Quality & Cost Benefits Example
QUALITY IMPROVEMENTS AND COST REDUCTION RESULTS REPORTED THROUGH LONG-TERM
APPLICATION OF COMPUTERIZED MONITORING AND REAL-TIME INJECTION CONTROL TO A
660-TON CONVENTIONAL DIE CASTING MACHINE.
INTRODUCTION
Existing conventional (defined here as without CNC real-time closed-loop injection controls) die casting machines, especially those more than 6 years old, often do not have adequate dry shot speed, flexibility or consistency to meet today's higher quality standards. Without computer monitoring and CNC real-time closed-loop controlled injection velocity, with their ability to provide any desired shot profile (including rapid deceleration) on an accurate, consistent basis, quality varies as the shots vary, setups are difficult, and flash is a constant concern. The impact of the shot end's kinetic energy is absorbed by the metal, forcing the die caster to de-rate the machine's clamping capacity, shortening tool life, and often requiring tools to be frequently reconditioned which causes downtime and repair costs. Yet the capital cost of replacing these machines often exceeds prudent budgets.
Fortunately, technology is available to upgrade those "workhorses" to state-of-the-art performance at considerably lower cost. Even if the machines are in bad condition, the combined cost of refurbishing plus upgradation with comprehensive computer process monitoring and CNC real-time closed-loop injection control can be done at substantial savings. Following are a series of reports on the long-term results achieved by this cost-effective approach.
FIELD EXPERIENCE
A conventional 660D series die casting machine was equipped with a comprehensive computer process monitoring and real-time closed-loop injection control system in September 1997. The machine was upgraded to computer control in order to improve casting quality and reduce rejects of a crankcase housing and other parts. Another objective was to improve tool life through elimination of flash and high metal temperatures required by the limitations of the original injection system.
The machine originally had only manually adjusted valves to set the slow and fast
shot speeds. A timer was used to determine the changeover position from slow to
fast speeds, and the intensifier was initiated by the buildup of pressure in the
injection cylinder resulting from back pressure created at the point of die cavity
filling. The monitors used to record process variables, store setups, perform SPC,
and program the injection controls are shown here.
In addition, the monitor provided graphic profiles of the injection ram velocity and pressures versus position and time. Master profiles were stored for each part, and were superimposed over the "Last Shots" to make setups faster and easier. Actual graphic profiles of the machine's injection variables are shown in Figures 2-3.
The photo on the left shows the complete shot velocity profile, beginning with the
initial acceleration ramp to pour hole close velocity (phases 1 and 2), next to the
critical slow shot velocity of 41.2 cm/sec (phase 3), then to the cavity-filling
velocity of 392.9 cm/sec (phase 4), then to impact elimination deceleration (phase
5), then to squeeze velocity (phase 6) and finally intensification (phase 7). The
plots are versus position until cavity fill, and thereafter versus time. This method
of plotting, called "hybridplotting" by its inventor, provides the maximum
information on one computer screen or paper copy so all the important variables are
observed simultaneously. This method also is the easiest to interpret. On the same
graph are shown the cylinder piston head pressure, P1 and exhaust pressure, P2.
These can be distinguished by noting that at the beginning of the shot, P2 is higher
than P1, whereas after cavity fill, P2 drops to essentially zero, while P1 increases
with intensification. The left and right vertical cursors provide numeric readouts
for all plotted variables and the differences from the left and right positions for
each variable. In addition to velocity, position and time, up to eight pressures,
vacuums, squeeze pin strokes, or other signals can be plotted for both Master and
Last Shot profiles.
The cursors show the fast response of the real time control, only 8.5 milliseconds
to accelerate from the slow shot to fast shot speeds. This allowed accurate setting
of the transition positions, since all velocity phases were programmed in position,
rather than time. The short distance and time to accelerate permitted complete
filling of the cold chamber without turbulence, thereby minimizing porosity, but
accelerated the metal up to the correct gate velocity before reaching the gate, so
missfills were also eliminated. The exhaust pressure peak shown at the end of
cavity was the result of the real time control system decelerating the plunger just
before cavity fill to eliminate flash.
