Work


I’m choosing to interpret the complete lack of response to yesterday’s helpful guide to making clasps as an urgent demand for more of the same. Today’s guide describes what to do with a cobalt-chromium denture framework once you’ve got it out of the casting machine.

We start by leaving the mould for half an hour. This is partly because the metal inside has a temperature of about 900 Celsius, and is therefore impossible to work on unless you have asbestos hands; but mainly because if you cool cobalt-chromium alloy too quickly it plays merry hell with the metal’s crystalline structure and the denture will break.

After about half an hour the metal has completely solidified, and can be immersed in cold water to cool it down. Doing this also damps down the investment, the dust from which contains silicone and other nasties that you don’t want to breathe in. When it comes out of the water, the muffle looks like this.

Ready to remove the investment. The delicate technical instrument we use to do this is in the top-right corner

Ready to remove the investment. The delicate technical instrument we use to do this is in the top-right corner

A few scientific taps with a hammer and most of the mould falls off. Once you can get your fingers around the cone at the top of the sprues, do so and smack it with the hammer until most of the investment falls off. If you put your hand underneath the framework to support it, it will get bent and you’ll have to start all over again. Presuming that this didn’t happen and that you haven’t broken a finger or something equally daft, you should be left with something like this.

Free of the investment, the metal clearly needs a lot of work.

Free of the investment, the metal clearly needs a lot of work.

One of the reasons I like this process is that I get to hit things with a hammer, then get out some power tools. In this case, it’s a high-speed grinding unit, with a carborundum disc rotating at 25,000 rpm. The sprues are cut off as close to the framework as is possible without cutting into it (that gets you belted around the head with the hammer). This leaves us with something like this.

The sprues have been cut off, so now we can get on with finishing

The sprues have been cut off, so now we can get on with finishing

Now we need to get rid of the investment and oxides that are all over the job. We do this with a sandblaster full of 25 micron aluminium oxide blasting compound. Two minutes of that, and you have sand everywhere and a chrome that looks like this.

Nice and clean. Time to start the real work

Nice and clean. Time to start the real work

Now we need to clean up the casting. There are the stumps of the sprues to remove and pieces of flash metal all around the edges, and the rests and other delicate shapes need to be defined and shaped. Once it fits on the model and we’re happy with the result, it goes back into the sandblaster to homogenise the surface. By now it’s looking more like something you’d put in someone’s mouth.

It fits. Time to make it shiny

It fits. Time to make it shiny

The sandblasted job goes into an electrolytic polishing bath for ten minutes at just under five milliamps. Then it’s smoothed off using hard rubber wheels and rubber points on the high-speed grinder. Once that’s been done it looks a bit shinier, but not shiny enough.

Better, but it needs more polish

Better, but it needs more polish

Next we get out the hand-held motor, fit a mounted steel wire wheel to it and set it to 9,000 rpm. The whole chrome gets a good going over with a fine metal polish and the wire wheel until any lines or marks have been removed. Since cobalt-chromium is quite a lot harder than steel, the wire does no harm to the surface. Actually the polish is doing all of the cutting, and the brush is only made from steel because anything else will wear down to nothing in seconds. After the first polishing our casting looks like this.

After polishing with the wire wheel, it's nearly finished

After polishing with the wire wheel, it's nearly finished

Now we need to get that extra bit of lustre, for which we use felt wheels and points and an even finer metal polish. Once this is done and the whole thing has been scrubbed in near boiling water and laundry detergent (that combination will get rid of anything – try it on that tannin-stained coffee mug), it’s ready to have the fit checked and go out to the client.

Polished and cleaned off, the chrome shines forth in all its glory. My work here is done.

Polished and cleaned off, the chrome shines forth in all its glory. My work here is done.

We do about five or six of these a day, so several other castings were going through the same stages alongside this one. That takes us from about eight in the morning, when the casting happens, to about three in the afternoon, when it’s time to start making the wax patterns that will be cast into frameworks the next day. It’s a busy life, but you do get to cut things up with power tools; and is that not what man has dreamed of since first he looked upon the stars? Probably not, but it’s not bad.

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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.

I live in the charity shop district of Exmouth, so quite often I see nice things going for a song. Unfortunately, I also start work at eight in the morning and don’t finish until at least six, which makes it difficult actually to buy anything. Yesterday, however, I saw something so compellingly cheap and useful that I enlisted some help from the family to get it for me.

The Oxford Library of Words and Phrases

The Oxford Library of Words and Phrases

As you can see, the set has a lovely red cloth binding with (presumably) imitation leather spines, and it’s going to come in useful for more than beautifying my groaning bookshelves. Volume One is made up of quotations, Volume Two of proverbs and Volume Three of etymologies. It will now be even easier to make myself look clever when I write things.

