In hot melt welding, more than half of weld quality comes down to temperature control — the most sensitive parameter in the whole process, causing problems when either too high or too low.
Too low: weak welds
Insufficient heating temperature or time leaves plastic surfaces incompletely molten, so pressing only sticks the surfaces together with far-from-adequate strength. Raise the heating temperature or extend heating time — adjusting one variable at a time and confirming with pull tests.
Too high: stringing and degradation
Excessive temperature makes molten plastic too fluid, causing stringing when the hot plate retracts; worse, the material thermally degrades — welds turn yellow and brittle while strength drops. Lower the heating temperature, shorten heating time, and check that the controller's displayed temperature matches reality — aging thermocouples cause the display to deviate from reality.
Uneven temperature
Large hot plates tend to differ in temperature between center and edges, leaving some weld zones strong and others weak. Check multiple points on the hot plate surface regularly with a temperature probe; if deviation is large, inspect heating elements for damage and verify insulation.
Establishing a temperature window
For each material, determine a "temperature window" through trials: the lower limit is the minimum temperature ensuring weld strength, the upper limit where degradation begins. Running production in the middle of the window with regular controller calibration keeps batch quality stable. The precise temperature and pressure control of servo hot melt welders makes holding this window more consistent.