Solutions to Cleaning Problems in Mold Manufacture and Maintenance

By Barbara Kanegsberg

As requirements for tight tolerances and minimizing contaminants increase, a documented, validated cleaning process will set you apart from generic suppliers and provide a competitive edge. A few months ago, we provided basic tips to getting a handle on your current cleaning processes, to understanding what cleaning agents you have in-house, and to spot and eliminate products that are likely to cause trouble.1 The series continues with some current and emerging contamination problems—along with solutions—that in our experience are critical for mold manufacture and maintenance.

The value for you is that by understanding contamination and cleaning issues, you can very often head off defects before they occur. When cleaning problems occur, you can resolve them logically rather than depending on commercial claims. Even when you need outside help with solving cleaning and contamination control problems, you are more powerful starting from an educated position.

Corrosion
Corrosion is a natural, unavoidable process, and a very costly one. The total annual cost of corrosion in the U.S. alone has been estimated at nearly $300 billion, more than 3 percent of the GNP. We find corrosion to be a double-whammy in that both the process of corrosion and the products of corrosion cause damage to molds and/or to molded products. The corrosion process can compromise the mold and therefore the configuration of the molded product. Contamination, even by minute quantities of corrosion products, can increase the reject rate.

Fortunately, corrosion can be controlled and forestalled. Knowing when to clean and controlling the cleaning processes can go a long way toward minimizing and forestalling corrosion, and preventing contamination of the molded product.

If we think of a fire triangle (fuel, oxidizer, ignition source), we also can consider a corrosion square. The four parts of the square are the anode, the cathode, the conducting path between the anode and cathode, and oxygen or an oxidizer. Just as eliminating one or more components of the fire triangle can prevent fire, eliminating one or more parts of the corrosion square can forestall corrosion.

Clean sooner. Clean often. Protect the mold. An achievable, documented preventive maintenance program is a must to forestall corrosion. Notably, polyvinyl chloride (PVC) can readily corrode molds. The successful solution is immediate soil removal. Many other soils—including oils, metalworking fluids and assorted particles—can serve as a conduction pathway for corrosion. Even water-soluble process fluids can damage molds. A related reason to clean sooner rather than later is that soils, which remain in contact with the mold for long periods of time, are more difficult to remove.2 If a rust preventative is used, make sure it is appropriate to how long the protection is needed and be sure it does not interfere with other molding operations.

Recovery Expeditions
There are cleaning problems, and then there are cleaning problems. Encapsulation of a manifold as a result of the hot runner system being left on over the weekend goes beyond cleaning to what might best be termed a recovery effort. In 2006, Steve Johnson wrote a fine series of articles3 about what happens when plastic ventures where it should not or when plastic is over- or under-heated. Mechanically chipping away at the residue often damages the mold; burning off the residue may be the best option in such instances. Training and proper procedures are certainly appropriate.

Mechanical removal and burning are reserved for gross, recovery operations. High heat may be the best and only solution for removing cured materials, but it is not an optimal solution, particularly for complex molds and micro-molding. Those who do heat treatment—particularly for critical applications—do not consider heat treatment or even passivation to be a cleaning process. The reason is that, depending on the mixture of soils, heating can result in permanent, unacceptable changes to the surface. Rather than depend on high heat for cleaning, many groups do extensive cleaning, with aqueous and solvent chemistries and with ultrasonics prior to heat treatment.

With increasing use of complex, molded products for critical applications, the surface of the mold itself becomes critical. Therefore, for critical applications, such recovery expeditions have to be minimized.

Match the Cleaning Process to the Soil
Other soils present problems for mold manufacture, maintenance and production. The general approach is to use aqueous (water-based) cleaning agents; and if that does not work, to try isopropyl alcohol. However, like water, isopropyl alcohol (IPA) is not a universal solvent. In our experience, investing effort in matching the cleaning agent and the cleaning process with specific soils results in superior, hassle-free manufacturing operations.

Silicones
Silicone removal is a perennial problem. Oftentimes, suppliers of silicone products recommend isopropyl alcohol; however, there are many silicone-containing compounds and IPA is not effective with all of them. In addition, the fabrication or molding process may itself involve a chemical reaction, so the silicone compound you started with may not be the same as the silicone compound that remains on the mold.

