Definition: Thermal, 1) as regards temperature and or heat   Control, 1) to command 2) a device for regulating and guiding a machine.

Thermal control is one of the issues with a heated sprue bushing, both monitoring and heating. This thermal control is based on watt density of the heater system within the bushing and further is enhance by where the thermocouple is  located.

At the upper end, is not a good choice as now we are measuring temperature at the nozzle end (machine) and not the exit end (sprue bushing).  The location of the thermocouple at the nozzle end results in a higher than normal setting/reading, this due to distance from the tip of the bushing.  Remember this is where our nozzle (heated) is in contact with the bushing, and the bushing is insulated from the mold at this point.  The material must now travel down the bushing and into our part/feed system, where it (the bushing is in contact with the Mold)  thus now we are measuring at the furthest point from this and may find that to get flow we are above recommended settings on temperature.

If our thermocouple is in the middle of the bushing than it is better than at the nozzle end but still not measuring at the end we wish. It may be found again to be higher than what is recommended on settings when used.

Knowing the thermocouple location is important as it determines what is measured, in relationship to our bushing and whether or not we are to use it as a monitor of actual temperature or just as a number and compare to previous runs as to what that number was. (This can only be determined by the operator/setup person)

The watt density refers to how the heater coils are spaced in reference to our bushing. There are formulas which state the density per square inch of surface, and these are correct, but from our point we need to know what it is, and how these coils are spaced around the channel of the bushing. Typically the density of the heaters is greater at either end, and spaced out though the body of the sprue bushing,  basically concentrating heat or energy at the entrance and exit point while maintaining heat in the body.  If one thinks about the mold contact areas and the heat sink effect this makes sense. 

One question to be asked is how the heaters are held in position and what contact do we have. The issue here is that if held there is no issue, this can be cast in place, cut in place or clamped in place. If in fact the heater is a coil which is allowed to grow and shrink because it is somewhat loose within the bushing, we can have an issue.  This issue is that watt density changes over time as the coil becomes shorter and shorter over use. This can be seen by the fact that the temperature set point  increases in temperature setting over time, (time is relative as this can take a year of use). What I have witnessed is that as the heat is increased it can reach a point where degradation starts with the materials being processed which leads to rejected parts and everyone wondering why.  Basically the watt density is now higher in one area, and heats are increased to maintain flow, due to lack of watt density in the tip area. (the heating coils have moved further from the tip area due to shrinkage)

An example was a polypropylene mold with a coil type heater. Over time the coli shrunk/ changed watt density to the point that temperature was increased greatly to maintain flow. The polypropylene started to degrade to a gas, resulting in tiny bubbles in the melt stream, and they trail into our parts. The results were rejected parts.  Upon disassembling the bushing and stretching out the coil, the temperature was lowered and the issue went away.

SLSilvey

silveysplastics@hotmail.com

The thermocouple is important in the location of its position within the heated sprue bushing. Is it located at the tip of the sprue bushing, at the entrance side or along the side of the heated sprue bushing.  What is important to know here is where it is. If the thermocouple is located at the tip or exit area of the sprue bushing than we have an accurate temperature reading of what the plastic is doing at this end which feeds our parts and or runner system. Given that we have contact to the mold in the same area it is a relative reading of what we have of melt temperature into our part, but keep in mind that it is measuring only the steel of our unit.  Ideally this is the position for our thermocouple.