Something goes wrong. You spot it in production. Or worse, a customer calls to complain about a shipment. You can’t imagine what happened. What do you do first? Where do you look? How do you find the problem?
While it’s important for a caster to have an understanding of the chemistry and metallurgy of his processes, sometimes the most important thing is to locate the problem, solve it and get back on line quickly. The practical what, where and when of the problem has to come first. Deeper analysis can come later.
In order to isolate and solve a casting problem, the caster must employ a practical problem-solving approach. Speed is of the essence, because downtime costs money.
As an example let’s say a good customer calls up to say he’s just discovered that 200 rings you’ve cast have unacceptable porosity. The problem didn’t show up until the last step of lapping and polishing, so he’s spent money in terms of labor for assembling, setting and polishing, and now the rings are unusable.
You’ve got to placate him, discover what the problem was and/or is, and stop it. But right off the top of your head, you have no idea what could have happened.
The average tendency is to panic. You start changing everything. This is a bad approach, because it’s not organized; it’s purely hit or miss. Even if you do by chance eliminate the problem, you ‘re never quite sure what actually did happen. So you’ve learned nothing.
Employ a scientific method. Take the problem piece, and change one variable per flask. For instance, set up ten separate flasks. Each one will have one difference: flask temperature; melting temperature; different types of spruing; different waxes; different people making the waxes; different batch of investment etc.
Then cast them up, finish samples from each lot and see which comes out best. This simple procedure alone will go a long way toward narrowing down what is and what isn’t the problem area.
While setting up this type of test, you should also delicately enquire of your customer whether or not he might be doing something different in his processes.
Look for patterns
In order to employ any kind of scientific problem-solving techniques, you must keep accurate records. A rule of thumb is when something starts going wrong, it continues, going from bad to worse. Things rarely get better or solve themselves.
If you keep accurate records, you may be able to see patterns emerge. For example at Atkin Casting, a chart recorder for burnout ovens was issued. This is basically a thermocouple that plugs into the oven to record time and temperature. It turns on automatically at the start of the burnout cycle, shows the time it takes to reach the proper temperature, and shows the exact temperature variation as the oven cycles on and off all night. At 8 a.m. the oven is turned down to casting temperature from its high of about 1400(F. This chart is compared with previous charts to check ovens once every six months. If any variations are noticed, something is happening; most probably the oven’s thermocouples are wearing out (thermocouples are sneaky; they go very, very gradually).
Look for patterns. In tracking down a problem, keep written records and look for common threads to emerge. Make up a problem chart and study it for patterns that show up rather like the connect-the-dots most of us did as children.
For example suppose you have prongs cracking. As you experiment to change the elements of the process, you find something showing up: An increase loss factor. You usually have X% loss per melt (you should know exactly what your loss factor is). All of a sudden, there’s a variation; the percentage of loss begins to go up. In all probability, you’re going to find other things happening as well.
Let’s say the gold is beginning to show pinkish or red. All of these things are beginning to point to alloy or melting trouble. The patterns show the way. Let’s say a customer calls and complains about incomplete or unfilled castings. This generally suggests a temperature problem-either oven or flask temperature. If it’s casting temperature, then perhaps your oven calibration is wrong-thermocouples have gone or are going. Maybe there was a radical variation in the ambient room temperature (the casting room will be cooler in winter and warmer in summer from the air circulating through the building ducts), which you didn’t compensate for in the casting temperature. If you are doing vacuum casting, it may be simply that the gold isn’t being drawn into the flask with enough force.
As you can see from the above example, any problem may have a great number of solutions. Your experience and expertise in casting will suggest immediate areas to study; but you must remember to keep an open mind. Don’t go onto autopilot and assume that the thing that went wrong is the thing that is always going wrong, or that it is the thing that most recently went wrong.
At Atkin Casting we once suddenly developed porosity troubles-a fine, misty porosity that showed up after polishing. After much anguished study, we tracked it down to our investment. While most casters believe that if the investment glosses nicely, it will produce good castings. But we now know that if the investment is not correctly mixed and vacuumed, there will be a change in the permeability of the plaster itself, resulting in an improper draw in your vacuum casting machine. And it doesn’t matter what the surface quality of the investment looks like.
