How to Shrink or Enlarge a Casting – Part 1 of 3

Introduction

This is a three-part article on how to enlarge (also called pointing up) and alternatively, how to shrink the size of castings without the use of laser scanning, CAD/CAM and computer operated CNC machinery. The procedures for both are much simpler than one would imagine and avoids the cost and the complexity of high-tech equipment. This knowledge comes in handy for many mold making and casting projects and artistic commissions, saving the time and aggravation of re-sculpting a model to a needed size.

Two Ways to Enlarge a Casting

There are two ways to enlarge a casting. The first is to enlarge the mold before the casting in created. While the second method is to enlarge the casting after the mold has been made. Both methods of enlargement, if properly done will yield a finished casting in correct proportion and detail as its original. Each method depends on a different material. To enlarge the mold, the material used is known as addition-cured silicone. To enlarge the casting the material used is a specially formulated polyurethane rubber.

Method One – Enlarge the Mold

We begin with instructions on how to enlarge the mold in order to enlarge the casting. The first method to create an enlargement of a casting, is approached the same as for making a duplicate size casting, which is to create a normal silicone mold. In this case the silicone material of choice is an addition cure (platinum catalyzed) silicone as condensation cure silicone will not work. Once the mold is created, the next step is to enlarge it. The mold is targeted rather than the casting because the enlargement process involves the solvent saturation of the rubber. Saturating the casting would make it awkward to work with and difficult to re-mold. So it is more efficient to enlarge the mold instead.

Before beginning, follow the usual mold making protocol. You should first de-gas the rubber using a vacuum chamber, lacking that, a pressure pot will do. If you don’t have either of those basic mold making tools, make certain you vibrate your mold while it is curing to drive air bubbles to the surface. A power tool such as a hand-sander placed against the mold is very effective. Once created, place the mold in a container that can be sealed, such as a 5-gallon bucket. Then add solvent to the container such as toluene, xylene or naphtha.

When handling volatile solvents, we recommend extreme caution as all of the recommended solvents are flammable. So you do not want to expose to open flame or a heat source such as an electric heater. In addition, the fumes are extremely toxic so that when pouring any of these solvents, you must have good ventilation and you should wear an approved NIOS mask.

Once the mold is fully immersed, the silicone rubber will begin to absorb it. Over time (weeks), you can expect an almost doubling in size could be reached with all its details in proportion to the original model. That is, providing that the mold is completely covered in solvent, if not the mold will distort. Patience is required when resizing the mold, as an example, a mold soaked in toluene takes seven days to expand to approximately 130% of the original size. Larger sizes require more time in the solvent. Molds that have been expanded using solvent are very fragile and must be handled more carefully than usual to prevent tearing or damage.

When you are satisfied with the size, or after three weeks the mold has ceased to expand it should be removed from the solvent bath, and the surface should be thoroughly dried. These are volatile liquids which immediately begin to evaporate when exposed to air. As evaporation continues the mold will begin to shrink. So you must be quick to create your casting. Once the surface is dried, a good casting choice is a two-part polyurethane casting resin. Though silicone doesn’t ordinarily need a mold release, we do suggest using a silicone-based release as it will be de-molding much easier.

Now that the new casting has been created, you can repeat the entire process of silicone mold enlargement using solvent immersion to further increase the size of the final casting.

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Versatility of Plaster in Mold Making and Casting

Plaster is one of the most commonly used casting materials as it is quite cheap, versatile, readily available, and user-friendly.. Once you start working with plaster, you will realize that it is easy to mix, sets quickly to a pleasant white color, can be sculpted and with proper pouring and application can yields few air bubbles in the cast. In fact, it suits almost all kinds of casting applications.

Little wonder then that plaster casting is especially popular among artists and life casters alike.

Plaster is manufactured as a dry powder and is mixed with water to form a paste or slurry The reaction with water creates heat through crystallization and the hydratedplaster then hardens. Let’s take a look at the role of plaster in regular mold making and casting works:

Life casting

While plaster cannot be used directly on the skin for making body molds, due to excessive heat during curing, the heat from the chemical reaction does not affect the casts in any way. In fact, you will find that most life casting artists favor plaster as the material of choice when making the final cast from the body molds. Plaster is also favored for casting using alginate, resin and silicone molds.

Latex casts

Due to the naturally porous characteristics of plaster, plaster molds are widely used in latex casting – such as making latex masks. This is because plaster will absorb the water from the latex and let it cure like a thin skin.

Shell molds

Alginate and rubber molds need rigidity to support their form during the mold making process. This is done by added a mother mold or shell mold. Plaster bandages are regularly used to reinforce alginate body molds and other rubber molds for this purpose, thus forming the shell or mother mold.

These plaster bandages come in long rolls and are formulated to become creamy with plaster consistency when wet. All you have to do is roll out the dry bandage, cut it to length, dip in warm water, squeeze lightly, and then apply over the original mold. Keep smoothing it down to prevent bubbles in the mold later. You should keep overlapping about half or one-third of the previous bandage so as to create a strong shell mold. The bandages will set quickly enough in 15-20 minutes.

Rough molds

Plaster bandages are sometimes even placed directly on the body to make molds. They have to be rolled out, wetted and applied carefully. Finally, fold the bandage in quarters to make rope-like lengths. Use it to reinforce the edges of the mold and make a secure mold.

