Glossary of Basic Molding and Casting

October 27, 2016

If you are new to molding making, the terms can be confusing. I’ve compiled this basic glossary of commonly used molding and casting terms and definitions. This is by no means ALL the molding making terminology, but a helpful list of the most common terms. If you have questions about molding and casting, terminology or otherwise, you can always give us a call, toll-free, at AeroMarine Products, 877-342-8860. We’re here Monday through Friday 9am to 4pm Pacific time, and we’re always happy to answer questions.

  • Air vent – A thin outlet, generally designed as part of the mold, but can also be later, to prevent pockets of trapped air.
  • Ambient – Relating to your immediate surroundings.  Used to refer to the ambient temperature or ambient humidity.
  • Block mold – A mold in a ‘block” shape. The molding material is poured around an original part which is anchored inside a box.
  • Cast/part – the end result of pouring resin into your mold. The copy of the original piece.
  • Cure inhibition – Failure of a material to cure properly. Generally this is caused by contamination, like using a sulfur containing clay with a tin-based catalyst silicone.
  • Cure time – The amount of time it takes a material to cure or harden to usefulness.
  • De-mold time – The amount of time it takes for the mold to be cured enough to be removed from the mold box or the amount of time it takes the resin in the mold to cure enough to be removed from the mold.
  • Degassing – To remove gas, in this case air bubbles, from a material. In mold making, this is achieved by using a vacuum chamber.
  • Durometer/Shore Hardness – The resistance of a cured material to indentation. Most commonly measured on the Shore Scale. Shore A materials are considered flexible and Shore D materials are considered rigid. The higher the number, the harder the material is. For instance, Shore A 10 silicone rubber is similar to a gummy bear, while Shore A 40 silicone is more like a pencil eraser. Shore D casting resin is like a hard hat.
  • Elongation – Recorded at the moment a product breaks from being pulled apart. It is usually stated as a percentage.
  • Flash – Excess casting material that runs over the opening of a mold or any excess casting material that seeps between where two parts of a mold meet. The flash can easily be trimmed away after the casting material has cured.
  • Glove mold – A mold that is brushed on to the original part, creating a thinner, floppy mold that can easily be pulled of the part. Because it is floppy, it needs extra support, usually from a mother mold, when casting. A glove mold can be made from brushable silicone or brushable latex.
  • Mold – The negative form made to reproduce objects.
  • Mold box – A container, not necessarily a square box shape, the holds an original for silicone or other molding material to be poured around it.
  • Mold release – a substance used to release one material from another. Some mold releases are particular to the material being released, like urethane mold release. Others are more general, like using petroleum jelly as a silicone to silicone mold release and also in a silicone mold to release the part being cast.
  • Mother mold – A rigid outer backing made to support a glove mold. Plaster and fiberglass are common mother mold materials.
  • Original/piece or part – The object that you are creating a mold of in order to reproduce it.
  • Parting line – The place where two halves of a mold meet. Parting lines are picked to facilitate easy removal of a part from a mold, usually where an undercut is located.
  • Pot life – The amount of time it takes for a material to double it’s viscosity.
  • Registration marks- Also called keys, these are indentations made in one side of a two part mold. When the other other side of the mold is poured, the mold material will flow into these indentations, creating a sort of placement lock that ensures the two parts of the mold will be held together properly for casting.
  • Room Temperature Vulcanization (RTV) – A two part rubber that requires no special treatment to cure, other than mixing the components together.
  • Rotational casting – A casting process that requires the mold to be constantly rotated while the casting material cures. It results in a hollow cast piece. Also called roto casting, spin casting, and slush casting.
  • SDS – Safety Data Sheet, information on the ingredients of a material, what it is used for, how to store it, how to handle spills and disposal, what safety equipment and first aid is needed when working with the material and other information about the material.
  • Sealant/Sealing agent – A material that is used to coat your original piece/part so that the mold making material does not come into contact with a material in may bond to or may inhibit curing.
  • Shrinkage – The change in dimension between the liquid product used and the finished cured piece/part.
  • Sprue – A funnel shaped opening in the mold where the casting material is poured. Any excess resin that cures in that area is also called a sprue and can be trimmed off after curing.
  • Tear strength/Tear resistance- A materials resistance to tearing. Usually expressed in pounds per inch.
  • TDS – Technical Data Sheet, the information sheet(s) that comes with a product/material. It contains mix ratios, cure time, pot life and other important information you should read before using the product/material.
  • Tensile strength – The force, measured in pounds per square inch, needed to stretch a material before it breaks.
  • Undercut – Any over hang, protrusion or indentation on a part/piece.
  • Viscosity – The measure of an uncured materials resistance to flow, measured in centipoise. Water’s centipoise is 1. The higher the viscosity, the thicker the substance.
  • Work life – The amount of time before the product becomes too difficult to work with. Used interchangeably with pot life, even though work life doesn’t have a defined value of viscosity.

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