This particular porosity problem eluded me. I set up my experiments, and they all pointed to the investment. I tore down our entire investing area-and found a leak in the vacuum system. The gauges all showed full vacuum, but a leak in a pipe was changing the rate of evacuation in the chamber. In effect it changed the entire sequence of everything in the investment process. But the investment looked great!
If you’ve reached a dead end, if you’ve taken every procedure and set up variables and still found nothing, then it’s time for a little help from your friends. Do all the steps of the process but one, then ask a friend from the industry to do this one step. For instance, I’ll make the waxes, set them up, someone else invests them, and I’ll cast them. Or, I’ll invest them and someone else will burn them out. Or, I’ll burn them out but someone else does the casting. In other words, if you can’t isolate something, maybe someone else can. Then when everything turns out well when they perform one specific step, you know you’ve isolated the area.
Many jewelry manufacturers and casters don’t have a very strong concept of preventive maintenance as a formal program. Many who do have some sort of program are misdirecting their money and their energies. Much of the industry believes in the philosophy “If it ain’t broke, don’t fix it.”
The trouble is that something’s already “broke” when an angry customer calls to say you’ve sent him unacceptable product.
At Atkin we’ve found that more often than not, problems result from mechanical rather than people or process failure, as illustrated by the examples cited in this article of oven/thermocouple and vacuum machinery failures. In fact preventive maintenance will solve many problems before they start.
You must remember that your machinery and equipment under-goes very heavy use, tough use under fairly adverse conditions-high temperatures, investment powder coating everything, corrosive atmospheres etc.
A machine costing thousands of dollars may have a $2 part-a spring that keeps a valve closed-go bad. For the most part the machine operates perfectly but that spring is ruining your castings. This happened at Atkin not long ago: A $2 spring lost its tension after prolonged use, and a valve stayed open-on the face of it a minor problem. But in fact, the result was that while we were building a vacuum up to 30 inches of mercury pressure during the vacuum casting, the rate of evacuation changed-taking probably an extra minute. The gauge read normally, but that extra minute or so was changing the properties of the cast. The result was bad castings.
Don’t assume that everything is working properly and that its technique that’s causing a problem. Don’t assume that everything’s working properly, even if everything looks like it’s working properly. A regular, planned maintenance check on different areas in your operation will bring to light many lax springs, weakening thermocouples, broken whisks, leaky hoses etc. We can all catch the big things-such as a compressor that stops running or a casting machine that doesn’t centrifuge. It’s the hidden little things that make an operation grind to a halt.
Here are a few of the most common areas where problems tend to be found. Certainly anything, anywhere can be a problem, but the three most common problem areas are:
- Wax making
- Temperature controlling (in nearly every process from waxing through burnout through casting and quenching)
What the wax looks like, the casting will look like. If the wax is in any way defective, the casting will be defective.
Wax problems fall into several basic categories: air bubbles; surface textures; shrinkage; and unfilled or very thin areas.
Bubbles: Sub-surface air bubbles are the most common problems with waxes. This type of bubble doesn’t look like a hole, and since it is internal, it doesn’t affect the actual surface of the wax. But if you look carefully at the wax while holding it up to the light, you’ll see a distinctive light spot inside the wax.
Many casters believe that it’s just the outer shape of the wax that melts out of the plaster, and that anything inside the wax makes no difference. But the fact is that if you have an air bubble trapped within the wax, and a relatively thin layer of wax between that interior bubble and the outer surface, there is a very good chance the wax will actually explode during the vacuuming process. The vacuum pulls the air to the surface of the wax, causing the wax skin to rupture outwards. The result is a hole, which will fill with investment. After casting and breaking out the investment, you will find a gaping hole in your casting.
Air bubbles can be caused several ways.
Wax temperature: The temperature of the wax may be too high, causing the wax to bubble, almost boil.
Mist or water in the wax: Water easily gets into your wax pot from your compressor. When you compress the air, condensation forms water, which goes down your airlines and drips into the wax pot.
Hot mold: The molds themselves can become too hot or too cold. A hot mold can cause bubbles. Sometimes when you have a big job to get out, you’ll give several molds to your best waxer and ask her to give you X hundred of these waxes by a specified time and forget about her other work for the moment. These few molds will heat up from the continuous hot wax injections, and the increased temperature within the mold itself will cause little air bubbles to form within the wax. Cycle your molds. Give them cooling down time.