Consider an entire class of cleaning products referred to as methyl siloxanes, which can be effective for removing silicones based on the like dissolves like principle or basic solvency. The linear methyl siloxanes have a relatively low boiling point and usually have a very low flashpoint—similar to acetone or isopropyl alcohol. While they evaporate rapidly, you have to be careful about all ignition sources. Cleaning equipment for low flashpoint solvents involves a relatively high capital investment. Cyclic methyl siloxanes have a higher boiling point and a higher flashpoint. Because they evaporate relatively slowly, residue can be a potential problem.

There are three additional bonuses:

1. Both linear and cyclic methyl siloxanes are exempt as volatile organic compounds (VOCs), so interactions with your friendly environmental regulatory agencies tend to be simplified.
2. We also are finding the cyclic methyl siloxanes to be effective as a replacement for high VOC mineral spirits for removing oils.
3. In our experience, methyl siloxanes can be unexpectedly effective in removing some mixed soils whether or not those soils are silicone-based.

You may need to test several different cleaning products, including blended cleaning agents, bio-based cleaners, chlorinated solvents and brominated solvents. Even if the silicone compound has not reacted to become something even more stubborn to remove, silicone residue may be present as a mixture with other soils. Mixed soils are always a challenge to remove.

Mold Release
The first step to solving the mold release problem is to get to the root of it, not simply to add more cleaning steps. Start by controlling the variety of mold release agents used. We strongly suggest that you not arbitrarily impose a single mold release compound. The plan will backfire. Instead, ask the technicians and manufacturing engineers (the people who have to walk the walk, not just theoretically talk the talk) why they are using specific mold release compounds. Involve them in selecting the most effective ones. Continue by controlling the mold release application process. More mold release is not necessarily better.

Only when the variety of mold release compounds are defined and minimized—and only when the level of mold release has been controlled—is it time to look at the cleaning process. At this point, you can achieve controlled, cost-effective cleaning. As a bonus, the overall manufacturing process is better controlled, too.

Pre-Plasma Cleaning
In automotive applications and other high-volume applications where defects are intolerable, one concern is handling the molded product prior to painting, in cases where plasma is used as a final cleaning step. We share concerns with line-of-sight plasma cleaners in that the plasma stream will not turn corners, a problem with complex molded products. Plasma chambers may be an option. Because the cleaning agent is being generated in-place, a plasma chamber effectively bathes the product with very reactive cleaning agents.

However, final plasma cleaning does not replace other, more traditional cleaning processes. One reason is that plasma cleaning is a double-edged sword. It also is used to modify the surface. If you do plasma cleaning, you have to be even more careful with cleaning during mold maintenance and with mold release and other chemicals used during the molding process.

Also, prior to plasma cleaning, you need to have what most of us would consider to be an extremely clean product. What if you have residual soil on the molded part? Soils can block access of the surface to the reactive plasma materials. Even more insidious, high levels of soils can themselves react with the molded product producing surface modification. To get only the cleaning or only the surface modification you want, you have to minimize what is in the plasma chamber.

A successful, rugged pre-plasma cleaning protocol involves defined, controlled cleaning during mold production, mold maintenance and product fabrication. You have to educate yourself about your own process, understand the traditional cleaning and plasma options available, and plan your cleaning and contamination control process independently of any commercial, vendor or supplier interest. Save time by asking for help. Use your technicians, engineers and outside advisors.

Clean the Molds You Produce
Keep your customers happy. If you want your mold to be more than a commodity item, make sure they are clean, free of undesirable residue and ready for your customers. Moldmakers sometimes assert that they do not need to clean the mold; the customer will take care of it. Tossing the cleaning process over the fence was a popular, but shortsighted idea of the 1990s and it succeeded for a while with the growth of generic supply chains.

We hear too many complaints that mold manufacturers “Don’t clean ‘em like they used to.” Aluminum molds retain metalworking fluids, because the surface is porous. The residue impacts molding and subsequent coating. The customer could avoid aluminum molds and he could bite the bullet and proactively clean aluminum molds and all molds as on arrival.

Complaints about the initial cleanliness of molds are likely to increase. Throughout the past five to seven years, surface requirements have become more exacting, your customers are concerned about their own process costs and mold configurations are becoming increasingly complex. In addition, because soil becomes more difficult to remove with time, your customer may have to use more aggressive cleaning chemistries that require specialized equipment, permitting and recordkeeping.

