Monday, September 3, 2012

05 Molding Design Nº 4

Principles of Mold Design

Plaster is a rigid molding material. The molds you make with plaster are not flexible, and cannot be flexed off the model being molded. There are some important things to know when designing plaster molds, and they all have to do with the fact that plaster is a rigid mold material.

Most doll parts will be cast in molds that have two or more pieces. However, to begin with, take a look at an open-face, one-piece plaster mold. One piece plaster molds may be used as press molds because the model is easy to remove. In all one piece plaster molds, the model must have draft.

If there exist any undercuts in a model that is molded in a one piece plaster mold, then withdrawal of the model results in damage to the model, the mold, or both.

If there are undercuts in the model being molded in plaster, a one-piece mold cannot be made, unless it is a waste mold. A waste mold is destroyed when removing it from the model.

To make a plaster mold of a model with undercuts, a mold with more than one piece must be made. The pieces of the mold are designed to change an undercut into draft.

A mold cannot embrace the model, or the model will be locked in the mold.

Note that the pouring hole in the above diagram of the sphere has the proper draft for removal from the plaster mold. For a sphere, the parting line is always at the diameter.

In the next series of diagrams, parallel arrows show unimpeded withdrawal in opposite directions.

1. The parting line of a sphere is at the diameter.
2. The heavy arrows show undercuts if the parting line stays the same.
3. Changing the parting line allows withdrawal from the undercuts.
4. The parting line is not always a straight line.
5. Setting up a bust for a two-piece mold.

A model must be studied for six major withdrawal sections.

It would be very convenient if the parting lines of a multi-piece mold always fell along the most prominent features of a model, but that isn't always the case. Each situation is different and only the general principles of draft, undercuts, and angles of withdrawal can be relied upon to guide the making of the mold. Always try to keep the number of sections to a minimum. Start with simple models and make some actual molds. It will become clearer as empirical experience is gained.


In molds with two or more pieces, it is necessary to register the various pieces of the mold with each other, so the finished casting will not have any offsets. On a two-piece mold, the first half of the mold is made, then the registration keys are carved into that half. When the second half of the mold is made, the plaster fills the carved areas to create keys. There should be at least two keys made to keep the two halves of the mold from shifting.

The Spare

The spare is a hole used for pouring wax or slip into the plaster mold. In a two piece mold, where pouring hole is on the parting line, the spare can be funnel shaped. Where the spare is not on a parting line, the top of the spare must be smaller than the bottom, to allow for draft.

This photo is an example of two simple molds with one parting line, no undercuts, and having plenty of draft. The angle of withdrawal, for both molds is straight out. The mold on the left is a plaster press mold for making simple basic face masks. The mold on the right is a plastic measuring spoon in the shape of a hemisphere.

This side view of the press-molded face mask shows that there are no undercuts, and that the face may be lifted straight out from the plaster mold.

This is the top view and bottom view of the simple face, showing that there are no undercuts, and ample draft.

If a nostril cavity and a nostril wing were modeled in the original oil-clay face, these would have created undercuts, and the face would not have lifted straight out from the plaster mold.

This photo shows that if the two hemispheres  were joined to make a complete sphere, the parting line would then be in the middle of the sphere. In order to mold a sphere, a two-piece mold would have to be made.

Plaster Mold And Model Making.
Charles Chaney and Stanley Skee.
NY: Van Nostrand Reinhold Co., 1973.
ISBN: 0442215118

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