Retention marks are caused by change in speed of plastic melt during filling process. Most on-site processes use temperature difference line on the surface of plastic part to determine whether there is a retention mark. Temperature difference line representation on actual plastic part is shown in Figure 1.

 

Figure 1 Plastic part retention marks
Plastic part shown in Figure 1 has an obvious color difference in the middle and rear sections. Because of color difference, it looks like a weld line mark. After adjustment, it can be confirmed that there should be no weld line at this position. Process believes that color difference here is caused by temperature difference, so it is called a temperature difference line.
Figure 2 shows gate design of decorative strip mold. Length of decorative strip plastic part generally exceeds injection length that can be performed by a single inlet point. Therefore, it is set to multi-point injection. Multiple sequential valve opening times can move weld line to ensure that there are no weld line defects on appearance surface. Generally, design of mold cavity of decorative strip plastic parts is 2-cavity symmetry to ensure balance of melt injected into mold cavity.

 

Figure 2 Decorative strip mold pouring system
After Moldflow analysis, location where temperature difference line appears is not location of weld line (weld line is at location of puncture hole), as shown in Figure 3. Because sequential valve is opened, third gate is opened first. After melt passes through second group of gates 2 and 4 (each group of gates will be opened at the same time), third group of gates 1 and 5, there will be no weld line. Through Moldflow analysis, contour lines are reopened and it is found that contour lines here become dense, confirming that retention marks are prone to appear here, as shown in Figure 4, so it is speculated that retention marks on site are temperature difference lines.

 

Figure 3 Plastic part weld line position (Moldflow analysis)

 

Figure 4 Plastic part contour analysis

Before mold design, mold flow analysis can be used to understand some problems of plastic part molding and avoid problems in time. Now Moldflow is used as analysis software for analysis and verification to find cause of problem.
In initial mold flow analysis, retention marks here were not noticed. Under ideal analysis conditions, software analysis recognizes that next set of gates will be opened as soon as gate passes. At this time, contour lines of newly opened gates will become relatively relaxed compared to contour lines of original gates, and displacement generated per unit time will be greater. If change of contour lines is small, corresponding change here will be ignored, and thus retention marks here will also be ignored.

During on-site debugging, color of relatively stable position of molded plastic part changed. It is considered that it may be caused by difference between temperature in hot runner and temperature in barrel. If temperature in hot runner is low and temperature in barrel is high, there will be a cold material in the front part and hot melt material pushed forward by barrel will show temperature difference of the two materials at a certain appearance position of molded plastic part. Hot runner temperature was adjusted from original 225 ℃ to 240 ℃ (material is general material 1071 of ABS plus masterbatch), and barrel temperature was slightly reduced (temperature of storage area was reduced from 240 ℃ to 235 ℃) to prevent temperature abnormality of barrel due to difference in temperature control. After above adjustment, color difference on plastic part (that is, retention mark discussed) still exists, then temperature of hot runner remains unchanged, barrel temperature was further lowered to 230 ℃. Injection verification was performed again, and color difference on plastic part still existed. Comparison test data is shown in Table 1.

At the same time, in order to prevent problem from being caused by material reasons, melting index of ABS material was tested to confirm that problem was not caused by material changes, as shown in Figure 5.

 

Figure 5 ABS material melting index
After field test, it was confirmed that cause of temperature difference line was not related to temperature difference between screw and hot runner, that is, guess about cause of field temperature difference line was wrong and had no corresponding relationship with melting temperature of material.

When conducting second group of tests, injection speed was changed to observe whether there was any change. When injection speed was adjusted from 25 mm/s to 70 mm/s, position of temperature difference line of plastic part changed. Therefore, speed and opening time of sequence valve were compared to determine that valve needle opening time caused appearance of this temperature difference line. Speed and gate opening time comparison test is shown in Table 2.

