【Abstract】Introduces reasons for dislocation of parting line of plastic parts of floor fan lift bracket, analyzes problem of rounded corners, dislocations between fixed mold cavity and moving mold core parting jaws, solves problem by improving mold manufacturing process.

In July 2014, invited by a mold factory, went to factory to inspect plastic parts of floor fan lifting bracket. Defect is that parting line of plastic part is dislocated, as shown in Figure 1 and Figure 2. Main reason for dislocation phenomenon is that mold cavity finishing, polishing and other processes are not reasonable. Original design did not take into account that polishing will bring some man-made problems, mainly because it is difficult to control position of jaw during polishing, or required finish cannot be thrown, and there are small rounded corners at jaw when required finish is thrown. Any situation will bring defects to appearance of plastic parts, and it is absolutely not allowed to involve appearance problems. However, after repeated rectification and maintenance according to engineer's idea, this problem could not be completely solved. Through long-term experience accumulated in factory's manufacturing and design of molds, after a comprehensive analysis of problem, a plan was developed and problem was finally solved.

 

Figure 1 Parting line dislocation (clamp) diagram

 

Figure 2 Parting line diagram of plastic parts

Plastic parts of floor fan lift bracket are made of diene-styrene (ABS) material, and surface is polished. The total height of plastic part is 57mm, width is 46mm, and length is 354mm (see Figure 3). Mold manufacturing process is shown in Figure 4 and Figure 5. First, the two openings of plastic part are core-pulled on both sides, then arc and oblique line are intersected to form upper and lower molds.

 

Figure 3 Schematic diagram of plastic parts

 

Figure 4 Mold manufacturing process diagram

 

Figure 5 Mold structure diagram

There are dislocations in trial molding parts and plastic parts that have been repaired several times, but size of dislocation is different. As shown in Figure 6, there are two types of dislocations (clamps): (1) upper small and lower large jaw; (2) upper large and lower small jaws.

 

Figure 6 Two types of dislocation (clamp)

(1) Structure of original injection mold is shown in Figure 5. Its design, manufacturing and forming processes are: first, design concept, using point gate, two cavities in one mold, upper and lower parting, left and right core pulling, parting surface and ejector rod exhaust, fixed mold cavity, moving mold core to circulate water, and manually take out plastic parts after core pulling. Second, manufacturing process, processing according to programming, processing of fixed mold cavity, processing of moving mold core, processing of panel, processing of gate plate, processing of fixed mold plate, processing of moving mold plate, processing of ejector panel and ejector bottom plate (simultaneous processing). Third, forming process, assemble parts and try out mold, install mold with a 500t injection molding machine, inject plastic parts according to injection parameters.
(2) There are some small flaws in mold design and mold manufacturing process. Reason is that it is too theoretical, neglects precision of machine and precision of manual operation. Fixed mold cavity and movable mold core cannot be fully processed in a mold factory at one time. They must be processed by milling machines, CNC machining centers, engraving machines, and EDM machining. Multiple processing requires multiple clamping and multiple edge seeking, accumulation of multiple processes leads to a larger cumulative tolerance. Especially during polishing, parting line is difficult to control, which often results in a small R on parting line. In this way, fixed mold cavity and moving mold core parting line are not completely matched, and clip is generated at this time, as shown in Figure 7.

 

Figure 7 Parting line dislocation

(1) Gap and difference are called parting line (parting line) by designer. There are two reasons for formation. One is caused by structure of plastic part, and difference is caused by unevenness of parting surface. When plastic part is in the hands of designer, surface itself is a straight and complete surface. Because parting surface is not flat, part of same surface needs to be taken out of front mold and part of rear mold, so this difference is caused by draft angle. Another situation is that there are processing errors in all aspects of mold processing, especially for non-precision molds, accumulation of multiple processes, cumulative tolerance is relatively large. When final plastic part comes out, jaws are more obvious, and mold repair master needs to repair jaws.
(2) Fitter repairs jaws.
a. Manual method. In the past, this method was used more when processing equipment was backward, precision was not high, and cost was too expensive. Current processing technology uses precision machining instead of manual repair as much as possible, but this method is still irreplaceable. When fitter repairs mold, one side is painted with white paint, and the other side is painted with red elixir. After repairing mold, observe degree of color of red elixir printed on the white lacquer surface. Based on experience, determine which parts need to add how much glue, and then polish them by hand. Of course, hand feel is very important when grinding. Fitter masters in the later stages of this grinding technology may not be able to keep up with the early fitter masters. After grinding is almost done by experience, reassemble mold, then use injection machine for proofing. After plastic parts are punched out, observe repair degree of jaws, then repeat previous process many times until jaws are repaired to meet customer requirements. .
b. Electrode method. According to position of jaw to be repaired, you can choose to use old copper electrode or remake copper electrode, measure average value of jaw by second element or other equipment, and then translate copper electrode or touch unilateral leaning, eccentric and glue to repair jaws. For plastic parts with high requirements, three-dimensional copper electrodes (that is, integral copper electrodes) can be used. Purpose is to discharge appearance and large surface as a whole as much as possible, which can well ensure consistency of processing errors. Difference between three-dimensional copper electrode and ordinary copper electrode is that it is best to process it in 5 directions. Ordinary copper electrode is processed in 3 axes, and three-dimensional copper electrode can be processed in 5 axes, or it can be processed in several directions.

