“Failing to plan” or “planning to fail?”
Click here for print version
(This article was also published in the June 2002 issue of
“Die Casting Management” magazine.)
Throughout the die casting industry, it is customary for the die cast operators to keep a putty knife handy to scrape flash off the die faces. Many operators even keep them in their hip pockets, for convenience. Sometimes, I have found it almost impossible to even find a putty knife in the possession of an operator.
While flash may be a fact of life of our process, it is not acceptable in the levels we often tolerate.
Some misconceptions concerning flash:
1. It helps vent the die. Wrong!, it allows uncontrolled loss of cavity pressure. It increases the thickness of the casting, the gates, and the overflows adding weight that is not in the quote.
2. It doesn't cost anything. Wrong!, Flash is only 20% recoverable, if that. On a recent automotive casting for example, we "spotted the die" to correct the flash problem. The machine had an auto ladle, so it poured the same amount of metal after the repair as it did before. However when we started back up, the biscuit was 1 inch longer than it was before we "spotted the die". We were running a 3 1/2" tip. Therefore, we were loosing 9.621 cubic inches of metal every shot. That is .9621 lbs. of metal every shot! The casting was scheduled to run 8,000 pieces. /wk. at 4 cavity (which we were seldom able to do during that period, because of the problems caused by flash.) 8,000 pcs./4 cavity = 2,000 shots X 50 weeks =100,000 shots/year. X .9621 lbs. /shot =96,210 lbs./ year X $.75/lb. metal = $72,157 /year in metal X .80 (80% lost metal) = $57,726 net loss /year due to flash on this single job.
A die that flashes .025 inch with a projected area of 94 square inches like this automotive casting, has 3.4 cubic inches of excess material. That is .34 lbs. per shot excess material. Let's say that the die runs 100,000 shots per year like this casting. That is an extra 34,000 lbs. of metal which we pay for and is not included in the quote. 34,000 lbs. X $.75/lb. =$25,500 /year!
Trim thickness can also be a major cause of quality problems. If a slide flashes, such as happened frequently on another automotive component, it can result in trim shear. If die repair relieves the trim die while the die is flashing, then when the slide blow condition is corrected, there will be excessive trim burr remaining.
In addition, scrap generated from lost cavity pressure results in internal porosity that is not visible from the surface. Generally, it is revealed when the casting is machined. There may be other processes prior to machining which add additional cost to the casting. These might include such operations as vibratory finish, as for the another automotive valve, shot blasting as performed on a variety of castings, or painting as done to other castings, etc. The painting on some castings nearly doubles the cost of the casting to that point! Machining occurs as a final operation. At this time 2/3 of the selling price is in the outside operations of paint and machining.
If the defects reach the customer before they are detected, the cost of correction is further increased. We must pay travel costs, plus wages for sorting or rework in a customers facility. Travel cost alone to a nearby customer can run at least $250. Time away from the die casting plant to address the problem could be a minimum of 1 1/2 days. In addition, there are corrective action meetings to explain the cause of the defect, and to describe procedures for prevention in the future. The meetings add additional cost to the defective castings.
3. "I can't afford to fix it". See above. The direct cost to correct the flashing problem described above, was one person, one shift. $15./hr X 8 hrs. =$120. "If you can't afford to fix it, you can't afford to run it".
Other problems resulting from flash:
1. On dies with slides, flash accumulates under the slides, keeps the slide from fully seating, and results in dimensional problems. Example, on an automotive control valve. When the slide is backed out, the valve seat has excessive machine stock, and we give away metal. The cam lock holds the die open allowing additional flash, which accelerates the accumulation of flash under the slide.
2. Flash accumulation under a slide can lead to compound damage on the die. One example of this is broken horn pins. This can also damage both the slide carrier and the key ways.
3. Safety: We have all experienced the discomfort of being burned by flying flash. In most cases, this is minor. However, worst case can result in lost time injuries and lost time.
4. Housekeeping: Much of the "trash" beneath the dies and on the floors around the die cast machines is flash. It becomes contaminated with die lube, tip lube, hydraulic fluid, die heater fluid, and water. This makes it virtually unusable as remelt. This is where a lot of the 80% number comes from in item 1. above.
5. Lost time from restarts: Accumulated over a 24 hour period, this can be a huge impact on quality, and productivity. Each restart can result in at least one and sometimes as many as three cold "start up" shots. They are either thrown out by the operator, which is the correct procedure, or later at machining after adding additional value to the casting.
If you need help solving flash problems and want to improve your bottom line profitability, contact us for on-site assistance.