Improper venting: The mold might not be properly vented with release lines or release cuts. If improperly vented, the trapped air has nowhere to go when the wax rushes in. The result is trapped air in bubble form.
Surface textures: If the wax’s surface isn’t perfect, the casting will not be perfect. People want a nice, smooth casting. Poor wax surfaces can result from a variety of things, most of them easy to check for and to spot.
The model: First things first. If you’re constantly getting a surface texture problem, go back and inspect the original model. Sometimes the model isn’t finished well; sometimes people put a layer of plating on a model and a poor plating job will build a texture right into the mold. Unfilled waxes or excessively thin areas may be traced back to a poor model as well.
Release material: Often the waxers will use too much mold release powder or silicon mold release spray (or worst of all, too much of both). The sprayed material in particular will actually mix with the hot wax entering the mold and form a coating on it. After investment and burnout, this coating is on the inside of the investment, and it will leave a discernible skin on the finished casting.
Wax shrinkage: Sunken surfaces or dishing within the wax itself; this generally occurs in a thicker piece. After the wax injection, the wax cools and contracts too much, leaving indented areas. Again there are several possible reasons for this sort of problem.
Mold temperature: Generally, cold molds will cause dishing. You come in on a Monday morning in the winter; the heat’s been off all weekend and the room temperature is chilly. When the waxer begins to inject the wax before the mold has warmed up, the hot wax will contract violently when it hits the cold mold. The result is dishing and indentations, along with unfilled areas. Excessively hot molds (see hot molds above) can also cause similar dishing troubles.
Large pieces: Often waxers aren’t used to working with unusually large and heavy pieces, so they might not give the mold enough injection time on the nipple. The waxer must be taught to hold the mold at the nipple long enough for the outer coating to begin to set up inside the mold.
Waxes: Preventive maintenance Careful care of wax and of waxing equipment and molds will often nip the above problems in the bud.
Give particular attention to the quality of the wax you’re using. If you reuse wax, be careful to make sure it’s clean. Never permit contaminants to get into your wax.
Sometimes your waxing problems can be the result of using the wrong wax for the job. Generally speaking, if you are casting finer, filigree pieces, you should use the freer-flowing waxes. On the other hand, these waxes are more brittle. Larger pieces require thicker and more elastic waxes; but while they don’t break as easily, they are not as liquid and free flowing. A little experimentation with waxes can give you the right trade-off between plus and negative qua1ities, and might well solve some of your filling or dishing problems.
Inspect the compressors to your wax pots regularly, and drain them of water, daily if necessary.
Delegate someone to inspect the molds for caked-up powder or silicone release. The person who pulls molds or puts them away is the most likely person for the job. If too much of these materials are on the molds, then your waxers must be taught to use less.
The investing process is probably the most important single aspect of the casting process; unfortunately it’s one of the easiest to make a mess of. Proper investing is very similar to a chef following a recipe-vary the recipe, and it won’t taste right; you’ll still come out with food, but it might not be as good as it ought to be.
The correct investing procedures must be followed rigorously, and by the book. Batch worktimes are specified by the manufacturers. Vary the specified procedures, and your castings will necessarily be of uneven quality.
Investment problems include pimples or nodules on the castings; water marks or streaking on the surfaces; gross porosity; cracking or finning; and surface roughness.
Pimples/nodules: This is a fairly common problem, almost always caused by air bubbles that attach to the wax. As a rule, the culprit here is poor vacuuming, though the cause can sometimes be traced to investment that sets up too soon.
Vacuuming: If pimples and nodules are caused by air bubbles adhering to the waxes, it stands to reason that you are not vacuuming properly. Everyone knows that vacuuming should be done twice-once in the bowl and once in the flask itself. However the temptation to shortcut processes is strong, especially when work has to be done in a hurry, and cutting out one of the vacuuming steps seems to be an easy shortcut to take. Don’t. Go by the book and vacuum in the bowl and in the flask. Failure to do so is sure to result in nodules.
Setting up too soon: Part of what we mean by “going by the book” is to follow the investment manufacturer’s specifications for work time. Failure to do so will result in all sorts of problems down the line. If you have a specified worktime of ten minutes, and your investing person is taking eleven minutes to get the investment into the final vacuum, or he’s using water that’s too hot, the investment material will be thicker than it should be during vacuuming. The thicker slurry makes it more difficult to pull the air bubbles out.
