In brief
Test results are used for various purposes, such as quality improvement, production process control, ensuring that the product meets customer expectations and regulatory standards, as well as comparison with competitors.
What is a plastics test?
Testing is the determination of one or more characteristics of a product, according to a procedure.
An authentic test result is the input for the next process and to:
- Evaluate improvement in product quality from new raw materials and additives
- Ensure that the manufacturing process is controlled within the “processing window”
- Ensure that the product has characteristics as per the customer’s requirements
- Ensure that the product meets national or international standards
- Prove design concept
- Compare product quality with competitor’s quality
- Verify how the product will perform under severe conditions
- Carry out research and development
Once the purpose is decided, it is necessary to have an “inspection plan” in terms of frequency of drawing samples, number of samples to be drawn, online sample drawing procedure, number of samples to be tested, conditioning to be performed, standard to be followed for the test method, requirements of the test results and reporting test results.
Conditioning of drawn samples:
Unlike metals, plastics’ properties are influenced by temperature. Conditioning of samples is performed to obtain reproducible results irrespective of previous history of exposure. This is done by following an applicable conditioning standard.
It is also necessary to report conditioning parameters, as well as test methods followed in the test report. It is imperative that the test equipment is calibrated or where necessary, calibrated before the testing. The testing must be carried out by a qualified person.
Common errors apart from conditioning to be avoided during testing:
- Calibration error & zero error of the testing equipment
- Accuracy error of the measuring instrument
- Stability error (applicable to reversion/shrinkage, hardness, curing)
- Repeatability error (when the same product is tested using the same equipment, same procedure, same person, on the same day, the result should be the same)
- Reproducibility error (different people testing the same product at the same or different places, using the same standard procedure and same equipment should give the same result)
- Linearity error (e.g. if the heating rate in VST differs, then VST will be different)
- Specimen shape error (those should be as per standard tolerance in dimensions)
The purpose of analysis is to ensure that the product conforms to the quality requirements of the standard followed, as well as customer requirements, and that it will not fail in agreed service life (guarantee period).
For every test result, corresponding process parameters recorded at the time of drawing samples are available. Comparison of test results vis-a-vis process parameters is a very useful tool for improvement in quality.
Such a comparison gives us information about the process (if it’s consistently giving consistent product test results), if there is deterioration (for corrective actions) and if there's an improvement in the desired test's result (to modify process parameters for better results).
The bests process parameters need to include:
- The collection of all the test results for a particular product
- The scrutinization of all the test results to find the best and the worst and their respective documentation of process parameters.
- The process parameters responsible for the difference in quality
- The repetition of the process as per the best parameters
- The standardization of the process parameters in the form of a “process control plan” for future production
