Not trusting your eyes is a good step towards Model accuracy. Always prove by measurement what you see. The diagram shows a good layout for checking particular points on the model for symmetry. This is important for the control of firing distortion as an unbalanced model will distort in the production process. There are of course other factors but for now I will concentrate on Modelling accuracy. First the table whether it be Plaster, Granite or Glass is drawn out. A suitable size is 1.25 x 1.75 m with borders drawn 12-15 cm from the edge at 90 degree and parallel is very important. The Blue bars are rulers or graduation scales, these are secured approximately 1 cm outside of the drawn borders again at 90 degree and parallel. The model or skin Cast (produced from mould or block) can then be placed on the drawn centre line of the table and the centre line of the model or skin cast.
.If it is possible to purchase a Digital height gauge and a Laser distance meter is accurately attached then several measurement can be taken at one time from the vertical axis.
Level or Height check
It is suggested that to accurately measure the height or level of a model the drawing attached should be followed. If the model is checked by point h/h then a reading can be obtained, if however the measurement is taken from point g/g do not be surprised there is a difference. Taking the reading from the furthest point from the centre line is a more accurate method Using the Ruler or Graduation scales equal distance from the W centre line the accuracy of any modelled in distortion can be checked for accuracy on any part of the rim. Use a Set or “L” square to check vertical position from the Ruler or graduation scale to the check gauge coloured Brown in the drawing. With the above suggestions, it is not necessary to draw Grid lines around the Model The next presentation will cover the use of plaster bats for contour and solid cast section accuracy. The fact that wet plaster distorts easily can be used to produce accurate clay wall thickness and very good quality mould seams.
Modelling, Blocking and Casing
- The Plaster mould is an integral part of clay-ware production. Its quality and condition play a vital role in achieving consistent clay-ware quality.
- Resin made cases have had a considerable influence on improved quality however there are greater gains if certain basic practices are complied with.
- Following are two sketches of a top view of a new mould (first cast) after the top core has been removed. If your product is as sketch one then all is well for the initial stage of quality moulds. If however your product resembles the second sketch then some process procedure has been overlooked.
- The Block or Mother mould should be as sketch one, resin cases will replicate well the initial quality of the Block or Mother mould but do not expect they will produce sketch one results from a sketch two situation.
- If the slip (clay) is allowed to enter the seam or joint area the this becomes a casting surface. Worse still the area wears at twice the rate of the main face of the mould. Leading to excessive seam smoothing as the mould ages. The staff have no control of the downward quality spiral, they just follow instruction “keep your moulds clean” In fact this adds to the mould quality decline as the thicker clay residue has to be removed somehow! In extreme cases and often well before the end of recognised mould life expectancy of 80 to 100 casts moulds are incapable of producing a quality clay-ware piece.