Table 2 Comparison test of speed and gate opening time
Comparing data of test groups 1 and 2, it can be found that temperature difference line moves to the end when speed increases, that is, the higher injection speed in the same time, temperature difference line will move to the end; comparing data of test groups 2 and 3, the earlier gate is opened, the further temperature difference line will move forward. Comparison of the first three groups of data can conclude that generation of temperature difference line corresponds to opening time of gate of group 2. On this basis, the fourth group of tests was added. By comparing data of groups 1 and 4, it was found that temperature difference line no longer appeared after weld line appeared, that is, probability of flow end appearing is high, and change of flow end causes problem point of temperature difference line. Given that materials are all gushing forward from middle, it can be confirmed that this problem is related to opening time of gate of group 2.
Adding test groups 5 and 6, comparing test group 5 with test group 4, it is found that retention mark will only appear when there is no weld line, that is, flow end, forming a closed-loop test with test groups 1, 4, and 5, confirming that temperature difference line is caused by opening of gate of group 2. Comparing test groups 5 and 6, it can be seen that when injection speed is slow, temperature difference line change will be reduced in the later stage. In actual debugging, injection speed cannot be too slow.

In summary, experimental conclusion is drawn: formation of temperature difference line is related to time of opening gate. At the same time, when opening time is same, the faster injection speed, the easier it is to push temperature difference line to the end of plastic part. Characterization at this time is same as retention mark characterization analyzed in Moldflow.
Use mold flow analysis software to analyze flow front temperature to check whether change of flow front temperature is consistent with change of isoline. If it is consistent, cause of temperature difference line is consistent with cause of retention mark. The two are different names caused by same problem. Therefore, melt flow front temperature test is carried out. Flow front temperature of decorative strip under retention mark state is shown in Figure 6.

 

Figure 6 Flow front temperature of decorative strip under retention mark state
Melt flow front temperature indicates a high temperature trend at dense contour lines. As shown in Figure 6, when melt is pushed forward by gate after opening, flow front temperature varies greatly, while change of melt flow front temperature in general mold design is 2~5 ℃. From flow front temperature of mold flow analysis, temperature at dense contour lines is more than 3 ℃ higher than temperature when contour lines are scattered. Temperature will drop suddenly here, resulting in generation of temperature difference lines on site, which is defect of retention marks in actual analysis report. For plastic parts with high appearance requirements, change of melt front temperature needs to be controlled within a smaller range to ensure a better appearance, that is, contour lines cannot show obvious sudden changes. Slow increase of melt front temperature will not appear in dense contour line area, that is, there will be no retention marks.
Confirming that temperature difference line is a manifestation of retention marks, it will occur when flow distance is set unreasonably when performing high-gloss surface and fine leather grain processing, can be verified by melt flow front temperature and density of contour lines.

After confirming that temperature difference line on site is retention mark, decorative strip is simulated by Moldflow again. This simulation is no longer set according to ideal situation, but tends to reality. Because ideal situation is that gate will open when melt flows through second group of gates (gates 2 and 4), but actual situation is that melt will open only after flowing through part of second group of gates. Therefore, simulation analysis is carried out according to actual situation on site. The two comparative tests are shown in Table 3, and contour lines are shown in Figure 7.

Table 3 Two comparative tests

 

Figure 7 Contour line comparison
At the same time, melt flow front temperature is detected, as shown in Figure 8. By comparing changes in melt flow front temperature, it is found that when next valve needle is opened, local temperature of melt will drop rapidly, and this sudden drop will cause color changes. When melt is pushed forward, front temperature keeps rising due to shear heat. After next gate is opened, injection speed and flow distance change, so temperature drops suddenly. In case of high-gloss surface, fine leather grain, and fine sandblasting, this temperature difference will cause defects.

 

Figure 8 Melt flow front temperature after debugging retention marks (℃)
According to above analysis results, it is found that analyzed retention marks are consistent with actual position of temperature difference line, indicating that temperature difference line is a special form of retention marks. Since the two gates transfer feed, pressure drops suddenly when rear gate is opened, shear heat also drops, and flow rate slows down, so a temperature difference will be formed at gate position. Boundary line of this temperature difference appears on the surface due to sudden drop in the front temperature. Representation of retention marks on plastic parts is popular name of temperature difference lines and color lines. It has been determined that these problems are caused by changes in melt flow temperature and will only appear on plastic parts with multiple gates.

