Types of Resin Moisture Measurement Devices
Moisture analysis is critical to knowing if resin is properly dried. Resin manufacturers recommend that their customers dry material to a maximum level of moisture for a stable process and to produce aesthetically pristine parts with the ideal mechanical properties. Monitoring the resin’s moisture content before you begin processing and during processing is the key to making high-quality plastic parts with reduced waste and increased efficiencies.
When selecting a moisture measurement device, it is essential to understand how they function and their differences. All moisture analyzers measure the percent of moisture in the resin.
Laboratory/Offline Devices
Laboratory devices can measure moisture content or even provide complete chemical analysis, which can be time-consuming, labor-intensive, and costly.
Loss-in-weight analyzers measure the total change in weight of the material when the resin sample is heated at a specific temperature. As the polymer heats up, it burns off moisture and volatiles, and when the weight stabilizes, it records the weight difference. However, volatile burn-off can lead to inaccurate moisture results. In cases where volatiles are considerable, the moisture readout will be higher than the actual moisture, which can lead to over-drying. The manufacturers of these units typically supply recipes that at least partially account for the discrepancy. Karl Fischer Titration is a well-known loss-in-weight laboratory device in the plastics industry and was the gold standard of moisture analyzers for many years before other types of devices became available.
Moisture specific analyzers use a chemical process that only detects moisture levels, not volatiles. These analyzers are expensive and need to be used offline in a lab setting; however, they are reliable and repeatable. It is recommended that plastic processors have one of these devices to confirm their resin's moisture level.
Inline Devices
Inline devices are an alternative to laboratory/offline devices. Inline devices are often less expensive and easier to use then lab devices.
Capacitive sensor moisture analyzers work by measuring the changes in capacitance caused by the changes in the dielectric. Water moisture presents a very high dielectric constant compared to the low dielectric constant of plastics, allowing the sensors to detect the presence of moisture reliably. These units are relatively inexpensive and simple to use on the shop floor. When installed after the hopper, tunneling in hopper can give an inaccurate overall moisture level.
Microwave moisture analyzers work on the principle that water has a significantly higher dielectric constant than most other materials. Due to a water molecule's dipolar effect, a microwave resonator's resonant frequency changes with variations in moisture content. These variations are detected by the sensor electronics, which are scaled by the calibration process to provide a precise readout of the moisture present.
Near-Infrared (NIR) moisture analyzers excite water molecules by bouncing light beams off the sampled resin and measuring how much light is absorbed in wavelengths. This measurement provides the moisture content of the resin and may also be used to measure residual oils or solvents, which can cause contamination issues if not detected and removed. This equipment can be handheld, desktop, online, or inline. These units are costly and rely on a small sample for an overall moisture reading, which will not represent the product's total moisture.
Pre-Drying Devices
Pre-Drying moisture analyzers are a new entry to the market. They include a patent-pending moisture analyzer that consists of a capacitance sensor, an accelerometer, and a gyroscope near the bottom of a material pickup lance to provide reliable inline sampling. The moisture is measured from the lance's cavity on a part per million (PPM) basis before it is drawn into the dryer for further processing.
Capacitance sensors measure the dielectric constant of plastics. Since moisture presents a sizeable dielectric constant compared with the low dielectric constant of plastics, the presence of moisture can be precisely and reliably detected. The plastic pellets' initial and ongoing moisture content is continually communicated to the dryer control, where drying parameters like blower speeds, temperature, and drying times are automatically fine-tuned to dry the resin to the manufacturer's prescribed dryness level.