As the temperature in the joint increases, polymer begins to melt and a fusion zone is formed. The molten polymer in the fusion zone exerts an outward force on the surrounding solid polymer material, referred to as "cold zones". These cold zones cause a pressure to develop in the molten fusion zone. The pressure in the fusion zone takes some time to reach its maximum value, usually not reaching the peak until about a quarter of the way into the joining process. After the current is shut off and cooling begins, the pressure slowly decreases until the joint is uniform temperature.