Resin Workshop Notes

Resin is a (usually liquid) casting plastic which sets in an endothermic reaction to a catalyst. Use acetone to clean from tools or fingers is necessary. For safety… Hair tied up, work overalls on and goggles are essential. All Resin casting must be performed on a tray on the vent units. Catalyst is oxadising so will presumably combust if it comes into contact with water.

Polyester available in the uni stores, medium hardness/brittleness. 30-60 minutes set time. Comes in opaque, clear and gel coat (thicker for painting a layer).
Polyurethane – low exothermic reaction, least hard/brittle and can be very clear. Only comes in opaque.
Epoxy most expensive, heat resistant, chemically stable and toughest & clearest. 2-3 days setting time. Comes with a slight yellow tint but not visible in small layers or pieces.
Vinylester – what surf boards etc are typically made out of.
Acrylic – industrial process not available at uni. Very clear and used for embedding objects (think scorpion in a key chain) and for jewelry.
Bio Resin – Lower ecological impact, very expensive.
Jesmonite –
Water based resin, still exothermic comes as a powder and liquid combo. Very hard and brittle with a off white opaque colour.

Resins come in the following purities/functions, these notes are based on polyester resin;

Clear Cast Resins
Most sensitive, toxic and expensive. Clear cast resin will go yellow if too much catalyst is added as the heat will colour the plastic.

General Casting Resins
Good for adding materials too like metal powders (iron which looks black, aluminum which looks silver, copper which looks coppery and can be corroded or oxidized with patination for green tones. Add 1:1 ratio of resin to metal powder for a fake metal appearance and 2:1 resin/metal ratio for things like patination.). Basically any materials which don’t contain water like broken glass, ground stone or oil paints (tip: when mixing oil paint in, first mix it into a small quantity of resin then add the rest as mixing into a large quantity takes ages)

Gel Coat
Gelcoat is the same as General casting resin but thicker so that it can be applied with a brush. It is often used as a first step in fiberglass to product the smooth outer layer. It can also be used to produce hollow castings or to fill a detailed mould surface which otherwise might trap air. I


Rubber band the silicone mould together or for moulds of other materials spray the mould surface with a release agent like Macwax or polish the mould surface with Mirror Glaze to prevent the resin bonding with the mould surface. Catalyst is added 2% for just resin, 4% for resin mixed with oil paint and between 8-15% when mixed with things like sand which will have a cooling effect on the reaction so require more catalyst. Mix very thoroughly and pour or inject resin into the mould. Bubbles from mixing will find there way to the top while it sets but if you have a complex mould it might be with rotating it and giving it a few taps as it settles to help any which are caught on details.

If casting a thick object pour in layers of 15mm to prevent a strong exothermic reaction (resin is oil based and therefor a fuel so highly flammable) which can cause yellowing, cracking and eventually a chemical fire (use powder or CO2 fire extinguisher only). Ideally leave 30 minutes between layers so that the resin is not completely set and creates a primary bond rather than a secondary bond which may be visible. When layering clear cast drop the catalyst for the 2nd layer to 1% to prevent heat yellowing.


Depending on quantity of catalist and thickness of cast you can demould 20-60 mins after casting. Surface tackiness will remain (especially on clear cast resin) for several days, careful handling will prevent finger prints and maintain a shiny surface until it fully sets (up to a week). Washing a demoulded object with acetone then leaving it to dry for 2 days can help expediate hardening.


ClearCastIt is possible to saw, file, drill, use a rotary tool, sand and machine polish hard resin. Be careful to do any machine tooling in a downdraft vent unit as inhaled dust is dangerous. Any Hand tooling may be done in the workshop, it is good practice to hoover the dust up rather than sweep it to prevent inhalation.

Surface tackiness resulting in finger prints while demoulding is often found using polyester resin and can be dealt with firstly by careful handling and patience (a week seems enough for small casts) or by washing the freshly demoulded cast in acetone to expedite setting and waiting for several days. It can also be helpful to warm your mould before casting to help the surface set or spray a wax release agent into the mould to put a barrier between the silicone (which is often condensation curing so can contain moisture) and the cast. It is often advisable to gently oven bake a silicone mould for many hours to rid it of any moisture. Over heating the silicone will result in splits and deformation.




Fiber glass

Comes in many forms, most commonly at uni as CSM (chop strand material). If using fiber glass in casting always wear gloves.  If not laying onto silicone first spray a release agent to prevent bonding. Apply a layer of gel coat and wait 10 minutes for it to go slightly tacky. On a vinyl sheet brush General Casting Resin onto the CSM on both sides so that it goes translucent. Layer onto the gelcoat and push air out with gentle brushing. 2 or 3 layers will be more than enough for a strong structure. When using other woven fiberglass the strand direction can produce a slight flex in the perpendicular direction if desired.


Casting resource This guy has comprehensive tutorials and information about different materials and mould making/casting process.

Suppliers cheap supplier of casting & mould making & sculpting products.
www.smooth-on .com brand of casting & mould making products. Bristol based resin and moulding company.