Principles of Centrifugal Rubber Mold Casting : Chapter 1

1. The pewter moldmaker’s job and qualifications
2. A word about safety
3. The pewter moldmaker’s tools
4. Classification of pewter model molds
5. The pewter moldmaker and the model
6. Classification of pewter production molds
7. Basic pewter moldmaking procedure
8. How to make a simple pewter squash mold
9. References
10. A note on pewter moldmaking and this book

1.1: The pewter moldmaker’s job and qualifications

A mold is a production tool. The fact that it is made of rubber does not diminish its importance or the skill required to make it. The moldmaker who makes the molds for CRMC must be an artist, a designer, and an innovator. He controls quality and costs, and contributes directly to the reputation of the company that employs him.

The United States Government Dictionary of Occupational Titles defines the moldmaker’s job in this way:

Rubber Mold Maker, D.O.T. 559.884. Forms rubber, wax injection molds, by vulcanizing pieces of rubber around metal samples. Places metal samples into frame and packs raw rubber around sample. Clamps sample, frame, and rubber into vulcanizer machine. Turns on machine, waits specified time and removes mold from machine. Splits mold into two pieces to remove metal samples using knife or scalpel. Inspects mold for defects. He may form investment mold around wax pattern. May operate centrifugal casting machine.

This definition provides government agencies with a basic description of the moldmaker’s job. The moldmaker looking for a job would register under the classification code number (559.884) at state employment offices. The company looking for a moldmaker would refer to the registrants under this same code number.

A good moldmaker has the following skills and attributes:

• High degree of manual dexterity
• Good or rectifiable eyesight
• Intelligence (of course) Arithmetical reasoning ability An understanding of modelmaking
• Practical experience with casting
• Some design ability
• The virtue of patience

Gender is of no importance in selecting the moldmaker candidate. The most important considerations are the candidate’s native ability and willingness to do the work.

Moldmakers do not form a guild. There is no exchange of ideas or technology among them. They usually learn their job in a haphazard manner by watching another moldmaker at work. Prior to the writing of this book, moldmakers had no central source of information to refer to. A giant step toward standardization of the craft was taken when the Manufacturing Jewelers and Silversmiths of America established the Jewelry Institute in Providence using part of the Oster complex on Sims Avenue as a trade school for moldmaking and allied jewelry crafts. A branch of the same school has been established in New York and is known as the New York Metro Academy.

With this book as a guide, a novice moldmaker should be able to learn rapidly how to turn out good, workable molds. The basic difficulties that he or she will encounter will be in cutting the molds, positioning the models in uncured mold sets, and, of course, in making the more advanced and complicated molds such as modulated and sectional types. Additionally, the Oster Group is available for consultation and help on any moldmaking problem that the moldmaker may run into.

1.2: A word about safety

The most important thing for a novice moldmaker to realize is the need for safety. Cutting molds is dangerous! Even years of experience are not always sufficient to prevent accidents. The basic tool in mold moldmaking is the scalpel. It is designed to cut cleanly, easily, deeply, and efficiently. The moldmaker must learn respect for the blade, for there are far too many cases of moldmakers slashing their chests and hands. Inattention, carelessness, and dull blades are the main causes of accidents.

1.3: The pewter moldmaker’s tools

To cut, a sharp blade is absolutely necessary. The duller the blade, the harder the moldmaker must bear down to cut, which increases the likelihood of the blade’s slipping or snapping and flying like a projectile into an eye or face. Do not try to save money on blades. Throw them away immediately when the effort required to cut begins to increase even slightly.

A moldmaker needs a clean, uncluttered work area with plenty of natural daylight and supplementary artificial light. Cluttered, poorly lighted areas make for sloppy work and many accidents. If possible, the moldmaker should work in a room entirely separate from the casting room, because the noise, heat, and dirt of the casting operation can reduce his efficiency and affect the quality of the molds he makes.

