COOLING | PROCESSING
Generally, we do not recommend selecting a cooling timer that only meets the HDT — a buffer should be allowed as a safety factor. A good rule of thumb is adding 20% to the cooling timer to account for variation on incoming material and slight shifts in machine performance. For tighter tolerance parts, the safety factor will likely need to increase. For the mould design, an 8-cavity mould with an
“H” pattern runner and a lapped edge gate is used. Cooling lines have been placed in the cavity, core, and runner block following established guidelines for diameter/depth/pitch. By using proven meth- ods for cooling line design, warp and cooling time are minimised. The mould is also fully instrumented with cavity pressure sensors at post-gate and end-of-fill in conjunction with in-cavity temperature sensors.
So where does that 80% number come from?
Let’s look at the data we collected. The process segments we looked at were: fill, pack/hold, cooling, mould open/eject/mould close. For this experiment, we developed a robust Decoupled II process, resulting in the following process parameters: 1. Actual Melt Temperature: 236°C 2. Actual Mould Temperature: 50°C with a flow rate of 11 lpm per cooling circuit
3. Fill Time: 0.26 seconds with a transfer pressure of 576 bar plastic pressure
4. Pack/Hold Time: 8.0 seconds at 286 bar 5. Cooling Time: 10.0 seconds 6. Overall Cycle Time: 21.43 seconds
If we add the process times together and divide by the overall cycle time, we reach a value of 0.85. That means that 85% of the cycle is spent cooling the part so that it can withstand the forces of ejection. The chart shows the actual part temperatures
measured with a surface probe at different time increments during process development. In our experiment, the tensile test bar cooled from 228°C to 103°C in 8.26 seconds (fill and pack/hold time). For the part to get below the HDT of 83°C, it took an additional 8.47 seconds. The temperature only dropped 20°C (to 80°C) in just under 9 seconds, which means that there was a lot of heat transfer efficiency lost. The chart also indicates that at some point,
leaving the part in the mould is of little to no added value. Based on geometry, material, mould, and processing, leaving the mould closed for longer than 24.73 seconds really is not cooling the part down much more. We must consider that using thermal imaging technology or a surface probe are not perfect representations of the actual part. The surface
www.injectionworld.com Part surface temperature
temperature of the part was measured several times over a 2-hour run at 74°C. Thermal imaging showed the part to be roughly
87°C, while software predicted 83°C at the same time in the cycle. The image also indicates that the thermal signature across all eight cavities is nearly identical. By utilising two different methods, we can feel confident that the part temperature is at about 80°C (3°C under HDT). In conclusion, part cooling is always a function of wall thickness and the material that the design engineer selects. It’s up to the mould design engineer to place the cooling channels in the proper location to allow for a minimum cooling time. As a process engineer, the old adage of “fill as fast as possible consistent with quality” still holds true. If the volumetric flow rate is very low, the chances of packing out a part are slim to none because the material will likely be frozen. For this geometry and material, most of the heat (125°C) is removed during the filling and pack/hold phases. However, to remove the last 20°C to be below the HDT, it takes longer than it did to fill and pack/hold the part. Cooling is a waiting game, but with better engineering (part design, mould design, and process), the time required to cool the material goes down. �
www.rjginc.com
About the author Jeremy Williams is based at RJG in Michigan, US and has over 17 years of experience in the plastics industry. Currently he is a Consultant/Trainer with TZERO®.
March 2019 | INJECTION WORLD 37
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