Plastic product business is becoming competitive as cheaper cost Asian countries including China continue to increase their share in the global business. To survive the competition, it is essential that the injection molder optimize the injection moulding process and enhance productivity, reduce wasteful operations and save time.
Any mold has an optimum filling time. The optimum filling speed is close to that which requires the least pressure. However, faster filling speeds are often compromised by problems such as gassing. The burning of the melt front due to entrapment of cavity air takes place. Vents should be cut where the gassing occurs. If vents do exist, and they are properly sized and clean, then reducing the locking force can help with venting and enable faster injection time. But the process should not be compromised because of the poor venting - the venting should be properly engineered to do the job required. Optimizing filling time; Probably 95% of all moulds should have a filling time between 1.5 and 3 seconds. If a mould takes longer than 3 seconds to fill, it should be investigated. Small parts with short flow lengths will fill quicker but only very thick parts or those with flow lengths more than 500 mm will need to be filled for more than 3 seconds. Optimizing filling may only make a small time saving, but getting the filling time set at the optimum will enable savings to be made elsewhere. The packing or holding time is a phase in which semi-molten mass in the filled mould is pressurized while it cools to allow additional melt into the mold to compensate for the shrinkage that occurs. With crystalline polymers such as Polypropylene, this can be significant as volume shrinkage is high. The packing phase reduces sink marks, improves replication of the mould finish, consolidates welds and controls gloss level. The gate must be sized correctly to remain open long enough to enable a correct packing. A gate freeze check will determine the exact time available for packing.
The cooling time for the part is determined by the wall thickness and the polymer type and is easily looked up from tables or determined accurately. The packing time is about 80% of the cooling time which itself is often less than that which is set. Cooling rate is an exponential relationship between polymer melt temperature and time. The rate drops continuously. Raising the temperature at which a moulding can be ejected reduces the cooling time by a disproportionately large amount.
Investigation will often find that the cooling time set on the moulding machine was established by experimentation and was the time that either the part did not distort, or it was the minimum that prevented ejectors digging in or some other reason that was more related to issues with the mold than related to process physics. Thereafter the cycle time is constrained by a tooling fault - which if fixed would reduce the cycle time forever.
Poor mold cooling is generally caused due to lack of coolant. It is important to know if the coolant is flowing in every channel. Circuits that are in parallel with others almost always mean that some channels get little coolant. Piping all of the channels in series is often the best solution. It does not cause any hot spots anywhere. The temperature rise in the coolant then becomes a function of coolant flow rate. The time taken to remove the part from the mold is another major area for time wastage. Typically it should not take more than 5 seconds irrespective of the size of molding. The mold should not take more than 8 seconds to open & close.If these times are not achieved then there is insufficient knowledge on injection molding process & optimization of cycle time. All parts of the mold movement should be accelerated and decelerated under control to minimize conditions & optimize time. The effort to optimize the molding cycle is rewarded with substantial cost reductions and greater productivity. The majority of production is far from optimum and the productivity of most molds can be improved by 10-30% without any capital expenditure.