Principles of Centrifugal Rubber Mold Casting : Chapter 2

1. The pewter caster’s job and qualifications
2. The pewter caster and the pot
3. Techniques for pouring molten pewter
4. The pewter caster and the mold
5. The pewter caster and the casting machine
6. Shimming
7. Pewter Casting Law Summary
8. Equipment and supplies for the pewter caster

2.1: The pewter caster’s job and qualifications

The caster’s job is central to the success of any pewter casting operation. The many variables for which he is responsible are reflected in the definition of the vocation given by the United States Government Dictionary of Occupational Titles:

Caster, D.O.T. 502.782 (Casting Machine Operator, Jewelry) operates one or more centrifugal casting machines to cast metal parts of costume jewelry. Dusts inside of rubber mold with chalk dust compound to facilitate flow of metal during casting and prevent rubber from burning. Fits halves of mold together and places them on mold setting. adjusting working pressure by placing discs under mold. Positions metal pressure plate on mold to level mold on machine. Sets dial for specified pressure and speed of rotation, closes lid and starts machine. Pours molten metal through funnel into mold using ladles. Stops machine and removes mold. Separates halves of mold to remove castings using pliers.

Casting is basically a trade. But it is also, to some degree, a craft. When the caster is a highly competent and creative individual, it can become an art. While it is the moldmaker who creates the rubber molds, success or failure depends largely on the caster. He controls the production speed, quality, metal costs, and scrap return. He can make a good mold bad, and a bad mold good. He can easily ruin expensive casting metal by running it inattentively. The way he handles the molds that he casts has a decisive effect on how long they will last. Therefore, the caster must be every bit as carefully trained as the moldmaker, and every bit as conscientious. He, too, must understand the entire casting process, from how the molds are made, to the physical characteristics of the alloys that he uses. Above all, a caster must possess a sense of responsibility, above average judgment, the ability to concentrate and ignore distractions, and the ability to remain calm and think clearly when processes do not go as planned or expected.

2.2: The pewter caster and the pot

The caster is entirely responsible for the pot of molten metal. Molten pewter is expensive, easily ruined, and dangerous. How carefully the caster runs the pot and how much attention he pays to the specific requirements - particularly for temperature of the alloy he is running affect significantly the quality of the finished castings, mold life, yield per pound of alloy, and even the overall safety of the casting shop.

Pot level: The temperature of the molten pewter in the pot is not always uniform throughout. The surface may be somewhat cooler than the bottom of the pot. Nor does the thermostatic control on the pot respond immediately to changes in temperature caused by the introduction of new pewter ingots, gates, or cool air blowing across the surface of the molten metal.

To help compensate for the variations that occur as pewter metal is used, a pot of molten pewter should be stirred occasionally as new metal is added, or its level drops. The pot should always be kept at least 3% full. In any case, the level of the pewter in the pot should never be allowed to drop below the tip of the temperature probe in the probe well because both the probe and the control may be damaged. Keeping the pot full not only helps keep the alloy’s temperature constant, it makes it easier to dip into the alloy when filling the ladle, and allows new pewter and gates to be introduced without causing a drastic drop in pewter temperature. Pre-heating ingots on the lip of the pot and preheating the ladle before using it to cast also help maintain uniform temperatures.

Temperature: The ideal temperature at which to run an alloy is one as little above the alloy’s liquidus (“completely molten temperature;” see Chapter 4.1) as possible. A rough rule of thumb is approximately 50°F above liquidus. But the cooler an alloy is run, so long as it is above its liquidus, the better the castings it produces will be, and the longer the life of the metal and the molds. Unnecessarily high temperatures ruin alloys and always lead to higher costs.

