Principles of Centrifugal Rubber Mold Casting : Chapter 5

1. Introduction: working with the alloyer
2. Factors that affect pewter alloy selection
3. Generic classification of pewter casting alloys as a guide to weight and price
4. The role of temperature in selecting a pewter alloy
5. Classification of pewter castings by type of design and function
6. Reference tables

5.1: Introduction: working with the alloyer

In selecting a casting alloy, the caster’s own past experience and the suggestions of a reputable alloyer are the most reliable guides. There are alloys made for every type of casting that is possible to produce by the CRMC process. If the caster follows carefully the procedures outlined in this chapter and the preceding one on Alloys, selecting the correct alloy for each mold that he casts will be simple.

There will occasionally be jobs, however, for which no alloy will seem right, and there will also be times when the alloy selected will not produce acceptable castings. It is then that the alloyer’s mold and alloy consultants can be the caster’s best friends. They can check both the mold and the general casting conditions, and usually pinpoint the cause of the problem quite rapidly. If it is the alloy that is at fault, the alloyer can determine why and suggest alternatives.

Fig 1: Pouring the alloy

The alloyer is only interested in supplying the correct alloy for a given job (Fig. 5.1) He knows that it is not always the most expensive alloys that do the best job, and he is more than willing to sell a less expensive alloy if that is the one that the caster needs. In a word, if an alloyer is reputable, he knows that it is in his own best interest to be his customers’ best friend. The caster should never forget that his alloy supplier offers him a team effort that is his for the asking, a team effort that has nothing less in view than making each casting operation successful and profitable for everyone involved.

5.2: Factors that affect pewter alloy selection

There are four important factors to consider before selecting an alloy to cast a particular mold with:

1. Cost
2. Weight
3. Temperature requirements
4. The general design classification of the casting

Cost: What the price range of the finished casting will be determines how expensive the alloy to be used can be. The cost of an alloy is directly proportional to its tin content. The higher the percentage of tin, the more expensive the alloy. When comparing alloys for cost, it is important to remember that some alloys which cost slightly more per unit of weight may produce a significantly larger number of pieces than an otherwise comparable but slightly less expensive alloy. (See Chapter 4.7) This is especially true in the case of so-called bargain alloys that are often touted as being equal to a particular Ostercast alloy. Bargain alloys may be cheaper, but they will cost more in the long run in down time, rejected castings, high temperature requirements, and high dross loss. How to compare the cost effectiveness of alloys is discussed in detail in Chapter 4.7.

Weight: How much weight the alloy for a particular mold or casting should have depends, very simply, on whether the finished piece should be heavy or light. The weight of an alloy is directly proportional to its lead content. The higher the percentage of lead, the heavier the alloy will be per unit of volume.

Temperature: The most important temperature characteristic to consider is how long a pasty range the alloy to be used should have. The design and size of the mold’s gating system will depend directly on the length of the alloy’s pasty range.

Casting design type: When selecting an alloy, the caster must have the design type of the casting and the physical characteristics appropriate to that type of casting’s requirements clearly in mind. Different casting design types such as ‘boutique’, ‘filigree’, or ‘tailored’ require alloys with different physical characteristics. For some casting types, ductility is important; for others, the capacity to take a highly polished finish.

Alloys must be selected with eyes wide open. Before the caster can even begin to make a realistic choice among alloys for use in a particular mold and casting, he must have clear and specific answers to a number of questions:

1. Price classification of the finished casting. High, medium, inexpensive.
2. Weight of casting. Heavy, medium, light.
3. Classification of casting. Tailored, semi-tailored, filigree, etc.
4. Strength requirements. Strong, moderate, not a factor.
5. Stone setting or pronging or bending requirements.
6. Surface finish. Polished, textured or ‘as cast’.
7. Plating requirements. Electro-plated or vacuum-plated, or ‘as cast’.
8. Size of casting. Large, medium, tiny.
9. Thickness of casting. Heavy, medium, thin.
10. Type of mold. Squash, modulated, sectional.

