MOLDING OF PLASTICS BY EXTRUSION

the barrels of extrusion with zone of channeled feeding they offer, with many resins, productivities that are between 20 and 40% more discharges, by each revolution, that those that can be obtained in a barrel of the same diameter but with smooth inner surface. A section of channeled feeding improves the transport of solids and increases the function of pump of the extrusores, increasing the productivity to a rate of given speed, in rpm, and reducing the amount of applied energy to polymer; consequently, the temperature of processing of the resin is also reduced. One has seen that the action of the channeled section can be neutralized warming up the channels so that they act with the versatility of a smooth wall. The section of feeding of a extrusor of 3,5 inches of diameter can contain from eight to 18 channels distributed uniformly around the barrel in the zone of feeding. In general, the resins of greater viscosity, as the Hmw-hdpe or polypropylene benefits when the number of channels is greater. When the resin has a low viscosity, he is preferable to count on a reduced number of channels. If too many channels are used, the resin can be fed a so high speed towards the zone of compression that can be presented/displayed inefficiencies in the process of smelting of the resin and problems of mixed of the same one. Typically, the channels have a wide one of 0.15 to 0.3 inches and its depth increases towards the part of back of the zone of feeding. In a beginning the depth of the channels is of 0.12 to 0.37 inches and they incline until arriving a depth from zero in a distance of three at four diameters after to have passed waters down through the mouth of feeding of the extrusora. The channels make parallelly to the axis of the screws. The feeding sections are designed with water jackets of cooling to provide an intense water flow around the feeding opening and it extends towards the channeled section. This comes up that the resin is based on the channels. On the contrary, to reduce the action of force feed of the channels and to diminish the function of pumping of the extrusor for materials like nylon or polycarbonate, heat (of 90 to 150 Cs) in the channels can be applied. This initiates the smelting of the resin and avoids a dangerous increase of the pressure towards the end of the channels. Selection of screws to operate with the channeled zones. The long channels always become hardened by means of nitriding of the barrel or, if more abrasive resins are due to process, covering them with tungsten carbide. The sections of shorter channels normally do not become hardened. The screws become hardened frequently with the same material used in the channels to have compatibility and wears away homogenous. The channeled sleeve is not common changing to process different types from resins. The sleeve is replaced solely due to the wearing down, for example, every five or ten years. Of course, the screw can be changed to manipulate different types from resins with the same system channeled in the zone of feeding. The escogencia of the hardening material not only depends on the abrasividad of the polymer that is processed, but also of the depth of the channels of the screw at the beginning of the zone of feeding. The screws for barrels with channeled zones of feeding of today relatively little have deep channels in the zone of feeding, to provide more efficiency in the feeding. In a screw of 3,5 inches, the channels with a section of channeled feeding, have a depth that varies between 0.25 and 0.4 inches, half of the depth of the channels of a screw for barrels of smooth wall (0.6 to 0.7 inches). The screws of less deep channels for barrels channeled in the zone of feeding forwards maintain a height of solid resin bed and a high force of advance low. This effect elevates the pressure at a level sufficient to reach the necessary compression in the zone of heating of the extrusor. The very small extrusores (of one inch and minors) frequently require the aid of channels in the zone of solid transports because the size of grains great is compared with the depth of the screws. The mechanical resistance of the section of channeled feeding must be the sufficiently high thing to handle high pressures in the startings or with certain polymers. The pressures at the end of the channeled sections can get to be of the order of 15,000 to 20,000 psi. In such a way that the equipment must be designed to support pressures until of 30,000 psi, although the present designs tend to maintain the pressure below 10,000 psi. The extrusores with zone of channeled feeding offer a greater productivity by rpm and, therefore, they require a box of gears of reduction greater than the extrusores of smooth wall, of similar diameter. Also they consume more torque in the startings. The pressure at the end of the section of channeled feeding is frequently greater than the one than it is had in the dice; therefore, the performance of the screw is not sensible to the variations of pressure in the dice, in the same way that is it in extrusores with smooth barrel. The extrusores of smooth wall operate with pressures in the dice from 3,000 to 4,000 psi and in them the speed of rotation of the screws is due to elevate to maintain the same rate of production that they have when the pressure of the dice is smaller. The energy to pressurize the fused resin, added to the energy due to the greater speed of the screw in a barrel of smooth wall, makes difficult the process in the measurement in which the temperature and the pressure of the fused resin rise. Resins adapted for extrusores of zone of channeled feeding the poliolefinas are the polymers that more success they have had in the processes of extrusion with channeled and cooled zones of feeding. The processors that have the greater opportunity to benefit from the extrusores with zone of channeled feeding are those that handle one or two resins with high rates of productivity. Examples of this are the lines operating with polyethylenes and polypropylenes of the high viscosity for the manufacture of pipe, laminae, blown film and blow moulding. The materials with high points of smelting and greater cristalinidad are not processed with as much facility in the extrusores with channeled zones of feeding. Their compresibilidades and their characteristics of smelting are different from the found ones in the traditional poliolefinas. In order to process them in conditions of security, the efficiency