The next image shows the end of cavity fill in detail for the same shot as earlier.
In this case, P2 is shown along with pressure at the plunger tip, P4. Here, the
cursors are positioned to show the extremely fast deceleration in 3.4 milliseconds,
a tiny fraction of the total cavity fill time. Metal pressure appears to actually
dip below zero as the plunger is rapidly decelerated, while the exhaust pressure
climbs to 192 bar to absorb the kinetic energy of the entire shot system. Without
this deceleration ability, all of the peak pressure would have to be absorbed by
the die, which is the cause of flash. It was found that deceleration must take
place in a small fraction of the cavity fill time in order to eliminate flash
without extending overall fill time, else surface finish and other quality
attributes dependent on a short fill time suffered. Moreover, deceleration to zero
velocity prevented intensification, so a tightly controlled fast deceleration to a
controlled velocity (the "squeeze" velocity, phase 6) was essential to
ensure that porosity would be lessened instead of increased. The intensification
system was also modified and enhanced to obtain the means to control it independently
of cavity fill back pressure. The graph shows P4 increasing at the right end of the
graph, controlled by any combination of metal pressure buildup, position and/or time
which resulted in the best castings.
The modifications to the original shot end are shown here. A manifold rated at 600 bar
containing the main servo velocity control valve was installed in series with the
existing telescopic exhaust tube. Changes were also made to the hydraulic circuit which
supplied oil from the main accumulator to the shot cylinder. The purpose of this was to
increase the dry shot speed of the shot end by approximately 34% to take full advantage
of the low impact capability, and permit the casting of larger parts.
CASE STUDY 1: HIGH QUANTITY STRUCTURAL AUTOMOTIVE CASTING
The castings on the right show high quantity production crankshaft housings weighing
3.66 Kg made using the actual shot profiles described above, courtesy of Bajaj
Electricals, Matchwel Unit. Scrap rates before upgradation averaged 7%, and required a
metal holding furnace temperature of 704 degrees C. After upgradation, cold flow was
eliminated and surface finish improved markedly. The die had been used and had suffered
flashing for some time, yet flash was substantially reduced on all castings, and completely
absent on many. Metal temperature was lowered initially by 45 degrees, and blow holes and
porosity in a machined boss which had previously caused many rejects was substantially
reduced. The elimination of impact permitted the use of much higher injection speeds,
improved surface finish and metal properties, and also allowed the reduction of metal
temperature.
Prem Singh, Manager of Maintenance, reports the following long term results:
- The Tymac SuperShot III was retrofitted on a 660D machine in September 1997. (See Figures 1 and 4). The system has been working satisfactorily since then.
- There have been no major failures.
- Special care needs to be taken to filter and prevent dust and contamination of the oil to ensure smooth operation.
- Rejection due to blow holes, pin holes and bearing seat for Bajaj Auto Crankcase reduced to 2% and 1% for continuous runs.
- Intensifier pressure can be adjusted as high as 700 bars, and fast shot speed is almost double that of other 660D machines.
- As all injection phases are driven by accumulated pressurized oil, the machine functions consistently even if the metal temperature is as low as 635 to 650 degrees at the holding furnace.
- Load testing by Bajaj Auto has passed up to 2100 Kg.
- Flashing was reduced even on old dies due to the low impact and precise computer control of all velocity phases and intensification timing.
- The monitoring resolution is 800 counts per inch.
- The monitoring system (see Figure 1) provides many facilities and controls, such as interfacing with auto ladle, auto extractor, hydraulic oil temperature, pressure, and correction of drive and velocity are done automatically during each shot.
- The monitoring screen displays all parameters on each shot, which can be seen and compared to the performance of each phase of the injection shot for the current shot and stored master shots.
CASE STUDY 2: LOW QUANTITY PLATED CONSUMER CASTING
A vacuum cleaner nozzle previously had presented quality control challenges due to the high standard for surface finish. In order to obtain the required finish, fill time had to be very short. Consequently, fill velocity had to be very high, which caused considerable flashing. In turn the flashing allowed more molten metal into the die, which increased the weight of the castings, and increased the heat input into the die. The increased weight was undesirable, and increased the cost of the parts. The increased heat input forced the chill time to be extended which decreased the number of shots per hour, also driving up costs. A complete CNC Real-Time Closed-Loop shot end was installed on a 400-ton machine to solve these problems and similar problems encountered on numerous short run jobs by this custom job shop die caster.
