Type of Patterns & constructions

Solid or single piece pattern: 

A single piece pattern is the simplest of all the patterns, is made in one piece and carries no joint, partition or loose pieces. Depending upon the shape, it can be moulded in one or two boxes. The pattern is the cheapest but its use can be done to a limited extent of production only since its moulding involves a large number of manual operations like gate cutting, providing runners and risers.

Two piece or split pattern: 

Many times the design of casting offers difficulty in mould making and withdrawal of pattern, if a solid pattern is used. For such castings, split or two piece pattern are employed. They are made in two parts which are joined at the parting line by means of dowels. While moulding one part of the pattern is contained by the drag and the other by the cope.

Multi-piece pattern: 

Castings having a more complicated design than above require the pattern in more than two parts in order to facilitate an easy moulding and withdrawal of pattern. Their pattern may consist of 3, 4 or more numbers of parts, depending on their design.

Match plate pattern: 

These patterns are used where rapid production of small and accurate castings is desired on a large scale. Their construction cost is quite high, but the same is easily compensated by a high rate production, greater dimensional accuracy and minimum requirement for matching in the casting, these patterns are made in two pieces, one piece is mounted on one side and the other on the other side of the plate called match plate. The plate may carry only one pattern, or a group of patterns mounted in the same way on its two sides. The plate may be of wood, steel, magnesium or aluminium. Gates and runners are also attached to the plate along with the pattern.

Gates patterns: 

They are also used in mass production of small castings. For such castings, multi-cavity moulds are prepared i.e. a single sand mould carriers a number of cavities. Patterns for these castings are connected to each other by means of gate formers which provide suitable channels or gates in sand for feeding the molten metal to these cavities. A single runner can be used for feeding all the cavities. This enables a considerable saving in moulding time and a uniform feeding of molten metal.

Skeleton patterns: 

When the size of the casting is very large, but easy to shape and only a few numbers are to be made, it is uneconomical to make a large solid pattern of that size. In such cases, a pattern consisting of wooden frame and strips is made called skeleton pattern. It is filled with loam sand and rammed. The surplus sand is removed by means of a stickle. The core can be prepared separately either with the help of a core box or another skeleton made for that, and assembled in position in the mould.

Sweep pattern: 

Sweeps can be advantageously used for preparing moulds of large symmetrical castings, particularly of circular cross section. This effect a large saving in time, labour and material. The full equipment consists of a base, suitably placed in the sand mass, a vertical spindle and a wooden template called sweep. The outer end of sweep carries the contour corresponding to the shape of the desired casting. The sweep is rotated about the spindle to form the cavity. Then the sweep and the spindle are removed, leaving the base in the sand. The hole made by the removal of spindle is patched up by filling the sand.

Pattern with loose-pieces: 

Some patterns usually single piece, are made to have loose pieces in order to enable their easy withdrawal from the mould. These pieces form an integral part of the pattern during moulding. After the mould it completes, the pattern is withdrawn leaving the pieces in the sand, which are later withdrawn separately through the cavity formed by the pattern.

Cope and drag pattern: 

When very large castings are to be made, the complete pattern becomes too heavy to be handled by a single operator. Such a pattern is made in two parts which are separately moulded in different moulding boxes. After completion of the moulds, the two boxes are assembled to form the complete cavity of which on e part is contained by the drag and other is cope. Thus, in a way, it is nothing but a two piece or split pattern of which both the pieces are moulded separately instead of being moulded in the assembled position.

Follow board pattern: 

A follow board is a wooden board used to support a pattern during moulding. It acts as a seat for the pattern. 

Such single piece patterns which have an odd shape or very thin wall require a follow board. In the former case, the hollow board is provided with a cavity corresponding to the shape of the pattern in which the pattern is seated for moulding. 

In the latter case, the follow- board carries a projection confirming to the inside shape of the thin walled pattern to support it during moulding.  If such a support is not provided, the pattern may sag or get broken due to less wall thickness during ramming.

Segmental pattern: 

These patterns are used for preparing moulds of circular castings, avoiding the use of solid pattern of exact size. If principle they work like a sweep, but the difference is that a sweep is given a continuous revolving motion to generate the desired shape, where as segmental pattern is a portion of the solid pattern itself and the mould is prepared in parts by it. It is mounted on a central pivot and after preparing the part mould in one position, the segment is moved to the next position. The operation is repeated till the complete mould is ready.

DESIGN CONSIDERATION IN PATTERNS

1. Proper allowance should be provided.
2. The parting line should be carefully selected.
3. Proper material should always be selected.
4. The wall thickness and sections should kept as uniform as possible. Abrupt changes should invariably be avoided.
5. The use of offset pairing, instead of cores, should be encouraged to as great extent as it is possible. Abrupt changes should invariably be avoided.   
6. For large scale production of small castings, the use of gated or match-plate pattern should be encouraged wherever the existing facilities permit.
7. All sharp corners and edges should be invariably provided with suitable fillets or otherwise rounded to enable an easy withdrawal of pattern, smooth flow of molten metal and ensure a sound casting.
8. All those surfaces of the castings which are specially required to be perfectly sound and clean should be so designed that they will be moulded in the drag.
9. The pattern should be given a high class surface finish as it directly effects the corresponding finish of the casting.
10. If gates, runners and risers are attached to the pattern, they should be properly located and their sudden contractions or enlargements should be avoided.

PATTERN LAYOUT

Pattern making consists of two different stages to prepare a layout and to shape the different parts. Layout consists of the following steps:
a. Initially study the drawing pattern and select a wood that can suitably accommodate two views.
b. Take on surface of the wood as base and plane the adjacent surface.
c. Take a folding rule and measure the dimensions as per the drawing.
d. Prepare the layout by using various measuring and marking tools locating position of core prints and surface to be machined.

PATTERN CONSTRUCTION

a. As per the layout select locating of parting line.
b. As per the shape of the pattern find out how many extra pieces to be made and how to construct them.
c. Select the wood and cut it so that direction of the grains lie along the length of pattern.
d. Provide sufficient draft allowance on the pattern.
e. Assemble all the separate pieces of the pattern by the help of dowels or glue and the check the parts completely by placing it over the layout.
f. Check the pattern as a whole for the specification.
g. Finish the rough surfaces and varnish it.
h. Fix the wax or leather wherever necessary then sand the complete surface and give final casting of shellac.
i. Different surfaces of pattern are colored by painting.

PATTERN COLORS

There is no universal method of coloring but following method is followed as a practice for coloring the patterns and core boxes.

1. Red for machining surface
2. Black for un-machined surface
3. Yellow for core prints
4. Red strip on yellow base for seat for loose pieces
5. Without colour for parting surface

**Core prints: In a casting if we want to have a hole then this is done by core. This core is placed in the mould on the impressions made in the sand. These projections are core prints.