PPP: Working with a Manufacturer, Part 2: Methods of Making Ceramic Reproductions

March 1, 2002
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A Ceramic Design Group employee uses a template to create a clay model for slip casting.
Many successful studio pottery operations use a combination of wheel throwing, slab and press molds, some jigger/jolly work, hydraulic pressing and slip casting. Doing this in-house requires a significant commitment in labor, equipment and overhead. You can also have many throwers hand-making each piece on a daily basis. All of this involves intensive training time. It is possible to earn a living making pottery in these ways in a studio situation, but it also involves significant capital and commitment. This semblance of a living wage might also be predicated on not having a life outside of the business, issues that were covered in the first part of this article (September 2001 Pottery Production Practices).

When you make the decision to grow your studio business by employing the services of a contract facility, these in-house issues are no longer your concern. You have made a wise economic decision to grow your business outside of your business. When you work with a contract manufacturer, your ceramic ware can easily and accurately be made in multiples using the appropriate assisted technologies.

Methods and Tooling Requirements

Three methods of making ceramic reproductions are used in a manufacturing environment: slip casting, jiggering/jollying and hydraulic pressing. Some contract facilities specialize in all methods of manufacturing ceramic ware, while some just slip cast, hydraulic press or jigger. Some facilities use only one clay body, and others have several.

Most open forms without returns or undercuts can be made on the jigger/jolly machine or hydraulic press. Forms that are undercut or have multiple returns and changes in surface orientation are usually made by slip casting. A form made by the jigger/jolly must be symmetrical, while forms made by hydraulic pressing or slip casting does not have to be symmetrical. All three methods of manufacturing produce ware with uniform wall thickness, and all three require tooling.

Tooling is the nomenclature for plaster or rubber molds, gypsum dies, impressions, die cases, roller form tools or profile tools. It can also include any jigs or fixtures necessary to make the above items. Without tooling, multiples cannot be effectively produced. Tooling represents a major expense for any project—the necessary start up costs so that the work can be easily reproduced. Tooling costs are either spread out on a per piece basis, so that the manufacturer can recapture its costs over time, or they are billed as a separate item in addition to the cost per piece. Most manufacturers prefer to have the tooling charged as a separate item and payable in advance.

Tooling—such as the air release coil and rebar die reinforcements for the hydraulic pressing operation shown above—represents a major expense for any project.

Making the Perfect Model

All tooling starts from a model, which can also be the original piece with the necessary shrinkage factor taken into consideration. The model must be oversized, so that the finished piece is of the correct dimension. If the model does not take into consideration the shrinkage of the clay, the resulting pieces will be too small, and the money spent on the tooling system wasted.

Since the finished product depends on the accuracy of the model, time and effort need to be taken to ensure that the model is perfect. Mold makers at the contract manufacturing facility will diagnose the model and determine its appropriateness for the next step. Defects can sometimes be corrected by the mold shop, but if they are too severe, the model may be returned to you to be remade. While a simple thrown form can sometimes make an appropriate, even-walled, perfect model, I have seen many “perfectly” thrown models with uneven wall thicknesses that were rounded, out of dimension and warped, with uneven feet and rims—none of which were suitable to manufacture tooling. In most cases, model making is best left to professional model makers.

Models can be drawn using traditional drafting techniques either by hand or on the computer. After they have been accurately dimensioned, these drawings are then turned into perfect models by turning or constructing them in plaster using various methods, or machining them on a CNC machine in an appropriate material.

All models need to be sealed so that the surface is non-porous. Models can be hand thrown, but again, they need to have perfectly even wall thickness throughout the piece, as well as sufficiently designed rims and feet. Models need to have a hard surface, since much work is done up to and including that surface. Soft models made of Styrofoam, modeling clay, etc. are not suitable, though there are situations where these materials can be used as adjuncts to a hard surfaced model. Every model must also have draft, which is deviation from the vertical. If there is no draft, the item will not release from the mold.

The more changes in direction on a surface of the piece, the more complicated the mold needs to be, and the greater the number of necessary mold parts. Outside edges or corners should have their edges relieved, and inside corners need to have a fillet. When designing for machine production, it is important to know how these machines function and how the clay needs to work with the specific tooling.

It is very important to stress that the model must be perfect in all ways. While skilled mold and model makers can usually correct minor defects, it is necessary that the potter prepare the model as perfectly as possible. Many hours of production time can easily be wasted if the model is not correct.

The Art of Slip Casting

Slip casting is perhaps a universal method of ceramic manufacturing. In fact, it is acknowledged to be one of the oldest methods of making duplications. Slip casting uses porous plaster molds and clay in a liquid state, known as slip. Simply put, the slip is poured into the hollow mold and left in the mold for a calculated length of time. The leftover slip is then poured out and the resulting ceramic shell shrinks inwards, releasing itself from the mold. After it has set up sufficiently, it is removed from the mold and trimmed. Parts can be assembled at this stage. After the greenware is dried, it is then bisque fired and glazed.

