Plastic Injection Machine
Inrodution
The plastic injection machine is one of the machines that needs power in motion because it relies on the hydraulic system to open and close the molds for forming plastics. It also needs a 380 volt electricity source because it relies on strong electricity to operate the heaters, the rollers, and the hydraulic unit motor.
Specifications of plastic injection molding machine
Each plastic injection machine has its own specifications, the most important of which are the following:
- Feed hopper capacity :- Feed hopper capacity is of great importance in fully automatic machines where infrequent material loading is desirable.
- The formed volume of each batch is the volume of the material displaced from the cylinder by the moving auger during a full journey, and is equal to the product of the cross-sectional area of the auger and the journey
- Product weight per batch: This value is estimated in grams per batch.
- Softening capacity: It is expressed in kilograms of plastic material / hour, and it depends on the size of the cylinder and the amount of heat available. Of course, the softening capacity will vary according to the material to be formed.
- Injection pressure: – It is expressed either in kilograms of force / square cm in the pressure of the auger, or as a total force in kilograms of force available over the auger.
- Mold Closing Force: It is the total force in tons available to keep the mold closed.
- Dry cycle time: It is the time calculated in seconds required for the machine to complete one cycle of operations in the absence of any other material.
- In general, there are some other specifications of the machines, including injection speed (cm / s) and injection rate (cm3 / s), mold plaque size, distance between tie rods, mold height, energy consumption rate for heaters and motors, as these specifications can be considered additional .
The main components of a plastic injection molding machine
- The base: It is called the machine bed because it holds the melting and closing units. It also contains the accessories of the hydraulic oils circuit, such as the oil tank, the oil pump, connections and oil valves.
Injection Unit: The injection unit consists of the following components:
1. Nozzle : It is the link between the two melting units and the forming mold, and it feeds the mold by adhering to the end by what is called the bushing or the outlet clearance.
There are two types of Nozzles :
- A. Open Nozzle : used in the case of forming high viscosity plastics.
- B: Closed Nozzle : used in the case of the formation of low viscosity plastics.
2. The cylinder: It represents the body of the melting unit and is surrounded by electric heaters. It is used to melt the material inside the cylinder.
3. The gearbox: which belongs to the screw and works either electrically or hydraulically.
4. Feeding hopper: which supplies the machine with the material to be formed
5. A hydraulic motor that rotates the screw.
6. Oil cylinder or pressure regulator.
7. Hydraulic connection pipes.
The melting unit may be fixed on the base of the machine, or it may be free to move in order to facilitate the change of screws, disassembly and installation of the Nozzle, and cleaning of the cylinder.
The functions of the fusion unit are summarized in the following points:
1. Melting the raw material through the heaters surrounding the cylinder.
2. Injecting the material into the vacuum of the mold by means of the Nozzle and the axial movement of the screw.
3. Determining the dosage required to fill the mold in each forming cycle. This is done by withdrawing the raw material from the hopper by rotating the screw and pushing the molten raw material to the front of the screw, so it bounces back until it stops at the predetermined calibration of the amount of raw material needed to fill the vacuum of the mold.
Mold closing unit
In the past, an injection machine of a size described in the weight repairs was used, which is formed in one batch Shot, but now the force of closing the mold is used as a measure of the size of the machine and from the basic laws of hydrostatics that the pressure applied over a certain area will cause a force equal to the product of pressure and area, and so on. A piston pressing with a force of 500 kg force / square cm is used to inject a product in the form of a flat disk of 500 square cm in area. It will require a closing force of 500 x 500 = 250,000 kg, a force of 250 tons on the mold. The process of closing the mold is the function of the closing unit and saves The mold is tight against the force of the injection pressure.
And also you open the mold at the end of the formation cycle. According to the method of supplying the closing force, the closing units are divided into the following units:
1 – Hydraulic unit used for small and medium machines
2 – Pneumatic units for the small machine
3 – Hydraulic units used for medium and large-sized machines (with oil).
4 – Mechanics using (electro mechanics)
The basic components of the mechanical locking unit
1. The knees are connected to the oil cylinder.
2. The movable block on which the slit of the mold is installed
3. piston stem
4. The front panel of the machine
5. Connecting nuts
6. Management gear group
The mechanical closing system includes the single and double knee closing method, and a system that combines the knee closing with the hydraulic closing.
The main components of the hydraulic sealing unit:
1- A cylinder that changes the distance between the two slits of the mold
2- The front panel of the machine
3- Oil cylinder to confirm the closure.
4- An oil cylinder to move the movable core slotting block
5- The block that supports the slotting of the movable mold
6- Slit the template
7- The Nozzle block
8- A group of rotating gears required for closing
9- Oil cylinder piston
Closing Unit Functions:
1- It performs the movement of opening and closing the mold quickly and without shocks affecting it
2- A sealing force is given during the injection process
3- Taking the production piece out of the mold using a benz or pushing with oil or air
The strength of the sealing of the mold depends on the following factors:
1- The surface area of the production piece with the mold
2- The actual internal pressure in the mold
Pressure System
Most of the modern injection machines are operated by using hydraulic fluid under pressure of about 70-140 kgf/cm². And the mold is pumped to the hydraulic cylinder that is connected to the auger, and it also operates the mechanism of closing the mold. The extent to which we need a certain injection pressure to give a good and safe formation depends on the viscosity of the melt.
