Page 29 - Plastics News May 2018

FeAtures

In a lean process setup, moisture content can be easily TA can provide specific insight into the properties and
determined directly on the production floor. Loss on quality of raw materials and molded parts alike.
drying, certified as efficacious by ASTM and frequently A variety of TA methods are available to study material
used by injection molders, is the easiest and cleanest quality. While thermomechanical analysis (TMA) and
available method, despite its lack of water specificity. dynamic mechanical analysis (DMA) are useful for
Also referred to as gravimetric moisture determination, evaluating volumetric or dimensional changes and stiffness
loss on drying can be undertaken by various means – drying (modulus), respectively, the most important TA methods
oven, infrared and microwave moisture analyzers, as used in injection molding are as follows:
well as thermogravimetric analysis (TGA) and distillation
solutions. TGA and other thermal analysis (TA) techniques Differential Scanning Calorimetry, or DSC, which measures
may be preferable to moisture analysis for sensitive heat flow as a function of temperature and/or time;
injection-molding operations or when a product has been Thermogravimetric Analysis, or TGA, which determines the
recalled due to flaws and diagnosis is required. quantity and rate of change in the weight of a material,
Thermogravimetric moisture determination via infrared as a function of temperature or time, in a controlled
moisture analyzer is both rapid and affordable, and can atmosphere.
easily be undertaken in a production environment without Operation of TA instrumentation requires laboratory
substantial operator training or a controlled laboratory infrastructure, and it may not, therefore, be suitable for
space. The sample is weighed and subsequently heated all production environments.
with an infrared radiator, and the loss on drying is
continuously recorded until mass stability is achieved. The Table 2 summarizes applications of DSC and TGA that may
moisture content then is calculated from the difference be of interest in injection molding.
in weight: Conclusions
% Moisture content = (Wet weight – Dry weight) / Wet Routine quality checks using the techniques described
weight * 100% in this article reduce the likelihood or reoccurrence of
The moisture contained in a material, therefore, includes product failure and its associated costs. Moisture analyzers
all substances that evaporate upon heating, thereby can be used for the quality assurance of goods received
reducing sample weight. The difference between the and ejected molds. Moisture content is determined
initial and dried sample mass, determined by a balance rapidly, and the compact instruments are easily situated
incorporated into the moisture analyzer, is interpreted as close to production – for example, next to the conveyor
the moisture content6. belt on which molded parts are dropped.
Comparison studies have shown that the results achieved Moisture is, however, just one aspect of IPC or QC testing.
via infrared moisture analysis are comparable to those High-performance TA instruments provide broader and
obtained by Karl Fischer titration (the reference method). more precise quantitative information on the purity,
Figure 2 displays results obtained by each method in glass transition, melting point, thermal stability and
assessing moisture in acrylonitrile butadiene styrene, or compositional analysis of materials. Knowledge of such
ABS. properties is critical for failure analysis and process
optimization. For example, the establishment of “pass-
Thermal analysis in quality assessment fail” criteria is not uncommon in the automobile industry
Among the suite of analytical tools available to ensure to ensure batch material received from suppliers is
product quality in injection molding, TA is well suited both indeed correct. In this case, DSC curves may be used as
to IPC during production and to diagnosis in the event of a fingerprint to identify materials. By checking the level
product failure. TA is the study of the relationship between of crystallinity and the magnitude of the glass transition
a property of a sample and its temperature as that sample of an injection-molded part, one can also measure the
is heated or cooled in a controlled manner8. As sample effects of cooling within the mold as a means to define
cooling rate influences the properties of a molded product, optimum cooling conditions.

29 May 2018 Plastics News