To arrive at the finished ware, however, a series of methodical steps needs to be followed. These steps each comprise a part of the entire slip casting mold system, which includes the model, the block mold, the case mold and the working mold.

The block mold is hand made from the model using Pottery Plaster #1 or an equivalent plaster. The mold maker must determine where the parting lines will be, and this determines the number of pieces that comprise the mold. The castings need to be able to be released from the mold, which is why multiple parts are necessary. Of course, some simple shapes may only require a two-part mold. The mold maker will also determine the location and configuration of the spare, or reservoir, for the slip. The block mold will also contain the keys, which are the locking devices on each mold part that “key” each piece together. The parts of the block mold, when assembled, are held together by rubber bands or mold straps with buckles, just as the parts of the case mold and ensuing working molds will be.

It is also important at this stage for the mold maker to project ahead and visualize how the case mold, also called the master or mother mold, will be made from each of these separate parts of the block mold. Some of the options include a hard case with side rails, a one-piece rubber case with integral side rails, or a combination case using both rubber and gypsum. The mold maker will determine which type of case mold will be most practical from a working point of view, as well as what will be most feasible financially. For example, rubber molding products are generally expensive, but the cost of labor to make the working molds from a rubber case is reduced as there is little surface preparation necessary compared to an all gypsum hard case with separate side rails.

The block mold looks exactly like a working, or production, mold. It is a negative of the model, since the model is a positive shape. When dry, the block mold is used to make a few sample castings that can be bisque fired and provided to the client for approval, though too many castings will degrade the mold surface and make it inappropriate for the next steps. Minor changes and adjustments can be made in the block mold at this time if necessary. Once approved, the block mold is then used to make the case mold, also known as a master mold or a mother mold.

Each mold part that comprises the block mold requires that a mold be made of it. For example, a two-part block mold will require two case molds, or one case mold for each mold part, and so on. The case mold can either be a hard case, made totally from gypsum cements such as Ultracal 30 or Hyrdrostone, or a pourable rubber mold, depending on financial and/or production constraints. These case mold parts are positive mold parts. When Pottery Plaster #1 is poured into the case mold, a working mold part, or a negative part, is produced. The number of production molds and the number of casting days will determine the weekly production.

Once the model and the block, case and production molds are finished, production of the ceramic ware can begin. The model, block and case molds are put away, since they comprise a system of manufacturing and should not be damaged. The working molds will eventually be degraded by the action of the slip as well as movement within the studio, but keeping the case molds in good condition allows more production molds to be made at any time.

Some facilities use newer materials in casting, such as porous resin molds that are used for pressure casting. In this process, the slip is injected under pressure and the water is quickly removed the same way, allowing for fast turnaround times. However, the resin molds and equipment for this process are expensive, making pressure casting best suited to high volume production.

Your manufacturer will discuss a production cycle and method with you based on the number of pieces that you need, the work required for de-molding, finishing and adding other cast parts if necessary; bisque firing; glazing; and finally, packing and shipping. Well before this time, you should have worked out any clay/glaze compatibility issues as well as received the fired glazed samples from your manufacturer. It is also important to work out issues such as design confidentiality, disposition of seconds, methods of packing and shipping, and payment terms.

The costs for slip casting tooling are usually billed by the hour for mold and model making, plus materials. Production molds are usually billed by the mold part, with straps and rubber bands included.

Some potters have been able to develop successful in-house casting operations. This requires a mold making shop and significant space for mold storage, casting tables, blungers and a learning curve necessary to compound and blend successful casting slips. Depending on your operation and available facilities, it might be a better allocation of resources to outsource the slip casting of your ware.

To Jigger or to Jolly?

Jollying refers to the process of making ware inside a revolving mold so that the mold shapes the exterior of the ceramic form and the profile tool makes the interior. Jollying makes the piece right side up. Jiggering, however, makes work upside down. The interior of the piece is formed over a revolving plaster mold, and the exterior is made by the profile tool. Roller form tools are used to jolly forms, while profile tools can be used for both jiggering and jollying. Both jiggering and jollying use plastic clay, and the clay must be of proper moisture content and sufficiently de-aired.

A jigger/jolly machine—sometimes just called a jigger machine—can either jigger or jolly ware, depending on the operator’s preference. These machines use a revolving plaster mold held in a bucket. The jigger arm pivots down on a vertical fixed arc so that it indexes precisely to the center of the spinning mold at a pre-determined height, which determines the thickness of the ware. The profile tool is indexed in the arm so that it shapes the interior/exterior of the piece at precisely the same place each time the arm comes down. Each piece requires one mold, just as in slip casting. The number of molds turned per day will determine the weekly production cycle.

