Tech Tip
                                        April 99

Hot Runner Manifolds, part 3
 

In the last few articles the subject of pressure drop and isolation of the manifold have been discussed and hopefully explained. To continue on that vane let us now discuss the idea or process of heating the manifold and where to place the thermocouples. Again we must state that what we do with the manifold is convey the melted or soften plastic to the drops. The manifold does not melt the material it only conveys it. The drops are what we use to convey from the manifold to the cavities.

In the heating of the manifold it again depends on what system is used, annular or cylindrical. With the annular system we have the use of distribution tubes and placement of the heaters and thermocouples are defined. The use of cartridge heaters in the distribution tubes is the form of heat provided, but note the watt density and the length of the cartridge that is used. In reference to watt density within cartridge heater it is also referred to as distributed wattage output. When discussing watt density we are talking
about the number of coils per inch or pitch of the coil wire within. Thus if the wire is close together it is of a higher watt density than when it is spread out, which within a cartridge heater is important to know so as to position the heater in the correct place. Further with cartridge heaters there are also cold sections, these being areas where there are no coils thus no heat. Note that the insulation / ceramic interior of the cartridge heater can absorb moisture and this is why a soft start is required. With a soft start we apply
lower voltage for a period of time to dry out any moisture that may have been absorbed, thus preventing a possible short and the replacement of the heater. Most cartridge heaters have an integral thermocouple within them so that the temperature is controlled for each heater.

With the cylindrical flow path tubular heaters are typically used. These are usually placed in a cut away channel in the manifold block and potted in place. We need to do this on both sides of the manifold block to get a uniform heat to the block. These heaters can be ordered bent from the factory or straight and bent in the shop. The potting compound typically has a high thermo-conductivity to allow the heat to transfer.

Again a slow start is recommended to dry out any moisture. A common problem with these is that they fail due to improper installation. That is, the ends are twisted when attaching the leads; this allows the wires inside to twist off, thus denying the circuit. X-raying non-working tubular heaters have found this.

The path cut into the manifold is such to allow as uniform heating as possible to the flow path. The simplest way is to look at a cross-sectional view of the block with the flow path and heater placement and have you surrounded the flow path and maintained a uniform distance from it?

Are there other ways to heat the manifold block and or maintain the temperature of the melt and the answer is yes. I have only listed the two most common methods per my experience.

In either of the two types of manifolds that are used we now need to control the temperature of said heater systems and must use thermocouples to do so. In the annular system with the use of cartage heater the thermocouples are integral with the cartridge heaters. Thus each heater would have its own thermocouple and control. With the cylindrical system using the tubular heater we would place a thermocouple on the manifold. The location / placement is very important. If placing this thermocouple in the wrong area say away from the center and close to the heater we see the temperature there. Thus we may have to have the manifold setting at 700 degrees F to maintain a melt temperature of 500-degree F. I may place two thermocouples on the manifold one on either side and control the temperature of each separately. If you buy the manifold from the supplier they will locate the thermocouple in the area best suited based on their knowledge and experience. If in fact you are building this than be careful.

Now you see the dilemma I there concerned that the thermocouple is reading 700 F to get 500 F, yes and no. The temperature of the melt is what needs to be controlled and the closer the location of the thermocouple is to the flow of material the better. It is best to know what your system needs to operate and that you are controlling properly. This again brings up a point in that if the manifold was designed to run at 500 F and we need to set it at 700 F what was the thermo-expansion of the manifold, drops etc calculated for?

Thanks for the time, to be continued.
 

Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Chemical