The following is a list of tools that the moldmaker will need:

• Lighting. Daylight type artificial light, preferably with a source of natural light close by the work bench. A bench mounted magnifier with florescent light is also recommended.
• Rubber solvent
• Steel wool, fine 00 grade
• Files
• Tweezers
• Modelmaking tools
• Soldering tools
• Modified shipper’s knife, scalpel, and blades for both (Fig. 1.1)
• Mold shim stock
• Supply of drills, burrs, needles
• Drill with flexible shaft
• Stand for drill (Fig. 1.2)
• Gloves, asbestos and canvas
• Pliers, screwdriver, claw hammer
• Talc
• Paint brush or talc applicator
• Graph and drawing paper
• Calipers in inches and millimeters
• Micrometer
• Alignment nuts
• Basin formers
• Preform sprues
• Modal storage drawers or racks
• Scissors
• Leather safety apron
• Safety gloves
• Pens, pencils, compass, dividers
• Container for disposal of used blades
• One half of a cured 18” or larger mold to use as a work holder during cutting

1.4: Classification of pewter model molds

The moldmaker produces two kinds of molds: model molds and production molds. The production mold is the mold that produces the finished product. (See below, Chapter 1.6, for a classification of production mold types.) Production molds are cured with models cast in preliminary molds known as model molds. There are two kinds of model molds:

Model molds: The model mold is the mold that makes the first reproduction of the prototype model. This stage is known as the “first shrinkage,” because the models cast from this mold undergo the first in a series of dimensional changes from the prototype model. (See Chapter 6.2 for a discussion of shrinkage and Chapter 7 for a complete discussion of models, especially 7.2) Because castings from this mold will serve as production models, they usually require more careful cleaning than production castings require, as well as the addition of detail by hand to make them ready for the production molds (or another model mold). When there are enough good models available to fill the production mold, there is no need to make a model mold. A model mold can be a full mold, or, when only one model is needed, a pie-wedge mold can be used. The model mold is used to cast out as many models as will be needed to cure a production mold or a production model mold. Models should be cataloged and stored separately for future use.

Production model molds: These are the molds that cast out the production models needed to cure the production molds. These production models should be clean and perfect in detail. Production model castings are usually rhodium plated to provide a measure of protection from scratches, deformation. and resistance to chemical reaction with the uncured rubber molds. It is important to cast enough production models that, if the production model mold is lost or destroyed, a new one can be produced. Production models should be labeled as such and stored carefully.

1.5: The pewter moldmaker and the model

The models that come to the moldmaker and from which he will make the model molds may often leave much to be desired. They may be scratched, gouged, missing details, and have flash or even porosity. Every one of these defects will be reproduced when the mold is cured. When the process includes all three steps, model mold to production model mold to production mold, the amplification of the original defect can produce a full blown production crisis.

The moldmaker should never accept a model without inspecting it carefully for all possible defects. A good moldmaker is expected to be able to do some cleanup work on the models that are given to him. It is his responsibility to ensure that model defects are not simply molded into production castings.

Two simple but very important tools used in cleaning up models are fine grade emery paper and steel wool. Polishing models to remove defects is not recommended, since polishing may also remove important details or sharp edges. The emery and steel wool will remove scratches without causing any loss of details. It is important to inspect all models after curing for damage and rubber buildup, and, if necessary, clean them with emery and steel wool after they have been used in the vulcanizer a few times.

If, after a production model mold has been made, the production models that are cast require any additional work or cleanup, it is wiser to rework the prototype model and then cure a new production model mold, rather than clean each model that is cast from the mold The time expended in making the additional mold will prove to be considerably less than that required for the additional cleanup operation on the production model castings.

1.6: Classification of pewter production molds

The types of production molds that are used and that a moldmaker must be familiar with are listed here. Each is discussed completely in Chapters 8 and 11 through 15. In selecting a mold type for a particular production mold, the moldmaker should keep it in mind that the easier it is to remove the castings, the faster the production pace and the longer the mold life will both be. Thus, the moldmaker must continually be encouraged to use his imagination to find ways to cast each job in the simplest type of mold possible.

Squash mold: A squash mold is a simple mold requiring no undercuts, severe cut-outs, build-up or other modifications. Squash molds are used for casting simple flat shapes such as advertising coins.

