Injection molding is one of the most common processes for transforming plastics from their raw material state into usable products. Most of the time, thermoplastic materials that can be melted, reshaped, and cooled at the same time are used in this process. From automotive products to food packaging, plastic injection mould maker -molded components are a feature of almost every functional manufactured item in the modern world. With materials that have revolutionized manufacturing technology over the past 50 years, we are able to produce high-quality, simple or complex components at high speeds using this adaptable process.
In order to comprehend the design and operation of contemporary injection molding machines, it is helpful to first examine the process’s relatively recent beginnings. The first injection molding machines were based on pressure die casting technology for processing metals. In the 1870s, celluloid processing patents were registered in the United States. In the 1920s, a series of hand-operated machines for the processing of thermoplastic materials were produced in Germany, bringing about further significant industrial developments. A two-piece mold was clamped together with a straightforward lever arrangement. The molded part was made by injecting molten plastic into the mold. Due to its inherent low pressure, its use was limited. To close the mold, pneumatic cylinders were added to the machine design, but little was improved. In the late 1930s, as a wider variety of materials became available, hydraulic systems were first used on injection molding machinery. However, the design of the machines was still largely based on die casting technology.
Injection molding machinery design did not develop to the scale of the machines we know today until the 1950s in Germany. Earlier machines used a simple plunger arrangement to force the material into the mold. However, as materials got better and processing requirements got more complicated, these machines soon became insufficient. The thermoplastic material couldn’t be easily mixed or homogenized with a straightforward plunger arrangement, which was the main issue. A polymeric material’s poor heat transfer properties made this even worse. The introduction of the injection barrel of a plunging helical screw arrangement was one of the most significant innovations in machine design to solve this issue, and it still applies to contemporary processing equipment. In the following years, the machine gained the moniker “Reciprocating Screw” injection molding machine.
The Injection Moulding Cycle
The modern process has developed and matured significantly to the point where fully automated, closed-loop, microprocessor-controlled machines are the “norm,” despite the fact that injection molding is still, in principle, a relatively straightforward procedure. The polymeric material in powder or granule form must be transferred from a feed hopper to a heated barrel for thermoplastic injection molding. The thermoplastic is melted in the barrel before being injected into a mold with a plunger arrangement of some kind. Within a platen arrangement, the mold is clamped shut under pressure at a temperature well below the thermoplastic melt point. Within the mold, the molten thermoplastic quickly solidifies, allowing the component to be ejected after a predetermined cooling time. Using a machine with a reciprocating screw, the following are the basic steps in the injection molding process:
Mould Closure and Clamping
During the plastic injection cycle, the mold is clamped with the necessary force to keep it closed and prevent plastic from leaking over the face of the mold. The clamping force of modern molding machines ranges from 150 to 4000 kN, or about 15 to 4,000 metric tons.
Mold tool clamping and opening/closing systems abound, typically falling into one of two categories. Direct Hydraulic Lock is a system in which a hydraulic piston arrangement drives the moving machine platen and generates the necessary force to keep the mold closed during the injection process. A mechanical blocking arrangement is utilized to transfer locking pressure from a pressure intensifier at the rear of the machine, which only moves by a few millimeters, through to the platen and tool. Alternately, smaller auxiliary pistons may be used to carry out the primary movement of the platen.
The Toggle Lock is the second type of general clamping arrangement. In this instance, a mechanical toggle device connected to the moving platen’s rear is actuated by a small hydraulic cylinder. When the toggle joint is finally locked over, it acts as a clamp and moves the platen, much like a knuckle arrangement.
