Patent Publication Number: US-6666999-B1

Title: Gas assisted moulding

Description:
This application claims the benefit of International Application Number PCT/SE00/02152, which was published in English on May 17, 2001. 
     TECHNICAL FIELD 
     The present invention generally relates to moulding and specifically to a method and an apparatus, respectively, for gas assisted moulding. 
     TECHNICAL BACKGROUND 
     Moulding is a manufacturing technique, wherein a polymer is injected into a moulding tool under pressure and solidifies. The moulding tool is substantially a negative of the detail which is to be manufactured. Conventional injection pressures vary from 5000 to 20000 psi. Since these high pressures occur, the moulding tool, which often consists of two mould halves, must be held in a clamped state during injection and cooling. This clamping force needs to be considerably high. 
     The moulding technique can be used for the manufacturing of a large number of details with a very high precision. Tolerances better than 0.0025 mm are easily achieved by using a suitable combination of mould design, material and detail design. Further, moulding is a high-capacity process. Cycle times vary from a few seconds to several minutes in dependence on the size and form of the moulded detail. 
     The moulding tools are relatively expensive and they have to be designed to offer the high precision and at the same time be sufficiently robust to withstand the high pressures that occur. They may be fabricated in aluminium but are preferably fabricated in hardened tool steel in order to be usable during a long time. Thus, the moulding technique is particularly preferable when a large number of details are to be manufactured. 
     Gas assisted moulding (also called GAM, gas assisted injection moulding, or GID, Gasinnendruck), which is an improvement of the moulding technique and which relatively recently has been commercialized, allows sections of the interior of a product to be removed or, simply, that hollow products can be manufactured. The technique, in the following denoted gas assisted moulding, is suitable for the manufacturing of thicker products, such as handles, as well as products having thinner walls. 
     The technique comprises that a moulded detail is pressurized with a gas via a network of flow channels or directly in the moulded detail prior to allowing the injected material to solidify. The pressurized gas provides the packing force required for the manufacturing of a hollow moulded quality detail. 
     An overview of gas assisted moulding including discussions of the technique as well as applications is found in “Gas Assisted Moulding”, T. C. Pearson, Rapra Review Report, No. 103, 1998. In the overview the choice of equipment is discussed, including e.g. positioning and time adjustments of gas injection, and dimensions and positioning of gas channels. 
     Advantages of gas assisted moulding includes that less material is consumed to manufacture the detail (up to 45% less consumption), that better dimension stability can be achieved, that shrinkage marks, i.e. hot spots, can be eliminated, that details having higher strength and rigidity can be manufactured, that cycle times can be reduced, that moulding induced stresses in the material of the detail can be heavily reduced, and that a lower clamping force is required. 
     The use of a pressurized gas for assistance during conventional moulding of polymer is believed to have been commercially applicable by means of an invention by Friederich, which was patented 1978, see U.S. Pat. No. 4,101,617. The invention relates to moulding of hollow articles in a single step, wherein a compressed gas is introduced together with, or directly after, the injection of molten polymer in the article-defining mould. 
     Further, different particular aspects of gas assisted moulding have been patented during later years, see e.g. the patents U.S. Pat. No. 5,728,329; U.S. Pat. No. 5,662,841; U.S. Pat. No. 5,558,824; U.S. Pat. No. 5,705,201; U.S. Pat. No. 5,411,685; U.S. Pat. No. 5,110,553; U.S. Pat. No. 5,069,858; and U.S. Pat. No. 5,204,050; and references therein. 
     The three first mentioned of these patents relate to the pressurizing of a gas in the mould prior to injection of the molten polymer therein and generation of a static pressure with the molten polymer. The third one particularly depicts a technique for controlling the exhaust of the initial gas, whereby the flow front and the expansion velocity of the injected molten polymer are modulated in order to achieve a uniform material flow in the mould to minimize marks due to non-uniform flow in order to ensure that no gas blows occur in the material flow and to achieve articles having a more uniform wall thickness. 
