Chemical compounds ease the pain of purging and add to bottom line and profitability

Published in Injection Molding Magazine on May 12th, 2009 by Clare Goldsberry

Purging between molds and material changes is time consuming and seems costly for molders. But is it a necessary evil? It’s necessary because molds and molding systems must be purged of residue materials, colorants, and other contaminants when changing molds, materials, or colors. And it’s evil because with the mechanical methods of purging, many times the abrasives used in the purging compounds cause surface wear on the mold and problems with the parts subsequently molded. 

No abrasion 

Chemical purging is being adopted by many molders, particularly those who use molds with hot runner systems, and it’s designed specifically for the material being purged. John Hoskins, sales manager for Chem-Trend LP’s (Howell, MI) Thermoplastic North America product line, says the company’s Lusin brand purging compound is specifically designed for cleaning hot runner systems in tools, and can do it safely and effectively. 

“Because of Chem-Trend’s 50-year background in release agents, we’ve developed chemical purge compounds that separate and remove contaminants from the metal mold components,” says Hoskins. “We know how to make it release from the metal surface without using surfactant or abrasives. Our material doesn’t damage the surface, yet it’s very effective and results in superior release.”  

He says the Lusin products are particularly effective in hot runner tooling where color is difficult to remove without damaging the delicate system. Additionally, Lusin leaves a fine coating that helps the parts release better and improves efficiency. “It makes the plastic less prone to stick to the metal surface, and if used as a preventive maintenance procedure, Lusin works better over time to protect the mold every time you run it,” he says. 

For purging high-temperature materials such as Ultem, Chem-Trend offers Lusin Clean G 410. Another Lusin Clean product is G 315, which is used in the automotive industry because it was developed specifically for polyolefin chemistry, including PP, PE, and TPO. The company also offers Lusin Clean G 320 for engineering-grade resins such as PS and ABS. Additionally, all of Chem-Trend’s purge compounds are NSF approved (except Lusin Clean 1500), making them suitable for products in all markets including automotive, medical, and packaging.

In one case, a custom molder needed to further improve efficiency while minimizing startup scrap. This molder evaluated Lusin Clean G 320 for transitioning from Xenoy black to yellow. (Xenoy is an alloy blend of semicrystalline polyester, typically PBT or PET, and PC.) The molder ran Xenoy (black) on a 550-ton Milacron machine with an 80-mm screw with an average barrel processing temperature of 540°F. The evaluation was done only on the barrel, as the next part (yellow) configuration required a mold change to a one-cavity mold with a part weight of 1.75 lb.

Using Lusin Clean G 320, the total time to clean the barrel and return to streak-free production was 22 minutes. The amount of purge compound consumed was approximately 9.7 lb. The molder then tracked Lusin over a three-day, 1700-part run that resulted in only 0.17% of scrap (three parts) containing color streaking, vs. the competing purge that had 3.17% (54 parts) of color/streaking scrap.

Chem-Trend’s newest purge compound, Lusin Clean 1500, is designed for the purging of clear polycarbonate. It removes carbon black specks, and won’t leave milky traces at startup. “We don’t have that problem because it’s a PC-based purging compound,” explains Hoskins. “You can purge PC with other things, but because the materials are dissimilar, it leaves a milky trace. And in today’s economic climate, no one wants to run scrap parts. Lusin Clean 1500 is specifically for those applications in which you can’t get rid of black specks or milky traces.”

Moldable purging compounds

Another purging compound formulation that differs from conventional versions by being a 100% chemical cleaning compound, not a mechanical cleaner, is Ultra Purge from Moulds Plus International USA Inc. (Costa Mesa, CA).“Because Ultra Purge is a chemical compound, we are able to clean the machine more efficiently because it reacts with the resin and the pigment, and can remove residue faster with less purging material required,” says Graziano Pestarino, Moulds Plus’s president.

Ultra Purge also has a formulation designed for use with hot runner systems. “Purging has become very important to companies using hot runner systems, and the hot runner business is growing because no one wants to create scrap with the high price of materials,” Pestarino says.

Rather than simply running purging compound through the barrel/screw and sprue, Ultra Purge is designed as an additive that, when added to the resin, can actually be molded into parts, reducing waste and machine downtime.

Moulds Plus worked closely with Husky to develop a PET grade of Ultra Purge for the manufacture of bottles, and this grade is recommended and distributed by Husky. “That’s where we got our knowledge of hot runner systems in PET applications, because Husky builds molds of 96-216 cavities,” says Pestarino. “Creating a hot-runner-safe product became very important to us.”

Purging in stack molds is also a challenge that Ultra Purge meets. “With a stack mold, there are two faces, so to clean out the hot runner system, you have to close the mold,” says Pestarino. “That means the formula must be moldable.”

Moulds Plus worked with a major packaging manufacturer to develop a polypropylene formula for thin-wall-container applications. The PP grade of Ultra Purge is concentrated with the active ingredient so that when the customer mixes it with its own material, it achieves a complete cleaning of the system. “Because the Ultra Purge PP formulation has the same melt temperature as the material being run, the melt index isn’t altered,” says Pestarino. “That’s important, because those molds are so well-designed and so technical, that you have to use materials of the specific properties for which the mold was made. We supply them with the active ingredient, and they combine it. We don’t alter the melt.”

Ultra Purge makes mold changeovers, material changes, and even color changes fast and easy, says Pestarino.

“Additionally, Ultra Purge carries a food-content certification from the FDA, as well as from the equivalent European group, which is stricter than the FDA, so it is entirely safe for use with food and drink containers.”

In high-tech applications, specifically with hot runner systems, customers are not looking for a basic purging product. “What we can achieve with our material can’t be achieved with a general-purpose compound,” says Pestarino. “These customers make medical parts, food containers, and drink bottles, and need what Ultra Purge provides.” However, the company also provides Ultra Purge to the automotive industry. “In that industry we can change color on the fly,” Pestarino explains. “Because we can mold parts with the compound, there’s no need to stop the press to change the color. They can also get more money for the scrap because the resin isn’t affected in any way by the purging compound.”

To help molders with this process, the company offers training to production staff as a package when molders purchase the company’s material. The training can be onsite at the molder’s facility (for large companies) or through a Web or DVD training program.

Pestarino notes that the market has changed a lot, and when the cost of resin went up last year, many molders realized they were throwing away money in purging. “Even though the cost of resin has dropped, we’ve taught them how not to waste money,” he says. “Ultra Purge reduces downtime and improves the quality of the production parts they’re making. Making parts with a clean machine gives you better-quality products. Even in preforms for bottle molding, you have brighter parts and won’t see the flow lines.”