Follow specified worktimes religiously.
Vacuuming equipment: If you are vacuuming twice, and if you are following instructions for work times, yet are still getting pimples or nodules, consider the possibility that you’ve got a problem in your vacuum system. Weak vacuum pressure or leaks will not always show up on gauges, yet they will adversely affect your vacuuming process.
Make sure your equipment is working properly.
Water marks/streaking: These are generally caused by improper measurement of the investment ingredients. A lot of people-especially the old timers-feel they can mix by eye. Measure your powder, measure your water, make sure the temperatures are proper (not only water temperature but investment temperature as well), make sure the mixing is good, and work within the mixing times specified-no longer, no shorter. You should also be sure your water is uncontaminated.
Gross porosity: This is the result of investment breakdown. This can be caused by contaminated investment. Investment is a natural material, and lots will vary in consistency and quality. There is also the possibility that your water can be at fault. In a lot of older buildings, especially here in New York where the building water is often stored in roof tanks, the water can pick up lots of contaminants-rust, fungi, algae etc. These can cause investment breakdown. You should consider some sort of water filtration if you suspect this is the area of your troubles.
Most often, however, investment breakdown comes from improper mixing. Follow the book.
Finning/cracking: These thin cracks around the edges of pieces are generally caused either by too much water in the mix (“thin mix”) or by an overloaded flask.
Again proper blending of the right amounts of water and investment and attention to work times will solve much of the problem.
In the case of overloaded flasks, care should be taken not to jam too many pieces into one flask. Remember heat expands; there is a moment during burnout when the wax will expand just before melting and going up in smoke. If you have a densely-packed flask full of waxes, you are increasing the expansion potential within the flask, as well as decreasing the amount of investment that can fit between the waxes. Too much expanding wax plus investment walls that are too thin and weak equals cracking of the investment. The result will necessarily be cracking and finning on your castings.
Surface roughness: This is often caused by the settling out of the investment. Again this is the result of not following proper investment procedure, in this case working too fast and not using the total worktime specified.
If it takes you ten minutes for the material once mixed to harden, and your investment person uses only eight minutes of that time before pouring the mixture into the flasks, you’re going to have two minutes of very wet investment sitting with the waxes. While in this liquid state, the investment will settle. The heavier particles will drop to the bottom while the lighter liquid rises to the top. You’ve lost your true mix.
Know your worktime and use it. You want the material to start setting up very soon after you put it into the flask.
Surface roughness can also be caused by moving the flasks too soon after they’re invested. Don’t assume that because the flask glosses off on top and begins to harden that it’s ready. It may look good, but if you move the flasks too soon, you’ll get all sorts of internal shifting of the plaster mold. Roughness-and sometimes finning-will result. Give the flasks 45 minutes to an hour to set up properly.
Also make sure that the surface you’re putting the flasks down on is absolutely steady and vibration free. Vibration is not uncommon in casting areas. For instance your investing area may be next to a room with a compressor; the resulting vibration can cause a slight shifting of the investment, resulting in a rough, dull, even fuzzy-looking casting.
Investing: Preventive maintenance Store your investment carefully. If you use large quantities, make sure that you rotate your stock, because the investment tends to settle out a bit in storage. Don’t get sloppy and let barrels sit for a long time in the back of your storage area because you’re too lazy to climb over or move newer and fresher material to get at it. You’ve heard of the FIFO method of accounting? That means first-in, first-out, and that’s how you should handle your investment. Always use your older material first.
You should also roll your barrel around a little before using it. This helps remix the ingredients that might have settled during shipping and storage.
Investing by the book is all well and good, but if your equipment is faulty or broken or gauges register improperly, mixing by the book won’t help much. Check your scales to see if they measure and register properly. We have water temperature gauges installed in our taps to be absolutely sure that our mixing temperatures are right.
We also measure temperature of the powder itself. Ambient room temperature changes from season to season; if you’ve got 50 barrels of investment stored out back in an unheated storage room, you’re going to have cold powder. Dumping cold powder into warm water is going to change the temperature of the mix, so the water has to be warmer to compensate for the cold powder. Conversely in the summer those barrels are sitting in a 115°F room. Then the water should be cooler to compensate for the hot powder.