Some of the effects that incorrect temperatures have on alloys, molds. and castings are:

Temperature too high: - Increased liquidity and possible flashing Premature mold burnout Heavy drosses (alloy burnout) Porosity - Excessively long time required for castings to solidify

Temperature too low: - Cavities do not fill properly the cavities - Alloy freezes in the gates before reaching - Heavy drosses - Lack of metal fluidity - Lack of alloy homogeneity causing porosity, weakness and plating problems

Dry metal: Wet metal is explosive! Putting wet, cold or damp metal into a pot of molten metal can cause the metal to explode. New metal and gates being returned to the pot must be absolutely dry and at room temperature or warmer. Ingots that have just been delivered from cold or inclement weather should be allowed to dry and warm up before they are used. Ingots can be preheated before use by placing them near the pot or on its ‘lip’

A conscientious caster should never allow drinks or other liquids near his melting pot. He should develop the habit of checking every piece of metal by sight before placing it in the pot because he will not be able to detect moisture on the metal if he is wearing gloves.

Sweetening: As a caster works, he constantly adds gates and new metal to the pot. This must be done systematically. The usual ratio is 50% new metal (‘sweetening’) to 50% old metal in the form of gates and defective castings. It is absolutely crucial that the caster return to the pot only gates and ingots of the same alloy that he is running. A single gate or ingot of a different alloy can ruin completely the entire pot of metal. (Ref 4.14)

Fluxing: Every bit as important as knowing how to cast is knowing how and when to flux the metal to maintain it properly. Fluxing cannot be done at random when the caster happens to think of it. It must be done methodically at regular intervals. The procedure is described in Chapter 4.8.

Summary outline: Maintaining a pot of molten metal

1. Always run the alloy at the lowest possible temperature, usually at least 50°F above its liquidus temperature.
2. Never skim, flux or stir a pot of molten metal that is below its liquidus temperature.
3. Stir the molten metal when adding new metal or gates. Stir as the level drops. Stirring helps even out the heat in the pot.
4. Calibrate the heat controls periodically to ensure that they are accurate. Use a thermometer to check their accuracy.
5. Keep the level of molten metal in the pot high enough to allow it to absorb new metal without a drastic drop in operating temperature, full at least.
6. Prevent drafts and cool air from blowing across the molten metal’s surface.
7. Use the ratio of 50/50 when adding new metal and gates.
8. Do not mix alloys and gates of different compositions or metal from different alloyers. Do not try to improve one alloy by mixing it with another.
9. Use a separate pot for each alloy so that pots do not have to be cleaned each time they are used. Never use a pot that has had a zinc alloy in it for any tin or lead alloy.
10. When fluxing, make sure the alloy is at least 50°F above its liquidus.
11. Always label each pot clearly with the alloy it contains to make identification easy and to prevent mixing alloys.
12. Always push the surface drosses aside when dipping the ladle for a pour.
13. Always maintain a heel of metal in the pot. It will speed up the melting of new metal since there is a larger surface in contact with the source of heat in the pot.
14. When molten metal must be left in the pot without any supervision, be sure that it is above its liquidus. Otherwise, half-melted crystals will rise to the surface and oxidize, which will cause loss of metal and changes in the alloy’s composition.
15. Metal should be melted as quickly as possible to prevent the formation of crystals.

2.3: Techniques for pouring molten pewter

Pouring or feeding molten pewter metal into the CRMC machine requires careful attention to technique. Different techniques produce different results.

Before dipping, the ladle should be preheated on the lip of the pot. A cold ladle will lower the temperature of the metal into which it is dipped significantly. When dipping the ladle into the pot, the surface of the molten metal in the pot should be pushed aside before the ladle is allowed to fill. This procedure helps prevent surface oxides from being poured into the mold along with the alloy. The ladle should always contain more metal than is needed to fill the mold. The extra amount helps prevent the metal from losing too much heat during its transfer from the melting pot to the CRMC machine’s funnel. Preventing premature cooling is especially important when a metal that has a short pasty range is being worked, since pouring metal that has begun to cool may result in the metal’s freezing in the gating system before the mold’s cavities have filled.

