Product of inner bracket of printer is shown in Figure 1. Maximum external dimension of product is 452.56 mm * 191.73 mm * 135.08 mm, average thickness of plastic part is 2.50 mm, plastic part material is PC/ABS, shrinkage rate is 1.0025 in Y direction, 1.0035 in X and Z directions, and weight of plastic part is 469 grams. Technical requirements for plastic parts are that there shall be no defects such as peaks, underfilling, flow lines, pores, warpage deformation, silver lines, cold materials, and spray lines.

 

Figure 1 Product map of inner bracket of printer
This type of stand, in printers, is a typical component. Many printers have components of similar construction. As can be seen from Figure 1, structure of plastic part is very complex, both front and back sides contain multiple bone sites and functional structural features. Two sides of plastic part also have very complex structures and shapes. When slider is directly designed, upper and lower levels of glue on the side will be tightly wrapped on slider, causing deformation of plastic part. Therefore, slider thimble needs to be designed on the slider.
For plastic parts with such a structure, side glue area is large and structure is complex. Multiple column positions and complex shapes need to be pulled several times if necessary to disperse adhesive force of glue position to slider. In these cases, it is often necessary to design a slider thimble or a secondary core-pulling mechanism. When large-area glue position on the side of plastic part is easy to stick to slider, it is often necessary to design multiple slider thimbles. In this case, it is necessary to design slider thimble plate. There are also some long posts on sliders of plastic parts, which require design of slider cylinders, which makes mold structure more complicated.
Slider thimble plate is same as movable mold thimble plate. It needs to design guide element guide column and guide sleeve, as well as limit element, driving spring and reset mechanism. Slider thimble plate and its structure are shown in Figures 6 and 7.
Three elements of mold design scheme are gating system, ejection system and parting surface design. Analysis and selection of mold opening direction is the most important link in determining mold design scheme. Analysis of mold opening direction is closely related to ejection and gate design of plastic parts. Before mold design, product must first be analyzed to determine a reasonable mold opening direction. Determining mold opening direction of a plastic part is generally divided into two cases. For some plastic parts, selection of parting surface is not complicated, and even parting surface is a plane in the middle of product. Difficulty lies in selection of opening direction, that is, which side of plastic part is placed on movable mold and which side is placed on fixed mold. This problem is sometimes difficult to judge, and even experienced engineers may make mistakes in judgment. Correct judgment will leave side with high sticking force in movable mold to facilitate ejection and demoulding of plastic parts, and design gating system on the other side. In most cases, mold opening direction can be determined by using factors such as wrapping force, and plastic parts can be left in movable mold. There are many bone positions on the front and back of printer inner bracket product, shape and characteristics are complex. It is difficult to distinguish which side is left with front mold and which side is left with rear mold by simply comparing front and rear wrapping force.

 

Figure 2 3D mold design

 

Wrapping force of plastic parts on the front and rear molds is very large. As shown in product picture in Figure 1, there are 31 arc bones on the top surface of plastic part that cross paper surface, and paper surface must be smooth. For printer parts, there are general technical requirements for paper surface, that is, there should be no insert clip lines, thimble traces and gate traces on this surface. Surface passing through paper is generally appearance surface. In order to reduce contact area between this surface and paper, resistance is reduced when paper moves. There are usually multiple parallel bone positions on paper, and top of bone positions needs to be smooth. Due to deep bone position on paper surface, venting of molten plastic is poor when flowing, and multiple inserts need to be cut. Cutting of inserts is shown in Figure 4. Different from ordinary inserts, inserts cannot be made on paper. Center of facial bone position to avoid influence of clamping line on the flow of paper.
On the other hand, wrapping force between inserts on paper surface and plastic parts is very large, and it is easy to stick to mold. Therefore, over-paper surface is designed in the front mold, and these inserts in over-paper surface bone position are first parted when mold is opened. See Figure 8.
Figure 3 Parts of printer inner bracket product

 

Figure 4 Method of inlaying on paper surface

 

Figure 5 Arrangement of point gate
Size of plastic parts is large, mold design cavity ranking is 1 cavity, mold base is a non-standard mold base DCI50100, gate of mold is a point gate, and 6 points are fed. Ejection of plastic part has a sloping top and a thimble ejection. Side that passes through paper surface belongs to appearance surface in printer, and thimble cannot be designed. Thimble can only be designed on the other side. Layout of point gate is shown in Figure 6. Since there is a thimble at the bottom of slider, mold must be designed with a reset device to ensure that thimble plate returns in time.

 

Figure 6 Design of slider thimble plate

 

Figure 7 Slider thimble plate design 2

 

Figure 8 First extraction mechanism of front mold inserts