For example, polyethylene has a viscous melt that needs a pressure of about 300-700 kgf/cm2. In general, the hydraulic cylinder has an area of about ten times the auger exposed. Thus, a pressure of 100kgf/cm2 gives an injection pressure of 1000 kgf. / cm2.
And the sprinkling of some plastic materials and some of the formation is desirable that the injection is done very quickly, and although the hydraulic pump is adequate for the purpose under normal circumstances, it cannot supply us with a high-pressure fluid quickly enough.
When a rapid flow of hydraulic fluid is required, it is released from the tank by opening the Solenoid Valve.
cylinder
The cylinder used in injection molding machines is designed to withstand the high pressure associated with the melting and melting of the polymer. The cylinder is made of wear and corrosion resistant steel.
And the cylinder is heated externally with electrical resistance heaters, and the heaters’ voltage is related to the size and thickness of the cylinder.
And a water cooling is done under the feeding hopper to prevent the raw material from being kneaded in that area
Cylinder ventilation:
Some plastic materials contain volatile substances, if they are not removed before forming, they cause defects in the product such as stains and splashes. It is usual to process this type of raw material before shaping by using a stream of hot air through the feed hopper or by preliminary drying of the raw material.
The depth of the ridges on the sugar can be increased, thus relieving the pressure on the melt and allowing volatile materials to escape through the vent hole.
Screw in the plastic injection machine
The design of the composition of the screw is very important in injection molding. For example, the amount of polymer transferred by the screw from the feeding hopper to the ready-to-injection melt depends on the depth of the screws bumps.
On the other hand, whenever the protrusions are less prominent, the amount of polymer transformed into a melt is less, and the heating and mixing speed is faster. Measuring augers are used to form thermoplastic materials, and variable values such as auger rotation and back pressure have a significant impact on the softening process.
And in machines that use only one screw. The sugar should have a good material handling capacity, a homogeneous heat distribution, and an excellent ability to melt.
The transportation and handling of the raw material depends on the length of the auger. In general, the auger can be divided into three areas: – (feeding area – pressure area – calibration area).
calibration area
This region of the screw transfers the cold plastic material coming down from the feeding hopper and transfers and compresses it to the pressure area. In the movement of the material forward it is subjected to heat and pressure and until the forward pumping is complete, the raw materials must not stop in the channels of the screw but must slide on the walls of the cylinder. The calibration area is the final mixing and heating area of the material, where the material becomes liquid molten. It is characterized by having a fixed depth of bumps
Pressure area in the plastic injection machine
This region compresses the raw material from the state of powder or granules and converts it into a homogeneous melt. It is noted that the bumps along this region decrease in the direction of the nozzle, in order to compensate for the change in the density of the material during this stage.
C:
This zone has a cross-sectional area larger than the other two zones. It is noted that the compression zone and the calibration zone have the same length.
The greater the length of the feeding area, the more efficient it will be in moving the raw material forward. The feeding area is located in the last part of the screw and has a cylindrical section. Its length can reach half the length of the screw. This area is also characterized by having sufficient depth, compared to the rest of screw areas.
And as we mentioned, the length of the screw has a great effect on the melting capacity, and the longer the length of the screw, the greater the effect of softening, but in this case it will become more expensive, and in modern injection machines, the ratio of the length of the screw L to its diameter D, this value L / d is equal to 15-20 And sometimes more than that.
The depth of extrusions in the feeding area H is about 0.12d, and in the middle area H2 it is equal to 0.03 to 0.08 D. The compression ratio of the auger is about 1:2 to 1:5, with an average value equal to 1:3.
Although the external heat is supplied to the cylinder through external heaters, a large amount through the movement of the screws in the small machines is sufficient to supply the thermal energy to the plastic material.
It is usual for the screw to be rotated using a hydraulic motor equipped with a control valve and flow control, or through an electronic motor with variable speeds.
Preform molds for plastic machines
The forming mold usually consists of two basic groups of components:
Mold base:
In the industrial environment, it is called the female mold, and this part forms the outer surface of the produced piece of plastic. Care must be taken to polish and smooth it well in order to help the product come out of the mold. This part is mounted on a bushing that meets the nozzle during the injection process.
Male mold: –
It is the part on which the inner surface of the production piece is formed, and it must be polished and smoothed well. Push pins, bushings, pusher pens, pneumatic castings, or push block are all means to remove the product from the mold because it is mostly held inside the mold. Because of the intense cooling factors, and there are some conditions that must be met when forming a piece of production, and they must be met during the mold manufacturing stage:
Taking into account the presence of a slight (negative) lateral inclination to facilitate the exit of the product from the mold.
Avoid stacking the material at the base and on the sides.
Avoid the presence of sharp corners or sharp edges that cause cracking or cutting the product.
Avoid the presence of folds or inclinations inside the product.
Observe the distances of the holes or openings at the letters.
Information that must be available when making a specific mold for a specific machine:
Know the capacity of the machine
Mold closing force
Product weight
mold pressure
The surface area of the product inside the mold cavity
The distance between the connecting columns
Dimensions of mold block and machine
Material shrinkage rate
