Pressure diecasting systems have become common in the production of complex aluminum parts for automotive structures. The larger, thinner-walled and more complicated the automotive parts—from powertrain to chassis parts or battery housings—the greater is the need to clear the tool form, not only from air and gases but also from other substances applied to the tool throughout the process. In principle, what does not exist in the form before the injection of the liquid aluminum, cannot end up as a deficiency of the part.
Today, many parts could not be produced without a deep vacuum, down to 10-20 mBar, to satisfy quality and acceptable scrap ratio. Dexwet Filters were contacted in 2017 by a German foundry to look at the identified filter problem caused by old-tech, steel wool pad filters that are applied in the vacuum suction line to protect the vacuum valves and pump from the debris that is sucked away.
Industry blind spot. As a common standard, these vacuum filters are exchanged preventively once per shift, with an average machine downtime of 30 minutes. With 20 shifts per week, this task leads to the loss of up to 10 hours of production time per week. Three warming shots are commonly done when the machine is resuming operation, leading to 60 “warm-scrap” parts, which will be remelted, causing a loss of overall production time and a waste of energy.
A deeper analysis of the occurrence of parts with quality deficiencies showed that the ratio of quality-deficient parts identified by quality control departments with X-ray increased in the second half of the shift, indicating that vacuum filter malfunction may be the root cause.
Solving the problem. Over three R&D stages, Dexwet Filters developed the aluminum-particle filter technology from APF1 to APF2, and the final industry solution SUPER-APF, which has been awarded OEM (Original Equipment Manufacturer) Part status for Pfeiffer Vacuum, a vacuum system supplier.
Dexwet SUPER-APF vacuum filters are permanent filters, with a stable filter inlet consisting of nine filter layers with a sophisticated and patented design. The first two APF versions allowed permanent operation of the diecasting unit for 2-3 days without machine downtime, and the third and current version using Dexwet SUPER-APF filters nearly doubled its effectiveness.
Normally, the Dexwet SUPER-APF will be exchanged once per week in the service shift and a maximum of one filter exchange during the week. Such preventive filter exchange is necessitated by the vacuum system itself and not by any general work direction, as standard nowadays, because such rules cannot take into account the specific conditions and emission behavior of each diecasting unit.
Old technology undermines output
The reason for fluctuations in output part quality caused by “old technology” steel-wool pad filters lies in the extreme conditions within the vacuum suction line when the vacuum valve opens. On the bottleneck of the suction line, particles hit the sonar wall at 671 mph. Air and gases have to be let through, while any oily particles and hot liquid aluminum particles have to be captured to protect the vacuum valve, the most critical component of any vacuum system.
When a vacuum valve fails, the diecasting units may fall out of operation for many days until replacement parts are supplied, which is a critical reason to have two filter housings and cartridges for each suction line.
When using steel pads as filters, and under extreme compression from being under pressure, in combination with the maximum possible particle speed, the orientation of the steel fibers of the pads changes from each shot to the next, resulting in fluctuations in the throughput behavior of the filters. As more debris gets collected by the form-side filter over the shift, the effectiveness of the vacuum system to always form the very same under-pressure conditions within the form is deteriorated, which causes quality fluctuations.
One-year ROI. Implementing Dexwet SUPER-APF is a one-time investment with a return-on-investment (ROI) guaranteed in less than one year. When the filter cartridges need to be exchanged, users simply remove the dirty filter cartridge and clean it at a central cleaning station, rotating the cartridges in the foundry between three locations (one in the machine, one at the machine, one clean in the central cleaning station.) This reduces waste, but much more important is that it allows users to gain valuable knowledge from the analysis of the filtered material or filter cake, which was not possible before.
Workflow optimization.One set of replacement filters is always provided in a transport box directly at the diecasting unit for easy and swift filter exchange. In case the vacuum system gives an alarm for preventive filter exchange, the machinist can replace all filter cartridges easily, with machine downtime within 5-15 min. The clean filters are taken from the box and the dirty ones are put back into the box.
This short downtime of less than 15 minutes avoids warming shots – and the diecasting unit can immediately resume production of good parts. This also will eliminate the necessity for warming shots and scraps due to vacuum filter exchanges during the whole shift week.
At the end of the shift, in case there has been a filter exchange alarm, the worker takes the transport box with the dirty filters back to the cleaning station, where a designated box with clean filters is always ready and should be brought back to the mount at the diecasting unit for the next exchange.
Filter cake analysis and process gains
The workers assigned to the filter cleaning station play a very important role, yielding the highest returns from using this technology. Because these filters are in operation for three days up to a whole week, the quantity and quality of the debris in the filter cake have to be looked at like a drilling core throughout its operation. This filter cake is a compound of aluminum particles with max. 0.5 mm particle size and various forms of oily or waxy particles coming from tool fats, separation liquids, silver fat, etc.
Within a few weeks of doing the filter cleaning, the worker designated for the job will learn how to differentiate normal filter cakes from abnormal or strange compositions. Then, such abnormalities must be reported to the machine operator, which allows early flaw detection in the tool and other system parameter calibrations and dosing.
Dexwet SUPER-APF technology has been designed to give valuable feedback to the machinist to optimize the overall effectiveness of each diecasting unit. With that, it helps to foster and improve process stability and security, and, by extension, helps to increase the overall weekly output of high-quality parts for each diecasting unit while reducing energy and production time waste.
Costs of low-tech filtration. The real costs of “old tech” steel wool pad filters are the blind spot of the industry because the direct costs of these inexpensive filters were not recognized in the overall cost structure of a diecasting operation. Technology-leading companies are recognizing the benefits; Georg Fischer, for example, started using the technology early on in 2017 and has since implemented the filters in all of their foundries worldwide.
Standard component. Dexwet Filters partners with Pfeiffer Vacuum, a global vacuum system supplier and the only supplier to date to measure the advantages of the build-up of vacuum within the form using Dexwet SUPER-APF technology. Pfeiffer Vacuum’s patented two-tank vacuum system allows for exact and accurate pressure measurement within the form throughout the microsecond injection cycle.
While the first tank sucks from the side of the injection chamber, the second cycle sets in the very same moment as the first closes, providing the full vacuum capacity of the second tank during the shot curve. Due to positive results, since 2018 Pfeiffer Vacuum has offered Dexwet SUPER-APF technology along with their leading-edge systems.
Clemens Sparowitz is the company's president and chief operating officer of Dexwet Filters.