Spin Doctor

An Expert Offers Cures for Centrifugal Rubber Mold Casting Ailments

Spin Doctor An Expert Offers Cures for Centrifugal Rubber Mold Casting Ailments Centrifugal rubber mold casting (CRMC) is a common jewelry manufacturing process, particularly in the production of costume jewelry.

CRMC is a method of producing castings using a rubber mold that has one or more cavities and that revolves around a central axis.

Molten metal is poured into the mold’s center, and is distributed by centrifugal force to the cavities through a series of runners and gates.

The centrifugal force is produced by a machine that spins the mold at a pre-selected speed.

The rubber molds that are used are reusable to certain point, low in cost and easily made. The metals typically used are alloys that melt at relatively low temperatures and have the strength, weight or ductility necessary for the design of the piece.

This article contains excerpts from the “Troubleshooting” chapter of a book called Principles of Centrifugal Rubber Mold Casting, written by Jerome Gonicberg and me. Here, I’ve identified 10 problems associated with CRMC, and provided solutions that are easy to implement.

Problem I: Metal Contamination
The alloy is sluggish, low in fluidity, and producing heavy drosses. The temperature required to cast it is abnormally high. You’re getting a high rejection rate.

Causes and Cures:

1. Check the alloy to see if it is contaminated. Check with the supplier to see if it is made of non-virgin (recycled) metals. If contamination is suspected, try a fresh ingot of control metal that is melted in a small pot floating in the large pot.
2. The alloy has been run at too high a temperature, and some of its constituents have burned out. Replace the alloy and check the proper operating temperature with the supplier.
3. The alloy may be too cool. Raise the temperature in 50°F increments.
4. Poor metal-maintenance procedures, especially when fluxing or adding metal, may be the cause.

Problem 2: Bleeding after Plating
Black spots or discoloration appears on your castings after they have been plated.

Causes and Cures:

1. The castings had porosity in which one of the plating solutions was trapped. The solution then bled through at a later stage. Porosity usually indicates poor gating or venting procedures. (See Problem 8.)
2. Inspect the models, the castings and the mold cavities to see if the problem is true porosity or roughness and pitting in the model.
3. Check the alloy to see if it is faulty. Melt a small control pot of metal that is known to be pure, and compare it with the original alloy.
4. Make sure the alloy is at the proper operating temperature.
5. Check the work of your plater by sending a batch of castings to another plating house. Poor plating procedures, especially during the cleaning cycles, or inattention to plating voltages and timing, may be the causes.
6. Some rubber compounds may have an excessively high sulphur content. Casting alloys react chemically with sulphur. It’s difficult to prove that the rubber is bad. Using a different rubber compound may correct the problem.
7. Contaminated (non-virgin) metal alloys may be the cause.

Problem 3: Insufficient Detail
Your castings fail to reproduce detail with sharpness, fidelity and clarity .

Causes and Cures:

1. The original model that was used to cure the mold lacked sharpness of detail, or may have lost some detail through handling. Models should be examined prior to each step in molding to ensure that they have not been damaged.
2. The alloy used for the models was incorrect and did not reproduce all of the detail. Certain castings may require an alloy that is higher in tin content.
3. A poor compression or timing during curing can cause poor reproduction of the model in the mold.
4. The alloy may have been below its liquidus, or too cool by the time it reached the cavities. (The liquidus is the temperature above which the alloy becomes completely molten.) Raise the temperature, pour with more metal in the ladle, or use a different alloy with a longer pasty range. The “pasty range” is the range of temperatures between an alloy’s solidus and its liquidus. (The solidus is the temperature at which the alloy begins to melt.)
5. Placing models face down in the mold before curing may enable the rubber to pick up more model detail.
6. Preheating the half of the mold that contains the detail before placing the models in it for curing also may enable the rubber to pick up more model detail.

Problem 4: Flashes or Fins
Your castings have a thin, uneven-flash or fin of excess metal that requires additional cleanup. The problem may include flashing, or “squirting,” of molten metal out of the molds while the CRMC machine is spinning.

Causes and Cures:

1. There is an incorrect air-pres- sure setting in the CRMC machine. Raise the pounds-per-square-inch (psi) setting in one-pound increments until the problem is solved.
2. The molds may require shimming. “Shimming” is casting the mold with a rubber, cardboard or paper form (the shim) to compensate for variations in a mold’s dimensions. (See diagram 1.)
3. The temperature of the alloy being used is too high, increasing the metal’s fluidity and causing squirting. Decrease the alloy temperature in 50°F increments to a minimum of 50°F above the alloy’s liquidus.
4. The maximum mold life has been reached. Make a new mold set.
5. The pressure plate, or hold-down plate, of the CRMC machine may be locked in the wrong direction, given the direction of the machine’s spin. (See diagram 2. )
6. There are not enough mitering nuts in the mold design.
7. Too much metal was poured during the casting sequence.
8. Metal is being poured too quickly. Pour more slowly.

Problem 5: Alloy Solidifying in Funnel
The alloy freezes too quickly. It is solidifying in the funnel on the pressure plate.

Causes and Cures:

1. The funnel or the pressure plate in the CRMC machine has been gouged or damaged, and the alloy has been freezing on the damaged area. Check the funnel or the pressure plate for damage, and grind or polish any damage that has occurred. If repair is not possible, replace the funnel or the pressure plate. Ceramic funnels are available, and may work better than metal.
2. Spray the funnel and the pressure plate sprue opening with a graphite or silicon spray.
3. The alloy may be too cold, which will allow it to solidify as soon as it reaches the cold metal funnel or the pressure plate.
4. The ladle may not hold enough alloy to keep the metal’s temperature high enough during the transfer from the pot to the machine.
5. The rubber mold was placed in the machine with the sprue hole facing down. (You really didn’t do that, did you?)
6. The mold may be too cold to cast properly. Pre-heat it on a vulcanizer or over the melting pot, or make a few more passes.
7. The mold may need more venting to allow the metal to enter the cavities.

