Workshop Practice

Module 7. Carpentry

Lesson 15
CARPENTRY AND PATTERN MAKING, MOULD MATERIAL AND THEIR APPLICATION

15.1 Introduction

A Pattern making may be defined as replica model of the desired casting which, when packed in a suitable moulding material, produces a cavity called mould. This cavity when filled with molten metal, produces the desired casting after solidification of the poured metal.

The ways in which a pattern differs from an actual component are

1. It carries additional allowance over those portions which are to be machined. (Machining allowance)
2. When a pattern is withdrawn from a mould it may injure the edges. To avoid this danger the pattern is made with slight tapes inward on the vertical surfaces. (Draft allowance)
3. It carries an additional allowance to compensate for metal shrinkage. (Shrinkage allowance)
4. It carries the necessary draft to enable its easy removal from the sand mass (shake or rapping allowance)
5. Some castings tend to warp or distort on cooling. To compensate the pattern is made with certain distortion allowances.(Distortion or camber allowance)
6. It carries additional projections called core prints, to produce seats for cores.

The important considerations which a pattern maker is to make in order to plan the successfully to yield desired results are the following

1. Number of casting to be made from the same pattern
2. The appearance and surface finish of the casting to be produced
3. Facility and ease in moulding
4. Method of withdrawal of the pattern from the mould

15.2 Pattern Materials

15.2.1 Wood

It is the common material used for pattern making because the following advantages

1. It is cheap and available in abundance.
2. It can be easily shaped into different forms and intricate designs.
3. Its manipulation is easy because of lightness in weight.
4. It can be preserved for a fairly long time by applying proper preservatives like varnish.
5. Good surface finish can be easily obtained by only planing and sanding.

Disadvantage of wood

1. It wears out quickly due to its low resistance to sand abrasion
2. It is very susceptible to moisture, which may lead to its warping or splitting.This needs its careful storing in a dry place and the application of preservatives
3. Its life is short as compared to other pattern materials. This confines its use to such cases only when a small number of castings are required.

The wood selected for pattern making should be straight grained, free from knots and other natural defects. It should be properly seasoned before use. Common woods used are Deodar, Teak, shisham and Mahogany.

15.2.2 Metals

Metals have a much longer life than wooden patterns.

They carry the following disadvantage

1. They are costlier than wood, and cannot be used with smaller number of casting are to be made
2. Metals are very heavy and in case of large casting the weight of the pattern always may be a problem.
3. Forgiving different shapes and fine surface finish they need machining operation.

Metals used in pattern making are the following

(A) Brass:Commonly used metal for small patterns particularly in bench and machine moulding. It has a high strength, high resistance to corrosion, sand abrasion,takes a good surface finish and can be cast into any shape.

(B) Cast iron: It is cheap and can be casted into any shape. It has a good machinability, high resistance to sand abrasion, good strength. Its excessive weight is a great drawback with it.

(C) Aluminium and its alloy: Larger patterns of metal are usually made from aluminium or its alloys because of their light weight and low cost. They can be cast into any shape and machined to give a good surface finish. They have a high resistance to corrosion.

15.2.3

Plastics and gypsum cement known as plaster of paris are also used for making patterns and core boxes because of their light weight, high strength resistance to wear and corrosion, moisture,surface finish and reasonable cost.

15.3 Factors Effecting the Selection of Pattern Material

The selection of a material for making the pattern is depend on the following factor

1. Number of casting to be made

2. Method of moulding to be used. It may be hand or machine.

3. Type of casting method to be used

4. Degree of accuracy in dimension and quality of surface finish required on the casting

5. Design of casting.

15.3.1 Types of patterns

The type of pattern to be used for a particular casting depends upon many factors like the bulk of casting, type of moulding process, number of castings required and the anticipated difficulty of moulding of the typical shape. The following types of patterns are commonly used

1. Single piece pattern

2. Two piece pattern or split pattern

3. Multi piece pattern

4. Match plate patterns

5. Sweep pattern

6. Pattern with loose pieces

7. Follow board pattern

8. Gated patterns

9. Cope and drag patterns

10. Skeleton patterns

1. Single piece pattern :This pattern is made in one piece and carries no joint, partition of loose pieces. It is simplest pattern. Depending upon the shape, it can be moulded in one or two boxes. This pattern is the cheapest but its use can be limited extent of production only. Because its involve large numbers of manual operation.

2. Two piece pattern or split pattern :When pattern making is difficult by single pattern. For such casting two piece pattern are employed. They are made in two part which are joined at the parting line. While moulding, one part of the pattern is contained by the drag and other by the cope.

3. Multi piece pattern :Casting 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. These pattern may consist of three, four of more number of parts depending on their design.

