When you select a moisture analyzer for low-moisture plastic resins you will need a model that that not only can quickly but also accurately gauge the moisture content of these resins. And as with many quality control requirements that involve moisture content, you’ll want a moisture balance that can meet standards set by trade associations and professional organizations.
In this post you’ll get a briefing on the following:
- why low moisture content is important in plastic resins
- a technical standard that applies to determining moisture in plastics
- how benchtop moisture analyzers work
- a standards-compliant moisture analyzer
for plastic resins
Why Low Moisture Content is Important
Owners of injection molding companies and operators of injection molding machines are aware that incorrect moisture content in plastic resins can result in an inferior finished product.
Product shipped from resin suppliers may have the specified moisture content but ambient conditions at the injection molding facility could raise or lower moisture content before it is placed in the injection molding machine hoppers.
Companies involved in producing quality products by injection molding take steps to ensure proper resin storage. This along with using commercial drying ovens are two important steps in quality control.
Periodic checks with a benchtop moisture analyzer for plastic resins provides assurances that moisture content is on target, or otherwise call for pre-processing adjustments.
A Standard for Determining Moisture in Plastics
The American Society for Testing and Materials (ASTM) has published a standard governing testing the moisture content of plastics*. It employs what is called the LOD method, or the loss of weight on drying. This is also described as the thermogravimetric method.
Taken directly from the published standard the test method is summarized here:
“The specimen is spread onto a sample pan that is supported on a balance in a heating chamber that has been preheated and equilibrated to the specified idle temperature. It is then heated to vaporize the moisture. The analysis is completed when the indicated weight loss falls below a rate specified in the test conditions. The total loss of weight is integrated and displayed as the percent of moisture. Both the analyzer’s balance and heater are calibrated to NIST** standards to achieve precise and accurate results.”
How a Benchtop Moisture Analyzer Works
The ASTM standard briefly describes the process. Here is a bit more detail.
A benchtop moisture balance employs a drying process to extract and measure moisture. It combines a precision balance and a heater. In a simple 4-step procedure:
- The drying process parameters are entered on a keypad . These are the drying temperature which may include how the heat is applied (a.k.a. heating profile) and shut-off criteria.
- The sample is placed on a weighing pan located in the drying chamber.
- The drying chamber is closed and the analysis begins.
- The instrument calculates the moisture content by comparing the initial weight of the sample to the weight after drying is complete.
As in the standard the total loss of weight is integrated and displayed as the percent of moisture.
Halogen quartz glass is generally the source of drying heat. These heaters provide constant even drying of the sample after temperatures are programmed in by the moisture analyzer operator. Temperature is generally maintained within 1⁰C of the selected setting.
While most analyses involve ramping the temperature to a maximum of 160⁰C or less, in special cases higher temperatures may be necessary to achieve complete drying. The analysis automatically terminates either when the dry weight is stable or at a time specified by the operator.
For more detail on how these instruments work please check our post on moisture analyzer questions and answers.
A Moisture Analyzer for Low-Moisture Plastics
A moisture balance complying with ASTM D 6980 – 04 is the DSC 71P available from Tovatech. This model provides precise moisture analysis and is equipped with a default program to test 75% of the most commonly used resins. The DSC 71P is highly recommended for:
- Molders processing a limited number of plastic formulations
- Molder having QC problems related to moisture content
- Molders who need to replace an old moisture analyzer
- Molders adding a new production facility and need another moisture analyzer to support it
It is also an ideal tool to check the moisture content of incoming resins, confirm moisture content before filling hoppers and check the performance of bulk dryers.
The DSC 71P Moisture Analyzer Close-up
Thermogravimetric moisture analyzers such as the DSC 71P calculate moisture content by loss of weight on drying (LOD). This is more environmentally friendly and far less complex than the Karl Fischer moisture analysis requiring a high degree of skill and the use of hazardous chemicals.
Yet due to the very small amounts of moisture that can be tolerated in resins such as the <0.02% for polyethylene terephthalate sensitivity is critical. To address this the DSC 71P has readout of 0.0001g or 0.001% moisture. The repeatability (standard deviation) is ± 0.01% moisture content for a 10g sample.
Other features include:
- 3 free tech support phone calls for optimizing test procedures
- Custom programming for your resins available prior to shipment
- Short measuring time. Get results quickly with fast and uniform heating
- High repeatability. Superior accuracy even when weighing small samples
- Real time graphic display. Watch drying take place via a connected PC screen
- Accuracy checks using sodium tartrate dihydrate with its stable moisture content
Simple Resin Moisture Calculation
Once programmed with up to 20 testing procedures all the operator has to do is call up the program for the particular plastic to be analyzed. The unit’s vacuum fluorescent display shows the drying profile (standard, quick, step or ramp), time, and temperature (30⁰ to 200⁰C in 1⁰ increments) as well as shut-off criteria.
The unit comes with a 9mm (3.5 inch) sample pan, WinCT Moisture software, 30 g sodium tartrate dihydrate for accuracy checks, tweezers, a spoon, an RS 232C cable connection, and the instruction manual. A dot matrix compact printer is available as an option for record keeping. Other options include a 50 g calibration weight and a certified temperature calibrator.
A Resin Moisture Analyzer at an Attractive Price
Plastic resin moisture analyzers equipped with large libraries can cost around $10,000 or more depending on features. This is a substantial investment for small to medium-sized injection molding companies. The DSC 71P moisture balance with its broad range of features and outstanding accuracy is available for under $5,000.
Caution should be exercised when looking at lower-cost units because of their inability to deliver the high accuracy and precision needed for quality control when working with low-moisture resins.
Contact the scientists at Tovatech for advice on selecting and using a moisture analyzer to support your injection molding operations.