Understanding and selection of injection molding machines

Injection molds must be installed on an injection molding machine to mold plastic parts. Injection molding machines are the primary equipment for producing thermoplastic parts and have recently also gained application in the molding of thermoset parts. Injection molding is characterized by high molding speed, short molding cycles, and easy dimensional control. It can produce complex, precise parts with inserts in a single process. It is highly adaptable to a variety of plastics, offers high production efficiency, stable product quality, and is easily automated. Injection molding is currently the most widely used plastic molding method, but the high cost of injection molding equipment and mold manufacturing makes it unsuitable for the production of single parts or small batches.

Germany has developed micro-injection molding machines with a shot weight of only 0.1g, capable of producing micro-injection molded parts weighing approximately 0.05g. my country has developed micro-injection molding machines with a shot weight of 0.5g, capable of producing micro-parts weighing approximately 0.1g. France already has ultra-large injection molding machines with a shot weight of 1.7×105g and a clamping force of 150MN. The United States and Japan have respectively produced super-large injection molding machines with injection volumes of 1.0×105g and 9.6×104g. The injection volume of domestic injection molding machines has also reached 3.5×104g, and the clamping force has reached 80MN.

I. Classification of injection molding machines

(1) According to the appearance, injection molding machines can be divided into three types: horizontal, vertical and angle.

① Horizontal injection molding machine The axis of the injection system and the clamping system coincides and is parallel to the bottom surface of the machine installation. It is currently the most commonly used and most important injection molding machine.

Horizontal injection molding machines have the following characteristics in terms of structure and operation.

a. Suitable for high-speed production and high production efficiency.

b. Easy to assemble, disassemble and adjust the mold.

c. The plastic part can fall down automatically after being pushed out, which is easy to remove and suitable for automated production.

d. The machine has a low center of gravity, is stable, and is easy to supply raw materials and operate and maintain.

e. The disadvantage is that it occupies a large area.

② Vertical injection molding machine: The axis of the injection system and the clamping system coincide and are perpendicular to the bottom surface of the machine installation. Its advantages are small footprint, easy mold assembly and disassembly, and simple and reliable installation of inserts and movable cores. Its disadvantages are high center of gravity, instability, difficulty in feeding, and the ejected plastic parts must be manually removed, making it difficult to achieve automated production.
③ Angular injection molding machine: The axis of the injection system and the clamping system of the angular injection molding machine are perpendicular to each other. The angular injection molding machine has a simple structure and can automatically remove threaded plastic parts by rotating the lead screw when the mold is opened. Its disadvantages are difficulty in feeding and inconvenience in installing inserts and movable cores. Mechanical transmission cannot accurately and reliably inject and maintain pressure and clamping force, and the mold is subject to greater impact and vibration. It is suitable for producing molds with asymmetric shapes and using side gates.
(2) Classification by plasticization method: Injection molding machines can be divided into plunger type and screw type.
① Plunger type injection molding machine: It is not suitable for processing plastic parts with poor fluidity and strong heat sensitivity. The structure of vertical injection molding machines and angle injection molding machines is plunger type.

② Screw injection molding machine The screw can rotate or reciprocate. The plastic entering the barrel is gradually melted and plasticized by the heat transfer of the barrel and the shear friction between the screw and the plastic; on the other hand, it is continuously pushed to the front end of the barrel by the screw. When the melt near the nozzle reaches the injection volume, the screw stops rotating and is pushed forward by the hydraulic system to inject the melt into the mold cavity. The structure of horizontal injection molding machines is mostly screw type.

(3) Classification by use Injection molding machines can be divided into general type and special type. General type injection molding machines are suitable for injection molding of general molds, and special type injection molding machines are suitable for injection molding of some special structure molds, such as gear molds, two-color molds, blow molding molds, etc.

