Mold Making Terms and Definitions

This is an excerpt from our free mold making and casting ebook available on our website. 

CASTING MATERIAL A casting material is a material that will take on and keep the shape that a mold gives.

COMPOSIMOLD ComposiMold is a reusable mold making material that works by melting and pouring. Many casts can be performed using the same mold. ComposiMold is good for the mold maker and craft artist because it can be reused to make different molds without worrying about wasting mold making material (such as urethanes, silicones, or Alginate). ComposiMold is an excellent material for those wanting to learn and become involved in sculpting, modeling, and mold making without having to worry about mistakes or wasting material. ComposiMold is flexible, rubbery, molding material that can be melted poured and reused. This material is a thermoplastic mold making material that is great for small mold parts using many different casting materials including plaster, cement, epoxy, polyurethane, and others. ComposiMold can be used for molds, mold making, and as a rubber casting material.

DE-MOLD TIME The de-mold time is the time required to pass before the casted part can be removed from the mold.

EXOTHERM An exotherm is the heat of reaction that occurs when a thermoset resin or rubber cures. This is of concern with ComposiMold because higher exotherms can melt the ComposiMold. Larger sized pieces create more heat. The type of resin used also affects the amount of exotherm. Plaster and concrete have low exotherms. Polyurethane is also relatively low. Epoxy typically has a higher exotherm. Fillers can be used to help reduce the amount of exotherm.

FILLERS Fillers are added to resins to reduce cost or provide specific properties. Many fillers are suitable to adding to casting resins including plaster, flour, sand, metal powders, fibers, sawdust, or specialty fillers. Heavy fillers can absorb some of the heat of the exotherm, but this will result in heavier parts. Lighter fillers can make parts float. An important use of fillers is to reduce the cost of a part. For example, adding ½ the resin volume with sand will reduce the cost of a casting by almost half.

MASTER/MODEL/ORIGINAL PART These terms are used to describe a 3-dimensional object you use as a pattern so you have something to make a mold of. Typically, the 3-dimensional object is initially made by sculpting, carving, construction, or other method. Found objects make great master parts as well.

MOLD A mold is something that will give a certain shape to a casting material. A way to look at this is as the three dimensional negative of your master.

MOLD BOX A mold box is any container you use to hold your master part and the melted ComposiMold. Be sure it is made of a flexible material such as a plastic food container and that it can withstand the heat of the melted ComposiMold. You can also build a mold box out of folded tin-foil or line a shipping box with a trash bag for larger molds.

MOLD RELEASE/RELEASE AGENT is a substance that prevents one material from sticking to another. Potential Mold Releases are water, mineral oil, vegetable oil, PVA mold releases, and specially formulated mold releases.

PAINT-ON/BRUSH-ON MOLDS Paint-on molds are made by painting the molding material directly onto a model’s surface. The advantages of a paint-on mold are that it is possible to use less material and you can mold parts that you could not do with a typical molding process. An example where you may need to use a Paint-on mold is making a mold of something attached to a wall.

PUSH MOLDS Push Mold is a technique used when you push the casting material into the molds. For example, you can push polymer clay into the ComposiMold mold to make your shape.

IMPRESSIVE PUTTY is our newest, reusable, mold making material that can be pressed onto your master part. You can also press your master part into the ImPRESSive Putty. This material is great for making molds of objects that cannot be moved to a mold box, easy one and two part molds, and handling higher temperature casting materials. It can be continually remelted and reused.

SPRUE A sprue is the opening in the mold where you pour in the casting material. Sprues are also added into the mold in locations where air would be entrapped into the casting. The sprues provide a way for the entrapped air to escape.

THERMOPLASTICS Resins or plastic compounds which in their final state as finished articles are capable of being repeatedly softened by increased temperature and hardened by decrease of temperature by means of physical change.

THERMOSETTING PLASTICS (THERMOSETTING) Resins or plastic compounds that solidify into a plastic that cannot be re-melted. Thermosetting resins are often liquids at some stage in their manufacture or processing, which are cured by heat, catalysis or other chemical means. After being fully cured, thermosets cannot be re-softened by heat.

UNDERCUTS are areas on your master part with indentations or severe angles. These areas will determine the type of process you use to get the best results. You will also want to be sure that air bubbles don’t get trapped in these spots.