Printer bottom shell product is shown in Figure 1. Maximum size of product is 381.91 mm * 287.33 mm * 99.25 mm, average thickness of plastic part is 2.00 mm, material of plastic part is HIPS, shrinkage rate is 1.005, and weight of plastic part is 507.40 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.

 

Figure 1 Printer bottom case product map
Bottom case of printer is bottom case of a certain Japanese brand printer. It can be seen from Figure 1 that structure of plastic part is a complex shell, movable and fixed molds have many bone positions and column positions, etc., size is relatively large, and mold is a large mold. In addition to marking product dimensions and tolerances, product drawings of plastic parts also mark detailed technical requirements. In addition, there are surface etching and matting areas, and surface polishing areas.

 

For large molds, difficulty of mold design lies in ensuring dimensional tolerances and technical requirements of plastic parts, while injection molding process is stable, gating system is reasonably designed, ejection process is stable, plastic parts are not deformed, and injection cycle meets requirements of mold design specification. Any small mistake will cause a lot of losses, because cost of trial molds for large molds is high.
Mold design cavity ranking is 1 cavity. There are 7 slide cores on four sides of plastic part, and two large slides are designed on the side of mold. Among them, slider 7 and slider 9 are limited by their positions, slider is used to drive slider to change core pulling direction.
Size of plastic parts is large, and design of gating system is also very difficult. Design of position and quantity of gate is the key. When bone position and structure of large plastic parts are complex, plastic process is longer, and balance of gating system is very important. Too many gates or unreasonable gate positions will cause excessive injection pressure, large plastic deformation, waste of plastic raw materials, prolong injection cycle and cause economic losses. If gate is too small, it will also cause filling difficulties.
In the early mold design, gating system completely relied on experience of mold designer to solve problem. Due to rapid development of computer technology, mold design has changed from traditional experience design to computer-aided design, auxiliary programming and processing. The most important of these is simulation of filling and flow of plastic melt in runner and cavity based on rheology of polymer. Mold flow analysis software provides convenient conditions for correct determination of gating system. In product development department of large companies, a department specializing in mold flow analysis has been established. For example, mold flow analysis engineer in product research and development department of Canon, Japan, specializes in mold flow analysis of plastic parts to provide technical support for product design.

 

Establishment of specialized departments and fixed mold flow analysts facilitates accumulation of big data and also facilitates determination of a reasonable injection molding cycle, which creates conditions for lean production. Through application of mold flow analysis software, bottom shell of printer needs to be designed with 3 gates, as shown in Figure 2 of mold design. YUDO hot runner system is used to feed glue.
Due to use of hot runner system, mold base is a non-standard mold base 7085. In order to improve positioning accuracy, four corners of mold base are designed with tiger mouths. Tiger mouths of A plate are protruding, and tiger mouths of B plate are recessed. Angle of tiger mouth is 5゜～10゜. For large molds, fixed and movable molds need to be designed with positioning pins separately. Positioning pins of movable mold are ø25, and positioning pins of fixed mold are ø40. Fixed mold uses guide posts instead, mainly for hot runner molds with multi-layer mold plates. Because temperature difference and thermal expansion of each mold plate are different, greater stress will be generated, and a large-diameter positioning pin is required to protect hot nozzle from force and prevent hot nozzle from being damaged when mold is installed. On operating side of mold base, two oblique positioning blocks are designed.
For a complicated case mold such as a printer bottom case mold, design of front and rear mold cores is also critical. Before mold design, review plastic parts, and carefully analyze trend of parting surface to determine parting surface where there are changes in the shape of edge of plastic part. Carefully check demoulding angle of plastic parts, and confirm with customer in time if there is any doubt. In Japanese product drawing, draft angle is called hook matching. To determine draft angle and step difference, you must get customer's confirmation. Generally, a special meeting is held to study specific issues of mold opening.
Length and width of front and rear mold cores are both 370*500. For mold cores with a length and width of more than 200, there are certain difficulties when lowering frame. Because size exceeds processing range of a general surface grinder, it is difficult to guarantee size of fine frame and dimensional accuracy of mold core. In this case, mold core needs to be positioned at a reference angle and compressed with a compression block. Specific method is to take base angle of mold base as base, make a 3 ゜ slope on two frame edges far away from base angle, and use squeezing block to compress mold core. Detailed block diagram of B board is shown in Figure 4. For situation of more movable mold bones, exhaust of mold during injection molding is very important. Therefore, in order to ensure surface quality of plastic part, fixed mold only divides insert partly. Movable mold cores are all broken up to make inserts. When designing mold, it is necessary to determine a few large pieces according to structure of plastic part, and then divide small inserts. When determining large insert, it needs to be combined with design of cooling water, direct water is designed as much as possible to ensure cooling efficiency. For corners of closed deep cavity that cannot be cooled, beryllium copper inserts need to be designed to dissipate heat.

 

 

A major feature of Japanese molds when dividing inserts and fixing inserts with a hanging table. After breaking up inserts, there are a lot of inserts in the back mold core. It is necessary to design plan assembly drawing of inserts to make mold inserts similar to building block assembly. Arrangement diagram of movable mold inserts is shown in Figure 3. Bottom of insert needs to be chamfered during production, bottom of mold frame is made of R6 to increase strength and rigidity of mold. All small inserts use hanging tables, only large inserts and inserts with a water-carrying rubber ring at the bottom are fixed with screws. For large molds, bottom of B plate is thicker. If small inserts are fixed by screws, screw holes are very deep, light is poor, and screws are easy to miss.
In order to prevent plastic parts from sticking to fixed mold, an ejection mechanism is also designed in fixed mold, including ejector plates 61 and 63, and driving element is a spring 60, which is reset by using a reset lever to hit parting surface.
All sliders are driven by oblique guide posts, wear-resistant plates are designed on the bottom and inclined surfaces of sliders for easy assembly and adjustment. Large sliding block has a relatively large lateral expansion force, original shovel base is used, and shovel base of the other small sliding blocks is designed as required.
Ejector components of mold are thimble, cylinder, straight top and inclined top. All inclined tops are designed with guide blocks on the back of B board, and wear-resistant blocks are designed on thimble board to extend life. Lifter adopts anti-loosening design, and spring washer is added under screw.
Mould steel adopts Japanese mould steel NAK80, standard parts adopt Japanese Misumi MISUMI brand standard parts.

 

 

 

 

 

 

 

Figure 2 Printer bottom shell mold diagram

 

Figure 3 Movable mold insert arrangement diagram

 

Figure 4 B board structure diagram

 

Figure 5 Locking diagram of inclined roof