Q1. Analyzewith the use of a system diagram the process.

Themolding machine has three pneumatic linear actuators, meaning all themovements associated with the functioning of the machine are in astraight line. All these movements are limited to forward andbackward directions only.

Theprocess constraints are:

  • When the mould is closed the barrel may move forward and back

  • when the barrel is forward the injector may be moved forward and back

  • the injector must be back before the barrel can move

  • the barrel must be back for the mould to open and close

  • When the mould is opened the part can be removed

  • Both the time the mould is closed and the time injecting linear actuator is forward should be adjustable.

Systemdiagram of the process

Thebarrel communicates with the mould to execute its functions. Uponreceiving communication from the mould, it can then move eitherforward or backward. The forward position of the barrel relaysinformation to the injector that is then at liberty to move forwardor backwards with the aid of the injector linear actuator. Theinjector moves forward and backward depending on the requirements ofthe process.

Theholding clamp is excited when there is a looming activity. Theactuator holds one side of the clamp tightly in position and preparesfor the locking side to move in position. The locking of the moldingsurface is facilitated by the barrel linear actuator. Once the barrelis in position, the locking is achieved and the injector can movefreely in a linear orientation to supply polymer whenever it isrequired.

Oncethe molding process is complete, the injector is released first. Thecircuit is designed in such a way that whatever came last in theinitial creation sequence has to leave first to allow for the reverseprocess to take place effectively. The injecting set-up paves the wayfor the barrel to allow for the unlocking of the molding section.Once the barrel opens up, the clamp loosens and allow the finalproduct to be retrieved from the set-up.

Thedesign operates with a closed looped control structure. The closedloop does not allow for the success of the proceeding process shouldthe preceding one fail. The arrangement is efficient in materialconservation since failure in one part of the system will result intoa halt of the entire operation.

Q2.Definethe inputs and outputs required for a control system.

Thegeneral structure of the control system is as shown below:


  • The electrical signal to the mould clamp actuator, Barrel movement linear actuator, Injection linear actuator: There must be an incoming signal to facilitate the operation of the mild machine. The electrical signal from the processor facilitates the closure of the mold. This is then followed by movement of the barrel which facilitates the injection of materials into the mold for designing the end material which is the desired output.

  • Pneumatic signal to the injector: The injector is operated by means of pressure to enable the supply of the molding material into the system. As such, there must be a pneumatic signal to facilitate the process. Actuators will move the required parts into position but cannot make the injection process successful for efficient molding to take place.

  • The molding raw material being supplied by the injector: The injector’s function is to provide the raw material (polymer) which is the main input to the system.


Theoutputs to the system is the shaped polymer according torequirements. The system depends on a number of outputs top deliver afinal product which is a well-structured molding from the system.

Q3.Developa suitable PLC or other electrical control circuit to control theabove with where all end conditions are fed back using limit switchesand the inject forward and mould close has a timer control.

Thestart button is connected back to the PLC through a stop button. Allthe information that leaves the PLC to the system are relayed backthrough a switching circuit. The overall operation is thereforecontrolled by a closed loop system. Failure in one sensor or processwill bring a halt to the operation of the entire system. The controlcircuit is a logical circuit and the PLC ensures that all operationsfollow a strategic plan with every process following the desiredflow. The system demands that the molding surface must be lockedbefore injection. The sensors in the system are responsible forensuring that all conditions are met before operations can proceed.

Sensorone (1) detects the situation of the mould whether open or closed andfacilitates the required operation. If it is open and there is a newmolding activity required to take place, it initiates closure.Otherwise if the work is done, it receives signal from the PLC toopen the mould and enable retrieval of the finished product. Sensortwo cannot call for closure of the locking side controlled by thebarrel unless there is communication from the actuator controllingthe clamp. Should sensor one (1) fail, there will be no operations inthe proceeding processes. Once sensor two has detected the requiredoperation and relayed the information to the holding clamp, sensortwo can then communicate with both the barrel actuator and the clampactuator and call for an action. Actions at this juncture can beeither closure by the barrel to create a new mold or open the set-upto retrieve the final product.


Incase of a new molding process that is just commencing, the barrel hasto close the set-up and call for the activities of the injectingapparatus. Sensor two (2) handles the barrel and its position. Thesensor is directly linked to the activities of the injector. Thesensor dictates all the operations of the movement of barrel movementlinear actuator. According to the constraints provided for thedesign, the injector can only move with the barrel in forwardposition. Once sensor three confirms that the barrel and the lockingsurfaces are in position, the injection of the polymer can then takeplace. The injection proceeds until the required quantity isachieved. After every injection, the actuator controlling theinjector provides space for the barrel to adjust its position inaccordance with the amount of materials injected and the model of themould being developed. The process continues until the desired resultis achieved. The molding surface locks with the help of the clampactuator and barrel movement actuator. This is followed by polymerinjection and reverse barrel movement. The achievement of the modelstructure being developed may then call for case two (2).


Retractionof the holding clamp relays information to sensor two, requiring itto move backward. The PLC will make this possible by sending a signalto the injector actuator to pull back. The reverse position of theinjecting arrangement allows the barrel to move backwards. Thebackward movement opens up the molding surface allowing foracquisition of the final product.


Allthe devices controlling the operation of the molding set-up emanatesfrom the PLC and are connected back via a switching circuit to theirpoint of origin. The purpose of the closed-loop system is to ensurethat there is no process that is proceeding contrary to the logicalarrangement designed for the set-up. If any abnormalities occurduring one of the processes involved in the cycle, the PLC detects itthrough the closed loop and puts a stop to the operation until theproblem is rectified. The action of the closed loop arrangementprovides an assurance for quality production and reduction of wastageof raw materials in a control system.

Q4.Devisea pneumatic circuit to complete the above operation. (Fluidsim may beused for this)

The24v control supply voltage is obtained from the PLC supply. Thiscircuit is a supplement tom the main PLC circuit in Question 4 above.The function of the pneumatic circuit is to ensure that thepressure-related demands of the circuit are fully met. All theactuators are connected to a control lever valve that moves inforward or backward direction depending on the pressure situation onthe actuator contacts.

Q5.Advisehow speed control on all linear actuators would be achieved.

Speedcontrol on the linear actuators can be achieved by varying the supplycurrent from the PLC. Lowering the current will reduce their speedwhile adjusting the current control knob to a higher current willincrease the operating speed. Alternatively, manipulating the supplyvoltage to the pneumatic circuit will help control the speed. Highvoltage leads to an increased air supply to the circuit that leads toa higher pressure. High pressure to the actuator control valves willresult into speed control for the actuators.

However,it is important to note that both methods cannot be applied at thesame time. Direct operation of the actuators via pressure controlledvalves will render electric control of the actuators impossible. Assuch, the best approach would be to determine one effective method toensure quick and accurate response of the actuators. Varying currentsupply to the actuators is therefore the best approach for the speedcontrol.

Speedcontrol is a very delicate issue in such a system and increment inone area should be accompanied by the same adjustment in all theareas. The signal being fed to any one actuator should be followed bya corresponding adjustment to all other actuators in the cycle. Ifthis is not adhered to, the process may not proceed as planned.Taking the electric current approach or the pressure adjustmentapproach should therefore be applied across the board to ensureefficient operations of the system.

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