Most car bumpers have large-area undercut structures on both sides, which can be either external or internal. The choice of these two types of parts mainly depends on the requirements of the end customer's auto OEM for Auto bumper mould. Generally, most European and American cars use internal parting technology, and most Japanese and Korean cars use external parting.

The two parting methods have their own advantages and disadvantages. The external parting bumper needs to deal with the clamping line, which increases the processing process, but the mould cost and technical difficulty of the external parting bumper are lower than that of the internal parting bumper. The inner parted bumper is perfectly injection-moulded at one time through the secondary track change track control technology, thus ensuring the appearance quality of the bumper and saving the processing procedures and processing costs of plastic parts. But the disadvantage is that the cost of Auto bumper mould is high and the technical requirements are high.