In injection mold design, stays (also known as support posts or ejector plate stays) are key components used to enhance mold strength and prevent mold plate deformation. The proper number of stays directly impacts the mold’s lifespan and the precision of the molded part. Stays primarily serve to protect the mold between the movable mold backing plate and the movable mold base plate, or at the bottom of the cavity, to withstand the significant clamping forces and cavity pressures generated during the injection process, preventing the mold plate from bending due to excessive forces. Insufficient stays can cause permanent deformation of the mold plate, leading to defects such as dimensional deviation and flash in the part. Excessive stays increase mold weight and cost, and may also affect the layout of the ejector mechanism. Therefore, determining the optimal number of stays is a crucial step in mold design.
The number of support pins is determined based on the load on the formwork, and the stress state of the formwork is assessed through mechanical calculations. Cavity pressure is a key parameter in determining the number of support pins. Melt pressure varies significantly among different plastics. For example, the cavity pressure of low-viscosity plastics like PE and PP is typically 30-50 MPa , while that of engineering plastics like PC and PMMA can reach 80-120 MPa . The total pressure on the formwork can be calculated based on the cavity pressure: Total pressure = Projected cavity area × Cavity pressure. For example, if the projected cavity area of a part is 500 cm² and the cavity pressure is 60 MPa , the total pressure = 500 × 10⁻⁴m² × 60 × 10⁶Pa = 3 × 10⁶N (approximately 300 tons). The thickness and material of the template will also affect the number of supports. When the template is thinner or low-strength steel (such as S50C) is used, the number of supports needs to be increased to distribute the load; when the template is thicker or high-strength mold steel (such as 718H) is used, the number of supports can be appropriately reduced.
The mold structure layout has a significant impact on the number and distribution of support pillars. For single-cavity molds, support pillars should be evenly distributed around the cavity, usually 4-6 in number, to ensure that the load is evenly transferred to the mold base. For example, a single-cavity mold for small and medium-sized plastic parts can be equipped with 4 support pillars, located symmetrically around the cavity; large plastic part molds need to increase the number to 6-8, and even add auxiliary support pillars in the center area of the cavity. The number of support pillars for multi-cavity molds needs to be determined according to the cavity layout. When the cavities are arranged in a matrix, the support pillars should be set at the periphery and spacing areas of the cavity group. The number is generally 1.5-2 times the number of cavities. For example, a 16-cavity mold requires 24-32 support pillars. In addition, when there is a large core pulling mechanism or a complex ejection system in the mold, additional support pillars need to be added near these components to resist local concentrated loads and prevent deformation of the template in weak areas.
The diameter and layout of the braces also affect the required number. A larger brace diameter increases the load-bearing capacity of a single brace, reducing the required number. Common brace diameters range from 10-50mm . The specific diameter should be calculated based on the load-bearing requirements using the following formula: Single brace load-bearing capacity = brace cross-sectional area × material allowable stress. For example, a 20mm diameter 45 steel brace (allowable stress 100MPa ) has a single load-bearing capacity of 3.14 × (0.01m)² × 100 × 10⁶Pa ≈ 31,400N (approximately 3.2 tons). Symmetrical layout is more effective than asymmetrical distribution for load distribution. Therefore, braces should be arranged symmetrically along the central axis of the formwork, with spacing between them maintained between 150-250mm . For larger formwork areas, a grid layout can be used to ensure even load distribution across all areas and avoid excessive local stress.
In actual design, the number of support pins should be optimized based on empirical data and simulation analysis. Industry experience suggests that for small and medium-sized molds (platen dimensions less than 500mm×500mm), the number of support pins is typically 4-8; for large molds (platen dimensions greater than 1000mm×1000mm), the number may be 12-20 or even more. However, this empirical data is for reference only. Finite element analysis software (such as ANSYS or Abaqus) should be used to simulate the platen stresses and observe platen deformation. If the maximum deformation exceeds 0.05mm, the number of support pins should be increased. Furthermore, platen deformation testing during mold trials is an important method for verifying the appropriate number of support pins. A dial indicator can be used to measure platen deformation during clamping and injection. If deformation exceeds the specified value, additional support pins should be added. Furthermore, mold installation space should be considered to avoid interference between support pins and components such as ejector pins, guide pins, and waterways. If necessary, special-shaped support pins or avoidance holes can be used in the support pins.
Determining the number of support pins also requires a balance between cost and performance. While increasing the number of support pins improves mold rigidity, it also increases material costs and processing time. For example, adding each 20mm diameter support pin increases material and processing costs by approximately 50-100 yuan. For large molds, excessive support pins can increase costs by 10%-20%. Therefore, while meeting strength requirements, unnecessary support pins should be minimized. Load-bearing efficiency can be improved by optimizing support pin diameter and layout. For example, using large-diameter support pins in areas with concentrated loads and small-diameter support pins in areas with lesser loads can ensure strength while controlling costs. Furthermore, for molds with small batches or low-precision requirements, the number of support pins can be appropriately reduced to reduce initial investment. However, for molds with large-volume production or high-precision parts, a sufficient number of support pins is essential to avoid frequent mold repairs and fluctuations in part quality.
