Archive for April, 2010
Castool Reduces the Number of Knock Offs
We are asked by many extruders and die casters to calculate the ROI for the purchase and installation of our tooling systems. If the ROI is less than one year, the system is usually purchased. In virtually all cases, the ROI is less than a year, and in most cases measured in weeks or months.
Extrusion Knock Off
A knock off is any die that goes to the press and is removed before completing the planned production run.
The typical 7 to 9″ extrusion press operates at $1,000 per hour. This includes packing and transport. Die costs are usually around $0.03 per pound. The cost of a knock off is typically thought to be $300. This neglects the impact of loss to customer service from failing to meet the expected delivery.
The argument is that if you are making $0.15 per pound profit, you need to extrude 2,000 pounds to make up for each and every die “knock Off”.
Press Delays and Billet Temperature
If the billet is at the chosen extrude temperature – say in the range 800-950F – any delay is of little concern as long as the temperature does not fall too low. Even within the above range there is some Mg2Si coarsening going on, but it is of little concern. Below this range the coarsening is more aggressive and there can be reason to start to worry.
Usually we’re not too concerned about some Mg2Si coarsening because it can readily redissolve during the actual deformation process (i.e. the extra extrusion heat generated as metal passes through the final deformation zone and then through the die).
Practically we can tolerate delays down to around 750F. But depending on how the delay may be and how much below 750F the billet temperature may be – it may be of concern. Most delays occur in the billet oven because (for example) the press might be down. Generally I recommend that delays below 750F for longer than 10 minutes require some corrective action.
The most practical corrective action is to reheat the billet above 900F and extrude. This ensures any coarse Mg2Si has a good chance to get back into solution. To reheat to 900F may be higher than the normal billet temperature for a given die, but if it is necessary to extrude slower until all affected billets are discharged from the billet heater, then it has to be, ’cause the risk of mechanical property failure is real.
These temperature limits vary with alloy and tend to be of more concern with the harder alloys such as 6061.
Billet Lubrication / Quench
They will be some reduction in billet surface inflow when using a taper heated billet or cooler dummy block.
The extent was quantified in Chris Jowett’s ET2000 paper on Flow of Billet Surface.
The % reduction in coring type inflow was influenced by dummy block temperature (or billet – dummy block temp), (billet – container) temp, difference between billet and container diameters (or amount of upset), the thickness of segregated shell on the billet, and whether or not the block is lubricated.
The singular contribution/effect of each of these variables can be assessed from the formulae in Chris’s report.
The two that matter in terms of dummy block and temperature are:
1. Dummy block temperature – if “x”oF below the billet, introduction coring will be delayed by 0.005 x “x”%. for example if 120oF cooler than billet, expect 0.6% saving on butt length or 0.6% later onset of coring. The same argument would apply with a colder back end on the billet.
2. Dummy block or billet lubrication – if lubricated, onset of coring is delayed by a further 0.85%.
This may not appear to be much of a gain, but I believe it’s quite significant. An almost 1.5% saving by just keeping the dummy block temperature cooler (or the back end of the billet cooler) can be a worthy gain -especially for hard alloy 6061 extrusions where an extruder is currently having to run long butt lengths up to 15% to avoid coring.
Interestingly the report shows that reducing the segregated shell depth in the billet by 1mm delays onset of coring by 6.5%. And reducing the upset by 1mm is worth another 0.55%. We can naturally assume the largest gain will come from billet surface skin thickness, but it’s interesting to see the benefit in reducing coring by working with a smaller upset – which also gives a big benefit in reducing part of the dead cycle – i.e. crush time.
Certainly, there are benefits related to reducing coring, or delaying its onset. But as big a gain if not more can be found by facilitating taper heating of the billet and improving the opportunity to reduce live cycle time – a significant factor in boosting productivity. We recently demonstrated on a 9″ press that a front to back taper of only -30oF on the billet resulted in a 7.5% increase in average extrusion speed. Imagine what 120oF can do!
This benefit applies to all extrusion, whereas reduced coring is only a benefit with the harder alloys used for structural or machining applications.
