Principles of Centrifugal Rubber Mold Casting : Chapter 7

1. The Prototype
2. Model Nomenclature and Classification
3. How to Design Practical Models
4. Shrinkage, What Causes It and How to Control It
5. Short Run Casting

7.1: The prototype

The prototype piece that is to be reproduced by the CRMC process is referred to as the “model.” A model may be anything, an original design hand carved in metal; a wax carving translated into metal by investment casting: or any object made of material that can stand up under the pressure and temperature necessary to cure a rubber mold.

Models made of organic materials: Objects made of wood, bone, and shell have been used as models. However, the moldmaker must be cautious when curing molds with these and other organic materials because they are likely to retain moisture. During curing, even small amounts of moisture can produce enough steam pressure in the mold frame to be potentially explosive. Models made of organic materials should be thoroughly pre-heated to drive off all moisture before they are used to cure a mold. Fragile models: Fragile models such as sea shells or pieces made of glass or plaster can be duplicated safely using molds made of RTV silicone rubber. The liquid rubber is poured around the model and allowed to cure. The mold that results can be cast with molten metal to produce a metal model which, after cleaning, can be used to cure a CRMC mold.

Pewter models: Sheet pewter is one of the most versatile materials available for model work. Oster’s Certified American Pewter (TM) is a lead free alloy that is easy to carve and form, yet has sufficient strength to enable the most delicate models made of it to stand up well under the temperature and pressure of vulcanization. Its solidus of 471° is well above the temperature at which the vulcanizer normally operates, 325°F. A list of sizes in which Certified American Pewter is available is given in Chapter 5.3. Pewter sheet is an excellent material for manufacturing both findings and jewelry. It is easily formed into almost any shape and will accept both a high polish and a quality plating. The stamp of ‘pewter’ is an accepted hallmark of quality in both hollowware and jewelry. It is also available in ingot form for casting under the Oster designations OR-8 and G-91. The tensile strength of cast pewter will not increase with heat treating.

Model failure during vulcanization: Modelmakers sometimes use solder and low temperature alloys to assemble, modify, or repair prototype models. Because these alloys have low melting points, they may melt during vulcanization. When the moldmaker suspects that this may have happened, he should allow the mold to cool for a half hour before he opens it. A water bath may be used to cool the mold more rapidly, but it should not be allowed to become too cool because shrinkage will make the models difficult to remove. It is important to remember that models destroyed or distorted during curing may still leave a viable cavity in the mold.

7.2: Model nomenclature and classification

It is extremely important to distinguish among the different kinds of models that are used at each stage of the CRMC process, and to understand which is which in order to avoid the serious mistakes that confusion can cause.

The models used to cure production molds are termed the ‘production models’. One is required for each cavity in the production mold. When the pieces to be cast are so large that there is room for only ten or fewer cavities in the production molds, then enough production models to cure a production mold can be produced by casting them one by one in a single cavity mold cured with the prototype model. (See section 7.5 on the ‘pie wedge’ mold.)

But when the production molds will have twenty, thirty, or more cavities, it is much more efficient to cast only seven to ten models initially from the single cavity mold cured with the prototype, and then use these ‘model-mold’ models to cure a ‘production model mold’. The production model mold can then be used to “mass produce” enough production models to cure the production molds.

An outline of the types of models includes three types for the three kinds of molds used in CRMC:

1. The prototype model: The ‘prototype’ model is the original design to be reproduced. Whether it is a hand made “work of art” that the modelmaker has carefully constructed, or just a “knockoff” from a previous run, it is still the ‘prototype’ if it provides the unique design that is to be reproduced. After careful cleaning and inspection, it can be used to cure a single cavity pie-wedge mold or a regular mold set.

2. Model mold models: This second step is optional and may be skipped if the number of cavities in the production molds is few enough-generally no more than ten that a number of models sufficient to cure the production molds can be easily cast one by one from the pie-wedge mold. If not, five to ten castings are made and used to cure a ‘production model mold’. These castings are then referred to as the ‘model mold models’. They are also known as the ‘first castings’, or the ‘first shrinkage’ because they will differ in size from the prototype model. They must be carefully cleaned and thoroughly inspected before they are used to cure the production model mold.