Fig. 2.1: Pouring Techniques

The technique used to pour the pewter into the CRMC machine funnel also affects heat loss. A good caster learns to gauge his pouring technique to the particular mold he is casting and the alloy he is casting it with. Pouring molten metal into the funnel steadily and fairly rapidly, straight down into the mold basin, prevents it from losing much heat. Pouring the metal against the funnel gives it a spinning action as it enters the basin which, in turn, gives the metal funnel time to absorb some of the heat of the molten alloy. Metal can be poured slowly or simply dumped all at once. (Fig. 2.1) Each of these pouring techniques is used by casters, and each produces different results. Different molds and different alloys require different pouring techniques. Trial and error is the only way to determine which is best for a particular mold. Once the best technique has been found for a particular mold and alloy, it should be used consistently throughout the run. An accurate record should be kept for future reference. (See Chapter 17.2)

2.4: The pewter caster and the mold

How carefully and conscientiously a caster works with the mold largely determines the quality of castings that it will produce and is also the single most important factor affecting its production life. A good pewter caster follows procedures religiously each time he puts a mold in the CRMC machine.

Talc: A mold should be dusted with talc each time it is cast. The talc used for molds is the same substance that talcum powder is made from, crushed magnesium silicate. Talc used for casting must have a very fine mesh, since larger particles may get trapped in mold cavities and cause casting problems.

Fig. 2.2: Talcing mold prior to casting (with talc bag)

Talc helps prevent the mold from burning out prematurely, facilitates the flow of pewter metal through the gates and into the cavities, and, like flour on a pastry board, helps prevent the finished castings from sticking to the rubber. Talc also seems to promote the escape of gasses, heat, and air from the mold cavities as the metal flows into them. Talc can be applied either with a talc bag or with a small paint brush. A talc bag can be made from an old sock filled with talc and securely tied at the opening. If a paint brush is used, the 2” width works best. The talc is usually applied directly to one half of the mold set, the half in which the cavities have the most detail. The two halves of the set are then clapped together to distribute the talc and remove the excess. The pewter molds should then be rotated 90° and clapped together a second time. Talcing must be done under an exhaust hood to keep talc dust out of the work area and protect worker health. The caster should always wear an OSHA approved mask to prevent his inhaling talc dust whenever he is in the casting area.

Care must be taken not to apply too much talc. Excess talc can build up in mold cavities and cause loss of detail in the castings. Should this occur, the molds can be cleaned with an air gun or washed with detergent. Solvents should never be used, since they may attack the rubber. Simply running the mold one or two passes through the CRMC machine without talcing will also eliminate excess buildup.

Fig. 2.3: Clapping mold halves to distribute talc

Mold cycles: To preserve and lengthen a mold’s production life, the standard mold cycle should consist of at least 9-10 molds that are cast in rotation. Because the heat of the metal in the molds directly affects the life of the rubber, using a 9-10 mold cycle lengthens mold life by allowing the molds to cool off between pours. In some casting shops, the caster’s work area is equipped with a forced air draft and open cooling racks on which the molds are arranged between pours to dissipate their heat. This arrangement contributes noticeably to longer mold life.

Fig. 2.4: Positioning of the mold set in CRMCM

Removing finished castings: Pewter casters usually use the period during the CRMC machine spin cycle to open previously cast molds that have cooled and solidified and remove the finished castings. Castings should not be allowed to get too cool in the mold before they are removed. Rubber shrinks as it cools, making it more difficult to remove the castings. It also increases the danger of the rubber’s tearing.

Most casters use a pair of needle nose pliers to remove completed castings along with their gates and runners from the mold. The procedure is to flex the mold slightly while working the casting unit free from the rubber. After the casting units have been removed, they are placed in a stack for break-off at a later time, although the caster can sometimes perform this operation as well. When the casting units are stacked, it is easy to observe the caster’s production rate.

Fig. 2.5: Placement of shim, mold set and hold down plate in CRMCM

2.5: The pewter caster and the casting machine

Machine RPM: The spin speed of the CRMC machine is variable, and ranges from 300 to 650 RPM. The casting machine should always be run at the lowest possible RPM at which a good casting can still be obtained. The only way to determine the speed for a particular mold is to make a number of trial runs. As a rough rule of thumb, the smaller the casting, the higher the RPM, and the larger the casting, the lower the RPM. Small molds require higher speeds, while larger molds require lower speeds. When experimenting with a new mold for the first time, work up or down to the correct speeds in 50 RPM increments. It is a good idea to record on the mold with a ballpoint pen the RPM and the direction of the spin that finally produced the best castings. An average spin cycle will last from 30 to 45 seconds.

