The raw materials of plastics fall into the charging barrel from the hopper and are sent forward by the rotating screw. While in the charging barrel, the plastics, on the one hand, receive heating and warming from outside of the barrel, and on the other hand, are compacted due to the gradual reduction of the volume of spiral flute.Meanwhile, strong stirring and cutting shall be applied to the plastics by the rotation of screw in the charging barrel, resulting in the intense friction between plastics and between plastics and the barrel as well as screw, and producing a lot of heat which leads to the gradual plastication and fusion of plastics while being pushed forward. The screw while rotating also moves
backward due to the pressure from the fusion of plastics by the head of screw, so that the plasticated fusant can be stored at the top of the charging barrel for use during injection. The backward stroke of screw is determined by the quantity of materials injected as required by each molding of plastic parts.
The earliest recorded injection molding machine dates back to 1872 in the United States, invented by J.W. Hyatt to address the plasticization and molding issues of a nitrocellulose and camphor mixture. This device, known as a “Packing Machine” (U.S. Patent No. 13329), marked a pivotal moment in manufacturing history.
Fast forward to 1921, the first machine that could be recognized as an injection molding machine was crafted by H. Buchholz. This manually operated, plunger-type machine bore similarities to the screw type compression presses of the time.
By 1926, the first series-produced injection molding machines were manufactured by Eckert and Ziegler. These machines featured manual mold clamping but introduced pneumatic injection—a significant innovation. Adopting a horizontal structure, the design principles of these early machines continue to influence modern plastic-making technology.
Injection molding harnesses the thermal properties of plastics, initiating the process by loading materials into the barrel from a hopper. A heating ring around the barrel melts the materials. Inside the barrel, a screw, driven by an external motor, rotates, pushing and compressing the material forward through its grooves.
The combined action of external heating and the screw’s shearing force gradually plasticizes, melts, and homogenizes the material. As the screw rotates, friction and shearing forces move the melted material toward the screw’s head. Meanwhile, the screw retracts under the material’s reactive force, creating a storage space at the screw head to complete the plasticization process.
Next, under the high-speed and high-pressure force of the injection cylinder’s piston, the melted material is injected into the mold cavity through a nozzle. After pressurization, cooling, and solidification in the cavity, the mold opens, and an ejection mechanism releases the finalized product as a qualified item.
Thermoplastics begin as a solid and are melted into a viscous fluid that allows the material to form into a mold. The process of taking raw solid material into the liquid process for molding is known as plastification. Many of the key properties of the final plastic part depend on the plastification process; if any aspect of the process is compromised, it could have a negative effect on the final outcome, so it is critical to maintain the plastification process to the highest standards.
In injection molding, the low-temperature plastic solid pellets are heated and transferred to high-temperature melt through the screw rotation and heater on the barrel. Melt temperature will affect the product quality. If the temperature is too high, material degradation and yellowing can occur. If the temperature is too low, the material fluidity will decrease and thus cause more flow resistance. The melt temperature cannot be directly controlled by the injection machine setting but can be controlled indirectly by the plastification process. There are three major factors in plastification that will affect the melt temperature, which are, the heater temperature, the screw speed, and the back pressure. How the plastification process affects the material temperature will be interpreted as follows.
The rotation of the screw will drive the plastic flow, and the shear heat generated in the process will increase the melt temperature. If the screw speed is too high, the material temperature will deviate too much from the heater temperature; if the speed is too low, it will increase the metering time and reduce production efficiency.when the screw speed is higher, the temperature at the center is higher; while the two sides are still at the temperature of the heater, resulting in uneven melt temperature. When increasing the screw speed to reduce the metering time, it is necessary to be aware of the effect of the consequent temperature rise on the plastic.
During metering, the screw will transport the melt to the front end of the barrel, and the melt stored at the front end will cause the pressure to rise. When the front pressure is higher than the back pressure, the screw will be pulled back. If the back pressure is too high, the screw cannot easily retreat, and the contact time between the plastic and the screw increases, generating too much shear heat that causes the material temperature to rise. If the back pressure is too low, the screw will retreat too fast, resulting in inaccurate and insufficient melt metering that may contain air. Proper back pressure can make a proper time for the plastic to interact with the screw so that the melt temperature can be well controlled.
Each heater generally has different settings, according to the temperature sensitivity of the material. Before the plastic melts, the heat majorly comes from the heater. At this time, the plastic temperature can be lower than the temperature of the heater. As the plastic melts down and enters the rear section of the screw, the shear heat gradually increases, and the melt temperature may be higher than the heater temperature.