Archive for December, 2009
Why do cold dies run lighter than properly heated dies?
1 Thermal gradients will result in stress gradients, be they radial or axial. The patterns of these stress gradients can best be determined only by modeling. However it is safe to presume that such stresses may well contribute toward premature failure of delicate features on dies, and thus is another reason why we should at all times avoid using colder dies.
2 The problem we see is not so much what is the mechanism causing thinner extrusions when using a cold die (although of course it is always preferable to better understand the consequences of their use), but more so what loss in product quality control we suffer when there is insufficient control of die preheat.
3 Today we should not be preheating dies for 18 hours or more in a chest -type die oven in an attempt to ensure an acceptable die temperature – because in reality we know dies preheated in such a manner rarely achieve the set point temperature, nor an acceptable temperature gradient throughout the tooling body. Therefore anyone preheating dies in this manner is likely suffering the consequences of cold dies. Whether the dies are loaded for 18 hours, 24 hours or more, it is now well understood that events such as repetitive opening and closing of chest oven lids and loading cold dies adjacent to hot dies, result in tooling preheat temperature variations that are unacceptable. The outcome is colder dies going to the press and the consequences are – thickness and shape control issues, poor extrusion surface finish due to die bearing oxidation, lower recovery, and lower net lbs/hr productivity. Preheating of single die ring assemblies to around 860F in single cell die ovens is the proven way to go. Bolsters are best preheated to around 550F, a process that can be achieved safely in a chest type oven.
4 Finally we recommend you also check your bolsters for condition (surface damage on the mating face, hardness) and also flatness to ensure full support is being provided.
We will continue to share our responses to technical questions, as they arise.
The Time Is Now For Vacuum-Assisted Die Casting
Here is Paul’s full article in PDF: The Time Is Now For Vacuum-Assisted Die Casting
Excerpt from Die Casting Engineer Magazing – July 2009
The market is there. The improved technology is available. The time is now.
Just now, immediately following the global economic meltdown, we are entering a period of unprecedented and virtually unlimited opportunity for the aluminum die casting industry. Automakers throughout the world are urgently redesigning and retooling to produce smaller, lighter vehicles that will cost less and use less fuel. The fact that if the weight of a vehicle is reduced by 10%, the fuel consumption is reduced by 6-8% means that the strength-to-weight ratio of steel, plastic and aluminum for every component of these new models is now being carefully compared. This will inevitably result in a much increased demand for die cast aluminum product. An additional factor is also involved in this equation.
A critical but as yet seldom articulated factor in the future use of aluminum products is the influence of “Generation Jones.” This is a term used to describe the generation of people born between 1954 and 1965, right after the well-known post-war “Baby Boomers” who dominated our culture for so long. Just now, Generation Jones, agest 45 – 55, has a commanding presence in both industry and politics.
Today, nearly any product can be profitable made with a vacuum-assisted die casting system.
More than a quarter of all adults in North America and Western Europe are Jonesers. To date, they have been a largely anonymous generation, but with some consistent and clearly defined characteristics. For the die cast industry, the most important of these is a strong sense of responsibility for the protection of our global environment. With regard to the use of aluminum in the automotive sector, for example, to a Joneser the fact that a lighter car using less fuel produces less greenhouse gas emmission is almost as important as its reduced cost of operation. He will also be attracted by the recyclable potential of the aluminum product. All of this influences the Joneser’s support of anything that will increase the profitable use of aluminum.
The best time ever for die casters to improve their productivity and profit is now. The opportunity is imediate.
Promoting Vacuum Again?
The use of vacuum is not new to die casters. It was first introduced a number of years ago. To date, the results have been rather inconsistent.
Every die caster knows the theory and advantages of vacuum-assisted casting. 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. Porosity causes more rejected castings than anything else.
The real cost of rejected products is actually very high and, more often than not, underestimated by the die caster. the total value of the machine time that is lost while producing the rejected product is never recovered. It should be calculated as the selling price of good product made in an equal period, less only scrap recovery.
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 a rejection. Porosity also affects the mechanical properties of the product. In structural applications, it 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 always be immediately apparent. If discovered after subsequent secondary processing, customer-dissatisfaction can be extreme. The only solution is vacuum assistance.
Before the injection shot occurs, a vacuum is drawn in both the shot sleeve and the mold cavity. The vaccum is maintained until the injection cycle is completed. Almost all 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 be guaranteed.
