Design of the overall dimensions of injection molded parts
When designing the external dimensions of injection-molded parts, the shrinkage characteristics of the plastic material must be considered first. The shrinkage rates of different plastics vary significantly, directly affecting the final dimensional accuracy of the part. Therefore, compensation for shrinkage must be made during initial dimensional design. Crystalline plastics such as polyethylene (PE) and polypropylene (PP) have relatively high shrinkage rates (1.0%-3.0%), requiring a large allowance for compensation during design. For example, if a PP part requires a final dimension of 100mm, the mold cavity should be designed with a dimension of 102mm (calculated based on a 2% shrinkage rate) to ensure that the final dimensions meet the requirements. Amorphous plastics such as polystyrene (PS) and polycarbonate (PC) have relatively low shrinkage rates (0.5%-1.0%), requiring relatively small dimensional compensation. For PC parts with high precision requirements, the shrinkage rate can be as accurate as 0.6%, and the mold cavity dimensions can be calculated accordingly. Fiber-reinforced plastics have a smaller shrinkage rate than pure resins and are anisotropic, with a small shrinkage rate in the flow direction (0.3%-0.8%) and a large shrinkage rate in the perpendicular direction (0.8%-1.5%). This difference needs to be taken into account when designing long strip parts. For example, for long rod parts made of glass fiber reinforced nylon (PA66+30% GF), compensation is applied at 0.5% in the length direction and 1.2% in the width direction to reduce warping and dimensional deviation after molding.
The dimensions of injection-molded parts must match the mold structure. Appropriate dimensional design can reduce mold manufacturing complexity, improve production efficiency, and enhance part quality. The maximum dimensions of a part are limited by the injection molding machine’s clamping force and tie-bar spacing. Designs must ensure the part fits within the machine’s processing range. For example, small and medium-sized injection molding machines (with a clamping force of 100-300 tons) are suitable for parts with dimensions under 500mm, while larger parts require equipment with a clamping force of 500 tons or more. Part wall thickness should be uniform to avoid areas of excessive thickness or thinness. Excessive thickness can easily create shrinkage cavities and bubbles, while excessive thinness can result in insufficient fill. Generally, the wall thickness should be maintained between 1 and 5 mm, adjusted based on the material’s fluidity. For materials with good fluidity, PE and PP can be designed to have a thickness of 1-2 mm, while for materials with poor fluidity, PC and PMMA require a thickness of 3-5 mm. In addition, the corners of the parts should be designed with fillets. The fillet radius is generally 0.5-1 times the wall thickness. For example, for a part with a wall thickness of 3mm, the fillet radius should be 1.5-3mm. This can reduce mold stress concentration and avoid cracks after the part is formed. At the same time, it can improve the melt flow performance and ensure uniform filling.
The dimensional design of injection-molded parts must consider assembly relationships to ensure accurate fit with other parts and functional realization, and to reasonably determine dimensional tolerances and fit types. For parts with interference fits, such as the fit between the bearing outer ring and the plastic housing, the housing aperture tolerance must be controlled to H7, and the bearing outer ring tolerance to G6 to ensure no looseness after assembly. Clearance fits are determined based on motion requirements. The clearance for sliding fits is generally 0.05-0.1mm. For example, for the fit between a slider and a guide rail, the guide rail width tolerance is H8, and the slider slot width is H9, ensuring smooth sliding while avoiding excessive shaking. When assembling multiple parts together, cumulative tolerances must be controlled. For example, in a transmission mechanism consisting of three plastic gears, the shaft hole tolerance of each gear is H7, and the shaft diameter is H6 to ensure that the total assembly clearance is within the design range. In addition, the parts’ dimensions must allow for assembly space. For example, for parts connected by snap fasteners, 1-2mm of operating space must be reserved around the snap fasteners to facilitate assembly and disassembly, and to avoid assembly difficulties or part damage due to overly tight dimensions.
The dimensional design of injection-molded parts must take into account the feasibility of the molding process to avoid molding defects caused by unreasonable dimensional design, which could affect production efficiency and part quality. The length-to-diameter (or thickness) ratio of slender parts should not be too large, generally not exceeding 10:1, otherwise they are prone to bending and deformation. For example, a 100mm long plastic rod should have a diameter of at least 10mm. If a thinner structure is required, reinforcing ribs can be added to improve rigidity. Large flat parts require reinforcing ribs or concave-convex textures to reduce warping. The rib height is generally 2-3 times the wall thickness, and the thickness is 0.5-0.7 times the wall thickness. For example, for a flat plate with a 3mm wall thickness, the rib height should be 6-9mm and the thickness should be 1.5-2.1mm. This enhances rigidity without causing sink marks due to excessive thickness. The external dimensions of the part must also take into account the ease of demoulding. The side walls should have sufficient demoulding angle, generally 0.5°-2°. The lower the surface roughness, the smaller the demoulding angle. For example, for a mirror-polished cavity, a demoulding angle of 0.5° is sufficient, while for a rough surface, a demoulding angle of 1°-2° is required to avoid scratching the surface during demoulding.
The dimensional design of injection-molded parts must meet the requirements of the operating environment, taking into account the effects of temperature, humidity, load, and other factors on dimensional stability to ensure reliable function throughout the part’s service life. Parts used in high-temperature environments, such as plastic parts in automotive engine compartments, must be made of high-temperature resistant materials (such as PA66+GF). The coefficient of thermal expansion must be considered during dimensional design to allow for thermal deformation. For example, a plastic conduit has a thermal expansion of 0.3% at 120°C; the length should be increased by 0.3% during design to avoid assembly interference caused by this expansion at high temperatures. Parts used in humid environments, such as bathroom accessories, must be made of materials with low water absorption (such as PP) and have appropriately increased dimensional tolerances, as the material will expand slightly after absorbing water. For example, the dimensional tolerance of PP parts can be relaxed to ±0.2mm to accommodate dimensional changes caused by water absorption. Load-bearing parts, such as plastic gears and brackets, need to improve their strength through dimensional design. The gear’s tooth width, module, and other dimensions must be determined based on the transmitted torque. The bracket’s cross-sectional dimensions must meet bending strength requirements. For example, for a plastic bracket that withstands a 50N load, the cross-sectional dimensions must be verified through finite element analysis to ensure that the deformation under load does not exceed the allowable range.
