Improving Machine and Process Efficiencies with Chemical Purge Compounds
More efficient color or material change long has been the benefit to companies that incorporate commercial purging compounds into their thermoplastics processing operations.
by Tony Schoendorff
More efficient color or material change long has been the benefit to companies that incorporate commercial purging compounds into their thermoplastics processing operations. And, while these products can substantially reduce scrap and machine downtime that eat into profits, many processors would rather be subjected to a time-share presentation or join a pyramid scheme than discuss a cost savings project with a new purge compound supplier. Over the years, technicians have used everything from soda pop and laundry detergent to popcorn kernels to clean their machines, rather than using commercially available purging compounds. Were these the best solutions? Certainly not, but they were cheap on the front end. More conventional methods of purging color-to-color that simply use the next resin or regrind to make the transition lack the efficacy of a commercial purging compound (CPC). CPC producers formulate products for specific polymer types, processes, equipment and operating temperatures to ensure the most effective removal of color, material and carbon encrustations.
Cleaning mechanisms explained
Commercial purge compounds incorporate cleaning mechanisms and, therefore, outperform production resins for purging. This might seem like an obvious statement, but there are plenty of processors today who primarily use the next resin to purge from color to color. In many cases, the results for a given color change are satisfactory, and certainly for light to dark color changes this is the industry norm. Over time, the layers of contamination build up and begin to bleed into production parts or degrade, causing black spots. At this point, there is a need for a better solution.
Mechanical purge compounds contain abrasive fillers that are intended to "push and scrub" contamination from the screw and barrel. This is considerably more effective at removing contamination than an unfilled polymer, yet there are limitations to what a purely mechanical product can achieve. Chemical purge compounds contain additives that initiate a chemical reaction at processing temperatures and penetrate into the layers of contamination. In the case of Lusin® purging compounds, the additives break the molecular bonds to release and separate deposits from the metal. Since chemical purge compounds are non-abrasive, they do not contribute to wear and tear, making them safe for use in hot runner systems. Many chemical purge compounds can be molded into parts, which generate pressure within the hot runner system for more effective cleaning as material is forced into potential dead zones. Molding also allows the operator or technician to remove purge from the press more easily (often utilizing robots).
A third classification of purge compounds simply is a combination of the chemical and mechanical technologies. While this would seem to be the best of both worlds, the added filler content could cause the material to be unsuitable for hot runner cleaning. This is a key benefit to using chemical purge technology. Due to the fact that – by definition – abrasive compounds need to be in motion to clean, purge consumption generally is higher with mechanical type products. Some mechanical purge compounds are marketed as safe for hot runner cleaning. If this is the case for a product that is intended to be evaluated, particulate size and suitability for molding parts should be confirmed with the supplier.
Realistic performance expectations
In ideal conditions, plastics processors would have the opportunity to skin the inside of hot runner systems with natural material during the inaugural run and then purge with an effective commercial purge compound from that point forward. In other words, processors would prefer to prepare the tool properly in the beginning and maintain the cleanliness without fail. Unfortunately, production demands, inherited tools and prior use of purge materials not suitable for running through the tool may have left the processor with buildup or carbonization to deal with. It is reasonable to expect that a tool/press combination that is heavily contaminated will take a bit longer to get clean than one that has been well maintained. While a bit cliché, it really is like keeping house.
Purge compounds are not magic. While removing all the contamination in one application would be preferred, it should be considered that the color and carbon layers that have accumulated may take a bit of time to remove. These deposits can be bonded very tightly to the metal surface, so even a chemical purge compound that is designed to penetrate into the surface of these residues may need a couple of applications to get the system truly clean. There are many times when black spots, for example, get worse before they get better. However, once the metal surfaces are rid of the deposits, regular application for color change or maintenance cleaning will keep things in check. An overnight or holiday shut-down can provide a great opportunity to fill the system with a chemical purge compound to allow for deep cleaning. Once such a program is implemented, processors will find that production should run quite a bit more smoothly.
Like any market in today's global economy, there is a great deal of competition in the purge compound industry. This competition drives innovation and development of new products addressing the direct needs of customers. For example, chemical purge compounds no longer need to be considered as ammonia-producing products that generate noxious fumes and smoke. The new technologies are both highly effective and environmentally friendly, allowing processors to take advantage of the superior cleaning effect without sacrificing operator safety.
There are many new products that have been introduced to the market over the last few years, including purge compounds for clear PC and acrylic. For years, processors of these materials struggled with color and material changes, as well as black spots. Unlike opaque materials, clear polymers show even the black spots that are buried beneath the surface. Until recently, use of a commercial purge compound meant milky traces that could take hours to eliminate from the system. Transition from clear PC to clear PMMA, or vice versa, presented similar results. Consequently, processors often elected to remove the screw and clean the system manually. If a hot runner was utilized, this further complicated matters. The solutions, which were developed by Chem-Trend, are products based on clear carrier resins. These new formulations are completely miscible with the production materials, safe for hot runners, highly effective and expelled from the system quickly. This is a huge step forward for processors of clear polycarbonate and acrylic.
The use of commercial purge compounds increases overall efficiency and lowers costs by reducing downtime between color and/or material changeovers. These products also extend the life of processing equipment and reduce waste. Many articles address the cost savings available through the use of commercial purge compounds and the bottom line is this: the best solution is the product that delivers the lowest cost-per-purge, which will be the lowest combined cost of machine downtime, material and rejects. Exceeding cost savings of 50 percent is possible, regardless of the unit price of the material, even when the incumbent product is another CPC. This represents tens or even hundreds of thousands of dollars to a processor's bottom line, depending on the size of their operation. A simple cost savings analysis (CSA) can be completed by the purge compound supplier following the evaluation.