Only If It Works Well…
There’s the rub. 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 create 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 or grease in an ill-advised attempt to seal the gap. This was constantly being pulled into the valve, and was often the 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 requirement of his product were so stringent that they couldn’t be satisfied without vacuum, he couldn’t afford it.
Today, nearly any product can be profitably made with a vacuum-assisted die casting system. We now have a valve that is stronger, has fewer moving parts and required minimal maintenance. Previously, most vaccum valves required maintenance after about 3,000 to 6,000 shots. With today’s valve, a die caster can expect 20,000 to 40,000 shots or more before maintenance is required.
This much improved valve is now working very profitable, while supported by a thermally controlled shot sleve and plunger tip combination that create a secure seal, using a minimal amount of a special benign lubricant.
Resetting the Die Cast Industry
Our economy has been reset. That is a fact. We can’t change it. We must accept it and manage to operate within the parameters of this newly reset business environment for an indefinite period.
How can the die casting industry best and more profitable reset itself?
The short answer has to be – by better die casting.
A huge and unexpected opportunity in a single market sector has developed almost overnight, and at the same time almost all other sectors are in sharp decline. Competition has suddenly become intense. Some die casting plants are already closed.
Vacuum-assisted die casting was introduced primarily to enable die casting to meet the demands of automakers and their Tier One vendors. Some die casters used it well and became very successful. Others didn’t. The reason most often was that they didn’t realize that the technology of the vacuum assist is so unforgiving. A die cast shot end system using vacuum either works very well or it is worse than useless.
Just now, the temperatures of both the plunger tip and the shot sleeve are being controlled better than ever before. Their interaction is improved and the integrity of the seal ensured. Also, the best vaccum valves now are indefinitely more relable than those of only a few years ago.
Yesterday, the choice was cheaper or better. This is no longer a choice. Today the product must be cheaper and better. It is fair to assume that in today’s business climate, most die casters who do not embrace the vacuum assist will possibly fail.
The market is there. The improved technology is available. The time is now.
We need more new ideas
In business circles just now, it‘s fashionable to be slightly downbeat . . to be a little pessimistic about the short-term and mid-term future. Especially if you did not anticipate the collapse and were burned by it. But if optimism is national, it is, of course better than pessimism. Optimism breeds consumer confidence, business investment, entrepreneurial risk taking, and job creation.
It is also true that depression helps to remove any misunderstandings, misguided goals, and the false optimism that have fuelled much goal frustration.
Are we really doing the right things?
Anything can happen.
Thomas Edison wrote, We do not know a millionth of 1% about anything.
A brief digression about quotations. Almost anything said by an outstanding scientist, for example, can be taken from context and used to reinforce an argument. The assumption being that if Thomas Edison actually said this, it must have some deep hidden meaning, usually unknown to the common man. On the other hand, when the old ballplayer Yogi Berra succinctly said, ”It aint over ‘till it’s over”, there could be no doubt about its intended meaning.
The idea is not to get into locked into the current reality, but to have strategies that can make you win, no matter what happens. We’re looking for win-win situations, And they exist. The truth is that the economy is never really as good as it looks in a boom, and never as bad as it appears to be in a recession.
Oliver Wyman made the comment, Everyone was running downhill for 15 years. Now we’ll see who the real athletes are.
We have to tell people what we’re going to do, and what we’re not going to do. The last thing we want is for people to be walking around, not knowing what’s going to happen. The companies that had to make large layoffs in the past year were usually mismanaged.
Manufacturing today, especially in North America and Western Europe, relies on ideas. The demand for ideas is increasing. Everyone knows that if you keep on doing the same thing the same way, you’ll keep on getting the same results. But if you can dream it, you can build it.
There is one thing we can be sure of, just reducing spending, and waiting for the economy to recover, will not be enough this time.
Trust me.
Why reline your container?
We reline containers when the occurrence of scrap increases from blisters, pick-up and die lines, and unscheduled down time increases from frequent dummy blocks changes and seal face cleaning.
I will briefly outline why this happens and how Castool’s QR container can extend liner life considerably.
