Principles of Centrifugal Rubber Mold Casting : Chapter 6

1. Uncured Rubber: How It Is Made, What Is In It, and How Long It Will Last
2. Rubber mold sets: lamination, shrinkage, and durometer
3. Vulcanization
4. Variables and problems and how to deal with them
5. A note on mold size and production efficiency

6.1: Uncured rubber: how it is made, what is in it, and how long it will last.

Basic to moldmaking is an understanding of the rubber from which the mold is made. The following is a guide to rubber formulation for the layman:

Elastomers: The rubber used to make CRMC molds is an ‘elastomer’. Elastomers are a group of materials with properties that have been described as “rubbery.” The term “rubbery” implies similarity to natural rubber. Some synthetic elastomers do have some of the physical characteristics of natural rubber, but chemically they are quite different. Therefore, not all elastomers can be used to make rubber molds.

The American Society for Testing Materials defines an elastomer as “a material that is capable of recovery from large deformations quickly and forcibly and can be, or already is, modified to a state in which it is essentially insoluble (but can swell) in boiling solvents such as benzene and methyl ethyl ketone. An elastomer in its modified state retracts within one minute to less than 1½ times its original length after being stretched at room temperature to twice its length and held for one minute before release.”

How rubber is made: Rubber is compounded on a “rubber mill” or in Banbury mixers. These are mixing devices that break down the structure of the raw rubber by passing it between rotating steel rollers to make it soft and plastic. The process requires several passes. During this process, the same equipment incorporates the various compounding ingredients into the rubber.

The compounded rubber is then extruded or ‘calendered’ into thin sheets. several of which are laminated or layered together to produce the desired rubber thickness. The laminated sheets of rubber are then blanked into the familiar circular mold halves using steel dies on a ‘clicking machine’.

Rubber fillers: Materials that increase hardness, stiffness, strength, and resistance to tear or abrasion must be added to the natural organic or synthetic rubber in order to produce a practical product and to control costs. Many of these are reinforcing materials which give the rubber important properties that it would not have without them. Other fillers are diluents that are necessary to keep costs down. Diluents include carbon black, zinc oxide, and clays. Some fillers, such as carbon black, are used as diluents but improve the quality of the rubber as well. Unlike metal alloys, reclaimed rubber is a perfectly acceptable material.

Color: Color has little or no effect on rubber performance. The carbon black that makes black rubber black is, generally, an excellent reinforcing agent for rubber. However, with proper care in compounding. light colored rubbers can be produced that compare favorably with black rubber. On the other hand, an improperly or carelessly compounded black rubber will be of inferior quality to a more carefully produced light colored rubber.

Accelerators: Accelerators are chemicals added to rubber compounds to ensure that the vulcanizing process takes place within the desired time limit and at the desired vulcanizing temperature.

Activators: Activators start the action of the accelerator at the desired curing temperature.

Retarders: Retarders delay the action of the accelerator. Heat is generated during the mixing of rubber compounds. Without retarders, this heat would cause premature vulcanizing.

Anti-oxidants: Anti-oxidants and other protective materials retard deterioration of the rubber during processing, storage, and use. Various chemicals are available to mitigate the effects of air, heat, and flexing, and to provide longer usefulness.

Processing aids: Various plasticizers and softeners may be added to a rubber compound to modify the properties of the cured and uncured material. Softeners may be used to control the hardness of the finished rubber. Tackifiers give the rubber “building tack” that allows it to be plied to various thicknesses.

Reinforcing pigments and resins: Various inorganic fillers and organic resins are used to increase the tensile properties and control the hardness of the cured rubber. These materials will combine with natural and synthetic elastomers to make a composite that is stronger than the elastomer itself.

Diluents: Diluents are added primarily to reduce cost. Some fillers, when they combine with elastomers, do not reinforce them, but simply dilute them, which, inevitably, dilutes somewhat the compound’s properties. A practical level of diluent is that which produces the best compromise between cost and desired physical properties.

