Archive for March, 2010
Is air cooling beneficial in containers?
Is air cooling beneficial in containers used during extrusion of aluminum alloys? This question has been debated for many years. Indeed, benefits of air cooling are questionable.
The objective of cooling by forcing air through spiral grooves at the interface between the mantle and liner, is to offset temperature increases within the container resulting from heat generated by the billet itself and heat of deformation when the billet is extruding. It has been argued that any temperature rise in the container will itself result in increases in temperature in the billet deformation zone prior to material passing through the die, and thus result in higher extrusion temperatures exiting the die. Without air cooling, to counter these temperature rises and avoid extrusion surface defects , the extruder will be forced to lower extrusion speed and extrude at lower productivity. At least that is the argument.
With conventional externally heated containers, this argument has some credibility and air cooling of containers may indeed offset increases in temperature, especially with high productivity extrusion of lean composition 6XXX aluminum alloys. However, residence time of a billet in a container during high productivity extrusion can be as short as 50-60s, and the influence of container temperatures on the billet is minimal – certainly in the early stages of the extrude cycle. However, there may be some benefit where the mass of a lower temperature container does help generate lower back end billet temperatures which may help approach isothermal extrusion conditions. It needs to be emphasized that this applies with conventional externally heated containers only, where the thermal gradient across the container thickness is influenced by both the externally applied heat and heat generated at the liner from successive extrusion cycles.
The thermal gradient across an externally heated container and the effect of the two heat sources aforementioned, can be detrimental to the service life of a container. First, externally applied heat has to create a high enough temperature (essentially a thermal head) on the external surface of the container to achieve the desired temperature at the liner. While the objective is to achieve container liner temperatures around 420-440oC, typically surface temperatures on the outside of the mantel can exceed the steels annealing temperature. The latter is supported by microstructural degradation of the mantel steel, heat crazing and loss in hardness, effects commonly observed with such container design. In addition, heat generated in the container bore from the billet and billet deformation sets up a counter-direction temperature gradient, that cycles with each pressure cycle and dead cycle. Even with thermocouples located close to the liner, thermal control is challenging with the primary heat source (the external elements) so remote from the control thermocouples, and the container mantel mass so significant.. Thermal cycling in the vicinity of the liner leads to thermal fatigue, and cyclical stresses patterns can result in cracking of liners and the mantel close to the liner. Thermocouple holes located close to the liner create stress concentrations that accentuate the effect. Machined grooves for air cooling are also stress raisers and contributors to premature container failure.
Therefore, while there is potentially some benefit in air cooling externally heated containers, the benefits are minimal and highly questionable. While it may be claimed air cooling provides a step toward isothermal extrusion, more significant gains can be achieved using taper heated billet.
More containers today have heating elements strategically located close to the liner – for example, the Castool QR container. By creating 4 separate heating zones in the mantel yet close to the liner, an extruder can apply a temperature offset between the set point temperatures for the top and bottom of the container, both at the die end and at the entry end of the container. These 4 zones, top, bottom, front and back, allow improved thermal management of the container, but more importantly improved thermal management of die.
A Castool QR container has elements applying heat close to the liner and without centre zone heating, specifically designed to create more uniform thermal gradients in the mantel. With thermocouples placed close to the liner, a QR container can better control the process with more rapid and improved heating response. Temperature gradients in the mantel are unidirectional, from inside out – either from the heating elements or from the billet in the container. Thermal fatigue effects are therefore virtually eliminated, and thermally generated stresses around stress raisers such as thermocouple holes are less. Heating elements located close to the liner considerably reduce the external temperature of the mantel, to the extent that external temperatures in the range of 225-250oC are common with QR containers.
Furthermore, the unidirectional and radial temperature gradient across the mantel negates any need to apply cooling to the liner, even in high productivity situations. Contrary to externally heated designs, the temperature gradients in a QR container allow for improved heat dissipation by conduction through the mantel wall without the need to introduce supplementary cooling.
In summary, a conventional externally heated container may benefit from air cooling, because opposing thermal gradients from the external heaters and from billet deformation, restrict efficient radial conduction through the liner and mantel. Stress concentrations at the cooling grooves may result in cracking and premature failure of the mantel or liner. A more efficient QR type container design benefits from a preferred radial temperature gradient encouraging conduction away from the liner into the container mantel. Air cooling is therefore not necessary. Reduced thermal fatigue effects with no unnecessary stress concentrations from cooling grooves, extend both mantel and liner life in a QR design container.
Bill Dixon
QED Extrusion Developments Inc.
3/9/10
Stepped Bolsters
Definition – the exit side of the bolster is more open (offset) from the entry side of the bolster. Typically the rear side of the bolster is offset .625″ from the entry side.
There are arguments supporting and arguing against the approach.
First I can support the idea – it makes sound sense especially for jobs prone to rub on a bolster. An experienced extruder or die designer can readily predict which jobs may benefit from a stepped bolster.
Having said that, and to argue against the concept of a stepped bolster, an improved die design with balanced flow and correct bearings, and improved temperature control of metal leaving the container into the die, should negate the need to create a solution to overcome an extrusion rubbing on a bolster. Also keeping bolster costs as low as possible by simplifying the “through hole” can only be a plus.
Finally, any single piece bolster whether stepped or not, is far superior to a “stepped” two piece bolster.
Stress Relieving Stems
The theoretical idea is as follows:
Subjecting tools to elevated service temperatures – especially in case of cyclic loading – creates thermally induced residual stresses which pile up cycle by cycle and superimpose with operational (mechanical) stresses. Residual stresses can be reduced by stress relieving in regular terms.