Base mold: A base mold is especially designed to control the position of the parting line on a casting, or to assure a flat surface on one side of a casting. It requires five operations to make. An example of a casting requiring a base mold would be a casting of an unusual shape, the parting line of which must run in a straight rather than irregular line. (See Chapter 11)

Flat back or dummy mold: When a casting is basically flat, has no design features on one side, and a parting line is not desired, it can be cast in a flat back mold. An example would be a medallion that goes on a wall plaque (See Chapter 11.2)

Modulated, ‘cut-out’ or ‘built-up’ mold: Modulated molds are molds made from mold sets modified before curing to accommodate very thick, highly built-up or modulated castings. An example would be a square or spherical casting. (See Chapter 13)

Sectional mold: To cast very complex pieces, it is necessary to design molds that have more than two parts in order to make it easier to remove the castings. Molds designed to cast heavy pieces or pieces that call for severe undercuts or difficult inserts almost always must be designed as sectional molds. An example of such a design would be a bangle bracelet. (See Chapter 14)

Insert mold: Insert molds are used to embed permanent inserts such as pins, beads, stones, chains or tacs into a casting in order to eliminate any additional operations such as soldering or swedging. Insert molds can also have reusable inserts which do not become embedded into the castings to cast holes, openings or to form unusual shapes in a casting (See Chapter 12, Inserts)

1.7: Basic pewter moldmaking procedure

The simplest and most basic mold design is the ‘squash’ type. The only consideration in setting up a squash mold is the position of the models in the uncured mold set. Thus, the squash mold is the best type for the beginning mold maker to start with, as he does not have to worry about cut-outs, parting lines, or the complexities of a sectional mold. From this most basic mold type, the beginning moldmaker will be able to advance to the more complex molds.

The basic procedures followed to make a squash mold are the same as those followed when making the more complex molds. It is the repetition of the basic squash technique that enables the novice to master the art. The more simple molds that he makes, the quicker he will master the complex types.

A simple coin type casting with few undercuts is the easiest type of casting to begin with. Refer to Chapter 8.1 where the gating and positioning of models in the mold are shown.

Refer to Chapters 9.8 and 10.2 un gating and venting for cutting techniques. Dexterity at cutting molds comes only with a great deal of practice cutting and handling many molds. Defective and burned-out mold halves provide excellent material for the novice moldmaker to practice on.

In making channels, runners, and gates, the important part of the technique is cutting without any hesitation to ensure a clean rather than a jagged cut. Self assurance comes only with experience, and as the novice moldmaker’s fear of the blade lessens. When cutting is mastered, however, the more difficult molding procedures come much more easily.

1.8: How to make a simple pewter squash mold, step by step

1. Pre-heat: Turn the vulcanizer on by setting the timer past the one hour mark and then returning the indicator to the desired time setting. Set the required temperature on the heat control. Place the bottom plate of the vulcanizer frame in the frame ring, making sure that the ring lip (if provided) is on the bottom. Place the frame cover upside down over the ring. Place the unit in the vulcanizer to allow it to pre-heat. (Fig. 1.3) Pre-heating allows the curing cycle to start as soon as the mold set and frame are placed in the vulcanizer for curing. A cold frame must heat up before it can transfer heat to the uncured mold. This time lag can throw off the time allotted for the squeezing phase in curing.

2. Rubber preparation: Begin with the rubber at room temperature. Separate the top and bottom halves of the mold set. Because grease, talc, and other foreign substances may interfere with curing, the rubber molds should be cleaned with a suitable rubber solvent (available from the rubber supplier), especially in the areas where the rubber must cure to itself.

While cleaning the mold set, inspect the rubber carefully for hidden blisters and hollows. If any are present, the mold half should be delaminated, if possible, until the defects are reached. Laminate the removed layers to the back side of the mold half, being careful to clean each of the layers with a solvent. (Fig. 1.4) To delaminate the rubber, first heat the rubber on a hot vulcanizer plate for 1-2 minutes. This softens the rubber sufficiently to allow the plies to be separated. Rubber solvent applied from a small plastic squeeze bottle through a nozzle greatly eases the delamination process. Simply squirt a little solvent between the plies of the rubber as they are separated. (Fig 1.5) A flat, spatula-like tool is also useful.

