CA1264005A - Thermal expansion resin transfer moulding - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of forming a composite sandwich core molded article, comprising forming a thermo-elastic rigid foam core into a desired shape, wrapping the pre-formed foam core with a fabric, placing said wrapped pre-formed foam core in a mold whose inner confining surfaces form the shape of the final article, injecting a liquid thermosetting resin into the mold such that the thermosetting resin surrounds and wets the fabric wrapped about the pre-formed foam core, heating the mold to a temperature sufficient to expand the rigid foam core to compress the fabric wrapped surface of the foam core against the inner confining surfaces of the mold; and cooling the mold and removing the article from the mold.

Description

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TITLE OF THE INVENTION
THERMAL EXPANSION RESIN TRANSFER MOLDING
~ack~round of the Invention Field of_the Invention The present ;nvention relates to a method of preparing shaped objects from a thermally expandable foam.

Description of the Prior Art One technique for preparing molded objects of various shapes is known as the thermal expansion moldin~ process (TEM). This method has the unqiue feature of providing pressure necessary for formin~
finished shapes by use of a thermally expandable material placed within the female cavity of a mold. In the process, a mandrel (of a thermally expandable material sucl- as silicone rubber) is wrapped with a resin preimpregnated Eabric material. The wrapped .
mandrel is then inserted into a closed cavity mold.

Then, the mandrel, mold or both man(1rel an;1 mold are ~ ~ heated with the result that the material of the mandrel ; . expands thereby causing the material to e~pand against the female cavity mold. Pressure is created witilin the cavity by the confining surfaces of the mold.

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Significant pressures can be generated by this process which leads to high quality molded object, even of complex shapes which are free of voids.
The conventional TEI~ process has several advantages over vacuum bag and autoclave curing of objects. An important advantage is that the TEM
process obviates the need for the capital investment required to support autoclaves. Moreover, significant labor savings can be realized by the fact that the expensive bagginy and debagging operations associated with autoclave processes are not necessary. Still further, the rate of rejection of product is reduced because of the above operational advantages, since bag leaks are a significant problem of autoclave curing operations. TEM processing is especially well suited in manufacturing parts of complex shapes because it is especially difficult to bag parts of complex shapes.
Disadvantages of conventional TE,~ processing which utllizes silicone rubber mandrels include the fact that rubber tooling does not have sufficient long term stahility. That is, during repeated thermal pr(?SSUre cycling, the rubber mandrels can experit?nce a permanent compression set, which limits their li~e. Provisions must also be made in the part being manuEactured to remove the mandrels aeter curing. This can compromise ~he shape or Eunction of the part. Another .
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-( disadvantage of the conventional TEM process is that a resulting hollow part of the object may lack suficient structural strength. Still another disadvantage of rubber tooling is that i~ the rubber tooling îs improperly sized, or if the cure cycle temperatures are exceeded, rubber .tooling is capable of generating pressures sufficiently high to deform the tooling or ruin the mandrelO Yet another disadvantaye-of conventional TEM processing is that since the mandrels provide an interior mold surface, any imperfections in ths rubber such as cuts, gouges! and the like will be transmitted to the part.
Several techniques are known for manufacturing paddles, which are useful in rowing, canoeing and the like, that are light in -~eight but yet structurally strong. Virtually all of these paddles are made either .
of wood, or they have an aluminum or fiberglass shaft with a blade of compression molded laminate or injection molded plastic. These paddles typically weigh between 1 to ~.5 pounds. In another method of light weiyht paddle construction as described in U.~.
Patent 4,061,106, a core material such as balsa wood is cut to tle shape of a paddle blade and then the blade is formed to ~he paddle sha~t. Thereafter, the core îs coated witll a first layer of resin which wat3rproofs the wood and provides the required strenyt;l for the .

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paddle. After the applied resin layer dries, the paddle blade is coated with a second layer of resin and then reinforcing fibers are laid into the surface of the still wet resin cbating. Upon drying of the resin coating, a complete paddle is obtained. While this method of paddle construction provides paddles of lighter weight construction than conventional paddles, the process still is relatively complex and involves a number of operational steps. A need therefore continues to exist Eor a relatively simpler way of manufacturing light weight paddles, as well as other structures, which involve ~ewer manufacturing steps.

SUM~IARY OF THE INVENTION
__ Accordingly, one object of the present invention is to provide a method of manufacturing light weiyht objects of complex shape by a simple molding process from synthetic resin materials.
Another object of the present invention is to provide a method of manufacturing paddles or oars of lighter weight which are use~ul in aquatic e~ents by a molding process involving synthetic resin materials.
Briefly, these objects and ot'ner objects o~ the present invention as h0reina~ter will become more readily apparent can be attained by a met`nod of formint~
composite molded articles-comprising formillg a tllerno--- , , , :

elastic rigid foam core into a desired shape, wrapping the pre-formed foam core with a fabric, placing said wrapped pre-formed foam core in a mold whose inner confining surfaces form the shape of the final article and which provides the capability of heating selected areas of said pre formed foam core~ injecting a liquid thermosetting resin into the mold such that the thermosetting resin surrounds and wets the fabrlc wrapped about the pre-formed foam core, selectively heating desired areas of t!-e mold to a temperature sufficient to expand the riyid foa-n core to compress the fabric wrapped surface of the foam core against the inner confining surfaces of the mold and completing the heating of the rigid foam core to complete the molding process, and cooling the mold and removing the article from the mold. This process is hereinafter referred to as the thermal expansion resin transEer molding process or TERTM.