Theoretically stress relieving is the more effective, the higher the stress relieving temperature is. The upper limit is obviously the lowest tempering temperature of the heat treatment. In other words: If the stress relieving temperature exceeds the tempering temperature the tooling material undergoes a change in microstructure and the hardness of the tool is reduced.
With H13 at 45 HRC, your tempering temperature is in the range of 630 °C (to be confirmed by your heat treatment shop). In order to prevent loss of tool hardness due to excessive stress relieving one would keep the stress relieving temperature to a maximum of 50 °C below tempering temperature, i.e. : You could increase your stress relieving temperature to 560 – 580 °C in an attempt to reduce residual stresses more effectively. The same could be achieved by increasing the soaking time or increasing the stress relieving intervals.
Back Flow across the Container with Excessive Inverse Segregation
Billet surface layers (i.e. inverse segregation) flow into extrusions in two ways. First there is flow mechanism known as type 1 flow where billet surface flows FORWARD into the die entry ports or feeder at early stages during extrusion. This influences the surface of the extrusion and contributes to pick-up, die lines and anodizing defects. The second mechanism is known as Type 2 flow where the billet surface accumulates into the butt end of the billet during extrusion and flows into the INTERIOR of the extrusion – commonly called back end defect or coring.
Until recently the industry believed ALL billet surface flowed into the butt, but we now better understand type 1 flow and the effect it has on extrusion surface quality.
Please review the 2008 ET paper and let me know if you have any questions, or the content does not fully address your immediate problem
Extrusion Surface Effects Resulting from Billet Surface Inflow
William Dixon, QED Extrusion Developments Inc., San Diego, USA
More Lessons from the Olympics
Economic crisis? Market crash? Ah, no. The Olympics.
With the exception of a certain hockey series in 1972, there has likely never been a more consuming sports event in Canada.
It would be a mistake to assume that all those daytime hours spent watching the Olympic was a waste of time. The games are nothing if not BIG business and watching them can give you all sorts of useful wisdom that applies to the world of commerce.
A little humility can go a long way. Bode Miller, “the biggest bust of 2006 Games in Turin” came back at the ripe old age of 32, minus the hype and showboating, and captured 3 medals. Some American banks could learn a thing or two.
Those who can do something unique can enjoy success that is disproportionate to their size. In the medal count, Norway, which has a population of less than 5 million, stand ahead of much bigger countries. The reason is specialization.
The same goes for business. The best companies tend to share the same characteristics. They focus on a small number of things in which they are superior to anyone else.
In a niche market, nobody does it any better than Apple. Apple makes most of their money from 4 products.
You want to own a monopoly, not do business with the monopoly.
NBC will lose 250 million and CTV has not disclosed their financial projections. The IOC will not sweat. Nice work if you can get it.
Drivers for Success
1) Be recognized as the best or be forgotten. Be remarkable.
2) Focus on the fundamentals, concentrate on the core.
3) You must be uniquely rewarding and indepensible to those you deal with.
4) Don’t get hung up on plans. Instantaneity is the new expectation.
5) Make it and keep it simple.
6) Daring and imagination must replace money and manpower.
7) Magnify other people’s talents. Everything can only be achieved through other people.
Butt Length
For most commercial applications – 6063/6060 type alloys the simple answer is no. Butt thickness is generally driven by what can be satisfactorily sheared (or curled) on the shear blade and ejected.
However it is known that billet surface layers and oxides etc do start to flow into the inside of an extrusion toward the back end (i.e. coring). This tends to start at around 15% butt length – much larger than typically used in most extrusion. The inclusions however are all contained in the extrusion interior and are generally considered to be of no consequence – certainly they do not influence surface finish.
Of course there may be concern with such inclusions for structural applications and therefore for many 6061/6082 extrusions where the higher strength alloys means there are structural or load bearing considerations, the butt length is usually kept to around 15-16% of the original billet length.
Therefore two cases:
1. Non-structural applications (most 6063/6060 alloys) – as short as you can get away with.
2. Structural (most 6061/6082 alloys alloys) – 15-16% minimum is recommended to avoid coring type defects
The Paralympics, no longer second tier!
Amputees are now regularly removing healthy tissue to make room for more powerful technology. They are getting second amputations-moving their amputation up their leg-to get the prosthetic equivalent of a hotter car.
Orthopedic surgeons often consider amputation the equivalent of failure. They usually try to save as much of a damaged, injured, or diseased limb as possible. Now, the allure of machinery has become so powerful that amputees are routinely taking the extreme step to have what the industry calls revisions.
Prosthetic feet act like leaf springs on a truck-the bigger they are, the longer the lever arms, the more energy storage and return you get. With enough clearance, you can go from a walking foot to a higher performance running foot.
In 2008, Pistorius wanted to compete for a spot on the South African Olympic team. The IAAF banned him from competition because of his carbon-fiber Cheetah legs.
During the court proceedings the following were cited:
His foot is in contact with the ground 14% longer on each sprinting step than an able bodied sprinter’s (disadvantage), but he spends 34% less time in the air between steps and takes 21% less time to swing his legs between steps, and has a “metabolic cost of running” that is 17% lower (in favor).
Pistorious was not allowed to compete, but they found that he was approximately equal to the able bodied competition. BUT, this is only for the moment. He could only be a couple of upgrades away from being able to leave the best in the world in the dust.
The Paralympics, which are essentially second-tier Olympics-held after the “real” Olympics-will be the place for sports fans to go to watch people really going faster, higher, and stronger.