What price did I pay for this little gem? The princely sum of three pounds. That makes a nice change from the branch of Oxfam that once charged me five-hundred pounds for a set of three second-hand books. Yes, I’m talking to you, Tony.

The Lord of the Rings, late first edition copies

Those expensive second-hand books. Late impressions from the first edition, 1961. You can just about see my favourite bookmark next to the phone.

I didn’t say I was overcharged, did I? Worth every penny.

I thought I might also share how I paid for my unwonted literary profligacy, so here’s a picture of a strengthener that I made for one of our clients. It’s made from medical grade Cobalt-Chromium alloy by the time-honoured technique of lost wax replacement casting, and I made it last week.

Full lower mesh strengthener, hand-made by yours truly.

Full lower mesh strengthener to stiffen an acrylic denture, hand-made by yours truly.

Plans are afoot to make my summer a lot more interesting than I expected it to be, so I’ve added a song about flying in airliners in honour of my impromptu holiday planning. It has virtually nothing to do with my summer plans, but what the hell? Parts of an aeroplane are mentioned in it, and that’s good enough for me.

I was going to end it there, but then I remembered a poem that I like, which is also about flying. This one is an ode to the Royal Aircraft Factory B.E.2C.

The Pilot’s Psalm (anonymous)

The B.E.2C is my ‘bus; therefore I shall want.
He maketh me to come down in green pastures.
He leadeth me where I will not go.
He maketh me to be sick; he leadeth me astray on all cross-country flights.
Yea, though I fly over No-man’s land where mine enemies would compass me about, I fear much evil for thou art with me; thy joystick and thy prop discomfort me.
Thou preparest a crash before me in the presence of thy enemies; thy R.A.F. anointeth my hair with oil, thy tank leaketh badly.
Surely to goodness thou shalt not follow me all the days of my life; else I shall dwell in the house of Colney Hatch forever.

Well, it’s finished. It took all day, but now it’s been processed, trimmed and polished. It’s not how I’d have chosen to celebrate the end of another year on the planet, but I’ve had worse.

Looking back a couple of days later, I realise that the best thing that happened all day was that my entire family, including my brother-in-law and my brother’s girlfriend, clubbed together to get me a ridiculously ostentatious car stereo. It’s a pity I was in such an appalling mood at the time, so could only think about the increased risk of someone breaking into the car to steal it. I can be a pillock at times.

My Full-Full, now finished

My Full-Full, now finished

Most people who’ve found this will already know that I’m training as a dental technician. To be perfectly honest, this is a ludicrous move for a twice graduate, since it means jumping through a lot of hoops to take a colossal pay cut. Still, there are one or two compensations, and one of those is being able to look back on a day’s work and see some practical result from it.

A full-full wax-up, as prescribed for edentulous patients

A full-full wax-up, as prescribed for edentulous patients

The end of the academic year looms large at the moment, and I’ve been taking some time to produce the practical coursework that will be required of me on Thursday. What you see in the picture is a full over full wax pattern, which will be used to make two plaster moulds. Those moulds will be filled with high-impact acrylic to make two dentures ready for trimming and final finishing, and I’ll be doing that tomorrow.

To be honest, I’ll be glad to see the back of this particular module, which has been rushed through to get it in before the end of the year. Come to think of it, I’ll be glad to see the back of the whole course, get registered and stop having to drive to Cardiff and back once a week. I’ve spent a lot of time in higher education, and never have I seen such a disorganised and rushed course of study as the one on which I’m engaged. It’s a sausage machine to turn out people with paper qualifications, whose existence the government can then put down in the official statistics to prove that they’ve made up the shortfall in technicians. As usual, a problem will appear to have been addressed without any practical difference being made. Don’t you just love politics?

Anyway, the depicted job isn’t great, but it meets the laughable standards required of students these days. I’m putting it here not so much to show how clever I’m getting as to mark a milestone of sorts: another piece of coursework that I don’t have to think about again; another faltering step on my long and meandering journey toward being useful.

Tuesday’s soundtrack

Today’s selection came out of my car’s delightfully outdated Minidisc player while I was risking life and limb on the byways of Devonshire, and has been augmented by what I put on when I came home.

Faith No More: The Gentle Art of Making Enemies
Extreme: Evilangelist
Led Zeppelin: Whole Lotta Love
Metallica: Poor Twisted Me
Offspring: Come Out and Play
Nick Cave and the Bad Seeds: Jangling Jack
Paradise Lost: Enchantment
Johnny Cash: One
Billie Holliday: That Ole Devil Called Love