Universal Cavity Pressure Sensor Installation
To ensure the successful application of cavity pressure monitoring and control technology, it is vitally important that sensors be installed properly. Proper installation requires two (2) fundamental things. First, the sensor should be located in the correct region of the cavity so that proper information will be obtained. Secondly, the mechanical installation must be done properly to ensure that the sensor can be installed and removed and that it will not have pre-loads, side loads, or other mechanical impediments to the successful detection of the pressure in the cavity.
Sensor Placement Strategy
The strategy for locating the sensor will be different depending on whether your primary goal is process monitoring or process control.
Process Monitoring Applications:
If the sensor is primarily going to be used for monitoring rather than control, the end-of-cavity is the best location. This specifically applies to accomplishing SPC, predicting good vs. bad parts, correlating cavity pressure to part conditions, or simply detecting non-fills (short shots). This location will show the maximum amount of variation of in-cavity pressure and act as a !°catch-all!± for the process detect change. A good way to find the sensing location at the end-of-cavity is to make a shot approximately 10% short by reducing the injection stroke and turning off 2nd stage pressure. The sensor can be placed beneath a pin (or flush with the cavity) in the unfilled region.
If the pressure drop across the cavity is to be detected or if the maximum amount of information is to be obtained using sensors, a sensor near the gate, in conjunction with one at the end of the cavity, will provide optimum capability. This will allow monitoring of the degree of packing across the entire part, as well as an estimate of average cavity pressure. It will also provide the maximum ability to monitor part quality.
In multiple cavity tools with cold runner systems, monitoring of one or two cavities will generally allow the prediction of quality in the other cavities. This assumes that proper attention is paid to mold balance, mold deflection, and mold temperature control so that each cavity is influenced in an identical fashion. It is easier to predict part quality in a balanced mold than an unbalanced mold. If an unbalanced runner system is used, installing a sensor in the first and last cavities to fill will usually be sufficient for monitoring purposes in most applications.
Hot runner molds are different because the temperature of the tips often varies over time so the balance is rarely consistent. This means that monitoring of one cavity will not necessarily predict quality in other cavities; therefore, more sensors are required for monitoring. If monitoring of all cavities is not practical, you can monitor one cavity in each zone instead (See Figure 1). Alternatively, you can identify problematic cavities and only monitor these. In single parts with multiple drops, the last point to fill beneath each drop should be monitored for maximum quality monitoring effectiveness
NOTE: In hot runner applications, if there is any question, contact RJG Customer Support for assistance before locating sensors in the mold.
Process Control Applications:
If control of the molding machine is the primary reason for the implementation of cavity pressure sensing or if flashed parts or gate seal/non-seal is always to be detected, placing the cavity pressure sensor near the gate end of the part inside the cavity is correct. RJG strongly recommends that end-of- cavity locations be avoided for control as the machine cannot react fast enough to use this for control.
Control sensors should be located in the "Area of Influence". This the region of the part where the last material was flowing. An easy way to find the Area of Influence is to change from a translucent material to a colored material. The region where the first color appears is in the Area of Influence
Mechanical Placement
Sensor placement in an injection mold should, whenever possible, be behind a moving ejector pin that is actually used to eject the part. This is always preferable to one which is stationary as the ejection movement will remove residue around the pin, minimizing friction. This is especially true in thermoset applications and when sensing thermoplastic materials that give off gases such as vinyl, acetyl, etc. (NOTE: Does not apply to Flush Mount sensors)
A sensor located behind a moving ejector pin will be able to sense and measure the force required to eject the part out of the mold.
The pin used must fully contact plastic in the mold runner or cavity and not contact or !°kiss off!± on mold ste in such a way as to impart a pre-load and cause false readings. The location of the sensor in the cavity is determined by the primary use of the sensor, as outlined previously.
RJG's strain gauge-based design sensors ar configured in a Wheatstone bridge, which converts mechanical energy into electrical energy. Strain gauges operate on the principle that the resistance of a wire changes as pressure on the wire changes
The size of the ejector pin impacts the amount of force applied. Therefore, the sensors are rated in terms of force (lb) rather than pressure. For a full scale pressure of 20000 psi in a mold, the amount of force on the sensor can be determined as follows:
F = 20000 psi * ?D2/4 Example: With a 1/8" pin, the force o the sensor equals 245 lb for a 20000 psi full scale pressure. This 245 lb is roughly half of the full scale rated load of the Model T-412 sensor (500 lb) yielding a rugged application.