Designing the draft angle for polyurethane foam molds is crucial for ensuring smooth demolding and avoiding damage. The key lies in determining a scientifically appropriate draft angle based on the product's structural characteristics, material properties, geometry, and functional requirements. This process requires balancing ease of demolding with product precision, preventing mold sticking due to insufficient draft angle or compromising structural strength due to excessive draft angle. The following analysis focuses on key dimensions of product structure.
Part geometry directly influences the choice of draft angle. For polyurethane foam parts with greater height or thicker walls, such as deep-cavity structures or pillars, a larger draft angle is required. This is because thicker wall areas shrink more significantly during cooling, increasing the force holding the core in place. Insufficient draft angles can easily lead to part tearing or mold wear during demolding due to friction. For example, the sidewall draft of deep-cavity parts is typically designed between 1° and 3° to reduce the contact area between the part and the mold, thereby reducing adhesion. For flat or thin-walled products, such as panels, the draft angle can be controlled within 0.5°-1° due to the reduced shrinkage, ensuring smooth demolding while maintaining dimensional accuracy.
The functional requirements of the product constrain the draft angle design. If the product requires assembly with other components, such as snaps, slots, or threads, the draft angle must align with the assembly direction. For example, products with snaps require a reverse draft at the snap location to ensure the snaps maintain their designed dimensions after demolding and avoid loosening due to improper draft. For products that require sharp edges, such as decorative strips or seals, the draft angle may need to be partially reduced to prevent rounded edges that affect functionality. In this case, a segmented draft design can be used, increasing the draft in non-functional areas and decreasing it in functional areas, achieving a balance between demolding and performance.
Reinforcing ribs and protrusions are key areas of draft angle design. Reinforcing ribs in polyurethane foam products enhance structural strength, but their thickness is typically less than the mainboard wall thickness, and their tops should be designed with a curved tapered shape. To prevent rib tearing during demolding, the slope should be adjusted according to rib height: 0.5° for low ribs (height <3mm), 1° for medium ribs (3-5mm), and 1.5° for tall ribs (>5mm). Raised features, such as buttons or logos, require slope to prevent scraping from the core during demolding. Typically, the slope on one side is designed between 2° and 4°, while the top corners should be rounded to reduce stress concentration.
Part wall thickness uniformity places special demands on slope design. In polyurethane foam molds, areas with sudden changes in wall thickness are prone to deformation due to differential shrinkage, requiring a slope transition to alleviate stress. For example, when transitioning from 2mm to 5mm wall thickness, the slope should be increased to 2° in the thicker areas and maintained at 1° in the thinner areas to smooth the shrinkage gradient. For locally thickened features, such as those around bolt holes, a reverse slope should be designed on the outside of the thickened area to prevent the thicker wall from binding during demolding.
The position of the mold parting surface is closely related to the direction of the slope. The parting surface serves as the reference for mold opening, and the draft angle must align with the parting direction. For multi-cavity molds, the draft direction must be consistent across all cavities to avoid tilting the part due to misaligned directions. For example, the parting surface for round parts is typically designed at the maximum diameter, with the slope decreasing inward or outward from the parting surface to ensure even distribution of demolding force. For irregularly shaped parts, simulation software is used to analyze the demolding trajectory and optimize the draft direction to prevent part shifting during demolding.
The surface quality requirements of the product guide the draft design. High-gloss or transparent products require strict control of the draft to prevent scratches during demolding. For example, the draft of transparent polyurethane foam parts is typically designed between 0.5° and 1°, and a release agent is used to reduce friction. For parts requiring post-processing (such as spraying or electroplating), the draft should allow for machining allowance to avoid excessive draft, which would make subsequent corrections difficult. Furthermore, for textured parts, the draft should be adjusted according to the depth of the texture. The deeper the texture, the larger the draft should be to prevent damage during demolding.