Some jigger/jolly machines are single speed, while some feature multiple or variable speeds. Older jigger machines were called “pull down” machines and were used as dedicated machines for a particular item. These machines were powered from a constant speed belt that was connected to a series of axles and bearings running off of a central motor. Newer machines are self-contained and quite efficient. Some have one arm pivoting into one bucket. Multiple jigger machines have many arms pivoting automatically into an equal number of rotating buckets. Clay pugs are automatically introduced into each mold, which are also robotically placed into each spinning bucket. The finished piece in the mold is then automatically ejected and placed into a dryer.

Tooling for this type of manufacturing usually starts with an oversized model or an oversized drawing, done as a section drawing. If the process starts with a ceramic model, the mold maker will cut the model in half and pull a sectional line drawing from that original. Either method is necessary to produce two profile tools. One profile tool is used by the production facility to produce a master mold, either from gypsum materials or pourable rubber materials. Cost and expediency determine which mold making material to use. Using mold making equipment such as a back case (the reverse of the bucket head), a faceplate and a ring, the facility will produce working molds either in Puritan Plaster, Pottery Plaster #1 or an equivalent, from the master. Puritan Plaster is used when the ware is dried in mechanical dryers and it also has a surface-hardening additive to increase mold life. Pottery Plaster #1 is used by most potters as it is very available and works well without automated or mechanical dryers.

There are many variations on jigger mold making equipment, but the important consideration is that each mold system produces perfectly concentric working molds. After the facility makes one working mold, a sample should be furnished to the client for approval before any production molds are made. Small changes can be tooled into a gypsum master mold, but if the master mold is made of rubber and the client requires a change, a new drawing, profile tools and mold need to be re-made. It’s easy to see how essential it is that the model be perfect.

To produce ware with the jigger/jolly technique, a deaired pug is shaped and oriented correctly and thrown into the revolving mold. Then the arm is pulled down, lubrication is applied, and the piece is shaped. With jollying, the plaster mold absorbs the water from the wet clay, and the piece shrinks inward. In jiggered ware, the piece shrinks over the plaster mold. When the ware has stiffened enough to hold its shape, it is removed from the mold, and the edge is finished. Many facilities have dryers that remove the water of plasticity in a very controlled fashion so that warping and cracking are eliminated. Proper design of the ware, clay selection, drying, and other plant procedures such as automated or mechanical ware handling, clay formulation and blending and proper forming techniques, will reduce the rate of seconds.

Jiggering or jollying is ideal for pilot as well as volume production. These machines can often be used for quick turnaround for hollowware samples, for example, so that a client can see quickly what the ware will look like before committing to a large volume. Most operations jolly rather then jigger as the ware shrinks in, unencumbered by the necessity of shrinking over a mold. It is a simple technique and requires an investment in tooling similar to that of slip casting. It is generally billed on a time and materials basis, with the working molds priced on a per mold basis.

Many potters have jigger machines, but the learning curve can be arduous and long. While these machines by themselves are somewhat affordable, they are merely expensive coat hangers without the necessary proper mold-making equipment. Also, these machines require adequate space for efficient operation and production, including mold making, mold storage, space for the machine, ware racks to hold the production, and drying facilities, not to mention the necessary kiln space for efficient and timely firing and project turnaround.

It may be prudent to outsource your work to a manufacturer that has the machinery, as well as the mold-making equipment and expertise, to make the molds. The facility should also have the knowledge and expertise to deal with production problems as they occur. Even in the manufacturing world, clay, glaze and production problems present themselves. The difference between the manufacturing environment and the studio is a matter of scale.

A 60-ton RAM™ press and jigger jolly machine in Ceramic Design Group's manufacturing facility.

The Hydraulic Pressing Option

Hydraulic pressing uses the ability of clay to flow under pressure into air permeable microporous gypsum dies. Gypsum can either retain or release clayware, as clay sticks to plaster. A hydraulic press uses the power of hydraulic fluid pumped under pressure to a cylinder that contains a moveable piston. The piston moves a platen either upward or downward in perfect alignment and in a calculated depth of travel. A working die set consists of a male die member and a female die member; the press bed contains one die member, and the platen the other. A deaired pug is put into the female die member, and the press is closed. After an adequate dwell time for proper compression of the ware, the press is opened. The ceramic part is retained on the upper die member and then released, by the application of compressed air to that die member, onto a catch board, and the cycle repeats itself.

These machines are big, heavy pieces of industrial equipment that represent a significant investment in money, time and space. The hydraulic power unit is noisy and large, and requires ventilation for cooling. Larger hydraulic power units are water-cooled. Small presses can have an integral power unit, while larger presses have external units connected by high-pressure hoses. A press can be “up-acting,” which means that the platen moves upward, with the hydraulic cylinder at the bottom of the machine. A “ down-acting” press has the cylinder on top of the machine and the platen moves downward. Machines have adjustable “jacks” that determine the depth of closure of the press. Some small presses have one cylinder, while larger machines have two cylinders operating together.

