【Abstract】With upgrading of consumption concepts, consumers have higher and higher requirements for appearance quality of automobiles. As an important appearance part of automobiles, OEMs have higher requirements for their exquisite perception. Welding line is a common appearance defect of bumper. During trial production stage, there will also be some welding line conditions that cannot be explained by conventional theory. Reasons and solutions for formation of unconventional weld lines are introduced with two practical cases.

Figure 1 shows schematic diagram of common welding lines of bumper plastic parts. Welding lines are mainly distributed in following areas: ① license plate area; ② lower grille area; ③ fog lamp area; ④ mounting holes on both sides of rear door; ⑤ rear bumper Reflector; ⑥ trailer hook hole.

 

Figure 1 Schematic diagram of bumper welding line
Common weld lines have two notable characteristics: ① Reason for formation is that plastic melt is divided into two streams when it passes through hole, and after bypassing hole, it rejoins to form, as shown in Figure 2; ② After melt bypasses hole, fronts of the two streams initially meet near hole, as angle of convergence increases, two streams merge into one, and weld line gradually disappears (invisible), such as shown in Figure 3.

 

Figure 2 Schematic diagram of welding line formation

 

Figure 3 Actual weld line status

Figure 4 shows rear bumper diagram of a pickup truck. Since upper side of plastic part is formed by an integral slider, gate is arranged on ground side, hot runner is transferred to cold runner, and side gate is filled with glue.

 

Figure 4 Rear bumper of a pickup truck
During trial production process, obvious weld lines appeared on reflectors on both sides (see Figure 5), which was different from weld line feedback from mold flow analysis (see Figure 6), surface of plastic parts was painted, light and shadow were not smooth after painting, so customer could not accept.

 

Figure 5 Welding line above reflector (surface effect after passing 300# sandpaper)

 

Figure 6 Mold flow analysis weld line location

 

Fig.7 Internal wall thickness distribution of reflector

Assumption 1: Welding line inside mounting hole of reflector, compared to defect pictures, welding line is closely related to mounting hole inside reflector, and it is suspected that welding line formed after material flows through hole rushes to A side.
Verification: Short-shot verification in trial production process shows that flow in reflector area is significantly slower than that of main surface, and flow in reflector is more consistent with that in mold flow analysis (see Figure 8 and Figure 9). Therefore, weld line installed inside reflector will not face A surface. Assumption one does not hold.

 

Figure 8 Actual short shot effect

 

Figure 9 Flow effect of mold flow analysis
Assumption 2: Undercurrent line, because gates are all on ground side, pressure on both sides of reflector is inconsistent, resulting in offset of weld line.
Verification: During trial production process, reflector is sending gate closed, and welding line above reflector is not improved; interior of reflector is thinned, initial position of welding line is frozen in advance, and welding line above reflector is eliminated, as shown in Figure 10 and Figure 11. Assumption 2 is established (similar projects use same means to eliminate problems)

 

Figure 10 Schematic diagram of undercurrent line

 

Figure 11 Effect of eliminating weld line problem

At present, mold flow analysis cannot accurately predict problem of subsurface flow line. By comparing mold flow results before and after rectification, it is found as follows: In terms of average velocity results, velocity direction on reflector before rectification will change obliquely upward, but after reduction and rectification, there is no change. Phenomenon occurs, as shown in Figure 12 and Figure 13, which can be used as an evaluation principle.

 

Figure 12 Average speed results before rectification

 

Figure 13 Average speed results after rectification

Figure 14 shows plastic part of rear bumper of an SUV model. Gate is arranged on the side of back door, and filling sequence is from middle to two sides. During trial production process, a weld line appeared on the middle gate facing exterior surface, surface of plastic part was painted, light and shadow were not smooth after painting (see Figure 15), which was unacceptable to customer.

 

Figure 14 Rear bumper of an SUV

 

Figure 15 Welding line at middle gate (surface effect after passing 300# sandpaper)

Figure 16 is a cross-sectional view of middle gate of plastic part, gate: 30 * 1.5mm, and basic wall thickness of plastic part is 2.5mm.

 

Figure 16 Cross-sectional view of intermediate gate

Assumption 1: Gate or runner problem.
Verification: Short shot verification, plastic enters cavity and shows a middle depression phenomenon (see Figure 17), indicating that there is a problem with runner/gate design. After adding glue locally in the middle of gate, it is verified that middle gate facing weld line has not been eliminated. Assumption one does not hold.
Assumption 2: There is a problem with structure of plastic part. Middle gate is facing a mounting rib. When plastic melt enters cavity and flows to rib, forward shunting leads to a depression in the middle.

 

Figure 17 Short shot effect
Verification: Intermediate gate facing rib is blocked for 120mm in the form of blocking, effect is verified and problem is eliminated, as shown in Figure 18. Assumption two holds.

 

Figure 18 Short shot contrast effect

Two cases of unconventional welding lines in this paper cannot be predicted in the early stage of development due to different reasons for formation of common welding lines, can only be verified by trial and error. Through rectification process of weld line problems in the two cases, understanding of causes of formation of unconventional weld lines can be broadened, and direction of thinking for similar defects can also be provided.