some sprues. This is done using 3mm sprue wax from a coil.

The finished sprue system

The finished sprue system

Now the two halves of the flask can be bolted together. The bottom half of the plaster mould is coated with water glass first to ensure that the whole thing separates neatly later on.

Ready to begin topping

Ready to begin topping

Now we put the flask on a vibrating table and slowly pour in more plaster, taking care to keep air bubbles away from the wax. Once the plaster has set the mould will look something like this.

The completed mould, ready for boiling out

The completed mould, ready for boiling out

After allowing the plaster to set completely (I leave it twenty minutes, which is a bit more than it really needs), the flask is immersed in boiling water for another fifteen minutes to remove the wax. When it’s opened after boiling there’s usually some residual wax, so it needs to be cleaned with boiling water and washing-up liquid. While the kettle’s boiling, it’s a good idea to check for feathered edges in the plaster that will break off in the resin and ruin the clasps. These are removed with a scalpel.

The mould after boiling out

The mould after boiling out

Nice and clean, and with any thin areas of flash plaster removed

Nice and clean, and with any thin areas of flash plaster removed

Now that the mould is finished, it needs to be coated with separating solution so that the resin won’t stick to it. While that’s drying, we prepare an injection cylinder.

The injection cylinder, resin ingots and plunger

The injection cylinder, resin ingots and plunger

This goes into the injection moulding machine, and a metal cap is placed over it. The hole that forms the beginning of the sprue system sits over the cap, which is all you can see of the whole cylinder assembly once it’s in the machine.

The cylinder in place, we're ready to add the flask

The cylinder in place, we're ready to add the flask

The flask is bolted back together and clamped down in the machine. The injection process is automated, so once you get to this stage you can forget all about it for forty-five minutes.

Ready to inject. The red elastic band is there for a very technical reason, and not at all because that metal shield won't stay up on its own. Oh no

Ready to inject. The red elastic band is there for a very technical reason, and not at all because that metal shield won't stay up on its own. Oh no

Now that I’ve done something else for three quarters of an hour, the machine has finished its cycle and the job can be removed from the mould. If the earlier stages were done properly, the mould should open cleanly, leaving us with something like this.

What you see on opening the mould after injecting. I've already cut through the sprues with a carborundum disc

What you see on opening the mould after injecting. I've already cut through the sprues with a carborundum disc

Now the investment can be removed, leaving the denture framework ready to be cleaned in the ultrasonic bath. Once it’s been in there for about half an hour, the clasps can be trimmed and polished, leaving two flexible clasps that are more or less the same colour as the patient’s teeth.

The new clasps, ready to be trimmed and polished

The new clasps, ready to be trimmed and polished

And that’s how you process a pair of acetal resin clasps. I know: it’s a wild and crazy whirlwind of excitement in my job, but I like it.

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