Printer paper guide product is shown in Figure 1. Maximum outer dimensions of product are 67.30 mm * 32.60 mm * 38.30 mm; average glue thickness of plastic parts is 2.00 mm, plastic part material is ABS, shrinkage rate is 1.005, and plastic part weight is 9.45 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling, flow lines, pores, warping deformation, silver streaks, cold materials, jet lines, etc. and they must comply with ROSH environmental requirements.

 

Figure 1 Product diagram of printer paper guide
Printer paper guide is an important part of a famous Japanese brand printer. Function of guide is to limit paper width position, guide paper, and limit paper loading height. Guide consists of a zero-degree outer surface, an adjusting piece, a bottom guide bar and a positioning triangular tooth. All sides of adjusting piece are suspended, and only bottom is connected to main body through two bones. Pinch paper adjustment tab hard to move paper guide in a small range. When it is adjusted to appropriate position just to accommodate passage of paper, release paper adjustment tab, the four positioning triangular teeth at the bottom will block adjuster to keep it in this position.
Working part of paper guide is paper passing surface for paper to pass through. This surface is generally a highly polished surface and needs to be smooth, without traces of inserts and line clamps, without marks such as gates and ejector pins to avoid affecting passage of paper. Outer side of paper guide is a 0゜ vertical surface, and a slider needs to be designed for demoulding. Long groove at the bottom requires a slider with a larger stroke for demolding. There is interference in the positions of these two sliders, and a reasonable slider structure needs to be designed.
Difficulty of mold design lies in analysis of the three elements of mold design, design to prevent plastic parts from deforming, and design to prevent ejection deformation. Analysis and selection of mold opening direction is the most important step in determining mold design plan. The three elements of mold design plan are gate, ejection and parting surface. Mold opening direction analysis is closely related to plastic part ejection and gate design. Selection of mold opening direction is basis and premise of parting surface design. Only when mold opening direction is correctly selected can parting surface be selected correctly. For parts with same shape and different sizes, size values will affect selection of parting surface. Selection of parting surface is related to mold structure. Qualitative analysis can only summarize general rules, while quantitative analysis needs to be combined with size of plastic part.

 

Figure 2 3D diagram of mold
For this set of molds, size of plastic parts is small. Due to special structure of plastic parts, it is difficult to select mold opening direction. Selection of mold opening direction should not only determine the three elements of mold design, but also take into account number of cavities. For small plastic parts, based on tonnage of injection molding machine and production requirements, number of cavities in this set of molds is determined to be 4 cavities.

 

Figure 3 Mold parting surface view
Through above analysis, 0゜ vertical surface and adjusting piece are designed in the direction of front mold, and 0゜ vertical surface slider is designed as front mold slider. Its stroke is very small, just to solve demoulding problem of 0゜ surface, as shown in picture 2. Due to large stroke of core puller with long groove at the bottom of plastic part, a rear mold slider is designed. Position of front mold slider coincides with position of rear mold slider, which brings great difficulty to mold design. Front mold slide blocks of the two holes on same side are merged together and driven by a T-slot shovel. Rear mold sliders of the two holes on same side are also designed on same slider seat, and rear mold slider is driven by an inclined guide post. Front and rear sliders are designed separately, which simplifies mold design and manufacturing difficulties, can effectively ensure quality of plastic parts. Strokes of front and rear mold sliders are quite different. Designing sliders separately is a very typical mold structure and is worth learning from.
Mold base is non-standard mold base 3545, and pouring system is a latent gate. Gate position is inside 0゜ vertical facade. Latent gate is hidden in front of mold, and glue is inserted at one point. Plastic parts are ejected using flat ejectors and dome pins.

 

Figure 4 2D mold structure diagram
In order to improve service life of flat ejector pin, mold ejection system uses graphite copper guide bushes for guidance. A travel switch is designed on ejector plate to ensure that ejector plate can return accurately. Front and rear molds of mold all use straight-through water delivery. Cooling system has fast flow rate and good cooling effect, effectively ensuring quality of injection molding.

 

Figure 5 Arrangement diagram of flat thimble and dome pins