How to accurately determine the key parameters when choosing an injection molding machine, is there any empirical formula or reference standard?

When choosing an injection molding machine, the determination of key parameters requires comprehensive consideration of many factors such as product size, weight, accuracy requirements, plastic raw material characteristics, and production efficiency. The following are some methods and reference standards for determining key parameters.：
Clamping force
Theoretical calculation: The clamping force should be sufficient to prevent the mold parting surface from opening during the injection and holding process.Generally, it can be calculated based on the projection area of the product on the parting surface. The formula is: Clamping force = projection area of the product × cavity pressure.Among them, the cavity pressure varies according to the different plastic raw materials and product structures, and is generally between 20-40MPa.For example, for ordinary plastic daily necessities, the cavity pressure should be about 25MPa; for high-precision and complex products, the cavity pressure may need to be 35MPa or even higher.
Empirical reference: For small and simple products, the clamping force can be estimated according to the projection area of 2-3 tons per square centimeter; for medium-sized products, the projection area of 3-4 tons per square centimeter; for large or complex products, the projection area of 4-6 tons per square centimeter.
Injection volume
Product weight: First of all, the weight of a single product must be accurately calculated, including the plastic weight of the ancillary parts such as the product itself, the runner and the gate.Then, the theoretical injection volume of the injection molding machine is determined according to the production requirements. The theoretical injection volume of the injection molding machine is generally required to be 1.2-1.5 times the weight of the product to ensure that there is enough plastic to fill the mold cavity and take into account a certain processing margin.
Raw material characteristics: The density and shrinkage of different plastic raw materials are different, which will affect the demand for injection volume.For example, some engineering plastics with higher density are heavier at the same volume and require a larger injection volume; while plastics with large shrinkage have obvious volume shrinkage during cooling, and the injection volume needs to be appropriately increased to ensure the dimensional accuracy of the product.
Screw diameter
Product size and shape: Large-scale products or products that require a large amount of plastic filling usually require a larger diameter screw to provide sufficient plasticizing capacity and injection volume.For thin-walled products, in order to achieve rapid mold filling, a screw with a smaller diameter but a larger aspect ratio (L/D) may be required to improve the plasticization quality and injection speed of the plastic.
Raw material fluidity: Plastic raw materials with poor fluidity, such as some enhanced engineering plastics, require larger diameter screws to increase plasticization efficiency, reduce screw torque, and avoid excessive screw wear or motor overload.For general-purpose plastics with good fluidity, a smaller diameter screw can meet the plasticization requirements.Generally speaking, there is a certain correspondence between the diameter of the screw and the weight of the product. For example, if the weight of the product is between 50-200g, a screw with a diameter of 30-40mm can be selected; if the weight of the product is between 200-500g, a screw with a diameter of 40-50mm can be considered.
Injection pressure
Product accuracy and raw materials: When high-precision products or high-viscosity plastic raw materials are used, higher injection pressure is required to ensure that the plastic can fully fill the mold cavity and obtain good appearance and dimensional accuracy.For ordinary precision products and plastics with good fluidity, the injection pressure can be relatively low.Generally speaking, the injection pressure is between 80-200MPa. For general plastics, such as polyethylene, polypropylene, etc., the injection pressure can be selected in the range of 80-120MPa; for engineering plastics, such as polycarbonate, nylon, etc., the injection pressure may be 120-200MPa.
Mold structure: Complex mold structure, such as molds with slender flow channels, multiple gates, or tiny cavities, will increase the resistance of plastic flow, and higher injection pressure is required to overcome the resistance and achieve smooth mold filling.
Mold opening stroke
Product height and release method: The mold opening stroke should ensure that the product can be smoothly removed from the mold, generally greater than the height of the product plus the ejection distance and the safe spacing of the mold.For products that require special mold release methods, such as rotary mold release, lateral core pumping, etc., the additional mold opening stroke required for these actions also needs to be considered.Empirically, the mold opening stroke is generally 2-3 times the height of the product, but the specific value should be accurately calculated based on the actual mold and product structure.