Cavity pressure sensors have been in injection molding manufacturing for over 30 years. There are various applications in industry. Today, let's talk about value of sensors to injection molding.
As we all know, quality of injection molded products is determined by three elements: pressure (P), specific volume (V), and temperature (T). Theoretically speaking, as long as balance and stability of these three elements are ensured, product quality can be ensured.
In actual molding process, specific volume cannot be directly measured, cavity pressure and temperature can be measured and controlled with high precision through sensors. Various molding process parameter values are set values on injection molding machine, not actual values. Due to influence of factors such as injection molding machine performance and raw materials, there is a deviation between actual value and set value, deviation is constantly changing. In other words, molding process is a fluctuating process. Therefore, cavity pressure is also constantly changing, and injection molding machine cannot finally control cavity pressure value.
Figure below is a graph of pressure measured at different parts of injection molding machine. It can be seen from this figure that pressure values measured at different parts of injection molding machine are different. Oil pressure and pressure at nozzle are far away from mold cavity, so they cannot truly reflect melt state information in actual mold cavity. Cavity pressure directly reflects melt state in cavity, and is most closely related to product quality.
A large number of scientific research and industrial practice have proved that among all process parameters, cavity pressure has the greatest impact on product quality. Cavity pressure and temperature are comprehensive indicators of product quality.

 

Main problem with this product is microprinting. Mainly due to uneven wall thickness caused by product design, thick wall part near end of filler is very far away from gate, and relatively thin wall part is caused. This defect is foreseen by volume shrinkage analysis in mold flow in figure below. However, due to limitation of product structure, problem cannot be solved by reducing thickness of glue.

There are miniature prints on both ends of molded product.

 

 

During production, pressure process window for packing stage is only about 20 Bar. Inherent fluctuations of injection molding machine, difference between different batches of materials lead to a very high rejection rate in mass production.
When this part/mold was first seen, cycle time was about 78 seconds. Every part has flash, and operator keeps scraping flash at the edge of machine. Parts are still being scrapped due to microprinting, with a scrap rate of about 22.5%.

 

Install sensor into mold
Variables in injection molding production, including people, machines, materials, methods, and rings. For process, key variables are plastic temperature, plastic flow rate, plastic pressure and plastic cooling, which can be acquired, analyzed and monitored by sensors.

• After installing cavity pressure sensor in cavity, process stability was first evaluated, in-mold pressure difference between molds was reduced by rationalizing parameters. Figure below shows in-mold pressure overlap curve. The higher overlap, the more stable process. On the contrary, if fluctuation is large, the first step in problem solving should be to improve process stability. Only in a stable process situation, it is possible to completely solve problem.

 

• Next, compare pressure curves between different cavities as shown in figure below, analyze differences between different cavities, and control difference between cavities to 5% by improving mold runners within (within 2% can be required for high-precision injection molding).

 

• Finally, real-time pressure monitoring is carried out through sensors. In addition to common pressure peak monitoring, pressure curve slope monitoring can ensure consistency of feeding rate and cooling, ensure quality of shipped products.

 

• Improved scrap rate of approximately 0.67% and cycle time of 50 seconds. Considering relatively small molding process window resulting from this poor part design, current improvement is quite noticeable.

 

 

 • Layout of the entire process control system is shown below

 

Through installation of cavity pressure sensor, pressure curve data in each cycle is obtained. Process stability can be analyzed by overlapping curves.

 

In-mold pressure curve
Reading and understanding in-mold pressure curve maximizes understanding of injection molding process.

 

Application of Sensors in Optical Injection Molding

To date, cavity pressure measurements have generally not been possible for optical components used in automotive engineering, such as lenses or light guides, and molded parts with Class A surfaces. First, surface needs to be protected from imprint of sensor mounting; second, mounting of direct sensor can cause small changes in thermal field near cavity wall, which can lead to changes in optical properties of plastic part. Kistler's sensor types 9243B, 9245B, 9243A and 9247A now provide perfect solution for measuring optical in-mold pressure.
Pressure to produce parts with extremely stringent optical and surface requirements. Sensor is inserted into mold insert behind cavity wall, and feedback change of pressure in mold by measuring steel compression caused by mold cavity pressure.
Sensor can be mounted away from cavity surface to prevent affecting thermal field near mold surface.
• Sensor measures mechanical strain (ε) in core caused by in-mold pressure.
• Finite Element Analysis (FEA) to determine stress concentrations; ie to obtain the best signal (red).

 

Special Applications - Pin Sensors
A mold for a projector lens with two pin-type sensors, sensor model 9245B, amount of charge is proportional to amount of force.