Castool’s new billet lubrication system uses “atomized mist” to break the thermal barrier around the back of the billet (also cooling the back end of the billet), and then applies a measured amount of liquid Boron Nitride.
High-performance individuals
1) They do not care about getting the credit for what they do. They just want to take a project to the next level. They have a driving need for achievement.
2) They insist on being in charge of their work. They cannot stand being stymied by corporate bureaucrats or having to beg for permission.
3) They want challenge and to be on the cutting edge all the time.
Radial and Thermal Temperature Gradients in the Die
- Any thermal gradients will also result in a 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.
- The problem I 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.
- 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.
- Finally I 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.
Transition
1) Until leaders can explain the change clearly, in a statement lasting less than one minute, they cannot get other people to buy into the change. Longer expectations and justifications will also have to be made, but this one minute statement will be the core of people’s undertsanding.
2) If the differences after the change is done cannot be spelled out, you need to tell people how those differences will be identified – by whom, and under what criteria – and when they will be explained.
3) You must also address the transition – the psychological reorientation that people must make for the change to be successful. Even when transitions that come from good changes begin with losses, as the old way is given up by people are used to it.
Is your company ready to rebound from the recession?
1) To continually identify gaps in the market, you need real-time data, the ability to share it with your company, and the wisdom to supplement that data with direct observations in the field.
2) Are the mechanisms that management uses to translate corporate objectives into aligned actions by individuals across the organization working?
3) Do you reward mediocrity and call it teamwork? Reward the individuals who do what they say they are going to do.
4) Management should breathe life into the company culture by hiring and promoting individuals on the basis of the adherence to values.
5) Is management making sense of volatile situations; deciding what to do, not to do, or stop doing; soliciting and monitoring commitments by others to deliver; making corrections in mid-course?
6) Is management attacking when they see an unprotected spot, and retreating when they realize that they cannot win?
7) Over the long haul, managers must build up your organization’s strength in a disciplined way, not dash from crisis to crisis.
Trust your brand.
We have already worried about all the details.
We do it day after day, so you don’t have to!
If a product is made in a certain way, there is a reason!
It works; use it and profit from it.
Castool is the first to tell you that perfection does not exist, but we strive on the thought to one day be able to attain it.
As a reality-check, could you please advise what practices regarding minimum temperature for the 1st billet and minimum die temperature do you use in your operations ?
I think we are guilty of trying to over-elaborate an answer to what in essence is a question with a fairly simple solution.
As we all know the secret of successful extrusion is temperature management and establishment and maintenance of consistent temperature conditions.
Ideally we preheat the die to the billet temperature, which for 6XXX commodity extrusion will be 860F – that is, an 860F billet temperature will allow extrusion without undue acceleration delay.
We should also be extruding in the preferred extrusion ratio range of around 40-80.
The reason some extruders choose to use higher billet temperatures or higher die temperatures to start up, is either they do not have sufficient temperature management or they are attempting to extrude at too high extrusion ratio – or both.
In an ideal world, to achieve near perfect temperature management and consistency, it could be argued we should preheat bolsters in ALL cases – to avoid that initial chill when a fresh die ring assembly is transferred from a correctly preheated environment and then sat next to a cold bolster in the die slide. But in reality we can get away with it for most dies except in some cases where dies are defined as “hard to push” – probably a difficult hollow. In these instances it is preferable to preheat a bolster to around 500F to reduce any heat loss and avoid problems pushing and the need to preheat initial billets any higher than 860F.
A billet preheat temperature of greater than 860F should only be considered if, when in steady state extrusion there is an acceleration delay of greater than 3 to 4 secs. After starting at 860F the billet temp 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 (for example 1050-1100F for 6063). These practices should allow ram speeds of no less than 26 ipm for any 6063 alloy extrusion.
Of course all this presumes the following:
Reliable billet preheat and a capacity to achieve 860F at the required billets/hr throughput.
Reliable die preheat (Castool single cell ovens).
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 extrusion at 26 ipm ram speed or above, we must be able to satisfactorily cool the extrusion to achieve both the required mechanical properties and the required dimensions without unacceptable distortion.
