Definition: Actual = 1. Existing in act or fact; real 2. Existing at present; current; real as now Temperature = 1. the measure of warmth or coldness of an object or substance with reference to some standard value So when combined together the definition of actual temperature they become what is the present measurement of warmth or coldness with reference to a standard value, at the present time or now.
For melt temperature which refers to the actual temperature of the plastic material as it is extruded, injected from the machine the industry standard has been the 30- 30 rule. Most material suppliers will detail this out in one form or another. To perform this test the following is needed.
1- pyrometer with needle probe,
a. Though there is debate on diameter of the needle try to go as thin as possible.
b. If using an infra red device please note the differences below and be sure it can measure the material temperature and has correct wavelength filters installed. Basically it was designed to be used with your materials.
2- Torch, heater, cigarette lighter.
3- Container, for capturing the polymer.
4- Safety procedures understood and good shop practices as to purging materials, and capturing them.
a. This includes gloves, glasses, possible sleeves, etc.
5- Notepaper for recording data.
The procedure for taking a melt temperature within the injection molding machine is as follows:
1- Wait till machine has stabilized in process.
2- Preheat the pyrometer to 30º F above what is thought to be the melt temperature
a. Maintain that heat point
3- Purge out a shot of material in a normal cycle into a cup, container or piece of cardboard
4- Immediately insert preheated pyrometer probe into the material
a. If using IR gun, break open material and take a reading of the inside of the material so as to record temperature.
5- Wait 30 seconds and record temperature.
a. If using IR gun there is not a 30 seconds time frame, record peak temperature or if possible have setting and recording of gun do it automatically and than recall and record.
i. To take temperature though break open the material to record inside temperature and not outside skin temperature.
6- This will be the actual temperature the material is being processed at and should be listed on the setup sheet.
As noted you can see that we are 30 degrees above the melt temperature prior to taking the reading and we wait 30 seconds after sticking the probe into the material to take a reading. This than gives the 30/30 rule, or procedure. Why 30 degrees above, it is because without knowing the pyrometer being used most pyrometers will count up from room temperature to the temperature being measured and this takes time at which point we have lost temperature within the material. The 2nd reason for the heat is to prevent a skin from forming (as fast) on the probe as quickly as it could and insulting the probe from the actual temperature. As far as the 30 seconds it is a time frame that is easily achieved establishes a reference point.
(Steel temperature would be done with a surface probe and takes a bit longer to achieve, this will be discussed in a separate article.)
Other temperatures which are to be measured may be the water bath temperature in extrusion and or the air temperature in blown film. Both extrusion and film lend itself to using an infrared gun or measurement device in that it is continuous and provides areas of measurement to record on an ongoing basis.
As mentioned before the filters in an Infrared gun need to be correct in order to measure plastics. In the original IR units they were either calibrated for a low range or high range. It had been my personnel experience to test the temperature of a plastic part coming out of a mould and finding it to be 98º F as I was measuring the temperature of my hand. The use of an IR camera at the time showed the part temperature to be 140º F and was confirmed with a surface probe.
The other aspect of IR temperature measurement is the emissivity of the unit and where it is set at. This will show up as a difference in temperature reading between two different units if their respective emissivities are set at different values while reading the same point for temperature.
Actual Temperature is what should always be recorded, this than used in conjunction of what is set on the machine or equipment will give insight into what is going on. Actual temperature should also be referred to as a result, and as such it needs to be documented for repeatability and quality purposes for that mold, process or manufacturing step.
Steven L Silvey
silveysplastics@aol.com
360-882-3183
Beaumont Technologies, Inc.
