MXPA98004825A - Apparatus for molding composite articles - Google Patents

Abstract

An apparatus for molding composite articles includes a pair of opposed matched-tool mold sections, each having a rigid housing (28, 30) and a thin, semi-rigid membrane (32, 34) removably and sealably mounted on the housing so as to define a fluid-tight chamber (38, 40) therein. The chamber of each mold section is filled with a noncompressible heat-conductive fluid (42) to provide fluid backing to the portions of each membrane defining the molding surfaces of each mold section (14, 16). A system of temperature control coils (56) extending within each chamber are connected to an external heater/chiller unit to permit circulation therethrough of a suitable temperature control fluid, whereby positive control of the temperature of the backing fluid (42) and, correlatively, the molding surfaces of the membranes.

Description

'L APARATO'JPARA MOLDING OF COMPOSITE ITEMS '*' TECHNICAL FIELD The present invention relates to the manufacture of composite articles, es. say, articles typically comprising a fiber reinforcing mesh within a curved resin matrix. More specifically, the invention is refers to the coupled tool molding apparatus suitable for injection molding composite articles at controlled temperatures with easily replaceable low cost tool surfaces. BACKGROUND OF THE INVENTION The injection molding of reaction and the molding of resin transfer are processes where the fiber reinforcement folds are ca / p_re shape are loaded into a mold cavity whose surfaces define the final configuration of the article to be manufactured, whereby a resin that can flow is injected under pressure into the mold. the mold cavity (mold enclosure) so that it saturates / moistens the folds / fiber reinforcement prism. After the resin preforms are cured in the mold enclosure, the finished article is removed from the mold. The prior art teaches that the injection molding apparatus consisting of a pair of complementary or "coupled" tools that provide those molding surfaces, with each of the tools being carefully machined, for example from a rigid metal that another form is relatively unreactive with respect to the resin that is used in conjunction therewith. Such metal molds are expensive to manufacture and are necessarily limited to the manufacture of a single article of a given design. Established in another way, even at small changes to the desired configuration of the item to be manufactured, the machining of a completely new replacement tool may be required. Additionally, such known metallic tools typically have substantial thermal mass which becomes increasingly problematic as the mold temperature is diverted from the desired process temperatures. In response, such tools are often provided with an integral internal heating system and / or cooling tubes or passages through which a fluid of ca 1 in t am i n t or / n f r i am i n t or supplied externally can be circulated. However, according to those prior art designs, the passages of c a 1 and n t am i n t o / e n f r i am i n t o are placed in relation to the tool surfaces to leave a minimum space of perhaps 5 cm. (2 inches) between them to ensure that the resulting article will be free of hot and cold lines or bands that might otherwise be generated in the article as a result of the different speeds of the windmills. during the curing of the resin. This inimitable separation, in turn, inherently limits the ability of those prior art tools to precisely control the temperature during the injection molding process, again, particularly where such processes are exothermic. And the control temperature of the mold enclosure also becomes more problematic when articles of varying thickness are to be manufactured, since the thicker portions of the article may be faster and will probably reach higher temperatures. In addition, when coupled metal tools are used in processes that employ reduced cycle times, the thermal mass dime nsi on ab 1 e of metal tools can frequently generate peak temperatures on the scale of approximately 190.5 ° C (375). ° F) up to approximately 204.4 ° C (400 ° F), resulting in "dry spots" which will probably become unusable to the finished article, Therefore, such coupled metal tools may have to be left at rest periodically for a sufficient time to Allow the mold to cool to an acceptable operating temperature, thus substantially increasing the cost of manufacturing the article using such tools. Finally, at the other end of the temperature scale, reduced mold temperatures are known to increase the rate of styrene accumulation when used with resins that employ styrene monomers thus precipitating the highest frequency of removal of styrene accumulation. and the associated work costs and equipment againg time, with an associated increase in the cost of the process. In an attempt to provide increased temperature control, while facilitating the removal of the finished article from the molding apparatus, the prior art teaches a modified molding apparatus wherein one of the molding surfaces is defined by a formed flexible member, for example, rubber. The other molding surface is still defined by a rigid thermally conductive metallic tool, which can be blackened by a pressurized fluid such as steam so that the heat of cure is transferred to the mold cavity for endothermic molding operations. Unfortunately, for such endothermic processes, the heating of one side of the mold cavity can limit the flexibility according to the surface finish and other characteristics of the resulting article and, furthermore, limits the degree to which the curing of the resin can be accelerated. Furthermore, when such a molding apparatus is used in exothermic processes, the resulting heat accelerates the deterioration of the flexible mold surface thus avoiding long-term use of the tool. In addition, such a molding stop often requires the evacuation of the mold enclosure prior to the injection of the resin therein, thus making the use and maintenance of the molding apparatus more complex and the processes employing such apparatuses with a more exhaustive time and more expensive. What is necessary then, is a coupled tool injection molding apparatus having replaceable molding surfaces that are easy and less expensive to manufacture than known rigid or flexible tools while offering improved temperature control during endothermic and exothermic processes thus providing articles of improved quality in times of a minor cycle.