4. Match plate patterns :This patterns are used where a rapid production of small and accurate casting is desired on a large scale. Their construction cost is quite high, but the same is easily compensated by a high rate of production, greater dimensional accuracy and minimum requirement for machining in the casting. These pattern are made in two pieces, one piece mounted on one side and the other on the other side of a plate. The plate may be of wood, steel or aluminium. Aluminium is preferred due to its lightness and cheapness.

5. Sweep pattern :Sweeps can be advantageously used for preparing moulds of large symmetrical casting, particularly of circular cross section. This effects a large saving in time, labour and material. The equipment consists of a base, suitably placed in the sand mass, a vertical spindle and a wooden template, called sweep. The sweep is rotated about the spindle to form the cavity. Then sweep and spindle are removed, leaving the base in the sand. Separately prepared core is placed in the mould, gates cut and the mould is ready for pouring.

6. Pattern with loose pieces :some pattern 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 mould is complete, the pattern is withdrawal leaving the pieces in the sand which are later withdrawal separately through the cavity formed by the pattern.

7. Follow board pattern :A follow board is a wooden board used to support a pattern during moulding.Such single piece patterns which have an odd shape or very thin wall require a follow board. The follow board carries a projection conforming to the inside shape of the thin walled pattern to support it during moulding. If such support is not provided the pattern may get broken due to less wall thickness during ramming.

8. Gate Pattern :Groups of patterns with gate formers attached to the pattern proper are called gated patterns; may be made of wood or metal and are used for mase production of small castings

9. Cope and Drag Pattern :For production large heavy castings, difficult to handle by single person.Separate cope and drag patterns are used to ease this difficulty. They are made in halves, split in convenient joint line.

10. Skeleton Pattern :Patterns for very large castings would require tremendons amount of timber fora full pattern. In such cases a frame work farming a skeleton pattern may be employed to give general contains and size of the desired castings.

Fig. 15.1 Various types of patterns

15.4 Moulding

Foundry or casting is a process of forming metallic products by melting the metal, pouring it into a cavity known as the mould and allowing it to solidify. Foundry engineering deals with the process of making casting in mould prepared by patterns. The principal material used in the foundry shop for moulding is the sand. For mould preparation special hand tools, mould boxes (flasks) and mechanical tools and equipments are used. Cores are separate shapes of sand that are generally required to form the hollow interiors of the casting or a hole through the casting. The care is left in the mould in casting and is removed after the casting moulding sands.

The main ingredients of moulding sands are (i) Silica sand grains (ii) Clay, (iii) Moisture and (iv)Miscellaneous materials like iron oxide, lime stone, soda and potash. Silicas and in the form of granular quartz is the main constituent, clay is of very small particles under 20 microns sand and may be up to 5 to 20 percent in the moulding sand. The moisture water should be of 2 to 8 percent. In general moulding sand must have good properties like porosity, flowability,collapsibility, adhesiveness, cohesiveness or strength and refractoriness, generally natural sands are mixed with clay, line, magnesia, potash, soda,horse manure, saw dust, cow dung, coal dust in small quantities to get the desired properties.

15.4.1 Moulding process and classification

(I) Moulding processes are commonly classified according to the different forms.

(i) Hand moulding : Employed for unit or small lot productions.

(ii) Machine Moulding : For large lot and mass production.

(II) Classification according to the type of materials used.

(I) Green sand Moulds

(II) Dry sand Moulds

(III) Skin Dried Moulds

(IV) Loam Moulds

(III) Classification according to the methods used. Which are based on the place / particular applications in the foundry.

(I) Bench Moudling

(II) Floor Moudling

(III) Pit Moudling

(IV) Plate Moudling

(V) Sweep Moudling

15.4.2 Green Sand mould

It is the most common process used for general purpose. Green sand moulds are prepared with natural moulding sands or with the mixture of silicon sand, clayand water. The moulding sand with moisture is called as green sand mould. The surface of the mould which comes in contact with the molten metal forms the most important part in green sand moulds. To get a clean casting and to prevent the sand from burning a layer of facing sand is given surrounding to pattern. Some times bonding materials such as molasses, a gelatinized starch is added to the facing sand mixture or sprayed upon the surfaces of the finished moulds. It is a common practice to coat the surfaces of the sand mould with refractory materials such as the carbonaceous materials known as blackings or mineral coating to produce a smooth skin on the castings. The materials commonly usedare graphite, coke, charcoal, gas carbon, plumbag, block lead, silicon, mica,tale etc. Fig. 15.2 shows the process of making green sand mould.

Fig. 15.2 Making a green-sand mould

15.4.3 Dry sand mould

It is similar to green sand mould except a different sand mixture is used and all parts of the mould are dried in an oven before being reassembled for casting. Here the binding materials suchas flour, resin, molasses or clay are thoroughly mixed and tempered. Dry sand moulds are often used for large works.

15.4.4 Loam mould

Loam is clay and sand mixed with water to form a thin plastic mixture from which moulds are made loam sand also contains fire clay or genistere. The loan must be sufficiently adhesive so that it can cling to the vertical surface.