Company insight Contact-free precision
Kistler offers a strain-based, contact-free method for measuring cavity pressure in medtech injection moulding, enabling precise, reliable monitoring without compromising high- precision components.
The miniature longitudinal sensor 9239B from Kistler measures the strain put on plastic moulds.
n injection moulding, cavity pressure measurement provides an advantage for highly regulated industries such as medtech by detecting deviations at an early stage and removing faulty parts. As a result, manufacturers can avoid costly rework, regulatory non- compliance, and product recalls. Regarding the measurement method, direct measurement has become the gold standard for precise cavity pressure measurement.
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While this technology is yet unmatched in terms of accuracy, it has some disadvantages: for example, the sensor comes in direct contact with the injected melt and thus leaves a small mark on every part. This can, even when minimal, still be an issue for high-precision products in medtech, including lenses, syringes and catheter parts. Also, the immediate contact with the melt and the exposure to high temperatures, aggressive melts and process gases impact the longevity of the sensors. As an alternative to direct measurement, some companies use indirect measurement for cavity pressure. However, this method also has its drawbacks. As the sensors are positioned behind the ejector pins, they are vulnerable to errors; for instance, if the mounting hole isn’t a perfect fit and either too small or large for the ejector pin. Melts with low viscosity such as liquid silicone may even flow into the ejector pin hole, which can damage the mould components or result in accelerated wear.
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Contact-free alternative Kistler has successfully worked on an alternative that avoids these disadvantages. It came up with a contact- free procedure that completely avoids melt contact. Instead of cavity pressure, it measures the strain that the pressure of the melt puts on the mould’s cavity wall. Most remarkably, although this contact-free measurement method does not measure absolute cavity pressure
CAD model, they evaluate at which maximum distance to the mould wall the sensor can be placed while still providing accurate measurements. The design of the sensor is also important. The Kistler portfolio includes several piezoelectric miniature longitudinal measuring pins suitable for compact moulds used for lenses, syringes and EpiPens. The 9239B miniature sensor, for instance, has a circumference
“The combination of specifically grown quartz, tailor-made sensors and engineering expertise means that manufacturers can get precise and reproducible measurements.”
values, it allows injection moulders to draw accurate conclusions about them. Measuring the strain results in a very similar measuring curve to other methods. Even more importantly, measuring strain leads to reproducible curves, thus meeting an essential requirement of quality assurance and regulatory guidelines.
Small and sensitive
In this contact-free measurement set-up, the sensors can be placed 2–4mm behind the cavity wall. This positioning is close enough to capture strain signals with high sensitivity but distant enough to avoid contact with the melt. Kistler engineers support manufacturers with a finite element analysis (FEA) to find the best sensor position. Based on their 3D
of only 2.5mm and can thus be used in small moulds.
It features a PiezoStar crystal specifically grown by Kistler, characterised by high sensitivity and temperature stability. As the molten material fills the cavity, the steel wall undergoes elastic strain in the micrometre range. This strain is converted by the PiezoStar crystal into an electrical charge with a sensitivity of up to 28.5 pC/N. The combination of specifically grown quartz, tailor-made sensors and engineering expertise means that manufacturers can get precise and reproducible measurements while avoiding disadvantages such as a negative impact on the sensors. ●
www.kistler.com/medical
www.medicaldevice-developments.com
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