2_PATTERN

MANUFACTURING PROCESSES

PATTERN1The Pattern A pattern is a fullsized model of the part which is made slightly large to account for shrinkage and machining allowances in the casting.Pattern materials:Wood - common material because it is easy to work, but it warps Metal - more expensive to make, but lasts much longer Plastic - compromise between wood and metal

21.Solid (one-piece) patternThe simplest is made of one piece, called a solid pattern same geometry as the casting, adjusted in size for shrinkage and machining.One-piece patterns are relatively cheap to construct, but the subsequent molding process is usually slow. As a result, they are generally used when the shape is relatively simple and the number of duplicate castings is rather small.31.Solid (one-piece) pattern

42.Split patternsWhen solid pattern is difficult for withdrawal from the mold cavity, then solid pattern is splited in two parts. Split pattern is made in two pieces which are joined at the parting line by means of dowel pins. Split patterns are appropriate for complex part geometries and moderate production quantities.The splitting at the parting line is done to facilitate the withdrawal of the pattern.52.Split patterns

63.Match-plate patternsFor higher production quantities, match-plate patterns or cope-and-drag patterns are used. In match-plate patterns, the two pieces of the split pattern are attached to opposite sides of a wood or metal plate. Holes in the plate allow the top and bottom (cope and drag) sections of the mold to be aligned accurately.73.Match-plate patterns

84.Cope-and-drag patternsCope-and-drag patterns are similar to match-plate patterns except that split pattern halves are attached to separate plates, so that the cope and drag sections of the mold can be fabricated independently, instead of using the same tooling for both.94.Cope-and-drag patterns

105. Loose-piece PatternLoose piece pattern is used when pattern is difficult for withdrawal from the mould.Loose pieces are provided on the pattern and they are the part of pattern.The main pattern is removed first leaving the loose piece portion of the pattern in the mould. Finally the loose piece is withdrawal separately leaving the intricate mould.115. Loose-piece PatternLoose piece125. Loose-piece Pattern135. Loose-piece Pattern145. Loose-piece Pattern155. Loose-piece Pattern165. Loose-piece Pattern176. Gated patternIn the mass production of casings, multi cavity moulds are used. Such moulds are formed by joining a number of patterns and gates and providing a common runner for the molten metal, as shown in Fig.These patterns are made of metals, and metallic pieces to form gates and runners are attached to the pattern.186. Gated patternPatternsGatesRunner197. Sweep patternSweep patterns are used for forming large circular moulds of symmetric kind by revolving a sweep attached to a spindle as shown in Fig.Actually a sweep is a template of wood or metal and is attached to the spindle at one edge and the other edge has a contour depending upon the desired shape of the mould. The pivot end is attached to a stake of metal in the center of the mould.207. Sweep pattern

218. Skeleton patternWhen only a small number of large and heavy castings are to be made, it is not economical to make a solid pattern. In such cases, however, a skeleton pattern may be used. This is a ribbed construction of wood which forms an outline of the pattern to be made. This frame work is filled with clay sand and rammed. The surplus clay sand is removed by strickle board. For round shapes, the pattern is made in two halves which are joined with glue or by means of screws etc.228. Skeleton patternStrickle boardSkeleton pattern239. Follow Board PatternThis type of pattern is adopted for the castings having some structurally weak portions which if not supported properly, are likely to break under the force of ramming.The bottom board is modified as a follow board to closely fit the contour of the weak pattern and thus support it during the ramming.249. Follow Board Patternstructurally weak pattern likely to break under the force of ramming