- In use, a container is subjected to stresses from a number of sources:
- Applied pressure during extrusion
- Shrink fit of liner in container body
- Thermal stresses from the temperature gradient both radial and longitudinal, in the container body
- In time the container body will distort from these stresses – bulging/barrel effect
- The liner will distort as the container body distorts and no longer provides ideal support, resulting in:
- The designed clearance between dummy block and container liner is exceeded
- Increased alloy skin thickness is allowed to accumulate on the liner bore
- Increased risk of blister
- The sealing surface of the liner will degrade over time due to high compressive sealing stresses between the liner face and the extrusion die or die ring, resulting in:
- Increased risk of flares
- More build up on the container or die sealing surfaces = increase in pick up, die lines and blister
- Attempts to clear flares with propane torches or chisels further damage the sealing surface.
- Recent design improvements that extend liner/container life are:
- Improved container seal geometry – modeling capability at Castool
- Improved temperature control with Castool’s multi-zone QR containers and elements located closer to the liner equals less thermal gradient in the container body and less thermal stresses.
- Improved Castool dummy block designs, better able to accommodate liner diameter changes due to the combined thermal and extrusion deformation stresses.
In normal operation Castool’s QR containers are achieving:
1. Liner life in excess of 1 year
2. Sealing face useful life (no flares) equal to liner life
3. Dummy block life in excess of 1 month
4. Mantle useful life (retention of hardness and minimum thermal fatigue) in excess of 5 years
5. Element life in excess of 2 years
6. Reduced scrap due to blisters, die lines, pick up, run out, dimensions and shape.
7. Reduced scheduled and unscheduled downtime.
8. And of course, today we must always consider the carbon footprint; energy consumption is regularly reduced by 50% or more.
What does customer service mean?
Consistently giving your customer more than he asks for. For example, at Castool, we not only provide outstanding products, we also provide technical advice, and help him to use our products to best advantage in conjunction with the other components of his production system. We help him to become a better, and more profitable, extruder (die caster). I’ve never met a customer that we couldn’t help in some way, nor one that didn’t appreciate our help.
How do you rate?
When a customer wants a product, he usually wants it as soon as possible. For most companies the priority, therefore, is usually to fill the order as quickly as it can. Our delivery time is always in line with the industry standard, but we realize that on time delivery every time is really much more important to our customers than fast delivery.
Few things are more frustrating to a customer than late delivery of a tool that he is expecting. Late delivery usually results from unscheduled downtime, caused by scrap being made at some stage in production. At Castool, scrap has been virtually eliminated. In the unlikely event that a late delivery does occur for any reason, a problem with the carrier for example, an immediate investigation is held to determine the cause. Steps are then taken to prevent lateness from occurring again from the same, or a similar cause. All details are of course fully documented.
Aside from deliveries that are on time – every time, we are dedicated to helping our customers to become better extruders and better die casters. For some time, we have been vigorously promoting a new approach to their production process.
Too often elements of the production process are evaluated individually, with little or no regard for the influence of other components.
No single part of the extrusion or die casting process operates in isolation. All components must function together as a system, and at a high level of efficiency, before maximum productivity can be approached. This is the essence of the Systems Approach to production that is now being advocated by Castool. The technology of Castool products is outstanding, but their effectiveness can best be maximized if several operate together as a minisystem.
This Systems Approach is, of course reflected in our selling strategy.
The relationship between extruders and die casters and their tooling suppliers began to change radically about 15 years ago. Prior to that time, the buyer enjoyed Relationship Selling by his suppliers. He bought from his friends. Both buyer and seller made an acceptable profit. Then after many years of growth the North American economy began to level off, and in an attempt to reduce their cost of production, buyers began to search for the lowest price for process tooling components on an individual basis. Commodity Pricing began.
Systems Selling, now being very successfully employed by Castool, is a logical and profitable successor to Commodity Pricing. The Plant Manager is encouraged to use the Systems Approach to his production process and measure the results. Many examples of its effectiveness in improving productivity can now be provided. When he decides to use several complementary components from Castool, a substantial increase in productivity can be assured. The Plant Manager also then has the benefit of undivided responsibility plus technical assistance.
The Purchasing Manager may have little interest in the technical benefits of the Systems Approach, but he usually does have a budget for the amount spent in the tooling category, likely based on past experience. When several of its tooling components are employed, we can guarantee an appreciable reduction in the total amount of money spent in this category.
This Systems Selling technique has enabled us to negotiate contracts with a number of very large multi-plant customers to provide them with several of our products at all their plants.
Die Casting, and Two Useful Laws of Physics
A law of physics can be defined as the mathematical relationship between measurable quantities that describe the physical state and properties of bodies. This is the fundamental concept.
Knowledge, by definition, must be factual. In the context of light metal die casting, most useful knowledge is based on the laws of physics. These rules were created empirically, over time. They are immutable. They cannot be changed.
