Unraveling the Mysteries of Melt Temperature

While the question about melt temperature appears straightforward, it actually also implies two other questions: What should the temperature be, and is the melt sufficiently uniform in temperature to specify a single number? Unfortunately, there are no simple answers. Even today there is no widely accepted and trusted method for measuring temperature during processing in injection molding. But you might find an infrared camera useful in getting some answers.

That said, how do you get a handle on what temperature to target and how to measure it? First, let’s tackle how to determine the target temperature for your resin. Start by getting information on the specific grade. Do not simply go with, “It’s a nylon” or “polystyrene.” Today there are grades formulated with additives that drastically change the processing temperature; you want to be sure you are operating in the recommended range for your specific material. Too high a melt temperature may destroy the UV or antioxidant stabilizer; go too low and you can shear off the screw tip or have unmelts flowing through your barrel and mold. So take the time to find the supplier’s recommended range and target the middle of that range, unless you have experience with this resin and already know the correct processing range.

Usually this data is available from the supplier’s literature or one of the online databases for resins such as SpecialChem’s Universal Selector. Look up the resin, find the specification datasheet and look for something along the lines of “processing details.” Once you have a target temperature, refer to my column in February 2014, which details how to set barrel-zone temperatures.

With the barrel zones set correctly, turn on the heater bands and watch to make sure that the barrel warms up uniformly. Ideally have all zones, including the nozzle body and tip, come up to temperature setpoint evenly. Use this method for your hot-runner nozzles, tips, and manifolds as well. Do not let the nozzle body or tips heat up to set temperature in 15 min while the barrel zones or manifold take 45 min. That is one way to start degradation and black specks. After the appropriate heat-soak time, roughly 20 min for smaller machines and up to 1 hr on larger ones, you can operate the screw.

Just don’t start by rotating the screw. My suggestion is to leave the screw 50 mm back, and manually inject to clear the check (non-return) valve. Then rotate the screw to purge and rotate to achieve shot size. As you purge, turn on your IR camera and allow some time for it to self-calibrate (most do this automatically). Make a purge and practice taking a picture (i.e., the temperature) with peak or max turned on. That is, find the angle that allows you to watch safely the purge as it is shot and sharpen the focus. Most IR cameras do not have auto focus, and focus is important for good results. Also, ensure the emissivity is at 0.95.