Optimising time cycle and machine productive time
Whatever ideas for cost saving are introduced into the injection moulding of plastic components，ultimately the time cycle of the machine and the rate at which it can be made to produce good saleable mouldings are the criteria for profitability. Unfortunately，the operator or setter who does not fully understand the working of the machine will try to snatch the odd second here and there in the time cycle, often reaching a stage where either faulty mouldings are being produced, or where，in multi-impression work，one or two impressions are not operating properly.
The continued production of faulty mouldings on，say，one impression of an eight-impression mould can soon reduce profitability to a very low level or even cause the machine to be running at a loss. It is necessary to consider not only the loss of one-eighth of the production，but also the cost of the material employeed，the cost of segregating the bad mouldings，and the cost of re-granulating the scrap material. There is also to be considered the possible wear on the mould if, for example，flashing is occurring，or of causing mould distortion if one cavity is not being filled. This latter practice can put the mould locking out of balance.
There is a strong case for setting a very rigid machine time cycle and some moulding companies do this by having the timers locked after they have been set. This may have disadvantages because，with the slightest variation there is a need for the operator to call the setter (who may be urgently employed elsewhere) to make the adjustment.The use of electronic sensing and feedback equipment may ultimately minimise the need for manual adjustment, but there will probably always be the need for properly trained operators or setters to carry out the ‘fine tuning’ of a machine to obtain optimum output.
The minimum time for each element is fixed either by the capabilities of the machine or by the properties of the material being moulded. ， however，machine functions are falling between these main elements which can be a cause of loss of time.
Consider，first，the locking of the mould ready for injection. This must be carried out swiftly but gently. The speed is controlled by a flow control valve in the hydraulic fluid line and the mould protection by a microswitch which only allows the high pressure for locking to be applied when the mould parts are virtually in contact. If the settings are not made accurately, the mould may take longer to close than necessary and valuable time is lost.
The next phase is injection. The minimum time for injection is fixed by the rate at which the hydraulic pump will supply fluid or by the rate dictated by the nature of the material being moulded and the configuration of the mould. But injection cannot take place until the cylinder nozzle is in contact with the mould sprue bush.
If the machine is designed so that the closure of the mould gives the signal for the cylinder carriage to move forward，time can be lost if the carriage moves slowly and over greater distance than is necessary. (Some machines have very slow-acting hydraulic cylinders to move the carriage, the object being to obviate damage to the mould sprue bush and nozzle.) Slight time delays occur at each stage of machine operation, but the good setter will anticipate such delays，caused by the inertia of the solenoid valves in the hydraulic system，and will carry out the fine setting accordingly.
If the machine is operating with sprue break, that is，the withdrawal of the nozzle from the mould sprue bush after each shot, the minimum of carriage movement should be allowed，so as to reduce the loss of time before injection can take place. The actual time during which sprue break occurs overlaps either cooling or the start of mould opening.
Unless a long plunger dwells time is required，as, for example, in the flow moulding of thick sections in some materials or the moulding of thick sections in nylons and acrylics, screw-back or plasticizing should start immediately the screw/plunger reaches the end of its forward stroke. Especially with high¬speed injection through a properly designed gate，there is little point in maintaining a long plunger dwell because the gate freezes immediately injection stops. The screw-back time should not be longer than the additional cooling time required and will rarely be so in normal moulding practice. At the end of screw-back or cooling，whichever is the later，mould opening will be signalled
The same criteria apply to mould opening as to mould closing，the mass of metal being controlled in its movement to minimise damage to the machine. During mould opening, ejection of the moulded parts takes place. When using a mechanical system, as the mould opens , the ejector bar comes up against a stop or pin fixed to the machine body and the ejector mechanism is actuated. Too rapid a platen movement at this stage can cause bad jolting of the machine，undue wear on the ejector system，and the ultimate breakdown of the machine or mould. Fine setting of the actuating gear is necessary to achieve speed with minimum strain. One advantage of hydraulic or pneumatic ejection systems is that they can be set to operate with very little jolting and can be timed so that ejector movement is complete and the moulding ready to drop from the mould at the instant that the mould becomes fully opened.
Before the commencement of the mould closing operation it is necessary to allow time for the mouldings to fall clear of the mould. On all machines operating fully automatically a time delay switch is fitted to give a suitable ‘pause’ before the signal to re-start the cycle is given- Frequently, the pause timer or interval timer is set to allow more time than is necessary.