We haven't had much luck finding someone willing to manufacture mylar balloons bigger than 1 meter (3 feet). Which got me thinking about how to manufacture balloons myself.
The highest quality seams are made with a bag sealer because with a bag sealer, the plastic is held perfectly still through the heating and cooling process. The professional equipment is the same and hasn't changed much in the past 100 years-- heat welding rubber or plastic is done with a gigantic, shaped bag sealer.
materials inside a bag sealer
The heating element is nichrome wire. covered in a sheet of PTFE (teflon). The plastic is pressed down against the heating element with high-temperature silicone, and the nichrome wire controlled by a simple timed on/off circuit. These products aren't too expensive from McMaster-Carr: Nichrome Wire-wire/=hlaehj) Conformable PTFE Tape Extreme Temperature Silicone-Backed Rubber Strips
To do more precision heating, a temperature control unit should be added. This could be a DIY setup with an arduino, a thermocouple, and a solid-state relay, but there are also commercial units that are quite affordable: digital temperature controller and another
making a curved jig
the bag sealer is easy because the heating element is straight. Nichrome wire needs to be handled gently while heating, and keeping it in a smooth curve (like we want for balloon making) is potentially problematic. My speculation is that it can be embedded into a routed channel in some material like MDF, so long as it is protected by more PTFE tape on both sides of the channel. I'm going to try this out.
Plastics that can be used
Mylar is not a thermoplastic, but laminated mylar(usually with LDPE) will work. EVOH does as well but is finicky about heat. Both of these are purchasable from Gary at balloonkits. Gary also sells reusable balloon valves that can be heat seamed in.