May. 06, 2024
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Placing a sensor within a mold can provide various types of data, but the value of that data depends on pre-determined goals. Identifying what you want the sensor to measure or control is crucial before deciding on its type and placement. Let's explore this subject by addressing these five essential questions sequentially.
Understanding the specific function you want the sensor to perform is fundamental before starting installation. Many people are unsure about what capabilities a sensor has, which can hinder decision-making. The reasons for wanting to instrument a tool may range from improving Decoupled Molding processes to resolving existing production challenges. Hence, clearly defining your objective is the first step in this journey.
Consider why you think a mold sensor is necessary and what you aim to achieve with it.
This leads to numerous potential applications for sensors, such as monitoring pressure during the cavity filling process, sequencing valve gates, or automating the sorting of defective parts. Defining your aim enables you to make more informed choices as you move forward.
The sensor’s installation location should be determined by the specific needs of your project. Common locations include post-gate, mid-cavity, and end-of-fill, among others. The chosen spot should facilitate accurate data collection aligned with your intended goals.
Temperature is also a factor to consider, as sensors are only rated up to specific heat levels. In hot molds, techniques can relocate the electronics away from high-temperature zones.
Post-gate sensors provide critical data on when plastic enters the cavity and at what pressure. This information is invaluable for conducting several studies, such as pressure loss analysis and gate seal effectiveness.
Mid-cavity sensors help gauge pressure loss across the cavity and can be instrumental in timing hot-runner valve gates for larger parts. These sensors can also be used for smaller parts where moving ejector pins are not positioned at the final fill point.
End-of-fill sensors are most commonly employed to automatically sort defective parts by monitoring cavity pressure. These sensors, when used alongside post-gate sensors, can offer valuable insights into the filling pressure and cooling behavior of plastic, especially crucial for semi-crystalline polymers.
Pressure sensors commonly come in flush-mount or button styles. Flush-mount sensors are installed directly in the cavity block, contacting the plastic, while button styles are mounted away from the cavity and transfer pressure via pins or sleeves.
Mounting the sensor in the clamping plate and using a transfer pin offers several benefits:
Two principal pressure-sensor technologies are available: strain-gauge and piezoelectric sensors. Strain-gauge sensors are cost-effective, require less maintenance, and are robust against contaminants. Piezoelectric sensors, however, are better suited for flush-mount applications and scenarios demanding ultra-miniature sensors.
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Determining the sensor's load capacity requires knowing the projected area of the sensing pin and the plastic pressure at the sensing location. Calculating load as pressure multiplied by area helps identify the necessary capacity. The chosen sensor should have a higher load capacity than expected forces but not excessively high to ensure accurate readings.
Flow simulation is the preferred method to predict pressure, but material datasheets and short-shot studies are alternative approaches to determining required pressure during filling.
Making informed decisions backed by actual data is essential for effective sensor integration.
About the Author: Brad Harvey has extensive experience in the plastics industry since 2006, including toolroom roles and production tool servicing. Since joining RJG in 2016, he utilizes his knowledge in tooling and processing. Contact: brad.harvey@rjginc.com; rjginc.com.
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