An evolution in gas injection molding

As competitive pressures and globalization continue to put a squeeze on plastics processors, it is becoming ever more important for manufacturers to explore ways to optimize production across the entire process flow. Seeking faster cycle times, lower energy consumption and a more economic use of raw materials, manufacturers are also looking at existing technologies that can offer benefits in these areas.

Gas injection molding (GIM) is a tried and tested technology that is used to mold hollow components with minimal internal stresses, reduced warpage and no sinkage — at lower machine clamp pressures. Common applications include handles for cars and appliances, automotive panels and similar parts with thicker cross-sections.

GIM involves injecting pressurized gas into the polymer melt. As gas flows through, it pushes molten resin against the wall of the mold cavity to create a hollow core. Typically, nitrogen is the pressure fluid used.

Over the past decade, water injection molding (WIM) has also grown in popularity as an alternative to GIM, especially for fluid pipes, due to its cooling performance. However, despite its high heat removal capacity and good pressurizing performance, water has various handling drawbacks, including the need for drying and the risk of water leakage, which can result in production stoppages, high scrap rates and even mold surface damage. Furthermore, WIM requires complex and expensive equipment.

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One company with experience in gas and gas-assisted production technologies is the German industrial gas supplier Linde Plastics. Linde not only supplies the atmospheric gases for GIM applications, the company also offers gases technology — such as pressure boosters and pressure control systems — to enable these processes.

Linde collaborated with Maximator GmbH, a German company specializing in high-pressure, testing, hydraulic and pneumatic systems, on the development of a new, patented technology that uses high-performing liquid carbon dioxide (CO2) instead of gaseous nitrogen. Called Plastinum GIM C, the system takes efficiency to the next level. At a pressure higher than 150 bars, CO2 is much denser than nitrogen and has a higher specific thermal capacity, plus a high expansion cooling capacity during depressurization at the end of the cycle. As a result, CO2 has a much greater cooling capacity than nitrogen. Parts cool far more quickly and are ready to release from the molds much faster.

While the adaptation of CO2 requires modifications to the pressure boosting, pressure control and injector equipment, Linde and Maximator jointly ensured optimization of the CO2 gas injection moulding equipment to deliver significant advantages over nitrogen-based GIM and WIM. While CO2 matches the heat removal and pressure performance of WIM, it has significant benefits compared to water in relation to process complexity and stability and equipment costs, with the added advantage of mitigation of an additional drying step in the fluid injection cycle. The high efficiency of CO2 makes it attractive for new molds as well as retrofitting existing molds, leading to higher production rates and more consistent quality. Existing GIM molds can often be converted with minor modifications to the gas injectors and no modifications of the tool geometry.

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One of the first companies to test the technology following its development four years ago was Sinsheim, Germany-based Engel Formenbau und Spritzguss GmbH, a manufacturer of high-quality gas injection molded components for cars and household appliances.

Engel Formenbau and Linde ​ had been working closely together for more than a decade in order to continuously improve the gas injection molding technology based on nitrogen.

In an effort to further enhance performance, Engel Formenbau decided to introduce Linde's new technology for enhanced gas injection molding based on CO2 for the production of high-quality refrigerator handles, to significantly shorten the manufacturing time frame.

Engel began by using a CO2 compressor control module for a test period. This module offers an easy entry point for users who are gradually converting to the new CO2 technology, allowing operators to initially supply CO2 from gas cylinders before switching to a tank at a later date. The process conversion, including trials and optimization, was completed in a short time period — and through employing CO2, the same high-quality product levels were achieved with CO2 as were with nitrogen, but with a 36 percent reduction in cycle time and better warpage behavior.

"The resulting increase in quality, productivity and efficiency means we are even better equipped to meet our customers' expectations," said Dietmar Engel, managing director of Engel Formenbau.

Since 2015, Engel has continued to work with the new system. Since then, several other injection molding companies have also adopted the technology in series production, including automotive suppliers of various parts. The technology is used both Europe and the United States.

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In addition to GIM technology, Linde has developed precisely engineered CO2 spot-cooling systems, including its Plastinum Temp that substantially reduces the cost of injection molded parts. Spot cooling helps achieve more uniform cooling across the part geometry, which is critical for quality and mold-release performance. CO2 spot cooling also targets specific areas that need cooling and can reach surfaces of the mold or areas that cannot be adequately cooled by water-based methods. The process has been shown in some cases to cut cycle times by up to 50 percent.