At a Glance:
- The high-tech die casting industry relies on old-school steel-wool pad filters for vacuum systems that lead to unnecessary and long machine downtimes and high cold and warm scrap ratios.
- Dexwet Filters devised a unique method of filtration that protect vacuum valves and pumps from debris that is sucked off components in the die casting process.
In the production of modern and complex aluminum structural parts for the automotive industry, vacuum support systems for die casting units have come into common use over the past two decades. The larger, thinner-walled and more complicated the automotive parts—from powertrain to chassis parts or battery housings—the greater 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.
Many parts today 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 the first foundry in Germany 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 off.
The 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 get melted in again, 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 final 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 their 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 the technology leading vacuum system supplier Pfeiffer Vacuum.
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 die casting unit for 2-3 days without machine downtime, and the third and current version using Dexwet SUPER-APF filters nearly doubled its effectiveness.
Dexwet SUPER-APF normally gets exchanged once per week in the service shift and a maximum of one filter exchange during the week. Such preventive filter exchange is mandatorily dictated 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 die casting unit.
Why Old-Tech Filters Deteriorate Output Performance
The reason for fluctuations in output part quality caused by “old-tech” 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, particle speed hits 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 die casting 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 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.
ROI Within One Year
Implementing Dexwet SUPER-APF is a one-time investment with guaranteed ROI in less than one year. When the filter cartridges need to be exchanged, users simply remove the dirty filter cartridge and clean them at a central cleaning station, rotating the cartridges in the foundry between their three locations (one in the machine, one at the machine, one clean in the central cleaning station). This reduces waste, but much more importantly, allows users to gain valuable knowledge from the analysis of the filtered material or filter cake, which was not possible before.
One set of replacement filters is always provided in a transport box directly at the die casting unit for easy and swift filter exchange. In case the vacuum system gives an alarm for preventive filter exchange, the machinist can easily replace all filter cartridges 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 die casting unit can immediately resume production of good parts. This eliminates 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 was 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 provided, and should be brought back to the mount at die casting unit for the next exchange.
Filter Cake Analysis and Process Gains
The workers designated to the filter cleaning station play a very important role yielding the highest returns from using this technology. Since these filters are in operation for three days up to a whole shift 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. Such abnormalities must then be reported to the machinist, which facilitates 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 die casting 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 die casting unit while reducing energy and production time waste.
Real Costs of Low-Tech Steel-Wool Pad Technology is an Industry Blind Spot
The real costs of “old-tech” steel wool pad filters are the blind spot of the industry because the direct costs of these cheap filters were not recognized in the overall cost structure of a foundry. Technology leading companies are recognizing the benefits; Georg Fischer (GF+), for example, started using the technology early on in 2017 and has since implemented the filters in all of their foundries worldwide.
Offered as a Standard OEM Component
Dexwet Filters partners with Pfeiffer Vacuum, a global technology-leading vacuum system supplier and the only supplier to date to measure the advantages of the buildup of vacuum within the form using Dexwet SUPER-APF technology. Their 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, Pfeiffer Vacuum has since 2018 offered Dexwet SUPER-APF technology along with their leading-edge systems.
This article was submitted by Dexwet Filters. Clemens Sparowitz is the company's president and chief operating officer.