Shell of printer and fax machine is shown in Figure 1. Maximum size of product is 478.97 mm * 234.44 mm * 148.05 mm, average thickness of plastic part is 1.80 mm, material of plastic part is PS-HI, shrinkage rate is 1.005, and weight of plastic part is 478.8 gram. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling of injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, etc. Nihong etching is required on the surrounding surface of plastic part. A matte pattern is used on the top.

 

Figure 1 Printer and fax machine shell product diagram
It can be seen from Figure 1 that plastic structure is a complex shell formed by an irregularly enclosed structure on four sides. There are large-area heat dissipation grid holes and mold undercuts on four sides of plastic part, 4 large slider cores need to be designed. Boundary of slider is on curved surface, therefore, it is necessary to design parting surface well. Connecting bridge in the middle of plastic part has deep bones, thin shape and poor rigidity. Structure of plastic part is irregular, parting surface is a complex irregular parting surface. Structure of plastic part is complex and size is large, but top area of plastic part is small, only edge part has a glue position, most of local area is hollow, which brings difficulties to design of mold ejection system.
Product size is large, mold structure is complex, and there are slider core pullers on all four sides. Mold design cavity ranks 1 cavity, injection molding machine has a 450-ton Nissan injection molding machine. Gating system is designed with a point gate design, 4 point gates are designed on the top of plastic part. Mold base is a non-standard simplified fine nozzle mold base 7094. For injection molds with large sliders on all sides, design adopts a simplified fine nozzle mold base. Due to absence of 4 guide posts, mold space is effectively saved. Due to long runner, long condensate of gating system, and irregular shape, in order to prevent nozzle material from falling when manipulator is used for injection molding, an anti-rotation column is designed on sky side of mold to locate nozzle material and facilitate gripping of manipulator.

 

Structure of plastic part is complex, there are many bone and column positions in glue position. Design of insert is a very critical part. By reasonably splitting inserts, processing difficulty is reduced and processing efficiency is improved. For complex molds, the most effective method for splitting inserts is full inserting method, that is, splitting to make inserts. Splitting of inserts is shown in Figure 5 and Figure 6.

 

Design of cooling system for large and complex molds is critical. For three-plate mold, cooling of nozzle plate cannot be omitted. If nozzle plate is not cooled, temperature of sprue sleeve is too high, wear of sprue sleeve and nozzle plate will increase, injection cycle will be unstable. Generally, cooling system of nozzle plate adopts 2-4 parallel straight-through cooling systems, as shown in Figure 3. Two cooling circuits in the middle should not be more than 50% away from sprue bushing, and they should be arranged in parallel.
Fixed mold, movable mold and slider all need to design a cooling circuit. When designing mold, design a 3D sketch for water transportation circuit to check whether water transportation design is sufficient, it is also convenient for injection molding operator to quickly pick up and transport water.
Slides on four sides of plastic part are all large slides. Three slides on horizontal and ground side are driven by two inclined guide posts; slider on sky side is driven by an oil cylinder, and two safety springs are designed. Stroke ends of four sliders are all designed with stroke switches, part 2, part 6, and part 9. Slider of large mold has a large stroke and a large force, which requires a high level of smoothness of slider movement. Therefore, height of hanging platform of sliding part of slider must be at least 15 or more to make movement stable. At the same time, bead of slider should be designed with positioning pins to ensure installation accuracy.

 

Width of slider is wide, so in order to make it run smoothly, length of tail along direction of movement cannot be too small. When designing mold, wear-resistant blocks are designed on the bottom and inclined surface of slider to facilitate mold assembly and adjustment. There are multiple heat dissipation grid holes on one side of plastic part. When slider moves, these holes will stick to slider. Therefore, in order to prevent plastic part from being deformed, a slider thimble is designed on corresponding slider.
Plastic parts are large in size, poor in rigidity, and uneven ejection force. For this purpose, thimble, inclined top and straight top out are designed. Purpose of inclined top ejection is to prevent product from sticking after ejection and prevent product from being clamped. At the same time, in order to get plastic part out of inclined top as soon as possible, an accelerated ejection mechanism is designed. Layout of accelerated ejection mechanism and ejection principle are shown in Figure 4.

 

 

Figure 2 Shell mold of printing and fax machine

 

 

 

Figure 3 Cooling system diagram
In injection molding production, some products cannot fall off automatically due to too deep or too much reinforcing ribs attached to mold after being ejected. It is necessary to reduce number of thimble attached to product to facilitate automatic production. Consider designing an accelerated ejection mechanism in injection mold. Realize that most of features are demolded first, then a small part of thimble is left behind. Add an accelerating top related device to thimble that needs to be accelerated. When ejector reaches limit pin, using principle of seesaw, centering on fixed rotating pin, it has a higher height than other ejector pins to achieve demolding needs. Since accelerating top device requires a certain amount of space, it needs to be considered before adding mechanism. Accelerated ejection mechanism can be processed by itself or purchased from related suppliers such as DME standard parts suppliers.

 

 

Figure 4 Schematic diagram of accelerated ejection

 

Figure 5 3D schematic diagram

 

 

Figure 6 Exploded view of front and rear molds