The photo to the left shows two consecutive shots under the identical operating
conditions with the sole exception that the impact elimination deceleration ramp was
turned off for the shot exhibiting substantial flash. This short run job shop also
reported that a major advantage is the system's ability to store computerized setups
and know that the first shot of each run is accurate, which reduced their previous 25
start up shots to between 1 and 3. Another advantage was that the machine could now
produce parts much larger than normal because the rated locking tonnage, without
de-rating reduction, can be fully utilized based on metal pressure times projected
area. The following table shows the results obtained from the fast fill/impact
elimination combination.
| VACUUM CLEANER NOZZLE, PLATED | Conventional | Real-Time CNC Shot Controlled |
| Cycle Rate per Hour | 51 | 68 |
| Expected Die Life | 100,000 | 115,000 |
| Melting/Holding Cost | 0.060 US$ | 0.054 US$ |
| Metal Cost | 0.60 US$ | 0.60 US$ |
| Scrap Rate | 10.0% | 1.0% |
| Total Part Cost | 1.74 US% | 1.29 US$ |
| Parts Shipped, Annualized | 91,980 | 173,547 |
CASE STUDY 3: HIGH QUANTITY POWDER COATED AUTOMOTIVE CASTING
A leading Eastern U.S. company specializing in high quality zinc automotive,
computer, and other die cast components manufactured thin wall side view automotive
mirror brackets requiring buffing, followed by powder coating and baking.
The following statistics were reported by executives of Sullivan Die Casting: A
conventional 650-ton hot chamber machine was originally operated at 106 shots per
hour, while scrap levels averaged 18%. The machine was dedicated to the production
of these parts on a 2-3 shift basis. Defects were caused primarily by poor surface
and porosity which caused blisters during the baking process. It was also necessary
to maintain an additional plant to buff the castings. After retrofitting a SuperShot
system to the machine, scrap was reduced to 4.5% and approximately 75% of the buffing
labor was eliminated. This permitted the consolidation of the scaled down buffing
operation into the die casting plant. In addition, the rapid deceleration provided
by the SuperShot within 0.2 inches of the cavity filling process, resulted in a
substantial reduction of fill time by allowing higher fill velocities. Previously,
fill velocities had to be reduced due to unacceptable flashing. However, with the
SuperShot, flash decreased from 0.012 inches to .002 inches. This generated cost
savings, as part weight decreased by 6.6% from 1.52 to 1.42 pounds. The combination
of reduced part weight and faster fill times resulted in substantially less heat
input to the die, and permitted production rates to increase to 142 shots per
hour.
| Automotive Side View Mirror Bracket | Conventional | Real-Time CNC Shot Controlled |
| Cycle Rate per Hour | 106 | 142 |
| Tooling Maintenance | $38,000-$48,000 / year | $5000 / year |
| Metal Temperature, C | 426 | 396 |
| Flash Thickness, inches | 0.012 | 0.002 |
| Scrap Rate | 18.0% | 4.5% |
| Part Weight, pounds | 1.52 | 1.42 |
| Metal Savings / year | 40,100 | |
| Parts Shipped, Annualized | 257,040 | 401,122 |
Other benefits included die life extended by 50%, and savings on tool repair previously caused by unavoidable flash. Before the SuperShot installation, the die required refitting and bluing every 6-8 weeks at a cost of $5-6,000 each time, averaging $38-48,000 annually. After SuperShot installation, tools required maintenance only once a year at an average annual cost of $5,000. As Table II shows, the combination of improved quality and faster cycle time resulted in an increase in parts shipped from 257,040 to 401,122, for a 56% increase. Material savings resulted from reduced flash amounted to approximately 40,100 pounds annually.
