by Steve Johnson, ToolingDocs
As noted in “The Cleaning Culture for Molds” (Plastics Business Fall 2012), the most popular method of cleaning plastic injection molds remains rooted in the antiquated process of scrubbing tooling and plates, one piece at a time. Ask why and the answer will be some form of “This is the way we have always done it,” “We cannot cost-justify other methods,” “Other methods do not clean as well” or “We’re too busy, and there’s no time to experiment or investigate other methods.”
These excuses cost companies thousands of dollars a day in wasted time, inconsistent results and worn tooling. This article addresses three examples of cleaning technologies that fit perfectly with specific aspects of cleaning typical molds and tooling components.
Ultrasonic Cleaning
Ultrasonic cleaning involves the use of high-frequency sound waves to clean mold residues and fouling from tooling and plates that are immersed in a heated (usually 160 to 180 degrees Farenheit) aqueous solution.
How It Works
An ultrasonic electric generator is used to convert standard line frequency current (60Hz or 50Hz) into high-frequency electrical energy (20,000Hz 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 (20kHz and above) when the current passes through, causing the bottom or sides of the tank to vibrate like the diaphragm in a speaker. This creates microscopic bubbles (a phenomenon known as cavitation) in the tank to scrub and loosen vent residue and grime. The energy released from these microscopic scrubbing bubbles produces forces at 10,000 degrees Farenheit at 7,500psi, which is powerful enough to loosen contaminates without damaging critical edges or surface finishes.
Key Considerations for an Ultrasonic Cleaning System
Cleaning Solution.The process and type of resin being run help determine the type of detergent required because of specific types of off-gassing residue that resins leave behind. Highly alkaline solutions – such as sodium hydroxide (considered “caustic” in raw form) – do the best job of removing stubborn contaminates, light rust and heavy grease on most tooling and plates, with no harm to most applied plating and coatings. 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 contaminates.
Slightly acidic solutions – such as those containing low concentrations of citric acid – excel at rust and oxide removal, but need to 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 problem, but doesn’t sit well with toolmakers.
Alkaline or acidic water-based solutions usually are completely biodegradable (environmentally friendly), but may require neutralization prior to disposal. This is an easy process and requires adding either acidity to an alkaline solution or alkalinity to an acidic solution. The detergent manufacturers will be able to provide information on how to properly neutralize their detergents.
There also are neutral pH-safe detergents being used that do a fair job 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 the ease of disposal, less mess and safer working conditions.
Ultrasonic Power and Frequency. Cleaning heavy mold plates and tooling requires the use of heavy-duty equipment and not your typical ultrasonic jewelry cleaner. The amount of power (wattage) required is dependent upon the tank size and the type of load to be cleaned. From my experience, a tank of 70 gallons (approx. 24″ x 30″ x 25″ deep), 3000 watts at 30kHz performs well on molds and plates up to 6,000 lbs.
Transducer Type and Construction. The heart of the system, transducers, come in two types: magnetostrictive and piezoelectric. Magnetostrictive transducers typically are more rugged and create the most aggressive cavitation action. Transducers may be specifically placed in the tank to correlate with the size and configuration of plates being cleaned.
Tank Design and Construction. Buy a 12-gauge stainless steel tank large enough to totally immerse the largest mold plates being worked with in the facility. Getting a smaller tank that will require someone to flip the mold plates over for total coverage just to save a few bucks up front is a move that soon will be regretted. Tanks can be found sized 10′ x 6′ x 4′ deep and even larger.
In the real world, mold plates will be 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 suddenly will have something that needs to be ultrasonically cleaned, as evidenced by the grass clippings and paint residue floating on the surface.
Heavy-duty, bullet-proof, robust, solid and big enough for the molds in use – all of these descriptions should apply when shopping for an ultrasonic system that will provide the value needed in a busy mold repair shop. Invest in all of these features before spending money on automatic loaders, washers, conveyors, dryers and fancy cabinetry – unless, of course, shop space and budget allow it!
CO2 Dry Ice Blasting
With the endless variety of resins, molds and processes being run today, there is no silver bullet for all types of mold fouling and corrosion. However, the dry ice cleaning system is extremely effective in performing certain duties that other systems do not accomplish.
Ice blasting is a non-abrasive cleaning method popular among many automotive rubber molders because of its ability to clean molds in the press while the mold is hot and without causing any secondary waste stream of its own accord. Injection molders that periodically clean their tools in the press will appreciate the speed at which dry ice will accomplish this.
When cleaning mold faces in the press, the mold tooling components are assembled at varying shut-off heights, forming nooks and crevices that are difficult to clean completely and without damage. Many times, the solvents used will migrate into the mold around the tooling and then leach out onto the part during production.
With ice blasting, solid carbon dioxide pellets (about the size of a grain of rice) or shavings (about the size of a grain of sugar) are introduced into an air stream and shot out at high velocities through a variety of state-of-the-art, aerodynamically designed nozzles to remove residue quickly and harmlessly from mold plates and tooling.
How It Works
Dry ice cleans using three criteria:
- Kinetic Effect. Ice pellets exit the nozzle tip at 900 feet per second and are responsible for most of the cleaning power. If less cleaning power is needed – for instance, when cleaning hot runner electrical boxes – simply dial down the air pressure, thus reducing pellet velocity. Ice pellets are soft (1.5-2.0 Mohs) as compared to other media noted here. Only talc (1.0 Mohs) is softer.
