Positioning rack is a paper-passing part in desktop office printer, which plays role of limiting width of paper, positioning and guiding paper. This article takes positioning rack of a Japanese brand printer as an example to introduce design points of injection mold for positioning rack.
Positioning rack product is shown in Figure 1. Maximum external dimension of product is 204.60 mm * 136.70 mm * 50.00 mm, average thickness of plastic part is 2.00 mm, material of plastic part is ABS, shrinkage rate is 1.005, and weight of plastic part is 30.70 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. Plastic part has an adjustment sheet, which is suspended on all sides, and only bottom is horizontally connected to main body. It can be slightly tilted to produce elastic deformation by hand pinching. Positioning racks are generally installed in pairs in printer. Pull positioning racks on both sides in reverse with both hands. Move, you can adjust width to fit paper, and guide paper together. Elastic lock after adjustment.
Parts in printer that come into contact with paper are all over paper. This face is also called through-paper face. Customers generally have higher requirements on paper surface of product. There should be no burrs, peaks, water points and thimble marks to avoid affecting passage of paper in machine. Contact surface should be as small as possible to reduce friction, such as smooth arc ribs, and contact surface should be as smooth as possible. There are 5 anti-skid teeth on side wall above paper surface, and fingers touch part when adjusting. Adjustment piece and adjacent parts constitute mold-inverted buckle, and it is necessary to design slider core-pulling. Anti-skid teeth also form mold undercut inside plastic part, and it is also necessary to design slider core. Difficulty of mold design is choice of mold opening direction and design of two sliders.
Difficulty of mold design lies in choice of mold opening direction. There are two types of mold opening directions. One is to determine movable and fixed mold side of plastic part, that is, which side is in fixed mold and which side is in movable mold. Plastic part in this example belongs to this type. Another option for opening mold is to design plastic parts down or vertically. Selection of mold opening direction is basis of parting surface design. In this case, parting surface is very simple, which is upper surface of rack.
Mold design cavity ranking is 2 cavities, because of special structure of plastic part, 2 cavities will balance force of mold cavity. Mold base is GCI4550 A: 110 B: 120 C: 120. In order to improve precision of mold clamping, a zero-degree positioning block is designed on four sides of mold base. A long tiger's mouth is positioned on the edge of front and rear mold cores. Injection method of mold is the side gate, and gate is located at toothless part of root of rack, which is easy to trim and does not affect function of plastic part after trimming.

 

Adjustment piece on outside of plastic part needs to be designed with a rear mold slider, as shown in Figure 3. This slider is driven by an inclined guide column, and a wear-resistant block is designed on slope of slider for adjustment and replacement. Anti-skid teeth are on inner side of plastic part and upper part of rack. It is necessary to design front die inclined elastic slider. This slider is driven by a shovel fixed on panel with a T-slot. Sprue sleeve is fixed on panel, avoids void with A plate, and fits with front mold core with a taper. A limit screw is added between panel and A plate, and a spring is set to assist mold opening. Resin shutters are designed between AB boards, so that front mold slanted elastic slider is opened first and then parting surface is opened. Rack part must be designed as an insert, which is key to design of this mold, so that toothed part can be processed by slow-moving wire.
Plastic parts are ejected with a thimble. Since ejector pin is at the bottom of slider, mold is designed with a return travel switch to monitor timely return of ejector system. Cooling water of mold core before and after mold adopts straight-through water transportation, which effectively ensures normal progress of injection molding.

 

Figure 1 Product Diagram of Positioning Rack

 

Figure 2 Positioning rack die diagram

 

Figure 3 Slider structure