5.3: Generic classification of pewter casting alloys as a guide to weight and price

Casting alloys may be classified in five categories according to their composition. When looking for an alloy that has the right cost, weight, and other characteristics for a particular mold, knowing the general characteristics of the alloys in each of these classifications will provide the caster with a guide that can narrow his search considerably. The five categories of alloys are:

High tin: This is also known as ‘white metal’, and white metals are the true aristocrats of casting alloys. Tin content ranges from 85% and up. By weight, they are the lightest and the most expensive of all the casting alloys. Under the same classification, but with a tin content ranging from 70-80% are a series of alloys, slightly less in cost. While fairly limited in their use, or where they can be substituted for higher tin alloys, their performance record has been excellent. 1.13 pounds is equivalent to 1 pound of white metal.

Low tin: Ranging from 16%-36% tin content. It fills a wide variety of casting requirements such as belt buckles, boutiques, rings, statuary, and both expensive and inexpensive jewelry castings. 1.35 pounds is equivalent to 1 pound of white metal.

High lead: A group of alloys ranging from 87% lead and up, balance antimony (antimonial leads). High lead alloys are used in both bronze and rubber mold casting. It also includes a new class of alloys developed by the Oster group that range from 2%-10% tin. They are used primarily for casting belt buckles, boutique items and advertising specialties. 1.5 pounds is equivalent to 1 pound of white metal.

CT: Originally known as ‘casket trim’ used for casting the trim and hardware for coffins. It now designates a specific Ostercast Alloy used in items for the living, such as hollowware parts (spouts, legs, handles), trophies, and lamp bases, using bronze molds only. Same weight factor as high lead alloys.

Pewter: The definition of pewter is specific: “a lead free alloy of high tin composition.” According to standards set by the American Society for Testing and Materials (ASTM) designation B-560-72 “Modern Pewter Alloys,” its nominal composition is: 92% tin, 6-7% antimony and 1-2% copper. These specifications meet the requirements of both United States Government Regulations and the American Pewter Guild (APG) for a “lead free composition alloy for use in food utensils.” The American Pewter Guild also requires members who supply or use pewter for jewelry to abide by this “lead free” requirement. The Oster Company is a member of both the American Society for Testing and Materials and the American Pewter Guild.

The name ‘pewter’ has in the past referred to any alloy made from tin and lead. Early English pewter and pewter from other countries contained as much as 30% lead in their composition, and were used primarily in eating utensils. In the 19^th century, a better grade of pewter called ‘Britannia’ was intro- duced in England. It was essentially lead free and helped to establish a new standard for pewter.

Today, however, in the costume jewelry industry, ‘pewter’ is still used to describe any alloy made from tin and lead, including alloys of lead/antimony only. While the United State Government Regulations restrict the term ‘pewter’ to a “lead free alloy for eating utensils” only, there are no restrictions on its usage for any other purpose. Thus, ‘pewter’ can be used to describe anything, including ‘pewter finish’ plaster of paris statues.

The Oster Company as a member of both the ASTM and the APG has concluded that a distinction must be made. Therefore, the term ‘pewter’ as used in this book will refer to a lead free alloy only, having a composition of 92% tin, 6-7% antimony and 1-2% copper. Alloy of this composition is sold under the trademark ‘Certified American Pewter’ in sheet form (Table 5.2) and in ingots for casting as OR8 and G-91. These alloys are certified to meet all ASTM, APG, and United States Government Regulations. In the European market. George Johnson and Company, West Midlands, England, a sister company of the Oster Group, manufactures and markets a comparable pewter alloy in both sheet and ingot. The following formulae are used to figure the weight in pounds of any sheet, blank or circle using Ostercast ‘Certified American Pewter’ (TM)

Pewter Sheet Metal Calculation in Pounds

5.4: The role of temperature in selecting a pewter alloy

In addition to weight and price, the third factor to be considered in selecting an alloy is the length of the ‘pasty range’. Both the moldmaker and the caster must understand clearly how a molten alloy behaves in a mold as it passes through the gating system to the cavity. When choosing an alloy, each must use this knowledge when judging which alloy will perform best in a given mold.

The pasty range of an alloy is the range of temperatures between the point at which the alloy first begins to melt and the point at which it is completely molten. (See Chapter 4.4, 4.5) What is important is not the specific solidus and liquidus temperatures, but the number of degrees between the two, the length of the ‘pasty range’. The length of the pasty range varies greatly from alloy to alloy.

