The most common causes include:

* Barrier not completely under die.

* Sharp edge on die or bed plate.

* Power control setting too high.

* Dirt on the plastic. Unrolling can cause particles to be attracted electrostatically.

* Poor condition of the barrier.

* Burn marks through the barrier.

* Die incorrectly fitted to the backing plate.

* Weld duration too long. The barrier will often be damaged by long welds.

* Too much overlap on ends of weld as in bar welding.

* Air gaps caused by uneven die. A newspaper can be used to check this, along the length of die while under pressure.

* Arcing on initial welds can be caused by cold dies and plastic. Warming artificially, or by trial welds at reduced power can solve this problem.

* On initial welds on a fresh set up, power must be gradually increased until a satisfactory weld is achieved.

* Surface arcing can be caused by metal filings falling from the die when pressure causes slight separation and release of trapped particles. Dies must be cleaned with an air jet after repair work or initial fabrication.

Dies should be stored carefully in a rack. A nylon cloth rubbed over the surface of a die will amplify any sharp edges, by catching on any points.

* Dies can skid when the plastic becomes molten. this would be caused by bad press bearings or die not located under the centre of effort of the press.

* Balance adjusting bolts being out of adjustment, can cause tilt of the die.

* Arcing can be caused by not enough preweld time for pressure to build up.

* Insufficient weld pressure. 50 - 100 PSI is normally used, but under ideal conditions less than 50 PSI can be used, while with more rigid materials, 150 PSI and up is essential.