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ASIMPLE MOULDING MACHINE
Asimple small moulding machine is used to force a heated and highlyplastic state polymer under high pressure to flow into a mold cavitywhere it solidifies to form a molding that is then removed from thecavity. The machine may have a mold containing more than one cavityto produce multiple moldings for each cycle or shot. The equipmentfor this simple small moulding machine emanated from metal castingand involves the application of pneumatic linear actuators which ismainly pneumatic cylinders (Budynas,Nisbett and Shigley, 2011).These forms part of fluid power actuators that receive fluid frompumping mechanism after that controlling the pressure, the flow andthe direction to convert the fluid energy into a reciprocating linearmotion to perform useful work.
Figure1: A Simple Moulding Machine
Thepneumatic cylinders are used in this small moulding machine with theadvantages of them being quieter, cleaner and require just a smallspace for fluid storage. These cylinders are applied in this mouldingin three stages as discussed below.
Thepneumatic linear actuator which is a pneumatic cylinder is used inmoving the barrel forward against the mould and back away from themould. This is typically the plastic injection mechanism of themolding process which is much like an extruder. It is made up of abarrel which is fed from one of its ends by a hopper that contains asupply of plastic pallets that forms the material for moulding in thecavity (Budynas,Nisbett and Shigley, 2011).The operation of this barrel is that at the time the mold is closedthe barrel is in a position to move both forward and back.Consequently, when the barrel is forward, the injector either movesforward and then back. Then for the operation to be complete, theinjector must always be back before the barrel can then move. Thebarrel also must be back for the mould to be again opened and closed.
Atiming diagram refers to a diagram applied to explore thecharacteristics of the object when exhibiting a certain rhythm. It isan interaction diagram that focuses on the time constraints in itsdisplay (Doolittle,2012).The three actuators of the simple moulding machine can, therefore,represented using the timing diagram to investigate their timeconstraint in relation to its motions while in operation. Here thethree components are assigned two behaviors of a timing diagram,which then indicates its motion constraints.
Themould actuator, the barrel assembly and the injector all representthe three components to be represented by the timing diagram. Theforward motion will always be represented by a high on the timingdiagram, the backward motion of each of the three representing a lowon the timing diagram (Dornfeldand Linke, 2012).The highs and lows of each of the three components will then be ableto tell the motion the will depend on another in either forward orbackward aspect for it to be executed and, therefore, depicting aconstraint and relation of the motions of the three components tofunction appropriately. The initial closure is taken to representback, and the opening is taken to mean forward (Jones,2009).Therefore, as the mould is closed it is on a low while the barrelbeing able to move forward and back means it can exhibit both highsand lows when the mould is on a low.
Whenthe barrel is forward on a high, the injector, it also can be movedboth forward and back meaning it can exhibit the high and the lowwhile the barrel is on the high. This is then the endpoint of asingle shot of moulding. At this point then, the components have toget back to their original position for the next round of the shotfor moulding of the machine. Therefore, the components need to followtheir immediate path for the motion of each element to be executedback. This on the timing will mean that the injector has to be at alow for the for the barrel to either exhibit a high or low indicatingthe injector has to move back for the barrel to have any motion. Theconsequently the barrel must be back before the mould closes or opensmeaning on the timing diagram that for the mould to exhibit either ahigh or a low the barrel has to be at a low. As the mould attains ahigh indicated by the opening of the mould, then the mold part isejected from the mold cavity (Lindsay,2012).This marks the end of a cycle of the machine moulding process. Thistiming diagram can, therefore, be represented as shown below.
Figure2.Error!Use the Home tab to apply 0 to the text that you want to appearhere..2:Timing diagrams (Nee,Song and Ong, 2013)
Thetiming diagram indicates that retraction process of the precedingpneumatic cylinders always takes longer as compared to thecompression and the retraction of the succeeding pneumatic component.This shows that the retraction often sucks air for the forward andbackward motion of the pneumatic cylinders ahead (Pham,Eldukhri and Soroka, 2008).The timing diagram above proves a sequence and order with which thethree pneumatic linear actuators of the machine operate to accomplisha given cycle of moulding and retract back for another cycle.
Itcan be deduced that the actuators are designed in such a way thateach actuators motion is dependent on one another both for the cycleand the retraction. During the cycle when the mould is closed, thebarrel can move forward and back (Nee,Song and Ong, 2013).This means the mould will have closed allowing more motion of thebarrel, which imparts its shape on the part being molded by actinglike an extruder.
Thetiming diagram also depicts the motion of the barrel being forwardindicated by a high to allow forward and backward motion of theinjector. This is to ensure that the molten polymer is continuouslyfed into the mold cavity to give out a superior molding.
Thetiming diagram also elaborates the aspect of retraction inconstraining the motion of the actuators in that the injector need tobe at a low for the barrel to either exhibit a low or a high. This,in essence, explains the motion constraint of the actuators on theoperation of the simple moulding machine. The injector has to getback before the barrel can carry on to avoid part distortion whenbeing squeezed by the barrel.