Even if you produce the mold and are never directly concerned with mold maintenance, supplying your customer with a clean mold will put your company at a competitive advantage.

Microcontamination
Mircocontamination is a term used to describe low levels of residue that are not always clearly defined or evident. This is an emerging issue and it is likely to become increasingly important where molded products are used in medical and pharmaceutical applications. Low levels of contamination are becoming significant for large and miniature molded products. Even where large surface areas on consumer products are involved, coating and performance depend on a clean, well-characterized surface.

A well-characterized surface refers not only to the physical dimensions but also the surface finish, surface properties and small amounts of what is sometimes referred to as desirable contamination. For high-end use, this goes far beyond the scraping, burning, and recovery efforts to restore molds and hot runner systems. Instead, those in the industry who want to get ahead achieve superior, defined, validated and controlled cleaning processes.

In the medical and pharmaceutical arenas, molded products are not a generic part of the supply chain. Instead, partnering and even total device outsourcing are becoming the norm. To assure reliability, critical cleaning processes that meet the needs of your customer are essential. Where micro-molding, medical devices and combination devices intersect, cleaning and contamination problems can increase exponentially. We observe increasing numbers of moldmakers and manufacturers of molded products at the technical portions of professional programs. The reason is that, while the FDA may hold the final assembler responsible for product quality, the final device assembler will be asking you the tough questions about cleaning and contamination control.

IPA—a workhorse chemical commonly used to remove microcontamination—will undoubtedly need to be supplemented with other specific solvents. Consider the compatibility of the molded materials with the cleaning process and the specific soils. While IPA wets better than water, it does not have adequate wettability to remove soils trapped in small spaces.

For microcontamination, you may have to think outside your immediate comfort zone. Where wettability is required, solvents such as hydrofluorocarbons (HFCs) or hydrofluoroethers (HFEs) may be required, particularly for final cleaning.

Wettability is not the same as solvency. Solvency is the ability to dissolve; wettability is the ability to spread out or to penetrate into small spaces so that solvency can occur. Because HFCs and HFEs are at best very mild solvents, blends may be needed. As soon as the solvent contains fluorine (F), think higher costs.

Consider n-propyl bromide (a brominated solvent that has recently been approved for solvent degreasing by the EPA SNAP group) or consider one of the classic chlorinated solvents. Such solvents aggressively dissolve a wide range of contaminants, provide good wettability and are low residue.

All effective solvents have the potential for worker and community safety issues. Therefore, for effective and/or costly solvents, consider well-contained solvent systems. Such systems are often inherently automated, resulting in better process control and allowing a single batch of valuable solvent to be used repeatedly, sometimes for several years.

Solving Cleaning Problems; Improving the Process
Look at options to update the aqueous process. The product line and the manufacturing soils may have changed. Washing, rinsing and drying are essential steps for most cleaning processes. With aqueous cleaning, the three steps require separate pieces of equipment, with distinct approaches to process control. Perhaps you have great washing steps, but the rinsing and drying steps were added as afterthoughts. This may be a good time to evaluate if the sub-optimal or non-existent rinsing and drying processes are costing you time, costing you product yield due to corrosion or residue, or costing you business due to dissatisfied customers.

Revisit solvent cleaning. Particularly if the aqueous cleaning process is not working reliably and the isopropyl alcohol is not doing the job. Look at a range of options. A cost-effective solution may be a more aggressive chlorinated or brominated solvent.

To avoid potential negative health or environmental effects, the equipment design is critical. Make sure your cleaning process is up-to-date and removes the soils. Perhaps you have stopped using chlorinated solvents or ozone-depleting compounds, and perhaps you have substituted an aqueous cleaning process. Is the aqueous process effective? Could it be better optimized?

Educate yourself in your customer requirements and about current cleaning agents and processes, and get knowledgeable advisors to help you. In our experience, education is the key to profitable, trouble-free cleaning that leads to high-quality product.

References
1 B. Kanegsberg, “Value-Added Critical Cleaning of Industrial Molds,” MoldMaking Technology Magazine, April, 2007.
2 B. Kanegsberg and E. Kanegsberg, “Critical Industrial Cleaning for Mold Manufacturing and Maintenance,” presented at MoldMaking Expo, Rosemont, IL, April 18, 2007.
3 S. Johnson, “Cleaning a Flashed Hot Manifold,” Parts I to III, MoldMaking Technology Magazine, June, August, and September, 2006.