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The Challenge of a Demanding Market

Just now, about 70 percent of all the light metal castings produced in North America are for the automotive industry. This figure will come no surprise to any die caster, nor will the tact that the auto industry is extremely demanding. Vendors must provide their product with perfect quality, in complete orders, and on time..every time. This is understood and accepted by all suppliers and would-be suppliers to the automotive industry.

The automotive market for light metal shapes continues to grow at an accelerating rate. This increase is driven mostly by the demand for lighter vehicles to reduce fuel consumption. Continued unrest in the Middle East has been less than reassuring with regard to the long-term availability of oil from that region. It can reasonably be assumed, therefore, that forseeable future, the automotive sector of our market will continue to grow.

This is good news for light metal die casters. But satisfying the needs of automakers will require die casters to make use of technology, which has been available for some time, but not universally employed, because it was seldom essential.

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.

Controlling Plunger Temperature

Almost all plunger tips are made of copper, because its excellent thermal conductivity expedites the transfer of heat from the plunger to the cooling medium. Copper, however, is not a strong metal. The face of the tip, that carries the full force of the shot, is therefore usually fairly thick. This reduces the rate of heat transfer from the molten metal to the cooling water circulating within the tip.

A very effective plunger tip, developed in Switzerland provides a method of maximizing the rate cooling without sacrificing strength.

This popular tip is not directly attached to the plunger rod. Screwed to the end of rod is stainless steel tip holder. The BeCu tip is attached to the holder with bayonet type coupling that provides firm and dependable connection plus quick and easy replacement. This steel holder lies in full contact with the inside face of the plunger, and Id absorbs the total shot pressure. This allows the face of the plunger to be very thin, and so improves the heat exchange. The tip holder is with four water channels that produce high velocity turbulent flow and maximize the cooling efficiency of the tip.

The plunger tip also has the tool steel wear ring, which floats in a groove machined in the tip. Easily replaceable the flexible ring expands to make continuous contact with the inside of shot sleeve. Providing the temperature of the shot sleeve is also well controlled, the possibility of tip/sleeve gap becoming too large is also virtually eliminated, and the seal necessary for an effective vacuum can be successfully maintained. As the producers of large castings control the gap with this plunger tip, the number of shots with a good at vacuum increases from a few thousand to tens of thousand. In some cases this plunger tip is actually the enabling technology that makes a new product feasible to produce.

Vacuum-Assisted Die Casting

In the phrase "vacuum assisted die casting" the operative word is naturally “vacuum”. 'The term 'assisted' is too often ignored. Vacuum alone will not make a mediocre casting system much, if any, more efficient.

Vacuum simply' supplements the efficiency of a well-designed casting process, and extends its capability. It depends on expertly engineered die cavities, runners, vents, and overflows. And, most importantly, a consistently tight seal is essential.

Vacuum Technology Still Evolving

Today's vacuum system is much more sophisticated and consequently more effective than when this technology was introduced several years ago.

The object of the valves in a vacuum system is, of course to maintain a high vacuum until the very end of the filling process. These valves are triggered by the flow of molten metal hitting the closing piston, which in turn closes the valve. Some systems now accomplish this in the astonishing time of two milliseconds.

A recent development controls the valves electro pneumatically means of an ultra-high response valve, which has a reaction time even less than two milliseconds. The control signals required this purpose are programmed directly on the die-casting machine.

An attitude in the vacuum industry that "anything can be improved" continues to forth efficiency of this increasingly necessary technology.

A Case History

Vacuum technology is continually improving. Today's best systems are considerably more effective than those of only a few years ago. As mentioned earlier, the best results from a vacuum assist is always from a well-run die caster. The large BMW variable cam timing gear housings were made by capable and experienced die casters on well maintained machines. The criteria of quality was as follows:

• After tooling- no micropitting
• Machined surface quality Ra 20-30 µm
• Casting Tolerances +/- 0.l mm
• Rough surfaces, sandblasted
• Body test pressure 400,000 Pa (4 bars)
• Boreholes test pressure, 200, 000 Pa (2 bars)

After upgrading to the state of art vacuum assist system, the productivity of the BMW variable cam timing gear housings increased by 10 percent.

A Holistic Process Adds Synergy

Cold chamber die-casting is a good example of a holistic production process. No single component of the DCM operates in isolation. Effective and profitable productivity depends on all parts of the process functioning at close to maximum efficiency at all times. Synergy really can result. This is an achievable goal for any die caster.

Most of the large and complex near net shapes that are now required can only be produced with vacuum assistance. The efficiency of any cold chamber casting process will be improved by the addition of vacuum. But the maximum benefit of vacuum can only be approached when the temperature of both the shot sleeve and the plunger tip are closely controlled throughout the injection stroke, and a completely secure seal between plunger and sleeve is maintained at all times.

A Challenge and an Opportunity

The need for light metal castings by automakers is really just beginning. It will continue to grow rapidly. Castings of it size and complexity never before envisaged will be required.

This is a challenge to die casters, but also an opportunity of unmatched magnitude. With the aid of vacuum, light metal die-castings are now being used in applications for which castings were never before considered. The potential market is virtually unlimited.

Primary Author - Paul Robbins

Paul's father was a toolmaker. He started his own shop, Extrusion Machine Company, half a century ago, making extrusion dies and die cast molds. Today, the EXCO group is publicly traded and is made up of 15 companies making tooling for extruders and die casters, and automotive interior components. Paul received his BBA from York University and MBA from the Schulich School of Business in Toronto. He has worked in this industry for 20 years and has been granted six U.S. patents for various extrusion and die cast tooling inventions. Paul has written many articles on die casting for industry magazines and has spoken at many international meetings, such as NADCA's Congress and Exposition.

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