Assisted Moulding
Gas Injection Moulding Technolo
The characteristic feature of gas injection moulding technology is the filling of a form with to two different materials. Plastic materials form the first component. The second component consists of a gas generally nitrogen .The two components do not mix. All standard injection moulding machines which are equipped with a device for introducing gas are suitable for GIT process. The injection of the gas may be performed by a machine nozzle or by a separate injection module in the mould.
Process Technology
The process sequence begins with the injection of the first component – the plastic. A dose of approximately 70% 20% of the volume of the cavity is proportioned and injected.
Shortly before the conclusion of the injection phase, the gas injection phase begins. The brief overlapping of injection and gas injection phases is intended to prevent a speed break of the melt front and the switch over marking which is related to this. The location for introduction of the gas is best in areas with large melt accumulations. The gas fills the cavity forces the melt forward. Design specific cavities in the moulded part are the result.
Once the cavity is completely filled, the gas – holding pressure phase begins. The pressure phase is applied until the part is dimensionally stable (maximum gas is 400 bar).Gas pressure during this time is constant throughout the entire canal. Because of the relatively low gas pressure, there are correspondingly low interior pressure in the mould, in turn leading to low clomping forces in the injection moulding machine.
After the initial solidification of the melt, the gas pressure is reduced. This takes place either by allowing the gas to escape into the ambient atmosphere, or by recovering a certain proportion (in to 90%) through the machine nozzle or the or the mould nozzle. The process is illustrated in Figure 10.17.
Figure 10.17 The gas injection moulding process
Patent Situation
Development in GIT, has been restricted to some extent by the number of patents that exist to this process. If a process this protected by a patent is employed, expensive licence fees must sometimes be paid. This section is written without regard for the patent situation, which must be clarified if considering adoption of this technology
Advantages and Disadvantages of GIT
Depending on the mould construction, the following advantages con be realized:
-Greater range of configuration option in the design of mould.
-Material savings (generally 20-30%).
-Cycle time reduction for moulded parts with thick walls.
-Increase in mechanical rigidity with equal weight.
-Mould without sink marks.
-More uniform shrinkage, lover residual stresses, significantly less distortion.
-Reduced clamping force.
-Implementation of long flow parts.
-Better surface in comparison to foamed parts.
-Simpler mould construction possible in some cases
-Possibility of integrating thick and thin areas.
The most significant disadvantage of gas injection moulding technology are:
-Additional costs for gas pressure generation equipment and pressure regulator modules, gas, machine nozzles or mould nozzles, and license fees.
-Frequent jetting.
-Empirical derivation of setting parameters.
-Greater weight deviations.
-Hole at the injection of point.
-Strength/tightness at sealing is frequently visible.
-Injector must be cleaned more often in some cases.
Process Variations in the Application of Gas Injection Moulding Technology
Gas injection through the machine nozzle
Here, the introduction of the gas is performed directly through the machine nozzle. The process sequence is:
The hydraulic GIT needle shut-off is open. The plastic material is injected. The material cushion for the sealing process remains in the screw pre-chamber as illustrated in Figure10.18a.
The hydraulic GIT needle shut-off nozzle is closed. Nitrogen is introduced. The melt core is displaced. The pressure holding phase. After the conclusion of the pressure holding phase, the recovery phase of the nitrogen gas begins. There is a brief retraction of the nozzle for pressure reduction. This stage of the process is shown in Figure 10.18.
Finally the hydraulic GIT needle shut-off nozzle is opened is and the material cushion is injected for sealing as shown in Figure 10.18.
Figure 10.18 Gas injection through the machine nozzle
Gas injection through an injector module in the mould
This can be carried out in two ways. The gas can be introduced either through sprue or directly into the moulded part. The hydraulic needle shut-off nozzle is open. The mould material is completely injected. No material cushion remains behind. This stage of the process is shown in Figure10.19.
