In industrial and municipal pipeline systems, demand for plastic bends is increasing. 90° pipe joint is a key component of pipeline. Especially in pressure pipeline, quality of pipe joints is more important, and it is difficult to obtain a high-quality and high-precision 90° pipe joint in production. Numerical simulation of shrinkage of pipe joint under different injection process parameters is carried out, and influence of injection process parameters on molding shrinkage of 90° pipe joint is obtained. Finally, three-dimensional inspection method is applied to high-precision detection of plastic parts to obtain shrinkage distribution.

 

Figure 1 Three-dimensional structure of 90 ° pipe joint

Structure of plastic parts is shown in Figure 1. Two straight pipes are 90° to each other. Two straight pipes are connected by same thickness thick pipe. Inner and outer diameters of straight pipe joint are φ40 and φ46mm respectively. That of elbow are φ30 and φ36mm respectively. Wall thickness of plastic part t is 3mm, bending diameter is R80mm, and bending diameter ratio R/D is 2.67 (D is inner diameter of elbow). In research of injection molding of plastic elbow joints, we use MoldFlow to simulate injection process of 90° plastic elbow joints. Shrinkage rate of plastic parts showed that shrinkage rate of main ends was the largest, which was 0.4%. ~0.7%, average molding shrinkage of plastic parts is 0.5%, which shows that uneven distribution of plastic parts is main reason that affects dimensional accuracy of plastic parts. Some scholars have studied influence of process parameters on shrinkage rate of plastic parts. Several different materials were used for testing. It is concluded that shrinkage rate of plastic parts is mainly related to holding pressure and melt temperature, while injection speed and mold temperature have little influence. However, various types of research have not studied dimensional change of 90° elbow joint structure. Now we use a reasonable test program to study effects of injection temperature, mold temperature, injection pressure, holding time and pressure on shrinkage of 90° elbow joint.

Number of injection model cavities needs to consider factors such as production efficiency, mold material and cost. According to structural characteristics and process requirements of plastic parts, mold is a 4-cavity structure. When mold is opened, side of straight part of plastic part is moved to complete core pulling in four directions. Elbow part is four curved cores, hydraulic system drives hydraulic cylinder piston rod, rack and pinion mechanism to rotate 90° bending core around rotating shaft by 98° to complete core bending of plastic part. Schematic diagram o mold device is shown in Figure 2.

 

(a) clamping mode

 

(b) Core extraction state

Fig. 2 Schematic diagram of mold device takes pipe joint as research object. Based on MoldFlow’s numerical simulation analysis of plastic filling, cooling and warping, results show that shrinkage is mainly distributed at the end of filling tube, and pressure holding time has no obvious effect on shrinkage of plastic parts. By optimizing five influencing factors (injection temperature T1, mold temperature T2, injection pressure P1, dwell time t2, holding pressure P2), optimal process parameter combinations are shown in Table 1.

 

 

In test, injection molding machine adopts semi-automatic operation mode. Test material Geon 87431 PVC is first added to drying barrel for preheating and drying, heating switch of injection molding machine is started to debug pushing mechanism, opening and closing mechanism, host is started when set temperature is reached. Before test, it should be injected twice by hand to observe color and uniformity of melt, prepare for plastic part test of formed elbow joint. Injection unit adopts an injection molding machine with a nominal injection volume of 500g, establishes research plan based on the results of numerical simulation analysis. As a result of simulation analysis, influence of dwell time on molding shrinkage rate is not obvious, test plan design does not consider dwell time factor. Molding test plan is shown in Table 2.

Injection molding cycle is that polymer material is added into hopper of injection molding machine from granular or powder, material is uniformly plasticized by strong extrusion, shearing, friction and external heat in injection molding machine, then injected into mold cavity to be cooled and shaped, and mold is pushed out to finally obtain glass-shaped plastic part of desired size and shape. Volume of plastic parts after cooling and setting is smaller than volume of mold cavity, which is molding characteristic of polymer plastic parts, measured by shrinkage index. Shrinkage of each part of elbow is different. Structure of test mold device leads to a large contraction at the joint of elbow joint. Dimensional change law of joint is studied from perspective of application and molding characteristics of plastic part.

 

Fig.3 Variation of outer diameter of plastic parts interface at different injection temperature

 

Fig. 4 Variation of inner diameter of plastic parts interface at different injection temperature

Figure 3 shows variation of outer diameter of plastic part interface at different injection temperature (mold temperature 45℃). Inner and outer diameters of 15 plastic parts are measured with a vernier caliper with an accuracy of 0.02 mm, average value of measurements was taken. Results show that with increase of injection temperature, shrinkage rate gradually decreases. When temperature rises to 170℃ at 160℃, dimensional change of plastic parts is obvious. After temperature is higher than 180℃, injection temperature has little effect on size of plastic parts. Figure 4 shows variation of inner diameter of plastic parts at different injection temperature. Shrinkage rate decreases with increasing temperature. When temperature reaches a certain temperature, dimensional change of plastic parts is not obvious.

