1.0 PRECISION CASTING PROCESSES

Transcription

1 1.0 PRECISION CASTING PROCESSES

2

3

4

5 An Introduction to precision casting processes The casting process is without equal as the manufacturing cable of producing fully shaped components of any size in any metal or alloy. Consumers are placing increasing demands on the foundry industry to produce castings of improved quality. This is reflected in the demands for improved dimensional accuracy and consistency, improve surface finish and improved metalurgical integrity. These are the recognised attributes of precision casting. Figure 1 shows the recognised attribute of precision in casting.

6 Figure 1 Precision in the Context of Metal Casting Production Factors influencing the attainment of precision in casting Tooling Cannot attain a higher standard of dimensional accuracy or surface finish than the pattern or die from which it is produced. Essential to achieve the highest standard of tooling. This equipment must be maintained regularly and inspected for damage or wear that impair the precision of the casting Mould Production Extremely important in the permanent pattern/expendable mould processes to which the mould conforms to the pattern shape. Poor conformity at this stage negates the advantage of accurate tooling. Seperation of the mould from the pattern is a critical stage. Damage of the mould is possible. The expendable pattern/expendable mould processes are not susceptible to this problem, but process must ensure that the moul conforms to the pattern.

7 Permanent mould processes mould production implies die production. Mould Materials Material chosen has a direct influence on both dimensional accuracy and surface finish. May also affect the metallurgical integrity by influencing the formation of shrinks, gas porosity, non-metalic inclusion and metal penetration defects. Mould Location Requirement of joint lines in the mould or die in permanent patterns or permanent moulds are invariably associated with a reduction in accuracy. Dimensions across a joint line are never maintained to the same tolerances as those a single mould or die half. Compounded by multiple joints and by the requirements for core location again has an effects like above. Further inaccuracies may arise from the need to locate the mould or die halves to one another. The pin and bush method employed inaccurate machined or wear of those can contribute to inaccurate matching. Pouring Stage A high metal pouring temperature promotes flidity, so too does a preheated mould or die. Alloy composition plays an important role near eutectic alloys (cast iron & Al-Si alloys) are especially good in this respect. Smooth surfaces aid metal flow but loss in permeability, and back pressure exerted which restrict the inflow of metal. The effect of heat on the mould or die material expand, enlarging mould cavity and casting. Casting Contraction Casting contraction is not so predictible in practice, even CTE is a constant for a given material, and is used as the basic for incorporationg a contraction allowance in tooling. Lack of predictibility arises from castings of complex shapes with section variation, and therefore, do not cool or contract uniformly. Mould or die may exert a constraint to contraction, building residual stresses that lead to distortion.

8 Combination of factors conspires against accurate prediction of and allowance for, contraction contribute to dimensional variations. Casting Finish Fettling (shot blasting, degating, feeder head removal and grinding), if not practiced with care can impair the precision of the casting. Measure Available to Improve The Precision of Castings Amongst them are; Attention to detail of each of the a forementioned factors will maximise the potential for improving the precision of a casting. Sand casting improvement in machinery and processes. Green sand moulding improve by understanding the interrelationship between clay and moist content, mould compaction and dimension accuracy. Introduction of cold setting processes (inorganic and organic) revolutionised the jobbing and short run production. Hardening of sand in contact with the pattern, a major feature of these.processes, improves the conformity of the mould to the pattern and consequently dimensional accuracy and consistency. The development of unbonded sand processes has also provided improvement. Reduced machining requirements are a feature of the castings produced by both the V-process and lost-foam process. The absence of water or binders from these sand reduces mould material related casting defects and results in improved metallurgical integrity. Subsitute of silica sand by Zircon (practice in shell moulding process and the Cosworth process) demonstrates dimensional accuracy as a benefit. The elimination of silica sand as the mould material is a feature of the plaster mould, investment casting, ceramic mould and die casting processes these processes can be expected to demonstrate improve precision. Precision is improved further in the invesment casting process because an expendable pattern is used and there is no mould joint line, and improve casting surface finishes with the use of very fine refractory particles. Mould preheat, as an aid to improved metal fluidity and mould or die filling characteristics improves thin section capability and detail replication.

9 Quiescent, non-turbulent filling of cavity, the maintenance of pressure difference during solidification of the casting enhances casting soundness in such processes as low pressure die casting, the counter pressure process, the Cosworth process and the Hitchiner CLA and CLV processes. Vacuum melting and casting (in production of nickel-base superalloy casting) eliminates oxidation and gas absorption improve metallurgical integrity. Vacuum suction improves mould filling, reduces the occlusion of gases during turbulent filling. Cost Implications of Precision Casting. Precision castings are more expensive than standard castings. This arises because of; o The more expensive plant and processes required. o The need for improved standards of process and QC. Cost justified when there is no suitable alternaive manufacturing route or when the savings which accrue from the reduced requirement for machining overweight Near Net Shape Capability Manufacturing processes that produce components to net or near net shape and size; minimum number of processing steps; and little waste have the potential to reduce manufacturing costs and improve manufacturing efficiency (e.g; flashless forging, fine blanking, and PM) The metal casting processes has an inherent near net shape capability and the precision casting processes can produce castings which come closer to meeting net shape and size requirement.