Today virtually all aspects of our lives are touched by injection molded plastics. These versatile plastic resins in a variety of formulations perform in products ranging from toothbrushes to automotive consoles to minute surgical implants. Because the moisture content of these plastics is so critical to performance, a moisture analyzer is an essential tool to ensure materials are within specification.
What is too wet or too dry?
Ascertaining the correct moisture content for a resin depends on what is being molded and the plastic employed. For example, a polymer manufacturer may state that the product being offered has a moisture content of 0.20 percent by weight or 2,000 parts per million. But atmospheric conditions, humidity and other factors may cause a change, especially if the product is hygroscopic or accumulates moisture on its surface due to adsorption. Similarly, storage in areas with very low relative humidity could result in lowering the stated moisture content of the product.
Generally excess moisture content does not immediately manifest itself in terms of the appearance of a finished product. Rather it will result in poor performance during the lifetime of the product. If the resin is too dry product quality will likewise suffer, perhaps more immediately, resulting in higher costs due to unnecessary remanufacturing time.
These are reasons why companies involved in injection molding must pay strict attention to moisture content, making certain it matches the manufacturers’ specifications. Dryers are employed to do this. Controlling drying time and drying temperature along with critical dew point monitoring are generally accepted procedures.
A Moisture Analyzer Provides Proof
Moisture analyzers or moisture balances such as offered by Tovatech are designed to dispel any doubts about the moisture content of a resin, and can be programmed to handle virtually any type of product. These analyzers are essentially compact drying ovens built on top of a precision balance. They are easy to use and produce results quickly compared with other methods of analysis such as weighing samples before and after drying in an oven.
Ideally each batch of resins will be analyzed before going into the injection molding operations. In the process the plastic sample is placed in the unit’s drying chamber, where halogen quartz glass heaters provide drying at pre-set parameters. The moisture analyzer software maintains the drying cycle to conform with the chosen setting as data are displayed to technicians monitoring the process. The cycle stops when drying is complete or when a pre-set drying time elapses.
LCD graphic displays on moisture analyzers typically show the moisture content (%) as the weight loss from the starting weight and the dry mass as the residual from the starting weight. A company’s quality control manual or standard operating procedures should require documentation of each run. Data on initial and residual mass, test parameters and results can be captured by a PC by connecting it to an RS-232 interface, and then archived for future reference if needed.
Calibration for Consistency
Periodic external weight calibration and temperature calibration should be performed to measure the accuracy of a moisture analyzer. Tovatech offers a temperature calibration set and certified test weights that can be used together with the calibration program built into the balance to adjust weighing accuracy.
How do you document moisture content in your injection molding operations? What types of products do you manufacture and what happens if moisture content is too low or high?