     Further, the fourth patent discloses a vibration-based process to modify the injection moulding and/or the properties of the mould material. 
     The fifth patent depicts a gas control unit for a gas assisted moulding system, wherein an amount of gas is introduced in the mould in combination with an amount of polymer material during injection moulding. 
     The sixth and the seventh patents show process methods to enhance the surface quality of the manufactured articles. 
     The first mentioned of these patents comprises first to inject a considerable amount of polymer in the mould and thereafter to simultaneously inject pressurized gas and a further amount of polymer. The introduction of the pressurized gas prevents first that the first polymer flow is stopped and thereafter, subsequent to the introduction of the additional amount of polymer, the gas distributes the total amount of molten polymer in the article-defining mould. Particular ranges for the gas pressure and for the mutual relation between the two polymer amounts are given. 
     The latter of these patents depicts a moulding process, wherein a first amount of pressurized gas is assisting during moulding, but does not enter the article-defining mould, but enters in a volume substantially adjacent to the mould in order to assist at the filling out of the mould. 
     Finally, the eighth patent depicts a method and a device, respectively, wherein an article is manufactured by injecting molten polymer into a mould and by injecting an amount of pressurized gas in the polymer in order to fill out the mould and create a cavity in the polymer. Thereafter gas is injected into the polymer in the mould at a second position, which either forms a separate cavity in the polymer or which forms a passage through the polymer into the former cavity. The first gas is preferably injected at the same position as the polymer is injected and the latter gas can be injected at a position having a direct communication with the mould. The gas is vented when the article has solidified enough such that occurring valves, discharge outlets or the like will not be clogged. This venting may be performed from either one of the injection positions or from both, simultaneously or in some sequential order. This technique exhibits advantages comprising i.a. short cycle times, minimization of operation stops, low material consumption and manufacturing of articles of high quality. 
     The present invention relates to a further enhanced technique during manufacturing by gas assisted moulding, which exhibits the advantage of the above mentioned technique but which also simultaneously provides for further reduced cycle times. 
     SUMMARY OF THE INVENTION 
     Thus, it is a main object of the present invention to provide a method for effective, fast and reliable gas assisted moulding having considerably shortened cycle times. 
     A further object of the invention is to provide a method, which provides for manufacturing of high-quality products having high strength and rigidity without any shrinkage marks or moulding induced stresses. 
     Still a further object of the invention is to provide a method for moulding of products, wherein the consumption of primary material is low. 
     Yet a further object of the invention is to provide a method, wherein the cooling of the moulded product can be controlled. 
     Still a further object of the invention is to provide an apparatus, wherein said method for moulding can be implemented. 
     These and other objects of the invention are attained by a method and an apparatus according to the appended patent claims. 
     An advantage of the invention is that the cycle times can be considerably reduced compared to prior art techniques, possibly by at least up to 30%. 
     Further advantages of the invention and characteristics thereof will be apparent from the following description. 
    
    
     SHORT DESCRIPTION OF THE DRAWINGS 
     The invention will be described closer below with reference to FIGS. 1-9, which are only shown to illustrate the invention and shall therefore in no way limit the same. 
     FIG. 1 is a flow scheme illustrating a schematic method in gas assisted moulding according to one embodiment of the present invention. 
     FIGS. 2-8 show schematically, in cross-section, an apparatus for gas assisted moulding during different phases of the inventive method shown in FIG.  1 . 
     FIG. 9 shows an inventive example for controlling the cooling of the moulded product. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description, for the purpose of explaining and not limiting the invention, specific details are set forth, such as particular applications, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to the man skilled in the art that the invention can be practised in other embodiments than these. 
     The invention will be described closer with parallel references on the one hand to FIG. 1, which is a flow scheme of a method for gas assisted moulding, and on the other hand to FIGS. 2-8, which illustrate a device for gas assisted moulding during different phases of the inventive method of FIG.  1 . 
