KEY TO BETTER DIE CASTINGIntroduction Likely the best die-casting being done today is for or by the automotive industry. That is, by die casters selling to the automotive market, or automakers in-house die casting departments. For die casters, the automotive sector is by far the most difficult to serve. It now demands castings that are bigger, more complex, and with closer dimensional tolerances than ever before … and with a perfect surface finish and absolutely no porosity. These tight specifications are difficult to fill, especially when the cost of casting must be kept to an almost impossible minimum. Fortunately the technology of die-casting has kept pace with the changing demands of the automotive industry, and made it possible to satisfy the increasingly stringent specifications. There are no “average” die casters serving the automotive sector. They are all “good” and most are excellent. Even with similar equipment, however, some die casters are getting better productivity than others. We often refer to them as Master Die Casters. I’d like to share with you one of the reasons. Light metal castings are not made by die-cast machines, they’re made by die-casters. This may seem like a simple-minded thing to say, but think about it. All die casters know how to make castings, but even with the same equipment, some are better at it than others. Two casters may share the same commendable corporate philosophies ... an obsession with temperature ... a commitment to ongoing improvement ... and so on. But there may be a critical difference in their basic approach to the production process. Let me explain. Analyzing the various forces that impact on the moment of casting, the truly successful die caster of today realizes that die-casting is, in fact, a holistic process. No single part of the casting production process operates in isolation. All components work together in common cause as a system. For the die caster, the real and immediate value of this approach is that it directly addresses the problem that no matter how well designed and how precisely produced any component of his production process may be, its real potential can never be achieved if it is interacting with another part that is less efficient. Three Critical Components The danger of exceeding a maximum allowable gap between the plunger tip and the shot sleeve has been understood by die casters for many years. The technology of both plunger tips and shot sleeves has improved appreciably in the interim. The parameters of the gap, however, remain unchanged. The interaction between the plunger tip and the shot sleeve is certainly the most critical in the die casting process.. It can only be most effective, however, if both components are operating at an equal level of efficiency. Also, an effective vacuum can only be achieved if the seal between plunger and shot sleeve remains intact at all times. If the seal is broken, an effective vacuum is obviously impossible. The challenge is to maintain a minimal gap without interference, while using the least amount of lubrication. Here we have the three critical components of a successful die casting production system – the plunger tip, the shot sleeve, and the vacuum system. For maximum productivity, the efficiency each must be approximately equal Maintaining the Gap Point one of a mm is the maximum allowable gap between the plunger tip and the shot sleeve during the casting process. If at any time during the shot, the gap exceeds point one, the alloy is likely to penetrate the space, and flash or blow-by will occur. This will inevitably cause excessive wear on both plunger and sleeve. It is essential, therefore, that a gap of less than point one of a mm be maintained at all times during the casting cycle. If the gap becomes much less than point one there is a danger of interference and inconsistent shot velocity. Scrap will result. The problem is that when metal is heated, it expands. Shot sleeves are getting bigger, but whatever the size of the sleeve, that critical maximum gap of point one of a mm unfortunately remains unchanged. Controlling the Plunger Tip Temperature Plunger tips were originally made of steel, primarily for their durability and economy. A steel tip, of course, has the same coefficient of thermal expansion as the shot sleeve in which it slides. Since the plunger tip is exposed to more heat than the sleeve, the expansion of a steel tip is difficult to control. The next step in the development of the conventional plunger tip was to make it of beryllium copper, which has a coefficient of thermal expansion more than 50% greater than that of steel. This made the expansion of the tip much easier to control. It was then possible to maintain the thermal, and therefore the dimensional, stability of the tip throughout the length of the stroke. There are some proprietary cooling-intensive plunger tips that utilize the cooling water much more effectively than conventional tips. The Evolution of a Plunger Tip The ARP plunger tip was developed a number of years ago by Allper of Switzerland. It has evolved over time in response to changing market demands, without compromising its original mandate of performing its function most effectively at the least cost to the die caster. Just a month ago, Allper became part of my company Castool Tooling Systems The body of the ARP is made of beryllium copper for its high coefficient of thermal expansion. With the ARP, a stainless steel tip holder is screwed onto the shot rod, and the copper tip is securely fastened to it with a quick release bayonet type connector. The front of the steel holder lies in full contact with the inside face of the plunger tip, and absorbs the total pressure of the shot. Beryllium copper is an ideal medium to dissipate heat from the plunger to the cooling water. It is, of course, not nearly as wear-resistant as the steel. Since the ARP tip is relatively dimensionally stable, and the gap controllable, this problem was resolved by the development of a replaceable steel wear ring. This tempered steel ring rests freely in a groove machined near the front of the plunger tip. It is split, and expands against the inside wall of the shot sleeve. Only the ring