Our welds are coming out all puckered, what is wrong?
This is caused by too much heat - i.e. you need to turn down the dial until the puckering stops but the weld is still sound.
Our welds are bursting open when we fill the bag. Why?
This is probably due to slightly insufficient heat. Try turning up the dial by just half to one point at a time until the welds hold properly.
After a few welds I can see a blackish build-up on the teflon over the sealing strip. What is this?
Its melted polythene. You may have your heat turned up just a bit high and this is causing excessive melting during the welding process. The excess melted polythene ends up on your teflon cover. Simply wipe it off and turn the heat down. Heat sealers should always be run at the minimum temperature at which a successful weld can be achieved.