A STUDY OF THE EFFECTS OF RAPID CYCLING PRESSURIZED WATER HEATING / COOLING ON COMPOSITE / INJECTION MOLD TEMPERATURES

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1 A STUDY OF THE EFFECTS OF RAPID CYCLING PRESSURIZED WATER HEATING / COOLING ON COMPOSITE / INJECTION MOLD TEMPERATURES Jim Fisher, Kip Petrykowski Single Temperature Controls Abstract There have been several technologies used to bring mold surface temperatures above/to a polymer s glass transition temperature (Tg) in order to improve part finish, appearance and mechanical properties. Steam, Cartridge Heaters, Induction, and High Temperature Pressurized Water have all been successfully applied. The intent of the study and consequent desire to publish or present the data was to help provide molders with better insight into the inherent benefits of Pressurized Water as compared to the other methods. Background Steam has been used for many years to heat molds as a carry-over from its use in other plant equipment and plant heating. Although steam carries large amounts of energy (at 10 bar/180 C, 2779 KJ/kg) and flows readily through traditional metal piping, it is difficult to keep as steam in complicated molding, is expensive to generate due to phase change and pristine water requirements, has limited precision (4 C) and has a limited upper temperature limit below the Tg of many of today s engineered resins. Steam does not offer same channel cooling and thus the overall system efficiency is greatly reduced. The cooling process typically has limited temperature control as the water is not pressurized and thus suffers an initial phase change and rapid temperature degradation until the temperature is reduced enough to flow as liquid again. This creates a situation where two unique systems are running, one for heating and one for cooling. Unlike pressurized water, steam is extremely dangerous if a leak develops. Electric cartridge heaters have also been used to heat molds. They are easy to install and operate, have a high temperature spectrum, and thus make a logical choice. Unfortunately, they consume large amounts of electricity to operate due to the nature of the cartridge construction (a heating element incased in magnesium dioxide inside a stainless steel tube with an air gap between itself and the mold, then add a somewhat conductive filler material to bridge the gap - all contribute to reduced conduction to the mold). They do not offer cooling, and are notorious for inconsistent heating (see figure 1 below). Like steam if cooling is utilized, it usually consists of a separate system incorporating unpressurized water through dedicated channels. Thus standard internally plumbed injection or RTM molds cannot be retro-fitted with the system. Page 1 of 11

2 Temp ( C) Temperature vs. Location - Water and Electric Heating with 1 Hour Soak Water - 1mm sample Water - 2mm sample Electric - 1mm sample Electric - 2mm sample Location Figure 1. (A Study Comparing Electric Cartridge, Oil, and Pressurized Water Heating for Composite Molding ACMA 2013) Induction Heating has also been successfully applied in University laboratories for the manufacture of composite and injection molded parts. Used extensively for metal surface hardening, it has great potential for high temperature thermoplastic composite molding applications. It unfortunately requires the molds be built specifically for this system. It also consumes large amounts of energy to operate, does not offer same channel cooling, and can require additional fees to operate the system with licenses and royalties. Although induction offers fast heat up rates, cooling rates are reduced as the channels have to be much further away from the mold surface in order to accommodate the induction coil location in the specially constructed mold. It has limited thermal inertia and thus is not able to offer resin rich surfaces with heavily fiber loaded parts. (see figures 2 and 3 below). Like steam, if cooling is utilized, it usually consists of a separate system incorporating unpressurized water through dedicated channels. Thus standard internally plumbed injection or RTM molding cannot be retro-fitted with the system. Pressurized Water, due to the inherent energy savings, high temperature spectrum, precise temperature control and fast ramp rates, offers numerous advantages over the above mentioned systems when applied to composites, rapid cure composite, and injection molding of various materials. Pressurized water is easily applied to a large percentage of the injection and composite molding markets, able to reach 220 C and use existing molds. Page 2 of 11

3 Figure 2. (Fibertuff PP 70% fill, Roc Tool, Polymer Center of Excellence Testing 2012) Figure 3. (Fibertuff PP 70% fill, Single ATT, Polymer Center of Excellence Testing 2012) Page 3 of 11

4 Unlike steam, pressurized water when decompressed shows dramatic reductions in temperature and thus offers a much safer system to operate. In the event of a line break causing depressurization the amount of water which can pass through an opening is diminished by 50% vs. cold water. Further cooling occurs through evaporation in the air. The combination of these effects reduces the amount energy able to be released at depressurization as compared to saturated steam to 1/10 (High Temperature Water Heating Systems, Steven Liescheidt, P.E., CCS, CCPR, 1991). Operating Principle Typical ramp rates for integrally heated molds, like most forced convection systems, are a function of flow (which drives heat transfer numbers), heater Kw, and conductivity rates for both the heated medium and the mold. If we abide by Newton s law of cooling, we are always at the mercy the delta T or the differential between the water flowing through the mold and the temperature of the mold. Thus the optimum method to increase heating/cooling rates would be to have a large tank of water at an elevated temperature to supply the lower temperature mold with energy. The same differential could be exploited for the cooling. The basic concept behind the rapid cycle pressurized water unit is a two tank accumulator system. Pressurization through volume control allows the unit to achieve temperatures in excess of the Tg s for most polymers. Many inherently beneficial features are derived from this arrangement: 1. Can be applied to any integrally plumbed mold. 2. Fast heating cycle. 3. Fast cooling cycle. 4. Re-claimed energy from the mold at change over through the use of robust valving and thermocouples. 5. All other benefits of a pressurized water system - high temperature spectrum and precise temperature control. Test Parameters Experimentation Goal: 1. Use the existing Weber mold that was utilized for published studies on standard pressurized water units (A Study Comparing Electric Cartridge, Oil, and Pressurized Water Heating for Composite Molding ACMA 2013). 2. Test in same facility as previous testing for published studies on standard pressurized water units used. 3. Test at same temperatures as previous testing for published studies on standard pressurized water units used. 1. Determine Temperature Control Unit (TCU) and mold temperature profiles. 2. Determine average and maximum ramp rates. 3. Determine the effects of pure conduction and forced convection on ramp rates. Page 4 of 11