wears, not the copper body. The die end of the shot sleeve is chamfered to compress the ring and guide it back into the sleeve. Because the ring is flexible, it makes continuous contact with the inside of the shot sleeve. Flash, which is a major cause of wear, is essentially eliminated. Shot speeds are consistent. In addition, since the expanding wear ring ensures a secure seal between the plunger and the shot sleeve, a much better vacuum can be drawn. As only the long-lasting steel wear ring is replaced instead of the copper body, the cost of consumables is considerably reduced. With a conventional tip, failure of the body is from wear. Failure of the ARP body is only from thermal and pressure fatigue. Operating life is therefore many times longer than that of most conventional plunger tips. An additional advantage is that the face of the ARP is considerably cooler than that of other plunger tips. This cools the biscuit much faster, and can reduce the cycle time significantly. This does not compromise compression, because since the wear ring remains relatively hot, while the face of the tip is much cooler, the outside of the biscuit tends to remain liquid slightly longer, allowing better than usual compression. It is not uncommon for die casters in Japan to attempt to reduce cycle time by cooling the die end of the shot sleeve. This unfortunately tends to shrink the sleeve at the point where the plunger tip is hottest, and is likely at its greatest diameter. A New High Strength Plunger Tip The ARP plunger tip has been successfully and profitably used for a number of years, but the market for castings is changing. Castings are now being required, that are larger, more complex, and with closer tolerances than ever before. They also require a faster rate of production than previously thought possible. Plunger tips used to seldom be larger than 150 mm. Now, 200 mm tips and even larger, are not uncommon. The demands on the plunger tip, particularly in strength and stability, outgrew the ARP. This led to the development of the high strength AMP plunger tip. This high-strength modular plunger tip has been designed specifically for the production of large castings. The challenge Allper was to develop a plunger tip that would remain relatively stable so as to consistently maintain the required gap with the shot sleeve, but would also be strong enough to withstand extreme pressures. The AMP plunger tip is more dimensionally stable than the ARP, and features a replaceable high-strength steel head. Cycle times are reduced. Cost of consumables is reduced. Operating life of wear-rings and tip bodies can usually be estimated, and downtime for replacement scheduled, so production runs are not unexpectedly interrupted. The AMP high strength plunger tip responds to the demands of an increasing and changing market. It makes better castings. Cooling the Shot Sleeve Typically, a shot sleeve may vary from 100 to 150Cº from top to bottom, and front to back. If the temperature of the sleeve is much higher at the bottom than at the top, unequal expansion will cause it to become oval instead of round. This will also cause the sleeve to become slightly bowed rather than straight. Either of these conditions will cause premature wear of both tip and sleeve. The extent of distortion is directly related to both the diameter and length of the shot sleeve. To avoid too much variance in thermal expansion, the shot sleeve should be controlled so that the difference in temperature does not exceed 50Cº. If uncontrolled, the temperature variance in the shot sleeve will result in distortion, which may allow some of the alloy to enter the gap between the plunger and the sleeve. This will cause premature wear and inconsistent shot velocity. Porosity and Vacuum Vacuum-assisted die casting is essential to any die caster who wants to eliminate porosity…who wants to make larger, thinner, more complicated parts, and with less pressure - but only if it works well. When any new product or technology comes to market, it is often difficult to immediately use it effectively, especially if it is assumed that the system with which it works will support its use. This is precisely what occurred when vacuum was introduced in die casting several years ago. The die caster needed an extreme application to justify its use, because at that time, the vacuum valve required so much maintenance. The problem was compounded when the shot sleeve and plunger tip often did not work together well enough to maintain a secure seal. This, of course, resulted in air being pulled into the die cavity. This unfortunate situation was further worsened when the die caster introduced a heavy lubricant in an ill-advised attempt to seal the gap. This was constantly being pulled into the valve, and was often a reason for its failure. Understandably, the amount of downtime was uncommonly high. Just a few years ago, if the average die caster invested in a vacuum system, his downtime often offset his profit, and unless the requirements of his product were so stringent that they couldn't be satisfied without vacuum, he couldn't afford to use it. Today, nearly any product can be profitably made with Castool’s vacuum-assisted die casting system. We now have a valve that is stronger, has fewer moving parts and requires minimal maintenance. This much improved valve is now working very profitably, while supported by a thermally controlled shot sleeve and plunger tip combination that create a secure seal, using a minimal amount of a special benign lubricant. Epilogue The market for light metal die-castings continues to grow. Castings are becoming bigger, more complex, and better in every way. Technology is keeping pace with the changing market, but castings are still made by die casters, not just by machines, and the die caster’s basic approach to the production process is just now more important than ever. Every die cast production system can be improved. There are no exceptions to this rule. With its fantastic strength-to-weight ratio and unmatched recycleability, both automotive designers and conservationists vote for aluminium as “The material of tomorrow” This is indeed a wonderful time to be a master die caster.