Injection molds used to form plastic products can represent sizeable investments and are characterized by complex shapes, precision finishes and vents. Cleaning plastic injection molds must be accomplished on a regular basis and in such a way that removes all traces of burned on deposits from surfaces and residual plastic in vents without scratching or otherwise mechanically damaging mold surfaces.
“Our investment in injection molds can reach tens of thousands of dollars each,” says Roger Wheeler, tooling manager at PAM Injection Molding based in Kernersville, NC. “While different molds have different surface tolerances, customer-demanded tolerances for a mold surface can be as precise as 0.0002 inch. That’s why,” he says, “longer injection mold life depends highly on not only keeping them clean but conducting mold cleaning operations in such a way that extends their life by maintaining surface finish tolerances.”
Manually cleaning injection molds doesn’t make the grade. “Frankly there’s no way that manual cleaning, no matter how carefully applied, can access complex shapes, vents, dumps and other mold surface configurations,” Wheeler comments. “Moreover, manual methods such as brushing with solvents run the risk of damaging surfaces, thereby rendering the mold unusable. Our search for a better way led to adopting ultrasonic cleaning as the optimum way to extend injection mold life.”
Why Ultrasonic Cleaning Works Best
Ultrasonic cleaning uses a process called cavitation – the implosion of billions of microscopic air bubbles – against surfaces immersed in a cleaning solution. Bubbles are created by generator-powered transducers bonded to the underside of a stainless steel tank. When excited the transducers vibrate at ultrasonic frequencies measured in kilohertz (kHz) per second. This causes the tank bottom to act as a membrane creating the bubbles.
The implosions blast loose and carry away contaminants on any surface immersed in the cleaning solution. The process is fast, safe and does not damage products that can be safely immersed in a biodegradable water-based cleaning solution.
While indiscernible to the naked eye the size of these cavitation bubbles varies based on the ultrasonic frequency employed. Lower frequencies such as 25 kHz create (comparatively) larger vacuum bubbles that implode more vigorously against surfaces and are generally used for removing stubborn contaminants; higher frequencies such as 45 kHz create (comparatively) smaller bubbles that more easily penetrate cracks, crevices and machined holes.
Mold Cleaning Cycles at PAM Injection Molding
The Ultrasonic Cleaner
“We employ an Elmasonic xtra ST 600H ultrasonic cleaner for the bulk of our operations,” Wheeler explains. “It has a 15-gallon tank capacity with internal dimensions approximately 20 x 13 x 14 inches deep. This size is ideal for our operations, as most molds are constructed as interchangeable modules that can be configured to make multiple parts and can be disassembled for the cleaning cycle.”
Operating features of the xtra ST ultrasonic cleaner that make this unit particularly attractive at PAM Injection Molding enable users to select either a 25 or 45 kHz ultrasonic frequency based on the extent and tenaciousness of burned on plastic residues. “For really tough deposits our operators can activate what is called the “pulse mode” that sends a more powerful blast of cleaning action into the bath,” Wheeler says. “Pulse is also activated to degas fresh ultrasonic cleaning solutions in order to quickly remove cavitation-inhibiting trapped air.”
A continuous “sweep mode” provides a slight ± variation in ultrasonic frequency that serves to avoid harmonic distortions resulting in areas of high or no cavitation. Too much intensity can possibly damage highly finished injection mold surfaces.
In addition to frequency and pulse the operating panel allows operators to set cleaning cycle time and cleaning solution temperature. Set and actual parameters are displayed. The unit will start when the set temperature is reached and can operate continuously with auto shut-off at 12 hours or if the solution temperature reaches 90⁰C.
PAM selected an insulated tank lid to muffle noise during the cleaning cycle and to reduce cleaning solution evaporation.
The Injection Mold Cleaning Cycle
Biodegradable cleaning solution concentrates are formulated for specific cleaning tasks. In this case PAM Injection Molding uses the aptly named Omega Mold Release Agent concentrate with a recommended dilution rate to 10 to 1 with water.
As described by Wheeler the cleaning cycle is quite straightforward:
- After filling the cleaning tank half way with water the correct amount of concentrate is added then water is added to the fill line.
- The unit is turned on and the pulse mode activated to mix and degas the solution.
- Time and temperature parameters are set. At PAM a typical cycle is 30 minutes at a temperature of 50 to 60⁰C.
- Disassembled modules are carefully positioned in the basket, taking care that they are not in contact with each other.
- Lower the basket into the tank and close the lid. Cleaning starts automatically and will shut off at the end of the timed cycle.
At the end of the cycle parts are removed, rinsed, dried and inspected. If all is satisfactory the molds are put back in service.
Cleaning Solution Maintenance
Contaminants removed during the cleaning cycle accumulate in the solution, which eventually must be drained and replaced. Spent solution should be stored and disposed of following local regulations. At this time the tank should be cleaned following user manual instructions before being refilled with fresh solution.
A Closing Commentary on Cleaning Injection Molds
“There are side benefits to using ultrasonic cleaners such as the xtra ST for cleaning plastic injection molds,” Wheeler points out. “Once we establish a cleaning cycle routine, our personnel simply load components in the basket, set the cleaning parameters and are free to attend to other tasks. No longer do they have to spend time manually cleaning these intricate and costly molds. The ultrasonic equipment does the job in about 30 minutes average – a task that could take hours if done manually.”
About PAM Injection Molding
PAM dates from 1983 and in 2004 launched its custom injection molding division complete with a full-service tooling shop. An ISO-9001 certified company, it provides molding capacity ranges from 110 to 950 tons and utilizes 3D printing technology to help customers with any prototyping and part realization required. Engineering support is also available.
Contact the scientists at Tovatech for expert help on selecting and using ultrasonic cleaners and the correct cleaning solutions to add efficiency to your operations.