< SAND PATTERNS FOR COMMERCIAL FOUNDRIES
COMMECIAL PATTERN MATERIALS & CONSTRUCTION
The high volume production techniques used by the commercial founder rely in no small part upon the use of carefully designed master patterns. Few patterns sent to commercial foundries are made in plaster of Paris, instead wood, metal, resin and plastics are generally preferred, these materials more easily meet the robust standards expected of a sand pattern in continuous use.
One of the most distinctive features of the commercial pattern is it’s typically modular layout. The typical pattern design is broken down into a series of individual pattern elements, each one of which can be moulded independently. The resulting sand piece parts are reunited only when the flask is finally assembled, just prior to metal pouring. For improved cost efficiency, pattern layouts often include provision for multiple copies of each design to be impressed with each moulding operation. In this respect a commercial sand pattern can be compared to the SPRUE TREE mounted CLUSTER, which is used to create many copies of a single jewellery design in a single investment mould.
As well as containing the image of the design itself, the commercial sand pattern often includes elements of the metal RUNNING SYSTEM. Part or all metal running system is automatically impressed into the sand as the pattern is moulded, later these impressed channels direct the molten charge into the flask’s mould cavity.
As well as a running system, there may also be a pattern part for forming a CORE. This is a useful feature if the cast has to be hollow. The part of the pattern system used to form a core is called a CORE BOX. The core box’s design usually includes a journal extension or CORE PRINT, this feature locates the core precisely into a corresponding location point within the moulding flask.
Taken as a whole, modular pattern systems enable a refractory sand mould to be produced any number of times both quickly and accurately, with little or no deviation or degradation in either the pattern’s accuracy or cast quality. Large quantities of metal reproductions can be produced in short order by this method and the commercial founder is normally able calculate moulding times, materials usage and production costs with considerable accuracy.
To further refine the founding process, master patterns for commercial sand casting are normally designed to be slightly oversize, allowing for predictable METAL SHRINKAGE and dimensional CONTRACTIONS in the cast metal copy. Any undercuts found on commercially cast designs are either accommodated into the separate elements of the pattern, or else formed independently in a separate moulding box (similar to a core box), the additional part is later keyed into the flask at an appropriate point in the mould’s assembly. To ease withdrawal of the pattern from the flask, any vertical edges on the design are slightly tapered by some 5-15° off true, this feature is referred to as the DRAW or DRAFT. Any excess metal deposit left by a draw is machined off the cast during finishing.
To enable a foundry moulder to clearly identify the function and finish of each part of the master pattern, a system of pattern colour coding is used; core print boxes are painted black (or with yellow stripes on a black background, according to finish). On the design itself, surfaces to be left AS CAST are usually indicated by a red or orange marking, machined surfaces on the other hand are highlighted in yellow.
From all this we can see that criteria that determines a good commercial sand pattern, contrasts somewhat with the attributes required of a good art foundry pattern. Because of the fundamental differences in approach between the two operations, it cannot be recommended that fine art type patterns be submitted to commercial founders – even if the rates charged by a ‘jobbing founder’ may appear at first sight to be significantly lower than those charged by an art founder. Few commercial founders can easily accommodate the types of pattern normally supplied to fine art sand moulding workshops into their production systems. Even fewer have specialist the finishing skills available ‘in house’ to complete the work to art foundry specifications (conversely, few art founders have direct access to skilled machine operatives required for close tolerance engineering projects). These differences in approach do not completely preclude the possibility of using a commercial based sand process, but with these differences in mind it is strongly recommended that a commercial foundry only be considered by artists and designers under the following circumstances:
• If the overall design, quality, and quantity of reproduction strongly lends itself to a commercial sand casting process.
• If no detailed finishing work is required, or if finishing can be carried out later in the studio, or by an experienced CHASER.
• Where a suitable master pattern design and construction material/method is agreed to, by both the founder and the artist.
As far as supply of a suitable master pattern is concerned, advice should be sought directly from the founder concerned, who in turn will normally be able to provide details of a preferred PATTERNMAKER – an artisan skilled in preparing commercial pattern designs to the necessary specifications. If the services of a professional patternmaker are used, then it will be essential to supply the technician with a detailed drawing, or an accurate scale model of the design, from which they can work a pattern from.
Whilst some commercial founders will be willing to take on artist made patterns, retaining the services of a knowledgeable patternmaker will often be essential, especially if a large number or very accurate reproductions are being seriously contemplated. The expense of commissioning a professional patternmaker is usually set against the total quantity of reproductions envisaged, with the cost allowed for in the unit price of each cast sold. Despite the potential for difficulties in obtaining satisfactory fine art and design casts from commercial sand founders, careful research, planning and selective use of available processes can allow artists to obtain very good quality results at moderate cost.
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