Gear pumps are also called positive displacement pumps. Positive displacement devices are small in size, light in weight and cheaper in price. The most basic form is that two gears of the same size rotate in mesh with each other. The problems that often occur in gear pumps are: unbalanced force, leakage, and oil trapping. It is widely used in the chemical industry for conveying, such as polypropylene, nylon and other molten resin high viscosity materials.