Cleaning a mold is a critical part of the repair process,but many myths must be dispelled.  

   By Steve Johnson

More time in mold repair is spent cleaning mold plates and tooling than in any of the other stages

(disassembly, troubleshooting or assembly) combined, for two reasons: (1) clean plates and tooling are

critical to maximize mold life and reliability and to produce a high-quality product; and (2) most molds are

cleaned completely by hand.

New (and old) cleaning equipment and mold design technology that could be used to drastically cut

cleaning hours and tooling damage is underused. The next articles in this series will discuss cleaning

attitudes, methods and myths, including how to justify the cost of buying new cleaning equipment. We will

also cover steel and plating improvements that will extend run cycles and clean molds easier and more



The Cleaning Culture

Mold cleaning is a process where major maintenance bottlenecks often occur because molds are pulled

faster than they can be cleaned and made production-ready. I have visited plants where molds waiting to

be cleaned line hallways and toolrooms, taking up valuable bench space.

Many times, in order to meet production demands, molds get reset dirty or the cleaning process is rushed, which subjects tooling to more damage through hurried handling. The question is asked, "Can we run it

the way it is, or does it really need to be cleaned?" In companies where firefighting is the accepted culture, the mold will be reset and started; if all the parts come out clean, it runs. Once this happens a few times,

management comes to assume that molds need to be cleaned only when the residue level is bad enough to migrate out onto the part, or until the mold locks (galls) up.

Some shops handle cleaning chores by enlisting non-skilled employees or toolroom apprentices to wash tooling and plates as quickly as the repair technician can take them apart. This practice works, unless the

product has critical flash, dimensional or aesthetic specifications, or the mold has a history of maintenance, reliability or quality issues. It is difficult for the repair technician to accurate troubleshoot mold and part defects when all the track marks are washed off from the tooling and plates.


Systemizing mold maintenance is based on establishing consistency in repairs required (performance)

and repairs performed (maintenance). Cleaning is another area where individual techniques (freelancing)

greatly affect mold reliability, part quality and the tooling budget. To be cost-effective, mold cleaning must

be performed:


At specified cycle frequencies

Using specific instructions for varying levels (in-press, wipe down, general, major)

After troubleshooting mold and part defects

After repairs have been made

After new tooling has been engraved with position number and has been installed


Instructions on how (and how frequently) cleaning must be performed should be determined by visual

inspection after a known number of cycles are run, looking for residue buildup in vented and non-vented

areas of tooling, plating wear and track marks. Supervisors should ask mold technicians for their input on

how many cycles molds can safely run. Observations concerning residue accumulation and wear should

be documented to underscore the significance of accurate observations and to ensure mold cleaning is

not taken for granted as a non-critical function, where expensive mold tooling is treated like rusted garden tools.

All molds should have in-press servicing procedures, including frequencies, and a maximum cycle count

set that is strictly adhered to. A number of areas are critical to reliable production, including the internal

grease level; the condition of gear racks, sliding cam blocks, internal pins and bushings, and other moving components; water line and bubbler contamination or blockage; manifold weepage; rust and corrosion from water leaks; or condensation. Excess grime can cause problems in many areas of a mold

that won't be first flagged by residue leaching out onto the part.


Cleaning is unpopular with many employees who prefer the challenge of troubleshooting or machining.

Cleaning is messy, sometimes monotonous, and can be a potential health risk. There can be prejudice in

a company culture that associates the job of cleaning with a lack of talent. On the flip side, other repair

techs live for the cleaning stage. It is an area where they can relax, crank up the radio and scrub tooling

for hours, while passing managers are impressed with the technician's diligence on the job.

However, a top-shelf toolmaker is usually not assigned to clean molds. This responsibility is often placed

in the hands of a technician who is not familiar with the mold-specific defects and function, or the critical

seal areas of the tooling. Doing so will often create continuous mold performance issues and inflate the

tooling budget through the addition of dings, burrs, rounded-over edges, premature plating or steel

removal and mixed up tooling. These problems, in turn, can ignite the fires that are fought in a reactive

system that does not monitor or count defects to correct root causes.

How Clean Is Clean Enough?

The type of fouling (chemical make-up and physical characteristics) or residue the molding process leaves behind on your tooling will help determine your cleaning requirements.

Many resins contain stabilizers, fillers or release agents that leave residue in the form of grease, light oil,

yellow waxy film or rust and white-colored dust. Some resins, such as PVC, create hydrogen chloride

gases that corrode many types of mold steels. Other resins with flame-retardants contain antioxidants, which will plate-out and over time attack steel. Some color pigments stain steels that build up and can be difficult to remove. Even plain water will do harm if left on untreated mold surfaces too long.

