Glossary Common terminology used in mouldmaking, casting, and the material world...

↑ Alginate (for lifecasting) -

a non-toxic non-allergenic moulding material which is safe to use on all areas of the body. It is a powder that you mix with water, this quickly sets into a firm gel. The body part is then removed and plaster is poured in to produce a positive cast. Although quite firm, the alginate can be easily torn off to reveal the finished cast

Alginate is a temporary mouldmaking material - as in, it dries out / loses it's water content over time, resulting in shrinkage and increasingly inaccurate casts. This means it must be cast into as quickly as possible after it's initially set, to ensure the cast is as close to the original body part as possible. It is also quite brittle and will tear when pulled at, so it is often destroyed during the demould of the first cast and cannot be cast into a second time

(Dental grade) alginate is best known for being the original material used to take impressions of teeth - although fast-setting, 2-part silicone systems are now the preferred product for that sector

↑ Block mould -

a mould made from a block of solid mouldmaking material - it is self-supporting and does not need a supportive shell/jacket. It can consist of 1 or multiple parts for ease of demould

↑ Cast -

Noun: the final object that is made from the casting material that was cast from the mould

Verb: to put a casting material into a mould, waiting until it cures/hardens and demoulding the result. 'Cast' is the correct word for all tenses, 'casting' can be used for present and future - 'casted' is not a word

Pictured above is a cast from an ice cube mould, water was the casting material and it hardened through the process of freezing

↑ Core -

a section of a mould which captures the internal shape of a part being cast. Example: a mould for an sfx cowl (over the head prosthetic) is made up of an outside mould section capturing the sculpted design and a core that is in the shape of the wearer's head. The casting material is then injected between these 2 sections to create a hollow cast of intentional varying thickness

↑ Curing -

a chemical process employed in polymer chemistry and process engineering that produces the toughening or hardening of a polymer material by cross-linking of polymer chains. There are many different catalysts that begin this reaction for each polymer.

The curing process is often mistook for 'drying' as that is the process the general public are more familiar with when it comes to witnessing substances harden

↑ Date stamp/mark -

a feature incorporated into injection moulds that records what date the part was manufactured. This is created using a 'mold date insert' that are inserted into the mould during the casting process. These sit flush with the insert of the mould and resemble a small dial/clock with an arrow(s) that indicate to the month/year or both. There are many other different types of inserts that provide further information about the part, such as material type and recycling symbols. See video below:

Further info

↑ Demould -

Verb: to remove an original shape or cast from a mould. Considering how an item will be demoulded is vital when designing a successful mould and, when working with more brittle casting materials, will help ensure your casts do not break when you try to take them out

↑ Displacement mould -

a 2-part mould that captures the outside of a shape (pt.1) , and the inside (pt.2). The shape of the object being cast is such that the casting material can be poured into pt.1 of the mould - and as pt.2 is lowered down to close the mould, the excess casting material is pushed upwards / displaced out the cavity by pt.2 (think of pushing a smaller bowl into a larger bowl that has water in it).

↑ Injection moulding -

the process in which molten material is injected into metal moulds. The material then cools and the part ejected. This is the fastest and most utilised process in product manufacturing

↑ Lifecast / lifecasting-

Verb: 'Lifecasting' taking an impression from a living being (often with alginate or lifecasting silicone), and casting a positive from that impression. The cast is often produced in plaster, either for the final outcome, or initially as part of a longer process to produce resin copies. 'Lifecast' is the correct word for all tenses

Noun: describes the final cast result from the lifecast process or a 'lifecasting session'

Above is a photo of Alternative Materials technician Emily Quinn at 19 having a lifecast done of her head whilst on work experience at Millennium FX

↑ Master -

the original object that the mould is created from. This is can be made from a wide array of materials. Its important that the master object has the same surface finish/texture as what is desired for the final casts, as mould making materials tend to pick up every tiny imperfection from the surface of the master. The only case where this is less important, is when multiple casts are not needed and the casting material is softer and easier to sand/finish than the material that the master was created with. E.g: someone has an object 3D-printed in PLA, they want to make a mould of it in silicone rubber and then cast 1 plaster version. PLA is very difficult to sand smooth (compared to plaser) so it is moulded 'as is' and then any visible print layer lines seen in the plaster cast can be sanded away easily

↑ Matrix mould -

any mould that has a thickness of mouldmaking material, but needs a supportive shell/jacket in order to keep it's intended shape and be cast into

↑ Mould -

the negative created from the master. Moulds can be made from a wide array of materials, but the most popular mould materials are: rubbers (silicone/polyurethane/latex), gypsum plaster, glass-reinforced epoxy or polyester, and metal.

In Alternative Materials, we most-commonly make moulds using the following materials (choice depends on the casting material and process):

• silicone rubber
• filament / FDM 3D prints
• resin / SLA 3D prints
• plaster
• grey styrofoam

↑ Parting lines -

the line or plane at which pieces of a multi-part mould meet and separate for the casting process. This line is a crucial aspect of mould design, as it can effect how the mould will be made and how easily the final casts can be demoulded from it

Parting lines are normally situated along the widest point of an object. Depending on the object being moulded, they can be a straight line along 1 flat plane, or an organic line where the angle of the plane changes slightly. Plane angles are chosen with the direction in which the mould parts will travel during demoulding in mind

You can often find evidence of where the mould parting line was when looking at plastic injection-molded parts because it is wear excess casting material (known as 'flash' or 'flashing') has flowed, either still remaining or signs it has been trimmed off. Sometimes when a mould is not correctly aligned, a step is noticable along the parting line and cannot be as easily rectified

↑ Rotocasting -

sometimes also referred to as 'rotational moulding/casting'. It is a method in which a casting material is put into a closed mould and rotated along all axis so the material coats the inside of the mould and cures to achieve a hollow cast with an even wall thickness

This casting process can be done by hand on a small scale with fast-curing casting materials, with slow-curing materials using a rotocasting machine, or on an industrial scale using very large scale machines that are designed to hold and rotate multile moulds at a time

Go to the 'Material and Manufacturing Videos' page to see the industrial process in action, or head to the 'Mouldmaking & Casting Tutorial Videos' page for help on creating your own rotationally-cast items

Short video of a rotocasting machine in motion, with a mould fixed in the center

↑ Undercut -

a section of an object being moulded which conflicts with the direction in which the object needs to be demoulded. Not considering undercuts during the mouldmaking process can result in 1) not being able to demould the original shape after moulding, 2) damaging the mould trying to get the master/subsequent casts out, 3) air becoming trapped in the undercut area and/or 4) breaking casts during demould

Small undercuts can be managed when the mould and/or casting material is flexible, but if rigid casts are being produced from rigid moulds (like in the plastic injection-molding process), then all unnecessary undercuts must be removed from a design and the mould must be properly designed to accommodate the undercuts that are necessary. These accommodations vary depending on the mould material but often involve splitting the mould into extra-pieces. The example image could be split along the right side to create a 2-part mould, where the larger piece is still removed upwards and the small piece is removed sideways. There is a section in this video that explain undercuts: