The model should be printed in PETG. 

To create a spring:

1. Cut a 30 cm long piece of PLA filament.
2. Soften the filament in 80°C (175°F) water for 3-5 min.
3. Insert a tip of the filament into the small opening at the top of the mold.
4. Press the filament into the groove from top to bottom of the mold.
5. Insert the second end of the filament into the small ring at the bottom of the mold. (Optional) Trim off the spare filament.
6. Put the mold with the filament on it into 80°C (175°F) water for 3-5 min.
7. Put the mold with the filament on it into room temperature water until cooled or run under the tap.
8. Remove the spring from the mold.
9. (Optional) Trim off the parts of the filament that were inserted into the opening at the top of the mold.