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What is actually happening
Definition: a measure of warmth or coldness of an object or substance with reference to some standard value.
Within the processing of plastics in whichever process, temperature is a key factor. This may be the temperature of the material, mold, barrel, throat, cooling bath, water spray, etc. The issue is; is it measured or is it measured at all.
In many processes the equipment allows one to set the temperature values and many actually yield an actual valve too. What this means is that there is feedback to allow the user to know what is set and what the actual valve is at this time. Within a process setup both of these valves may be recorded and or what that temperature setting is actually yielding, such as the melt temperature if one was looking at barrel settings.
Another example of temperature settings and actual values may be chiller or mold heater in use with the process. One can set the temperature, and in most one can view the actual return temperature but in most cases the piece of equipment or thing (mold, rolls, cooling bath) is not instrumented or actually checked for temperature. What size unit was used, hose diameter, fittings and was it plumbed the same as last time, can all have an effect on the actual temperature which the plastic sees.
In many cases the issue is that the tools are not available (pyrometer, IR gun, etc.) for the operators, process techs to use, and also because they do not think it is important to measure. As some may say "I set the machine up same as last time, or I set it up the same as the setup sheet stated so it must be the same." In many cases it can be agreed that all shall operate satisfactory with no issues. But when an issue arises the temperature of the process should be checked and checked first.
The temperature of the melt is important and to what degree is really dependent on whether one is processing a semi-crystalline material or an amorphous material. With a semi-crystalline material one must be above the melting point, and hopefully mid-range of recommendation. While the viscosity does change with increased temperature, (not that great) the flow-ability is affected due to the heat content and cooling rate of the materials.
With the amorphous materials the melt temperature plays a greater role in the viscosity, which is directly related to the flow-ability.
What is meant is that minor temperature differences in an amorphous material well have a greater impact on flow than what one would see
from the same temperature variation in a semi-crystalline material.
The melt is affected by not only the barrel settings, but also by screw surface speed, and back pressure, thus the result of all these setting equals the melt temperature. At present to measure this result the use a pyrometer/IR gun, is needed unless one has installed a nozzle or die with a probe which measures the actual melt.
An example of an issue with melt temperature and or uniformity of melt is a change of machines, both the same tonnage but one used a 40mm screw and the other a 25mm screw. If all was set the same in regards to machine setting, (disregard to screw size change) than going from the 40 to 20 the material temperature may be colder, but without measuring the actual melt one would not know. The reason for this is that setting RPM the same (as most setup list RPM values) the actual shear/surface speed is lower than in the 40 if the 40 was the original.
Within the mold / cooling side of things, there is so much thermal mass with a mold that setting the temperature on the equipment without checking the actual results typically yields a great disparity in temperatures. In most cases the technician well change the settings to a lower valve on the control unit and when and if measuring the actual mold, bath or roller, find that they are not near the manufactures recommendation. But if the actual is not measured than there is no reference except what was set, and this may not be good enough.
Temperature of the mold affects the crystallinity of the semi-crystalline polymers, and stress in both amorphous and semi-crystalline materials. Further in semi-crystalline materials with color addition the melt as well as mold temperature can reduce the issue caused by nucleation.
Temperature of the part upon ejection, or the completion of a step in the extrusion process is critical and can lead to troubleshooting of an issue either within the mold or on the line. Typically one can eject a part out of the mold when it is below the HDT of the material, whether this is 5 or 10°C below said temperature. Also if one were to use an IR camera one may find areas in the part which were not similar in temperature, which can lead to warping and or shrinkage issues, and if back tracking lead to either part (wall thickness), or mold (cooling) issues.
In conclusion temperature is critical to processing due to the fact that first the plastic material must be heated up and then cooled to be extracted. The temperature at each step should be checked for both efficiency and quality, thus it must be documented so that it can be controlled.
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