Pattern Design and Manufactur

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IPE 205Manufacturin

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Pattern is a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during the casting process.

Function of the Pattern:1. A pattern is used to make a mold cavity2. The pattern is like the original object with

some dimensional allowances. 3. Runner, gates, and risers used for feeding

molten metal in the mold cavity may be a part of the pattern.

4. Proper patterns with good surface finish reduce casting defects.

5. A properly constructed pattern minimizes the overall cost

The imp. Properties which the pattern material must possess are:

6. Light in weight7. Strong, hard and durable8. Resistant to wear and corrosion9. Dimensionally stable(unaffected by

temperature, humidity)

Pattern

Pattern Materials

• The following material are used for making pattern: Wood

Advantages: (i) Easily available (ii) Cheap (iii) Light in weight (iv) Easy to work and (iv) Good finish.Disadvantages: (i) readily affected by moisture (i) wear out quickly by sand abrasion (iii) warp badly if not stored properly (iv) less strength, tends to break on miss-handling (v) shape changes when dries out and (vi) does not last long

Metals: Aluminium, Cast iron, Brass, White metal etc.Advantages: (i) no effect of moisture and (ii) no cracking, bending etc. due to improper storingDisadvantages: (i) less easy to shape and work (ii) heavy in weight (iii) costly and (iv) affected by rust or corrosion

Plastic Wax Rubbers Plaster of Paris

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Variety of patters are used in casting and the choice depends on the configuration of casting and number of casting required. The type of pattern selected for a particular casting will depends on the following several conditions:◦ Shape and size of casting◦ Number of casting required◦ Method of molding used◦ Difficulty of the molding operation.◦ Characteristics of castingsDifferent types of patterns are Single piece pattern Split pattern Loose piece pattern Gated pattern Match plate pattern

Sweep pattern Cope and drag

pattern Skeleton pattern Shell pattern Follow board

pattern

Types of Patterns

•Single solid piece without any subparts or joints•Most simple of all and is used to make simple shapes•Determining the parting line between cope and drag is more difficult for the foundry worker.

Solid Pattern

•Shapes which are more intricate are made out of 2 or more pieces•When placed in the mold properly the plane at which the two parts are assembled should coincide with the parting line of the mold•Aligned together with the help of dowel pins•Mold setup is easier since the patterns placement relative to the parting line of the mold is predetermined

Split Pattern

•Two piece pattern representing the casting, and divided at the parting line, similar to the split pattern•Each of the parts is mounted on a plate. The plates come together to assemble the pattern for the casting process•More proficient and makes alignment of the pattern in the mold quick and accurate•Used in high production industry runs for casting manufacture.

Match Plate Pattern

•Two piece pattern representing the casting and divided at the parting line. Each of the two halves is mounted on a plate for easy alignment of the pattern and mold. •In match plate pattern the two halves are mounted together, where as in the cope and drag pattern the two halves are separate•The cope and drag pattern enables the cope section of the mold, and the drag section of the mold to be created separately and latter assembled before the pouring of the casting.

Cope and Drag Pattern

•Gated pattern are used for mass production of small castings. The passage through which the molten metal flows into the mold is called gate.•In mass production if the gate is made by hand for every small mold, it will take a lot of time. Therefore, a number of small castings are produced in a single multi-cavity mold by joining a number of patterns through gates.

Gated Pattern

•The sweep pattern are used to prepare mold of symmetrical and regular shapes particularly in large sizes.• It could be economical to save the money and efforts of making the full pattern•A sweep pattern consists of a wooden board fixed to metal rod. The outer contour of the board is similar to the contour of the castings.•Cavity in such a case could be made by sweeping the pattern (which is a part of the full shape) around a central axis

Sweeping Pattern

•Loose piece pattern is made of loose component pieces assembled together by dowel pins. •The whole pattern can be removed from the sand mold by taking out all the component pieces one by one. •The main piece is usually removed first, after that the separate loose pieces, which may have to be turned or moved before taking out, are removed.

Loose piece pattern

In a two-part mold, the upper half, including the top half of the pattern, flask, and core is called cope and the lower half is called drag.

The parting line or the parting surface is line or surface that separates the cope and drag.

Factors, which determine the number of parting lines required, are:

Geometry of the component to be molded.Number of cavities required within the mold

sections.The nature of the runner system.The kind of gating required andThe method of ejecting the final product from its

mold

Parting Line

Figure: Guidelines of the parting line

Process of Pattern Design

Pattern allowances• Shrinkage or contraction

allowance• Draft or taper allowance• Machining or finish allowance• Distortion or camber allowance• Shaking or Rapping allowance

Fillets Cores

The molten metal cools and solidifies it will begin to contract. This means that although the molten metal completely filled up a mold, by the time the casting was cold, the casting is smaller than the mold. Pattern must be made larger than the design drawing. The difference between the size or dimensions of the desired casting and the size of the pattern used to create the mold is called a shrinkage allowance. A master pattern from which metal patterns are cast must have double shrinkage allowance.

