10.0 Cycle Diagram
10.1 Introduction:
A mould cycle for a typical screw injection machine is shown in the figure. Mould cycle is made up of a number of elements, any one of which can be shortened or accelerated, thus condensing the entire cycle.
It shows the machine condition needed for production of satisfactory parts in minimum time
10.2 Preparation of Mould Diagram:
The mould diagram shows the machine conditions needed for the production of satisfactory parts in minimum time. Maximum and minimum presses for various cylinder temperatures are determined. The mould temperature and the other elements of the mould cycle are kept constant. Starting at allow temperature and low pressure a series of short shots will be obtained. By increasing the pressure, several satisfactory shots will be produced. At some point as the pressure continued to be increased for each shot, the mould will begin to flash or stick. This then becomes one point on the flash or stick line.
A second series of points are generated at a higher temperature. This procedure is continued with successive temperature increase until a temperature is reached at which polymer deterioration starts. By connecting all the last short shot points, a curve is drawn which indicates the minimum pressure temperature combination at which satisfactory parts can be obtained. The curve connecting all the flash stick lines indicates the maximum pressure temperature combination which can be used to produce satisfactory parts. The area between the two curves is the mould area. Theoretically combination of pressure or temperature within this area should produce satisfactory components. But in actual conditions, some of the filled parts may show streaking, bubbles or other defects. If a record is kept of the conditions of moulding each part, it is found that some defects are consistently present when parts moulded in the particular area of the diagram. Other areas will produce consistently good parts. The optimum area will be the one characterized by the highest temperature and pressure necessary to produce parts which are completely satisfactory. The use of highest temperatures and pressures will result in a minimum fill time. The object of determining the minimum cycle time is to produce parts with maximum efficiency. The mould diagram will establish the optimum pressure temperature combinations.
To determine the minimum cycle time, the process is started at a given mould temperature with the highest pressure and cylinder temperature which will give satisfactory parts. Series of shots (at a constant pressure and temperature) with decreasing ram forward line is tried. Below a certain point, no satisfactory parts can be made. During this test, the cooling tome should remain constant. After establishing the minimum ram forward time, the same procedure is repeated by decreasing the cooling rate. The ram forward time is kept at the minimum. This will establish a minimum moulding cycle for any given cylinder and mould temperature. The procedure should be repeated with either higher or lower mould temperature until an optimum is reached.
The establishment of an optimum fills time and a cooling time effects the total time the plastic remains in the cylinder. The net effect should be a lowering of the time in the cylinder so that possibly a higher cylinder temperature may be feasible. Another series of test shots should be made at various increased cylinder temperature establishing a minimum cycle for each cylinder temperature. A graph of the minimum cycle time verses the cylinder temperature is shown in figure. The minimum cycle time diagram will be very useful when establishing conditions for a new mould or new resin.
The mould cycle time can be again reduced by the following procedure
The fill time can be reduced to a minimum by correct choice of gate size and location. The second element, ram forward line depends on the time required for the gate to solidify. This time depends on the parts thickness, the gate size and the runner size. The mould should have the smallest gate possible. The screw retract time should be slightly less than the cooling time. This can be achieved by the increasing the screw speed or to reduce the back pressure. A machine with larger shot capacity can also be tried. The mould open time can be reduced by keeping the minimum distance for the mould to open just enough so that the parts can be ejected or removed from the core.
The two disadvantages of reducing the mould cycle time is
1) The total shrinkage of the part will be more if a longer cycle is used.
2) The component will have higher moulding stresses.
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