Bottom shell of scanning, fax and printing machine is shown in Figure 1. Maximum size of product is 378.60 mm * 354.36 mm * 43.69 mm, average thickness of plastic part is 2.20 mm, material of plastic part is HI-PS, shrinkage rate is 1.005, and weight of plastic part is 367.43 grams. 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.
Product is an all-in-one model for scanning, faxing and printing from a certain Japanese brand. It can be seen from Figure 1 that plastic part is a flat shell, with two raised vertical ears on the top surface and a through hole on vertical ear, which requires design of a fixed mold tunnel slider core. There is a set of mounting hinge holes on the edge of rear wall of plastic part, and the two hinge holes need to be concentric. The other two side walls each have a hinged pillar, and inner holes of two pillars are hollow. It is also necessary to design slider core and maintain coaxiality.

 

There are large parallel bone positions on inner edge of rear wall of plastic part, which play a role of strengthening and installation. It is necessary to design a large inner sliding block to pull core. Plastic part has a curved surface on the edge of main square shape, there are multiple pillars and bones on the back. Five pillars are inclined to bottom surface, inner slider needs to be designed to pull core diagonally. To sum up, there are slides and large slides, oblique slide core pulling in many places of plastic parts, and fixed mold has tunnel slide core pulling. All this makes mold design difficult.

 

Mold is a large mold, mold design cavity rank is 1 cavity, and mold base is a non-standard mold base 7085, which uses hot runner molding, so mold base is a non-standard mold base. On the other hand, mold is a large mold whose size exceeds specification range of standard mold base. Four ø30 positioning pins are designed between four plates of fixed mold to increase rigidity of mold system. Force here is very large, so do not design a locating pin smaller than ø30, otherwise, it will bend during injection molding. Four positioning pins of movable mold have a diameter of ø20 and are divided into two sections. They are square iron and bottom plate, square iron and movable mold backing plate to locate. Positioning pin here is short in length and located inside mold plate, which is difficult to take out. Therefore, threaded holes are designed at both ends of positioning pin to facilitate taking out.
When ordering mold base, 4 sets of inclined positioning blocks are designed to be located on four sides of mold base and arranged in a vertical direction, which is convenient for offsetting lateral force during injection molding, so that mold can be accurately closed. Due to large size of mold, in order to stabilize code mold, a code mold support block is designed on the side of mold, and a V-shaped block is designed for fixed mold.
Mold gating system adopts YUDO needle valve hot runner system, 2 points of glue is injected, and gate is located on the surface of plastic part. Model of needle valve hot nozzle is BIM35VCA, hot nozzle with color change sleeve. Color change sleeve in front of hot nozzle will form a loop of clamping lines on the surface of plastic part. Advantage of adding a color change sleeve is that it is easy to clean and easy to change colors. This set is heat-treated precision parts, generally provided by hot runner manufacturers. Angle of tapered surface in the sleeve that matches valve needle is generally 2 ゜～6 ゜ larger than angle of valve needle tapered surface, so contact surface of valve needle and inner tapered surface of sleeve is in line contact. Only line contact can ensure that valve needle cuts off molten plastic and realizes gate seal. If there is no experience in mold design and production, the two angles are made equal, valve needle will squeeze cone surface of color change sleeve when gate is cooled and sealed. At this time, it is often found that there is fine iron powder near gate of plastic during injection molding. Although needle valve hot runner system has a high cost, it is easy to realize timing control of each gate, gate marks are smooth and flat. It has a wide range of applications in molds of large plastic parts.
Cavity and core steels are all made of Japanese steel NAK80, which belongs to pre-hardened steel, which is harder kind of pre-hardened steel, with a hardness of 38-41HRC, its structure is fine and uniform, suitable for polishing. Rough machining and finishing of mold cavity mainly rely on CNC cutting. Under current technical conditions, the most suitable hardness for steel processing is 38HRC. Cutting tool with this hardness has high durability and the highest processing efficiency.
Front and rear mold cores are larger in size, front mold core adopts an overall design with small partial inserts. Rear mold core adopts method of full inlay. Both front and rear mold cores are tightly positioned by squeezing blocks.

 

Design of various sliders is focus and difficulty of this set of molds. For the two through holes on vertical ears on the top surface of plastic part, it is necessary to ensure their coaxiality. Front mold tunnel slider is used to pull core. Since four fixed mold plates of hot runner mold are all fixed together, they cannot be separated. Therefore, it is impossible to design a fixed-mold slider core-pulling mechanism with a separate mold plate, and an oil cylinder must be used to pull slider core. Specific design is shown in Figure 2 of mold design.
Since 5 pillar positions on back slope of plastic part are inclined to bottom surface, it is necessary to design inner side oblique core pulling, inclined slider 23 is driven by spring 24; there are large parallel bone positions on inner edge of rear wall of plastic part, and a large inner slide core pulling needs to be designed. Here, inner sliding block 31 is driven by an inverted inclined guide post, and shovel base 32 provides a moving space for inclined sliding block. The two kinds of inner sliding blocks here are realized by floating of back mold plate. Power of mold plate floating comes from 4 oil cylinders 38. This oil cylinder is a Japanese Taiyang Iron brand oil cylinder, which has a simple and compact appearance and takes up a small space. It is suitable for installation and use in mold.
Ejection of plastic parts adopts thimble, cylinder and straight ejection. MISUMI ball bearing guide is designed on thimble board. Return position of ejector plate adopts in-mold cylinder 33 to return position. After ejection, under action of compressed air, piston 33 at the bottom of cylinder in mold will rush out to return ejector plate.

 

 

 

 

 

 

Figure 2 Bottom shell mold diagram of scanning, faxing and printing all-in-one machine

 

Figure 3 3D drawing of dynamic mold

 

Figure 4 Fixed mold 3D diagram