Figure10.19. Gas injection through an injector module in the mould(1)
The hydraulic needle shut-off is closed. Nitrogen in introduced into the mould through the injector module. The gas displaces the melt core. The screw can proportion doses, since the holding pressure is provided through the gas pressure phase. Upon conclusion of the gas holding phase,the recovert of the nitrogen is activated. This process stage is shown in Figure 10.20.
Figure 10.20. Gas injection through an injector module in the mould(2)
Gas injection in the moulded part
In this process, the introduction of the gas takes place directly into moulded part through an injector module in the mould. The gas introduction point can be at any selected location on moulded part in this process. The Disadvantage here is that the hole that is created in the part cannot be is automatically sealed.
In this process, it is possible to work with the nozzle open at intervals. The mould material is completely injected. No material cushion is left behind.
Figure 10.21 Gas injection in the moulded part.
Nitrogen is introduced in the mould through the injector module. The holding pressure is realised through the gas holding phase. The delayed dosing phase runs until the sprue is sealed. Upon conclusion of the gas holding phase, the recovery of nitrogen is activated. The screw can simultaneously begin dosing again. The process sequence is shown in Figure 10.21
Melt blow moulding technology
Melt blow moulding technology is also divided into two subprocedures, melt back pressure technology and melt extrusion technology in a secondary cavity. The advantage of these processes is the avoidance of switch over markings on the surface of the moulded part.
Melt back pressure technology/gas injection in the moulded part.
Ln melt back pressure technology, the entire moulded part is first with material (Figure 10.22a).Nitrogen is then introduced at the flow part end, which forces the plastic melt back into the plasticizing cylinder (as shown in Figure 10.22b). With the hydraulic needle shut-off nozzle open, the mould material is injected (the moulded part is filled completely). The hydraulic needle shut-off nozzle remains open. The gas is injected. The screw is withdrawn. After the completion of the gas injection the hydraulic needle shut-off nozzle is closed. The gas holding phase and recovery of the gas follow; the screw can simultaneously begin dosing again. The process is illustrated in Figure 10.22.
Melt extrusion technology /gas injection in the moulded part.
In melt extrusion technology, the cavity is first completely filled with material (Figure 10.23a). Concurrent with the start of the gas injection, one or more secondary cavities into which the excess meit may be displaced are opened (Figure 10.23b).
The process is performed with an open nozzle. The material is injected, and the mouled part is filled completely. The second cavity is closed. A hydraulic slide gate opens the secondary cavity. The gas injection begins concurrently and the excess material is forced into the secondary. The gas holding sequence runs completely through. The recovery of the gas follows after the gas holding phase. The screw can simultaneously begin dosing again. The process is shown in Figure 10.23
Figure 10.23 Melt extrusion GIT
Core pull technology
In core pull technology, the cavity is first completely filled and the part is subjected to melt back pressure (FIgura 10.24a). Before or during the introduction of the gas, an additional displaced volume is created in the main cavity by the withdrawal of one more the cores (Figure 10.24b).
In core pull technology, work also proceeds with an open nozzle. The core is inserted. The material is injected, and the moulded part is completely filled. The core is withdrawn from the moulder part hydraulically, this opening into the moulded part occurs simultaneously. The gas holding phase follows. The recovery of the gas follows after the gas holding phase. After the gas is sealed, the screw can begin dosing again. The process is illustrated in Figure 10.24.
Figure 10.24 Core pull technology GIT
Systems Technology for the Implementation of Gas Injection Technology
Gas pressure regulation
Gas pressure regulation consists of the pressure regulation and the control unit. The pressure regulation module reduces the established system pressure to the desired gas pressure. The electrical control of the pressure regulation module is provided either the machine control unit or through an external memory with programmed controls. The witch over from melt injection to gas injection can take place either as a function of time, stroke or pressure. Either the hydraulic pressure of the injection moulding machine or internal pressure in the mould can be used for the pressure dependent impulse.
Two different design concepts exist for the control unit:
(a)Stationary unit: the stationary unit consists of the pressure regulation modules and an electrical control unit which is integrated in the machine controls. This process method is implemented with the control unit. External operating elements ar