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Dongguan Jinchen Precision Mold Co., Ltd. is a manufacturer specializing in high-precision mold parts processing and technology research and development. It has more than ten years of production experience and has solved many supply and technical problems for customers.

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Key points for manufacturing precision gear molds [Part 1]

  1. Concept and application of precision gear mold

(1) Precision gears refer to gears with very high parameters, size and performance requirements. Parameters include: tooth tip circle outer diameter, tooth root circle outer diameter, module, indexing circle diameter (also called common normal), indexing circle tooth thickness, pressure angle, helical tooth inclination angle, gear ring runout tolerance, etc.; Performance includes: selection of materials and accuracy requirements for other assembly dimensions, etc.

(2) In terms of application, representative products such as internal transmission parts of printers and copiers, the common one is the VCD disc drive bracket. If the gear cannot meet the precision requirements, the bracket will move in and out unstable, accompanied by vibration or swinging. Shaking and extremely noisy. Through these, we can have a perceptual understanding of the concept of precision and its necessity.

(3) Precision gears are graded in accuracy. The national standard is that the higher the level, the more precise it is, and the Japanese standard is that the smaller the level, the more precise it is. Taking the JIS standard as an example, most precision mold factories can only produce level 5 and level 4 gears. Being able to produce level 3 plastic gears is already a very high level.

Types of precision plastic gears

  1. Types of precision plastic gears

(1) According to the axial inclination angle of the teeth, there are spur gears, helical gears, combined spur gears, combined helical gears and combined spur helical gears. The concept of combination is that spur gears or helical gears are distributed stepwise in the axial direction and are molded in one step.

(2) In terms of transmission, there are two main types: meshing transmission gears and belt transmission gears.

  1. Selection of mold base

The accuracy of the mold base will directly affect the manufacturing and injection molding accuracy of the mold, as well as the life of the mold. Experience shows that the accuracy of general-purpose mold bases is generally difficult to meet the requirements of precision gear molds.

  1. Selection of plastic raw materials

There are two main types: POM and PA (PA + GF%). Other raw materials are not excluded, such as polysulfone, etc.

The most fundamental reason why POM and PA raw materials are preferred is that the surface of POM and PA products has good self-lubricating properties. Other common reasons are: good wear resistance, good impact resistance and fatigue resistance; secondly, POM products have high rigidity and excellent creep resistance; PA products have good toughness, and GF% can be added to enhance their rigidity if necessary. In general, POM gears can be meshed with each other for transmission, and PA gears can be meshed with POM gears for transmission.

5,Relevant points of mold structure design

(1) Gate: Generally, 3-point or 6-point balanced pouring is used; for micro gears, only single-point pouring is usually used.

(2) Mold cavity ranking: 1-4 cavities are common. Small gears can reach 6 or 8 cavities; gears of different shapes can form 3 cavities; the most representative one is 4 cavities.

(3) Mold ejection method: Common ones include center cylinder and ejector pin ejection. For helical gears, bearings must be added to assist ejection. The bearings can be placed on the cylinder needle or on the mold core CORE. The two ejection principles are the same. Because the product is small and the mold base is also small, there is generally no need to add center support and EGP on the ejector plate.

(4) Parting surface tube block: Cylindrical parting surface tube block must be installed.

(5) The front and rear mold meat and inserts are cylindrical to facilitate processing and ensure the coaxiality of the product. The material is generally SKD61 and hardened to HRC52-56. SKD61 is preferred because:

① After the material is hardened, the deformation is small and the heat treatment margin is small, which naturally saves cost and time in semi-finishing;

②After hardening, the wear resistance is high. For gear demoulding without draft angle, wear resistance is naturally important;

③Hot work mold steel, which changes with the temperature difference, has stable steel performance and is suitable for long-term precision injection molding;

④SKD61 contains high Cr content, and after “electroslag remelting” treatment, the phosphorus (impurity) content is reduced from 0.03% to 0.001%. Therefore, after electrical processing, the mirror effect is good;

⑤ After hardening, it has good abrasion resistance, especially corrosion resistance to formaldehyde gas decomposed by POM.

However, it should be noted that if higher hardness requirements are required, cold work die steel materials such as SKD51 and SKD11 can also be used, and 8407 can be used for the center insert (pin).

(6) Regarding the accuracy of the cylinder and cylinder needles in key parts, or the small central inlay pin, the design accuracy usually requires a coaxiality of 0.02mm, and some requirements are higher. In addition, the upper end of the center cylinder pin or insert pin should be designed with a cylindrical straight tube position, and the upper mold insert should be inserted and positioned to achieve higher coaxiality requirements for the front and rear mold glue positions.

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