Molds should only be cleaned as much as necessary to carry them through a predetermined number of

cycles. Scrubbing or blasting all oxidation stains and discoloration off non-critical tooling and plates every

time a mold comes out opens pores and slowly erodes the surface and edges, requiring replacement long before it should. Unfortunately, it only takes minutes of run time to reacquire the initial staining. This

was proven to me by observing molds that were thoroughly cleaned, run for a couple of hours, then

pulled for a change-over. In areas where a lot of time had been spent scrubbing, some surface stains reappeared.

Many molds have self-cleaning vent passages, which means highly polished in toolmaker terms. Cleaning, then polishing vents to an SPI #A3 finish or better prevents residue from adhering to a rougher

surface that milling or grinding leaves, allowing residue to be blown into the larger vent dump area. This

keeps vents cleaner for a longer period of time and also allows potential cycle increases between cleanings.

Over-cleaning results from abrasive hands-on methods using scrubbing pads that are too coarse, emery

cloth or sandpaper, stones, and brushes with bristles made of assorted compositions such as nylon, brass and steel.

High-pressure blasting units that use media like hard plastics, glass beads, walnut shells and aluminum

also can abrade the surface of molds. If used frequently, or in an unregulated maintenance environment,

these abrasive methods slowly pound the surface of steel like microscopic chisels, causing residue to

adhere to the now-porous surface. Overly abrasive cleaning methods are the primary reason molds

experience quick residue build-up, excess wear, premature tooling failure and flash defects.

Finding the right cleaning equipment required for your molds and processes, combined with documented

methods and frequencies, can reduce repair hours by as much as 50 percent and reduce tooling wear.

The ROI on cleaning equipment is less than 60 days in many cases. Typically, the best method will

involve two or three different technologies designed to clean specific types of residue.

The most commonly used method for cleaning plastic injection molds today remains the some what

antiquated process of scrubbing tooling and plates, one piece at a time. Ask why, and the answers will

probably be:

"This is the way we've always done it.”

"We can't cost-justify other methods.”

"Other methods don't clean as well, they don't leave a bright and shiny surface."

“We're too busy. We don't have time to experiment with or investigate other methods."

These excuses can cost companies thousands of dollars a day in wasted time and worn tooling. To realize a major reduction in the time required to clean molds, to maximize tooling life, to systemize cleaning and make it more consistent and predictable, more modern methods should be introduced in the

mold shop, and hand cleaning should be limited to the areas where it works the best and damages the least.

Hand Cleaning

It is a safe bet that hand cleaning will never be completely eliminated, but it is an enormous waste of time and money to remove, clean, rinse, dry and replace every piece of tooling contained in a mold whenever it needs to be cleaned.

Hand cleaning tooling is a slow and damaging process. It is an effective cleaning method, however, for

areas of plastic molds that normally have the most corrosion and contaminant build-up (rust), including:

front and rear clamp plates that directly contact the press platens

bubbler plates (non-stainless)

bushings and wear plate grease grooves

water lines (non-stainless) with heavy build-up

plates subjected to heavy internal condensation and clamp pressure (non-stainless)

Managerial inertia sometimes leads a shop to keep using old, familiar (and ineffective) methods rather

than deal with the mayhem that a change in cleaning culture can cause. Choosing the wrong type of

cleaning system or having repair technicians reject the new method can cause disastrous repercussions,

so they avoid making a change. I spent many years during my career hand cleaning my own plates and

tooling (and being pushed to "hurry up") while the company leisurely searched for effective alternatives.

We experimented with different types of spray cleaners, solvents and brushes, but they all required

handling each piece of tooling several times, so none resulted in significant savings in cost or labor. Even

extremely caustic cleaners, such as sodium hydroxide, sulfuric acid baths or MEK, were limited in the

amount of rust build-up they would remove on unplated surfaces in a specific time frame. In the end, I

found that the answer to heavy rust build-up is prevention—through more frequent cleanings, use of

stainless steel plates and corrosion-resistant platings, and consistent application of spray-on rust inhibitors.

Cleaning Systems—CO2 Dry Ice Pellet Blasting Cleaning Systems—CO2 Dry Ice Pellet Blasting

There is an almost endless variety of resins, molds and processes available today, so there is no silver

bullet that is effective against all the types of mold fouling and corrosion that attack molds, whether

running or racked. An ice blasting cleaning system, however, is a very effective method.

Ice blasting is a non-abrasive cleaning method that first became popular among automotive rubber

molders because of its ability to clean molds in the press, while the mold is hot, without causing an

appreciable secondary waste stream of its own. 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 are shot out at high velocity through a variety of state-of-the-art, aerodynamically designed nozzles to remove residue quickly and harmlessly from mold plates and tooling.