     Reference numeral  21  in FIG. 2 refers to a mould cavity of a moulding tool intended for gas assisted moulding. In the illustrated case a mould cavity is shown, which is intended for moulding of hollow handles but the present invention is applicable for moulding of a large variety of products including both wider and thinner parts. It shall be appreciated that any kind of moulding tool known within the technical field can be modified to incorporate the present invention. A moulding tool as disclosed in said U.S. Pat. No. 5,204,050 may e.g. be used for that purpose. 
     The moulding tool comprises an opening in the one end of the mould cavity, where a valvle-controlled nozzle  22  is arranged. Further, the nozzle is in communication with a controllable feeding system of a standard type (not shown) for injection of the raw material. 
     In a step  11  (FIG. 1) an amount of molten polymer of a typical temperature of about 200° C. is injected into the mould cavity  21  through the above mentioned nozzle  22 . In FIG. 2 the polymer is indicated by  23 , the front of which has here reached about halfway into the mould cavity  21 . The fact that the injecting is under progress is indicated by the arrow POLY. Further, the moulding tool comprises a second opening, wherein a second valve  24  is mounted, said valve having an important function in the present invention, which will be described in detail further down in the description. 
     Subsequently, preferably after the injection of the molten polymer having been completely ended, a predetermined amount of pressurized gas, preferably an inert gas, such as nitrogen or a nitrogen-containing gas mixture, is in a step  12  injected into the molten polymer in the mould cavity through the same nozzle  23 . The nozzles shall then be connected to a controllable gas system for controlled supply of gas. Alternatively, the moulding tool is provided with a separate gas supply nozzle at a separate opening for supply of the pressurized gas. Time adjustment, pressure, temperature and velocity of the supplied gas are critical parameters, which have to be accurately controlled. 
     The pressurized injected gas is in FIG. 3 indicated by  25 . An arrow denoted GAS indicates that the gas injection is in progress. Since the polymer is cooled from the inner wall of the mould cavity, a temperature gradient arises perpendicularly to the flow direction with the highest temperature and thereby the lowest viscosity in the center. Thus, the gas chooses the natural way through the low viscous and hotter portions of the polymer towards areas having low pressure. 
     In this manner the gas creates a cavity, which extends along a central axis from the nozzle  22  and towards the front end of the polymer and thereby forces the molten polymer towards the inner wall of the mould cavity. This step is indicated by  13  in FIG.  1 . 
     In FIG. 4 is shown the process at a stage where the injection of the polymer is ended and where the polymer has also reached the end of the mould cavity at  24 . The gas is though still being injected, which is again indicated by an arrow denoted GAS, and the gas has reached a substantial distance in the mould cavity. By controlling the amount of gas injected into the cavity the pressure, which puts load on the cooling polymer, is controlled. Preferably, a further supply of gas through the nozzle  22  is stopped when the gas has reached a predetermined, estimated distance, e.g. such as the one illustrated in FIG.  4 . 
     Further, in a step  14 , a predetermined amount of pressurized gas is injected through nozzle  24  located on an opposite side of the mould cavity compared to nozzle  22  and into the cooling polymer. In FIG. 5 this step is illustrated by an arrow denoted GAS indicating gas injection in progress through the nozzle  24  and by a gas amount  26 , which extends along the above mentioned central axis from the nozzle  24  and in the direction towards the cavity defined by the former amount of pressurized gas. In this respect a further pressure towards the polymer is achieved, which allows for the fabrication of higher-quality details having a more accurately defined form and more uniform surfaces. 
     By controlling the supply at  24  so that the pressure in gas  26  is higher than the pressure in gas  25  a pressure gradient along the axis is achieved. Hereby, in a step  15 , a passage to cavity  25  can be achieved. In FIG. 6 this is clearly illustrated, wherein said passage is denoted by  27 . 
