When processing semi-crystalline engineering polymers, choosing the right melting temperature is critical to ensure part quality. Tolerances are typically smaller compared to processing amorphous resins. A processor's machine has a direct impact on the quality of the end product. In the fifth part of the series of ten questions to be discussed, the author discusses the melting temperature of POM (acetal), PA (nylon), PBT, PET (polyester) molding.
What happens next?
The melting temperature is too high or too low: both cases are wrong. Keep in mind that the temperature distribution is also a factor that must be taken into account during the melting process.
Too high a temperature can break the molecular chains and degrade the polymer. Another consequence is that additives such as pigments and impact modifiers will decompose during the melting process. These can lead to reduced mechanical properties (due to shorter molecular chains), surface defects (caused by decomposition of the product) and undesirable odors.
If the temperature is too low, the product structure will not be homogeneous, which will significantly reduce the impact resistance and in most cases the mechanical properties will change.
In addition to the melting temperature, the residence time of the polymer during the injection process is also very important. Experience has shown that the dwell time should usually be 2~9 minutes. If the residence time is too long, thermal decomposition will occur in some cases, even if the melting temperature is correct. If the residence time is too short, there will not be enough time for the melt to homogenize.
What are the signs?
In the case of POM, excessive thermal stress can break down the product and bubbles appear in the melt, which can be clearly seen in cleaning. Other phenomena include an increase in molding deposits and the appearance of undesirable odors. However, too high a melting temperature will not affect the mechanical properties of POM homopolymers.
PA will fade in case of overheating, including overheating due to excessive temperature of the injection nozzle. In all PA type products, a decrease in mechanical properties is a sign of thermal decomposition. In the laboratory, thermal decomposition can be determined by measuring the viscosity of the solution, but this method is not commonly used by manufacturers.
PBT and PET react more strongly to overheating, which leads to a decrease in toughness. During machining, these defects are barely visible. If not determined using appropriate quality control methods, such defects are usually not found until the assembly stage, or in use. Discoloration of the product indicates a serious quality problem. In practice, a random check of a product's resilience-related performance can reveal problems. Testing the viscosity of molded parts is time-consuming and expensive.
For unreinforced PA and PBT, if unmelted particles are found during cleaning, the melting temperature is too low and, in rare cases, is caused by too much injection.
Correct melting temperature
The appropriate melting temperature range for each material is listed in the engineering polymer data. Often, the temperature setting of the barrel heating zone is unreliable because in addition to the temperature increase caused by the heating belt, the friction generated by the rotation of the screw also heats up. The amount of heat generated in this way depends on the screw geometry, rotational speed and back pressure.
The following recommendations will help you determine your temperature accurately:
• Melting temperature probe diameter less than 1,5 mm (reaction characteristics);
• Pre-heating probes;
• Collect the melt in an insulated vessel;
• Stirring for temperature measurement.
When the initial temperature measurement is carried out, or if the temperature value is not known, the temperature range of 10~15 °C above the melting point should be selected in the feed area, and the temperature range of 5~10 °C below the required melting temperature should be selected in the measurement area. Depending on the measured melting temperature, the temperature can be fine-tuned. In the case of long residence times and short metering strokes, it is advisable to gradually increase the temperature setting. In the case of short residence times and long metering strokes, the best results are obtained with temperature settings that are usually laid flat. Do not set the temperature zone below the melting point of the polymer