of the zone of feeding is due to reduce, to protect the barrel and the channeled section of the so high increases of the pressure. The extrusores with channeled zones of feeding have a better performance if the resin cools off. When colors in form are added of concentrated is necessity to obtain a greater mixed level of in the screw and therefore it is due to resort to greater temperatures of operation, which goes in contravía of the benefits that can be obtained of the channeled zones of feeding. The screws of smooth barrel offer a better performance with resins of high point of fusion like nylon, PET, polycarbonate and fluoropolímeros. If these materials must process be in a machine with channels in the zone of feeding of the barrel, the channels must be warmed up. This reduces the efficiency in the feeding because it allows a degree of smelting of the resin in such. Also, the screws with deeper channels in the zone of feeding can be used to reduce the tendency to overfeed the extrusor. The extrusores with venteo are not compatible with the channeled zones of feeding in the barrel because the productivity in this case usually depends on the pumping capacity of the second stage. One first very efficient stage can take to flood the venteo or to cause a poor smelting of the resin, to the height of the venteo. The resin that arrives at the height of the venteo must be completely fused to allow the evacuation (usually by emptiness) of air, volatile humidity and resins. In spite of the potential advantages of the extrusion with zone of feeding channeled in the barrel, most of the processors in North America they seriously do not take it like an alternative for the extrusion with barrel from smooth wall, that already have used during years. Nevertheless, a careful revision of resins, their conditions of process, and the procedures of operation, can reveal opportunities to improve the productivity and the quality of products, using the channels in the zone of feeding of the barrels. MOLDED OF PLASTICO BY TRANSFERENCE In this process, two different raw materials are injected consecutively in cavities different from molds, through separated fuzes, to produce a molded part individual. In first term, in an appropriate cavity the first raw material is injected, soon this one is transferred to another cavity, where the second raw material is injected. Fulfilled this last step, the finished product of the mold is evacuated. In these special processes it is necessary to obtain a narrow coordination between the machine and the mold, through the control system. The mold and its mechanisms determine the size of the machine for the process. If a transference mechanism exists it must be specified if a rotation in the same mold must become or if of a robotizado mechanism or a movable system the function takes control of transference. Another possibility is the one to use a rotatory table with several molds incorporated in her. If the mold must rotate in the machine, the diagonal of the mold must be smaller than the distance between the two diagonals formed between the bars of union of the machine. If a rotational table is used it must increase the height of the mold. In any case special attention to the design of the ejection system is due to render of part molded them. It is absolutely necessary that the ejector is connected with the mold to be used. The extractors of the molded centers and the program of the drive sequence are points to consider itself. The adjustment of the injection units, on the other hand, is determined by each application in concrete. Figure 5 Process of sobremoulding of Ferromatik Milacron and Gram Technology, with piled up mold and revolving central plate In the process, the unit of injection 1, (figure 5), injects a first material in the cavity conformed by the plates central and the fixed plate of the injection side. Immediately afterwards, the mold is opened and the central plate slides towards the center to turn, for example, 180°. The face that still received and maintains in its interior the first material watches the later plate now and to her it conforms the second cavity which the second originating material of the second unit of injection enters. Simultaneously, it is happening the filling of the first material in the opposed face of the central plate. When the mold is opened to allow the turn of the central plate, it happens the ejection of the piece finished with the second material. In each opening of the mold it happens the ejection of a finished piece. Whenever the mold is closed happens the simultaneous filling of both material in the opposed faces, respectively. Between the most important benefits of the technology it is in the first place the fact that the revolving molds can be installed in the multicomponent machines standard. The machine can operate a balanced process of injection, comparable to which it is had in the traditional injection of a component. Another additional advantage is derived more from the reduced time of injection, that takes to the duplication of the productivity of the machine, with a minimum of maintenance. The revolving central plate of the mold can contain two or more sections, doing than the route distance of the lateral plates always are smaller. The material that can be considered power station injects with the main extrusora of the inyectora machine. The second material is plasticized in one second extrusora, that normally is added to first. Because the entry point of the second material to the molds can change of location of mold mold, it is necessary to provide with a comfort of movements for the location of this second extrusor. They exist, therefore, several configurations of machines that contribute particular solutions for positioning of the second unit of injection: On guard vertical; On guard in "L"; fixed in parallel next to first; and fixed in an angle with respect to first. Figure 1 Figure 2 In fig 1 is the Unit of vertical injection that can be slid horizontally to locate the injection on the second material, on the injection mold. In fig 2 is the second unit of injection placed in "horizontal L" with respect to the first unit. The system of sliding for location in any lateral point of the injection mold can be observed. In figure 1, it appears a unit of injection placed On guard vertical, in which is the mechanism of sliding of the unit. Due to this mechanism, the unit can be located in any point on the mold to carry out the injection of the second