For plastic parts that do not allow traces of push rods on the surface of plastic parts, such as large transparent plastic parts, plastic parts such as printer covers and other plastic parts with front and back surfaces can be pushed out at the edges of plastic parts. The other type is box-type plastic parts, with larger size, deeper cavity, and large ejection force. If only push rod and push tube are designed to be ejected, ejection force is not enough to demold plastic parts, and push plate cannot be designed to be pushed out. At this time, push block is also used to be push out. Push blocks are mostly used in large and medium-sized molds. Similarly, for flat-shaped flanged plastic parts, if mold is adhered when pushed out with a push plate, push-out ejection mechanism should also be used for ejection. Pushing out with a push plate is not easy to deform plastic parts, which will not affect appearance, and there is no trace of push rod.
Push-out mechanism of push block is fastened on push block by using a round push rod to increase push-out area, so that push-out is easier to ensure accuracy of plastic parts and not easy to deform. It can be considered that push block is an extension and simplification of push board mechanism. Large plastic parts, sometimes unable to manufacture push plates, mainly consider steel waste and difficult to process, molds are difficult to assemble, then push blocks can be used to eject.
Push block is part of cavity and participates in molding of plastic part, so it should have higher hardness and lower surface roughness. Push block should have a good fit gap with cavity and core, which requires both flexible sliding and no overflow. Push rod and mold plate used in push block need to have a matching length of 20-30mm or more, and the rest of place should be avoided.
Ejection mechanism of push block must have a ejector pin plate guide mechanism, and the first reset mechanism is designed as needed. If return spring of ejector pin plate is strong, the first reset mechanism can be omitted. When push block is pushed out at the edge of plastic part, because upper plane of push block collides with cavity, if push block cannot be reset before mold is closed, cavity will come back every time mold is closed, and cavity will be easily hit by marks of depression.

 

Figure 1 Schematic diagram of pushing out block

Schematic diagram of push block ejection mechanism is shown in Figure 1, (a) is mold clamping state, and (b) is ejection state.
Assembly form of push block is shown in Figure 2. When designing push block, push block is very close to cavity, and strength of cavity needs to be considered. Cooperation between push block and cavity is H7/f6, mating surface roughness is Ra0.8～0.4µm, material of push block is H13, and heat treatment is 48～52HRC.

 

Figure 2 Push block assembly form

Sealing surface of push block and core can be fitted with a straight body. The other three sides that cooperate with movable mold insert should be designed as slopes. It is not suitable to use straight body. Slope is 3゜~5゜, as shown in Figure 2. Battery tank shown in FIG. 3 is a large closed shell with a large ejection force. A push tube is designed to push out at internal column position, and a push block is designed to push out at the edge. Clearance between push block, insert and core requires a sliding fit, subject to no flash. For a longer push block, more than two push rods must be designed. When push block is small and only one push rod can be equipped, special attention must be paid to anti-rotation problem of push rod and push block, so as not to bump mold when push block is reset. When designing, pay attention to make push rod at the bottom of push block extend into push block 3~5mm, and fixing screw should be added with a spring washer to prevent loosening.

 

Figure 3 Push block of battery slot is pushed out

Push box of instrument box cover is shown in Figure 4. Material of instrument box cover is PP, maximum external dimensions of plastic parts are 414mm*150.8mm*30.69mm, and bone depth is 30.5mm. Because middle bone is very long, push block is used for ejection. This type of push block inside plastic part, top surface can not be connected with screws, instead use T-slot connection. Snap head of large diameter push rod into side of push block, and tap the other end to fix it on push plate. Design putter on the other side of bone position to keep bone position balanced.

 

Figure 4 Instrument box cover mold drawing