It is important to be aware that not all rubber mold formulations can be de- laminated because of qualities that are specially manufactured into some rubber compounds. (See Chapter 6) This is especially true of the silicone rubbers, for example, which have a clay-like consistency. In the case of a mold that has a design that makes delamination necessary, the rubber supplier should be consulted for the proper rubber compound.

3. Models: Have the models to be used ready for positioning in the mold set. Make sure that they are clean, well made, and able to stand the heat and compression of the vulcanizer. Always make sure that production models are inspected for dirt, rubber buildup, corrosion, and breakage and then cleaned after each use. On intricate models, especially filigree, inspect the openings for dirt, sharpness of detail and breakage. Remember that the finished casting can be no better than the model. But it can easily be a lot worse.

4. Layout: Position the models in the mold set to make the most effective use of available space while allowing room for gating, venting. and locators. It is not the number of models that you can get into the mold that is important, but, rather, how easy it is to release and handle them. Use a ballpoint pen and compass to indicate the layout of the models on the rubber and to scribe the outer and inner limits of model placement

5. Registration nuts, locators, or alignment nuts: Registration nuts are cured into a mold along with the models. They ensure correct and easy alignment of the mold set each time the mold is assembled for casting. The most popular style of nuts are the “pal-nuts’’ which are a stamped and formed rather than a machined nut. They are inexpensive and readily available Moldmakers’ preferences in registration nuts range from their own cast designs to heavy nails, but the basic purpose of aligning mold halves quickly and efficiently should be the only consideration in deciding which type to use.

The number of registration nuts that can be used in a mold is basically unlimited. In especially heavy or large castings, the more that are used, the better the mitering of the molds.

6. Placement (hot mold frame method): After the mold set has been prepared. begin with the half of the mold set that has the die-cut center hole. Remove this die-cut and discard. Position the models on the pre-scribed mold half along with the registration nuts, the basin former, and the runner preform, if one is used. (Fig. 1.6) Work with the mold outside of the mold frame, since the pre-heated frame is hot. It is best to place the models face up on the mold half so that they will lie face down when the mold set is placed in the CRMC machine. Because dirt and dross rise during the CRMC process, a face down position in the CRMC machine insures that dirt and dross will end up in the backs of the castings. Whenever possible, the part of the mold cavity that contains the face of the model or the details that will be most visible should be in the mold half that contains the basin.

7. Frame: The novice moldmaker must become acquainted with the mold frame before making his first mold. (Fig. 1.7) The three parts are: the bottom plate, the top pressure plate, and the ring frame. Close examination will show that while the plates may appear the same, there is an important difference one of the plates is thicker and serves as a pressure plate during the compression stage of curing. The ring frame also has a top and bottom. The bottom has the ‘lip’ for insertion of a tool to pry the plates apart after curing.

8. Placing the mold in the frame: After the models and locators have been positioned on the mold half with the hole, grasp the mold by placing the fingers of one gloved hand in the die cut-out hole. and place it carefully on the bottom plate inside the ring frame. Position the basin former in the die-cut hole, basin side up. Position the other mold half over the mold and models being very careful not to disturb their location. With heavy gloves, fit the frame pressure plate - which is very hot - over the mold set and align it in the ring frame. (Fig. 1.8)

Tilting the pressure plate into the ring frame helps to align it within the ring. (Fig. 1.9)

Place the frame set in the vulcanizer being careful to hold it steadily so that the models and locators do not move or “shift.” (Figs. 1.10. 1.11) Position it in the center of the bottom platen to ensure that the platens will exert equal squeezing pressure. A scribed guide mark on the bottom platen will make positioning easier. Make sure that the frame pressure plate is mitered correctly into the frame ring.