(1) According to results of above analysis and traditional solutions, it is almost impossible to repair jaw problem by traditional manual methods according to existing mold structure, because mold is relatively large to assemble and disassemble, proofing takes time and material resources, and it is difficult to achieve a complete fit. According to second electrode method, parting line (clamp) can be repaired, but it takes a lot of time and money, and this method is not acceptable in small businesses with thin profits. Therefore, it is necessary to find and analyze cause of problem, and design a new process structure. New process is to use fixed mold cavity and moving mold core parting line for laser welding, add a polishing plate to fixed mold cavity and moving mold core for processing, so that fixed mold cavity and moving mold core can be polished independently as a whole.
(2) On original structure, fixed mold cavity and movable mold core are designed, installed with polishing plates, as shown in Figure 8.
(3) Design and install a polishing plate on moving mold core, as shown in Figure 9.
(4) Install a polishing plate in fixed mold cavity design, as shown in Figure 10.

 

Figure 8 Polishing plate
 

 

Figure 9 Design of moving mold core with polishing plate installed

 

Figure 10 Design of fixed mold cavity and installation of a polishing plate
(5) Advantages of new process.
a. Add some processes to the original mold, which can save materials.
b. Process of the overall mold remains unchanged, but a polishing plate is added to cavity, which is convenient for assembly, disassembly and maintenance.
c. Save costs and improve efficiency.
(6) New technological process.
a. Demolition → design fixed mold cavity, moving mold core and positioning holes of polishing plate, screw holes → fixed mold cavity, moving mold core drilling positioning holes, screw holes → manufacturing polishing plate according to drawing → fixed mold cavity, moving mold core parting line burning laser welding→grinding parting surface→polishing plate fixed on fixed mold cavity and moving mold core→fixed mold cavity, moving mold core in computer processing center gong cavity (with polishing plate)→fixed mold cavity, moving model core with polishing plate and polished to 600# sandpaper → disassemble polishing plate → fix mold cavity and movable mold core using bite position to fix cavity → fix mold cavity and movable mold core are matched with finely trimmed parting line (clamp) → fixed cavity and movable core are disassembled, polishing plates are installed → polish to mirror surface → assemble mold → try mold (20PCS) → customer confirmation → hand over mold.
b. A schematic view of fixed-mold cavity and moving-mold core parting line-burning laser welding, as shown in Figure 11.

 

Figure 11 Sketch map of dynamic and fixed parting line burning laser welding
When burning laser welding, pay attention to drawing thick line part to weld about 2mm to cavity. Material of electrode is same as that of fixed mold cavity and moving mold core.
c. Manufacture fixed mold cavity, movable mold core, positioning holes and screw holes of polishing plate.
After mold is disassembled, first measure peripheral dimensions of movable and fixed mold core, cavity → design positioning pins and screw holes with CAD → design polishing plate with Pro/E → use domestic 718 material to manufacture polishing plate, and use vertical milling machine to process peripheral dimensions, pin holes, and screw holes according to design drawings (cavity holes do not need to be processed) → the two two planes of polishing plate are ground with a surface grinder → fixed mold cavity and moving mold core are corrected with a vertical milling machine, then drill positioning holes and screw holes .
d. Fixed mold cavity, moving mold core and polishing plate are processed by a computer processing center.
Moving and fixed mold cores, cavities are respectively equipped with polishing plates, positioned with pins, and screws are tightened → on machining center, calibration, edge finding (centering) → according to programming rough, fine, milling fixed mold cavity, moving mold core and polishing plate (pay special attention to connection between moving and fixed mold parting lines and polishing plate cavity), as shown in Figure 12.

 

Figure 12 Schematic diagram of fixed mold
(7) Die polishing.
After core, cavity and polishing plate are CNC completed → fixed mold cavity and movable mold core are polished separately (polishing plate cannot be removed) → polishing is first polished with 150# oilstone to 600# oilstone → polished with 200# sandpaper to 600# sandpaper → Disassemble polishing plate → merge fixed mold cavity and moving mold core → finely cast parting line position (clamp position, see Figure 13) → after fine polishing, disassemble moving and fixed molds → install polishing plate on fixed mold cavity and movable mold core → Throw core and cavity from coarse to fine to 3# metallographic sandpaper → polish cavity to a mirror surface with abrasive paste.

 

Figure 13 Parting line position for fine casting (clamp position)

Through improvement of above process, appearance defects of plastic parts are solved, and mold manufacturing cycle is shortened. This process has been widely used in same type of mold structure by a mold factory, which has solved many technical problems. In the future work, I will continue to learn, constantly sum up experience in my work, improve my technical level to a new height, and serve society better in the future.