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Before and after shots show how well ultrasonic vibrations remove grease and molding residues with no abrasion and no need for operator attention. (Photos: Blue Wave Ultrasonics)
Is there a “perfect” cleaning system for molds and tooling that would allow a significant reduction in labor hours and tooling damage while providing consistent cleaning results? If it exists, it would meet these 10 criteria:
One cleaning method that comes close to meeting all of the above requirements is ultrasonic immersion cleaning. Nothing has more impact on reducing cleaning time, improving mold-cleaning consistency, and reducing tooling damage than this non-abrasive, user-friendly method.
Ultrasonic cleaning uses high-frequency sound waves to remove mold residues and fouling from tooling and plates that are immersed in a heated (usually 160 to 180 F) aqueous solution. An ultrasonic generator converts standard line-frequency current (60 or 50 Hz) into high-frequency electrical energy (20,000 Hz or more). The generator is connected to small transducers that are mounted to the bottom or sides of a wash tank. These transducers vibrate at ultrasonic frequencies (20 kHz and above) when the current passes through. This causes the bottom or sides of the tank to vibrate like the diaphragm in a speaker, which creates microscopic bubbles that form and collapse in the tank (a phenomenon known as cavitation), which scrub and loosen vent residues and grime. The energy released from these microscopic scrubbing bubbles produces forces at 10,000 F at 7500 psi, which is powerful enough to loosen contaminants without damaging critical edges or surface finishes.
The cleaning ability of ultrasonics depends on several key factors:
Mild alkaline solutions also are available that perform well on many residues without the caustic issues. But in general, the friendlier the detergent, the less effective it will be on stubborn contaminants.
Slightly acidic solutions, such as those containing low concentrations of citric acid, excel at rust and oxide removal, but must be used with caution on some tool steels because they can react with the iron in the steel, turning it gray. This normally does not cause a technical problem but doesn’t sit well with toolmakers.
There also are several environmentally safe detergents being used that do a fair job of removing most contaminants left from resins in use today. Combined with a minimal amount of hand scrubbing on heavily contaminated areas, these user-friendly solutions are gaining popularity with companies simply because of their ease of disposal, less mess, and safer working conditions.
The higher the frequency rating, the less aggressive the cleaning power. Common systems use either 40 or 30 kHz and a few use 25 or 20 kHz. Thirty kHz is a good compromise between cleaning ability and noise level. Lower frequencies can be extremely loud and irritating.
The best transducers also are zero-spaced and silver-brazed to the tank rather than bolted or epoxy-glued, because the latter have more tendency to fail slowly over time. Transducers may be placed at specific locations in the tank to correlate with the size and configuration of plates being cleaned.
Purchase or build a hinged cover for the top of the wash tank to slow down the evaporation of 180º F water. Also fabricate a tall (about 4 ft) stainless backsplash shield that will run the length of your system and will keep your walls clean and direct any runoff back into the tanks.
When you fabricate or purchase tooling baskets, be sure the handles are tall enough to extend above the surface of the hot cleaning solution and incorporate an eyehook to lift with an overhead chain hoist. Design the baskets to fit side by side across the bottom and make sure the basket material perforations are 0.25 in. or larger so cavitation is not impeded.
In the real world, mold plates will get slammed into the sides of the wash tank and baskets full of tooling will be dropped in too fast. Exposed knobs will be ripped off. The tank will run 24/7 because everyone will suddenly have something that needs to be ultrasonically cleaned, as will be evident by the grass clippings and paint residue floating on the surface of the wash tank. And, due to the nature of ultrasonic operations—high heat and constant vibration, the system’s parts can shake loose, wear through, and burn out.
Heavy-duty, bullet-proof, robust, and solid are the words to keep in mind when shopping for an ultrasonic cleaning system that will perform reliably in a busy mold-repair shop. Unless absolutely necessary, stay away from bells and whistles such as automatic loaders, conveyors, dryers, and fancy cabinetry.
However, do make sure of the following:
To see if ultrasonics will work for you, send samples of contaminated tooling to an ultrasonic equipment supplier as a test. This also will help the supplier match your specific contaminants with the proper detergent for efficient removal. Some suppliers will even let you use a trial unit.
Once the system is installed, you will see an immediate reduction of labor hours simply because of the speed with which tooling will be cleaned compared with laborious hand cleaning of one piece at a time. As a bonus, there will be no more rounded-over edges or scrubbed-off platings and coatings.
Steven Johnson worked as a toolmaker for 26 years, rebuilding and repairing multicavity molds for Calmar Inc. and then as mold-maintenance engineer for Hospira Inc., a medical device manufacturer. Today, he is the maintenance systems manager for Progressive Components and has his own business, MoldTrax in Ashland, Ohio, which designs and sells software for managing mold maintenance (www.moldtrax.com). He can be reached at firstname.lastname@example.org or (419) 289-0281.
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