Shelf life: The shelf life of uncured rubber varies for different compounds. A practical rubber life would be several months, but a shelf life of a year or more is possible.

Uncured rubber must be stored in a cool area, away from heat. After a long period of storage, a slight degree of cure may develop that could interfere with the flow of the uncured rubber during vulcanization. Uncured rubber that is stored in very cold warehouses may freeze. Removal to a warm place will restore it to its original plastic state.

The shelf life of a cured rubber mold depends on many factors such as storage conditions, the number of times the mold has been cast, the intricacy of the mold, and the care with which the caster handled it. The rubber compounder can compensate for service conditions by incorporating anti-oxidants and protective agents in the rubber.

Ozone has a particularly deleterious effect on rubber. Molds should, therefore, be stored away from electric motors and fluorescent lights. Both put considerable amounts of ozone into a shop’s air.

6.2: Rubber mold sets; lamination, shrinkage, and durometer

Rubber laminations: There are approximately eight to ten layers of rubber in the standard 17/32” half of a mold set. These can be added to or removed, within reason, as required. Many kinds of rubber can be delaminated mechanically after warming them, or chemically with a mild rubber solvent. Benzene should not be used due to the risks involved in working with the chemical. Some rubber compounds, however, precisely because of their desirable properties, may prove impossible to delaminate.

During the process of laminating the new rubber in the rubber plant, blisters or air pockets may form between laminations. If these are not corrected, they may cause problems when the mold is cured. They are often not visible until the moldmaker has completed a set of molds, but may sometimes be visible or felt before the rubber is used if the moldmaker inspects new mold sets carefully enough. Defective rubber should not be used, but put aside for return to the supplier, or corrected by delaminating. Unfortunately, most of the time defects will show up only after the mold is cured. (See Chapter 1.8, for how to delaminate rubber.)

Durometer: An important tool in molding is the durometer, a device used to measure rubber hardness. (Fig. 6.1) It is calibrated from 0 to 100 (soft to hard). Cured rubber products range in hardness from values below 30 for pencil erasers, to 40 for rubber bands, 60-70 for tire treads, and 80-90 for shoe soles and typewriter platens. Jewelry molds lie in the 60-80 range. The durometer can be used as a guide for the moldmaker in selecting a rubber compound that is softer for highly modulated castings, or harder for flat objects such as coins.

To use the durometer, place the device on the cured rubber mold and press down against it. A spring loaded indenter point, which projects from the instrument’s bottom, forces the indenter point extension back into the case. The action on the spring is indicated on the calibrated dial which gives a rubber hardness reading. The higher the reading, the harder the rubber ‘durometer.’

Rubber shrinkage: Because rubber and metal have different coefficients of thermal expansion, a cavity cured in a rubber mold will not preserve the precise dimensions of the model used to form it with as the rubber cools following vulcanization. Casters use the term ‘shrinkage’ to describe the dimensional difference between the cured mold cavity and the model. The change of volume during heating or cooling-or ‘thermal coefficient for natural rubber is approximately 0.035% per degree of temperature change, while that for steel is 0.002%. Average shrinkage for CRMC molds is usually figured at 10% in the vertical dimension and 1.5% in the horizontal dimension, but it can be more or less, depending on the type of rubber and its hardness. With some kinds of rubber, shrinkage can be as great as 30%. In general, the lower the durometer of the rubber being used, that is, the softer the rubber is, the greater the shrinkage will be. But the problem is further complicated by the expansion of the rubber when molten metal is poured into the mold. It becomes particularly difficult to predict thermal expansion in sectional and modulated molds and molds with un- dercuts, because these molds must be made with rubber with lower durometer readings.

Fig 6.1: Shore Durometer. Placing the durometer on a cured rubber mold will indicate its ‘durometer’ or hardness.

Most shrinkage occurs in the vertical dimension of the mold cavity rather than the horizontal. It is especially important to compensate for shrinkage when preparing models for casting that will be heavy or highly modulated. Experienced moldmakers develop intuitive foresight that enables them to gauge shrinkage however and consider shrinkage a minor problem.