Fig. 2.6: Locking of the hold down plate over a mold set in the CRMCM

Machine rotation: The normal direction of spin of a CRMC machine is clockwise. Sometimes, however, when a pewter mold will not cast properly, simply reversing the direction of spin may cause the mold to cast beautifully without any alterations being made to the mold itself.

Machine pressure: It is the air pressure in the CRMC machine that miters the mold halves together during the spin cycle, keeping them in alignment and preventing molten metal from forcing its way out of the cavities and between the parting planes. Most of the CRMC machines in use today are air operated. Mitering pressure is controlled by air which forces the spinning table against the hold-down plate. Modern machines compensate automatically for varying mold thicknesses and unevenly cured molds.

Fig. 2.7: Pouring alloy into the funnel

The best mitering pressure is the lowest required to prevent distortion. flashing, and leakage of metal along the parting planes. Determining the best pressure setting for a particular mold is, again, a matter of making a number of trial runs with that particular mold. A change in any aspect of the casting process affects the pressure setting: metal temperature, the type of mold used, the piece that is being cast, the durometer of the rubber, the type of pewter alloy, and the shims that are used, if any. The only general rule that can be offered is that, as mold diameter increases, the pressure requirement increases as well since more pressure is required to hold larger molds in alignment.

Fig. 2.8: Initial fill - cold mold. Fig. 2.9: Second pass - warm mold. Fig. 2.10: Results of casting after mold has reached operating temperature

Casting machine funnel: The casting machine funnel is the part of the machine into which the metal is poured and which guides the metal into the mold basin. The funnel can be either metal or ceramic. Metal funnels are the most common type in use, but the caster must be careful with them because they are easily scratched. Once a funnel has been scratched, it is difficult to remove molten pewter metal that solidifies in it. To prevent metal from sticking, a graphite spray release should be used periodically with metal funnels. Silicones do not work so well. Ceramic funnels are sometimes used because molten metal does not stick to them. Their great disadvantage is that they are fragile.

Fig. 2.11: Opening the lid of the CRMCM at the end of the spin cycle stops the machine (on CRMCM’s so equipped)

Fig. 2.12: Casting unit

Fig. 2.13: Opened mold with completed casting unit intact

Summary of casting procedure:

1. Set the speed of the CRMC machine to the appropriate RPM. High speed for small castings, low speed for large castings.
2. Have the pewter alloy at the lowest temperature above its liquidus at which it will produce an acceptable casting from the mold.
3. Set the air pressure of the RMC machine as suggested in Chapter 1.10.
4. Pick up the mold set and apply talc. (Fig. 2.2)
5. Under an exhaust hood, clap the mold set together, rotate 90° and repeat. (Fig. 2.3)
6. With sprue hole in mold facing up, position mold set on mold plate in the CRMC machine. (Fig. 2.4)
7. Position pressure plate over the mold set and lock into bracket arms opposite the direction of rotation of the CRMC machine Figs. 2.5, 2.6)
8. Close CRMC machine lid, and start the spin cycle.
9. Fill with molten pewter metal a ladle that is the proper size for the mold being cast and pour down the funnel. (Fig. 2.7)
10. Complete at least 4-5 spin cycles to bring the mold set up to heat before making corrections in the mold. A cold mold will not fill initially. (Figs. 2.8, 2.9, 2.10)
11. If the mold does not fill or cast properly after it has heated up, try the following modifications: a. Increase or decrease RPM in 50 RPM increments. b. Increase or decrease pewter temperature in 50°F increments. Use a thermometer to check the temperature, since the casting pot will not respond instantly to temperature adjustments. Temperature can be lowered more rapidly by placing new pewter ingots in the pot. Stir before using. c. Reverse the direction of rotation of the CRMC machine. d. Refer to Chapter 17, Troubleshooting, to pinpoint the specific casting problem.
12. After the spin cycle has been completed, lift the machine lid, (if machine is not automatic, first shut off the motor) allow the rotation to stop, remove the pressure plate, and lift out the mold set. (Fig. 2.11)
13. Open the mold, set a remove casting unit (Fig. 2.12); or place the mold set aside for later opening. (Figs. 2.13, 2.14)
14. Repeat the cycle.

