Pressure measurement on injection molding machines

Injection molding machines work with the highest precision. The Swiss company Netstal-Maschinen AG offers high-performance, high-precision injection molding machines and system solutions for the beverage, packaging and medical technology industries. Pressure sensors from STS are installed in the high-quality systems.


A plastic injection molding machine is used to produce finished plastic parts from plastics in granular form. Such a system consists of two roughly simplified components, the injection unit and the clamping unit. The raw material is prepared in the injection unit. This is usually heated and homogenized in a screw shaft located in a hydraulic cylinder. The clamping unit contains a mold that represents the negative mold of the plastic part to be produced. The molding compound prepared in the screw is injected into this negative mold under pressure.

Monitoring the necessary pressure ratios is essential to ensure a fault-free injection molding process. For this purpose, the sensors are mounted in the hydraulic circuit of the injection axes. The specific melt pressure can be calculated on the basis of the measured chamber pressure during the injection process. It is particularly important that the measurement deviation of the sensors is very small, otherwise the plastic pressure is calculated too low or too high.

If the melt pressure is too high or too low,
-this will affect the injected filling quantity,
-the finished plastic part may be defective,
-material loss or mold damage may occur,
-the system may come to a standstill.

High-precision systems such as the injection molding plants of Netstal-Maschinen AG require pressure transmitters that deliver absolutely reliable results over the required measuring range. In order to find the best solution for the high demands, extensive tests were carried out with instruments from various manufacturers. The aim was to test not only the accuracy of the measuring instruments, but also their long-term stability under high temperatures. The following measurement intervals were performed on the test rig:

Test method for the evaluation of pressure transmitters - pressure measurement technology

Figure 1: Standardized test procedure for the evaluation of a suitable pressure transmitter. After four, six and eight million pressure cycles, the pressure sensors were each subjected to temperature stress (aging - artificial aging).

STS's ATM.1ST high-precision pressure sensor achieved top marks in these heart-and-kidney tests in the aspects of tolerance, long-term stability, precision and accuracy over the entire pressure and temperature range. Particularly decisive was the fact that the pressure sensor gives the high temperatures the cold shoulder, even over long periods of time, and is characterized by very high accuracy in the low-pressure range.

Result Evaluation of a pressure transmitter - pressure measurement technology

Figure 2: Evaluation of a STS pressure transmitter over time and temperature. OZ (original condition - in red, dash dotted) was used as the starting position, the drawn lines after a fixed interval each, the dashed lines including the aging process according to the test procedure from Figure 1. The value Tolerance field transducer refers to the manufacturer's specification (data sheet), the fixed lines Tolerance field NM were the targets of the evaluation.

Another advantage of the ATM.1ST is that it can be adapted to individual applications without effort due to its modular design. The data at a glance:
•Druckmessbereich: 100 mbar ... 1000 bar
-Relative and absolute measuring ranges
-Characteristic curve: ≤ ± 0.10 / 0.05 % FS
-Operating temperature: -40 ... 125°C
-Total error: ≤ ± 0.30 %FS (0 ... 70°C)
-Materials: Stainless steel, titanium