Engine housing is a key component of a small engine and main functional component of a small engine. Main requirements for this part are sufficient strength, fatigue resistance, abrasion resistance, good heat dissipation and noise reduction. Therefore, shape of engine shell is very complicated, and for a long time, part has been produced by means of casting. In recent years, with improvement of performance of plastic products and improvement of plastic molding technology, in order to meet light-weight requirements of electromechanical products, engine housings are gradually designed and manufactured with engineering plastics. This article introduces key points of injection mold design using PA+30GF to make engine shell.

 

Figure 1 Product picture of engine housing
Maximum size of engine shell product is 268.60 mm * 155.25 mm * 107.40 mm, average thickness of plastic part is 2.0mm, material of plastic part is PA+30GF, shrinkage rate is 1.006, and weight of plastic part is 338 grams. Technical requirements of plastic parts are that there must be no defects such as peaks, under-injection, flow lines, pores, warpage deformation, silver streaks, cold materials, spray lines, bubbles, etc., and no mold release agent shall be used in molding of plastic parts. Plastic parts have complex shapes, with multiple ribs and heat dissipation holes on each side, holes on each side are mounting holes for functional parts. Therefore, these holes and structural shapes require high machining and geometric accuracy. Plastic part is irregular, three sides need to be designed with slider core pulling, and slider is in an inclined position in horizontal plane, which brings great inconvenience to mold design and processing.

 

 

Size of plastic part is large and structure is complex. Therefore, mold design ranking can only be 1 cavity, mold base is non-standard CH6065A100B220C180. Two tapered positioning blocks 28 (pair lock) are added at the edge of mold to play an accurate positioning role when closing mold, and can offset lateral force during injection molding and improve accuracy of plastic part. If space is sufficient, design one on each side of lock. In this example, space is limited. In order to avoid interference with slider, only two pairs of locks are designed, which are set at 90°.
Material of plastic part is PA+30GF, and glass fiber content is high. Therefore, plastic wears mold cavity greatly. In order to meet mold's service life requirements, mold must be designed as a hard mold. Mold core material is S136, and heat treatment is HRC48~52. Principle of determining hardness of mold core is based on clamping stress of parting surface. When clamping stress of parting surface is below 3.5T/in2, you can choose a pre-hardened mold steel with a hardness below HRC45. When clamping stress of parting surface is above 5T/in2, selected steel must have a hardness above HRC45.

 

Mold is more complex, and analysis of mold opening direction is very critical. Design inclined slider in the direction of three sliders that mold needs to design, and design part with a round hole on the top surface of plastic part in fixed mold, which is convenient for designing gate. Gating system is designed with side gate 3 point glue. 3-point side gate is located in round hole on the top of plastic part. PA+30GF material has good fluidity, but due to addition of glass fiber to material, shrinkage rate of material is anisotropic, that is, along different directions of plastic flow, plastic parts have different shrinkage rates, which will cause size of plastic parts to be inaccurate.. There are usually two ways to solve this problem. If you are familiar with physical properties of plastics and have shrinkage data in 3 directions, you can use 3D software to design different shrinkage rates in 3 different directions to solve problem. For example, commonly used 3D software PROE and Ug have this function.

 

For large and complex plastic parts, flow process is more complicated. Choosing multiple points of glue in round holes of plastic parts can significantly improve dimensional accuracy of plastic parts. In injection molding process, in view of high content of glass fiber, it is necessary to appropriately increase barrel temperature, improve fluidity of plastic, solve problem of floating fiber, and make surface of plastic part beautiful. Due to large area of plastic slide block and large tightening force, two large slide blocks use oil cylinder to pull core, and one small slide block uses inclined guide post to pull core. Oblique wedge of each slider is designed with an anti-locking mechanism to increase locking force of slider. Mold design drawing is shown in figure 2.

 

 

 

 

 

 

Figure 2 Mould diagram of engine shell