Surface Finish
Die Cavities
All die
cavities must be smooth enough to permit easy ejection of the casting. Aside from
this, the required surface finish of die cavities depends mainly on the application
of the casting. When the casting is to be painted or plated, particular care
must be given to the die-cavity finish.
Die Castings
For good appearances, a casting must be homogenous, be free of surface imperfection due to uncontrolled floe of metal (imperfections such as heat-sink marks, pits, porosity swirls, cold shuts and misruns) and exhibit no imperfections caused by oil deposits and dross inclusions. A normal amount of buffing seldom removes surface flaws. Excessive buffing is likely to break through the dense skin and expose un-dallying porosity.
Die Wear
The rate
of die wear is influenced chiefly by the temperature of the casting metal and
by the design of the die. When the metal has a casting temperature no higher
than that of zinc alloys and the die is of simple design, it is not unusual to
obtain more than 5,00,000 shots before there is significant amount of die wear.
As metals with higher and higher casting temperatures are used, progressing
from zinc to aluminum and thence to copper alloys, die wear increases rapidly, regardless
of die design. As the configuration of the casting and gating system become.
es more
complex, wear in localized portions of the die also increases, especially as
the temperature of the metal rises. Die erosion (wash) is likely to be severe when
the hot metal goes around a corner.
Die Lubricants
Lubricants for Zinc alloys:
Solvent soluble lubricants: - are mixtures of mineral
spirits and oil or kerosene and oil, with graphite sometimes added. These give
excellent die release and at the same time, lubricate the ejector pins. They do
not; however, provide cooling for the die.
Water mixed die lubricants: - (colloidal graphite or silicon emulsions) are excellent for cooling the die and provide good release, but do not adequately lubricate the ejector pins. When water mixed lubricants are used, it is desirable to use a separate pin lubricant and apply it regularly.
Lubricants for Aluminum alloys:
Graphite greases are best for dies that operate at cool-to-normal temperatures. The waxes in these compounds act, as the wetting agents and the oil and graphite are the lubricants. These compounds are usually mixed with kerosene in a ratio of fifteen parts kerosene to one part lubricant and then sprayed on the die. The mixture should be agitated frequently to prevent the graphite from setting out.
Colloidal graphite in oil is used when die temperatures range from normal to hot. Carbonizing takes place slowly with this type of compound and if the dies are not enough, oil stains will show on the surface of the casting.
Water-mixed die sprays are colloidal-graphite or silicon emulsions that cool the die and act as a lubricant. Proprietary concentrates are usually mixed with water in ratios (concentrate-to-water) of 1-to-30 to 1-to-100.
Variables that Determine Cycle Time
Die-opening & closing time: - depends on the travel distance and the average speed of travel of the ejector half of the die, on the number and complexity of movable components used in the die and on the speed travel distance of the individual movable parts. Die-opening time is usually slightly less than die-closing time.
Pouring time varies depends on the type
pouring system used and on the amount of metal per shot. Time for hand lading
is influenced by operator efficiency. Automatic pouring time varies for the
different
Injection time varies with the volume of the metal cast (longer time for larger-volume castings) and the configuration of the gate used in the die. Generally, a longer time is required for a more constricted gate.
Dwell time depends on the cooling rate of the die (slower cooling rates require longer dwell times and vice versa), casting weight (especially if walls are thick) and metal temperature (the hotter the metal, the longer the dwell time).
Extraction time depends on the type of
actuation, speed and travel distance of the die and whether extraction can
partly overlap the die-opening operation. Extraction time also varies with
operator efficiency in withdrawing the ejected casting from the machine.
Location, size and number of ejectors contacting surfaces, together with size,
weight and configuration of the casting, influence the ejection portion of the
extraction time.
Die-cleaning time is affected by the method
and equipment used, the amount of residue left in the die, operator efficiency,
and cavity and
Lubricant-application time is influenced
the method and equipment used, the amount of lubricant required, the area and
configuration of die cavity to be lubricated, operation efficiency and
condition of die cavity surfaces.
Estimation of Die and Product Costing
- Calculate cost of all raw materials, which will be required for desired die.
- Consider additional cost of 10% for variation in market price, a availability of size and handling cost of material.
- Department overhead for 1 to 6 above.
- Add risk allowance as 10% of total of all above
- Add fitting and polishing charges as 8% to 10% of total of all above.
- Add design cost as 8% to 10% of total of all above.
- Add cost of heat treatment depending upon nature of heat treatment required
- Add material & manufacturing cost of electrode required for making cavities by spark erosion operation.
- Calculate cost for manufacturing of these items in shop as well as brought out items such as mould base, ejector pins and standard hardware required.
- General administrative charges, which included sales cost and profit – usually not included in overhead.
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