A wild GUESS at answers to your possible question.
If you treat in water or a very humid environment, your parts will very slowly absorb water up to equilibrium. At the temperatures you mention, this will take a long time. You mention no time. You will get some degradation due to hydrolysis, but this will be very very slow at those temperatures.
If you dry at 60 deg C in normal humid atmosphere, you will never really dry, but you will approach equilibrium with the atmosphere at whatever moisture content it is. This will take a long time. You do not say what time you intend.
If you dry at 60 deg C in an extremely dry atmosphere you will eventually dry the material, but it might take months or even years depending on how dry you need and section thickness of plastic to be dried. You will suffer some thermal degradation during this process
If you dry at 60 deg C in vacuum, this will slowly dry with no damage to polymer.
If this is a preparation to condition parts before a physical test, the sequence depends on what condition you want.
My best GUESS is that you want to establish equilibrium to standard laboratory conditions within a reasonable tolerance in I GUESS a commercially viable time.
The normal method for conditioning is to determine what the equilibrium moisture content is. This takes a long time to accurately test, but can be normally be obtained from existing data.
Once you know this the best method to condition is to treat in water until the surface of your sample is over conditioned, then to dry in normal atmosphere until the centre has picked up moisture to equilibrium and the surface has dried back to equilibrium. The time taken is very dependant on section thickness and accuracy of even distribution required. Moisture absorbs onto the surface very rapidly and reaches equilibrium virtually instantly, but migration of water through the polymer is very slow and to practically approach equilibrium of normal test bar thickness takes a week or two.
If you wish to do tests on freshly moulded parts to test the raw material used for the mouldings you mention, I would suggest that drying 24 hours under vacuum, then normalising overnight then treating in water until a specified weight gain is achieved then normalising 24 hours then do physical testing.
As Demom3 says, elongation is a much better indicator of initial damage than tensile. In fact the tensile might initially increase due to damage and cross linking.
If you actually want to test the mouldings, I would start by weighing some, then drying under vacuum then reweighing until no practical loss in weight. I would the either dry or moisture condition until the parts were at a weight to suggest equilibrium was obtained. I would then test the mouldings for elongation.
PBT, especially glass filled will not change properties to any significant degree due to moisture content, so the above is really mostly theoretical.
PBT is very susceptible to post moulding shrinkage. This shrinkage can cause internal stress. Glass fibres help reduce this, but a lot depends on the orientation of the glass fibre in the moulding.
Drying or moisture conditioning might anneal the parts to some extent, depending on temperatures and time.
Premature cracks in PBT are most likely caused by:-
Stress from uneven post moulding shrinkage.
The original moulding being done with material not sufficiently dried.
Exposure to water over 80 deg C for a reasonable time, (from memory, like maybe 1000 hours).
Exposure to UV light for a reasonable amount of time.
The original raw material being to low in molecular weight.
The compound not having the correct formulation re grade of glass, colours, stabilisers, flame retardants or whatever.
End of beginning of thesis.
End of trying to GUESS.
I sincerely hope you really are starting to understand.
Regards
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