Not all bags are equal. Due to the different materials, and in some cases, multiple layers of material like a paper or foil bag lined with poly, the bags may be very difficult to seal. Depending upon the type of bag, you might need an Impulse, Double Impulse or Constant Heat Sealer.
Impulse sealers will apply a timed about of heat and then shut off for the seal to congeal. Constant heat sealers keep heat applied to the bag until you remove the bag from the sealer. We offer handheld, table top, and floor type sealers. Band Sealers automate the process, check them out.
For heat sealers to work the bag must have some sort of poly interior otherwise you will need to consider using a pressure sensitive tape, staple or wire twist tie.