Control rack product diagram is shown in Figure 1. Maximum size of product is 134.70 mm * 29.70 mm * 6.4 mm, average thickness of plastic part is 2.20mm, plastic part material is POM, shrinkage rate is 1.014, and plastic part weight is 6.29 Grams. Technical requirements of 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 Control rack product map
Plastic part is control rack part in paper travel mechanism of a certain Japanese brand printer. Plastic part is shaped as a flat strip, with a rack on one side and 4 arc-shaped shrapnel on the other side. Plastic parts inside printer have high requirements. Geometric tolerance of control rack is mainly to control bending deformation of plastic parts. Within full length range, bending deformation in upper and lower directions shall not exceed 0.3. Another geometric tolerance requirement is tooth profile accuracy of rack, which mainly affects smoothness of running mechanism.
For nearly half a century, precision design and inspection of plastic gears have been referring to basic concepts and ideas of metal gears. Precision requirements, processing technology, testing methods and methods of metal gears have also been mature, and mechanical design manual has also been widely introduced. Processing technology and equipment of metal gears with soft and hard flank gears depend on gear accuracy. Gears with different accuracy grades are processed by different equipment and techniques. Gear accuracy has also been formulated for international standards (ISO). However, production of high-volume plastic gears can only be accomplished through an injection molding process. Due to anisotropic shrinkage characteristics of plastic injection molding, stability of injection molding process parameters, rationality of mold structure design and cavity accuracy, accuracy of molded gears is generally low.
Precision design of plastic gears has not yet a unified specification to follow. If precision level is set too high, production cost will greatly increase, and it is too low to guarantee quality requirements of product. At present, accuracy requirements of domestic plastic gears are basically still at the stage determined by experience of designers.
Precision of plastic gear used for paper transport of American color printer adopts AGMA Q9 precision molded gear. Main parameters and accuracy requirements of this gear are as follows:
Main parameters of gear: DP=48 (m=0.5292), z=58, dZ=1.25〃(31.75mm).
Gear material: Polyoxymethylene (POM).
Gear accuracy requirements: Fp=0.0015〃(0.038mm) is equivalent to GB/T 10095 level 8 accuracy (Fp≤0.041mm); fpt=±0.0071〃(±0.018mm) is equivalent to GB/T 10095 level 9 accuracy ( fpt= (±0.02mm) requirements.

Japanese JGMA 116-02 standard plastic gears. For high-volume moulded gears (Fi〃, fi〃), general gear is JGMA5~6, precision is 3~4, and high precision is 1~2 . For high-precision molded gears, there are very strict requirements on performance of injection molding machine, accuracy of injection mold, and production environment.
Polyoxymethylene (POM), as the most commonly used and important engineering thermoplastic for gears, has a history of more than 40 years. Due to its extremely low hygroscopicity, polyoxymethylene can ensure long-term dimensional stability of gear and its excellent properties such as fatigue resistance and corrosion resistance in a wide temperature range and self-lubricating performance. It has always been preferred engineering plastic in plastic gears.
Nylon (PA66) has advantages of good toughness and durability, and is another commonly used engineering plastic for gears. However, nylon has strong hygroscopicity, which can cause changes in properties and dimensions of plastic parts. Therefore, nylon is not suitable for gears for precision transmission. Therefore, POM materials are mostly used for precision printer gear racks.
It can be seen from Figure 1 that plastic part has a flat and long shape. Mold cavity is 8 cavities, and mold design drawing is shown in Figure 2. Due to large mold core size, front and rear mold cores are divided into 4 pieces, each with 2 holes. Mold base is CT4055 A70 B100 C100, four mold cores are squeezed and positioned with a squeezing block.
Design of gating system needs to consider structure of plastic part. Long plastic part needs to be glued from one end to ensure that plastic part is not deformed. Therefore, gate is designed as a side gate, as shown in Figure 2.
Key to mold design lies in design and processing of rack part. Rack part cannot be directly machined to size with a milling cutter. If EDM is used for finishing, accuracy of cavity cannot be guaranteed. Therefore, inserts must be designed, as shown in Figure 3 for tooth-shaped inserts, and slow-moving wire cutting can ensure machining accuracy.
Both front and back mold cores of mold are designed with a wrap-around direct water transport to ensure injection cycle.
All plastic parts are ejected by a thimble.

 

 

 

 

 

Figure 2 Control rack mold diagram

  

Figure 3 Rack insert