TYPES OF PATTERNS

(i) Solid or single piece pattern: Such patterns are made in one piece and are suitable only for very simple castings. There is no provision for runners and risers etc. Moulding can be done either in the foundry floor (called pit moulding) or in a moulding box. There is no difficulty in withdrawing the pattern from the mould as the broadest portion of the pattern is at the top. As an example, if a cylindrical pin with a circular head has to be cast, a one piece pattern shown in Fig. 6.1 will be adequate.

(ii) Split pattern: It is not practical to have one piece pattern for parts of complicated shapes, because it would not be possible to withdraw the pattern from the mould. For example, if a circular head was added to the bottom of the pin shown in Fig. 6.1, it would make it necessary to go in for a split pattern as shown in Fig. 6.2.

One-half of the impression in the mould will be made by using piece no. 1 in one moulding box and the other half of the impression will be made by using piece no. 2 in a second moulding box. After withdrawing the pattern halves from the respective moulding boxes, the two boxes will be assembled and clamped together, so that the complete impression is available for pouring the metal.

The two pattern halves are provided with locating dowels, so that one-half may sit on the other half in the exact position required with no mismatch. Also two tapped holes are provided on the flat mating surface of each part. These tapped holes are used to provide a grip to lift the pattern halves from the sand without damaging the mould-impression.

The line along which the pattern is divided into halves is called “parting line” and it usually follows the broadest cross-section of the casting. Deciding where the parting line should be is a matter of considerable skill and experience.

Some of the more complicated castings may require pattern to be split in three or even more pieces.

(iii) Loose piece pattern: In some cases, the casting may have small projections or overhanging portions. These projections make it difficult to withdraw the pattern from the mould. Therefore these projections are made as loose pieces. They are loosely attached to the main part of the pattern and the mould is made in the usual way.

When the main pattern is withdrawn from the mould, the loose pieces slip off and remain behindin the mould. After removing the main body of the pattern, the loose pieces are taken out by first moving them laterally and then lifting them through the space vacated by the main pattern. The method is illustrated in Fig. 6.3.

(iv) Match plate pattern: Match plate is a metal plate, usually made of aluminium. The two halves of the split pattern are mounted on this match plate one on either side. While fixing them to the match plate, care is taken so that there is no mismatch. These patterns are used in conjunction with mechanically operated moulding machines. Bottom side of match plate pattern is used for making the bottom half of the mould impression in one moulding box (known as the drag). The upper side of the match plate pattern is used for making the mould impression in another moulding box. Finally, the two moulding boxes are kept on top of each other, the bottom box is known as the drag, whereas the top one is called the cope.

(v) Gated patterns: Sometimes alongwith the pattern for the casting, another portion is added so that when the impression is made in the moulding box, the cavity contains a shallow channel along with the main cavity for the object to be cast. This channel will be used for feeding molten metal into the main cavity and is known as the “gate”. Such patterns where provision for gating has been made are called gated patterns. It removes the necessity of making a gate separately.

(vi) Other pattern types include skeleton pattern, sweep pattern and segmental pattern etc. In these patterns, the full pattern is not made and the mould is completed with an improvised pattern. This is done to reduce the cost of pattern making. This procedure is resorted to, if only one or two moulds are to be made.