Add a group of short shot settings on site to be consistent with gate opening and closing sequence in Moldflow. After setting is completed, finished product test is carried out. Comparison of gate opening time of second group is shown in Table 4.

Table 4 Comparison of gate opening time of the second group
Plastic parts with retention marks on site are shown in Figure 9. Retention marks of plastic parts are manifested as an obvious temperature difference line. Direction of temperature difference line is similar to temperature of melt flow front. Mold flow analysis warning direction of such problems can be determined, thereby confirming that representation of retention marks on plastic parts is temperature difference line.

 

Figure 9 Retention marks on trim strips (temperature difference line and color line)

Same problem exists on uneven trim strips. Retention marks are obvious at small end of trim strips. Using logic of adjusting temperature difference line, retention marks are placed at the farthest end before opening next set of gates. Because length between gates has been designed in the early stage, it is inconvenient to modify. Problem can only be solved by moving it back. Plastic part is shown in Figure 10. Moldflow's first analysis of retention mark position (contour line diagram) and melt front temperature is shown in Figure 11. Adjusted analysis contour lines and melt flow front temperature are shown in Figure 12.

 

Figure 10 Plastic Part

 

Figure 11 Moldflow's first analysis of retention mark position (contour line diagram) and melt front temperature

 

Figure 12 Adjusted analysis contour lines and melt front temperature
To solve temperature difference line problem represented by retention mark, this secondary mold can only shorten opening time of second group of gates at small end, push retention mark at small end out of surface of plastic part before next injection. Except for a slight extension of injection time, the other changes are not significant, but distribution of contour lines no longer has retention marks. At the same time, change in melt flow front temperature no longer changes from high to low. Plastic parts with strict appearance requirements are more sensitive to sudden changes in temperature. By adjusting process parameters, melt front temperature can be slowly increased and no sudden drop occurs, solving temperature difference line problem.

Through study of retention marks, as a representation of temperature difference lines, it appears more on highlight surface, and generally this obvious temperature difference line defect will appear on the basis of mirror surface. In some frosted leather textures or after sandpaper polishing, there will be no retention marks. This situation is similar to sun spots. It appears on high-gloss surface, but it will be hidden on sandpaper surface or leather surface.

When solving retention mark problem, first consider whether appearance of plastic part can be changed to a non-high-gloss mirror surface. This is a simpler solution. High-gloss surface will magnify color defects, and sandblasted surface will magnify defects of flowing gas. Both are color problems, so lowering color of high-gloss surface of plastic part can improve it.

There is no specific conclusion on when retention marks appear when molding plastic parts, but theoretically solving this problem requires defining the longest flow distance of melt, using retention marks as a defect for early warning, and requiring gate design of mold to be as low as possible from the longest flow distance of melt. Therefore, considering solution of retention marks from perspective of mold structure, gate distribution should be appropriately changed during mold design, rather than being evenly distributed at equal distances. Next set of valve needles should be opened in advance when melt does not need large resistance to advance, avoiding sudden changes in injection speed, which can fundamentally solve generation of retention marks.

From process point of view, after gate is confirmed, retention marks are still generated, resulting in surface defects. In terms of process, they should be pushed out of surface of plastic part or pushed to a position where they cannot be found, and process parameters should be debugged. As in above comparative test, after all gate positions are confirmed, position of retention marks is moved back, that is, opening time of second group of gates is moved back, so that melt can flow farther and defect is adjusted.
Silted contour line is used as an intuitive evidence for software to judge retention mark. It is supplemented with melt flow front temperature, which can find defects of retention mark and accurately correct it. Density of contour line also needs to be determined, so software needs to be updated in real time. In addition, other appearance plastic parts with multiple gates may also have same problem during early analysis, and this solution can be used as a reference.