(4) Classification by transmission mode Injection molding machines can be divided into mechanical type, hydraulic type and mechanical and hydraulic combined type. 2. Basic Structure of an Injection Molding Machine

Regardless of the type of injection molding machine, it consists of four main components: the injection mechanism, the clamping mechanism, the hydraulic transmission system, and the electrical control system. The injection mechanism consists of a hopper, barrel, heater, screw (for plunger-type injection molding machines, the plunger and diverter shuttle), nozzle, and injection cylinder. Its function is to uniformly melt the solid plastic into a molten state and inject the molten plastic into the closed mold cavity with sufficient pressure and speed. The clamping mechanism includes a fixed platen, a movable platen, tie rods, a cylinder, and an ejector. Its functions are: first, to lock the mold; second, to open and close the mold; and third, to eject the molded part when the mold is opened. The clamping mechanism often uses a combination of mechanical and hydraulic forces, sometimes fully hydraulic. The ejector mechanism is available in both mechanical and hydraulic types.

Basic Structure of a Horizontal Injection Molding Machine
The hydraulic transmission system and electrical control system ensure that the injection molding process is carried out accurately according to the predetermined process requirements (pressure, temperature, and time) and the operating sequence. The hydraulic transmission system is the power system of the injection molding machine, while the electrical control system is the system that controls each power hydraulic cylinder to complete the opening, closing, injection and ejection actions.

III. Technical parameters of injection molding machine

The basic parameters of the injection molding machine are the basic basis for its design, manufacture, selection and use. The basic parameters that describe the performance of the injection molding machine include injection volume, injection pressure, injection speed, plasticizing capacity, clamping force, basic dimensions of the clamping device, etc.

my country's SZ series injection molding machines use the theoretical injection capacity and clamping force that can be injected at one time to characterize the production capacity of the injection molding machine. For example, SZ-160/1000 means that the theoretical injection capacity of this model of injection molding machine is about 160cm3 and the clamping force is about 1000kN.

(1) Technical parameters of injection device

① Screw diameter: The outer diameter of the screw (mm), represented by D.

② Effective screw length: The length of the threaded part of the screw (mm), represented by L.

③ Screw compression ratio: The ratio of the volume V2 of the first screw groove in the feeding section to the volume V1 of the last screw groove in the metering section, i.e., V2/V1.

④ Theoretical injection volume: The product of the cross-sectional area of ​​the screw (or plunger) head and the maximum injection stroke (cm³).

⑤ Injection volume: The maximum volume of the screw (or plunger) that can be injected in a single shot (cm³) or the maximum mass of PS material injected in a single shot (g).

⑥ Injection pressure: The maximum pressure applied by the screw (or plunger) head to the pre-plasticized material during injection (MPa).

⑦ Injection speed: The maximum speed of the screw (or plunger) during injection (mm/s).

⑧ Injection time: The shortest time the screw (or plunger) takes to complete the injection stroke during injection (s).

⑨ Plasticizing capacity: The maximum mass of PS material that can be plasticized per unit time (g/s).

⑩ Nozzle contact force: The maximum contact force between the nozzle and the mold, i.e., the injection seat thrust (kN).

11 Nozzle extension: The length of the nozzle extending from the mold mounting surface (mm).

In addition, there are technical parameters such as nozzle structure, nozzle aperture and spherical radius.

(2) Technical parameters of mold clamping device

① Clamping force: The maximum clamping force applied to the mold to overcome the expansion of the plastic melt (kN).

② Molding area: The largest projected area of ​​the cavity and gating system on the parting surface (cm2).

③ Mold opening stroke: The maximum distance the movable mold of the mold can move (mm).

④ Template size: The outer dimensions of the mounting plane of the fixed template and the movable template (mm).

⑤ Maximum (minimum) mold thickness: The maximum (minimum) thickness of the closed mold that can be installed on the injection molding machine (mm).

⑥ Maximum (minimum) template opening distance: The maximum (minimum) distance between the fixed template and the movable template (mm).

⑦ Tie rod spacing: The distance between the inner sides of the tie rod of the injection molding machine in the horizontal and vertical directions (mm).

⑧ Ejection Stroke: Maximum displacement of the ejection mechanism during ejection (mm).

