The heat deflection temperature is a very tricky measurement. It gives you the temperature at which a defined shape is indented into your material when applying a fixed load. Unfortunately, this translates difficultly to real-world applications, as forces and loads are often not well known. Besides that, the stiffness of your geometry can help withstand certain deformations. There are two typical standards for heat deflection temperature: HDT 0.455 MPa and HDT 1.82 MPA, both performed with different pressures applied to the test specimen. Therefore the HDT should be interpreted with care: an HDT of 60 degrees doesn’t automatically mean that your part will withstand 60 degrees. It depends on how much force is applied when reaching that temperature.