Pneumatic Linear Actuators - Assignment Example

 Pneumatic Linear Actuators Analysis of the Process The Mechanism of injection moulding operation is largely based on the individual functions of each of the three pneumatic linear actuators that make up the small simple moulding machine. According to Malloy (2004, p.34), pneumatic linear actuators utilize compressed air to generate their force which is then converted to linear motion. The sequence of events involved in the injection moulding cycle begins with the closing the mould, followed by the injection of the polymer into the mould cavity and retraction as the next molding shot is prepared. Once the final product is sufficiently cooled, the mold opens and the product is ejected. Injection Linear Actuator This is the first of the three linear actuators and is situated at the rear end of the moulding machine. Once the injection hopper has been refilled with the liquid plastic polymer to be moulded, the injection linear actuator uses compressed air to generate linear force sufficient enough to push the polymer liquid into the moulding unit before returning to its original position to refill for the next shot. In the next step, the first linear actuator retracts back to the original position as the next shot is prepared. Barrel Movement Linear Actuator This pneumatic linear actuator particularly works by moving the barrel forward against the mould and back away from the mould in successive sessions. The linear force applied on the injection piston forces the molten plastic polymer to flow into the sprue hole of the mould where molten thermosetting polymer assumes the shape of the contours of the mould (Malloy, 2004, 41). For example, during the mechanical activity of the machine, the piston in the hollow cylinder is pushed by air causing it to produce energy on the form of motion. Mould Clamp Linear Actuator This is the last pneumatic linear actuator used that operates by opening and closing the mould. The mould cavity then determines the shape of the polymer before the final product is pushed by the linear actuator into the final stage involving the cooling of the product. For example, while the cooling of the already moulded product is in progress, Mould Clamp Linear Actuator is used to hold and clamp the plates of the mould together until the final product cools and hardens into the desired shape. In this regard, the clamping procedure is primarily used to determine the finished product’s final shape. Inputs and Outputs Required for a Control System There are a number of inputs and outputs required for a control system that will ensure that to ensure that the injector is always back to its original position before the barrel can move while at the same time controlling the barrel and ensuring that it get back to allow the mould to open or close (Todd and Allen, 1994, p.102). In this regard, some of the inputs required for the control system include timer control button, pressure control button, moulding shot size limit switches and clamping force control button. On the other hand, the outputs required for the control system include solenoid valves and motor starters. PLC Program The PLC program involve connection to an outside plant as well as input and output devices that are needed for identification inside the program. A PLC program could be considered to act as a permanent running loop in which user instructions are obeyed sequentially. After the last instructions are obeyed, the operation starts once again at the first instruction. The program does not communicate with the outside world continuously. The PLC program instead acts by taking a snapshot. The PLC program is as shown below. Source: http://www.plcprogramming.org/ How Speed can be adjusted in Penumatic Circuit From the diagram, only exhaust air can be used to initiate control of the cylinder. This case is shown in the above diagram, which has a five-port valve. Time Delay Sometimes in a circuit one wants a pause or delay before something else happens. To create a delay one needs to use two components that include a unidirectional restrictor and a reservoir. Time delay is achieved through connection of a valve that is unidirectional-flow control and a reservoir. This connection has to be in series form as shown below. Source: http://www.bbc.co.uk/schools/gcsebitesize/design/systemscontrol/pneumaticsrev4.shtml It follows that bigger the reservoir, the longer it takes to fill up with air. To make the delay longer one needs to use a unidirectional restrictor in front of the reservoir. This slows down the air so that the reservoir takes even longer to fill. The length of time it takes to fill creates the delay as shown below. It is possible to change the length of a delay by changing the size of the reservoir or adjusting the restrictor. Whenever the push button is pushed, the 5/2 valve adjusts in its status and the cylinder out-strokes. As the cylinder out-strokes, it pushes the former together. The plastic sheet, which is now hot is pressed back into shape. When this happens, it the roller also gets actuated. Air now flows through the restrictor and starts to fill up the reservoir. After the reservoir is full, the 5/2 valve changes its status, and the cylinder in-strokes, set for the process to begin another time. This happens as in the diagram below. Increasing speed High speed of a pneumatic cylinder can be achieved by pre-exhausting the other chamber before the movement takes place. This implies that the cylinder is first vented prior to the actual movement. This results to a higher travel speed than in the case of conventional solutions. Suitable PLC for the System The suitable programmable logic controller (PLC) for the system should be able to automatically regulate and adjust both the time the mould is closed as well as the time the injecting linear actuator is moved forward. The proposed PLC system will significantly enhance the processes and functions of all the three pneumatic linear actuators working simultaneously in the small simple moulding machine by continuously monitoring the state of the input devices as well as controlling the state of the output devices. According to Sclater (2007, p.48), one of the major benefits of incorporating PLC into the control systems is its ability to easily change or replicate operations and processes while at the same time collecting and communicating critical information regarding the operations of the system. Additionally, by the virtue of being modular, it will be relatively easier to mix and match various types of input or output devices that enhance the functionality of the moulding machine. Lastly, to ensure that all the end conditions of the injection moulding machine are effectively fed back using limit switches and the inject forward and mould is controlled by a timer, the proposed PLC for the control system of the moulding machine will include operator terminals, networks as well as a number of other distributed input and output devices. Pneumatic Circuit to Complete the Operation The FluidSim simulated pneumatic circuit for the operation of the small simple moulding machine consists of three cylinders with each cylinder intended to support and control the operations of each of the three pneumatic linear actuators that make up the moulding machine. For example, the first cylinder is for moving forward injecting polymer and then returning (injection linear actuator), the second cylinder for moving the barrel forward against the mould and back away from the mould (Barrel movement linear actuator) and lastly, the third cylinder if for opening and closing the mould (mould clamp linear actuator). The simulated circuit model allows for easy interaction between the various inputs such as the buttons and switches as well as output components like the motors of the control system. Lastly, the simulated circuit displays a warning in the event that there is a logical failure in the pneumatic circuit system. How Speed Control on all Linear Actuators Can Be Achieved The speed of each of the three linear actuators can effectively be controlled to maximize the operations of the moulding machine through a number of ways some of which include using shock absorbers and flow controls. For example, flow control devices can be installed on each of the cylinders specifically at the rod end of the cylinder with the free flow towards the cylinder which is then extended against a cushion of air that is slowly getting bled into the atmosphere. According to Callister (2009, p.66), when the valve is actuated, pressure is applied to the blank end of the cylinder thereby slowing down, decelerating or providing control speeds to the pneumatic linear actuators. However, despite the effectiveness of using this approach to control the speed of linear actuators, it may significantly increase the complexity of the installed system thereby invariably raising the costs and maintenance expenses of the simple injection moulding machine. References Callister, W. D. 2009. Materials Science and Engineering: An Introduction. New York: John Wiley and Sons. Malloy, R.A. 2004. Plastic Part Design for Injection Molding. New York, NY: Hanser Publishers. Sclater, N. 2007. Mechanisms and Mechanical Devices Source book, 4th Edition. London, UK: McGraw-Hill. Todd, R.H, Allen, D.K. 1994. Manufacturing Processes Reference Guide. New York: Industrial Press, Inc. Read More