According to the technical conditions and different application requirements of the pipe parts, the corresponding plastic processing methods should be selected. In actual production, although the shape, size and application of the pipe fittings are different, the basic processing procedures are the same, mainly including cutting, punching, bending, bulging, shrinking (diameter), flaring, flanging, hemming etc. Each processing procedure can be completed by different plastic processing methods. For example, the tortuous processing of pipes can be divided into methods such as winding, pressing, pushing, and rolling. If it is heated or not during bending, it can be divided into two categories: cold bending and hot bending.

Another example is bulging processing. According to the layout characteristics of the mold, it can be divided into two types: rigid mold and soft mold bulging, while soft mold bulging can be divided into rubber bulging and PVC plastic according to different pressure transmission media bulging, wax bulging, hydraulic bulging and pneumatic bulging. Another example is the shrinking process, which can be divided into punching shrinking, screwing shrinking, impact shrinking and heating shrinking, etc., and pipe wall punching can be divided into punching with concave die, punching without concave die, rubber die punching Kong et al.

Although there are many similarities in the injection molding of PP air ducts in terms of deformation properties and deformation characteristics, but in terms of process methods, process difficulties that need to be solved, tooling layout design, process parameter selection, and in order to avoid product quality defects. There are great differences in the process methods adopted. The hollow section of the pipe is the reason for these differences.

The injection molding of PP air ducts is often prone to the following quality defects, especially when the pipes are bent.

1. The thickness of the wall becomes thinner and breaks

For example, the outer wall of the zigzag deformation zone and the tube wall of the bulging deformation zone, and the tube wall of the flaring and flanging deformation zone will experience thinning of the wall thickness. The part with the largest amount of thinning is at the largest deformation, and it will cause a break when the thinning is excessive. From the point of view of deformation mechanics, this is because of the effect of tensile stress that the deformation zone loses its bearing capacity, so it belongs to the problem of plastic tensile instability. Such quality defects all occur in the elongation forming process.

2. Thickening and wrinkling of wall thickness

For example, the wall thickness of the side wall in the zigzag deformation zone and the tube wall in the necking (diameter) deformation zone are increased. If the degree of deformation is too large, the stability of the pipe wall will be lost, resulting in wrinkles. Therefore, the instability does not only appear under the effect of tensile stress, but also under the effect of compressive stress, there is also the problem of plastic instability. Such quality defects all occur in the compression molding process.

3. Transverse and shape distortion

In the bending process of the pipe, if the necessary measures are not taken (such as filling material in the pipe or placing a mandrel support, etc.), the cross section after bending will be distorted (approximately elliptical) in a large or small way. Another example is that when the mold is used to block the pipe, the pressure effect of the cutter on the pipe wall will also cause the block section to collapse and become non-circular. thus. In order to avoid the occurrence of the above-mentioned quality defects, it is very important to select a reasonable process method and adopt the necessary process methods. Production practice shows that eliminating or minimizing quality defects to meet the application requirements of pipe fittings is the key to the injection molding process of polypropylene air ducts.