Considering application environment of predecessor of toy man, since it is a daily necessity, it must be made of non-toxic, odorless, and odorless materials, and it is often touched by hand, molding material needs to have strong acid resistance, not easy to wear and harden. According to characteristics, application, price, performance and plastic properties of plastic parts, material selected is acrylonitrile-butadiene-styrene copolymer (ABS). ABS is an opaque non-crystalline polymer, which is not easy to deform, wear and tear, has high hardness, small shrinkage after cooling, convenient for later processing. Surface of molded plastic parts has good gloss, dyeing and spraying properties It can be adjusted to any color of plastic parts, and can also be plated into metallic luster.

 

 

(B) Three-dimensional structure

Figure 1 Predecessor of toy man

According to analysis in Figure 1, plastic parts are deep in recess, but shape and structure are relatively simple. Wall thickness at each position is uneven but meets minimum wall thickness requirements. Inner surface of plastic parts is equipped with an assembly position, but quality requirements for outer surface are very high. In design and analysis, we must pay attention to ensure quality of outer surface.

Because there are many uneven structures on the surface of front body of toy man, wall thickness is unevenly distributed, it is easy to shrink and cause local position of surface to sink. Relevant data is obtained through mold flow analysis to reduce number of mold trials. In injection process, it is possible to increase surface forming accuracy of plastic parts by increasing holding pressure and extending cooling time. Surface structure of plastic parts is more complicated, some shrink marks will also be generated due to deformation during injection process. For example, cooling shrinkage of plastic parts after molding will cause surface warpage and deformation, mainly due to insufficient cooling time. Using Moldflow analysis can predict surface warpage of plastic parts in advance. Wall thickness of plastic parts of toy figure is unevenly distributed. During injection molding, when two streams meet, it is easy to produce weld marks at their junction, and sometimes there will be obvious color marks. These are caused by two streams meeting and not completely melted. Main reason is uneven distribution of wall thickness.

As application of plastic parts continues to grow in various industries, it is particularly important to reduce manufacturing costs and advance time to market, companies are increasingly demanding precision in plastic parts. Number of cavities is determined according to geometry of predecessor of toy figure and production batch. Due to simple structure of plastic parts and large production batch, in order to improve production efficiency, structure design of 2 cavities is adopted, cavity is arranged in a balanced manner. After Moldflow analysis is completed, a large amount of text, images and animation results will be generated, displayed in classification task window. Because there are too many analysis results, some images will be used to summarize the whole.

 

Figure 2 Filling results

It can be seen from Figure 2 that time from melt injection nozzle into main runner of mold to fill  cavity is 3.605s. During injection process, matching of gate, flow resistance and pressure are within a reasonable range, melt reaches ends of toyman's predecessor cavity at the same time, without material shortage.

 

Figure 3 Cavitation location

During injection process, in the area where ends of two or more streams meet, air bubbles are suppressed to form air pockets. As shown in Figure 3, it can be seen that air pockets mainly appear at the edge of predecessor of toy man.

 

Figure 4 Location of welding marks

When melt is injected, several streams are split and merged in the cavity of mold. Melt will form a weld mark before melt is completely fused at the interface. As shown in Figure 4, weld mark mainly appears at the edge of toy man's predecessor.

 

Figure 5 Speed/pressure switching

Clamping force is force that injection molding machine must have to lock mold when injecting. It will gradually change as melt fills mold cavity. If you want to optimize mold structure, you must reduce maximum value of clamping force. The key is to reduce filling pressure. XY graph shows situation where clamping force changes with time. At the same time, pressure at the time of speed and pressure switching can reflect situation of clamping mold, as shown in Figure 5.

 

Figure 6 Warpage deformation in all directions

Uneven shrinkage of plastic parts during molding will produce warpage deformation in all directions. Through comparison of warpage deformation in all directions, injection process parameters can be continuously adjusted to adjust amount of warpage deformation within a reasonable range, and then test mold again. Through adjustment of injection process parameters, analysis results shown in FIG. 6 are obtained, which show that total deformation of molded plastic parts, deformation in X direction, Y direction, and Z direction are all within a reasonable range.

 

Figure 7 Parting surface and demoulding mechanism

In design process of injection mold, first is to determine parting surface, which is related to whether plastic part can be demolded smoothly, overall layout of gating system, overall layout of mold and processing method of mold parts. Predecessor of toy man is a shell type, which has high requirements on surface quality, especially in terms of roughness. When selecting parting surface, it is important to consider appearance quality of plastic parts produced by mold. Parting is prone to flash, and it is necessary to consider how to easily remove flash. Considering all aspects, mold adopts a two-plate mold structure with a single parting surface, which can well control manufacturing cost of mold. Parting surface is selected at the place where shape and contour of plastic part is largest, which is conducive to exhaust of gas. At the same time, after mold is opened, plastic part is left on the side of moving mold, parting surface is shown in Figure 7(a), and ejection mechanism is shown in Figure 7(b).

 

Figure 8 Structure of cavity plate

Side gate is used in mold design to make mold parts easy to process and maintain, and easily remove condensate in runner. Due to small size of predecessor plastic parts of toy man, in order to reduce manufacturing costs and improve production efficiency, 2 cavities are used for injection production according to design requirements of gating system. According to structure, cavity plate can be divided into: integral type, integral embedded type, partial mosaic type and four-wall mosaic type. Integral cavity plate design can improve strength of mold parts, it is directly processed on cavity plate, and cavity plate is not easily deformed during mold work. On the premise of ensuring that mold can work normally and runner of gating system is as short as possible, it is designed as a balanced mold cavity, so that melt can fill each cavity evenly when filling, to ensure molding quality of plastic parts. Due to relatively simple structure of plastic parts, no lateral parting is required, and a symmetrical arrangement can be adopted, as shown in FIG. 8.

 

Figure 9 Core structure

Design of molded parts including core and cavity plate, because molded parts directly contact with high temperature and high pressure melt, its quality directly affects quality of molded plastic parts, molded parts must have higher strength and hardness, also need better wear resistance and stiffness. Generally, a better die steel is selected for manufacturing, and at the same time, heat treatment is performed to make hardness reach 50~55HRC. Core is a part that forms inner surface of plastic part. Core structure of toy man's predecessor mold is more complicated, inner surface of plastic part is relatively low. Core structure adopts an integral core, as shown in Figure 9.

 

Figure 10 mold structure

1. Movable mold base plate 2. Push plate guide post 3. Support post 4. Pull rod 5. Movable template 6. Core 7. Cavity plate 8. Fixed mold pull 9. Sprue bushing 10. Positioning ring 11 .Fixed mold base plate 12. Screw 13. Push rod 14. Push plate 15. Push rod fixing plate 16. Pad block 17. Guide post 18. Guide sleeve 19. Nozzle
Using UG software to carry out three-dimensional modeling of parts of predecessor of toy man, and at the same time assemble each part to obtain three-dimensional structure of mold, and export three-dimensional structure to AutoCAD to obtain two-dimensional structure, as shown in Figure 10.