Additional savings that have not been calculated, but are substantial, include reduced downtime due to decreased tool maintenance, energy savings due to a higher percentage of acceptable shots, reduced startup shots, reduced re-melt material losses and energy costs due to less re-melt and lower runner, overflow, and flash weights due to the flash reduction. Substantial energy savings also were realized as furnace holding temperature was reduced from 426 to 396 degrees C. The largest savings came from the dramatic increase in the number of good castings shipped per year, and the consequential reduction of all the fixed costs amortized over shipments. The effective plant capacity was increased substantially without adding machines, operating personnel utilities or floor space.
CASE STUDY 4: HIGH PRESSURE HYDRAULIC COMPONENTS
A prominent Midwestern custom die casting company obtained substantial scrap reductions and quality improvements were by retrofitting Tymac SuperShot CNC real-time closed-loop control systems to several older die casting machines. These included a 600-ton Lester cold chamber die casting machine of late 1970's vintage and a 1978 650-ton B&T. Both machines were also equipped with Tymac Intensimax fast response intensifier systems. In the first application, the specifications for the barrel for a portable "nailgun" required close tolerance machining of the inside diameter followed by chrome plating and heat treating. Production rates ranged from 3,000 to 4,000 per month. Due to the draft required on the 7-inch long bore, between .100 and .125 inches of material had to be machined off, exposing the interior of the casting, where porosity is most difficult to eliminate. According to executives at Top, prior to installation of the SuperShot system, scrap was 80%. After it decreased to less than 2%. Meeting the leak requirements was particularly challenging due to the extreme pressure requirements, and the fact that any surface porosity exposed by the machining was expanded during the plating process. Porosity control was also essential because the parts required heat treating.
In a second application, a hydraulic adapter between the motor and pump of a
personnel lifting device (also known as a "cherry picker", see Figure
8) required 5,000 psi leak testing after machining and impregnation.
The previous supplier had experienced scrap levels as high as 80%, and an average
of 35% over a three year period. Top's Tymac MTU-9000 Central Computer was used to
analyze filling conditions and determine the optimum gating. The parts were then
made on a SuperShot-equipped 1978 650 ton B&T machine. In the first month of
operation a scrap level of 8% was achieved. In the second month after gating
changes were made in accordance with the MTU-9000 recommendations, scrap decreased
to 1%. Further improvements were made in the process settings, resulting in scrap
levels consistently below 0.4%. The die casting purchaser's confidence increased as
a result of the long term reliability of the process to the extent that today they
no longer required leak testing. This resulted in considerable savings, because they
were able to rely on the consistent process control provided by the SuperShot real
time closed loop control system, and the MTU-9000 Central Computer monitoring of 100%
of the parts produced. Top also reports that they successfully do 800-ton jobs on
their SuperShot-equipped 650-ton machines, due to the effectiveness of the low impact
deceleration.
CONCLUSIONS
Comprehensive computer process monitoring combined with upgrading the machines with CNC real-time closed-loop control provides proven substantial quality and cost improvements. In each case, proven results led to additional installations. As was the case with each of the companies with the long term experiences described in the case studies, based on the positive results achieved on the 660D at Bajaj Electricals Matchwel unit, the decision was taken to install a complete new shot end equipped with a CNC real-time closed-loop shot monitoring and control system similar to the system retrofitted to the 660D on an 1100-ton die casting machine in their Chakan unit. This new shot end was designed from the beginning for shot control. Dry shot speed exceeds 10 meters per second due to the high-efficiency, low-inertia design. Figure 9 shows a similar complete shot end. Its entire injection process will be monitored and computer numeric controlled instead of via valves, timers and limit switches adjusted by hand.
The additional power, free of impact constraints, will increase the effective
capacity of the machine to the range of 1300 to 1500 tons, allowing the production
of larger parts than would otherwise be possible at lower cost, while maintaining
world-class quality and faster production rates. The process variables for each
shot will be compared to tolerances, and stored, keeping track of the process
integrity of every part, and will provide automatic warnings of any out of tolerance
process condition. In this way, defects are prevented, substantially increasing the
number of good parts actually produced per hour's production, the key to long term
success. These examples of long-term success in modernizing and upgrading older
machines show a proven path to realizing world-class quality and productivity levels
at lower capital investment levels by saving the major portion of previous investments
in die casting machines.