- Thermal Effect. Dry ice is cold (-111 degrees Fahrenheit), and when it strikes contaminants that are hot, like on a 400 degrees Fahrenheit mold face, the large delta-T between the two causes micro-cracks in the contaminant as it shrinks rapidly, thus breaking the bond with the mold substrate.
- Sublimation Effect (gas expansion). The force generated by the ice turning into gas also aids in contaminant removal.
Key Considerations for a Dry Ice Cleaning System
Non-abrasive. Ice blasting is non-abrasive to all tool steels and hardened aluminum, even over an extended period of time. Companies can ice-blast molds several times per shift with no damage to parting lines, applied plating or surface finishes. It works on textured and polished cavity surfaces.
Clean operation. Ice blasting creates no residual dust or waste stream through its own operation as all other media blasting units do. Air quality tests have been performed in cleanroom operations using ice blasting with no measurable effects from the sublimation of the carbon pellets. The residue blasted off the surface of the mold may collect over a period of weeks or months on surrounding equipment, but if it’s a problem, install air extraction hoods over the molds that generate heavy contamination.
Portability. Small, lightweight units now are available that can be pushed right up to a mold on the bench or in the press, which results in a considerable cost savings over bringing the mold to the cleaner. Keep in mind that an air supply will be needed close by (within 20 feet) to tap into. Standard line pressures of 70 – 90 psi with a 3/4-inch feed line will suffice.
Versatile. Ice blasting systems can be used to clean a variety of molding equipment other than molds. They are effective cleaning press screws, barrels, internal mixers and other equipment when not in use on the mold.
Low operational costs. Ice pellets for single line systems are about $0.20 cents a pound. It takes approximately 20 minutes to go through 30 pounds of ice pellets, which is about how long it takes to clean both halves of a plastic mold measuring 2’x3′. Plan on spending at least $25,000 for a single line portable unit and a couple of different nozzles.
Ease of use. It only takes a few minutes to familiarize a operator with hooking up the hoses and nozzles and where to load the pellets. Non-abrasiveness eliminates the fear of surface degradation if the nozzle is moved too slowly.
Mold safety. Mold plates can blow over and tooling launched from bores when proper precautions are not taken or the nozzle tip is misdirected. Plates must be securely braced or laying down. Tooling must be backed up or secured. Do not stack tooling in a basket and blast away.
Personnel safety. As hunting dads always say, “Watch where you point that thing, son.” This holds true in ice blasting. Flesh is no match for ice pellets moving at 900 feet per second. All necessary safety equipment must be worn, to include heavy-duty gloves, full face mask, long sleeves, etc.
Heavy rust/residue removal. The non-abrasive nature of ice blasting limits heavy stain and rust removal. Ice blasters also are a line-of-sight cleaning system, meaning operators can clean only what they can see.
Ice blasting excels as an in-press cleaning system, reducing cleaning time and eliminating premature tooling wear when compared to the typical solvent-soaked rag and abrasive pad “wipe down” method. It also works great to remove LSR (liquid silicone rubber) colorant plate-out and when cleaning tooling with textured and polished surfaces.
Plastic Media Blasting
There will come a time when ultrasonic cleaning systems and ice blasting just dont have the aggressiveness needed to remove the heavy contaminants, rust and plate-out (colorants) that some molds suffer. The typical approach is to use an aggressive (hard) media such as glass beads or sand in a standard blasting unit. It’s time to consider plastic. Most toolrooms are not aware that they can use plastic as a blasting/cleaning media, but plastic is softer, lasts longer, is cleaner to use and will not harm mold steels.
Friedrich Mohs developed a scale (Mohs) to measure hardness in minerals (media) from 1 (softest) to 10 (hardest) to determine what type of media is capable of scratching what type of surface. For instance, a fingernail is rated at 2.4, a copper penny is 3.2, window glass is 5.5 and a diamond is 10. There are literally dozens of different types of media used in blasting, so research thoroughly. Safely removing contaminants from molds requires an understanding of the hardness of the media to be sure it does not etch or erode the steel in any manner during cleaning.
The three most commonly used medias found in tool rooms and their hardness scales are as follows:
| Type | Mohs Hardness |
|---|---|
| Glass Beads | 5.5 – 6.0 |
| Silica Sand | 6.0 – 7.0 |
| Aluminum Oxide | 8.0 – 9.0 |
| Plastic Media | Mohs Hardness |
| Polyester Soft | 3.0 |
| Urea Medium | 3.5 |
| Acrylic Medium | 3.2 – 3.5 |
| Melamine Hard | 4.0 |
Not only is plastic media much softer (4 or less) than other media, but it also does not break down as fast, nor does it produce the “dust” that seems to cover everything within 20 feet of the blast unit. The polyester media also will not harm a polished surface on most tool steels, which makes it a good choice for removing heavy grime on cavity tooling and plates.
The Bottom Line
All three cleaning systems perform specific jobs with great speed, precision and results over hand cleaning. To help justify the ROI, track hand cleaning hours separately from other repairs made on molds. Technicians will see that 75 percent of their time will be spent on this stage of a typical mold repair, so why not consider investing in a more efficient system? In addition to the quick payback, there are major advantages over hand scrubbing in the form of increased speed, consistent results, eliminating tooling damage and – just as important – a happier and more productive group of repair technicians.
Steve Johnson is the operations manager for ToolingDocs, a provider of mold maintenance training and consultation based in Ashland, OH. He designed and developed MoldTrax™, a documentation software system for tracking mold performance and maintenance. To learn more, call 800.257.8369 or visit www.toolingdocs.com.