Two types of systems are used in ice blasting:

1.Venturi (double-line hose) system—pulls or vacuums shaved ice particles into the air stream at the nozzle that is then directed toward the contaminated surface. This system is less costly than the single-line hose, but does not generate the nozzle velocities necessary to remove some of the more stubborn residue and grime left through the molding process of plastic molds.

2.Direct acceleration (single-line hose) system—introduces larger ice pellets into the airline at the hopper instead of the nozzle, allowing for much faster moving pellets exiting the nozzle tip (900 feet per second versus 300 for the double line), which translates to better cleaning performance. If less cleaning power is needed, the air pressure is simply dialed down, reducing pellet velocity.

Pros of Dry Ice Pellet Blasting

Non-abrasive—Ice blasting is not abrasive to all tool steels and hardened aluminum, even over extended

periods. Molds can be ice blasted several times per shift with no damage to parting lines, applied platings

or surface finishes. The technique works well on textured and polished cavity surfaces.

Clean operation—Ice blasting is the only media blasting that does not produce residual dust or waste stream through its own operation. Air quality tests performed in cleanroom operations using ice blasting

showed no measurable effects from the disintegration of the carbon pellets. Residue blasted off the surface of the mold may collect on surrounding equipment over a period of weeks or months, but air extraction hoods can be installed over the molds that generate heavy contamination to alleviate the problem.

Portability—Small, lightweight ice blasting units are available that can be pushed up alongside a mold on

the bench or in the press, which is consideraby less expensive than bringing the mold to the cleaner.

Keep in mind that an air supply will be needed close by (20 feet) to tap into. Standard line pressures of 70

to 90 psi with a 3/4 feed line will suffice.

Versatile—Ice blasting can be used to clean molding equipment other than molds. It is also effective for

cleaning press screws, barrels, internal mixers and other equipment.

Low operating cost—Ice pellets for single line systems cost about 20 cents per pound. It takes approximately 20 minutes to use 30 pounds of ice pellets, which is approximately the time it takes to clean both halves of a plastic mold measuring 2 feet by 3 feet. Plan on spending at least $15,000 for a single-line portable unit and a couple of different nozzles.

Ease of use—It takes just a few minutes to become familiar with procedures for hooking up the hoses

and nozzle and loading the pellets. Not having to be concerned about abrasiveness eliminates the fear of

surface degradation from moving the nozzle too slowly.

Cons of Dry Ice Pellet Blasting

Noise level—At approximatley 102 decibels, good ear protection is required when ice blasting, as when

using any compressed air cleaning system. However, even with earplugs, ice blasters are are loud and

the noise grows more irritating when used for extended periods. It helps to use earmuffs with earplugs.

Mold safety—Mold plates can blow over and tooling can be launched from bores if proper precautions

are not taken, or if the the nozzle tip is misdirected. Plates must be securely braced or laid down and tooling must be backed up or secured. Do not stack tooling in a basket and start blasting.


Personnel safety—As hunting dads say, "Watch where you point that thing, son." Human flesh is no match for ice pellets moving at 900 feet per second. In addition to ear protection, required safety equipment includes heavy-duty gloves, full face mask, long sleeves, etc. Typical workshops are set up with benches and other machines in close proximity (three to five feet), so the operator must be aware at all times where others are located, or risk being accused of violating good shop etiquette through unsolicited exfoliation.


Rust/residue removal—The non-abrasive nature of ice blasting limits its effectiveness for heavy stain and

rust removal. Ice pellets also can bounce off elastic or soft residues left in some natural rubber molding operations. Ice blasters are a line-of-sight cleaning system—you can clean only what you can see.


Ice blasting is best used as an in-press cleaning system where it can reduce cleaning time and eliminate the premature tooling wear that results from a typical solvent-soaked rag and abrasive pad wipe down method. It is especially effective for removing liquid silicone rubber plate-out and for cleaning tooling with

textured surfaces. Our 32-cavity, quarter cap mold receives an in-press ice blasting every seven days

throughout its production run, cutting the cleaning time required for the next phase of residue removal... the ultrasonic bath.


If asked to describe the perfect cleaning system for molds and tooling that would allow a significant reduction in labor hours and tooling damage while providing consistent cleaning results, it would meet the 10 criteria listed below.

1. Repair technicians would like using it.

2.The cleaning method would be totally non-abrasive to all types and hardness of steel, platings, coatings and polishes.

3. It would not require a repair technician to manually control or stand watch over.

4. Operation would not pose a safety hazard to man, machine or the surrounding area.

5. The cleaning solution would be user- and environmentally-friendly.

6. The cleaning unit would be easy to clean and maintain.

7. It would not be loud or irritating.

8. It would have the capability to clean subassemblies and tooling contained in plates.

9. Must remove 90 percent of mold fouling.

10. Must not leave behind any type of residue or film on tooling or plates.

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.

What Is It?

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 180oF) aqueous solution.

