Moldmakers in today's competitive environment are called upon many times to be more than skilled professionals at building quality tooling for a customer. Because they are asked at times to be a plastics expert ("What shrink should we use?"), process technician ("What mold temperatures do we need to run and how fast?") and automation inventor ("Is there another way we can get the part out of the mold?), moldmakers need basic knowledge on the benefits of pressure sensors, options of styles and placement within the mold. The adding of value to customers is important in today's competitive environment.

 The Purpose of a Pressure Sensor
 The pressure sensor measures the plastic somewhere between the machine nozzle to the cavity area as the part is filled, packed, held and cooled. Sensors can be placed in the machine nozzle, hot runner systems, cold runner systems and the cavity area of a mold. However, we will focus on the cavity area.

  The sensor reads the pressure at the location it is placed in the runner system or cavity area, and that information is recorded on a monitoring system. This information, depending on auxiliary equipment, can be used to adjust the molding press real time, be reviewed after the mold trail or to troubleshoot problems that develop during a production run. Also, with the information collected, parts can be reproduced on a machine-independent basis.
Pressure Sensor Options

 There are two styles of pressure sensors being used in molds today—flush mount and indirect. The flush mount uses a sensor which is installed close to the cavity by machining an installation hole from behind the cavity to allow the sensor to be flushed with the cavity surface. Then, cabling is brought out through the mold, as appropriate, to the connector on the outside surface of the mold. One advantage of this type of sensor is that it does not see the forces of ejection; however, in some high-temperature applications it can be damaged, which makes the installation problematic.

  The indirect sensor comes in two configurations, the slide and the button. These sensors use the force exerted by plastic on an ejector or stationary pin, which transmits the force to the sensor which is either in the ejector plate or the clamp plate of the mold. Indirect sensors are most commonly placed in the ejector plate of a mold beneath an existing ejector pin. However, in high-temperature applications or when low force sensors are used for small pins, many times the sensors are placed in the clamp plate of the mold and the pin is used through an ejector sleeve or uses a transfer pin, The benefits of the transfer pin are that an existing ejector pin can be used while the sensor is not exposed to the forces of ejection. Also, in fast-cycle, high-speed ejection situations the sensor is not exposed to the rapid acceleration and deceleration of the ejector plate.

 The size of the ejector pin that sits on top of the sensor will dictate what sensor size will be used. It is best for the mold designer to use the same size pin for all sensors in a mold to eliminate setup or calibration errors by the user. Since a pin is used to transfer the pressure of the plastic to the sensor, one size pin will not work for all applications.
The button-style sensor is held captive by its pocket. To move this sensor around in a mold, plates would have to be separated or provisions would have to be designed in advance.

  Usually when this style is used, a sensor is placed where required.
Depending on types of button sensors and where they might be located in a mold, collector pockets for the cable may need to be added in the mold plates. The button sensor is more reliable because of its pocketing and is preferred whenever possible.
The slide type, which is no longer in wide usage, can be removed from the mold while in the press. The downside is that the slide sensor can move in its pocket and not give an accurate reading.
Where to Locate and Why

  Sensors
It is important that pressure sensors are properly located to get the maximum amount of information for molders. While there are many specific exceptions to the rule, sensors for process monitoring should be located near the last one-third of the cavity to fill. Sensors for controlling the process by transferring the press should be in the first one-third of the cavity. For very small parts, sometimes the sensor is located in the runner system. However, this is a compromise that does not allow gate seal to be observed using the sensor. It is important to realize that when a short-shot is developed, zero pressure at the end-of-cavity is present; the end-of-cavity sensors are key for sensing short-shots and automated part containment. Today, using digital sensors makes it easy to put sensors in every cavity and simply have one network wire from the mold to the press. This allows 100 percent insurance against short-shots being delivered to the customer by simply installing the sensors at the end of cavity without any further process control interface to the machine.

  Pockets
 Mold designers or moldmakers will have to determine during the design process where to place pockets for the sensor, cabling and exiting from the mold. It is not desirable for moldmakers to let the cable or wire exit the mold and be free to move around. The connector should be fastened to the mold base and an additional cable used to connect the mold to the molding press or auxiliary equipment.

   Mold Tryout
  One of the biggest values provided for and by moldmakers is the rigorous mold tryout using the instrumented mold. This allows a process to be set up and optimized on the first or second tryout and then used for every subsequent tryout to minimize the number of trials necessary. Once this is done, and the mold has been qualified with correct dimensions and capabilities, the documented process can accompany the mold to molders and be set up. Mold builders who understand this process help molders make the transfer directly to their production equipment as part of the value of the mold. This could be handled directly by mold builders, or in some cases, by a third party who has the equipment and understands the methodology. This truly gives mold build a new set of values to molders and, ultimately, to OEMs who own the tool.                              

About First-rate Mold Solution Co., Ltd.
First-rate Mold Solution Co., Ltd. in China, provide industry design,mold design,mold manufacturing,molding analysis,project management service.

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