Achieving a reliable joint in many of today's plastic compounds is at best an adventure. Many of the recommended rules and guidelines become just that, rules and guidelines. You have to exercise common sense and empirical testing. The first step in common sense fastening is to understand the material you are working with. They fall into two general categories, thermoplastics and thermoset plastics.
Thermoplastics become soft and melt at elevated temperatures.
They retain their shape and will not become soft or melt like thermoplastics. You must also be aware that all plastics possess a characteristic called 'creep'. The tendency and amount of creep can be 'ballparked' by knowing the flexural modulus of the material you are working with. Flexural modulus is a measure of the material's softness or brittleness. The higher the flex modulus the more brittle or rigid the material will be and the less creep it will normally have.