BRIEF DESCRIPTION OF THE INVENTION It is an object of the present invention to provide an injection molding apparatus having inexpensive reusable molding surfaces. It is another object of the present invention to provide an injection molding apparatus that has improved temperature control of its molding surfaces, whereby improved control of the molding process and the characteristics of the article can be achieved. Under the present invention, an injection molding apparatus includes a pair of mold sections wherein each mold section includes a rigid housing and a semi-rigid membrane removably mounted to the housing to define a fluid-tight chamber therein. The membrane of each mold section which, in turn, defines its molding surface is preferably formed of an inexpensive composite such as glass fiber or reinforced nylon, or other suitable material; and, in accordance with the present invention, different materials and / or membrane characteristics can be selected for the respective membranes of each mold section. When the two mold sections are assembled with their respective mold surfaces in opposition to each other, a molding enclosure is defined within which the desired article is manufactured. Therefore, under the present invention, the design changes for the article are easily accommodated through the alteration or replacement of the low cost m orm. Established in another way, under the present invention, the given mold section housing can be equipped with a wide variety of relatively inexpensive composite membranes, useful in the production of composite articles of different shapes, sizes and characteristics thus greatly reducing measured tool costs compared to the prior art. According to the present invention, a non-compressible fluid is deposited inside and fills the chamber of each mold section, whereby its respective membrane is held to ensure adequate sizing of the finished article while allowing slight dimensional flexing during the injection of resin evenly distributing any pressure load by injection of the membrane through the entire surface. The latter feature can prove to be especially advantageous when a peak is encountered in the injection pressure during the resin injection step. As a further advantage, such additional light bending of the membrane during the injection of the resin is considered to improve or increase the flow of resin through the mold enclosure. An expansion chamber in fluid communication with the chamber of one or both sections of the mold serves to accommodate the thermal expansion of the fluid back from the membrane prior to the injection of the resin into the mold enclosure, and subsequently to cure the finished article. , with a valve that operates to isolate the chamber from the expansion chamber during the injection of the resin and curing. And, according to another feature of the present invention, the return fluid is preferably thermally conductive and the molding apparatus further includes means in thermal communication with the return fluid within one or both of the mold sections to regulate the return fluid temperature. For example, in a preferred embodiment, the temperature regulating means includes a system of windings that extend into each chamber, and an externally cooled / cooled unit of conventional design that is connected to the system. winding and is operative to circulate a temperature control fluid at a predetermined temperature therethrough. In this way, the temperature of the return fluid and correlatively, of the 1 or Surfaces and molding of each mold section can be regulated closely, thereby offering improved characteristics of the finished article and / or improved control of process parameters such as curing time and temperature. Additional benefits of such temperature regulation of the molding surfaces include, for example, reduced styrene accumulation with a concomitant reduction in mold repair time and mold maintenance costs compared to the prior art molding apparatus .