At this point in the machine cycle, the helical form injection screw (Figure 1) is “screwed back,” and there is a charge of molten thermoplastic material in front of the screw tip that is about the same size as the amount of molten material needed to fill the mold cavity or slightly larger. The length to diameter ratios of injection molding screws typically range from 15:1 to 20:1, and the compression ratios from the rear to the front are typically around 2:1. 1 to 4 : 1 to make room for the thermoplastic material’s gradual densification as it melts. The front of the screw is fitted with a check valve that prevents material from flowing back over the flights of the screw during injection but allows material to pass through in front of the screw tip for metering (or material dosing). The screw is contained within a barrel with a hardened inner surface that resists abrasion.
Due to the fact that most of the heat required for processing is generated through shear imparted by the screw during processing, ceramic resistance heaters are typically installed around the barrel wall. These heaters are used to primarily heat the thermoplastic material within the barrel to the required processing temperature and compensate for heat loss through the barrel wall. In order to provide a reliable indication of the melt temperature, thermocouple pockets are machined deeply into the barrel wall. A Proportional Integral and Derivative (PID) system can therefore be used to control heat input in a closed loop. The thermoplastic material is discharged from the injection barrel through the injection nozzle, which serves as an interface between the mold and the injection barrel, and into the moulding tool itself by driving the non-rotating screw forward under hydraulic pressure.
Holding Pressure and Cooling
The screw is held in the forward position for a predetermined amount of time, typically with a molten thermoplastic “cushion” in front of the screw tip. This allows a “holding” pressure to be maintained on the solidifying material inside the mold, allowing compensating material to enter the mold as the molded part solidifies and shrinks. There are three ways to start holding pressure: by a predetermined number of seconds after the injection fill phase began; by the distance, in millimeters, that the screw is from the end of the injection stroke; or by the rise in hydraulic pressure, which is measured by a pressure transducer in the injection hydraulic system or in the mold itself.
The hold pressure may decay to zero as the material solidifies to the point where it no longer affects the mold packing. This will help reduce residual stresses in the resulting molding. After the end of the hold pressure phase, the mold must be kept closed for a certain amount of time to cool. During this time, the molding’s heat can escape into the mold tool, lowering the molding’s temperature to the point where it can be ejected from the mold without causing too much distortion or shrinkage. This typically necessitates lowering the molding’s temperature below the thermoplastic’s rubbery transition temperature, or Tg (glass transition temperature). This can be within a few degrees or over a temperature range, depending on the kind of plastic. The tool typically incorporates channels for pressurized water flow for mold temperature control. Depending on the material being processed, component type, and required production rate, the mold may be connected to a cooling unit or water heater.
Material Dosing or Metering
During the cooling phase, material is added to the barrel to prepare it for the subsequent molding cycle. Material in the form of granules or powder is drawn into the rear end of the barrel from a hopper feed by the rotation of the injection screw, which is helical in design. Typically, the throat that connects the hopper to the injection barrel is water-cooled to prevent material bridging and early melting, which would disrupt the feed. A proximity switch at the screw’s back is used to measure the rotation speed, which is typically specified in revolutions per minute (rpm). During metering, screw rotation can be set to one constant speed or to multiple speed stages.
When it reaches the screw tip, the material should be completely molten and homogenized due to the gradual transfer of the material forward over the screw flights. The screw is gradually pushed back by the molten material transferred in front of the tip until the required shot size is reached. By restricting the flow of hydraulic fluid out of the injection cylinder, the backward movement of the screw results in increased shear imparted to the material. This is referred to as “back pressure,” and it aids in the homogenization of the material and reduces the likelihood that unmelted material will transfer to the screw’s front.
Mould Open and Part Ejection
Following the completion of the cooling phase, the mold is opened and the molding is ejected. Either an air-operated ejector valve on the face of the mold tool or ejector pins in the tool that are connected to a hydraulic actuator via an ejector plate are typically used to accomplish this. The molding can either be manually removed by a robot or free-fall into a collection box or onto a transfer conveyer. In the second scenario, the molding cycle is entirely automated. In semi-automatic mode, the operator can manually remove the molding at this point in the cycle. The entire molding cycle can be repeated once the molding is removed from the mold tool.
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