A key factor in this market sector is that automotive design engineers today are in no way constrained by the previously accepted light metal cold chamber die casting production process. They simply decide what would be the best shape and size for the part they want. Their criteria are usually limited only to strength and weight. They now want die cast products that are larger, thinner, more complex, and stronger, than have ever been commercially produced before. If they cannot be satisfactorily provided by existing die casters, the automakers will simply make these large near net shapes themselves.Consider the dashboards of several new cars. They are light metal castings, and they are large, complex, and unusually thin. They are proof that with vacuum assistance, this type of product really is possible.This is the challenge facing North America's light metal die casters today.
The Problem:
Porosity causes more rejected castings than any other reason.In cold chamber die casting of light metals, because of the turbulence of the alloy as it is forced at a high pressure into the die cavity, and the complex shape of many casting molds, air and other gases are often trapped in the metal. This, of course, results in porosity in some parts of the casting.If the casting is to be chromed, painted, or powder coated, or if any part of the casting is very thin, any air or gas inclusions usually result in rejection. Porosity also affects the mechanical properties of the product. In structural applications, porosity can act as a stress concentrator and therefore create a site where cracks may occur.An additional problem is the fact that porosity in a casting may not be immediately apparent. If discovered after subsequent processing, customer dissatisfaction can be extreme.
Vacuum-Assisted Casting is the Solution
Before the injection shot occurs, a vacuum is drawn in both the shot sleeve and the mold cavity. The vacuum is maintained until the injection cycle is completed. Almost all of the air is positively evacuated from the mold.A good vacuum in the Mold cavity enables the alloy to flow into blind recesses in complex shapes. It also allows the fronts of the molten metal to merge freely without forming shuts.Whatever vacuum method is employed, if it works well, improved quality and reduced scrap can he guaranteed.
BENEFITS of Vacuum Assisted Die Casting:
- Rejections due to porosity are virtually eliminated.
- Rejections after secondary processing are virtually eliminated.
- Excellent surface quality is ensured.
- Product density and strength are increased.
- Larger, thinner, and more complex, castings are made possible.
- Less casting pressure is required.
- Tool life and mold life are extended.
- The die closes better.
- Flash is reduced or eliminated.
Value Added
A metal die caster is not, by definition, really a manufacturer. He does not actually make anything. He simply converts metal from one form to another. He changes liquid metal into a solid casting. In doing so, he adds value. His success or failure therefore results entirely amount of added value economically generated by his casting process.Product that is rejected is unusually costly to the die caster. The value of the machine time that is lost while producing the rejected product is never recovered. It be calculated at the selling price of good product made in an equal period, less only scrap recovery.Adding a vacuum system to his operating process benefit die caster in several ways. First it reduces his rate of rejection. Second, by lessening the force required on the plunger, it increases the life of almost all components of the DCM. But most importantly today, by allowing the die caster to produce thinner, stronger, and more complex castings, it provides an opportunity for him participate in a fast growing market sector to which he would otherwise be denied.
The Essential Seal
It is an obvious fact that a vacuum can only be created in a totally enclosed space. This makes the seal between the plunger and the shot sleeve critical to effective vacuum assisted die-casting.The gap between the plunger and the wall of the shot sleeve is necessarily very small..only 0.004 in. It at any time during the slow part of the shot, the gap becomes much greater than four thou, air is likely to be sucked through the gap, During the fast part, with the sleeve full of metal, the alloy may penetrate the gap and flash or blowbv will occur. If alloy collects on the plunger tip, rapid deterioration of the vacuum seal will result, as the tip becomes galled and the sleeve becomes soldered.If the gap becomes much less than four thou, there is then a danger of interference. Inconsistent shot velocity will inevitably result. This gap must therefore remain virtually unchanged during the entire casting cycle to guarantee the secure seal that is necessary if an effective vacuum is to be drawn. If close control of this tip/sleeve gap is lost, a good vacuum can be easily destroyed in less than 1,000 shots.A difficulty, of course, is that when metal is heated it expands. If the ID of the shot sleeve is no greater than 3 or 4 inches, expansion is minimal, and usually creates no great problem. But large castings require large shots, and the coefficient of thermal expansion remains constant. The same increase in the temperature of a six-inch sleeve, for example, will cause it to expand exactly twice as much as three inch sleeve. Unfortunately, the critical allowable is still only four thousand of an inch.Another problem is that the shot sleeve is of steel and the plunger tip is usually of copper, and copper has much greater coefficient of thermal expansion than steel. This difference in coefficients makes close control of the gap, from the start of the shot to the finish, extremely difficult. At the start, the tip is coolest and the sleeve is hottest. At the end of the shot, the tip is hottest and the sleeve is often water-cooled.To further complicate gap control there are other concerns, such as the difference in temperature from the top of the sleeve to the bottom.The bottom of the sleeve directly below the pour spout is where flash most often occurs. Interference will occur when metal gets on the plunger tip from penetration of the gap that often results from the erosion of the steel beneath the pour. And there is also the possibility of blow back of metal at the end of the fast part of the shot.As well, the alloy being poured into the sleeve is at about 1300°F, while the annealing temperature of H13, the shot sleeve material, is only 1085'F. If the shot sleeve is not adequately cooled, it will likely lose some of its hardness. Soon wear will result from the abrasive action of an alloy that penetrates the gap.With temperatures constantly changing throughout the stroke, the size of the gap doesn't depend on the actual temperature, but only difference in temperature between the plunger and the shot sleeve at any point. For larger castings, effective temperature management of both the plunger tip and the shot sleeve is therefore absolutely essential, if a consistent gap and a secure seal is to be maintained.to find complete information go to our site
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