Before and after shot of a mold being ultrasonically cleaned. Photo courtesy Blue Wave

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 (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 (a phenomenon known as cavitation) to form in the tank that scrub and loosen vent residue and grime. The energy released from these microscopic scrubbing bubbles produces forces at 10,000oF at 7,500 psi, which is powerful enough to loosen contaminates without damaging critical edges or surface finishes.

Key Factors of an Ultrasonic Cleaning System

The cleaning ability of ultrasonics is dependent upon several key factors:

Cleaning Solution

The type of detergent required is dependent upon the process and type of resin you run, which leaves

behind specific types of off-gassing residue.

Highly alkaline solutionsÑsuch as sodium hydroxideÑdo the best job of removing stubborn contaminates,

light rust and heavy grease on most tooling and plates with no harm to most applied platings and

coatings. On the downside it can be caustic, posing a potential health risk and may require neutralization

before disposal, depending upon local codes.

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 solutionsÑsuch as those containing low concentrations of citric acidÑexcel 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.

An ultrasonic cleaning system. Photo courtesy Blue Wave

There also are several environmentally safe detergents being used that do a fair job removing most  contaminates 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. For a tank of 70 gallons (approx. 30" x 30" x 30" deep), 3000 watts @ 30

kHz performs well.

The higher the kHz rating, the less aggressive the cleaning power. Common systems use either 40 or 30

and a few use 25 and 20. 30 kHz is a good compromise between cleaning ability and noise level. Lower

ratings can be extremely loud and irritating.

Transducer Type and Construction

The heart of the system, transducers come in two types: (1) magnetostrictive and (2) piezoelectric.

Magnetostrictive transducers are typically more rugged and create the most aggressive cavitation action.

The best transducers also are zero-spaced and silver-brazed to the tank versus a bolted or epoxied

transducer, which has more of a tendency to slowly fail over time.

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 tank large enough to totally immerse your largest mold plates. You will regret getting a small tank

that will require you to flip the mold plates over for total coverage just to save a few bucks up front.

Also make sure the tank is constructed of at least 12-gauge stainless steel and be sure to have the

manufacturer weld a few support bars across the bottom to keep plates and tooling baskets from

directing resting on the bottom of the tank.

Purchase or build a hinged cover for the top of the wash tank to slow down the evaporation of 180-degree water.

Also fabricate a tall (about four feet) stainless back-splash that runs the length of your system and will keep your walls clean and direct any run-off 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 1/4" or better so cavitation is not impeded.

Buying Considerations

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 evidenced by the grass clippings

and paint residue floating on the surface. And, due to the nature of the way ultrasonic functions Ñhigh

heat and constant vibrationÑsystem parts can shake loose, wear through and burn out.

Heavy-duty, bullet-proof, robust and solid all apply when shopping for an ultrasonic system that will perform reliably in a busy mold repair shop. And unless absolutely necessary, stay away from all the bells and whistles in the form of automatic loaders, conveyors, dryers and fancy cabinetry.

However, do make sure:

The transducers are warranted for the life of the system.


The tank is warranted for 20 years or more against cavitation wear through (which can drastically cut

transducer life).


The wash tank has a pump filtering system and is easily accessible.


The pump intake is screened to keep small dowels, o-rings etc. from damaging the pump impeller.


The tank drain valves are easily accessible.


There are no protruding knobs, handles, switches or buttons.


Tank heaters are easily accessible.


Ultrasonic Pros.


Ease of use


Simply place the plates or tooling in the tank, set the timer, turn it on and walk away; doesn't need to be baby-sat


Cleans the mold while repair technicians work on other issues; like doubling your workforce


Cleans some tooling without requiring removal from plates


Predictable and consistent result


Cleans hard-to-access areas like counter bores, threaded holes, pockets, water lines and bubbler tubes


Does not harm sharp edges, applied platings, coatings textured or polished surfaces


Extremely fast ROI



Requires light scrubbing of heavily rusted areas before immersion


Will not remove some types of residue (using an environmental safe solution). Step up to sodium


hydroxide if your cleaning requirements demand it and your local codes permit it.


Requires really hot water 180¼ to 200¼ which won't kill you, but does get your attention


The high frequency noise can be irritating over a period of time if working close by

Ultrasonic Benefits

To see if ultrasonics will work for you, send samples of contaminated tooling to an ultrasonic supplier as a

test. This also will help the supplier match your specific contaminates with the proper detergent for efficient removal. Some suppliers will even set you up with a trial unit to use.

Once the system is installed, you will see an immediate reduction of labor hours simply because of the

speed in which tooling will be cleaned compared to the laborious task of hand cleaning one piece at a

time. And as a bonus, there will be no more rounded over edges, or platings and coating scrubbed off.


Co-injection molding
Choosing and using the right mold cleaners



Cleaning molds