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partly exploded, partially diagrammatic isometric view of an injection molding apparatus according to the present invention; and Figure 2 is a cross-sectional view of the apparatus shown in Figure 1 along the vertical plane passing through line 2-2 thereof subsequent to the assembly of the upper mold section on the mold section. bottom of it.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Referring to Figure 1, an illustrative apparatus 10 under the present invention for molding a composite article includes a mold insert 12 containing an upper mold section 14 and a lower mold section 16 which defines, to the assembly of the upper mold section 14 on the lower section 16 with the help of locating pins 18 and complementary locating grooves 20, a mold enclosure 22 with the engaging molding surfaces 24, 26 thereof. Specifically, the upper and lower mold sections 14, 16 each include a rigid housing 28, 30 and a relatively thin semi-rigid membrane 32, 34 that is removably and sealably secured to the respective housing 28, 30 along the edge peripheral of. the membrane by means of a clamping ring 36. In this way, the assembly, the housings 28, 30 and the membranes 32, 34 of each mold section 14, 16 cooperate to define the fluid-tight chambers 38, 40 therein. In accordance with a feature of the present invention, each membrane 32, 34 is formed in itself preferably from a composite layer, which, in its most elegant form may simply comprise the smooth segment of a model of the article that is going away. to manufacture. And, while each brane 32, 34 can conveniently be formed of fiberglass or reinforced nylon, the present invention contemplates the use of semi-rigid membranes 32, 34 from other suitable materials such as light sheet metal whose membranes 32, 34 can be manufactured conveniently and inexpensively and configured and economized in a pressure chamber in a manner known to those skilled in the art. In this regard it is noted that the present invention contemplates the use of the same or different materials for the respective membranes 32, 34 of each mold section 14, 16 depending, for example, on the desired characteristics of the sheet (for example, its thermal conductivity, its formability and its useful life), the desired characteristics of the manufactured article (for example, surface finish and gloss) and / or the parameters of general processes (for example, resin injection pressures) , resin curing time and time of a mold assembly cycle).