     During the continued solidifying and cooling process it is important that a uniform pressure is obtained in the mould cavity  21  in order to produce a high-quality detail. The time for this cooling process is normally in the order of 75% of the total cycle time. As mentioned above the cycle time is a very critical parameter for the productivity of a tool and it can vary tremendously in dependence on i.a. tool, mould cavity, detail design, raw material and cooling. Typical values of cycle times are 90-120 seconds. During experiments applicant has established that it is possible to lower the cooling time with up to 50%, which will give a shortened cycle time of up to about 35%. Even if much lower values are achievable in production, every possible reduction of the cycle time is desirable. Also a reduction of 5-10% is regarded as considerable in this respect. 
     In order to reduce these cooling times, and thereby the cycle times, a further controlled amount of pressurized gas is supplied through nozzle  24  at the same time as an amount of the same size is allowed to flow out of the mould cavity through nozzle  22 . The temperature of the supplied gas shall be lower than the temperature of the gas removed in order to being able to convey heat in the mould cavity from the mould cavity via the obtained gas flow. This step is indicated by  16  in FIG.  1 . 
     In such a manner gas is utilized in gas assisted moulding for two different main purposes: partly as a means to deliver necessary packing force of the polymer according to conventional gas assisted moulding, partly as a means to convey heat from the mould cavity through e.g. convection for the purpose of speeding up the cooling. In the latter case the pressurized gas is operating as a heat exchanger. 
     To be able to operate such a function in practice accurate control is required to keep the pressure in the mould cavity within acceptable tolerances during such a flow through the mould cavity. Parameters which can be controlled include amount of gas flowed through the cavity, its velocity and the temperature of the supplied gas. In FIG. 7 the flow through the mould cavity is indicated by arrows denoted GAS at the respective nozzle  22 ,  24 , the heat conveyed is indicated by an arrow denoted Q and the through cavity formed by the pressurized gas is denoted by  28 . 
     Thereafter, in a step denoted  17  in FIG. 1, the pressurized gas is vented from the mould cavity. This is also illustrated in FIG. 8, again by an arrow denoted GAS at the nozzle  22 . 
     The venting can certainly also be performed through nozzle  24  but the polymer ought to be coolest at the end at  22 , whereby it ought to be an advantage to vent the gas there, since the risk of clogging the nozzle is the lowest there. 
     Finally, in a step  18 , the mould cavity is opened and the finished, moulded detail is removed. 
     The present invention may alternatively in step  16  comprise that gas is supplied at  24  och removed at  22 , such that a flow in the opposite direction is instead achieved. Normally it is, however, most preferable to remove the gas at  24 , since the polymer is coolest and has reached furthest in the process of solidifying. The more molten the polymer is, the higher probability that the outlet valve will be clogged or that polymer particles will follow out through the outlet. This is certainly not desirable. In order to prevent this process from occurring the cavity and the nozzles are formed thereafter and further the complete process has to be accurately controlled. 
     Another critical point concerns the piercing, i.e. the formation of the passage, to the cavity  25  from nozzle  24 , which is shown in FIGS. 5 and 6. In practice, the polymer material may limit the size of the passage, such that a through cavity is achieved, which has a structure more similar to the one shown in FIG. 6 ( 25 ,  26 ,  27 ) than the one shown in FIG. 7 ( 28 ). If the passage has a smaller cross section area than the cross section area of the opening of the nozzle  24 , problems may arise, as in such an instance the flow resistance in the mould cavity is unknown. 
     Thus, in order to avoid this, it must as far as possible be arranged so that said passage is achieved with a smallest cross section area, which is larger, preferably considerably larger, than the cross section areas of the openings of the nozzles  24 ,  22 . In the second place, one has to try to form said passage with a smallest cross section area, which is repeatably achievable, such that in each cycle the same flow resistance is obtained. The purpose of this procedure is to achieve a repeatable cooling of the polymer in the mould cavity from cycle to cycle, which is achieved by a repeatable gas flow through the mould cavity, which in turn is achieved by a repeatable flow resistance in the mould cavity. 