In the die casting process, for example, much of the critical interaction between the plunger tip and the shot sleeve depends on the following two basic laws:
“The coefficient of thermal expansion is the fractional change in length of a material for a unit change in temperature.”
If you know the coefficient of thermal expansion for the materials you are using for both the plunger tip and the shot sleeve, for example, and the amount of change in temperature (ΔT), the amount of expansion of each can be accurately calculated and therefore predicted.
“The coefficient of thermal conductivity is the rate at which heat is transferred through a unit cross-sectional area of a material when a temperature gradient exists perpendicular to the area.”
If you know the coefficient of thermal conductivity for the material of the shot sleeve, by using finite element analysis the changing flow of temperature throughout the shot sleeve during the casting cycle can be accurately determined.
The results of each of these two laws are predictable, measurable, and precise.
Redefining Risk
The Gullies, Big Sky Montana
Just a few months ago four men were seriously gored during the traditional running of the bulls in Pamplona, Spain.
But the risk of serious injury failed to keep hundreds more from running with the 1,300-pound (600-kilogram) behemoths the very next day during the city’s famous annual festival.
What it is that drives some to embrace extreme risks, while the rest of us settle for the safety of the sidelines?
Not everyone has the mental makeup to excel in dangerous pursuits. It takes a certain kind of person. Most of us hit a natural ceiling that limits our appetite for extreme risk and, as a result, our ability to perform well in dangerous conditions.
But others have a much higher tolerance, if not an actual craving, for risk. For example, a top U.S. downhill ski racer spends the summer off-season racing in motocross competitions. He enjoys the challenge and the risk.
The high element of risk makes you feel alive, tests what you are made of and how far you can take yourself.
I’m not looking for danger. I’d be a fool if I was. I ski downhill for the challenge, my heart thumping as I finish, the feeling of really being alive. I’ve definitely been scared on some courses. It just makes me try harder. The more turns the course has the better. That’s when I do best.
The fear that drives many people away from the risks of extreme sports may be the same ingredient that keeps others coming back for more.
Having skied for more than 40 years, I no longer push to the extremes as I once did—but the feeling is still vivid.
I can remember getting into situations where I thought that at any moment I could be killed. I’m not particularly religious, but I would think, “Oh God, don’t let me be killed here. I’ll never do this again.”
But we’d get back down, and when we were safe we’d say, “Man was that great!” You soon forget how scary it was, and you go back again.
When Risk Becomes Real
As your expertise grows, so do the stakes. In a sport where skiers make lines down steep faces the height of a five-story building, the consequences of a mistake can be serious.
As I started doing steeper runs, I was drawn by the fear factor. There are definitely moments when you’re up there doing a new line and it seems like the stupidest thing in the world. But overcoming that fear is just the coolest feeling in the world. Doing something that you know most people wouldn’t do is likely part of it.
I was forced to stop when risk became reality—I broke both tibia and fibula of my left leg during a much too dangerous run.
How did the injury change my outlook on risk?
As an injured athlete coming back, generally … my initial reaction was to stop and reduce the risk a bit. I’ve had to change my mind set a little now.
I’m moving up to a steep decline that was natural before the injury, but now there is a fear of pain, injury, and even the fear of not being able to do it like I could before. Your body seems to remember how to do these things. But your mind sometimes gets in the way.
Redefining Risk
We redefine risk according to our skill, experience, and environment.
I’ve met several people that have climbed Everest. To me that just seems like the ultimate risk. But the climbers take every possible precaution. To them it was the next step in a dangerous activity that they’ve enjoyed for years. They weren’t going out there to get hurt.
The perspective of extreme athletes is very different from our own. Most people look at a risky situation and know that if they were in that situation they would be out of control. But from the athletes’ perspective, they have a lot of control. There are many things they can do to minimize the actual risk.
Often other apparently “dangerous” activities are statistically not nearly as risky as outsiders assume.
The Zone
Another key aspect of risk perception may be something referred to as “the flow” or “the zone.” It is a state in which many athletes describe becoming absorbed in pursuits that focus the mind completely on the present.
Something that makes you begin road racing, perhaps, is that your adrenaline flows and you become very concentrated on what you’re doing. After it’s over there’s exhilaration. You wouldn’t have that same feeling if the risk hadn’t been there.
People of different skill levels experience “flow” at different times. As a result, some may always be driven to adventures that others consider extreme or foolhardy.