5 Figure 4. (Weber Nickel shell test tool, Weber Manufacturing, Ontario, Canada) Figure 5. (Weber Nickel shell test tool, Weber Manufacturing, Ontario, Canada) Page 5 of 11

6 Test Equipment: Figure 6. Item Description Used Single STWS 200_2-36_6-36_B10_20-36 kw Heating / 60 kw Cooling H1.2 Hours 5 Hours on unit Power Supply 460 V/60 Hz (3 phase) Flow 100 liters/minute rated flow Heating Lines out to mold 13 mm I.D Line Length 1.5 m Number of Lines 2 Cooling lines into TCU 16 mm I.D Line Length 3.5 m Number of Lines 2 Mold Mold Weight Weber externally plumbed Nickel, single sided test mold 35.6 Kg 2012 NI 4 channel data logger K type TC Measured flow through mold (water) Method 69 l/minute Differential pressure Ambient air temperature for test 18.3 C Chilled water supply temperature 25 C Experimental Procedure Flow (water) (20 l/minute) 4. A nickel shell externally plumbed mold was provided by Weber Manufacturing, Ontario, Canada, for the test. The Weber mold was chosen because of its ability to be heat balanced by controlling the location and number of tubes used to deliver the liquid medium. 5. Temperature readings confirmed that variation across the mold at temperature was within the desired <2 C. The mold was completely open to the 18 C room temperature and was not insulated. This was believed to represent the worst possible scenario for heat loss. 6. The water unit was taken up to a temperature of 100 C and allowed to soak at that temperature for 5 minutes in order to purge air from the system. The unit was then taken back down to 25 C. The 6th-14th runs were used to plot the data. 7. Mold temperature readings were taken using a National Instruments 4 channel data logger with K type Thermocouples on the molding surface at the center, left, middle right and outer right edge of the mold. Page 6 of 11

7 8. The outgoing and returning fluid temperatures from the mold and Kw s were read directly from the controller provided on the unit. 9. One of the thermocouples was placed on the corner of the mold plate where there were no water circuits to provide insight into the difference in ramp rates for forced convection versus conduction only. 10. The data was plotted using Labview 2012 and Excel *Note during the operation of cycling between 200 o C temperatures the middle right side thermocouple adhesive failed and the thermocouple released from the mold surface. Observations From figure 6. below we see that the hot tank temperature (as shown by the upper red line) was degraded from 200 C to C during the cycle. This is typical of the system as the heaters tried to input more energy to the outgoing water. In this example, the unit would have benefited from more heating capacity than the 36 Kw that was available. Also from figure 6. below, we see that the cold tank temperature (as shown by the lower blue line) was increased from 25 C to C during the cycle. This is typical of the system as the heat exchanger tried to extract more energy from the outgoing water. In this example, the unit cooling capacity was almost ideal at 60 Kw. It should be noted that the true output for the exchanger was closer to 45 Kw as the water supply was not at the recommended 15 C. Typical temperature differentials between returning water and mold temperatures average around 20 C for larger (1000 kg) properly designed P20 tools. For this test, with the open tool, we observed a maximum returning water temp of 178 C and a maximum tool temp of 165 C, leaving a differential of 13 C. Not too surprising considering the 5x faster conduction rate for the Nickel versus P20 tool steel and as expected the radiant component to the room was minimal. Page 7 of 11

8 Figure 7. (Screen shot of hot/cold tank water temperatures and returning water temperatures from Single STWS 200_2-36_6-36_B10_20-H1.2) Figure 7. below is presented in an effort to give a baseline for typical ramp rates of traditional pressurized water systems as compared to a Rapid Cycling Pressurized Water Heating/Cooling system. Page 8 of 11

9 Flow rate: 25 L/minute Pressurized Water - Temperature Control Unit vs. Tool Heating/Cooling Profiles kw Heating 42 kw Cooling 60 l/min. pump Temperature ( C) Average temperature delta between TCU and tool: Time ( Minutes) Water Cooling Tool Cooling Water Heating Figure 7. (A Study Comparing Electric Cartridge, Oil, and Pressurized Water Heating for Composite Molding ACMA 2013). The testing was performed under the same conditions with the same mold as this study. Maximum ramp rate observed was.3 C/sec. If we correct for flow and heat capacity differences we would have an observed a maximum ramp rate of approximately 2.7 /sec. From Figure 8. below we observe a significantly improved ramp rate with the Rapid Cycling system. Maximum ramp rate observed was C/sec. Page 9 of 11

10 Second cycle Temperature (5 C) Left TC Middle TC In-op Right TC Outer Right TC Time (10 sec) Figure Second cycle Temperature (5C) Left TC Middle TC Right TC Data Points (20 sec) Figure 9. Page 10 of 11

11 Conclusions Pressurized water rapid cycle heating/cooling systems offer of robust technology that can be readily applied to the majority of currently used injection and integrally heated composite tools without modification. It offers fast cycle times, precise temperature controls and energy recovery in a technology that is familiar to most organizations. Acknowledgements We wish to acknowledge Tom Schmitz and all the people at Weber mold for supplying the test mold for this project. Bibliography 1. Kip Petrykowski, Jim Fisher, A Study Comparing Electric Cartridge, Oil, and Pressurized Water Heating for Composite Molding ACMA 2013, 2. Steven Liescheidt, P.E., CCS, CCPR, High Temperature Water Heating Systems, 1991, Page 11 of 11