As a pewter casting alloy cools in a mold cavity, solidified shells full of slushy metal form against the cavity walls. As these cool and shrink, the casting becomes a mixture of solidified and slushy metal with voids and pockets left by the shrinkage. Unless the cavity continues to be fed with additional molten metal from the gates while it is solidifying, these voids and pockets will remain in the finished casting, especially in the last portion of the casting which fills and solidifies the area around the gate.

Fig. 5.3: Tailored castings and suggested Ostercast Alloys. A classification of castings with few holes or openings, usually requiring a high polish and good plating ability. Strength requirements are usually secondary since pronging or bending are not part of design. Suggested Ostercast Alloys: Excelsior(pat’d). Special K, Special KP, Special BS, 025.

This is why it is important that the pewter not freeze in the gates until the cavity has filled completely. So long as the metal in the gates remains molten, it can act as a ‘solidification shrinkage feeder head’ and feed additional molten metal to the cavity to fill in the voids left by shrinkage during solidification. The gate must continue to feed the cavity until complete solidification has occurred. From this description, it is obvious that alloys that are eutectic; that is, alloys such as linotype metal that do not have a pasty range, will produce poor castings with shrinkage and voids in the finished pieces. The problem then, is to select an alloy that has a pasty range that matches the mold’s gate and cavity design, one that is long enough to enable the metal in the gates to re- main molten until the cavity is com- pletely filled and the casting solidified.

Fig. 5.4: Semi-tailored castings and suggested Oster cast Alloys. Similar to tailored type castings, they will usually require additional strength in order to compensate for bending and pronging of the raw castings for stone setting or forming. Polishing requirements are usually secondary in finishing as they often have a textured cast surface. Suggested Ostercast Alloys: Excelsior(pat’d). Special K. Special KP, OR8E, Special 0, 77 Alloy. 0365, 0363, 032.

The size of a mold’s cavities and the size, length and configuration of its gates all play a part in determining whether the mold will require a casting alloy with a long pasty range. An alloy with a short pasty range should only be used in molds with large gates or small cavities. A mold with large cavities that are back gated so that the gates are long and curved will require an alloy with a longer pasty range than a mold with cavities that are small and front gated with large gates. In general, the smaller and more constricted the gates, or the longer they are, or the larger the cavities, the longer the pasty range of the alloy used to cast the mold must be.

Fig. 5.5: Filigree castings and suggested Ostercast Alloys. Filigree castings are characterized by a lace-like appearance. They have many small openings or holes and are usually very thin. Because of this, they will require alloys with exceptional strength. Suggested Ostercast Alloys: Excelsior(pat’d), OR8, Special 0, 77 Alloy, 0365, 0363.

When a mold produces castings that show the characteristic signs that the alloy’s pasty range is too short for the mold design (voids, shrinkage, distortions and pockets, especially in the area of the gate) it is very important that the caster not try to solve the problem by working the alloy at a temperature higher than its normal casting range in an attempt to increase its fluidity and lengthen the pasty range. Higher temperatures do increase metal fluidity and lengthen the time it takes for the metal to freeze. But the cost is coarser grain structure in the finished castings and a drastically reduced mold life. Further, alloys that run above their normal casting range produce heavy drosses with consequent unnecessarily high loss of tin and other constituents. This requires much more frequent fluxing. When metal freezes in the gates before the mold cavities have filled completely, the gates must either be enlarged or redesigned.

Fig. 5.6: Buckle castings and suggested Ostercast Alloys. The majority of buckles are full bodied and tailored in design. Where light weight, and the ability to take a high polish and plated finish are important, high tin alloys are suggested. Where low cost and weight are prime considerations, high lead alloys are recommended. Suggested Ostercast Alloys: Excelsior(pat’d), Special K, 0363, Special LX, 6AL.

5.5: Classification of pewter castings by type of design and function

The fourth factor that a pewter caster must consider when he selects an alloy for use in a mold is the design type or classification of the castings that the mold will produce. If the caster keeps in mind that the casting which a particular mold will be producing is, for example, a ‘boutique’ type casting, and if he knows that the most important characteristics of boutique castings are low price and high weight, he then needs to consider only alloys that are low in cost and high in lead when selecting an alloy to use in that mold.

Fig. 5.7: Advertising specialties and suggested Ostercast Alloys. Advertising specialties have two main considerations: low cost and good reproduction. Because of this, low tin and high lead alloys are recommended. Suggested Ostercast Alloys: 0365, 0363, 025, 020, 016, Special LX, 6AL, 4AL.

