Hello and welcome to a new post. Today we discuss considerations for injection moulding of Polybutylene terephthalate (PBT). In previous posts we discussed the DMA behavior of PBT and other thermal properties.
General properties of PBT
PBT belongs, together with PET, to the class of Polyesters. Polyesters are made by polycondensation reaction under the release of water. Unreinforced PBT can be used from -60°C up to +110°C and reinforced PBT up to +200°C. In case PBT is exposed to hot water above 60°C it shows hydrolysis reactions (degradation reaction). It has good electrical properties such as electrical breakdown strength / dielectric strength at higher temperature (aliphatic Polyamides show much lower dielectric strength at higher temperature). Also the Comparative Tracking Index (CTI) is high with PBT and therefore it is often used in electrical and electronics applications (housings, connectors, busbars, e-Mobility). PBT has good mechanical and stress cracking properties. It burns with yellow-orange sooty flame. It needs to be protected against UV and to reach a certain flame classification, flame retardants need to be added.
Injection moulding of PBT: Dos and don'ts
Drying before moulding: min. 3 hours at 120°C; this ensures optimal mechanical properties in your final part at a later stage. Remaining humidity should be not more than 0.04%. If there is 0.1% humidity left, tensile strength may be reduced up to 12% from the optimum value. I created a table with the different maximum moisture levels after resin drying to ensure proper processing of 36 polymers.
Residence time: max. 8 to 10 min, otherwise material degradation will take place; thin wall parts optimal residence time is around 5 minutes and for thick wall parts around 3 minutes; if you are around 1 to 2 minutes, risk is given that there are not molten plastic pellets.
Packing: long packing is beneficial to prevent shrinkage cavities.
Injection speed: as high as possible, especially for thin wall parts.
Tool temperatures: PBT needs lower tool temperatures due to the high crystallization rate of the polymer. Mould temperatures between 30-60°C are sufficient. If you produce precision parts, mould temperatures up to 120°C are better, since the post shrinkage will be negligibly small.
Post shrinkage: low mould temperatures up to 0.3% and with high tool temperatures between 0.01 and 0.022%.
Melt temperatures: In table 1 an overview on the different processing temperatures is given.
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| Table 1: PBT processing temperatures |
Conclusions
During the design and material selection phase, polymers are selected due to their specific properties which they can bring to the table. However, if certain things such as proper drying before processing, crystallization rate and residence time during processing are not considered, the final part will not have the desired optimum properties.
Thanks for reading and #findoutaboutplastics
Greetings,
Herwig
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Literature:
[1] http://www.kunststoff-meister.de/mediapool/132/1320009/data/PBT.pdf
[2] https://www.plastverarbeiter.de/wp-content/uploads/migrated/docs/348_27934.pdf
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