Note: A spare funnel and pres- sure plate always should be handy.

Problem 6: Incomplete Castings
The cavities are not filling, producing what is known as “misruns.”

Causes and Cures:

1. The alloy may have been poured too quickly, thus choking the mold channels before it could be distributed to the cavities. Try pouring the alloy more slowly.
2. There’s not enough alloy in the ladle. Dip more metal, or use a larger ladle.
3. You may be pouring so slowly that the alloy is freezing in the gating system before it reaches the cavities. Try a different pouring method, one that is more rapid and direct.
4. The alloy is not at the correct operating temperature. Metal that is freezing-mainly in the mold’s sprue and basin area-is a good indicator. Raise the temperature of the alloy in 50°F increments.
5. The mold is not hot enough. Allow it to heat up by making a few consecutive passes before making any corrections in its design.
6. The revolutions per minute (rpm) rate of the CRMC machine is too slow. Increase the speed in 50- rpm increments.
7. Try reversing the direction of the machine’s rotation.
8. There’s too much air pressure in the CRMC machine. Lower it in one-pound increments.
9. The mold may require shimming. If it already is shimmed, try removing the shims.
10. There are not enough gates or vents. Metal is not flowing properly. Cut additional vents to the part of the cavity that isn’t filling. If this does not solve the problem, enlarge the gating.

Problem 7: Mold Burnout
The mold has failed because of tearing or burnout.

Causes and Cures:

1. The service life of the mold has been exhausted.
2. There is premature mold failure because of too few molds being used in a casting cycle. Nine to 10 are recommended.
3. There is an incorrect pressure setting in the CRMC machine.
4. There is incorrect shimming that allows metal leakage and flashing during the spin cycle. If not corrected, this can quickly ruin a mold.
5. The rubber is defective, which is difficult to prove.
6. The mold is designed poorly. It may require additional venting, larger gates, or a different in-gate location in the cavity .
7. Combining thin and heavy castings in the same mold requires a higher pouring temperature, which may burn the cavities of the thinner castings. Redesign the mold, or accept a shorter mold life.
8. The casting design may necessitate redesigning the mold as a sectional or modulated type, especially if rubber-tearing is the problem.

Problem 8: Porosity
Parts of your castings have porosity and discoloration.

Causes and Cures:

1. The alloy may be contaminated. Melt a pure ingot of the same alloy in small pot floating on the surface of the large pot, and trial-cast the mold to check it.
2. The alloy temperature is incorrect-probably too high.
3. The alloy may be made from non-virgin metal.
4. There has been poor alloy maintenance, usually caused by the inadequate, incorrect or unsystematic fluxing or adding of metal.
5. The rubber mold may be burned and transferring its roughness to the casting. If so, pieces of burned rubber may be embedded in the casting. Examine the mold with a magnifying glass to confirm this.
6. There is incorrect gating or venting in the mold.
7. The original model or the production model may be rough or pitted. Examine both with a magnifying glass. If they are faulty, correct them and make a new mold.
8. The models that were used in molding may have reacted chemically with the mold rubber. Plate the production models with rhodium, and cure a new production mold.
9. Reverse the rotation of the CRMC machine. If the porosity appears in a different part of the casting, it indicates a problem with the venting. Correct it by adding vents to the part of the casting where the problem occurs.
10. There is too liberal an application of talc. Cast a few spins without talc, or wash the mold with a mild detergent solution.

Note: Do not polish away porosity or roughness. They will remain and cause bleeding after plating.

Problem 9: Rubber Flash in the Vulcanizer
During curing, a large amount of rubber has oozed or flashed out of the mold frame, making the mold unusable.

Causes and Cures:

1. Check the mold frame for wear or damage. Frames that are worn or gouged may allow rubber to flow out during curing. Repair or replace the frame set.
2. In placing the frame cover over the uncured molds, or when placing the frame into the vulcanizer, shifting of the models, locators or basin-formers may have occurred.
3. If the models are highly modulated or very large, and excess rubber is not removed from the uncured mold sets to accommodate the models, some rubber will flash out of the frame. The space in the mold frame is limited, and displaced rubber must go somewhere.

Note: Some rubber flash is normal during the curing operation because rubber expands when it is heated.

Problem 10: Shrinkage
Your castings display extreme shrinkage. They are too thin.

Causes and Cures:

1. The original model design did not allow for shrinkage. Make the models heavier or thicker to compensate for this.
2. The alloy used for the models was different from the alloy used in production. The difference in the coefficients of shrinkage was not anticipated.
3. The mold halves may not be parallel, and may require shimming.
4. Inadequate gating may not be providing a solidification feeder head. Enlarge the gates.
5. More vents may be required.
6. The durometer of the mold set may be too high.
7. The air-pressure setting of the CRMC machine may be too high. Lower it in one-pound increments.
8. The alloy maybe too hot or too cool.
9. The gate entrance may be incorrect.
10. The speed of the CRMC machine may be too slow. Increase it in 50-rpm increments.
11. The operating temperature of the molds may be too hot because too few molds are being used in a cycle. The molds may require a special cooling table.

AJM, June 1993 Marshall Ritch

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