Bob McClintic
February 28, 2000
Other technical articles on die casting by Bob McClintic
Return to Home page
E-mail Bob at: RMcClintic@DrDieCast.com
Copyright Bob McClintic and Associates: 2000, 2002
Copyright: McClintic and Associates, 1998 - 2003
Revised: February 8, 2003
(Published June 2003 issue, Die Casting Management” magazine)
The die casting clamping mechanism is a mechanical marvel that multiplies force by approximately 33::1 while providing a die stroke roughly equal to that of the closing cylinder. Despite the demands on the system, most clamping mechanisms make millions of cycles without failure.
Success is not an accident. In order to produce die castings with quality consistent enough to approach PPM levels, it is necessary to have a method to maintain the entire clamp system. It is a key component of process control.
Components of a clamp mechanism:
- Foundation and levelers
- Tie bars
- Tie bar adjusting nuts
- Die height mechanism
- Platens
- Moving platen shoes or guides, also known as traveling plate and rails.
- Toggle linkage
- Cross head
- Closing cylinder
- Hydraulic system
What are some of the common causes of failure?
- A level machine begins with a solid floor/machine pad:
- An un-level or weak floor that does not support the machine at all points will create stress points in the machine frame and clamp system.
- This will accelerate machine wear and can cause operating problems in the die.
- Frequent hydraulic leaks from cracked hydraulic lines are common.
- Leveling pads may compress over a period of time.
- Recheck machine level annually to insure that the machine is not stressed and remains within tolerance of 0.002 in/foot. (0.0508 mm/0.305 meters)1
- Tie bar has end play.
- Cover end: The tie bar should be secure and have no end play. If end play is allowed the tie bar nut will "coin" both the nut and the platen each cycle. If this is only occurring on one tie bar, eventually the reduction of pre-load on this tie bar will result in an uneven stress on all the tie bars. This can cause flashing dies, and failure of the other tie bars. As the tie bar nut becomes coined it will be difficult or impossible to remove or adjust. In addition, coining the hole through the platen can reduce the hole diameter making the tie bar difficult to remove. Damage to the platen is permanent and requires extensive welding and/or machining to repair.
- An "end bell" is used on some cover end tie bars. A pre-load is essential to prevent end play. Machining or welding may be required to eliminate end play and prevent future wear.
- Adjustable end: The tie bar adjusting nut covers should be secure and proper operating clearance maintained.
- Loose covers:
- This is often the sign of a more serious problem. It is a common practice to loosen the covers in order to free a locked up die height system. The cause of the bind is overlooked and the covers remain loose further accelerating the wear.
- The most frequent reason covers are allowed to operate loose is an out of square machine caused by worn linkage.
- The failure mode is similar to the cover end. If allowed to operate loose for extended periods, the gears, nuts and platen will become coined and require machining or replacement of the tie bar and nuts and welding and machining to repair the damaged platen.
- Tie bar bushings:
- Tie bar bushings provide both lateral and vertical alignment. They are not, however intended to support the weight of either the platen or the ejector die. That is the function of platen shoes/guides.
- Regular lubrication is essential.
- See platen shoe/guides below for additional information.
- Moving platen shoes/guides:
- The platen shoes and wear plates must support the platen and ejector half of the die. Die carriers are recommended for larger dies 600 ton and above. This will reduce the wear on the tie bar bushings and guide pins in the die.
- Adjust the shoes as required to maintain platen support.
- Replace rail or wear strip if necessary.
- Regular lubrication is essential.
- Platens:
- Platens should provide a smooth parallel surface to support the die.
- Surfaces that are coined or not structurally sound can allow the die to flex during the fast shot, impact and intensification.
- Short term solutions include welding and hand grinding the surface (if small enough area) to create a flat supporting area. For larger surfaces see below.
- Machine the platen to re-qualify the surface using portable milling equipment. (For more on portable machining see next months issue.)
- Longer term solutions include tear down, removal, welding and Blanchard grinding.
- Toggle linkage:
- The toggle linkage uses mechanical advantage to develop the lock up force by placing the linkage, platens and die in compression while elongating the tie bars.
- Worn pins and bushings result in excess clearance and uneven lock up. Wear accelerates as the clearance increases, further coining them with each cycle.
- Problems with uneven lock up/out of parallel:
- Flash (wasted metal)
- Inconsistent casting thickness
- Slide blow (flash) and stuck slides.
- Safety issues from flash
- Die damage from above.
- Damaged die guide or leader pins.
- Frequent interruptions to clean up flash.
- In addition to placing a relatively equal force on the entire surface of the die, it also must maintain parallelism while opening the dies.
- Problems from out of parallel opening.
- Broken or shearing link bolts on Harvill machines
- Drags on castings
- Broken or damaged core pins
- Castings sticking to the cover half of the die.
- Galled die guide or leader pins
- Cracked castings from out of parallel ejection.
- Toggle systems live on lubrication. A constant supply of clean lubricant is essential for reliable service.
- Maintenance keys:
- Routine check and maintain the level in the automatic toggle lubrication system.
