Injection Unit Components in Injection Mold Machine
The injection unit is a critical part of an injection mold machine, responsible for heating plastic materials to a molten state and injecting them into the mold cavity. This comprehensive guide explores the common types and structures of components that make up this essential system in any injection mold machine.
Function of the Injection Unit
The injection unit of an injection mold machine performs the crucial task of transforming solid plastic resin into a molten state and delivering it into the mold cavity. As shown in the diagram, resin is fed from the hopper into the barrel, where it is heated by heaters surrounding the barrel. In an injection mold machine, the combination of external heating and shear stress generated by the rotating screw melts the plastic, converting it into a homogeneous molten state.
The molten plastic is conveyed to the front of the barrel through screw rotation. During the injection phase of the injection mold machine cycle, the screw moves forward under controlled pressure and speed to inject the molten material into the mold cavity. As the molten plastic flows within the mold, precise control of the screw's movement speed (injection speed) is essential. Once the mold cavity is filled, pressure (holding pressure) is maintained to ensure proper packing and compensation for material shrinkage.
In a modern injection mold machine, the transition from injection speed control to pressure control occurs when either the screw reaches a predetermined position or the injection pressure reaches a specified value. This precise control mechanism is what distinguishes a high-quality injection mold machine from lesser alternatives.
Main Types of Injection Units
There are two primary types of injection units used in injection mold machine technology: the plunger type and the reciprocating screw type. While plunger-type injection units were once common in early injection mold machine designs, they have become increasingly rare in modern manufacturing due to their limitations in plasticizing efficiency and material homogeneity. For this reason, we will focus our attention on the reciprocating screw type, which has become the standard in contemporary injection mold machine technology.
Reciprocating Screw Injection Unit
The reciprocating screw system represents the most advanced technology in modern injection mold machine design. This system operates through a two-stage process: plasticization and injection. During plasticization, the screw rotates within the heated barrel, melting and mixing solid plastic pellets (or powder) through a combination of heat and mechanical shear.
As the molten plastic accumulates at the front of the barrel, it pushes the screw backward, creating a reservoir of molten material. When sufficient material has been prepared, the screw stops rotating and moves forward axially, injecting the molten plastic from this reservoir into the mold cavity. This dual action – rotation for plasticization and axial movement for injection – is what gives the reciprocating screw injection mold machine its name and efficiency.
Key Components Related to Plasticization
The plasticization process in an injection mold machine relies on several critical components working in harmony. These include the screw, barrel, spreader, check ring, nozzle, flange, and hopper. Each component plays a vital role in ensuring proper plasticization, mixing, and delivery of molten plastic in an injection mold machine. Let's examine each component in detail:
1. Screw
The screw is perhaps the most critical component of an injection mold machine, responsible for conveying, compacting, melting, mixing, and pressurizing the plastic material. All these functions are accomplished through the screw's rotation within the barrel of the injection mold machine.
As the screw rotates in an injection mold machine, friction and relative motion occur between the plastic and the barrel inner wall, the screw flight surfaces, and between the plastic particles themselves. The forward movement of plastic is the result of these combined motions, while the heat generated by friction contributes significantly to raising the plastic temperature and facilitating melting in the injection mold machine.
The design of the screw – including its diameter, length-to-diameter ratio, flight depth, and compression ratio – is carefully engineered based on the types of materials to be processed and the specific requirements of the injection mold machine application. Proper screw design ensures optimal plasticization efficiency and material homogeneity in the injection mold machine.
2. Barrel
The barrel is essentially a cylindrical tube with a feed opening, serving as the housing for the screw in an injection mold machine. The driving force for plastic advancement and mixing during plasticization comes from the relative rotation between the screw and barrel in the injection mold machine.
Based on the different states of plastic as it progresses through the screw, the screw is generally divided into three sections in an injection mold machine: the solid conveying section (also called the feeding section), the melting section (or compression section), and the homogenizing section (also known as the metering section).
The barrel is equipped with external heating elements that provide controlled thermal input to assist in the plastic melting process. The temperature profile along the barrel length is carefully controlled in an injection mold machine, with different zones maintained at specific temperatures to optimize the plasticization process for different materials.
3. Spreader (Glue Head)
The spreader is a torpedo-shaped component mounted at the front end of the screw in an injection mold machine. Its primary function during plasticization is to split and mix the molten plastic, ensuring further homogenization of the melt in the injection mold machine. Additionally, the spreader helps to position the check ring during plasticization.
