Making Molds For Metal Casting At Home

Making Copies – Made Easy

Have you tried it? Building and pouring molds yourself is an exciting hobby that is a lot of fun. Depending on the nature of the original, the wealth of details , processing times and options, there are different molding and pouring compounds .

No matter what you want to make an impression: baby feet, holiday memories, decorative parts for restoration or fossilization … DAWANGCASTING recommends the right material for the best result!

The Material

in the ‘crosslinked i.e. solidified state, is a silicone rubber elastic, but quite dimensionally stable compound, which is ideally suited for molding decorative objects as well as technical parts. A cast object (model) is molded true to detail and without measurable shrinkage. However, the dimensional stability also has a certain disadvantage: if a rubber mold is overstretched, it breaks. So rubber is nowhere near as elastic as “rubber”. This property must be taken into account when creating / designing the shape.

The Types in the Selva Range

Silcolan NV

The  NV  rubber type is liquid, but not of a very thin consistency, but rather”creamy “. The basic color is white. The variety  NV(Low viscosity = liquid) is used for forms that are made of solid rubber. These will be smaller forms of everything for parts no larger than a fist. For larger cast parts you have to think about a different molding technique, mainly because of the otherwise excessive material requirement, because silicone rubber is not exactly cheap! Type  NV  can withstand temperatures of up to 190 ° C.

The silicone type “HB” (HB means “heat-resistant”) behaves in the same way as rubber type ” NV ” during processing, Is an idea less elastic, but can withstand temperatures of a good 400 ° C, but only for a short time, not as a permanent load. The basic color of light rust red comes from additives (iron oxide), which have the task of absorbing and dissipating heat energy.  HB  rubber is used to manufacture casting molds for low-melting metals (e.g. tin, lead or alloys thereof). A molten metal at around 400 ° C can therefore be poured directly into the rubber mold without damaging it.

Silicone Compound

A moldable 2-component silicone, in an elastic, soft, highly viscous and pliable, kneadable setting. It is characterized by its exact molding accuracy and fast curing (within 5 minutes). Silicone compound is used wherever a one-part or multi-part, ready-to-use mold has to be produced with a rapid impression material within a few minutes, without any release agent pretreatment and without a casting box. The finished form can be used for molding with all casting compounds.

The silicone compound is skin-neutral (pH value 5) and is also ideally suited for body impressions, which require an impression material that hardens quickly.

Silcoval MVE

Universal, condensation-curing RTV silicone rubber compound, pourable, with medium elasticity and high tear resistance.

Well suited for the production of complex one-part and multipart negative molds in which decorative candles, figures and precision parts with undercuts are to be molded quickly and safely.

Which Models are Suitable?

Compact, fully plastic figures of slim shape up to approx. 20 cm in height or compact, clumsy shapes up to approx. 1 liter volume are molded more easily (and cheaper) with latex, provided that the exact depiction and the observance of dimensions are not so important . Parts that cannot be removed from a one-piece form due to their shape and structure (e.g. a human figure with arms spread apart or legs apart) up to 15 cm in height or fist size, or technical parts that require dimensional accuracy and evenness of surfaces and the reproduction of fine surface structures is important, it makes sense to mold them in silicone rubber.

If you want to reproduce the objects with, for example, hard plaster or casting resins, you choose rubber NV.

If you want to metal casting the molded parts in tin, lead or their alloys, choose the rubber type  HB . It should be mentioned here that metal parts should not be too big or too heavy, because the thermal energy introduced with the liquid metal is absorbed by the rubber of the mold and has to be “digested”, so to speak.

A weight of approx. 1 kg of metal can be assumed as the limit value. The limit can be higher for flat and filigree parts, but lower for very compact bodies.

Preparing the Model

First, the decision must be made how the shape will be laid out. Reliefs with a flat back only need a one-piece shape.

If a flat object has two structural surfaces (such as a coin), a two-part mold must be made. Slight undercuts in the mold can withstand the elasticity of the rubber, but if the mechanical loads are too great when a casting is being formed, the rubber will break out. Depending on the shape of a body, it may therefore be necessary that the shape has to consist of more than two parts, that for example “inlays” have to be used.

The casting box is designed according to the size and shape of the model. Of course, care is taken not to use up material unnecessarily, and accordingly the casting box should be as small as possible or not larger than necessary. The distance between the model and the wall of the casting box should not be less than approx. 5 mm.

Here is Another Tip

If you press a recess in the investment material in 3 or 4 places when investing the model (e.g. with the back of a pencil), you will get a “register” that holds the two halves of the mold precisely in position.

Determine the Material Requirements

The amount of material required for a casting mold can only be determined using the volume. Often you can’t really measure the volume, but rather – with the help of the measuring stick – you have to estimate it. Another possibility is to “fill in liters” with a clean, crumbly filler, e.g. styrofoam granulate (or semolina or rice), which is poured into the prepared casting box and then poured into a measuring cup. The determined volume is then multiplied by the specific weight of the rubber, e.g. 440 cubic centimeters x 1.35 (for type ” NV “) = approx. 594 grams. From the determined rubber requirement of 594 grams, the amount of the required crosslinker, the second, is then calculated back Components that are added for processing: normal addition 2%, here 594 x 0.02 = 11.88 gCrosslinker . Since it doesn’t really matter, the factor of the specific weight of the cross linker can be neglected and, for the sake of simplicity, the calculated value in milliliters would be assumed, which would mean approx. 12 ml (ccm) crosslinker .