Setting injection temperature is 180℃, mold temperature is set to 30, 35, 40, 45℃ respectively, other parameters input fixed value, study influence of mold temperature on shrinkage of plastic parts. Variation rule of outer diameter of plastic part interface under different mold temperature is shown in Fig. 5. Size of 15 plastic parts interface is measured by vernier caliper with precision of 0.02mm, average value of measurement is taken. Measurement result shows that mold temperature has little effect on outer diameter of interface, and mold temperature is ideal at 40~45℃.

 

Fig.5 Change rule of outer diameter of plastic parts interface at different mold temperature

 

Fig.6 Variation of inner diameter of plastic parts interface at different mold temperature

Figure 6 shows measurement results of inner diameter of interface at different mold temperature. Effect of mold temperature on inner diameter of interface is small. Mold temperature is ideal at 40℃. In combination with influence of mold temperature on test index, mold temperature was chosen to be 40℃.

 

Fig. 7 Change of outer diameter of different injection pressure plastic parts interface

 

Fig. 8 Variation of inner diameter of different injection pressure plastic parts interface

Injection pressure has a great influence on contraction of elbow. Generally, increasing injection pressure, plastic parts are more compact and shrinkage rate is reduced. Injection pressure is high, melt enters mold cavity at a high speed and enters pressure holding stage earlier. According to molding test plan, injection temperature is set to 180℃, mold temperature is 45℃, and holding pressure is 90 MPa. Effect of different injection pressure (90, 100, 110, 120 MPa) on forming shrinkage at the joint of elbow joint was studied. Measurement results are shown in Fig. 7. As injection pressure increases, outer diameter shrinkage shows a decreasing trend, which is obvious at 90~100MPa, and outer diameter varies from 45.6mm to 45.92mm. Measurement results of inner diameter of interface are shown in Fig. 8. Injection pressure has a great influence on inner diameter shrinkage, especially at 100~110 MPa. When injection pressure is 90 MPa, inner diameter of interface is 39.4 mm. After 110 MPa, dimensional change is not large, and value is close to size of 40 mm.

 

Fig. 9 Change rule of outer diameter of different pressure-bearing pressure plastic parts interface

 

Fig. 10 Variation of inner diameter of interface of different pressure-holding plastic parts

Forming test shows that holding pressure plays a key role in shrinkage rate of plastic part before gate is condensed. Increasing holding pressure, shrinkage rate of plastic part is significantly reduced. According to molding test scheme, injection temperature was set to 180℃, mold temperature was 45℃, and injection pressure was 100 MPa. Effects of different holding pressure (80, 90, 100, 110 MPa) on forming shrinkage at the joint of elbow joint were studied. Outer diameter measurement results are shown in Fig. 9. As holding pressure increases, shrinkage of plastic parts shows a decreasing trend, which is obvious at 80~100 MPa. When holding pressure exceeds 100 MPa, effect on shrinkage disappears. Inner diameter of interface is shown in Figure 10. Holding pressure has a great influence on the shrinkage at inner diameter, especially at 80~100MPa. After 100MPa, size is about 40mm.

Figure 11 shows plastic parts molded under optimal process parameters. Surface of plastic parts is smooth, free from flash, scratches, sink marks, pores and other defects. Appearance of color is uniform, and there are small protrusions at the gate, which meets requirements.

 

Figure 11 molded plastic parts

 

Figure 12 3D scanning of elbow joint point cloud

In order to evaluate high-precision dimensions of plastic parts, three-dimensional optical scanner is used to measure size of plastic parts, point cloud data of qualified plastic parts surface is collected to obtain high-precision point cloud of curved pipe joint, as shown in Fig. 12. GeomagicStudio is used to process point cloud noise and in vitro isolated points. In order to improve contrast accuracy, GeomagicQualify is used to directly compare actual production point cloud data with 3D model to obtain a test report. 3D comparison collected 199980 points, results show that deviation range is -0.074~0.357mm, outer diameter of plastic parts is negative deviation, actual size is smaller than design size, and inner diameter of plastic part at elbow interface is slightly larger than actual size. Maximum value is 0.357 mm, and 3D detection result is shown in FIG.

 

Figure 13 plastic parts 3D test results

 

Table 3 shows distribution of plastic parts deviation. Deviation between point cloud data of elbow joint and original design model is mainly -0.1~-0.05mm, and deviation is -0.074~-0.018mm, accounting for 33.7%, and deviation is -0.018~ 0.018mm accounted for 39.5%.