When looking at injection molding cavity pressure sensors, the sheer number of products to choose from can be a bit overwhelming. In order to choose the proper sensor for your application, it’s vital to fully understand all of the options and what will work for your specific project.
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Some sensor selection considerations include:
We’re going to focus on the type of sensor technology.
There are two main types of sensors: strain gage and piezoelectric (piezo).
Strain gage sensors have a full or partial Wheatstone bridge strain gage sensing element (fun fact: it inspired our logo). When force is applied, the gage is deformed causing its electrical resistance to change. The resulting changes in current is interpreted by a process control system (such as CoPilot) where it can be displayed in graphical format.
Piezoelectric sensors have a quartz crystal in the sensor that, when force is applied, emits an electrical charge. This electrical charge is amplified to a signal that can then be sent to a process control system (such as CoPilot) where it can be displayed in graphical format.
Both technologies are reliable, and we recognize benefits each may have over the other depending on the application; however, we have identified strain gage sensors as our preferred technology for several reasons. Let’s review the benefits of strain gage sensors and why they are preferred in most scenarios.
Let’s review the benefits of strain gage sensors specifically and why they might be a good option for your application.
Contamination is any outside substance that causes inaccurate data readings. It happens when debris, water, or moisture sneaks into the electrical sections of a sensor system, greatly skewing the data.
Piezo sensor systems as a whole have many locations that can be exposed to contamination, causing drift in data and potential failure of the sensor itself. An example of this is any connection point, either at the sensor or on the side of the mold.
Strain gage sensors are highly robust and are inherently immune to all but the most severe forms of contamination. This means you can rest assured that your hardware is collecting accurate data.
It’s important to take into consideration how hot you will be running your mold. Each sensor head has a maximum temperature that it can withstand—going above that will cause damage to your sensor and inaccurate data. Piezo sensors can withstand up to 392°F (200°C), whereas some strain gage sensors can withstand up to 425°F. That means they can be used in a wider array of machines and applications without risk of damage to the sensor head.
Strain gage cables are permanently mated to the sensor head, providing more shielding against ambient electrical currents or build up through the tool. Each connection point for the Piezoelectric sensors is a break in the shielding, which may allow stray electricity to interfere with the picocoulombs of signal generated by the piezoelectric crystals under load. This signal interference is then magnified by the signal amplifiers necessary to boost the crystals’ generated current to a level detectable by modern computers (such as process control systems).
A frequently overlooked benefit of strain gage sensors is the ability to diagnose the health of the sensor while it is installed in the mold. The readings from a strain gage sensor at rest can determine whether a cable has been crushed during installation, the sensor is improperly installed, or the sensor has been overloaded or damaged. On the other hand, piezo sensors only provide very limited data for diagnosing sensor problems. This can be helpful in detecting problems in the tool room before the mold is installed in the press, saving significant downtime if a problem occurs.
One of the biggest things that sets strain gage sensors apart from piezo is that they are a lower cost option with the same data quality as a piezo sensor. When looking at equipping all of your molds with multiple sensors per mold, the savings add up fast. Though this is important, we want to make sure that you’re receiving the proper equipment for your application based on all factors, not just cost. But if those align, this is a huge benefit and win for your budget.
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Overall, strain gage and piezo sensors are very comparable in performance and final data displayed on a process control system. Their resolution and accuracy are in line with one another, and they both offer a large array of sensor head sizes. Strain gage sensors offer a more robust design that is more likely to provide accurate data, ease of debugging, higher temperature threshold options, and a lower cost solution with comparable results.
Tooling design, materials, and cavitation all impact tolerance. In general, the more simple the process, the more likely it is to achieve and maintain tight tolerances.
Conversely, complex parts can put tight tolerances at risk since there are more variables to consider, such as the number of cavities in the mold or the need to precisely heat or cool the tools. For example, if tooling is not designed for consistent cooling, shrink rates will vary and tight tolerances will be harder to achieve. Mold cooling and part cooling are also critical for determining surface finish.
Logically, it follows that sophisticated injection-molded components require sophisticated tooling. Added features such as undercuts and threads are common. In the case of complex geometries, injection molding tooling could require the addition of rotating mechanical racks/gears, rotational hydraulic motors, hydraulic cylinders, floating plates, or multi-form slides.
Directly correlated to scientific molding is the use of strategically placed transducers in multi-cavity and hot manifold tools to monitor and control the process in real time. Sensors can also be placed on the surface of the tool as a cooling lines or unit failure back-up. Upper and lower limits can be set on these sensors to monitor the cooling rate and the cavity pressure profile.
Establishing a production-capable process with in-tool sensors is also important for benchmarking. Having verified data to reference when making tooling material, process, or molding machine changes is essential for accurate monitoring, documentation, set-up, and future production repeatability.
A critical but sometimes overlooked injection molding tooling design consideration is proper tooling materials selection. The choice impacts several aspects of tool longevity:
Details matter in plastic injection tooling. Engineers play pivotal roles in ensuring an optimized production process, reduced costs, and unparalleled product quality.
Among chief responsibilities for tooling engineers is:
Likewise, design engineers are accountable for several key tooling decisions. Principle among them is determining gate types and locations in order to:
Contact us to discuss your requirements of Injection Molding Gauge. Our experienced sales team can help you identify the options that best suit your needs.