Basic parameters of injection molding machine can better reflect size of injection molded product, function of injection molding machine, evaluation of type of material to be processed, grade range and quality of product. It is basis for design, manufacture, selection and use of injection molding machine.
1. Injection volume-maximum injection volume that injection molding system can achieve when injection screw or plunger makes a maximum injection stroke under condition of air injection.
This parameter reflects processing capacity of injection molding machine to a certain extent, and marks maximum quality of plastic products that can be molded by injection molding machine. It is an important parameter of injection molding machine. There are generally two ways to express injection volume, one is based on PS (density ρ=1.05g/cm3) expressed by mass of injected melt (g), and the other is expressed by volume of injected melt (cm3)
According to definition, theoretical value of maximum injection capacity that injection screw can inject at one time is: product of maximum projected area of screw head in direction perpendicular to axis and stroke of injection screw. QL =π/4 D2 S
Injection molding machine cannot reach theoretical value during working process, because density of plastic changes with temperature and pressure. Therefore, injection capacity needs to be appropriately corrected. Revised injection capacity is Q=αQL=π /4 D2 Sα, α——Injection coefficient, generally 0.7∽0.9,
When processing plastic products on an injection molding machine, quality of general products and total material consumption of gating system should not exceed 25%∽70% of injection volume of injection molding machine.
2. Injection pressure-force of end face of screw or plunger acting on unit area of melt, size of injection pressure and structure of injection molding machine, flow resistance, product shape, plastic performance, plasticization method, plasticization temperature, mold structure, mold temperature is related to requirements of product accuracy and other factors. In actual production, injection pressure can be adjusted within allowable range of machine. Size of injection pressure should be selected according to actual situation. General injection pressure selection range is as follows.
A. Material has good fluidity, product shape is simple, arm thickness is large, and injection pressure is generally less than 340∽540Kgf/cm2. Suitable for processing of LDPE, PA and other materials.
B. Melt viscosity of material is low, precision of product is average. Injection pressure is 680∽980Kgf/cm2. Suitable for processing materials such as PS, HDPE, etc.
C. Viscosity of material melt is medium or high, product precision is required, shape is complex, and injection pressure is 980∽1370Kgf/cm2. Suitable for the processing of PP, PC and other materials.
D. Material melt viscosity is high, product is thin wall, long process, high precision requirement, complicated shape, injection pressure is 1370∽1670Kgf/cm2. It is suitable for processing of materials such as reinforced nylon, polysulfone, polyphenylene ether and so on.
E. When processing high-quality precision miniature products, injection pressure can reach more than 2260∽2450Kgf/cm2.
In order to meet processing requirements of injection molding precision products or products with complex molding structures and engineering structural parts, processing adaptability of injection molding machine is enhanced, molding cycle is shortened, product quality is improved, and injection pressure has a tendency to increase.
3. Injection time (injection speed)-melt begins to cool after passing through nozzle. In order to inject melt into mold cavity in time and obtain a dense, uniform and high-precision product, melt must be filled in mold cavity in a short time. In addition to sufficient injection pressure, there must also be a certain flow rate, which is a parameter used to express characteristics of melt filling speed.
4. Plasticizing capacity-amount of material that can be plasticized per unit time. Generally, plasticizing capacity of screw is related to screw speed, driving power, screw structure, and material performance. Plasticizing device of injection molding machine should be able to provide a sufficient amount of plasticized and uniform melt within specified time. Plasticizing capacity should be coordinated with entire molding cycle of injection molding machine. Generally, theoretical plasticizing capacity of injection molding machine is greater than about 20% of actual required amount.

1. Mold clamping force-maximum clamping force exerted on mold by mold clamping mechanism. Under action of this force, mold should not be pushed open. It reflects to a certain extent size of product that injection molding machine can process, which is an important parameter. Our company uses maximum clamping force as nominal specification of injection molding machine. Before melt is injected into mold cavity at a certain speed and pressure, it needs to overcome resistance flowing through nozzle, runner, gate, etc., part of pressure will be lost, but melt still has a relatively high pressure during mold filling. It is called melt pressure in cavity, or cavity pressure for short. Expansion force formed by cavity pressure during injection will cause mold to open. In order to ensure that molding of product fully meets accuracy requirements, mold clamping system must have sufficient clamping force to lock mold.
During injection, in order to prevent mold from being pushed back by expansion force formed by cavity pressure, clamping force F=KPcpA(F clamping force Kg; K safety factor, generally taken as 1∽2; average pressure in Pcp cavity Kgf/cm2; maximum projected area cm2 of A product and gating system on mold parting surface.). Average pressure of Pcp cavity is a relatively difficult value to determine. It is affected by injection pressure, molding process conditions, material properties, mold structure, nozzle and runner form, mold temperature, product shape, accuracy requirements and other factors.
Average pressure of cavity can be selected by referring to Table 2-1.

 

Table 2-1 Relationship between average cavity pressure and molded products
Select average pressure of cavity according to Table 2-1. Result is rough. In most cases, flow length ratio (i) (ratio of limit flow from gate to edge of product to wall thickness of product) can be used to reflect resistance of runner, viscosity coefficient a is used to represent flow characteristics of material. Use chart (table) calculation method to determine average pressure of cavity.
Pcp= aPm (a plastic viscosity coefficient; Pm is cavity pressure value found in Figure 2-1 according to flow length ratio).

 

 

Selection of clamping force is very important. If clamping force of injection molding machine is not enough, it is easy to cause flashing of product during molding, and thin-walled products cannot be formed. If clamping force is selected too large, it is easy to crush mold, increase internal stress of product and cause unnecessary waste. In recent years, due to improvement of efficiency of plasticizing mechanism, improvement of clamping mechanism, increase of injection speed and realization of its process control, clamping force of injection molding machine has been significantly reduced.
2. Basic size of clamping system-directly related to range of products that can be processed, installation and positioning of mold, including: mold plate size, tie rod spacing, maximum distance between mold plates, movable mold plate stroke, mold thickness, mold adjustment stroke.
Mold plate size and distance between tie rods-main parameter indicating mold installation area. Mold plate size determines length and width of mold. It should be able to install mold of general products whose product quality does not exceed injection volume of injection molding machine. Mold plate area is about 4∽10 times maximum molding area of injection molding machine, and mold can be mounted on mold plate by conventional methods. It can be said that mold plate size limits maximum molding area of injection molding machine, tie rod spacing limits mold size.