Determination of injection mold frame size
The size of an injection mold frame should be determined based on the part size and mold structure. The primary consideration is the impact of the cavity layout on the mold frame length and width, ensuring sufficient clearance between cavities and between cavities and the mold frame edge (typically ≥30mm). For a 100×80mm part using a symmetrical two-cavity layout with a cavity center-to-center distance of 150mm, the mold frame width needed to be ≥150 + 2 × 30 = 210mm. A 250mm-wide mold frame was ultimately chosen, leaving ample space for components such as guide posts and cooling nozzles. For large parts (>500mm), the mold frame size must consider the tie rod spacing of the injection molding machine. A 600mm-long automotive door panel mold failed to fit because the 580mm tie rod spacing was not considered. A split mold frame was ultimately used to resolve the issue. The mold frame length is typically 100-150mm greater than the longest cavity distance. For a 300mm-long part, a 450mm mold frame was chosen to ensure sufficient space at both ends for the locating rings and ejector mechanism.
The mold frame thickness (closed height) must be determined based on the cavity depth, mold plate thickness, and mechanical clearance. The calculation formula is: mold frame closed height = cavity depth + core height + upper and lower mold plate thickness + backing plate thickness + mechanical clearance. For a part with a 5mm wall thickness and 30mm depth, the cavity depth is 35mm, the core height is 40mm, the upper and lower mold plates are 30mm each, the backing plate is 20mm, and the mechanical clearance is 50mm. The total closed height is 35+40+30+30+20+50 = 205mm. A 210mm standard mold frame is selected. The closed height must be between the minimum and maximum mold thicknesses of the injection molding machine. For some injection molding machines with a mold thickness range of 150-300mm, the 210mm mold frame mentioned above is fully suitable. If the mold frame is too thick (for example, 320mm), spacers must be added to adjust the thickness. In one case, adding 20mm spacers increased the total thickness to 340mm, exceeding the range and preventing mold clamping. The thickness of the template must meet the strength requirements. Using the formula t=√(6×F×L/(b×[σ])), for a template with a cavity subjected to a force of 100kN and a span of 200mm, the thickness must be ≥50mm to ensure a deflection of < 0.1mm.
The size of a mold frame’s guide and support components influences overall stability. Guide pin diameters are typically 1/4-1/5 the thickness of the mold plate (e.g., 8mm guide pins for a 30mm mold plate). Large mold frames (thickness > 50mm) require 12-16mm guide pins. A 250×200mm mold frame uses four 12mm guide pins, located at the four corners. This achieves a guiding accuracy of 0.01mm/m, reducing mold positioning error by 60% compared to a mold frame with two guide pins. Tie rods (Corinthian pins) must be positioned to match the injection molding machine, with a spacing error of less than 5mm. In one mold, an 8mm misalignment in the tie rod hole position resulted in uneven stress on the mold plate after installation, causing deformation during production, which required rework to restore to normal. The number and diameter of support pins are calculated based on cavity pressure. Each 20mm diameter support pin can withstand 50kN of pressure. A mold with a cavity pressure of 150MPa requires six 20mm support pins to prevent mold plate deformation and dimensional deviation in the molded part.
Special mechanisms (such as slides, lifters, and hot runners) increase mold frame size requirements, requiring ample space to avoid interference. For molds with slides, the mold frame width must be increased by the slide stroke plus 20mm (for safety reasons). For molds with a 50mm slide stroke, the mold frame width increases by 70mm compared to a mold without a slide. The lifter mechanism requires additional height clearance. For a mold with a 100mm lifter length and a 15° angle, the mold frame thickness must be increased by 30mm to avoid interference with the ejector plate. For hot runner molds, the mold frame must consider the size of the manifold. For a four-cavity hot runner mold with a manifold measuring 150×120mm, the mold frame must be 20mm×20mm larger than a standard mold frame to ensure sufficient installation space. The hydraulic cylinder for the side core pull mechanism must be located on the side of the mold frame. For a mold with a 50mm hydraulic cylinder diameter, the mold frame length must be increased by 60mm (including connectors) to avoid collision with the injection molding machine rails.
Standardization of mold frame dimensions and cost-effectiveness must be balanced. Standard mold frame series (such as those from Longji and HASCO) should be prioritized to reduce costs. A standard 200×250mm mold frame is 30% cheaper than non-standard mold frames and reduces delivery time by 50%. When part dimensions approach the upper limit of a standard mold frame, a larger mold frame should be selected. For example, a 190×240mm part was forcibly constructed using an 180×230mm mold frame instead of a 200×250mm one. This resulted in insufficient mold edge strength and cracking after 10,000 mold runs. Mold frame dimensions can be optimized through CAE analysis. Simulations for one mold revealed that the original mold frame width could be reduced by 30mm, maintaining strength while reducing costs and increasing material utilization by 15%. The final mold frame dimensions must be verified through trial assembly to check for smooth component movement and proper installation of cooling and exhaust systems. During a trial assembly, one mold was found to be too narrow, preventing water pipe connections. Widening the mold frame by 20mm resolved the issue, avoiding complications during mass production.
