【Abstract】Structure type of injection mold and way of ejection of aggregate are different due to different gate forms. Aiming at problems of low production efficiency and poor shearing quality of side gate aggregate cut off outside mold, injection mold structure of cut side gate aggregate cut off in mold was proposed, its design points were analyzed.

Gating system is a section of melt channel from nozzle of injection molding machine to entrance of mold cavity. Its function is to guide molten plastic from nozzle of injection molding machine to closed mold cavity. Generally, it can be divided into a point gate gating system and a side gate. Gate gating system is shown in Figure 1 and Figure 2 respectively [1].
After injection molding is finished, plastic in the gating system is cooled to form waste and needs to be separated from plastic part. Therefore, removal of gate and convenience of trimming should often be considered in design of injection mold.
Removal of aggregates of gating system is generally determined according to gate type and requirements of plastic parts. Different types of gates have different mold structures, methods of cutting and removing aggregates of gating system are also different.

 

Figure 1 Point gate gating system

1. Condensate of gating system 2. Plastic parts 3. Point gate

 

Figure 2 Side gate gating system
1. Condensate of gating system 2. Plastic parts 3. Side gate

Point gate, also known as fine nozzle, often adopts a three-plate mold structure. As shown in Figure 3, a small pull rod 1 is designed on fixed mold base plate. When mold is opened and mold is divided along II, pull rod 1 pulls point gate aggregate 4 out of fixed mold plate, so that gate aggregate 4 and plastic part 5 are automatically cut off; when mold is parted along II-II, runner push plate (de-casting plate) 3 is separated from fixed mold seat plate 2. At this time, runner push plate 3 pushes gating system aggregate to remove it from pulling rod 1, so as to realize in-mold cut and ejection point of gate gating system aggregate.
Due to small size of point gate, gating system condensate and plastic part are often separated by in-mold cutting and ejection. Therefore, mold needs to be opened multiple times. It is necessary to design a fixed distance parting mechanism to ensure mold opening sequence and mold opening distance. Mold structure is complex, but it saves man-hours of removing gate outside mold, so production efficiency is high [1].

Side gate is also called ordinary gate, or large gate, generally adopts a two-plate mold structure. When mold is opened, mold is divided once, side gate gating system condensate and plastic part (as shown in Figure 2) are connected together, released from mold, then secondary processing is carried out, and plastic part is separated from condensate with help of a tool outside mold. Due to large size of side gate and higher connection strength with plastic part, if side gate and plastic part are to be cut and separated in mold, cutting structure with a blade must be designed in mold structure.

 

Figure 3 Release of aggregate at point gate
1. Pull rod 2. Fixed mold seat plate 3. De-casting plate 4. Condensate of gating system 5. Plastic parts

Figure 4 shows mold structure of single shear block to cut aggregate. When injection is completed, injection molding machine nozzle moves backward, sprue bushing 1 retreats under force of spring 2, so that sprue aggregate 3 comes out of bushing 1 (shown in right half of Figure 4). When mold is closed, due to cavity pressure, spring 6 in movable mold is in a compressed state. When mold is opened, plastic part 4 wraps core 8 and does not move, compression spring 6 pushes shear block 5 to move upward to cut side gate, so as to realize in-mold separation of plastic part 4 and condensate 3 of gating system. When pushing mechanism pushes out plastic part 4, limit screw 7 moves upward under action of connecting push rod 9, condensate 3 of gating system is simultaneously ejected from mold.
Amount of movement of cutout 5 is controlled by limit screw 7. During design, stroke of shear block should be determined according to thickness of different gates. This structure relies on plastic part to shrink and pack core without moving, while shear block moves relative to plastic part under action of elastic force to cut off aggregate, so it is generally suitable for box-shaped, lid-shaped or shell-shaped plastic parts.

 

Figure 4 Single shear block cuts off aggregate mold structure
1. Sprue sleeve 2. 6. Spring 3. Gating system aggregate 4. Plastic parts 5. Shear block 7. Limit screw 8. Core 9. Connecting push rod

Figure 5 shows mold structure of double shear block to cut aggregate. Movable mold cut block 3 and fixed mold cut block 5 are designed on both sides of movable and fixed molds. When mold is opened, aggregate in sprue bushing 1 is pulled out under action of pull rod 7 and left on the side of movable mold. At the same time, movable and fixed mold shear blocks move relative to each other under action of UniGlue (ie polyurethane PU elastomer) 2 and 6, to separate gating system condensate from plastic part 4, and realize cutting of condensate in the mold. When pushing out, push tube 8 removes plastic part from core 9, and at the same time pulling rod 7 makes gating system condensate escape from mold. Moving amount of fixed mold cut block 3 and movable mold cut block 5 is controlled by stroke space on fixed mold plate and movable mold plate, respectively, and is generally determined by thickness of gate. This structure relies on relative movement of two shear blocks to separate aggregate from plastic part, so it is generally suitable for tubular, cylindrical or plastic parts with a hole in the center.

(1) In order to enable gate aggregate to be separated smoothly, stroke of cutting edge relative to mold plate should be greater than gate depth, and it is generally recommended to be 2 to 3 times gate depth.
(2) There should be a proper gap between two shear blocks. Recommended gap value is 0.05～0.08mm. You can also refer to gap value between punch and die in cold stamping die.
(3) In order to avoid cutting burrs, spring force or elastic force of superior rubber should be sufficient, cutting edge should be sharp.
(4) Spring should be used to push elastic element of shear block. When temperature of mold is low, you can use Uniglue.
(5) Form of shearing cut off side gate can be flexibly changed according to structural characteristics of plastic part, cut block can also be replaced by a die insert (shear insert) with a cutting edge.

 

Figure 5 Double shear block cuts the aggregate
1. Sprue bushing 2. 6. Youli glue 3. Shear block for fixed mold 4. Plastic parts 5. Moving die cut block 7. Pull rod 8. Push tube 9. Core

In design of injection mold, because side gate has a simple structure, is easy to process and repair, processing cost is low, it is easy to ensure molding of plastic part, so it is widely used. However, separation of aggregate and plastic part of side gate pouring system requires manual separation outside mold after molding. Production efficiency is low, surface quality and geometry of gate part are affected by human factors. There may be uncleaned burrs and plastic debris, which will adversely affect subsequent assembly of plastic parts and quality of finished product. Sometimes it is necessary to increase gate cutting and rolling process after deburring, so there are many subsequent processes in molding process, which increases processing cost, reduces production efficiency and quality is not easy to guarantee.
Adopting shear block injection mold structure, aggregate can be separated from plastic part when mold is opened and plastic part is pushed out, side gate aggregate and plastic part can be cut off in mold. Production efficiency is high and cutting quality is good. It is worth promoting and applying.