The following points highlight the fifteen main patterns used for casting of materials in an industry. The patterns are: 1. Single-Piece or Solid Pattern 2. Split Patterns 3. Loose Piece Pattern 4. Gated Pattern 5. Match Plate Pattern 6. Follow Board Patterns 7. Sweep Patterns 8. Segmental Pattern or Part Pattern 9. Skeleton Pattern 10. Shell Pattern 11. Built-Up Pattern 12. Boxed-Up Pattern 13. Lagged-Up Pattern and a Few More.
Type # 1. Single-Piece or Solid Pattern:
This type of pattern is made without joints, partings, or any loose pieces. Such a pattern is also called as a loose pattern since it is not attached to a plate. With these patterns, the moulder has to cut his own runners, feeding gates and risers. Thus moulding operation takes more time. Single-piece pattern is generally used for large casting of simple shape and for limited production.
Such a pattern is usually ‘flat-back’ type (Refer Fig. 3.2), which may have a few or no irregularities and no core print. The large horizontal cross-sectional area of such a pattern serves as the parting surface in the mould. The mould cavity of this pattern will be either entirely in the drag or entirely in the cope.
Type # 2. Split Patterns:
Many patterns can’t be made in a single piece because of the difficulties encountered in removing them from the mould. To eliminate this difficulty, some patterns are made in two parts, so that half of the pattern will rest in the lower part of the mould and half in the upper part. The split in the pattern occurs at the parting line of the mould.
The two parts of the pattern are aligned with dowel pins. Sometimes a pattern is constructed in three or more parts for complicated castings. Such a pattern is called ‘multi-piece pattern’. (Refer Fig. 3.5).
Type # 3. Loose Piece Pattern:
It is a pattern with loose pieces which are necessary to facilitate withdrawal of the pattern from the mould. It is used to produce undercuts. Loose pieces are removed separately through the cavity formed on main pattern, after the pattern has been removed. These loose pieces may have to be turned or moved before taking them out. These also need to be fastened loosely to the main pattern by wooden dowel pins. (Refer Fig. 3.6).
Type # 4. Gated Pattern:
In production work, where several castings are required, gated patterns are used. Such patterns are made of metal to give them strength and to eliminate any warping tendency. To save time, a number of castings are produced in a single multicavity mould by joining a group of patterns.
The gates or runners for the molten metal are incorporated in the integrated pattern. By this arrangement the time ordinarily spent by the moulder in cutting gates and drawing patterns is eliminated. These groups of patterns with gate formers attached to them are called gated patterns.
Type # 5. Match Plate Pattern:
Match plate pattern is made by fastening each half of a split pattern to the opposite sides of one plate. The plate provides a substantial mounting for patterns and is widely used in machine moulding.
The gates and runners are also attached in their correct positions. On the drag side of the plate, the plate is equipped with locator holes, which fit into the pins provided on the drag portion of the flask.
When the match plate is lifted off the mould, all patterns are drawn, and the cope or upper half of the mould matches perfectly with the drag or lower half of the mould. The gates and runners are also completed in the same operation. Match plate patterns are suited for mass production of small castings in moulding machines. Match plate patterns are expensive to construct, but the initial cost is justified in mass production.
Type # 6. Follow Board Patterns:
The patterns having thin sections, (Refer Fig. 3.8), tend to get distorted or collapse during ramming. Sagging of thin pattern due to ramming can be easily overcome by constructing a supporting block (follow board) which may fit inside the pattern to serve as a support during ramming.
Type # 7. Sweep Patterns:
Many patterns of symmetrical and regular shape, usually of large size, may be constructed by the use of sweep pattern (which has the shape corresponding to the shape of the desired casting) which sweeps the desired shape into the sand mould thus eliminating the need for costly three dimensional patterns. The sweep pattern is arranged to rotate about a central axis on a needle.
Type # 8. Segmental Pattern or Part Pattern:
Segmental patterns are sections of a pattern arranged in such a manner as to form a complete mould by placing the segmented patterns in the mould suitably. These are generally used for circular work, like rings, wheels, rims, gears, etc.
Type # 9. Skeleton Pattern:
For very large castings solid patterns would require a tremendous amount of timber, which may not be economical, particularly, if the castings required are less. In such cases the pattern is made of wooden frame and rib construction (skeleton) so that it will form a partially exterior or interior outline of the casting and provide the general contour and size of the desired casting.
Type # 10. Shell Pattern:
Shell pattern is usually mounted on a plate and parted along the centre line, the two halves being accurately doweled together. Usually, these short bends are moulded and cast in pairs. This type of pattern is largely used for drainage fittings and pipe work. (Refer Fig. 3.12).
Type # 11. Built-Up Pattern:
Built-up patterns are composed of two or more pieces. For special pulleys, patterns are built- up of segments of wooden strips. The segments are made by cutting strips of wood to the curvature required. The thickness desired is built-up by gluing them in layers. Flanges are also made similarly. Such patterns are used to make intricate shapes because it is easier to build-up the shape by gluing or joining number of segmental pieces together.
Type # 12. Boxed-Up Pattern:
It is a box-like pattern in which planks or strips of wood are joined together either by glue, nails, or screws. This method economises wood for large patterns and makes them lighter in weight.
Type # 13. Lagged-Up Pattern:
Cylindrical patterns e.g., barrels, pipes or columns are built up with lag or stave construction to ensure proper shape. Longitudinal strips of wood, called ‘lags’ or ‘staves’, are bevelled on each side and glued to the wooden end pieces called ‘heads’ as shown in Fig. 3.13.
Type # 14. Odd Shaped Pattern:
An odd side cope or false cope is used, when the shape of the casting does not produce a convenient flat split in the pattern, such as in a cranked lever. The pattern is solid and an odd side (or false cope) is used to assist in providing of a contoured split between cope and drag as shown in Fig. 3.14. The odd side is often made of plaster of Paris.
Type # 15. Skeleton Pattern:
It resembles from outside corresponding to the shape of the casting but otherwise is a simple wooden frame. It is used for large castings having simple geometrical shapes and its purpose is to guide the moulder for hand-shaping the mould.
In the design of such patterns, which make use of cope and drag, the large portion should be accommodated in the drag because the moulding sand has greater strength in compression than in tension. Further, since the loose sand defects are more frequent in cope, the critical surfaces should also be included in the drag.