< POUR MOULDS
BRUSH OR HAND APPLIED BLANKET MOULDS
The blanket mould, as the name suggests, involves the even distribution of a moulding compound across a master pattern’s surface, to a thickness that enables the set and cured rubber skin to be removed without tearing. The process is rather more involved than that for pour moulding as initially the master pattern being blanket moulded is divided into convenient sections using clay walls. The division of a pattern in this manner enables more complicated three dimensional forms to be reproduced in a manageable way.
A simple blanket mould can be created by brushing layers of LATEX over a master pattern, then peeling away the fixed rubber impression once the latex has set, cured and supported with an outer [plaster] case. In most cases latex is not really suitable and the master pattern is treated with a release agent, divided up into two or more sections using clay walls, and a RTV rubber compound applied instead. The rubber compound is applied either by hand, or by brushing onto the pattern. Some provision can be made for the registration of adjoining mould sections by either making depressions into the clay dividing wall, and/or by cutting keys into the set rubber’s edges before the formation of the support case begins. An entrance hole into the mould also has to be established at some convenient point, this feature later enables a reproduction material such as wax to be introduced into the mould cavity.
Once SET and CURED, the rubber is supported by an outer plaster or resin CASE which is built up over the skin. Some means of locating and retaining the rubber in it’s case must also be added if the existing registration points do not make this possible – some mouldmakers use pre-formed rubber keys, which they place on the rubber blanket’s surface whilst it is still tacky.
Each section of the divided master pattern has it’s own rubber blanket and plaster support case completed before moving on to the next adjoining section. This allows for a continuous registration of all the mould parts as the work progresses. The mouldmaker applies a release agent to each exposed rubber surface and plaster face that abutts an adjacent mould section – this prevents each moulded section from sticking to it’s neighbour. Once one side of the pattern has been moulded and the plaster case(s) applied, the whole assembly can be turned over and the process repeated on the other side of the ‘parting line’.
Care is taken to ensure that the rigid case which supports the rubber does not become LOCKED IN to any pattern undercut that has not been removed by the overlaid moulding compound. Large undercut depressions can be filled with a foam padding, this material is sandwiched between rubber layers and avoids excessive use of a potentially expensive moulding medium. Alternatively, mould cases can be split to allow easy removal from a heavily undercut feature, this latter solution is somewhat reminiscent of the rigid piece part moulding technique described above. Each section of the plaster case is designed to accept a temporary clamp, this is often either in the form of a strong ‘elastic band’ (often cut from a rubber tyre inner tube); or else a ‘U’ shaped metal clamp known as a DOG. Either or both of these devices can be used hold an entire mould case together during use.
The BRUSHED ON moulding technique is especially useful when working with master patterns under difficult conditions, for instance if the pattern is fixed in a permanent position, or if it is fragile and unsuitable for manipulation into a convenient position for restrained case moulding. Blanket moulding allows for a THIXOTROPIC thickened RTV compound to be applied directly to a pattern which has to be worked on in a vertical or even overhead position, without first constructing a restraining case to prevent the compound from slumping off.
A brush/hand applied blanket mould can usually be constructed both quickly and at reasonable cost – though any advantage gained here is usually dependent upon the grade of moulding medium used and the length of time that the mouldmaker might have to spend controlling any slumping. The quality of the finished mould is also very much reliant upon the care taken in applying the rubber, especially in minimising trapped air pockets.
Where circumstances permit, the detail and quality of a brushed or hand applied blanket mould can be significantly enhanced by first applying a very fluid layer of the moulding compound to the pattern’s surface. While still tacky to touch, this first layer can be backed up with a thicker thixotropic layer, building up to an adequate skin thickness of rubber.
RESTRAINED CASE MOULDS >
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