3. Production models: The production models are the castings that the production model mold produces. They are used to cure the production molds. Depending on how many are needed, they are produced either directly from the pie-wedge mold or from a production model mold. When they are produced from the production model mold, they are referred to as the ‘second shrinkage’, because each additional mold stage in the moldmaking process introduces additional shrinkage. When production models are produced directly from the mold cured with the prototype model, they are then termed the ‘first shrinkage’.

The production models must also be cleaned and inspected carefully before being used to cure the production molds. To improve their surfaces and protect them from damage, it is a good idea to have them rhodium plated before they are used.

After use, the production models should be labeled and carefully stored for future use. Care should be taken that they are not mixed in with and used inadvertently as production castings. The production model mold should be carefully labeled and stored to prevent its being used accidentally as a production mold.

4. Production castings: These are the castings that come out of the production molds. They will become the finished products.

7.3: How to design practical models

Law of the Model: “What you see is what you get.”

A finished production casting will never be any better than the worst model used in the process that produces it. No model is more “important” than the others. Each model must be perfect: prototype model, model-mold model, and production model. Rubber, with its remarkable capacity to reproduce details, will pick up every blemish and imperfection that is allowed to remain at each step in the pre-production process.

It is important that both the modelmaker and the moldmaker appreciate how extremely critical it is to success in CRMC that they begin with models that are as nearly perfect as possible. Potential problems must be worked out, defects repaired, and blemishes eliminated before the production molds are made, not after. The modelmaker should give “nothing but the best” to the moldmaker as a model, and the moldmaker must accept nothing less. It is the moldmaker’s responsibility to inspect every model carefully and, if necessary, either reject it or clean it and perfect it himself.

Practical design for CRMC: The CRMC process is one of the most versatile production methods ever devised. But like all production processes, it cannot “do everything.” Rubber molds have their limitations, and they simply will not perform their best if the modelmaker disregards practical design requirements when he builds a model. Some of the most important practical design requirements for the model- maker to keep in mind are:

Blind holes: If possible, blind holes should be rounded so that they are slightly concave at the bottom. This promotes better casting and plating and helps prevent the mold rubber from tearing.

Flat surfaces: Because of the flexibility of rubber, large flat surfaces are difficult to reproduce in casting. A slightly convex surface on a model will cast flatter and plate better than a model with a perfectly flat surface.

Holes: A hole or opening in a casting requires that the rubber have a male registration of the hole. In especially small holes such as occur in filigree work, the rubber may tear at these points as the casting is removed. Where possible, the use of inserts is advised. Any hole or opening in a model should be made as smooth as possible. (See Chapter 12)

Edges: Sharp edges on models should be rounded for better plating and reduced tearing of the rubber in the mold.

Parting lines: The modelmaker should design models to conceal parting lines as much as possible. Gating to unobtrusive areas, and using texture in the parting line area are two effective methods of concealing parting lines. (See Chapter 11)

Complicated castings: When designing models, the modelmaker must work by the principle that cost, design efficiency, and practicality are as important as aesthetic considerations. It may be possible to make a particular mold, but highly impractical because of its cost. For example, undercuts or holes that may look attractive may severely limit mold life. Complicated sectional molds and insert molds that are difficult to prepare for casting may reduce drastically the speed at which the caster can work and perhaps make it necessary to assign extra workers to the casting operation.

Polishing: The modelmaker must plan models with the needs of the polisher in mind. For example, the modelmaker should try to avoid positioning an area that will require polishing immediately below a textured area. Such close proximity makes it impossible for the polisher to “hit” the clear area with the polishing wheel without “hitting” and ruining the texture. Also, models must be designed with the various mechanical operations that will be performed on the finished castings clearly in mind.

Design: Texture is an important part of casting design because it conceals many defects that are difficult to avoid in the casting process. It also serves to reduce time and work during the cleanup operations. When castings are textured, polishing may in many cases be eliminated. Small nicks and scratches that result from handling castings are usually then not as obvious. Textured production models can withstand more abuse than plain ones. The backs of castings are frequently textured since the surface requirements are not as critical. Texture may consist of thin lines, cross-hatching, a hammered effect, or any other regular or irregular pattern. It is important that a polished area adjacent to a textured area be designed to allow for polishing without the textured areas being affected. On flat back or dummy molds, texture may be cured across the whole face of the dummy mold half. (See Chapter 11.3)

Model quality: The surface of the model should be free of scratches and pits before the moldmaker receives it. Models must be handled carefully. Production models should be rhodium plated to protect them against scratches. Models should be labeled and stored properly, not simply thrown together in a box. Plating the prototype model is not suggested since modifications are often required.