Shrinkage or Contraction Allowance

Shrinkage allowance for metal casting varies by the type of metal. Shrinkage allowance for metal casting is linear meaning that these allowances apply in every direction.

Other factors on which shrinkage in the casting depends:

• Dimension of the casting• Design and intricacy of castings• Resistance of mold to shrinkage• Molding material used• Method of molding used• Pouring temperature of the molten metal.

Material Dimension Shrinkage allowance (inch/ft)

Grey Cast IronUp to 2 feet

2 feet to 4 feetover 4 feet

0.1250.1050.083

Cast SteelUp to 2 feet

2 feet to 6 feetover 6 feet

0.2510.1910.155

AluminumUp to 4 feet

4 feet to 6 feetover 6 feet

0.1550.1430.125

Magnesium Up to 4 feetOver 4 feet

0.1730.155

Table: Rate of Contraction of Various Metals for Shrinkage Allowance

Machining allowance is the amount by which dimensions on a casting are made oversize to provide stock for machining This added amount of extra material depends • Machining operation• Characteristics of metal• Methods of casting• Size and shape of the casting• Degree of finish required in casting

Metal Dimension (inch) Allowance (inch)

Cast ironUp to 1212 to 2020 to 40

0.120.200.25

Cast steelUp to 66 to 20

20 to 40

0.120.250.30

Non ferrousUp to 88 to 12

12 to 40

0.090.120.16

Machining or Finishing Allowance

The casting shown is to be made in cast iron using a wooden pattern. Assuming only machining allowance, calculate the dimension of the pattern. All Dimensions are in Inches

For dimension 18 inch, allowance = 0.20 inchFor dimension 14 inch, allowance = 0.20 inchFor dimension 8 inch, allowance   = 0.12 inchFor dimension 6 inch, allowance   = 0.12 inch

The pattern needs to incorporate suitable allowances for draft, which means that its sides are tapered so that when it is pulled from the sand, it will tend not to drag sand out of place along with it. Its value lies between 0.50-30 depending upon the size of the external faces. The amount of the draft needed depends on the following factors:

Shape of the patternLength of the vertical side of the patternMethod of moldingIntricacy of the pattern

A pattern having no draft allowance being removed from the mold. In this case, till the pattern is completely lifted out, its sides will remain in contact with the walls of the mold, thus tending to break it.

Draft or Taper Allowance

A pattern having proper draft allowance, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity.

Pattern material

Height of the given surface

(inch)

Draft angle(External surface)

Draft angle(Internal surface)

Wood

11 to 22 to 44 to 8

8 to 32

3.001.501.000.750.50

3.002.501.501.001.00

Metal and plastic

11 to 22 to 44 to 8

8 to 32

1.501.000.750.500.50

3.002.001.001.000.75

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It is found that big castings tend to warp or distort during the cooling period due to their size, shape and type of metal. Uneven shrinkage also causes distortion. To overcome this effect, the pattern is made initially distorted in opposite direction. Such an allowances depends on the judgment and experience of the pattern maker who knows the shrinkage characteristics of the metal.

Distortion or camber Allowance

Shaking or Rapping Allowance

When the pattern is rapped or shaken for easy removal from the cavity, it is found the cavity in the mold is slightly increased in size. To compensate this increase, the pattern should be initially made slightly smaller. In small and medium sized castings, this allowance can be ignored, but in large sized castings or in those that must fit together without machining or where high precision is required, shaking allowance is provided by making the pattern slightly smaller.

Fillets

The sharp edges are rounded for the process of molding and fillingThey vary in size from 3 to 25 mm (1/8 to 1 in.) radius depending on size, shape and material of the casting.

Cores

Cores form the internal geometry of a casting. Cores are also used for producing complex features which cannot be produced otherwise. Cores are placed in the mold, and remain there during the pouring phase of the manufacturing process. The metal casting will solidify around the core.Cores must be strong and permeable

Sometimes a reinforcing material will be placed in a sand casting core to enhance strength. Core may be manufactured with vents to facilitate the removal of gases.

Core prints are used to support the core inside the mold cavity. The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the mold.

The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation. Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity.

Core Design Analysis

To make the core stay in place during the process of molding assembly, the print must balance the body. While the process of mould filling is working, the print must not shift. Minimized diversion of the core is to be achieved. Maximized heat transfer from the core print to the mould is required. The inner gases should be allowed to be produced within the core to flee away to the mould. To prevent incorrect units, uneven holes should have full proof prints.

Pattern Design Suggestions

Pattern Design Suggestions