component. This it is a modular design since the second unit of injection and its corresponding hydraulic system can be added to a machine of standard injection, particularly in those with force in the press of until 2000 kN. The vertical position of the second unit of injection allows to have a freer access to the mold in the machine, for example, for effects of changes. Figure 2, sample that the second unit of injection is placed On guard of "horizontal L". In this case, the second unit can be slid to locate the point of injection of the second material in a lateral point of the mold. This adjustment is preferred in the machines of more than 2000 kN since he is not advisable to place very heavy units in the vertical position, on the one hand, and because the molds of greater size occupy a greater height. MOLDED PLASTICO BY INJECTION the idea of the moulding of several materials to form a unique product it must need in detail so that it can receive an appropriate denomination. This, because several technological possibilities exist to make a product molded by injection with several materials, that nowadays are considered excluding you enter yes. In general terms the processes that use multiple fuzes of injection or stations different from moulding and the processes exist that use a single fuze of injection with a single station of moulding. To the first class the processes of moulding with transference of mold or sobreinjection correspond. To the second type of process the coinjection and the injection belong type "sándwich". Figure 3 Figure 4 fig 3 shows the second unit of injection is located diagonally on the first unit. Fig, 4 shows the schematic drawing of an ejection system for molds with filling in two halves. The mold in this case broken within the machine. In figure 3, the direction of the second unit of injection acquires knowledge On guard diagonal with respect to first. As one is the injection of two materials in different cavities each, both extrusores they feed different, separated fuzes to each other approximately 55 mm This way, both material fused are controlled independently. Figure 4 is a diagram that shows the ejection mechanisms for the two cavities of a mold that broken within the injection machine. The ejectors can control one of another one independently. This variant offers a maximum of flexibility for the implementation of an ample variety of designs of molds that consist of two halves, like which they are used for the combination of thermoplastic materials with elastomers of cured, which must have a thermal separation between the two cavities. Processes of molded associated to the coinjection the coinjection process, by definition, are the injection of the materials fused through a unique fuze to form a product in a cavity, also unique, ediante which the injection, for example, of a central material and a second material that does the times of skin can make through a fuze. The construction of this fuze allows the closed opening and independent of the individual components. The fuze places both material the individual phases of injecting according to the description presented/displayed in figure 6. In phase 1, the needle of the fuze closes the passage of the material B, that is the power station. The entrance of the material To, that it conforms the skin of the product, stays open and flows within the mold. In a subsequent phase, the needle is opened to let simultaneously pass the central material to the cavity of the mold. This adjustment eliminates the formation of flow marks on the surface of the product, particularly in products of intrincate forms. In the third phase the passage of the material is closed To and flows solely the central material in the mold. This type of fuze allows to assure the application a pressure of support in the end of the cycle of injection or with the material of the skin, To, or with the power station, B. the machine of so large minor in which this 600 process can be applied is one of kN in the press. Phase 1 Phase 2 Phase 3 Figure 6 Injection of multiple materials through a unique fuze. Phase 1: Injection of the skin through the fuze opened for the material To the fuze for the material B is closed by means of a needle. Phase 2: Simultaneous filling of the skin and the central material. The fuzes To and B are open. Phase 3: The injection of the material of the Injection skin is suspended. Closed fuze To. Open fuze B. The coinjection offers because the possibility of contributing combinations of properties in a same product, from the functional point of view, aesthetic, ergonomic, and to reduce the operations of joint and finished in end items. On the other hand, when the central materials are abrasive because they contain fillings or reinforcements, can be used an external skin that encapsulates the central material and avoids the contact with the surface of the mold. Reports of substantial savings in the maintenance of molds due to the use of the coinjection with this intention have appeared. MOLDED OF PLASTICO BY COMPRESSION Or SANDWICH the injection in sándwich is a variation of the coinjection process as it is possible to be observed when the process described in figure 6 is compared, already mentioned, with the sketch of the process in sándwich of figure 7. Figure 7 Process of coinjection in sándwich. The skin is introduced in the storage cell of the main extrusor (material power station), before the moulding by injection of the product. Both material fused they are accumulated of a consecutive way in a common cylinder and soon they are injected in a single passage of injection inside the cavity of the mold. Due to the action of push of the central material that it forwards pushes the material that conforms the product skin, automatically form the layers of external skin and central material of a way put in in the mold. The central material is plasticized in the main extrusor of the inyectora machine and the material that conforms the skin is plasticized in an auxiliary extrusor that is added to the machine. The fused material of the skin introduces in the barrel of the main extrusor of the machine doing use of a system of volumetric metering or with base in the control of the pressure of the resin. Normally, the introduction of this material in the barrel takes control of little resistance of the pressure of the central material. This is the only modification that becomes with respect to the process of traditional moulding and for that reason it is so simple to apply.