Because fillers and elastomers have different coefficients of thermal expansion, the compounder may reduce shrinkage by increasing the amount of filler in the rubber compound. Different fillers expand and contract at different rates. Therefore, changing the fillers in a rubber compound will alter the rate of shrinkage. This is one of the reasons there are so many different mold rubber formulae.

6.3: Vulcanization

Vulcanization comes from the name of the ancient god of fire, Vulcan. The heat and sulfurous odors associated with volcanoes are similar to those that the rubber curing operations produce.

Because rubber in its raw state is tacky, thermoplastic, and relatively soft, it takes faithful impressions of models and lends itself to making rubber molds for jewelry casting using low temperature alloys. Vulcanization, or ‘curing’, is a method of altering crude or synthetic rubber chemically to change these characteristics, to give it different useful properties such as greater hardness or elasticity, more stability or durability, and to reduce its sensitivity to heat and cold. Once the molds have been cured, they become tough and elastic.

Rubber for molds must be compounded to meet certain requirements in the uncured as well as the cured state. The objective is to produce rubber that will take the model impression easily, cure at a reasonable temperature in a short period of time, and then have the durability to stand up to the stresses of casting. This is accomplished by adding various ingredients to the rubber such as accelerators, accelerator activators and retarders, age resisters. reinforcing pigments and resins, inert fillers, diluents, and coloring pigments. Vulcanization is then accomplished either by mixing this compounded rubber with any of a number of other chemicals and then subjecting it to heat and pressure.

The most widely used chemicals are sulphur and sulphur compounds which are mixed with the rubber during manufacture. Vulcanization occurs when the rubber containing the sulphur compounds is heated. The heat produces a chemical reaction in which cross linking of the rubber molecules takes place. This cross linking may be visualized by imagining the molecules of uncured rubber as a structure consisting of lengths of string. In vulcanization, the strings or rubber molecules become inter-connected - or knotted together to form a mesh network. It is the sulphur that “ties the knots.” This mesh structure makes the rubber hard. By increasing or decreasing the amount of sulphur used the number of inter-connecting links can be increased or decreased and the density of the mesh moderated to produce varying degrees of strength and flexibility.

Since chemicals other than sulphur and agents other than heat are some- times used to accomplish these changes, the term ‘curing’ is often used rather than “vulcanization.”

The Vulcanization process: Semi-positive compression molding, as is done in a standard vulcanizer using heat and chemical vulcanization agents in the uncured rubber, is the best curing method known at present. For a step by step description of the vulcanizing procedure, see Chapter 1.8.

Pressure is not necessary to cure the rubber; however, without pressure, the rubber cures to a porous, spongy texture and does not reproduce detail in the casting cavity.

After relieving the rubber enough to accommodate the casting models and control the parting line, the loaded frame must be locked tightly and positively in the heated vulcanizer. It is extremely important that the rubber be clean before starting.

As the rubber is heated, it begins to soften and expand. It is the thermal expansion of the uncured rubber that creates the pressure required to expel trapped air from the frame and force the rubber into complete contact with the model. This pressure is essential for good reproduction of detail.

As the rubber heats, it becomes at first more plastic, that is, it begins to “free-flow.” It reaches its greatest plasticity after it has been in the vulcanizer for approximately 10-12 minutes. The squeezing or compression process must be completed prior to this time. The actual chemical reaction that is vulcanization starts 15-20 minutes after the rubber has been placed in the vulcanizer. Full cure is usually completed within one hour. Vulcanization should be completed fairly quickly to reduce the possibility of overflow from the frame and distortion of the mold.

Curing temperatures: Rubber compounds are available in a wide range of curing temperatures. The length of time required to cure the rubber is “temperature dependent.” Curing is a chemical reaction that approximately doubles in rate for each 18°F of increase in temperature. Thus, reducing the vulcanizing temperature from 300°F to 280°F would double the time necessary for curing. Conversely, raising the temperature from 300°F to 320°F halves the curing time.