IV. Injection Molding Machine Selection

1. Selection Based on Nominal Shot Volume

Nominal shot volume refers to the maximum ejection volume that the injection unit can deliver when the injection screw makes one full injection stroke, under empty injection conditions. It is one of the key parameters of an injection molding machine and is expressed in grams or cubic meters. The shot volume indicates the injection capacity of the injection molding machine and reflects the maximum mass or volume of plastic parts the machine can produce.

There are two ways to express shot volume: one is based on the processing of PS raw material (density 1.05 g/cm³) and expresses it in grams of molten material injected. When processing other materials, density conversion should be performed. The other is expressed in terms of shot volume, which is the volume of molten material injected in one shot (cm³).

The nominal shot volume in the basic parameters refers to the volume or mass corresponding to the maximum screw stroke, assuming empty injection. In practice, due to factors such as temperature, pressure, and melt backflow, the shot volume may not reach the theoretical value. The actual injection volume is 0.7 to 0.9 times the nominal injection volume.

Production practice shows that the total material volume of the plastic part should ideally be 25% to 75% of the machine's nominal injection volume, with a minimum of no less than 10%. Exceeding this range will result in underutilized machine capacity or reduced part quality.

my country's standard series of injection molding machines specifies injection volume specifications of 16cm³, 25cm³, 30cm³, 40cm³, 60cm³, 100cm³, 125cm³, 160cm³, 250cm³, 350cm³, and so on, up to 64,000cm³.

2. Selection Based on Injection Pressure

Injection pressure refers to the maximum pressure at the screw head during the injection process. Its function is to overcome the resistance of the melt flowing through the nozzle, runner, and mold cavity during injection, while also exerting a certain amount of pressure on the melt injected into the mold cavity to ensure material replenishment and a dense molded part. Currently, the injection pressure of domestic injection molding machines is generally 105 to 150 MPa. When selecting equipment, consider whether the required injection pressure is within the machine's theoretical pressure range.

3. Selection Based on Plasticizing Capacity

Plasticizing capacity refers to the amount of material the plasticizing unit can plasticize per unit time, measured in g/s. Plasticizing capacity determines screw speed, drive power, screw structure, material properties, and other factors. Plasticizing capacity represents the machine's production capacity. However, plasticizing capacity must be coordinated with the entire injection molding machine's molding cycle and shot volume to ensure a sufficient and uniform amount of plasticized melt within the specified timeframe. Generally, the theoretical plasticizing capacity of an injection molding machine is approximately 20% greater than the actual required amount.

4. Selection Based on Clamping Force

Clamping force refers to the maximum clamping force applied by the mold clamping mechanism to the mold, measured in kN. Clamping force balances the expansion force of the melt on the mold cavity during injection, ensuring that the mold does not expand during injection and holding pressure. When selecting equipment, it is important to determine whether the equipment's clamping force is sufficient. Selecting the clamping force is crucial. Insufficient clamping force can cause flash on the part and prevent thin-walled parts from being formed. Excessive clamping force can damage the mold. Clamping force F ≥ bulging force divided by 80%. Bulk force is related to melt flowability, part wall thickness, and material temperature. The formula for calculating bulging force is as follows: Bulk force = part projected area A × cavity pressure P. Common cavity pressure values ​​used in plastic injection molding are detailed in Table 5-1; typically, 20 to 40 MPa are used.

5. Select based on maximum mold dimensions

The basic parameters of the mold clamping system determine the mold's installation dimensions and, therefore, the planar dimensions of the part that can be processed. These basic parameters include the dimensions of the movable and fixed mold plates, tie rod spacing, the maximum opening between the movable and fixed mold plates, mold height, and the travel and speed of the movable mold plates. The dimensions of the movable and fixed mold plates refer to the maximum center-to-center spacing between the screw holes in the plates in both the length and width directions. The tie rod spacing refers to the maximum center-to-center spacing between the tie rod holes in the plates in both the length and width directions. The mold plane size must be limited to the range specified by the fixed plate size and the tie rod spacing.