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Cleaning dies and moulds is labour intensive and also often abrasive. By using ultrasonics for mould maintenance, the result is a cleaner mould with reduced manual input and no wear leading to
large cost savings, extended mould life, and shorter payback time. The technique suits plastic, rubber and die
cast moulds and tools.
Ultrasonic cleaning uses high-frequency sound waves
to remove mould residues and fouling from tooling and plates that are immersed in a heated (usually 80 degree C) 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 degree F at 7500 psi, which is powerful enough
to loosen contaminants without damaging critical edges or surface finishes.
Ultrasonic cleaning gives the following advantages:
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Short wash process leading to reduced time spent in cleaning |
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No manual labour required. An operator is not required when the machine is cleaning the mould |
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High levels of cleanliness give longer life in production |
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No abrasion of precise edges and surfaces gives longer mould life |
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No solvents needed because of aqueous process |
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Suitable for all moulds, large and small |
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Machinery is simple and easy to use |
The ultrasonic cleaning of molding equipment, such as injection molds and blow molds, will require the same three processing
stages as any other type product when cleaned with aqueous cleaning agents; the Ultrasonic Cleaning Stage,
where contaminants are removed from the product, the Rinsing Stage, where the detergent residues are removed
from the product, and the Drying Stage, where rinse water is removed from the product. Although the 3 main
stages are the same, the ultrasonic cleaner must be properly configured with additional equipment to clean these items successfully.
Most molding equipment is composed of a number of different metals, which may include steel, stainless steel, brass,
aluminum, and exotic metals. Therefore, the cleaning fluid selected must be multi-metal compatible. This makes the
cleaning of these items a bit more difficult, since most cleaning agents which are compatible with aluminum are rather
gentle, and can not loosen the contamination as well as cleaners which are more aggressive. Since steel is typically
included, the ultrasonic cleaner must also include a Rust Preventative stage after the final rinse.
The cleaning ability of ultrasonic depends on several key factors, of which the cleaning solution is the most important
variable. The type of detergent required is dependent upon the process and type of resin, which leaves behind specific types
of off-gassing residue. Highly alkaline solutions such as sodium hydroxide do the best job of removing stubborn contaminants,
light rust and heavy grease on most tooling and plates, with no harm to most plating and coatings. On the downside- it can
be caustic, posing a potential health risk, and may require neutralization before disposal.
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.
Another difficulty involves the complex design of the molding equipment being cleaned. Most molds include a large
number of internal channels which are used to release gasses during the molding process. In order the effectively
clean inside of these holes, these areas must be filled with liquid during the ultrasonic cleaning process.
This may require that parts be submerged in a particular orientation, or perhaps the item must be rotated once
submerged to fill these areas with cleaning agent, and allow the ultrasonic cleaner to effectively cavitate inside
of these critical areas. However, these areas must also drain properly prior to entering the rinse cycle and the
drying cycle, or the dirt which has been loosened during the ultrasonic cleaning stage will remain trapped within the part.
Most molding equipment is also very heavy and dense. These items tend to absorb a significant amount of ultrasonic
energy, and the ultrasonic cleaner must include a higher power level than an ultrasonic cleaner designed to remove
oils from sheetmetal parts. The ultrasonic cleaner must also include the proper ultrasonic frequencies to provide
the fastest and most consistent cleaning results.
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