Fig. 2.14: Removing the casting unit using needle nose pliers

2.6: Shimming

Centrifugal force and the pressure that molten metal exerts as it is forced into the mold cavities sometimes cause molds to distort. This distortion can prevent some cavities from filling and cause others to produce pieces that are disfigured by flash. (Fig. 2.15) Pieces that are unusually thin or unusually heavy are most often affected. Some- times distortion can be corrected sim- ply by adjusting the air pressure that miters the molds in the CRMC machine.

Fig. 2.15: Example of extreme flash

However, when a mold contains cavities for pieces that are quite dissimilar perhaps a combination of very thin castings with very heavy ones-adjusting machine pressure may not be enough to compensate. The solution is to cast the mold with shims placed beneath the affected cavities. Shims can be made of rubber, cardboard, or paper. For a simple coin casting, a circular shim is cut for the single cavity. The shim should resemble a doughnut. The round hole of the shim should be about ½” greater in diameter than the diameter of the cavity for which it is intended. This shim is positioned in the CRMC machine under the affected cavity so that the cavity is centered over the shim’s “doughnut hole.” (Fig. 2.16)

When the machine pressure is brought up, the shim will push the rubber around the cavity upward and miter the mold halves more tightly together around the cavity. The rubber directly under the cavity will expand into the shim’s “doughnut hole,” which will cause the cavity to enlarge slightly and allow more pewter to flow into and fill the problem areas. When using this technique, the center of the shim should be cut out slightly larger than the cavity for which it is intended, and should reproduce the cavity’s general shape.

Shims are also frequently used inside model molds to build up models that are too thin, or to allow corrections to be made in prototype castings. (Fig. 2.17)

Mold sets that are 12” or larger in diameter usually require shimming around the entire outside perimeter to ensure uniform clamping pressure. Without shimming, they tend to produce flash and to allow molten pewter to escape during the spin cycle. The perimeter shim for a large mold should be the same diameter as the mold and from 1” to 2” in width. (Fig. 2.18)

Fig. 2.16: Doughnut shim (external)

Shimming may be required when the thickness of a mold varies too greatly. After curing, the thickness of a mold set may vary as much as ½”. A simple cardboard shim of the right thickness cut to the diameter of the mold should provide adequate compensation. (Fig. 2.19) A thin sheet of rubber cut to size may also be used instead of cardboard or paper.

Most shims are applied to the outside of the mold. But when the mold is too thick for the shim to produce any effect through the rubber, the shim can be applied inside the mold between the parting planes. This method is good only for a few spins, and is not practical for long production runs.

Reinforced nylon strapping tape is also an excellent shimming material, and does an especially good job when applied to the outer edges of a mold set to reinforce against distortion. It makes a permanent shim and does not need to be repositioned each time the mold is cast as is necessary with shims made of other materials.

2.7: Pewter Casting Law Summary

Always aim for:

• The lowest pouring temperature above the liquidus of any alloy used that will produce an acceptable casting.
• The slowest possible spin speed of the CRMC machine.
• The lowest pressure necessary to miter the mold halves together.

Fig. 2.17: Internal shim. Fig. 2.18: Perimeter shim. Fig. 2.19: Full shim.

2.8: Equipment and supplies for the pewter caster

Basic equipment and supplies for a caster should include:

1. Workbench. Comfortable height for standing. 3 × 6-8’ long, with a backstop
2. CRMC machine with grounded 110V outlet
3. Pewter metal Furnace (160 lbs lead), stand mounted
4. Proper exhaust and “make-up air” ventilation system
5. Various size ladles
6. Skimmer
7. Flux for drossing (ammonium chloride, trade name salammoniac)
8. Supply of materials in various sizes for shims
9. Gloves, cotton and heat resistant
10. Containers for scrap and drosses (preferably with tight lids)
11. Talc and applicator (brush or bag)
12. Dust brush
13. Needle nose pliers
14. Safety glasses, safety apron, safety shoes, dust mask (OSHA approved)
15. Pot black
16. Good artificial lighting, preferably near source of natural daylight
17. Mold racks
18. Air compressor capable of 100 PSI isolated for noise
19. Wire brush for pot cleaning

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