Laboratory Assignment – Sand Casting of an Aluminium 13wt% Silicon Alloy
1) Process Attributes
Waste shown yellow*
Waste shown yellow*
After taking part in the sand casting laboratory session it was made clear that the process possessed many advantages and disadvantage, both from the process itself and from the final part produced. Observing the process step by step it was noted that the tasks carried out by the operator were not very difficult (i.e compacting sand, pouring liquid metal) and so giving the advantage of only requiring a low skill level. However, although the process did not involve a large amount of skill it was observed to be very labour intensive and took a long time to complete one cycle of the process (around 30mins). Having a long cycle time is clearly a disadvantage as time is money in the manufacturing industry but this could be slightly reduced if multiple moulds were prepared simultaneously. Another advantage observed during the sand casting procedure was that the equipment used was not very expensive and unlike other casting processes the mould was reusable. With only a low capital investment needed this would make short production runs viable. Probably the most noticeable disadvantage to the sand casting method was the low material utilization. In the labs example (seen left), almost a third of the material used is lost to the riser, runners and feeder. Particularly in today’s waste conscious culture this can cause many problems, however it can be tackled by better design.
2) Process Problems
Shrinkage: Shrinkage defects in the final product are usually the result of a feeding defect. As shrinkage naturally occurs during the solidification process, if liquid metal is not sufficiently fed through the feeding system to compensate then it will result in an indented surface. The surface defect was clearly shown in the lab example (shown left). A shrinkage defect normally occurs on the last part to solidify, so logically in the labs case the shrinkage occurred on the surface of the thickest part Furthermore the big defect problem of a surface sink was observed in the sand casting lab session (shown left). A sink such as this would give major dimensional inaccuracies and would probably have to be redone as it would be unsuitable and uneconomical for further processing.
Solution: One way in which an engineer could reduce the effects of shrinkage is to design the mould to compensate for the shrinkage during solidification (i.e making the mould larger). Also due to shrinkage being mainly associated in being a feeding problem, the redesign of the feeding system would reduce the set back. Making the feeding sprue thicker would ensure that the system continues to feed liquid metal to the cavity, thus preventing shrinkage. Furthermore utilizing a uniform part thickness would also tackle the problem reducing residue stresses and leading to uniform cooling.
Rough Surface Texture: The defect causes an undesirable distorted surface finish and is formed due to the texture created from the compacted sand used to create the mould cavity. The effect was observed in the part created from the lab session and can be seen right. Solution: The rough surface effects created from the mould sand can be significantly reduced by using sand with very small grain size. Particularly for the face sand which lines the surface of the mould cavity, if the grains used are very fine then surface finish will be improved dramatically.
Porosity: The defect of porosity occurs because of the liquid metal used to create the part can hold large amounts of dissolved gasses. As the liquid metal solidifies, the materials new solid form can no longer hold these gasses and so they form bubbles. These bubbles can be generated both on the surface of the material and internally, thus they effect the strength of the materials solid form and resulting in a decrease in mechanical...