Keep in mind that while plaster is renowned for capturing intricate details well, the plaster bandages will only capture the form and not the detail. However, they are skin-safe and will not cause any kind of discomfort to the model. It is often used for belly casting or pregnancy molds.

Above all, do not be mistaken that regular plaster or Plaster of Paris can be used for plaster casting. That is because plaster of Paris is formuled for application to wallsit is naturally soft and chalky and does not hold detail well. You need to purchase specially-treated and high definition plaster powder that will not chalk, chip or crack. Only then can you get durable molds and casts that hold finishes well and set hard on drying.

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How to Shrink or Enlarge a Casting – Part 2 of 3

This is the second part of a three-part article on how to enlarge (also called pointing up) and alternatively, how to shrink the size of a casting without the use of laser scanning, CAD/CAM and computer operated CNC machinery. The procedures for both are much simpler than one would imagine and avoids the cost and the complexity of high-tech equipment. The first part of this article described the steps on enlarging a casting by enlarging the mold. Part two provides an alternate method of casting enlargement by actually enlarging the casting, rather than the mold as previously described, in the first part in the enlargement process.

This second method of casting enlargement employs the use of a specially formulated flexible polyurethane rubber that when immersed in water expands proportionally over time to create the enlargement. Polyurethane formulations and water usually don’t mix. For example, in high humidity environments, cure times are adversely effected. If these formulations come in contact with moisture during curing, the surfaces that are in contact, will foam leaving an imperfect casting or mold. However, once cured polyurethane is then impervious to water.

But with this particular enlargement formulation, something different takes place. The polyurethane is actually attracting water to and through it. When water comes in contact with its surface, water molecules are attracted to the atomic charge of the polymer molecules. The polymer molecules actually pull water away from the adjacent molecules of the surrounding water and absorb the water internally. As water is absorbed, the casting stretches to accommodate the incoming water. The water is so tightly held in the casting that it feels dry to the touch. Even cutting or crushing the polyurethane casting will not easily release liquid water. Migrating water molecules uniformly surround each polymer molecule, evenly stretching the polymer matrix in all directions. Through this process, this specialty polyurethane formulation produces a proportional expansion of molded shapes by soaking in ordinary tap water.

To create the enlarged casting, the expanding polyurethane rubber is mixed together at a one-to-one mix ratio. For best results it should be de-gassed as described above. After degassing it is poured into a mold that has been treated with a mold release. Silicone is the best candidate for the mold material, as polyurethane sticks fast to polyurethane, so a polyurethane mold should be avoided. The polyurethane rubber cures overnight. Cured rubber should not be tacky to the touch.

Be careful in de-molding so as not to tear the cured rubber. Fill a container with tap water and submerge the cured casting in the water. Make certain that it is completely covered with water. As any part of the casting that is left out of the water will not expand properly. Cover the container and leave it undisturbed for three weeks to allow the water to be absorbed.

After three weeks the casting should enlarge to approximately 160%. Remove it from the container and pat dry. A mold must quickly be made of the enlargement as just as with enlargement of the silicone mold process, evaporation will shrink the casting. Usually a silicone mold is recommended of the enlargement so when a finished casting is created it will be now be 160% the size of the original and in proper proportion. The enlargement process can be repeated many times, limited only to the size of container.

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Casting Under Pressure With Injection Molding

Mold making and casting is a fascinating activity which can take varied forms. From clay modeling to making simple one-part molds or the more complicated approach of two-part molds. In fact, mold makers even make multi-part molds at times to capture all facets and other details of the model. The procedure of making molds also varies from pouring the mold making material to spraying it or even brushing it on the model.

Apart from the regular blanket, glove and even shell type, another making and casting technique is injection molding. It is actually a simple procedure in which the casting material is forced into this under pressure!

Injection molding can be done in single or multi-part molds. Since it is favored for capturing minute details, generally complicated two-part molds feature in injection molding. Tiny holes (called sprues) are bored into the mold all along its surface. This is where the casting material will be injected into this.

The casting material like polyurethane resin is filled in a syringe and the large hypodermic needle is inserted into the mold sprues. Pressure is applied to ensure all cavities are filled. The procedure continues until the casting material spews out of all the sprues. This shows that the casting material has displaced the air inside the mold and adequately filled all the nooks and crannies in the mold.

It is allowed to cure before being demolded carefully. You will find a thin-bodied cast that is an exact replica of the original model, down to every tiny nook and crevice. Other casting methods fail to achieve such a high level of precision. The demolded piece does not need to be cleaned or finished much either. However, models with large undercuts cannot be replicated with injection molding.

Injection molding is a commonly-used manufacturing process for producing thin-walled parts with great accuracy. It is generally used to make small plastic, metal and even glass items. Model parts for miniature cars, trains and complicated superhero figures are made with the injection molding process. It will yield accurate copies very quickly and in large numbers too.

The manufacturing process involves feeding the casting material into a machine which crushed it to a liquid state before injecting the same into the mold under high pressure. This mechanical process is repeated again and again to make multiple copies. The plastic in the mold is allowed to cool before being ejected from the mold.

While injection molding is a simple process that proves cost-effective, making the molds can turn out very expensive. It has to be made with steel or aluminum and precision machined to capture all the details properly. However, when artists use injection molding technique in their studios, they make use of clay modeling techniques and silicone rubber to make a precise block mold. This serves well for injection molding!

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