This page contains a Flash digital edition of a book.
veloping as a sharp, grit-like feature on the surface of the casting. Low levels of lead, tin and

cadmium have a severe effect on zinc casting quality and must be controlled down to a maximum of 50 ppm or less. Te elements sit at the grain boundar- ies of zinc and promote inter-granular corrosion through the formation of a minute galvanic cell stimulating the zinc to sacrifice itself at the grain boundaries. Tis can lead to the total collapse of the casting structure, converting it to oxide dust. In less severe cases, corrosion stress cracks are formed, promoting premature part failure. Te presence of these impu- rities often is indicated by hot short- ness cracks during casting. However, these cracks also can be caused by other process-based problems.

Melting Practice Te zinc diecasting process per-

forms optimally when all features are consistent. Tis particularly applies to melt temperatures. Low melt tempera- tures, large variations in melt tem- peratures, and low metal levels in the furnace are potential sources of defects that should be eradicated by process control procedures. The diecaster should attempt to maintain a minimum pot tempera- ture appropriate for the alloy being used. To avoid excessive variations, the diecaster should use the ap- propriate furnace technology, avoid charging ingots directly into the melt and ensure the maximum level of metal is present when making castings. This prevents dross entry into the goose neck and helps main- tain temperature stability. Low levels of alloy allow the pot

temperature to increase and necessitate large additions of cooler material. Tis promotes instability and inconsistency in the process. Using two cell furnaces can eliminate this problem, but the diecaster always should be aware of the problems associated with machine furnace operation. Where single furnace crucibles

are in use, the diecaster should avoid feeding scrap, sprues and runners directly into the melt. Sprues and runners may have a die lubricant surface residue, which will increase


Adopt curved surfaces to disguise blemishes. Large flat surfaces exaggerate edge build up from plating and powder coating.

dross levels and cause fumes. Some of the oxides released can become entrained in the melt, causing cast- ing defects and increasing the risk of failure or fracture. The entrained oxides will appear on the surface of castings as oxide folds, which cannot be removed by polishing and will show through a plated surface.

Die Lubrication In zinc alloy die casting, die spray is

a lubricant, not a coolant. It is normally a water-based soluble oil (for health, safety and environmental reasons) and is sprayed in mist form onto the die faces in sufficient volume to maintain the die temperature while evaporating the water content of the spray.

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60