Mould board259. Follow Board PatternFollow board269. Follow Board Patternweak pattern is supported by the follow board during ramming27Pattern AllowancesThe patterns are not made the exact size as the desired casting because such a pattern would produce undersize casting. When a pattern is prepared, certain allowances are given on the sizes specified in the drawing so that the finished and machined casting produced from the pattern will conform to the specified sizes.Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting.28Pattern AllowancesThe selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns are as follows:Shrinkage or contraction allowanceMachining or finish allowanceDraft or taper allowanceDistortion or camber allowanceRapping allowance291. Shrinkage or Contraction AllowanceIn practice it is found that all common cast metals shrink a significant amount when they are cooled from the molten state. The total contraction in volume is divided into the following parts: Liquid contraction, i.e. the contraction during the period in which the temperature of the liquid metal or alloy falls from the pouring temperature to the liquidus temperature.301. Shrinkage or Contraction AllowanceContraction on cooling from the liquidus to the solidus temperature, i.e. solidifying contraction.Contraction that results there after until the temperature reaches the room temperature. This is known as solid contraction.The first two of the above are taken care of by proper gating and riser design. Only the last one, i.e. the solid contraction is taken care by the pattern makers by giving a positive shrinkage allowance.311. Shrinkage or Contraction AllowanceThis contraction allowance is different for different metals. The contraction allowances for different metals and alloys such as Cast Iron: 10 mm/mt. Brass: 16 mm/mt.Aluminium Alloys: 15 mm/mt. Steel: 21 mm/mt. Lead: 24 mm/mt321. Shrinkage or Contraction AllowanceThe casting shown is to be made in cast iron using a wooden pattern. Assuming only shrinkage allowance, calculate the dimension of the pattern100160253033http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/MANUFACTURING-PROCESSES/metalcasting/lecture3.htm331. Shrinkage or Contraction AllowanceFor dimension 100mm, allowance = 100 X 10 /1000 = 1mmFor dimension 160mm, allowance = 160 X 10 /1000 = 1.6mmFor dimension 25mm, allowance = 25 X 10 /1000 = 0.25mmFor dimension 30mm, allowance = 30 X 10 /1000 = 0.3mm

341. Shrinkage or Contraction AllowanceThe pattern drawing with required dimension101161.625.2529.7352. Machining or Finish AllowanceThe finish and accuracy achieved in sand casting are generally poor and therefore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining.Machining allowance is a positive allowance given to compensate for the amount of material that is lost in machining or finishing the casting.If this allowance is not given, the casting will become undersize after machining. 362. Machining or Finish AllowanceThe amount of this allowance depends on the size of casting, methods of machining and the degree of finish. In general, however, the value varies from 3 mm. to 18 mm.373. Draft or taper allowance

383. Draft or taper allowanceDraft is the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder. Figure shows a pattern having no draft allowance being removed from the pattern.Till the pattern is completely lifted out, its sides will remain in contact with the walls of the mold, thus tending to break it.39Removal of Pattern without Draft40

Removal of Pattern without Draft41




3. Draft or taper allowanceNext figure shows an illustration of a pattern having proper draft allowance. Here, 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 cavity45Removal of Pattern with Draft46

Removal of Pattern with Draft47




3. Draft or taper allowanceThe normal amount of taper on the external surfaces varies from 10 mm to 20 mm/mt.514. Distortion or Camber AllowanceSometimes castings, because of their size, shape and type of metal, tend to warp or distort during the cooling period depending on the cooling speed. This is due to the uneven shrinkage of different parts of the casting.Expecting the amount of warpage, a pattern may be made with allowance of warpage.It is called camber.524. Distortion or Camber AllowanceFor example, a U-shaped casting will be distorted during cooling with the legs diverging, instead of parallel . For compensating this warpage, the pattern is made with the legs converged but, as the casting cools, the legs straighten and remain parallel.Warpage depends on the thickness and method of casting and it is actually determined by experience. Generally 2 to 3 mm is considered appropriate for 1 metre length.534. Distortion or Camber Allowance

Example of camber: (a) Casting without camber, (b) Actual casting, (c) Pattern with camber allowance545. Rapping or Shaking AllowanceBefore withdrawing the pattern it is rapped and thereby the size of the mould cavity increases.Actually by rapping, the external sections move outwards increasing the size and internal sections move inwards decreasing the size. This movement may be insignificant in the case of small and medium size castings, but it is significant in the case of large castings. 555. Rapping or Shaking AllowanceThis allowance is kept negative and hence the pattern is made slightly smaller in dimensions 0.5-1.0 mm.56

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2_PATTERN