The fluid-tight chambers 38, 40 defined within each mold section 14, 16 are completely filled with a substantially non-compressible heat conducting fluid 42 supplied by a fluid supply network 44 prior to injection of the resin into the enclosure of the mold 22. The fluid 42 within each chamber 38, 40 thus provides the support for each membrane 32, 34 in compression during the injection of the resin in a manner to be described below. In the preferred embodiment shown in Figure 1, the membrane return flange 42 is conveniently tap water which is supplied by the network 44 to the upper and lower mold assemblies 14, 16 as well as through the control valves 46 respective input and quick connect couplings 48. Other suitable return fluids useful on different operating scales (for example having high vaporization temperatures) will be recognized by those skilled in the art. A pressure gauge 50 can be used downstream of each inlet valve 46 to monitor the flow rate of the return fluid 42 within the chamber 38, 40 of each mold section 14, 16. To facilitate filling and emptying of each chamber 38, 40 each mold section 14, 16 has an air hole 52 through which air within each chamber 38, 40 it can escape to fill it with return fluids 42. Once filled, each chamber air hole 52 is sealed with an air hole plug 54, thus imparting the requisite stiffness for each mole section membrane of / The mold size 24, 26. As seen in Figure 2, where the relative dimensions of, for example, the membranes 32, 34 and the mold enclosure 22 are exaggerated for ease of illustration, each section of mold 14, 16 includes a winding system of ca 1 in t ami ent / cooling to 56 that extends within the hermetic chamber of fluid 38, 40 of this, which are themselves coupled by means of couplings. of ** quick connection 58 to a unit ca 1 in t ad / enfri ad or r to external 60 of conventional design. As such, the windings 56 operate in conjunction with the heater / chiller unit 60 to accurately regulate the temperature of the return fluid 42 and thus, the mold surface 24, 26 of each membrane 32, 34 through the process by injection. And, while the windings are illustrated in Figure 2 being located close to the back side of the composite membrane, under the present invention, the thermal conductivity of the return fluid 42 allows for substantial design variation with respect to the placement of the windings 56 within the chamber 38, 40 of each mold section 14, 16 which, in turn, facilitates the use of a given mold section housing 28, 30 and the winding system 56 with a wide variety of membranes 32 , 34. In fact, under the present invention, while the membranes 32, 34 of the illustrative apparatus 10 are shown in Figure 2 the efficiency with which each mold temperature can be controlled under the present invention is of relatively uniform thickness the use of membranes of variable thickness 32, 34 as may be desirable, for example, when the finished article is provided with re-ribs. To the point that the return fluid 42 with which each molding portion 14, 16 is filled, is supplied at a temperature different from the desired process temperature, the fluid supply network 44 further includes an expansion chamber. of low pressure 62. Thus, upon subsequent heating or cooling of each mold section 14, 16 to the desired temperature, any thermal expansion resulting from the return fluid 42 within each chamber 38, 40 will be accommodated by the expansion chamber. 62, thereby avoiding distortion and / or damaging tension of the membranes 32, 34. Returning to the drawings, an injection element 64 can be seen in Figure 2 extending through the upper mold section 14 to provide a path through which the desired thermosetting resin from a resin supply 66 can be injected under pressure by a suitable pump 68 into the mold enclosure 22. The or and placement of such elements 64 depend on the desired characteristics and configuration of the article to be molded and the flow characteristics of the resin that are employed, in a manner known to those skilled in the art. In this regard, it will be noted that a series of small air holes 70 is provided between the opposing clamping rings 36 of the upper and lower mold sections 14, 16 through which trapped air can be spilled into the atmosphere during the injection of the resin into the mold enclosure 22. According to another feature of the present invention, the illustrative molding apparatus 10 further includes a mechanism generally indicated by the reference numeral 72 in the lower mold section 16 for vibrating the mold assembly 12 or, at a minimum of the return fluid 42 contained in the lower mold section 16. The vibration of the mold assembly 12 / f returning fluid 42 during the injection of the resin is considered to facilitate the flow of the resin through the resin. of the mold enclosure 22 as well as improving the saturation and wetting of the fiber reinforcement preforms (not shown) located therein. In accordance with the present invention, the illustrative mold apparatus shown in the drawings can be used as follows: one or more of the fiber reinforcement preforms are located within the mold cavity defined by the "female" molding surface 26 of the lower mold section 16. The upper mold section 14 is then lowered onto the lower mold section 16 such that it engages with each location pin 18 with its respective location slot 20 (wherein the upper mold section 14). , desired can be secured to the lower mold section 16 through the use of suitable fasteners, not shown). Each mold section 14, 16 is then connected to the return fluid supply network 44 (water) and its respective air hole 52 is open and the inlet valve 46 is operated, such that it completely fills the chamber 38, 40 of it with water. Once the chambers 38, 40 are completely filled, each mold section air hole 52 is sealed with its respective air hole plug 54 and the ca. 1 cold / air operated unit 60 operated to carry each mold section 14, 16 at the desired process temperature. The inlet valve 46 towards each mold section 14, 16 is thus closed to isolate its. respective chamber 38, 40 from the fluid supply network expansion chamber 62 (which otherwise accommodates any thermal expansion of the return fluid 42 during temperature normalization). By way of example only, where the resins are injected there is a thermosetting polyester or vinylester resin, the desired operating temperature which is needed to provide the desired flow characteristics for a given thermosetting polyester or vinylester resin which has been shown to be 60 ° C (140 ° F) to approximately 65.5 ° C (150 ° F). The desired resin is then injected under pressure into the mold enclosure 22 through the injection element 64. Where the membranes are formed, for example, of glass fiber with a normal thickness of. perhaps about 0.95 cm (0.375 inches), a typical injection pressure used in the injection of a thermosetting polyester or vinylester resin having a viscosity of between about 400 and 500 centipoise within the mold enclosure 22 is preferably less than 690 kPa ( 200 psig) and more preferably, less than about 410 kPa (60 psig). Of course, the optimum flow rate at which the resin is injected is based on a number of factors well known to those skilled in the art. Once the mold enclosure 22 is completely filled with the resin, as visually confirmed by the decay of the resin through the air spills formed in the clamping rings 36 of each mold section 14, 16, it ends the injection of resin. The temperature of each mold surface 24, -26 is subsequently regulated by the operation of the heater / chiller unit 60 to preferably provide an optimum curing speed with which the desired surface finish and / or other desired characteristics of the article is obtained. finishing or otherwise optimizing the molding process. The molding sections 14, 16 are subsequently separated and the finished article is removed from the mold cavity in a conventional manner. According to another characteristic of the present invention, due to the semirigid character of the membrane of the lower mold section 34, the membrane 34 will flex lightly dimensionally during the resin injection as the return fluid 42 distributes the pressure load; of the resulting injection through the entire surface of the membrane 34. In this way, the semi-rigid membrane 34 avoids the concentration of harmful stress on its molding surface 26 during resin injection. In fact, the slight flexing of the mold surface 24, 26 of one or both membranes 32, 34 during resin injection is considered to further improve or increase the flow of resin through the mold enclosure 22, which effect can to be further improved by deliberately pressing the injected resin, all without damaging impact of the molding tools (membranes 32, 34). While the preferred embodiments of the invention have been described, it should be appreciated that the invention is amenable to modification without departing from the spirit of the invention or the scope of the appended claims. For example, while the preferred embodiment employs the membrane return fluid 42 which is itself completely restricted within the chamber 38, 40 of each mold section 14, 16 to be heated or cooled by the ca 1 unit at In accordance with the invention, the present invention contemplates the use of a closed circuit temperature regulation system wherein the return fluid 42 is calculated between each internal mold section chamber 38, 40. and the unit ca ent ent ent / en fri ad to 60.

