Main contents of injection parting surface design
The location of the injection molding parting surface is a primary consideration in the design process. Comprehensive considerations must be given to the part’s structural shape, appearance, and molding process to ensure smooth demolding while maintaining part precision. For parts with critical appearance requirements (such as appliance panels), the parting surface should be located away from visible surfaces and located on the bottom or inner side of the part. For example, the parting surface of a television housing should be located at the back edge, making the parting line invisible from the front. The surface roughness should be controlled to below Ra0.8μm. For deep-cavity parts (such as plastic cups), the parting surface is typically located at the mouth edge, coinciding with the part’s opening plane. This ensures a deep cavity depth that facilitates moldability and demolding, while also preventing the parting line from interfering with the part’s functionality. When a part has lateral projections or holes, the parting surface’s location must be coordinated with the core-pulling direction of the slide mechanism. For example, for parts with lateral bosses, the parting surface should be located below the bosses to allow for molding through the lateral core-pulling mechanism, minimizing the number of parting surfaces and mold complexity. The position of the parting surface also needs to consider the exhaust effect of the mold. The parting surface should be set as far as possible at the position where the melt is last filled, and the gas in the cavity should be discharged by using the small gap of the parting surface. For example, the parting surface of a box-type plastic part should be set at the edge of the box mouth to ensure that the gas can be discharged in time when the melt is filled to the box mouth.
The shape of the parting surface must be designed to match the contour of the part, ensuring the mold cavity can fully form the part and facilitating manufacturing. A flat parting surface is the most common design and is suitable for simple, flat parts, such as mobile phone battery covers. The parting surface is a flat surface parallel to the mold’s mounting base. During machining, a surface grinder is used to maintain a flatness tolerance of no more than 0.01mm/m. A curved parting surface is suitable for parts with curved contours, such as automotive instrument panels. The parting surface must conform to the curvature of the part’s outer surface and is machined using a CNC milling machine . The contour tolerance of the curved surface is controlled within 0.03mm to ensure a tight fit when the mold is closed. A stepped parting surface is suitable for parts with flanges or steps, such as washing machine control panels. The parting surface is segmented according to the step height of the part, with the height difference between each segment equal to the step height. Each segment remains parallel, and spacers are used to adjust the mold closing height to ensure that all segments fit together. For plastic parts with complex shapes, a combined parting surface (plane + curved surface + step) can be used. For example, the parting surface of an air conditioner casing has a plane as the main body, a curved transition at the edge, and a stepped structure at the bottom. The segmented design takes into account the molding requirements of various parts of the plastic part.
The fitting accuracy design of the parting surface is the key to ensuring the dimensional accuracy of plastic parts and preventing flash. The clearance and fit of the parting surface need to be strictly controlled. The fitting clearance of the parting surface is usually 0.01-0.03mm. If it is too large, it is easy to produce flash. If it is too small, it may cause difficulty in closing the mold due to mold deformation. For high-precision plastic parts (such as optical lenses), the flatness error of the parting surface must be controlled within 0.005mm/m, and the fitting area must be no less than 95%. The color coating inspection is used to ensure uniform contact of the parting surface. The guide positioning design of the parting surface is essential. Usually, positioning pins and positioning holes are set around the parting surface. The fitting clearance between the positioning pins and the holes is 0.005-0.01mm to ensure that the parting surface is accurately docked when the movable and fixed molds are closed, and the coaxiality error does not exceed 0.01mm. For large molds, auxiliary positioning devices (such as conical positioning blocks) need to be added to the center area of the parting surface. The matching accuracy of the conical surface is H7/h6 to enhance the positioning stability during mold closing. For example, in addition to the locating pins at the four corners of the parting surface of the automobile bumper mold, two conical positioning blocks are set in the middle to ensure that the misalignment of the parting surface after mold closing does not exceed 0.02mm.
Venting design on the parting surface is crucial for ensuring part quality. It must provide a smooth path for gases trapped within the mold cavity. Venting grooves should be designed at the end of the parting surface (where the melt is last filled). The groove width should be 5-10mm and the depth should be 0.01-0.03mm (adjusted to the plastic type: 0.01-0.02mm for low-viscosity plastics like PE and PP, and 0.02-0.03mm for high-viscosity plastics like PC and PMMA). These grooves should extend to the outside of the mold to ensure direct evacuation of gases. The number of venting grooves should be determined by the size of the cavity. Small parts should have 2-4 venting grooves, while large parts should have 6-12. For example, for a small gear part with four cavities in a mold, two venting grooves should be installed at the end of the parting surface for each cavity to ensure timely evacuation of gases during filling. For deep or complex cavities, a venting gap can be added where the parting surface meets the slide mechanism. For example, a 0.015mm gap should be left between the slide and the fixed platen to serve as a venting channel and prevent trapped air in the cavity. A dust screen must be installed at the outlet of the exhaust groove to prevent external debris from entering the cavity. At the same time, the exhaust groove must be kept at a distance of more than 5mm from the cooling water channel to prevent cooling water from seeping into the exhaust groove and affecting the exhaust effect.
The strength and rigidity of the parting surface must be designed to ensure that the mold does not deform under the clamping force, thereby extending the mold’s service life. The pressure-bearing area of the parting surface must be calculated based on the clamping force: Clamping force (kN) = part projected area (cm²) × melt pressure (MPa) × safety factor (1.2-1.5). For example, for a part with a projected area of 500 cm² and a melt pressure of 40 MPa, the required clamping force is 500 × 40 × 1.3 = 26,000 kN. The corresponding pressure-bearing area of the parting surface must be large enough to avoid mold deformation caused by excessive pressure per unit area. The thickness of the parting surface template should be determined based on the cavity depth and clamping force. The thickness of the template for small and medium-sized molds should be 2-3 times the cavity depth, and 3-4 times for large molds. For example, for a medium-sized mold with an 80 mm cavity depth, the template thickness should be 200 mm (2.5 times) to ensure sufficient rigidity. The edge of the parting surface should be reinforced with ribs. The rib height should be 1/3 of the template thickness and the width should be 0.8 times the height. For example, if the template thickness is 200mm, the rib height should be 65mm and the width should be 52mm. This will enhance the deformation resistance of the parting surface edge. For large molds, support columns can be installed on the back of the parting surface. The diameter of the support columns should be 1/4-1/3 of the template thickness, and the spacing should be 300-500mm. They should be evenly distributed to further improve the rigidity of the parting surface and ensure that the flatness error during mold closing does not exceed 0.03mm/m.