Presses are manufactured to produce significant pressure upon closing. Small presses can generate 5, 10 or 15 tons of pressure spread out over the entirety of the press bed. Larger presses produce upwards of 30, 60, 90 and 120 tons or more of pressure. Press capacity is also measured by the stroke of the cylinder and the maximum opening of the press, also known as daylight.

Presses require two-handed operation, and some have light curtains or other safety devices to prevent injury. A semi-automatic press requires the operator to open and close the press, operate the air to the top and bottom dies, and dwell the press. These types of presses have one speed. An automatic press can be programmed for fast or slow approach; dwell time; and programmable air at specific pressures and times, such as early air to the male die member assisting in the release of the ware. The operator simply has to insert the pug of clay and catch the released ware. Hydraulic presses can produce large volumes of ware. The ware does have a seam and it must be removed and the edge finished, just as in casting and jiggering.

The key to hydraulic pressing is the die. These dies are made from gypsum cements, such as Ceramical, that have a very low expansion factor and resist abrasion. A single die set, properly cared for and maintained, can produce upwards of 750-1200 reproductions, more or less, depending on the clay body used and the design of the die. Newer porous resin die materials can produce thousands of reproductions.

A press die is contained in precision-machined steel die cases that can be circular, square or rectangular. These steel cases are machined so that their edges are perfectly parallel, which ensures that when the press closes, the ware is perfectly shaped and uniform. The working die set contains an air system that makes the die microporous when made.

The die making process is lengthy and costly. Again, it begins with a perfect model, which is a positive piece. A male and female master die set is made from the model. These are the negatives, or the reverse of the model. They are perfectly tooled and dressed and are exactly like a working die set, except they are not purged. From this master die set, a set of impressions is made. The impressions are gypsum-backed and rubber-lined positive forms. From these impressions, a working die set is made, which is a negative. When the clay is pressed between these two die members, a positive clay form is produced. The impressions are a valuable part of the die system, in that after the working dies wear out, a new working die set can be easily remade using the impressions. The working die set contains an air coil made from special cotton tubing. When the Ceramical is poured into the die cases over the impressions, a sequence of purging is done so that the resulting working die set is air-permeable and microporous. Dies can be either of single cavity configuration, making one piece per pressing, or configured as a multiple cavity die, which creates more than one piece per press closing.

Once the die is set up on the machine and properly indexed, aligned and spaced, ware is produced. Some shops charge a fee for press set-up for small runs. As it is a timely endeavor to set up a press, manufacturing facilities usually like to maximize press time by producing as many pieces as possible. Some facilities charge for press time or just the cost per piece. Finishing the seam can be done automatically on a finishing machine, or manually on a potter’s wheel or other finishing device.

A ceramic piece must be properly designed for the press. As the clay flows under pressure, it needs to flow unencumbered. Ware that is not properly designed is subject to cracking and warping. Die makers will diagnose the model and make sure it is properly designed, especially in the foot and rim area. Again, ware must have adequate draft. The die will be designed to have the necessary gutters in place to provide compression at the rim. The clay must be of press consistency, contain less moisture content than the plastic clay for throwing or jiggering, and be adequately de-aired. Clay that is too wet or not properly de-aired cannot be successfully pressed. Also, clay bodies that are not properly designed with sufficient plastic material are prone to developing air bubbles when the press is opened after pressing the piece.

Dies need to be cared for and purged periodically when pressing. It is not enough to simply place a pug in the die and let the press do its thing. It takes skill to operate a press and produce first quality ware; it does not happen by fiat.

A press can be a valuable production tool, but if the ware is poorly designed or of inferior quality, the resulting pressings will also reflect this and, in many cases, more seconds will be produced than first quality ware. A facility that is well versed in hydraulic pressing can also offer consulting information that is necessary for making correct tooling and quality pressed ware. Again, everything starts with a perfectly designed and executed model.

Taking the Next Step

Producing ceramic ware using assisted technologies such as slip casting, jigger/jolly or hydraulic pressing can be an invaluable resource to a potter looking to move a small studio business up to the next level of production. But do not be easily deceived. If the demand for your ware is greater than what you can produce, a contract facility can be of an immeasurable value to you. Making this step to the next level demands not only thoughtful consideration but also a redefining of the pottery-making paradigm.

If you are ready for this step, make educated and well reasoned choices. Get the help you need from the beginning by choosing a production facility that is flexible, proactive and can work with you throughout the manufacturing process. The manufacturing facility can help you determine the best method of making reproductions based on your design and your budget. By working with others, the growth that your business will undergo can be shared gratefully many times over—as can the profits you can potentially reap and the goals you can attain.

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