Cold frame method: Place the mold half without the die-cut hole in the frame on the bottom plate. Then position models, locators, proform sprue if used, and basin former, basin side down, on the mold half (Fig. 1.6.) Since the frame is cold, gloves are not required. The balance of the procedure is the same as that described for a hot frame. The hot frame method is preferred by Marshall Ritch, as it allows better control of the timing in the compression stage of curing.

9. Preventing rubber build-up in the frame: Rubber can build up in the mold frame and make it difficult to cure a mold with good parallel surfaces and perpendicular sides. Some moldmakers use newspaper as a mold frame liner to prevent rubber build-up. Many moldmakers also believe that the paper acts as a gas release during the curing operation. However, because newsprint and printing ink contain sulfur and other chemicals, using newspaper in the frame may prevent proper curing and shorten mold life.

A better method of preventing rubber build-up is to clean the inner surfaces of the frame frequently with rubber solvent. Also, prior to each use, the inside of the frame set should be rubbed with a release agent, such as talc, or a mixture of talc and powdered graphite in a ratio of 3 parts talc to 1 part graphite. This will prevent the rubber from adhering to the frame. Spray silicone release agents, because they are liquid, may leave a liquid residue in the frame which can interfere with the curing process) and also cause porosity.

10. Closing the mold frame: Care must be exercised in closing the mold frame over the mold set. Careless closing can cause the models and the mold to shift which will cause mold distortion and excessive rubber leakage during the curing process. It is sometimes difficult to fit and close the pressure plate of the frame properly because the uncured mold set with the models in it is often higher than the ring frame. Tilting the pressure plate as it is positioned in the ring frame will help to align it in the vulcanizer. If distortion problems persist with a particular mold design, it may be necessary to redesign the mold as a modulated type so that it will accommodate the models.

The mold frame should appear as in Fig. 1.12 prior to the lower platen being jacked upwards in the vulcanizer. If it does not, the jack must be released immediately, and the mitering of the frame pressure plate corrected. It may also be necessary to re-open the frame and inspect the mold set to see if any model shift has occurred. If mis-aligned molds are not corrected, the mold will be ruined. It is obvious that continuous checking of the alignment of the mold set in the vulcanizer is important. For this reason, the vulcanizer should always be installed on a stand or a bench so that the platens are at eye level. This makes it easier for the moldmaker to check the mold while it is being cured. A vulcanizer on the floor is a major cause of poor molds, backaches, and worker fatigue.

11. Vulcanizer: Lock the vulcanizer jack ratchet and begin to raise the platen and mold frame by hand until contact is made with the top platen. Check to ensure that the frame pressure plate remains properly aligned with the ring.

12. Time Temperature: The mold supplier will provide an approximate time and temperature schedule for curing the particular compound of rubber being used. The compression timing will depend on the type of models and mold style selected.

13. Compression, squeezing, or jacking: After the mold frame containing the uncured mold set has been positioned in the vulcanizer:

1. Engage the jack lock ratchet trip of the vulcanizer by hand.
2. Jack the vulcanizer’s lower platen until the mold frame is in contact with the top platen. Check mold alignment.

1. Insert the jark handle. Make sure the handle provides sufficient leverage for the height and weight of the mold maker (Fig. 1.13)
2. Make two complete jacking movements initially, trip the jack lock ratchet, and jack the platens apart slightly. This will help to release any entrapped air in the mold set and frame
3. Re-engage the lock and jack the platens closed until a slight resistance is met Remember that the mold set is still cold at this point, so the rubber has not yet begun to soften. Too much pressure or “squeeze” applied to cold rubber before it softens can distort the models, especially if they are fragile.
4. Within a minute or so, as the rubber heats, it will become more plastic and begin to yield. Compression should now begin. The only indicator for the moldmaker of how much pressure to exert will be the “feel” of the jack handle during jacking. It is this ability to “feel” the rubber through the jack that is most important to the moldmaker during this stage of moldmaking because it provides him with the only clue to when and how much pressure to apply. The pressure or “squeeze” must be applied in steps during the mold supplier’s suggested time for the compression stage. The final squeeze must be completed before the rubber has gone into its cure or ‘scorch’, because additional pressure applied after this happens will not make any further impression of the models in the mold set.