- Daily, weekly and monthly checks of the lubrication hoses, lines and metering blocks.
- Cross head:
- The cross head and bushings maintain the linkage alignment. Depending on the manufacturer of your machine, the cross head bearing surfaces could vary greatly in design. Regardless of design, the cross head must maintain its center throughout the lock up in order to develop force on all four corners of the platens.
- Four corner lock up designs such as B&T, Prince and Quantum have a center bearing.
- Other linkage systems use stationery guide bars attached to the bumper or knock out plate on one end and the adjustable platen on the other. Examples include Birch, Cleveland, Ajay/Dejay, National/Avnet and Kux.
- Moving cross head guide bars are attached to the moving platen and move through bushings in the adjustable platens. Examples include Harvell, HPM and UBE.
- Worn cross head components also create excess side loads on closing cylinder rod bushings and seals. This results in frequent blown seals, lost fluid and more down time.
- Other problems created by worn cross head components include "over-locking". In this situation, the main linkage is pushed past center and must tighten during the opening or "un-lock" portion of the cycle. In extreme cases, the cylinder may not have enough force to open the die and is stuck with a casting in the closed position.
- Cross head maintenance and lubrication requirements are similar to the toggle linkage.
- Closing cylinder and hydraulic system:
- The closing cylinder and hydraulic system do not affect balance, but they do affect force and cycle time. Examples include:
- Closing cylinder piston rings/seals must be maintained to prevent blow-by. A defective piston seal can reduce the lock-up force by as much as 50%.
- An improperly adjusted hydraulic system can also reduce the locking force. An example would be a regenerative circuit that remains active also reduces the lock-up capability of the machine by 50%.
- Improperly adjusted pressure controls can drop off early resulting in reduced lock up force or excessive lock up time.
In summary, the maintenance of the mechanical system of the machine is essential to producing quality casting consistently year after year. While it does require effort and investment, the cost is minimal in comparison to the cost of neglect.
Notes:
1 Alofs, W., Carstens, J., "Mechanical Maintenance and Evaluation of Die Cast Machines," NADCA, 1987
Bob McClintic
May 28, 2003
Other technical articles on die casting by Bob McClintic
Return to Home page
E-mail Bob at: RMcClintic@DrDieCast.com
Copyright Bob McClintic and Associates: 2000, 2003
Copyright: McClintic and Associates, 1998 - 2003
Revised: November 24, 2003
It's an old adage, true even in the die casting industry. Where does it start? As I have stated in previous articles, it should start with product design and development. It also continues in the quoting stage. "What?" you say! Make the sales department responsible for the success of a casting project? Why not! Some castings are much easier to sell then they are to produce. Is there a simple solution? Success is more likely with a mutual commitment between the designer, purchaser and producer. Complex, engineered castings have forced the die casting industry to improve their machine, tooling, engineering and process capabilities in recent years. In many ways, we have not all adapted to the shift from castings resulting from collaboration between the customers design and manufacturing engineers and the die casting supplier. Commodity buyers have replaced technical purchasing specialists. Brokers have introduced communication layers between the supplier and the final user that can inhibit the engineer and sales from properly defining quality and functional requirements. This can result in disappointed customers and frustrated suppliers. Going "off-shore" for solutions only makes it more difficult.
What then does a good project look like?
- The designers will solicit design and manufacturing input from the casting suppliers during the concept stage of the design.
- Purchasing departments will select qualified suppliers based on "on-site" visits and surveys. Qualifications will be based on:
- Quality
- On-time performance
- Engineering support
- Price
- A record of successful project management
- When the quote arrives the sales department will solicit input from the key departments including:
- Quality
- Manufacturing
i.
Die Casting
ii.
Machining/finishing
- Engineering
i.
Tooling
ii.
Process
- Finance
- If the above team deems the project as technically "high risk" it may be necessary to perform a flow and/or solidification simulation before completing the quote. Since this represents a significant financial commitment, this may be an item the customer would be willing to pay for.
- At the least, the quote procedure should include calculating the process parameters. This will often forecast the ability of the machine to successfully produce the castings. This would include the following parameters:
- Number of cavities
- Physical size of the die (mold) and fit in the machine(s) to be used.
- Cavity pressure
- Locking tonnage
- Cavity fill time
- Metal melting and holding capacity.
While the above steps are not a final guarantee of 100% success, they do greatly improve the company's chances for a successful build and launch.
For assistance with your project, contact us by one of the following methods.
The above article was first published in "Die Casting Management" magazine, October 2002.
Return to "Technical articles Index page".
Return to Home page
Contact information:
Bob McClintic and Associates
Grand Rapids, Michigan 49428 USA
Ph: 616-292-0454
Fax: 616-669-7884
E-mail: RMcClintic@DrDieCast.com
Copyright Bob McClintic and Associates: 1993 to 2003
Revised: February 8, 2003
