by Mettler Toledo
Quality issues in injection-molded products can range from purely cosmetic to serious structural defects that affect product performance and function. These may be caused by problems, such as contamination, related to the raw materials themselves, how they are stored and/or handled by the manufacturer prior to the production process or to the molding process.
A technical discussion of injection-molding machinery is not within the intended scope of this article. Rather, the article focuses on selected in-process control (IPC) and quality control (QC) measures for the testing of raw materials and ejected molds.
Injection molding of thermoplastics
More than 80 percent of molded plastics sold today are made of either thermosets or thermoplastics. The absence, or extremely low density, of cross-links in thermoplastics makes them capable of flow, and accordingly easy to process by a variety of methods, among which injection molding is the most frequently used. Thermoplastics also possess a unique physical property: They can tolerate melting, solidification and re-melting without significant alteration of their chemical composition1-3.
All processing methods have in common melting, forming and cooling, and careful control of these steps is vital to the quality of the final product. IPC tests are typically positioned throughout the manufacturing process to ensure the finished product meets required quality standards (Figure 1). In the remainder of this article, we present techniques for the IPC and QC of raw materials and ejected molds, i.e. before and after the injection-molding process.
Moisture analysis of raw materials
The hygroscopic nature of many plastic polymers poses the risk of surface or structural damage in molded objects and also may engender corrosive wear on components of the injection unit. For optimal melting and forming, a polymer resin must be within a specified water-content range. Hence hygroscopic resins in particular must be dried in an oven before being processed in the injection mold. Excessive water content resulting from ineffective drying in the hopper may lead to splays and delamination of the final product or even cause partial hydrolysis of the polymer chains during the melting process, whereas lack of water content may cause brittleness in, or incomplete formation of, the final product1, 4.
Standard assay methods (Table 1) developed by the American Society for Testing and Materials (ASTM) can be used for water and moisture determination in plastics. These guide injection molders on ensuring polymer resins are in the specified water-content range, for efficient processing and high-quality products5.
In a lean process setup, moisture content can be easily determined directly on the production floor. Loss on drying, certified as efficacious by ASTM and frequently used by injection molders, is the easiest and cleanest available method, despite its lack of water specificity. Also referred to as gravimetric moisture determination, loss on drying can be undertaken by various means drying oven, infrared and microwave moisture analyzers, as well as thermogravimetric analysis (TGA) and distillation solutions. TGA and other thermal analysis (TA) techniques may be preferable to moisture analysis for sensitive injection-molding operations or when a product has been recalled due to flaws and diagnosis is required.
Thermogravimetric moisture determination via infrared moisture analyzer is both rapid and affordable, and can easily be undertaken in a production environment without substantial operator training or a controlled laboratory space. The sample is weighed and subsequently heated with an infrared radiator, and the loss on drying is continuously recorded until mass stability is achieved. The moisture content then is calculated from the difference in weight:
% Moisture content =(Wet weight Dry weight) / Wet weight * 100%
The moisture contained in a material, therefore, includes all substances that evaporate upon heating, thereby reducing sample weight. The difference between the initial and dried sample mass, determined by a balance incorporated into the moisture analyzer, is interpreted as the moisture content6.
Comparison studies have shown that the results achieved via infrared moisture analysis are comparable to those obtained by Karl Fischer titration (the reference method). Figure 2 displays results obtained by each method in assessing moisture in acrylonitrile butadiene styrene, or ABS7.
Thermal analysis in quality assessment
Among the suite of analytical tools available to ensure product quality in injection molding, TA is well suited both to IPC during production and to diagnosis in the event of product failure. TA is the study of the relationship between a property of a sample and its temperature as that sample is heated or cooled in a controlled manner8. As sample cooling rate influences the properties of a molded product, TA can provide specific insight into the properties and quality of raw materials and molded parts alike.
A variety of TA methods are available to study material quality. While thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA) are useful for evaluating volumetric or dimensional changes and stiffness (modulus), respectively, the most important TA methods used in injection molding are as follows:
- Differential Scanning Calorimetry, or DSC, which measures heat flow as a function of temperature and/or time;
- Thermogravimetric Analysis, or TGA, which determines the quantity and rate of change in the weight of a material, as a function of temperature or time, in a controlled atmosphere.
Operation of TA instrumentation requires laboratory infrastructure, and it may not, therefore, be suitable for all production environments. Table 2 summarizes applications of DSC and TGA that may be of interest in injection molding.
Routine quality checks using the techniques described in this article reduce the likelihood or reoccurrence of product failure and its associated costs. Moisture analyzers can be used for the quality assurance of goods received and ejected molds. Moisture content is determined rapidly, and the compact instruments are easily situated close to production for example, next to the conveyor belt on which molded parts are dropped.
Moisture is, however, just one aspect of IPC or QC testing. High-performance TA instruments provide broader and more precise quantitative information on the purity, glass transition, melting point, thermal stability and compositional analysis of materials. Knowledge of such properties is critical for failure analysis and process optimization. For example, the establishment of pass-fail criteria is not uncommon in the automobile industry to ensure batch material received from suppliers is indeed correct. In this case, DSC curves may be used as a fingerprint to identify materials. By checking the level of crystallinity and the magnitude of the glass transition of an injection-molded part, one can also measure the effects of cooling within the mold as a means to define optimum cooling conditions.
- Bruder, U. (2015). The Users Guide to Plastic. Munich: Carl Hanser Verlag.
- Kunz, R. (2016). Market Analysis for Water Content Determination in Plastic Granulates. Zurich: University of Zurich
- NobelPrize.org (2016). Retrieved 15 July 2016, from https://www.nobelprize.org/educational/chemistry/plastics/readmore.html.
- Plastics Technology Online (2016). Retrieved 10 July 2016, from http://www.ptonline.com/knowledgecenter/Plastics-Drying/Resin-Types/Hygroscopic-VS-Non-Hygroscopic-Resins.
- ASTM.org (2017). Retrieved 6 October 2017.
- Wernecke, R. (2003). Industrielle Feuchtemessung. Weinheim: Wiley-VCH.
- METTLER TOLEDO (2014). Plastics Methods for HX204, https://www.mt.com/us/en/home/library/product-brochures/laboratory-weighing/05_Moisture_Analyzer2/00_Family/05_Method_Collection/Plastics.html.
- Lever T., Haines P., Rouquerol J., Charsley E.L., Van Eckeren P. and D.J. Burlett. (2014) ICTAC nomenclature of thermal analysis (IUPAC Recommendations 2014), Pure Appl. Chem. 86(4): 545-553