Vertical injection molding machines can be classified using two main nomenclature systems: 1) Tonnage, which indicates the clamping force, and 2) Gram weight, which reflects the capacity of the extruder screw barrel.
The clamping force is crucial as it determines whether the injection molding machine can handle the pressure generated by the mold’s product area. Meanwhile, the screw barrel’s capacity indicates whether the machine can adequately process the raw materials required for your products.
1) Choosing an Injection Machine Based on Clamping Force
The formula for calculating clamping force is:
Clamping Force = Clamping Force Parameter × Product Area (PE).
Typically, the clamping force parameter is set at 0.32. The product area can be determined by multiplying its length by its width. For vertical injection molding machines, it’s advisable for the clamping force to exceed the required force for the product by a factor of 1.17.
2) Selecting Based on Screw Barrel Capacity
The specific main parameters regarding the screw barrel’s capacity are included in the equipment’s user manual. For instance, if your product has a weight of 70g (like a feeding handle), the screw barrel’s minimum capacity should be at least 100g. Each injection should ideally use about 70% of the screw barrel’s total capacity, as overloading can lead to excessive waste, while underloading could result in insufficient material, shrinkage, and other complications.
It’s important to note that the tonnage of the clamping force does not have a direct correlation with a product’s net weight. Different manufacturers offer varying tonnages for their injection molding machines, and the screw barrel capacities are also not uniform. However, rough estimates can be made. For example, a 360-ton injection molding machine typically has a material capacity of around 1000g, while a 120-ton machine has a capacity of about 150g, and a 90-ton vertical injection molding machine may have a capacity around 90g.
The tonnage correlates with the respective gram weight, which is generally based on the standard net weight of polystyrene (PS) material. However, during actual production, strict adherence to gram weight specifications may not always be feasible. Some machines might exceed the gram weight, while others might not meet it. This variability does not necessarily indicate a flaw in the injection molding machine’s design, as each manufacturer targets specific market needs with distinct products.
In scientific research related to these machines, the primary focus is on tonnage, followed by gram weight, spacing of support rods, capacity strain rate, and arrangement of the mold opening stroke, all of which help in selecting the most suitable equipment.