1. After the compression stage is finished (average time for a 9” x 1” mold set will be about 8-12 minutes), allow the rubber to cure for the time suggested by the supplier.

14. Opening the frame and removal: At the end of the selected curing cycle, release the jack lock ratchet and jack the platens apart using the jack handle. When they move freely, remove the handle and, for speed, continue to jack them open by hand. Wearing heavy safety gloves, remove the “pie.” (Fig. 1.34)

Because the vulcanizer is timed automatically, it will shut the heat off at the end of the cycle. It will not harm the molds should they inadvertently be left in the vulcanizer for longer periods of time under heat and pressure. However, the mold should be removed from the frame and opened while it is still hot, because it is difficult to open it after the rubber has cooled due to a vacuum created in the frame during curing (Fig 1.15) The models should also be removed from the rubber as soon as possible because if they are left in the mold too long, the shrinking of the cooling rubber will make them difficult to remove (Fig. 1.16) If a mold requires cooling to prevent damage to soldered or low-temperature alloys, it should be removed from the frame and allowed to air cool, or cooled by quenching in cold water. Before opening the mold set, cut an alignment notch on the side of the two mold halves, being careful to avoid cutting too deeply and possibly disturbing a registration nut or model. Trim the rubber flashing on the mold edges.

The tools most moldmakers use to open the mold frame are a heavy screwdriver or a crowbar. (Fig. 1.17. 1.18) The tool is placed in the frame ring lip and gently pried in several places to open the frame plate. This will eventually gouge and rain the frame, even with the greatest of caution, since the frame is made of non-hardened steel. A mold opener that will not damage the frame can be made by modifying a small foot press to hold the frame and force the plates apart.

15. Identification: Molds should be identified on both the outside perimeter and the top mold half. A white or black marking pen is usually used.

16. Trimming flash: When a cured mold set is removed from the frame, it will have rubber flash along the mold circumference on both edges. This must be trimmed with either the shipper’s knife or a single edge razor blade, by rotating the mold while guiding the blade against the edges. Care must be taken not to bevel the mold edges while trimming the flashing. Beveled edges allow a mold to collapse slightly during casting and will require shimming. The mold set should not be separated before flash has been trimmed. (Fig. 1.19)

17. Cutting: Gates and vents should be cut while the mold is still hot since cold rubber is more difficult to cut. (Experienced moldmakers can cut cold molds!) Gloves are suggested because of the heat and as a safety factor. See Chapters 9.6 and 10 for detailed procedures. Ignore the experienced moldmakers who say not to wear gloves. After all, they’re your fingers!

1.9: References

Clamping pressures: Clamping pressures in the CRMC machine vary depending on the mold. For a standard 9” mold, 20 pounds is a good pressure to start with. If the mold produces flash, clamping pressure in the CRMC machine should be increased in one (1) pound increments until the mold produces castings without any flash.

Average vulcanizing times:

1" thick mold: 1-1½ hours
1½ thick mold: 24 hours
2 thick mold: 3 hours
2% thick mold: 34 hours
3 thick mold: 4½ hours

1.10: A note on pewter moldmaking and the ‘Principles of…’

Every mold is unique because every model used to make a mold is unique. Every moldmaker will interpret the model and how the mold for it is to be made in light of his or her own experience (or lack of experience) and will, no doubt, look askance at each and every mold used in illustrating this text. A 90° rotation of models from a model mold to a production model mold to a production mold is not an absolute - only a suggestion. There will be times where determining factors for rotation will be based upon where the model is to be gated, how thin it is, and how metal must flow in order to fill the cavity. The need to place 50 models in a mold, rather than only 30 may preclude any possibility of a side or back gate. What is a simple squash mold for one moldmaker, may be better as a sectional mold for another moldmaker.

What this book can do is provide another viewpoint, another map of how to get from point A to point B, that perhaps a moldmaker may not have realized previously. “Where is it written that…” should be read in this book only as “Where is it suggested that…”

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Chapter 2: Introduction to the moldmaker’s job and basic moldmaking