< IMPRESSION MOULDS
POUR (DROP & BLOCK) MOULDS
Pour moulds, sometimes also called DROP or BLOCK moulds, are somewhat more sophisticated than impression moulds – but still fairly basic and easily constructed. This type of mould is well suited to the moulding of low bas reliefs, and simple small scale volumetric forms, all of which can be fixed down or secured to a flat working surface.
When pour moulding, the pattern form is secured and sealed to a flat work surface, then surrounded by either a clay wall, or a secure shuttering. A gap is allowed between the pattern’s edges and the shuttering wall, the air gap enabling moulding compound to later run around the sides of the pattern. If a mould case is to be added, the clay wall or shuttering may taper slightly towards the top, this puts a ‘draught’ clearance on the moulding compound and allows for the easy insertion and removal from it’s case when later used. A rubber or other moulding compound can then be prepared and poured into the shuttering, running over the master pattern until the design is fully covered.
When using any fluid moulding medium, care must be taken in to avoid trapped air pockets. The standard technique to minimise trapped air in a fluid rubber (or PVC plastic), is to pour the medium from a container held at some height above the mould thus allowing a thin extended stream to run over the enclosed pattern. Whilst this is somewhat more laborious than rapidly decanting the contents of a bucket into a mould, this extended pouring technique tends to help disperse any air bubbles that may have become trapped during mixing. Excluding air bubbles at this stage is time well spent, especially when compared to the alternative of retouching multiple faults in each every copy taken from an inferior mould. A vacuum degasser may be used to evacuate air from mixed RTV compounds, likewise, proprietary sprays that disperse air bubbles can also be applied to the master pattern’s surface before pouring, though this last measure is no real substitute for initial care.
Once the poured moulding medium has set and cured, the restraining clay wall or shuttering can be stripped down and removed. If needs be, a rigid case can be constructed around the mould to replace the shuttering and minimise distortion of the rubber, though in practice most small pour moulds are adequately self supporting.
Mould support cases when used, are usually formed in plaster of Paris, reinforced internally by a woven scrim bandage laminate. Alternatively, support cases can also be made from lightweight resins or resin plasters and similar materials. Plaster for the case can either be poured into a retaining clay wall built up around the rubber, or else applied directly by hand until a suitable case thickness is achieved. Once the case has hardened, it can be disengaged from the underlying rubber mould, which in turn is stripped away from the moulded master pattern.
If the drop moulded master pattern a simple 3D form (rather than a flat based relief design), the resulting ‘block’ will probably require cutting to release both the enclosed master pattern and later reproduction copies. Any cutting of RTV rubbers and PVC plastic is normally done with the aid of a sharp craft knife, using a technique similar to that used to release master patterns from vulcanised rubber dies. Cutting a ‘zig-zag’ line into the rubber block down to the enclosed pattern helps register the two halves of the mould. Cutting towards the top of the rubber, but not all the way through, enables the mould to hinge like a bi-valve shellfish, each half can be peeled back to remove the underlying master pattern, and later, the wax reproduction.
Because this type of mould has a restraining clay wall or shuttering, highly mobile moulding media, such as GELATINE, PVC, and low viscisity RTV rubbers are suitable for use when pour moulding.
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