If the thought of a lukewarm drink makes you shiver but the price of an insulated flask makes you burn with anger, creating your own insulated flask is a great option to keep your favourite drink cold for extended periods of time. Assembling your own Thermos flask can save you money while allowing you to explore how insulation is achieved in thermoses. This Thermos design is simple compared to vacuum Thermos designs and can be made using household tools.
Select two metal flasks to use create the insulated metal flask. To simplify this project and create a more durable product, it is important that you choose a small flask that will fit inside of the larger one.
Saw off the bottom of the large metal flask using a hacksaw. Saw using long, even strokes to create a straight cut. This will make the flask's reassembly process easier.
Wrap the mouthpiece of the small metal flask with saran wrap to protect it from the epoxy. Apply a strip of masking tape between the edge of the saran wrap and the metal mouthpiece to keep the saran wrap in place.
Insert the small flask into the large flask. Push the small flask up so that the mouthpiece fits through the large mouth piece, giving you enough clearance to screw on the small flask's lid. Tape the two flasks in place, using masking tape.
Hold the two flasks at an angle so that you can look through the opening you cut into the large flask. Insert the tube of expanding insulation epoxy in the hollow space between the small flask and the large flask.
Spray expanding insulation epoxy into the hollow space, starting at the far end of the large flask (where both mouthpieces meet) and working your way towards the base of the large flask. Pause once you have filled the hollow space 1/3 and 2/3 with foam, to allow the foam to expand. Make sure that the foam completely covers all portions of the small flask, but avoid overfilling the cavity.
Apply a bead of epoxy around inside of the piece you cut off of the bottom of the large flask. Line up the bottom piece with the large flask and place it onto the flask. For best results, hold the piece onto the flask until the epoxy dries.
Remove the masking tape from the flask. If there is a small gap between the mouthpieces of the small and large flasks, apply epoxy into the opening.