To enhance the mixing effect, it is recommended that injection mold machine with a clamping force of 250 tons or more utilize a spreader with a barrier-type mixing structure. This advanced design not only improves the color uniformity of the final product but also enhances the mechanical strength of the molded parts produced by the injection mold machine.
The spreader also plays a role in controlling the flow path of the molten plastic, ensuring that it is properly distributed before entering the nozzle section of the injection mold machine. Its design can significantly impact the pressure loss and temperature distribution of the melt as it exits the screw.
4. Check Ring (Glue Ring)
As its name suggests, the check ring in an injection mold machine serves a non-return function, preventing the backflow of molten plastic during the injection phase. During injection, the check ring comes into contact with the check washer (glue washer), forming a seal that prevents molten plastic from leaking backward along the screw in the injection mold machine.
The precision of part weight in an injection mold machine is significantly influenced by the response speed of the check ring. The reaction speed of a check ring is determined by factors such as its stroke, sealing contact time, and the time it takes to separate from the spreader in the injection mold machine.
Through careful experimentation and optimization of check surface parameters, contact parameters between the check ring and spreader, and clearance parameters between the check ring and barrel, high-precision injection volume control can be achieved in an injection mold machine. Proper maintenance and replacement of worn check rings are essential for maintaining consistent part quality in any injection mold machine.
5. Nozzle
The nozzle serves as the transition component between the barrel and the mold in an injection mold machine. During injection, molten material from the barrel is forced through the nozzle at high pressure and speed into the mold cavity of the injection mold machine.
The design, orifice size, and manufacturing precision of the nozzle significantly affect pressure and temperature losses, flow distance, packing efficiency, and the occurrence of drooling in an injection mold machine. A well-designed nozzle minimizes pressure drop while ensuring proper melt temperature control.
While there are numerous nozzle designs available for different injection mold machine applications, three types are most commonly used: open nozzles, shut-off nozzles, and reverse-taper nozzles. Each design offers specific advantages for different materials and molding conditions in the injection mold machine, with considerations for viscosity, heat sensitivity, and flow characteristics of the plastic being processed.
6. Flange
The flange is the component that connects the nozzle to the barrel in an injection mold machine, primarily serving as a passageway for the molten plastic. While its function seems simple, the flange plays an important role in maintaining the integrity of the melt flow path in the injection mold machine.
If significant gaps or grooves form between the flange and nozzle or between the flange and barrel in an injection mold machine, plastic can become trapped in these spaces. Over time, this trapped material will degrade due to prolonged exposure to heat, leading to black spots or discoloration in the molded parts. Proper maintenance and alignment of the flange in an injection mold machine are therefore essential for ensuring part quality and preventing material degradation.
7. Hopper
The hopper is the component that stores the plastic raw material in an injection mold machine. Many hoppers are equipped with heating and blowing devices, functioning as drying hoppers to remove moisture from hygroscopic materials before processing in the injection mold machine.
The typical hopper design features a conical lower section and a cylindrical upper section. The angle of the conical slope significantly affects the feeding efficiency in an injection mold machine, with optimal angles determined based on material particle size, shape, and friction characteristics.
Two common feeding problems can occur in an injection mold machine hopper: "bridging" and "funneling." Bridging happens when plastic particles form a stable arch above the feed opening, typically with larger or irregularly shaped regrind materials. Funneling occurs when flowing particles fail to带动 adjacent particles, often with smaller particle sizes in the injection mold machine.
If heat from the barrel transfers to the hopper in an injection mold machine, causing excessive temperature rise, plastic particles can soften or agglomerate on the surface, also leading to bridging or blockages. Common solutions include installing vibration devices on the hopper or adjusting the cone angle in the injection mold machine. For heat transfer issues, proper temperature control at the barrel feed zone and enhanced cooling at the barrel feed opening are recommended in the injection mold machine.
Summary of Injection Mold Machine Components
Each component in the injection unit of an injection mold machine plays a critical role in the plasticization and injection process. From the screw and barrel that work together to melt and convey the plastic, to the spreader and check ring that ensure proper mixing and prevent backflow, every part contributes to the overall performance and efficiency of the injection mold machine.
The nozzle, flange, and hopper, while seemingly simpler components, are equally important in maintaining material flow, preventing degradation, and ensuring consistent feeding in the injection mold machine. Understanding the function and importance of each component is essential for optimizing the performance of an injection mold machine and achieving high-quality molded parts.
Proper maintenance, selection, and configuration of these components based on the specific materials and applications will significantly impact the efficiency, quality, and cost-effectiveness of the injection mold machine operation.