In the injection molding of plastics, localized hot spots such as thin steel conditions or small core pins often increase cooling time and therefore significantly increase the overall molding cycle. Water cooling reaches its limits where the space for cooling channel bores is restricted. In such cases, the degree to which overall cycle time can be reduced is dependent on the degree to which cycle time is controlled by the cooling of localized hot spots.

Linde, a leader in industrial gas applications, together with ISK GmbH in Iserlohn, Germany, has developed a proprietary process (patent pending) that uses liquid carbon dioxide to supplement the existing water cooling system for effective localized cooling of hot spots in molded parts. When properly applied, this technology can reduce cycle time up to 50% while improving quality and lowering costs.

Background: Costs & Cooling

The growing demand for inexpensive, mass-produced plastic products has led to the creation of a multi-billion dollar industry, with more and more plastics processed using injection molding. The plastics injection molding industry has rapidly evolved over the years, with great improvements in design flexibility as well as in the strength and finish of manufactured parts. Manufacturers have achieved this while reducing production time, cost, weight, and waste.

Injection molding is now used to produce a vast array of products for industries such as automotive, medical, aerospace, consumer products, toys, plumbing, packaging, and construction. Throughout this evolution, the demands placed on cycle times and the high quality of injection molded parts have increased multi-fold.

Cooling time of injection molds often accounts for a large part of the production cycle and hence impacts the quality, efficiency, and profitability of the manufacturing process. Uniform temperature distribution through the complete mold is crucial for high quality and short cycle times. The temperature of the mold is normally controlled with the help of water passed through cooling channels. However, cooling time is often determined by hot areas on the mold, so-called hot spots, which cannot be cooled with water to an adequate extent.

The cooling of long, thin cores, fillets, slides, ejectors, or other difficult-to-reach parts often leads to problems. For example, cooling confined areas with water can become inefficient as the thin water channels become clogged by deposits, thus restricting the flow of coolant. Small cooling channel diameters also cause high pressure losses, and as a result, conventional temperature control methods may not be suitable.

Alternative Cooling Cuts Cycle Time

Linde’s spot mold-cooling technology cools the hard-to-reach hot spots of the injection mold. Carbon dioxide spot cooling can produce up to a 50% reduction in cycle time for a manufacturer of plastic parts, although the degree to which cycle time can be reduced depends on the degree to which cycle time is determined by the cooling of localized hot spots. 

The technology uses liquid carbon dioxide (LCO₂) for effective cooling of hot mold parts, and is used in addition to the existing water temperature-control system. The system uses special CO₂ solenoid valves to time the injection of LCO₂ through stainless steel capillary tubes inserted into small chambers in the metal of the mold. The chambers may be either drilled or eroded (with electrical discharge machining (EDM)) into the mold itself, or they may be formed by hollow core pins (see Figure 1).

When the cooling cycle begins, LCO₂ is fed under high pressure (approximately 850 psi (58.6 bar)) through the thin, flexible stainless steel capillary tubes to the points where cooling is required. The expansion of the CO₂ at the end of the capillary tube in an “expansion room” (see Figure 1) creates a CO₂ snow and gas mixture with a temperature of about
-109°F (-78°C) and a high cooling capacity.

The high sublimation energy of the CO₂ from solid to gas phase, along with the resulting cold gas, provides a very high local cooling capacity. The CO₂ withdraws heat from the steel of the mold and escapes out of the expansion room in gaseous form through an annular gap between the hole and capillary tube. Given the purity of LCO₂ supplied for this application (typically >99.98%), there is little danger of residue build-up or contamination of the hole as there would be with water cooling.

Engineering Spot Cooling: An Example

Consider the production of a headlight housing (Figure 2). Cooling the very thin mold fillet in the middle of the part proved to be a challenge. The mold for two cavities initially provided no cooling of the fillet.

Consequently, this fillet needed a very long time to cool, which extended the cycle time. The solution lay in the integration of a supplemental cooling process into the existing mold to achieve the accelerated cooling and cycle times. The manufacturer, a well-known maker of vehicle headlights, chose the CO₂ spot mold-cooling process. As a result, cooling time was reduced by 45%.

Existing molds can typically be retrofitted with the spot cooling temperature-control system. Due to the low hardware requirements, the investment costs are low, and Linde engineers can assist with the installation of the thin and flexible capillary tubes, as well as the optimization of the control system. Through thermal analysis of the mold using an infrared thermal-imaging camera, Linde application engineers can help establish where the capillary tubes should be placed.

For repeated cooling, the CO₂ must be supplied at the right pressure and at the right temperature without gas bubbles. Linde engineers adapt the supply concept to the requirements of the particular application, considering LCO₂ consumption, withdrawal profile, and site conditions (i.e., the space available and pipe lengths needed). Small plants can be supplied with cylinder bundles for supplying LCO₂. Cylinders must be specially adapted for liquid withdrawal. For higher consumptions, a CO₂ tank with suitable pressure-boosting equipment supplies the liquid and bubble-free carbon dioxide.

In order to adjust the opening and closing times of each solenoid valve, a control device is required. With a control unit (Figure 3), application engineers can help adapt the CO₂ injection cycles to the specific needs of each hot spot.

The CO₂ control unit has to communicate with the PLC of the injection molding machine. An electric signal from the injection molding machine is required to indicate “mold closed” and initiate the spot cooling cycle. Linde can also provide demonstration systems to easily test and optimize the CO₂ spot cooling process on specific parts, polymers, and molds. Cycle-time reduction and cost savings can be proven in a short time, with little disruption to existing processes.

About the authors… Andreas Praller, application manager for Linde AG, Manufacturing Industry-Plastic & Cryogenics, has been with Linde for 20 years. Jim Stanley is a program manager and has been with Linde LLC for 20 years. Contact Linde at U.S. 800-755-9277 or sales.lg.us@ linde.com.

Note: This article was based on an original whitepaper published by and available directly from Linde ( www.lindeplastics.com) and used here with permission.