Responsive force control and precision metering are the critical elements in this application of specialized fluid power.
A hydraulic system is a key feature in plastics injection molding machines. The system may be running around-the-clock and
must provide consistent operation through its many years of continuous use. Nozzle approach, injection of the plunge screw, extruder screw rotation, plus the closing of the mold require a great number of motion activated sub-circuits. Granular plastic material requires an extremely steady motion to move smoothly through the heated plasticized state as it flows into the mold, during the screw rotation and plunge phase. The quality of the molded product could come into question if there are any irregularities caused by the hydraulic motions.
A necessity for force control sub-circuits also exist to keep the mold from opening and to hold nozzles in place along the sprue of the mold. During the injection process these functions are critical. Adjusting and monitoring hydraulic pressure controls the mechanical pressure.
The mechanism for the screw which is moved by the hydraulic motor, coupled with the cylinder that propels the screw forward, make up a complex mechanism that require exacting hydraulic seals during the injection and packing process of the mold. Improvements in modern seal technology produce leak-free hydraulic machines suitable for molding plastic products that can even be used in the medical and food industries.
Although many see this as a simple machine with basic hydraulic applications, manufacturers of injection molding machines, face considerable design challenges. The market for molding machines is extremely competitive. Existing customers as well as prospective customers are demanding. They expect efficiency, reliability, accuracy, and ease of maintenance.
In order to provide smooth flow across many working pressures while decreasing heat build-up and input energy waste, making the right choice for your primary hydraulic pump is critical. Some molders have utilized a variable volume piston pump as moving to higher efficiency pumps present a greater challenge. In order to minimize flow pulsations, many piston pumps require the use of a system accumulator.
A popular choice because of its smooth flow and high volumetric efficiency has been the vane pump. When paired with a VFD motor some fixed displacement variety of vane pumps perform very efficiently (compared to variable volume vane pumps that have been previously used) because of its abilities at high system pressures. Older injection molders that may seem outdated and inefficient have been retrofitted with a servomotor-driven pump or VFD controlled pump of either the internal gear or vane design. By changing the speed of the prime mover you can control the pumping rate of a VFD pump. During the end cooling stages of injection molding, virtually no motion is required. The pump’s rotation is slowed by the VFD providing only the needed flow. This will maintain the needed pressure for such functions as mold clamping and provide only the necessary flow to compensate for internal system leakage.
As a minimum requirement for many years, a pressure-compensated, variable-volume pump has been a valuable standard. Even though the shaft on this type of pump continually turns at full speed even when no flow is needed, decreasing the size of the pumping chambers (when equipped with a vane pump) or by diminishing piston stroke (if using a piston pump) minimizes the displacement. Until a valve is opened to move a cylinder that resumes flow demand, maximum system pressure will be maintained.
The goal of minimizing pump displacement in the previous scenarios will assist in saving considerably on input energy when minimal or no motion is needed for the molding machine. In an industry as competitive as injection molding, savings such as these are critical to profit margins. Excess heat is also minimized. Reducing the amount of heat produced is extremely important to the longevity of a machine that already radiates and absorbs the heat generated from plasticizing heaters.
The system pressure can be brought down to near zero even when the machine is in an idle state using an unloading valve. This valve routes the pump’s outlet back into the tank through a solenoid operated relief valve which is connected to the relief valve’s vent pilot line.
Previous versions of injection molders controlled hydraulic and cylinder motor speeds with the use of simplistic needle valves. Proportional, electrohydraulic, or servo valves are currently used in most modern day machines. Acceleration and deceleration ramps along with final actuator speeds are controlled by these zero lapped directional valves or minimally overlapped valves. This method of operation results in the removal of pressure spikes and the accompaniment of shock waves through the frame of the machine during decompression or during start and end of a cylinder stroke.
Steady speed and precise motion control can ensure smooth, quality oriented delivery and heating of plasticized material into the mold by adding a linear positioning transducer to the screw plunge cylinder. Additionally a tachometer/shaft encoder should be placed on the injector screw. A fast acting, programmable controller is needed for all of these sensors and proportional valve inputs in order to fulfill a closed-loop control scheme.
Priority processing and high resolution adaptation of the analog signal, provided from the most dire sensors, requires a suitable controller. The features of this controller should include tunable proportional-integral-derivative (PID) error correction as a standard for accurate, responsive loop control. Controllers that meet these are widely available. They are planned with purpose-built, hydraulic valve drive outputs.
Maintaining hydraulic pressure is paramount when using clamping force applications during the injection cycle of a molder. A signal is fed to the controller from a transducer on the hydraulic line to a large diameter clamp cylinder. Controlling the proportional directional valve is similar to that of a pressure reducing valve. Adjust the clamp pressure during the clap-closed part of the cycle to a setting suited for the molding job being produced. Previously a pressure reducing valve, that was manually adjusted, was often set permanently to a pressure that was too high for the compressing of some small molded products. Cylinders and their seals can be stressed by excessive pressure. Such pressure can shorten the life cycle of the machine’s components.
The nozzle approach cylinder and others like it are usually smaller in diameter. In order for them to maintain their position, this may require higher system pressure during the injection phase of the cycle.
The motion of the screw plunging is where carefully monitored and controlled hydraulic pressure makes a difference in the quality of the product produced. The injection of the plasticized material into the mold is the plunge motion. As the initial material comes in contact with the walls of the mold, it begins to cool. Thus it becomes harder to push the residual plastic yet to be delivered. This results in higher pressure being put on the screw plunge cylinder. Precise servo-valve or proportional control can be obtained by a transducer sensing the pressure and a linear position transducer maintaining the cylinder speed which will then continually push the screw forward all the while slowly maintaining or adjusting to a suitable pressure for the already chilling plastic being injected. This critical phase of injection is known as packing. It is vital to production as it ensures the molded product does not shrink during the cooling process nor warp once it is separated from the mold.
Optimal hydraulic flow and pressure are major factors in what mold designers have gone to great lengths to create for plastic to flow smoothly into the mold cavities. Modernized sensors, electrohydraulic valves, and programmable controllers make up a total control suite process. This process will produce a high-quality product.
Hydraulic system and component makers must highlight their finest, most efficient products and solutions to the machine manufacturers because of the popularity of electric-motion injection machines. This is especially true in the category of smaller machines. The force needed to mold large products such as massive multi-piece molds or automobile bumpers require highly unbeatable hydraulics systems. For the required injection screw torque, radial piston motors are still a prime consideration. For linear motions with high forces cylinders are best. However, just a few modifications and adjustments to what one might think is an outdated machine, well-built hydraulic molding machines are running reliably decades after their installation date.