Until now, there has been no industry-wide standardized classification of casting types. What one moldmaker habitually refers to as a ‘semi-tailored casting, another will insist it to be ‘filigree’. The classifications and illustrations (Figs. 5.3-5.11) that are offered in this chapter are in- tended to organize this confusion and provide some standardization for types of castings. They provide terms for describing particular castings when selecting alloys, when ordering metals from the Oster Company or Fry Metals Inc., England, and when describing problem molds to the mold consultant. These terms are, however, only suggestions, and are by no means intended to represent present standard terminology in the industry.

Fig. 5.8: Pewterware and suggested Ostercast Alloys. Ostercast ‘Certified American Pewter’ (TM) meets all applicable ASTM, APG and United States Government Regulations for a “lead free alloy for eating utensils.” Oster’s special casting and rolling process contributes a unique grain structure to the pewter that makes spinning and forming easier and better reducing rejections to near zero percentages. It is the largest selling brand of pewter in America. Suggested Ostercast Pewter Alloys in sheets, blanks and circles: ‘Certified American Powter’ (TM) In ingot: G-91, OR8. (Comparable pewter alloys in sheets and ingots are manufactured and sold in the European Market by George Johnson Company, England, a sister company of the Oster Group. an L.I.G. Company).

The following illustrations represent castings of a particular classification and suggest a range of Ostercast alloys capable of fulfilling their requirements. A similar range of alloys for the European market are manufactured by a sis- ter company of the Oster Group. Fry Metals Ltd., London, England.

Fig. 5.9: Boutique castings and suggested Ostercast Pewter Alloys. Also referred to as ‘full-bodied”, boutique castings are used for decorative items such as pill boxes, mirror and picture frames and paper weights. The prime requirements are low cost, weight and ability to reproduce detail. Suggested Ostercast Alloys: 025. 020, 016, 6AL, Special L. Special LX.

Miscellaneous Pewter Castings and suggested Ostercast Alloys

Model work: Models must pick up a great deal of detail and at the same time be easy to work and form. High tin alloys are recommended for model work. but production models can be cast with alloys containing as little as 36% tin. Models cast from alloys with lower tin content may have difficulty in picking up detail. The newer low-temperature Ostercast Alloys are not recommended for models as they may melt or distort during the process of curing the molds.

Fig. 5.10: Hollowware and suggested Ostercast Alloys. Legs, handles and spouts for hollowware; trophies; and lamps are usually cast in bronze molds and thus require alloys that are either eutectic or have a short pasty range. If the casting is to be a part of a pewter item, it must only be cast from a lead free alloy. Suggested Ostercast Alloys: OR8, G-91, CT, 12AL, 10AL, 6AL.

For carving models, Ostercast ‘Certified American Pewter’ (TM) is the easiest to carve, form or shape, and in addition takes an excellent polishing and plating.

Fig. 5.11: Statuary castings and suggested Ostercast Alloys. Statuary must be able to reproduce all the details of the original model with both fidelity and sharpness. Surfaces must be capable of a high polish since statuary is not usually plated. Alloys with high tin composition are recommended but low tin alloys may also be used for less expensive pewter castings. Suggested Ostercast Alloys: Special BJ, G-91(Pewter), Excelsior(pat’d). Special K. 025, 020.

Suggested Ostercast Alloys: Casting: OR8, OR8E, Special BS, 0365, 0363. Sheet: ‘Certified American Pewter’ (TM).

Rings: Rings may be classified as ‘tailored’, ‘semi-tailored’ and ‘filigree’, and use the appropriate casting alloy for that category. When the ring design requires the casting to have the shank as part of its design, it will require a stronger alloy in order to form the shank with sufficient strength to resist deforming.

Suggested Ostercast Alloys: Excelsior (pat’d), Special KP, 0363, 025.

5.6: Reference tables

Once a caster has mastered the principles of pewter alloy selection presented in this chapter, he can then use the following tables as a quick reference guide for selecting the proper alloy for a particular mold that he is casting. Table 5.12 gives the generic classification of Ostercast Alloys by general alloy composition. Table 5.13 shows the classification of castings by design type and lists the suggested Ostercast Alloys to use. Table 5.14 is a chart of the solidus, liquidus and pasty range of selected Ostercast Alloys.

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