Page 28 - Plastics News May 2018

FeAtures

From Melting to Molding – Analytical Techniques to Support Process

Optimization and Quality Control in the Injection Molding Industry

Mettler Toledo

The article focuses on selected in-process control (IPC) and quality control (QC) measures for the testing of
raw materials and ejected molds.

uality issues in injection-molded products can range standards (Figure 1). In the remainder of this article, we
Qfrom purely cosmetic to serious structural defects present techniques for the IPC and QC of raw materials
that affect product performance and function. These and ejected molds, i.e. before and after the injection-
may be caused by problems, such as contamination, molding process.
related to the raw materials themselves, how they are Moisture analysis of raw materials
stored and/or handled by the manufacturer prior to the
production process or to the molding process. A technical The hygroscopic nature of many plastic polymers poses
discussion of injection-molding machinery is not within the risk of surface or structural damage in molded objects
the intended scope of this article. Rather, the article and also may engender corrosive wear on components of
focuses on selected in-process control (IPC) and quality the injection unit. For optimal melting and forming, a
control (QC) measures for the testing of raw materials polymer resin must be within a specified water-content
and ejected molds. range. Hence hygroscopic resins in particular must be
dried in an oven before being processed in the injection
Injection molding of thermoplastics
mold. Excessive water content resulting from ineffective
More than 80 percent of molded plastics sold today are drying in the hopper may lead to splays and delamination
made of either thermosets or thermoplastics. The absence, 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 product.

Standard assay methods (Table 1) developed by the

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 American Society for Testing and Materials (ASTM) can be
and cooling, and careful control of these steps is vital to used for water and moisture determination in plastics.
the quality of the final product. IPC tests are typically These guide injection molders on ensuring polymer resins
positioned throughout the manufacturing process to are in the specified water-content range, for efficient
ensure the finished product meets required quality processing and high-quality products.

Plastics News May 2018 28