Why are our products successful?
We’re not just selling products to the extrusion and die cast industries, we’re very much part of the industries, and have been for about 25 years. The Castool slogan is “For Better Extrusion” and “For Better Die Casting”. That is our overall philosophy. And along with a sincere commitment to ongoing improvement, it’s reflected in the products and service we provide.
We know that there’s a fairly wide gap between the productivity of some major companies that we call the Superextruders (Superdiecasters), and that of the average extruder (diecaster). Most extruders (die casters) mistakenly assume that this somehow results from economies of scale. It doesn’t. These people are really better extruders or diecasters. They understand the process thoroughly, and operate at close to maximum efficiency at every step in their production. They also realize the real value of Castool products, and usually buy from us.
Much of our time is spent educating average extruders (die casters) . . showing them how to make better use of the production equipment they already have, as well demonstrating the advantages of Castool products. Teaching them to become better extruders (diecasters). This approach is very successful. I’m often amazed at how much the productivity of some plants can very quickly be improved simply by going back to basics, considering the production process as a system with all the components complementing each other and working together in a common cause, and measuring everything. We tell our clients that anything that can be measured can be improved. Usually this is true.
Robotic Die Expediter (die scheduling and heating system)
Why are our products successful? They’re as good as any in the world, and better than most.
Yes, There Still are Superextruders
In 1996 I wrote an article called, “Who are the Superextruders?” Who are the extruders whose average rate of productivity clearly exceeds that of almost all others who are making similar product on similar presses?
The answer, of course, was those few extruders for whom having the correct temperatures all the time, every time, was almost an obsession.
That article received more favourable attention than anything I had previously written, and its catch phrase, “Temperature, Temperature, Temperature!” is still often quoted today.
In my travels , visiting extrusion plants in all parts of the world, the variance in productivity between plants is quite apparent.. For example, am average extruder with an 8” press extruding 6063, produces about 3500 net lb per hour (1587 kg per hour) whereas a Superextruder usually makes about 5300 lb/hr (2404kg/hr), over 50%more.
The principal factor in successful extrusion is temperature establishment and management, and the maintenance of consistent temperature conditions. Ideally we preheat the die uniformly to the billet operating temperature which for 6XXX commodity extrusion will be 860ºF(460C). That is, an 860ºF(460C) billet temperature will allow extrusion without undue acceleration delay. We should also be extruding in the preferred extrusion ratio range of around 40 – 80.
Some extruders choose to use higher billet temperatures or higher die temperatures to start up, either because they have inadequate temperature management, or they are trying to extrude at too high an extrusion ratio, or both.
In a perfect world, to achieve near perfect temperature management and consistency, it could be argued that in every instance we should preheat bolsters to avoid the initial chill when a fresh die ring assembly is transferred from a correctly preheated environment and placed next to a cold bolster. Usually, however, we can get away with it for most dies. An exception is with dies known to be hard to push, such as some difficult hollow dies. In this instance, it is preferable to preheat a bolster to around 500ºF(226C) to reduce any heat loss and avoid problems pushing, and also the need to preheat initial billets any higher than 860ºF(460C).
A billet preheat temperature of greater than 860ºF(460C) should only be considered if, when in steady state extrusion, there is an acceleration delay of more than 3 – 4 secs. After starting at 860ºF(460C), the billet temperature should be adjusted to allow breakthrough at peak pressure without significant acceleration delay. The job can then be run at a speed that generates the ideal platen exit temperature for the alloy being used. For example 1050 – 1100ºF (476 – 500C) for 6063 alloy. These practices should allow ram speeds of no less than 26 ipm for any 6063 alloy extrusion.
Of course, all of this assumes the following: Reliable billet preheat and a capacity to achieve 860ºF (460C) at the required billets/hr throughput. Reliable die preheat (such as Castool single cell die heaters.). Adequate control of temperature losses in the die stack (Offset container heating – Castool QR technology). The die design is correct for the target ram speed.
This finally brings us to another aspect of thermal management – quenching.
If we can control the process temperatures to create acceptable product at a ram speed of 26 ipm or more, we must be able to satisfactorily cool the extrusion to achieve both the required mechanical properties and also the required profile, without unacceptable distortion.
Yes, there still are Superextruders who can do it all.
December Vacation
I am taking a much needed vacation for the next couple of weeks.
Happy Holidays and New Years to All,
I will BLOG again in the New Year,
Paul