To produce a good, usable mold set the uncured rubber must flow well enough to expel the entrapped air and fill out all the details of the models. Vulcanization should then take place rather quickly to reduce the possibility of rubber overflow and possible distortion of the models and mold rubber. Jewelry mold compounds are manufactured with a generous cure time and temperature safety factor, which precludes problems of under or over cure.

The mold set thickness is an important variable which must be considered. The thermal conductivity of rubber is low and the entire mass must be heated to a uniform temperature. Also the heat is applied only to the external surfaces of the rubber. Therefore, the thicker the mold set the more time required for the heat to penetrate to the interior of the rubber mass and to produce a ‘cure’.

6.4: Variables and problems and how to deal with them

Out-of-round molds: Rubber molds, because they are elastomers, are often out of round after they have been blanked by the manufacturer. They should be matched up prior to use so that both the top and bottom of the mold set, if slightly oval, will match and lie in the same direction when placed in the frame.

Mold temperature: Cold rubber molds will not cast initially. Before looking for problems in a mold that is being run for the first time, at least three passes should be made in the CRMC machine to heat up the mold.

Rubber molds should be run in a cycle of at least 9-10 sets to allow time for cooling between spins and to assure long life.

Model release: When models stick to the rubber of the mold they will cause tearing as they are removed. This is especially true of filigree castings. Spraying the models with a release agent, or, preferably, plating them with rhodium prior to use will help. Unplated models, especially of brass, may react chemically with the rubber during curing.

Venting and drilling: Rubber that is obtained from different suppliers may nominally be of the same formula, but will nevertheless vary in many ways. A mold design that has been made in the past with rubber from one supplier and that has always required extra drilling and venting may not require it when made with rubber from another supplier. Therefore, when preparing new molds with rubber that is different from that used previously to make the same mold, it is always wise to try the new mold with a few passes on the CRMC machine before drilling and cutting vents that may not be needed.

Talc: Talc, in addition to providing a fine clearance in the molds that allows gas to escape during casting, is also important in making the molds. It prevents rubber from adhering where it should not, such as where the two mold halves join together. To make a release agent for use in the curing process, mix 3 parts talc with 1 part fine graphite. Rub the mixture over the mold faces until it is well impregnated. Apply it lightly on the back and sides of the molds as well. Do not apply it where provision has been made for cutouts or where the rubber must bond to itself.

Many release agents are effective in rubber molds. However; some, such as the silicones, may interfere with the flow of the rubber during curing. They may also trap moisture in the mold.

6.5: A note on mold size and production efficiency

An 18” mold is not twice as efficient as a standard 9” mold. The fact that the number of casting cavities available in the larger mold is double does not mean that its production capacity will be double that of the smaller mold, There are many reasons for this.

• First: it is more difficult to squeeze the larger size in a standard vulcanizer. Heat will not always be uniform on the outer edges of the vulcanizer platens.
• Second: shrinkage in the larger molds is much more difficult to figure.
• Third: the mold is heavier and more tiring to use in production. It takes longer to open and close, and, since it is not as flexible as the smaller size molds, it requires more time and effort to remove the casting from it. Also, a larger ladle of metal is required during casting. Problems with larger molds are more costly. It is more expensive to make them, and gating and venting to the pieces that are farthest from the center are more difficult to design because the metal must flow through more runners and longer gates to reach the outermost cavities.

The list of mold sizes and their respective weights below helps to illustrate the disproportionate relationship between mold size and efficiency. To these weights must be added the weight of the castings in the mold set.

• 9” Mold set - 3.7 pounds per set
• 12” Mold set - 6.6 pounds per set
• 18” Mold set - 15.5 pounds per set

An 18” mold is only twice as large as a 9” mold, but it weighs four times as much. Experience has shown that the smaller molds will produce more castings per day with less operator fatigue. The larger molds can serve important purposes, especially when a particular casting design will not fit in a smaller mold. In the main, however, it is the smaller molds that are the most efficient and adaptable.

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