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14. Molding and casting

Group assignment:

  • Review the safety data sheets for each of your molding and casting materials
  • Make and compare test casts with each of them
  • Compare printing vs milling molds

Group Work

The chemicals available for molding can be quite varied, and many of the could be toxic. Therefore, it is important to read and understand the material data safety sheets associated with them. These should be read and understood prior to working with the chemicals, and particularly important to note any safety precautions that need to be taken.

Two part silicon rubber

Our lab has a supply of silicon rubber from the Zhermack company. Particularly, we have ZA 13 MOULD WT 45, which is noted as RTV2 Silicone Rubber for industrial use. For details on this product, we can look at the data sheet.

This is a two component silicone which is mixed in a 1:1 ratio (by weight) to catalyze a reaction to make the silicone rubber. According to the documentation, it has the following features: low viscosity, high mechanical resistance, dimensional stability, precision in reproduction.

It is noted to be useful in a wide variety of application, specifically noting:

  • Concrete moulding (good compatibility with concrete formulas)
  • Podiatry (low viscosity, high mechanical resistance, dimensional stability, precision inreproduction)
  • Artificial Stones (good compatibility with concrete formulas)

Usage

For use the two bi-component products should be shake before use in order to homogenize each component prior to mixing. Weigh equal amountsof catalyst and base. According to the data sheet, this can tolerate 5% variance in the amounts. Once the product is weighted in equal quantities, the base and catalyst are mixed in a container with a paint stirrer (or similar disposable tool). Mix until the blend is homogeneous. Once the product is thoroughly mixed, it is ready to be casted and the data sheet recommends pouring the silicone from a 30 cm height into the recipient mold.

The working time after mixing is 45 minutes, and the data sheet gives a curing time of 8 hours.

Data safety sheets

Since this is a two part mixture of base and catalyst, each will have separate chemical properties and each has material data safety sheet.

Data safety sheets:

ZA13 Catalyst ZA13 Base

The primary notes from these sheets recommend the following:

  • Wear protective goggles
  • Wear protective clothing
  • Work with gloves
  • Respiratory protection is recommended when ventilation is poor

Consequently, when working with these materials in the lab these steps are taken. The room we use for this is under negative pressure, and consequently has very good ventilation. The compounds are not marked as dangerous, and following the simple safety steps noted in the safety sheets is straightforward. With these steps, it is a good molding compound to work with in the lab.

Jesmonite AC100

What Is Jesmonite?

Jesmonite is a versatile composite material originally developed in the 1980s as an environmentally friendly alternative to traditional resin-based products. It consists of a mineral base (typically gypsum or calcium sulphate) and a water-based acrylic resin. When mixed together, the result is a material that sets quickly, is durable, and can mimic many surfaces like stone, metal, or ceramic.

There are several types of Jesmonite, such as:

  • AC100 – Commonly used for casting and molding.

What Can Jesmonite Be Used For?

Jesmonite is extremely popular among artists, designers, and architects for a wide range of creative and practical applications, including:

  • Casting decorative objects: Such as vases, coasters, planters, and sculptures.
  • Architectural elements: Like columns, tiles, panels, and facades.
  • Prototyping and model making: For industrial design or theatrical props.
  • Surface finishes: Imitating stone, metal, wood, or other textures.
  • Interior design: Countertops, furniture inlays, and ornaments.

Its appeal comes from its low toxicity, versatility, lightweight nature, and environmental friendliness compared to traditional resins.

Dangers and Safety Concerns

Although Jesmonite is considered safer than many other casting materials, it still comes with precautions:

1. Dust Inhalation (Dry Powder)

  • The base powder (before mixing) can be harmful if inhaled.
  • Always wear a dust mask or respirator when handling the powder.
  • Use in a well-ventilated area or under a fume hood.

2. Skin Contact

  • Jesmonite is water-based and less harmful than solvents, but prolonged skin contact may cause irritation.
  • Wearing gloves is recommended when mixing or pouring.

3. Mold Safety

  • Some molds may release fumes, especially if heated or made from certain materials.
  • Ensure good ventilation when using silicone or plastic molds.

4. Mixing Hazards

We made some examble test from jesmonite A100 and Asocret M30 cement.
This was the result As seen on the pictures we put some jesmonite and cement in thin layers in to some aluminium holders, the jesmonnite just got stuck in the holder but the cement just went in parts when try to take it out. We also testet silicon in the aliminium form , the silicon was easy to get out of the form fast and in one piece

Comparison of printing vs. milling molds

In discussion, we were encouraged and asked to make our molding process not show evidence of the method used in molding. We were particularly discouraged from using 3D printing in making a mold. In cases where 3D printing might be used, post-processing of the print would be necessary to hide the evidence from 3D printing. FFM 3D printing typically have evident layer lines, and those lines can be propagated to molds derived from this print.

In contrast, with appropriate bits and milling protocols, there can be little evidence of the specific process,

In Albert’s molding work he made a mold based on milling, as well as one based on 3D printing.

In the milling process, he made a printing stamp in silicon. The mold was cut from wax on a CNC (Shopbot). There were a smooth mold, and there was no evidence of the process used to make the mold.

For the 3D printing based method, he made 3d prints which represented the positive shape to make bars of soap (in halves). Subsequently, a silicon mold was made in 2 parts. This was then the mold to make a bar of soap.

In the 3D print, there were clear layer lines evident, which can also be seen in the silicon mold. Examined very closely, some very faint layer lines could be seen in the bars of soap. One did need to look very closely to see these lines, and they were not particularly evident.

For the specific use case of 3d printing to eventually make a bar of soap (via molding), there didn’t appear to be any strong need to post-process the 3d print. The process itself largely removed the layer lines. Perhaps the process of making soap is more forgiving towards the appearance of layer lines. Both the silicon and soap have some softness, and that might be forgiving.

In contrast, it is anticipated that harder materials in molding might be less forgiving towards layer lines. That was not tested. If one does not want layer lines in molding based on underlying 3d printing, one should be careful and perhaps need to post-process the 3d print.