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Ambient Temperatures Every rotational molder in the world struggles with this one at some point. There are many great molders who have worked their way through the problems and have found cycle settings, cooling equipment and operating procedures to solve specific problems. What I've done here is gathered some extracts from articles and used some measurements to highlight some of the things to consider. I am always open to being corrected or being shown a new perspective, so please let me have your thoughts.....
Basic Numbers Plant (factory) temperature range: 60°F to 120°F (15°C to 49°C) Demolding temperature range (polyethylene - typical): Molds: 60°F to 180°F (15°C to 99°C) Parts: 80°F to 220°F (27°C to 104°C) Polyethylene shrinkage range: 1.8 to 4.0% Polyethylene thermal coefficient of expansion range: 55 x 10-6 to 111 x 10-6 in/in/°F (100 x 10-6 to 200 x 10-6 cm/cm/°C) Average cooling rates (depending on point in cooling cycle): Static air 2 - 5 °F / min (1 - 3 °C/min) Forced fans 5 - 20 °F /min (3 - 11 °C/min) Water 30 - 60 °F/min (17 - 33 °C/min)
Overview Ambient temperature has a major effect on the rate of cooling of a mold. The temperature differential between the mold surface and the surrounding air coupled with the speed and turbulence of the cooling fans used, are the primary factors that dictate the length of the cooling cycle. When water is used, there is a dramatic increase in the energy that can be drawn from the surface due to evaporative and boiling effects. This can be controlled through the use of very fine misting systems that allow molds to be cooled at rates between forced air fans and heavy water showers (shock cooling).
Figure 10.1: Temperature Differential Between Mold Surface and Processing Environment for Typical Molding Cycle (Rotational Molding: A Practical Guide © 2001) Figure 10.1 from Rotational Molding: A Practical Guide demonstrates the reducing differential between the mold surface and it's environment during a cycle and how this process is accelerated by the use of water. If ambient conditions are hotter, the cooling process can be slowed dramatically (i.e. summer conditions). Effect of Ambient Temperature on Cooling Rate
Figure 8.50: Mold Surface Cooling Rates vs. Ambient Temperature (4x 18” (0.46m) Aluminum Cube Molds – 0.150” (3.8mm) LLDPE Parts) (Rotational Molding: A Practical Guide © 2001) Figure 8.50 from Rotational Molding: A Practical Guide clearly shows the effect of ambient conditions on the rate of cooling experienced by the molds. These measurements were taken using continuous infrared thermometry measurements. Mold Temperature During Production Ambient temperature has a big part to play in the length of a cooling cycle but so too do:
The rhythm of the molding machine will often have as much effect as the ambient conditions on cycle time and part demolding temperature. Consider the following data for 139 consecutive molding cycles from an independent arm machine:
Figure 1: Arm Delay Time Prior to Entering Main Cooling Station (Time in Wait Station Between Oven and Main Cooler) Only 45% of the cycles move at the preprogrammed delay of 5 minutes - the rest are delayed due to changes in product mix due to scheduling, changes of operators, training and problems in the demolding station.
Figure 2: Mold Entry Temperature to Main Cooling Station Delays in the pre-cooling station affect the temperature at which molds begin cooling. Figure 2 shows how the mold entry temperature (measured continuously using infrared thermometry) is affected. Long delays mean that the molds are cooler when they enter the cooling station.
Figure 3: Mold Exit Temperature from Cooler Figure 3 shows the exit temperature of the molds from the cooler - this is affected by the delays in production as the molds sit in the cooler before being moved into the demolding station. Note the rise in demolding temperatures around cycles 70, 90 and 110.
Figure 4: Ambient Temperatures Ambient conditions were reasonably constant for this set of tests except for the three days around cycles 70, 90 and 110 when the ambient conditions rose substantially. This clearly affected the final demolding temperatures shown in Figure 3.
Figure 5: Time Required to Cool Molds to 100°F (38°C) During cooling, most machines are oblivious to the actual temperature of the molds and a preprogrammed cycle is normally executed. Figure 5 shows the actual time that was needed to cool the molds to 100°F (38°C). The cooling cycle could have been stopped at this point.
Figure 6: Potential Recovered Cycle Time Delays in the cycle and a more balanced schedule will help to eliminate problems; a smooth production cycle in which the molds move continuously is the goal. However, in situations where problems do occur, such as above, the machine can automatically compensate for the changes in ambient conditions, operator problems, etc. Figure 6 shows the time that could have been recovered for production if the machine had been given control of the cycle by temperature and not by time. There are a number of ways of providing continuous control for processing. Read Garth Galloway's excellent article in the latest edition of RotationTM magazine (November - December 2001 www.rotationmag.com) about using direct (RotologTM) temperature measurements inside molds on rock and roll machines. For biaxial machines, IRT (Infrared Thermometry) can be used for continuous mold surface temperature sensing that can be tied directly back into the logic of the machine (contact Ferry Industries, Inc. at www.ferryindustries.com for more information). Part Size vs Demolding Temperature Part size is also affected by:
Figure 7: Part Size vs. Demolding Temperature The ultimate goal of monitoring the effect of ambient conditions is to use the information to control the final properties of the part for consistent product quality. The temperature at which a part is demolded can have a dramatic effect on part size. The hotter the part is when removed from the mold, the more it will shrink. Figure 7 shows this trend for one particular part tested over a period of time. There are, of course, many other variables such as mold release, pigment and additives, part shape, material properties, surface texture, rate of cooling, etc. that will also affect part size.
Key Factors Some factors to be considered for part size control:
All of these and more are considered in depth in Rotational Molding: A Practical Guide.
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