Watch your mixing machines. The steel whip, which mixes the investment just like cake batter, is composed of a series of elliptical wires, which will break after a lot of use. Naturally the longest wire-the outside one-is likely to break first; then the next one, and so on. Pretty soon you’re not, going to be reaching the slurry at the bottom of the mixing bowl. This will radically change the proportion of the rest of the slurry mix. Keep your eye on that whip, and effect repairs or buy a new one when wires begin to break.
Inspect your vacuuming equipment regularly. Check hoses for leaks that can reduce your vacuum pressure; also make sure that your rate of evacuation is proper. Remember not everything shows on the gauges, so check hoses, couplings, etc., all the way back to your vacuum pump.
Make sure your water is clean and uncontaminated. Filtering systems are easy to install and inexpensive.
The temperature of materials, equipment, processes and even the ambient room temperatures within the factory are all extremely important, and many casting problems can be tracked right down to improper temperatures, from wax that’s too hot to burnouts that are too cool. Here’s a quick look at the areas where temperature is critical.
Mold making: Watch the temperature of your vulcanizers. If they aren’t hot enough, your rubber mold may not cure properly; if too hot, you can burn the rubber.
Waxing: Your wax pots can be too hot, causing dishing or bubbles; or too cold, resulting in unfilled areas. Check the temperature of your wax regularly. Open the top of the pot and immerse the thermometer into the wax itself; you’ll get a much more accurate reading than by using the thermometer tube most wax pots have built in. The wax manufacturer will specify the ideal temperature. Know the specs and follow them.
The temperature of the compressed air feeding the wax pots can be too hot, causing excessive moisture and condensation. The temperature of the molds being injected is also important; if the mold is too hot, you can get bubbles in the wax; too cold and you can have filling and dishing problems.
Ambient room temperature can be a problem, since waxes are more brittle when it’s cold. If the room is chilly, particular care must be taken while handling the waxes-treeing, set-up, or working with stock waxes from the shelf. Not only can cold waxes break during handling, but they can snap right off the sprues when investment is poured over them.
Investing: Water temperature is critical. Powder temperature is critical. Again room temperature can be critical, because it will affect the worktime of the material. Compensate for room temperature.
Burnout: We find that the best burnout temperature is between 1350ºF and 1400ºF. Make sure of the accuracy of your oven temperatures. Don’t burn out too hot, or you’ll get some investment breakdown; don’t burn out too cool, or you’ll get carbon residue within the cast itself (a giveaway here is a gray residue on he investment, a black residue on the castings). Incomplete burnout always leaves carbon residue.
Also don’t let the oven flame hit the flask directly; adjust your flasks in the oven so that the flasks can heat evenly. First direct flame will shorten your flask life, literally burning up your flask. Second inconsistent heating will probably cause one area of the waxes on the tree to melt before the center sprue melts. If this happens, the melting wax will have nowhere to go, so it will boil within the mold and break down the investment.
You should also consider using different burnout temperatures for handmade waxes; heat them slower than your regular injection waxes. You should probably have them on a different temperature cycle.
Casting temperature: Both flask and metal temperature are important. Keep metal temperature consistent and vary flask temperature; don’t vary both.
Consider the ambient room temperature-you’ve got probably between 15 and 45 seconds from taking the flask out of the oven to the actual casting: take flask out, put it in casting machine, go to melter, take gold out, pour gold in etc. Room temperature will affect that flask and thereby affect the casting.
Quenching is very important. Improper quenching in water too soon will cause the castings to crack-very much like immersing hot glass in cold water. Let the flasks cool off (ten minutes to a half-hour) before quenching.
Consistency of procedure is critical at Lou Atkin Casting. The only possible way to get consistent quality in your product is to have consistent procedures day in, day out from beginning to end.
Consistent procedures also are very important in helping the caster track down a problem. If the caster is so inconsistent that everything is done a little differently every time, he will never be able to figure out what he did wrong, and thereby be unable to prevent the problem from reoccurring.
At the risk of sounding corny, casting is a chain only as strong as its weakest link. If that chain breaks down anywhere, it’s shot. If the model is bad, the casting is bad; if you have a good model but a bad mold, the casting is bad; if you have a good mold but a bad wax, the casting is bad and so on down the line.
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