Heater

Don't buy from cheap suppliers.

By using heaters and thermocouples that are designed for your application (or at the very least not being provided enough protection). If the thermocouple sleeving is getting burnt and heating elements are failing due to mechanical stress these are just some indications that they may not be fit for your application.

In many cases you can use the cheaper alternative thermocouple and heater (to save money, especially in OEM manufacturing where a small saving per unit equals a large overall saving), but you must ensure they are protected from damage that can shorten their life.

A little thought and design go a long way.