Onthe part of the mould the barrel has to get back before the mould canopen. This ensures that the part can be removed from the mould andwithout distortion. This therefore on the timing diagram is indicatedby the low for the barrel and the mould is free to exhibit the lowand high.
Figure.3.1:Pneumatic circuit diagram (Lindsay,2012)
Compressedair often powers the Pneumatic devices and for the case of this smallmoulding machine between 15 to 20 psig is ideal. This compressed air,which is due to the pressure from an external source that moves thepiston inside the cylinder, acts as the input to the control systemwhich brings about the corrective action to regulate the working ofthe actuators (Nee,Song and Ong, 2013).The eventual output of the control system for the machine is workdone by the actuator which is the movement of the piston inside thecylinder. Thus, the output of the control system is the distancethrough which the piston moves.
Thepneumatic linear actuator which is a pneumatic cylinder is used inmoving the barrel forward against the mould and back away from themould. This is typically the plastic injection mechanism of themolding process which is much like an extruder. It is made up of abarrel which is fed from one of its ends by a hopper that contains asupply of plastic pallets that forms the material for moulding in thecavity (Lindsay,2012).The moulding machine can have more than one barrel thereby implyingit can have two pneumatic actuators that serve as injection units.The one with two barrels serve two different purposes in the moldingprocess. One barrel is utilized in the plasticizing the polymerwhereas the other barrel serves in the injecting of the polymer inthe mold cavity to form the molding. Therefore, this moulding machineis referred to as the two-stage machine.
Inthe barrel there is a screw which operates faster than the extruderin respect to the following functions: turning for mixing the polymerand heating the polymer and also acting as a ram which swiftly andquickly moves in order to inject molten plastic into the mold at ahigh pressure for the formation of the mold in the cavity. Theprocess is aided by the existence of the no return valve that ismounted just near to the tip of the screw whose function is toprevent the molten polymer from flowing back to the screw threads.The when a single cycle is complete the ram will then retract back toits original position.
Theother part of the part of the equipment for the simple small mouldingmachine is the pneumatic cylinder that is also a pneumatic linearactuator in nature whose function is the opening and the closing ofthe mould to enable the operation of the moulding machine (Jones,2009).This is achieved by the clamping unit, which is pneumatic in itsoperation in that the two cylinders are directed towards the twohalves of the mould. The clamping unit also functions to keep andmaintain the mould closed during the injection process through theapplication of clamping force hence a pressure sufficient to resistthe pressure from the injecting force.
Theclamping unit additionally functions to facilitate the opening andthe closing of the mold at the suitable or appropriate timesdepending on the molding cycle. The clamping mechanism consists oftwo platens. One is a fixed platen, and another is a movable platen.
Alongsidethe two platens, there exists a mechanism, which facilitates thetranslational motion of the movable platen during the closing and theopening of the clamping unit (Jones,2009).This mechanism is typically a power press pneumatically operated suchthat a pneumatic piston or a mechanical toggle device is applied ininside a cylinder (actuator) to accomplish the task of the clampingmechanism.
Theactuator moves the crosshead forward, thereby extending the togglelinks. This then pushes the movable platen towards the closedposition of the mold. Initially, the mechanical advantage of theactuator is low whereas the speed is high. But towards the end of theactuator stroke, the mechanical advantage is high as the speeddiminishes.
Theinjection tool can be different types depending on the type ofmolding or shape being produced by the moulding machine. It is pistonlike that is built into the cylinder and employs the workingprinciple of the pneumatic cylinder. One of a kind is theconventional two plate mold that is made up of two halves fastened tothe two platens of the moulding machines clamping unit. The momentthe clamping unit is opened, the two halves open. The cavity oftenmade by metal removal from the mating surfaces of the two halves. Thetwo plate mold has a side gate which facilitates the flow of thepolymer melt around the core and is joined on the opposite side whichmost likely results in the creation of the weak line at the junction.The mold also has a distribution channel. This channel is made up ofthe sprue, the runners and the grates. This distribution channelserves to allow the molten polymer to flow from the nozzle of theinjection barrel into the mold cavity (Dornfeldand Linke, 2012).The speed control on linear actuators can be achieved by the use of aseries of adjustable stops on each of the strokes made by the piston.The piston may drive a valve directly or be used to operate a damperby using a linkage. Alternatively, a spring may be used to act as adamper.
Budynas,R., Nisbett, J. and Shigley, J. (2011). Shigley`sMechanical Engineering design.New York: McGraw-Hill.
Doolittle,J. (2012). Mechanicalengineering laboratory.5th ed. New York, etc.: McGraw-Hill.
Dornfeld,D. and Linke, B. (2012). Leveragingtechnology to a sustainable world.Heidelberg: Springer.
Jones,P. (2009). Budgeting,costing and estimating for the injection moulding industry.Shrewsbury: ISmithers.
Lindsay,J. (2012). PracticalGuide to Rubber Injection Moulding.Shrewsbury: ISmithers Rapra Pub.
Nee,A., Song, B. and Ong, S. (2013). Leveragingtechnology to a sustainable world.Heidelberg: Springer.
Pham,D., Eldukhri, E. and Soroka, A. (2008). Innovativeproduction machines and systems.Dunbeath: Whittles.