How Long Is An Open Container Of Formula Good For How to Shrink or Enlarge a Casting – Part 1 of 3

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

Introduction

This is a three-part article on how to increase (also called pointing up) and how to reduce the size of castings without using laser scanning, CAD/CAM and computer-controlled CNC equipment. Both procedures are much simpler than you might think and avoid the expense and complexity of high-tech equipment. This knowledge comes in handy for many mold making and casting projects as well as artistic commissions, saving time and effort in converting a model to the required size.

Two ways to enlarge the casting

There are two ways to enlarge the casting. The first is to enlarge the mold before creating the casting. While another method is to enlarge the casting after the mold is made. Both methods of enlarging produce a properly made cast in the correct proportion and detail as the original. Each method depends on a different material. The material used to increase the form is known as addition hardened silicone. A specially made material is used to enlarge the casting polyurethane rubber.

Method One – Increase the form

Let’s start with instructions on how to increase it mould to increase the casting. The first method of enlarging the cast is approached in the same way as when making a repeat size cast, which is to create a regular silicone mold. In this case, the silicone material of choice is extra-curing (platinum-catalyzed) silicone, as condensation-curing silicone will not work. Once the form is created, the next step is to enlarge it. Mold is targeted instead of pain, as the enlarging process involves saturating the rubber with a solvent. Saturating the casting would make it uncomfortable to work with and difficult to remold. Therefore, it is more effective to increase the form instead.

Before you begin, follow the standard mold making protocol. First, the rubber should be degassed using a vacuum chamber, if it is not available, a pressure vessel is suitable. If you don’t have either of these basic mold making tools, make sure you vibrate the mold while it cures to introduce 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 sealable container, such as a 5-gallon bucket. Then add a solvent such as toluene, xylene or petroleum to the container.

We recommend extreme caution when handling volatile solvents, as all recommended solvents are flammable. So you don’t want to come into contact with an open flame or a heat source like an electric heater. Additionally, the fumes are extremely toxic, so you must have good ventilation and wear an approved NIOS mask when pouring these solvents.

Once the mold is completely submerged, the silicone rubber will begin to absorb it. Over time (weeks), you can expect the size to almost double in all its details in proportion to the original model. This means that if the mold is completely covered with solvent, the mold will deform. Patience is required when resizing the mold, for example a mold soaked in toluene takes seven days to expand to approximately 130% of its original size. Larger sizes require more time in the solvent. Solvent-blown molds are very fragile and must be handled with more care than usual to avoid tearing or damage.

When you are satisfied with the size or after three weeks the mold has stopped swelling, it should be removed from the solvent bath and the surface thoroughly dried. These are volatile liquids that immediately evaporate on contact with air. As evaporation continues, the mold begins to shrink. So you need to be quick when creating your casting. Once the surface is dry, a good casting option is two-part polyurethane resin. Although silicone does not usually require a mold release, we still recommend using a silicone-based release because it is much easier than a mold.

Now that the new cast is created, you can repeat the entire process of enlarging the silicone mold using solvent immersion to further increase the size of the final cast.

This concludes the first part How to shrink or enlarge a casting. The second part provides details on how to enlarge the cast yourself. The third part contains details about pain contraction.

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