Through analysis of box cover products, mold structure of box cover products with inserts is introduced, design of supporting and fixing structure of metal inserts by floating ejector rod of cylinder is described to ensure correct position of inserts in the product, provides a loan for mold design of similar products.

Box cover product shown in Figure 1 is made of PBT+GF30, with a shrinkage rate of 0.5% and its color is black. Product is like an open drawer. Filling box and filling ellipse are key control dimensions, of which No. 1 size 134±0.08mm is center distance of two steel rings with a diameter of 6.8mm. This size must match center distance of accessories, otherwise box cover cannot be installed. Height and position of terminal are key points of this mold design. Because terminal is affected by injection pressure during injection molding, terminal will move in product, resulting in unqualified height and position, which directly affects function of product. This article introduces design ideas and manufacturing process of box cover in detail through design and manufacture of box cover.

 

Figure 1 Product map

Box cover has a plastic bracket, 2 steel rings, and 33 copper terminals by injection molding (see Figure 2). There are 3 types of copper terminals, and there are also 3 strips. Each strip is processed by stamping die. Plastic bracket is arranged through vibration of vibrating plate. With cooperation of automated manipulator, insert three types of terminals into plastic bracket, and then perform terminal bending. Finally, assembly, 2 steel rings and 3 single-head terminals are put into mold for injection molding.
A 1*0.4mm groove is added in the middle of steel ring to ensure pulling force of steel ring in plastic part; also in order to ensure retention force of terminal in product, a barb of 0.75*0.125mm is designed on cutting surface, as shown in Figure 3.

 

Figure 2 Box cover product structure

 

Figure 3 Terminal structure

As product is formed and solidified, degree of cooling of various parts of plastic part is inconsistent, plastic part is bound to have internal stress that involves each other. The greater internal stress, the more severe deformation of plastic part. This product is GF30 with fiber. During filling process, molecular orientation direction is generally consistent with melt flow direction. Oriented molecules must return to their original state after solidification, internal stress must occur, plastic is oriented and perpendicular to orientation direction. Shrinkage rate is also quite different. All of these will inevitably lead to inconsistent shrinkage and deformation of plastic part. Combining above factors results in a size variation of 134±0.08mm, which is center distance of two steel rings with a diameter of 6.8mm (plastic part mounting hole ). If size does not meet requirements, plastic parts cannot be installed and accessories will fail to function. In order to facilitate correction of this size, root of column of positioning steel ring is made into a rectangle (see Figure 4), position can be moved back and forth to achieve purpose of adjusting size. Chamfering of head of positioning column is beneficial to guiding effect when steel ring is placed. Steel ring is prone to produce metal chips during positioning of column. In order to better solve this problem, positioning column is cut on three sides to reduce contact area and polished at the same time, similar to 3 points to determine a circle center.

 

Figure 4 Positioning steel ring column

Terminal shown in Figure 5 is suspended in product. In mold, terminal is a blind hole. Affected by injection pressure during injection molding process, terminal will move up and down in mold, causing product terminal to be high or low or mold terminal hole is glued, which increases product defect rate and seriously affects product output. In order to solve this problem, a floating ejector mechanism is specially adopted. Working process of floating ejector rod (see Figure 6): In normal mold opening state, cylinder 1 is connected to push block 5 through connecting rod 1 and connecting rod 2, push block 5 is connected to upper cover 6 of ejector rod fixing block and bottom plate 7 of ejector rod fixing block with a "T-slot". Ejector rod 9 is connected to parts 6 and 7 through floating ejector rod fixing block 8. Ejector rod moves up and down along guide post 4 under action of push block, so as to realize fixation of ejector rod to terminal, movement track of pushing block 5 is horizontal movement. Since terminal is suspended in product, cylinder is on far right side when mold is opened. At this time,  horizontal distance between push block 5 and upper cover 6 of ejector fixing block, lower backing plate 7 of ejector fixing block is 0.5mm ( See Figure 6a). When mold is started, cylinder moves to push floating ejector 9 against terminal to ensure that terminal will not move during injection molding process. At this time, horizontal distance between push block 5 and upper cover 6 of ejector fixed block, lower backing plate 7 of ejector fixed block is 6mm (see Figure 6b). Floating top rod 9 moves vertically downward by 0.87mm, plastic flow time to terminal is simulated by MoldFlow software. At this time, terminal is basically covered by plastic. Since there is a glue position at the root of terminal (and there is a sealing requirement), ejector pin must be retracted. Response time of normal injection molding machine is within 0.2s. All simulation time minus 0.2s is time when ejector rod retracts. At this time, cylinder moves from 6mm to 0.5mm, which is an action cycle process.

 

Figure 5 Terminal is floating in product

 

Figure 6 Working process of floating ejector
a — — mold opening state b — — mold clamping state
1. Cylinder 2. Connecting rod Ⅰ 3. Connecting rod Ⅱ 4. Guide column 5. Push block 6. Top cover of top rod fixing block 7. Lower backing plate of top rod fixing block 8. Floating top rod fixing block 9. Floating top rod 10. Product 11. Terminal

Role of cooling water circuit design:
(1) Improve productivity. Cooling water circuit is used to reduce mold temperature during thermoplastic injection molding cycle, thereby accelerating freezing of plastic parts. Effective cooling water circuit circuit design can reduce cooling time, thereby increasing total production.
(2) Improve quality of plastic parts. Uniform cooling can reduce residual stress caused by uneven heat transfer. Thus, warpage of finished product can be controlled to maintain accuracy and stability of size of plastic part, thereby improving quality of plastic part. This mold is designed with 4 water paths, upper and lower mold water paths, slider water path, plus lower cavity and core water tower. Slider waterway and movable mold core waterway have been extended into accessories in the wall of plastic part, which can more effectively reduce residual stress caused by uneven heat transfer and reduce warpage of product. Figure 7 is 3D and 2D diagrams of water circuit of this mold.

 

Figure 7 Cooling water system
1. Fixed mold waterway 2. Plastic parts 3. Slider waterway 4. Cavity waterway 5. Core waterway

After plastic injection molding, plastic parts shrink inevitable, shrinkage rate directly affects size and shape of mold cavity, final shape of plastic part. In injection molding process, because plastic macromolecules are oriented along flow direction during injection process of melt into cavity, shrinkage rate of plastic in flow direction is greater than in vertical direction. At the same time, shrinkage rate of crystalline plastics in flow direction and vertical direction, difference in shrinkage rate is larger than that of amorphous plastics, so superposition of shrinkage rate and anisotropy of shrinkage results in a great tendency to deform commonly used crystalline plastic parts. If plastic part design and mold design phase do not consider impact of shrinkage during injection molding process, then plastic part will not shrink in set direction, geometric shape of plastic part will be very different from design requirements, serious deformation will cause plastic part to be scrapped. In order to facilitate adjustment of terminal position, plastic part is made into a rectangular split plastic part. X and Y directions of terminal are controlled by changing left and right dimensions of terminal fixing groove, front and rear dimensions, stop bar controls Z direction, so as to meet requirements of adjusting position.

Through design and manufacture of product, it is proposed how to design and position product with suspended inserts, optimize design and manufacturing to ensure that mold is accurate and reliable, is more practical, which can serve as a reference for development of similar products.