Heat Energy Removal/Time to Cool
Thus far, for the Vial Case Study, we have reviewed the Core Designs, Simulation, & Stress. Next is Heat Energy Removal & Time to Cool
During the injection molding process, heat energy is removed from the molten plastic in several ways:
Conduction: The mold, typically made of metal, acts as a heat sink, drawing heat away from the molten plastic, allowing it to solidify.
Convection: Cooling channels within the mold facilitate the circulation of cooling fluids, such as water or oil. These fluids absorb heat from the plastic and help maintain a consistent temperature throughout the mold.
Radiation: Some heat energy may be dissipated through radiation, although it is not the primary mechanism for heat removal in injection molding.
The time it takes for the molten plastic to cool and solidify in the mold depends on several factors:
Material: Coolant flow versus temperature rise was estimated based off generalized material properties for polypropylene. Meaning, we looked at different grades of polypropylene materials: There is a range in specific heat, latent heat of fusion, etc. We then went down the middle of those values to get an estimate of where things were at.
The Part/Molding: We evaluated how much heat energy would need to be taken out of the part to get it from its processing temperature down to a reasonable ejection temperature. We used a processing temperature of 450 degrees Fahrenheit down to 160 degrees Fahrenheit for ejection temperature.
Time to Cool: This is how much time it will take to get the heat energy out of the mold into the water system, assuming it is capable of pulling that much heat out. We are defining Time To Cool as how much “total” time it is going to take to cool. Time to cool here would include first stage injection, pack and hold, cooling time before mold open until mold open. We estimated the range of time to cool from 7 seconds to 12 seconds, and settled on the eight second mark.
Heat Energy: We assessed how much heat energy was needed to pull out of the mold, and how much heat energy would be going into the water if the system is capable of pulling that much heat out.
Mold Components: We then had to decide, out of that heat energy, how much proportionally, is going into the core, cavity and the two slides. We basically broke down the heat energy like this: 50% of it goes into the large cavity insert because it’s the largest insert in the stack up, the biggest heat sink, and a lot of water flow coming into it. 10% of the heat energy into the slides (5% per slide) would leave an estimated 40% heat load going into the core.
The Graph: We then built the below graph:
Assumptions:
% of total heat energy within the PP part will be removed by the core coolant circuits
∆T of 5F
Time to Cool: 1st Stage Inj, Pack & Hold, Cooling Time before open, and Mold open
Process Temp 450◦F
Ejection Temp 160 ◦F
With an 8 second cooling time, what we are able to extract from the mold with the goal of increasing no more than five degrees Fahrenheit.
40% requires .43 gallons per minute through each core. If we feel the core represents 50% of the heat energy coming into the core, then we would need half a gallon per minute.
The graph represents these two lines. 40% of energy represents 40% of the heat energy of the plastic going into the core and a cooling time of 7 to 12 seconds. From the trend line, it can be seen that the longer we have to cool the part, the less the temperature rise will be.
At 8 seconds, we know for 40% of heat energy we have .43 gallons per minute. So now it is how many cores are we going to string together and are we going to use a series circuit or parallel?
Multiple Cores:
Now we simply took that last value for each one of these times and multiplied it by the number of cores. We are putting them in series and will need 1.7 gallons per minute coming through a series circuit that will feed 4 cores.
The next installment, Part 5 will be around Pressure Loss
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