Poor models: When a prototype model is substandard, too thin, a “knockoff,” or when it needs to be changed or redesigned, it is not always necessary to make a new one. A model can be repaired or altered using solder, tape, or epoxy strategically placed. The model can be made thicker in the mold by shimming, by cutting out the mold cavity, or by drilling it with a burr, drill, or wheel to increase its size. After casting the modified model and making the necessary corrections to the resulting casting so that it can be used as a model, a new mold may be made.

7.4: Shrinkage, what causes it and how to control it

One of the most serious problems encountered in any kind of casting is shrinkage. CRMC is no different. Factors at every step in the process can contribute to the problem. This is why there are no precise formulas for predicting shrinkage. There are simply too many variables involved. The contribution that rubber makes to the problem has already been discussed in Chapter 6.2.

The moldmaker can help to minimize shrinkage from the outset by keeping in mind all of the factors that contribute to the problem and by using his knowledge to design and build models with features that help to minimize it.

In small, thin castings, shrinkage is fairly low because the metal is usually poured at a lower temperature and will freeze or solidify more quickly. Because of their low mass, small castings also have less heat to dissipate.

In heavier or larger castings, the metal remains molten longer and the shrinkage of the mass is, as a result, greater. In addition, the greater pressure necessary to clamp the mold during the casting cycle has a greater effect on heavy pieces. Because of its flexibility, the rubber itself is affected by the pressure and the weight of the piece being cast. Rubber from different suppliers and rubber with different durometers will also produce differing degrees of shrinkage. Different alloys will differ in the amount of shrinkage each undergoes as well. It is especially important to keep in mind that models are usually cast with a completely different alloy from that which will be used in production. Consequently, model shrinkage is not a reliable guide to production shrinkage.

Shrinkage is usually more severe in the thickness cross section of a casting than in the length, since the mold is far more elastic in its thickness than it is in its diameter. Castings may vary in thickness from the model by as much as 30%. Gating is also an important factor, since most model molds have much larger gates than production molds. Shrinkage in a mold is proportionately less severe as the gates are enlarged.

Important size limits for models are:

a. Minimum castable thickness for a model is .050” (2 mm) b. Metal shrinkage will be at least 10% from the model to the production mold.

In summary, the important factors affecting shrinkage are:

Rubber: Differences in supplier, durometer, mold size, mold thick- ness, and mold type will all result in differences in shrinkage.

Model: A minimum of 10% shrinkage from model to production mold should be allowed for.

Alloy: Both the percentage of tin in the alloy and the temperature at which it is cast are important factors to consider when predicting shrinkage. The higher the casting temperature, the more the shrinkage. The cooler the casting temperature, the less the shrinkage. In general, high tin alloys will shrink much less than low tin and lead alloys.

Casting: The larger or heavier the casting, the more severe the shrinkage. Weight is especially important.

Machine RPM: The higher the RPM at which the metal is spun, the greater the possibility of distortion because of centrifugal force.

CRMC machine: The higher the pressure needed to mitre molds together in the CRMC machine, the greater the shrinkage.

Gates: The larger the gates in the mold, the less the shrinkage will be. However, large gates can usually be used only in the model mold since they can create problems in degating and in concealing the gate area on the production castings.

Mold cycle: The more molds used in a casting cycle, the less severe the shrinkage will be because the molds will have time to cool down between spins. Hot molds are more likely to produce severe shrinkage since hot rubber molds will vary more from their original dimensions than cooler rubber.

7.5: Short run casting

When casting out one or two models for revision, or when casting the model-mold models, it is not necessary to use a full rubber mold. A pie-wedge mold frame which holds only a quarter of a full mold can be used. The pie wedge requires a special adapter to be cast in the CRMC machine. This adapter is available from the casting equipment supplier.

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