I posted recently what a typical day in the workshop looks like and received some excellent feedback from different people who were interested in some of the nitty gritty that happens during a typical day. I've had a couple of weeks to reflect on that blog entry and there was one thing missing. What happens when things go right.
You see the most common problem I have in the day to day work when I'm in the workshop is that most often the problem is that very little goes right. And by right I mean "up to my perfectionist-ic standards of what could as a quality cast". The issue is not that I have a problem when things go right but rather than the rarity of it happening means that when the opposite happens it can be it's own issues.
An example with some background. There are three main issues that can go wrong when you are casting models. There can be too much or too little pressure. Too much or too little speed. And there is the issue of temperature.
When you put a mold into a centrifugal spin casting machine the first thing you have to do is lock the plate onto the mold (see pic of Pod 2 machine). When the lid is closed it triggers a switch that then allows pressurized air from the compressor to push the mold up against the locking plate using the bottom plate to push it. There is a control that allows an operator to adjust the pressure. Exactly what pressure is used is a highly guarded secret and I'd have to kill you if I told you…
Just kidding. The pressure to use is dependent on a number of factors. The first is the age of the mold. This might seems weird but just like people the rubber molds that we use get stiffer and less flexible over time as they have more and more castings made with them. All that hot metal flowing through them eventually overcooks the mold and causes the rubber to get brittle and eventually break. When that happens small details break off and we get a problem that we call 'gunk'. This most often happens in molds that have severe undercuts like in the gaps between shoulder plates and under legs or any place where there is a tight angle that can pinch the rubber. The older the rubber mold the more pressure needs to be applied because the gap between the sides stops naturally flexing against the other side and allows metal to flow between them. A little bit of metal seeping is normal and is called flash. This is inconsequential when it is 0.1mm or less thick but if the mold is old and the sides are not pushed together with at least 20psi (pounds per square inch) then more metal will seep, maybe as much as 1mm which is tricky to remove and does not look good at all.
A new mold will require anywhere from 13 to 17psi depending on how large the parts in the mold are. Smaller parts require less pressure. Typically the first spin of a mold is a test spin. This is where the speed becomes a factor. Too much speed and metal will be forced between the mold halves regardless of the pressure applied and result in flash. Not enough speed and the metal may not be pushed through the mold with enough force to enter all the cavities and result in a miscast.
In terms of issues that there can be a casting miscasts are far worse than flash as a problem to have. Flash is actually a good indicator. It means that the mold filled with enough metal to push some extra metal out. Rarely this can be false so I always have to check every part carefully. But if I see flash I am usually pretty happy. A small amount can be good since it can be removed easily. Too much and the part if not worth the time it takes to clean it, I just adjust the settings and spin it again. A spin only takes 30-45 seconds or less if the mold is cool/cold so a corrected spin does not take a lot of time. This is the main advantage of metal casting, the speed.
The last issue that happens is something that must be pretty unique to the situation in Canada. Temperature.
And when I say temperature I do not mean the temperature of the metal, though metal that is not at the right temperature can cause flash or miscasts identically to issues with speed or pressure.
When I make a mold I make cuts to allow metal to flow into the mold cavity – called gates – and to allow the air in the cavity to escape – called vents. I always try to cut the space as small as possible. Like many hand crafting skills it is pretty easy to cut something more but it can be very hard to cut something less than it already has been cut. When I cut the mold after vulcanizing it the mold is usually between 200 and 250F because I dunked it in some cold water to get the mold plates off. As the mold cools the mold can shrink by 3-5% so a vent that is cut too small can be blocked causing air to be trapped and prevent the mold cavity from filling in all the way. In the winter in the workshop the temperature can dip to 15-16C degrees overnight which can cause those gates and vents to shrink and close. Typically a single pour of metal is required just to heat up the mold and the first casting goes straight in the recycling bucket without even getting checked since the chance of miscasts is nearly 100% in those conditions.
The opposite problem can happen when it's summer and the temperature swings up to 35-40C. In that case not much happens since the molds easily hit those temperatures when operating. When it gets hot the problem is usually more that the operator gets disgustingly sweaty, tired and very very grumpy to be missing the excellent summer weather.
When there are so many things that go wrong it can be as much of an issue when the pressure, speed, and temperature the biggest problem is complacency. It can be too easy to stop checking and rechecking the parts and settings. 'Being in the zone' can be a bad thing in this case.
So there's another snapshot of things that matter when making models. If you are a kickstarter backer don't forget that you have until the 16th of May 2016 to get additional orders added to your kickstarter rewards. Time to put the pressure on!