Claims (19)

1. An injection molding apparatus comprising a pair of sections of the opposite one, wherein each of the mold sections includes a rigid housing and a semi-rigid membrane mounted vibrationally to the housing to define an air-tight chamber. fluid within each of the mold sections, the membranes of each of the mold sections defining a molding surface therein, the opposing molding surfaces of the molding sections defining a mold enclosure, and a first non-compressible fluid placed inside and filling the chamber of each of the mold sections.

2. The apparatus of claim 1, wherein the membrane of one of the mold sections is formed of a composite material. V

3. The apparatus of claim 1, wherein the membrane of one of the mold sections is formed of a first material and the membrane of another of the mold sections is formed of a second material.

4. The apparatus of claim 1, further including an expansion chamber in fluid communication with the chamber of one of the mold sections, and valve means for isolating the expansion chamber from the chamber of one of the mold sections. .

5. The apparatus of claim 1, wherein the first fluid is thermally conductive and further includes means, in thermal communication with the first fluid placed within the chamber of one of the mold sections to regulate the temperature of the first fluid.

6. The indication apparatus 5, wherein the temperature regulation means includes a winding system extending inside the chamber, inside one of the mold sections, a temperature control unit connected to the winding system operating to provide control of a second fluid at a controlled temperature through the winding system.

7. An injection molding apparatus comprising a pair of opposed mold sections, wherein each of the mold sections includes a rigid housing and a semi-rigid membrane removably mounted to the housing to define a fluid-tight chamber within each of the mold sections, the membranes of each the mold sections define a molding surface therein, the opposed molding surfaces of the mold sections having a mold enclosure, a first non-compressible fluid placed therein and fills the chamber of each of the mold sections; means for injecting the resin under pressure into the mold enclosure, and the temperature control means for controlling the temperature of the first fluid within the chamber of each, one of the mold sections.

8. The apparatus of claim 7, wherein the membrane of each of the mold sections is formed of a composite material.

9. The apparatus of claim 7, wherein the membrane of one of the mold sections is formed of a first material and the membrane of another of the mold sections is formed of a second material.

10. The apparatus of claim 7, further including an expansion chamber in fluid communication with the chamber of one of the mold sections and valve means for isolating the chamber expansion chamber from one of the mold sections.

11. The apparatus of claim 7, wherein the first fluid is thermally conductive and further includes means in thermal communication with the first fluid placed within the chamber of one of the mold sections to regulate the temperature of the first fluid.

12. The apparatus of claim 11, wherein the temperature regulation means includes a winding system that extends into the chamber of one of the mold sections, and a temperature control unit connected to the winding system. operative to supply a second fluid at a regulated temperature through the winding system.

13. A closed system injection molding process comprising forming a juxtaposed pair of coextensive semi-rigid surfaces spaced apart from a part to be molded and thickness defined by the space it bears; close the space; reinforce the surfaces externally of the enclosed space with the fluid that leans against it; injecting molding resin into the enclosed space and providing heat / cool through the fluid to the surface and the space to produce a molded part cured in such a manner.

14. The process as claimed in claim 13, and in which the surfaces serve as the internal juxtaposed junction and closing walls of opposite coupled tool mold housing chambers and the fluid is introduced into the chambers.

15. The process as claimed in rei indication 14 and in which semi-rigid surfaces are supported to ensure proper sizing of the molded part thereof while allowing slight dimensional bending during resin injection uniformly to distribute any operating load of injection on surfaces.

16. The process as claimed in claim 15, wherein the thermal expansion is effected externally of the chambers prior to injecting the resin, and is avoided during the injection of the resin and curing by the isolation of the expansion chambers external

17. The process as claimed in claim 13, wherein the temperature of the surface return fluid is regulated - during molding to achieve the desired variations in the characteristics of the molded part and in process parameters, such as the curing time and the resulting mold detection time.

18. The process as claimed in rei indication 17 ,. in which the temperature is controlled ca 1 in t / t / in f r i and the fluid.

19. The process as claimed in claim 14, wherein the surfaces are removable from the housing chambers and replaceable by joining wall surfaces formed in a similar or different manner for respective replacement and molding of a different shaped part within the common molding tool housing cameras.