     Further, it is possible first to flow the pressurized gas in one direction and then in an opposite direction. Since the cold gas supplied will cool most effectively at the inlet, a more uniform cooling is hereby achieved, which further reduces the cooling time and improves the quality of the moulded product. It is even possible to make the gas flow repeatedly and alternately in opposite directions, e.g. 2-6 times. 
     The gas flow system for supply and removal of pressurized gas can be controlled by means of flow regulation. 
     The amount of pressurized gas, which flows through the mould cavity  21 , the velocity of this flow and the temperature of the supplied gas are chosen in one embodiment, such that at least substantially as much heat Q is conveyed from the mould cavity via the pressurized gas as is conveyed through heat conduction in the moulding tool. 
     The temperature of the supplied gas can range from 10-30° C., more preferably from 20-25° C. and most preferably the temperature is at room temperature. 
     In another embodiment, where the cooling must be performed rather slowly, the temperature of the supplied gas is ranging from 30-80° C., more preferably from 40-70° C. and most preferably from 50-60° C. Within these temperature ranges the moulding tool is typically held at conventional gas assisted moulding in order to allow the polymer to “flow out” sufficiently, such that details having uniform surfaces without waves or the like are obtained. Further, the risks of clogging of the nozzles can be reduced. 
     In a further embodiment, wherein the flow velocity must be low (e.g. in order to avoid the problems of clogging), the supplied gas may be cooled to a temperature below room temperature, e.g. to a very low temperature by means of liquid nitrogen. In this respect, the same heat transport may be obtained by using a much lower flow velocity. It is also possible to choose another gas, e.g. helium, which has different heat conduction properties. 
     The pressure of the pressurized fluid has to be accurately controlled during the moulding. Typical pressures range from 50-200 bar. 
     Further, the moulding tool may comprise additional openings to the moulding tool for further supply and/or removal of pressurized gas. It is possible to image an arbitrary number of openings, which serve as inlets and/or outlets for the through flowing gas, particularly for the manufacture of more complicated products. 
     FIG. 9 shows schematically an inventive example for regulation of the cooling of the moulded product. A valve-based tubing system comprising valves  29 - 32  and tubing  33 - 36  is connectable to a controllable gas flow system (not shown) via switches  37 ,  38 . Pressurized gas is supplied at  37  and removed at  38  in a regulated manner, such that a uniform flow of pressurized gas is achieved in the tubing system and a substantially constant pressure is achieved in the mould cavity  21 . 
     The above described alterations of the flow direction through the mould cavity  21  may be achieved by opening and closing the valves  29 - 32  in the following way. First, valves  29  and  30  are held closed, while valves  31  and  32  are held opened. The pressurized gas entering at  37  is forced through tubing  33 , through nozzle  22 , through cavity  28  in the mould cavity  21  and out through nozzle  24 , through tubing  34  and finally out from the system at  38 . Thus, by simultaneously opening valves  29  and  30  and closing valves  31  and  32  the flow direction is altered in the flow cavity  21 . The gas is here flowing in at  37  through tubing  35  and in through nozzle  24 , through cavity  28  in mould cavity  21  and out through nozzle  22 , through tubing  36  and out at  38 . By opening valves  31  and  32  synchronously by closing valves  29  and  30  the flow direction is again altered. The procedure may be repeated a suitable number of times. 
     According to the present invention a technique is achieved, which increases the filling out of the polymer in the mould cavity in order to increase the quality of the moulded detail, at the same time as the cycle time can be most considerably shortened. 
     In summary, the technique includes the use of pressurized gas in gas assisted moulding for conventional, inner pressurizing of polymer injected into a mould cavity and further as a transportation medium in order to remove considerable amounts of heat from the mould cavity. 
     The present invention as hereby depicted solves the problems, which are associated with prior art. Certainly, it is not limited to the embodiments described above and illustrated in the drawings but can be modified within the scope of the appended claims.