Printer stand is the most critical structural component in printer, and plastic structure is very complicated. There is a smooth paper-passing surface on front side, and complex glue positions on both sides to form a column, which requires lateral core pulling. These pillars on the side are basis for installation of gears and other motion mechanisms. There are verticality and parallelism requirements, and dimensional tolerances of entire plastic parts are very strict.

 

 

Desktop office printers include six systems, including power supply systems, DC control systems, interface systems, laser scanning systems, electronic imaging systems, and paper-drawing systems. Printer holder is the largest support part inside printer and is also a functional part. Printer holder is a base for mounting parts such as an electronic imaging system, a paper feed motor, a circuit board, and a control component. Left and right vertical side walls have a plurality of column positions, which are mounted with rotating parts and gears. Different parallel bone positions are paper-passing surface through which printing paper passes. Design requirements for paper-passing parts of printer are flat, smooth, no burrs, no apex marks and no gate marks. Inserts must not be designed on paper surface, difference between moving and fixed mold joints is less than 0.03mm. Technical requirements for plastic parts: All unfilled dimensional tolerances are ±0.1mm, appearance requirements are no shrinkage marks, whitening, charring, gas lines, chromatic aberrations and material defects. Plastic parts must not be contaminated with oil and dust, and no release agent should be used during injection process. Due to complicated structure of plastic parts, heights of left and right end faces are different. After being placed naturally, plastic parts are inclined, which makes it difficult to measure size. Therefore measurement of size of plastic parts requires use of specially designed fixtures. Through this example, design process of Japanese mold is elaborated. All materials, including product drawings, mold DFM, printer mold design style book, printer mold design style confirmation, 3D mold design are the best reference materials for learning Japanese mold in moulding manufacturing process.

Printer stand product diagram is shown in Figure 1. Plastic material is HIPS, shrinkage rate is 1.005, wall thickness of plastic part is 2.00mm, size of plastic part is 301.90mm X162.50 mm X 176.80 mm. Weight of plastic parts is 218.50 grams.

 

Difficulties in moulding die design:
1. Design of sealing position of slider and movable mold, slider and fixed mold, design of insertion and wearing position, processing of parting surface details, and selection of reasonable draft angle.
2. Design of slider thimble, which is necessary to avoid column of slider sticking to slider.
3. Coaxiality tolerance of column on both sides of slider is ±0.018, which brings great difficulty to design and moulding manufacturing process.
4. Die ejection system design.
5. Gating system design.
6. Design and processing of paper-passing surface.
7. Multiple inserts in the front of plastic part.

 

 

 

Printer bracket is large in size, and both sides need to design a large slider core. From die structure, cavity can only be designed 1 out of 1. Mold base size is non-standard die 750*950mm.
3.2 Opening direction and parting surface: Selection of mold opening direction and parting surface is relatively straightforward. Paper passing surface is in the front mold of printer mold, and two side rear mold sliders are cored.

Plastic parts are large in size, glue position is not tightly connected, plastic parts are steep in shape, flow resistance of molten plastic is large, and process is long. To this end, a needle valve hot runner system was designed, using YUDO brand. A set of flow plates with 4 hot nozzles . Advantages of needle valve hot nozzle:
(1) Entrance gate is flat on the appearance of plastic part, and there is no protruding gate mark, which does not affect performance of paper passing surface.
(2) Use of larger diameter gates can speed up cavity filling, further reduce injection pressure and reduce deformation of plastic parts.
(3) It can prevent phenomenon of drawing and flowing when opening mold.
(4) When screw of injection molding machine is retracted, material can be effectively prevented from being sucked back from cavity.
The most common control methods for needle valve hot nozzles are air pressure control and hydraulic control. Because air pressure control uses air, it is environmentally friendly and has obvious advantages. Therefore, needle-type hot runner of printer holder adopts a pneumatic control method. For cylinder needle valve type hot nozzle used in this example, in order to ensure high precision (coaxiality) matching with cavity plate, flow channel plate, and upper plate of mold, it is necessary to improve processing precision of corresponding mating portion of printer mold.

Ejection of plastic parts is mainly use of straight top and dome needles. There are two types of straight tops, one is fixed directly on ejector plate, and some are directly used for floating ejection. See moulding die design drawing 2.

Both slider mechanisms are inlaid with a mosaic structure. In order to avoid glue position of two sliders, slider thimble and slider cylinder mechanism are designed. Because there are more thimbles on slider and cylinder mechanism, slider ejector mechanism is designed to be driven by spring and guided by guide post.

Cooling system is designed for both dynamic and fixed molds, which is convenient for temperature adjustment and shortens molding cycle. For parts that are difficult to dissipate locally, a beryllium copper insert is made in back mold.

 

Complex printer mold (printer bracket molds), division of insert is very important. It is necessary to closely combine company's moulding manufacturing process, combined with existing machine tool's stroke, parameters and processing range, comprehensive consideration of injection molding exhaust, mold polishing, unreasonable insert splitting, it may be difficult to exert functions and advantages of insert, waste processing time, weaken mold cooling, may also affect strength and rigidity of printer mold, and bring irreparable damage to mold. Poor insert segmentation also makes mold assembly difficult and lacks basis for assembly. When dispersing design of insert, adopt principle of first big and then small, generally first determine position of large insert, consider circuit design of cooling water, and arrange position of fixing screw.