Here are a pair of videos that describe a plastic welding process that I have some experience with. Hot plate welding is a type of plastic welding used in many applications, but the one I am most familiar with is automotive lamps (my LinkedIn profile will give you more insight on that). This process works on thermoplastics and it is sometimes referred to as ‘mirror welding’. You should refer to weld compatibility charts or weld equipment suppliers to see if hot plate welding will work for your material selection and design.
The first video is a quick animation from Branson Ultrasonics. It shows the two halves that are to be joined being held in the upper and lower fixtures. The first step in the hot plate welding process is for the heated platten to come between the two halves of the part. The work pieces are each brought into contact with the heated platten where the weld legs that are designed into the part are heated to melting point. The work pieces are removed from the heated platten and it in turn retracts.
In the second step of the process the two halves are then brought together and clamped precisely with the molten weld legs in a state of significant interference. As the weld legs cool the material returns to solid state and the weld legs are joined.
The process does create weld flash or boil out as indicated in the last few seconds of the animation. Normally a flange or lip is designed into the part to disguise the weld flash.
The second video, from Dukane Ultrasonics, shows an automotive fluid reservoir being joined using hot plate welding.