BRIEF DESCRIPTION OF THE DRA~ING~
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

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FIGURE 1 is a cross-sectional view of a mold for conducting the TERT~ process of the invention; and FIGURE 2 is a top vie~ of the mold of Figure 1.

DETAILED_DESCRIPTION OF THE_ PREFERRE~ EMBODIMENTS
The fabrication process of the present invention can be successfully used to prepare consumer products of widely differing shapes which are usually made from wood, metal and plastic. In ~act, an advantage of the present TERTl~ process is that it provides for flexibility in the manufacture of new products which are unsuitable or impractical ~or manufacture from wood, metal or many plastic materials. Moreover, the TERTM process effectively utilizes lightweight materials in an economicall~ attractive manner to prepare sandwich core products of light weight, strength and stiffness. Thus, the utility of the process is in the manufacture of articles and objects of widely varying design and shape which are themselves useful in many applications such as parts in the construction of aircraft, motorized, as well as unmotorized vehicles, recreational Aevices, and the like~
In the first stage of the TE~TM process oE the invention, a thermo-elastic rigid foam core is or~ed into a desired shape by any convenient metllod.

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Usually, the rigid foam is shaped by direct molding or compression molding with heat. Since the TERTM process can be utilized to Eorm products oF widely varying shapes, it is apparent that the shape of the riyid foam core can be of any convenient shape to expand into the ..... . . _ .. . , . . . , .. . . . . _ . _ . _ . .. .. . . .
shape of the final product within the mold.
In selecting the polymer material for the core, factors which enter into the determination are t'ne heat deflection temperature of the finished product, the weight desired Eor the final product, the cost oE the polymer material, and the cycling time desired. The ~oam core material must be a m~terial which is heat expandable at elevated temperatures and yet is stable at ambient air temperatures. Suitable polymer ~aterials include polyvinyl chloride, selected polyurethane materials, polyimides, and the like. All of these materials have thermo-elastic properties which make them suitable ~or use as core materials.
Polyvinyl chloride and the polyurethanes have the advantages that they can be cycled much faster at lower processing temperatures and they cost less than the polyimides. However, these materials pro-1uce heavier products because oE the increased .3en~sities re~uire-1 to compensate for lower compressive and shear properties. The other Ina jor diEEerences hetwee~ the foa~able materials are the expansion pressures -hich :. , - .: , ;

: ~' ;~ ' ( they exert on the internal laminating materials as they expand within the mold. The higher density polyimide material having a density up to 7 lb/ft3 is capable of exerting over 100 psi of pressure, while polyvinyl chloride and polyurethane pressures are significantly lower with polyurethane expansion pressures being as low as several psi.
When either polyvinylchloride or polyimide is used as the core material, a shaped object is cut fro.n slab stock of the material and roughly shaped if necessary. The roughly shaped material is then heated in an oven for a time sufficient to soften the cell structure of the object, and the object is tnen quickly transferred to a room-temperature compression mold for more precise shaping. In fact, for articles of relatively complex shape, several compressions Molds my have to be used to fonn segments which may be joined to provide a complete core for later use. On the other hand, lf a polyurethane material is selected as the core or mandrel material, mecilanical ~re-shaping of the foam mandrel ~rom slab stock can be eliminated by molding the mandrel directly Erom the closed ~ cs~iQ~ mold. Polyurethane is availahle in free rlse densities ran~ing from 2 to 10 lb/ft3. Upon subsequent exposure to heat in a production mold, the polyurethane material will expand in a ~as'lion similar ,.. . .

-~ r c to pre-compressed polyvinyl chloride and polyimide, but at significantly lower pressures.
After appropriate core material selection, the preformed rigid core is ~rapped with fabric reinforcement. The fabric can be dry or prepregged if desired or a combination of both dry fabric and preprey may be used. The dry fabric reinforcing material can be unidirectional, woven, knitted or braided material. In fact a suitable structure can be attained by wrapping the core with filaments. The dry fabric which is selected can be prepared from a variety of materials such as graphite, ~evlar, fiberglass and polyester. Fabric selection also depends on how they perform when impregnated with the tlermoset resin used subsequently in the process. Awareness of the various forms of fabric such as braids, knits, woven r unidirectional materials and traditional woven materials will allow the manufacturer to more fully utilize speclfic properties of the fabric to the best advantage possible, particularly with respect to raw material costs and processing.
The utilization o~ hybrid braided f~bric reinforcements presents particular opportunities in the fabrication of complex tabular-shaped pro~ucts by ~ERTM. The typt?~ orient~tion and quantity o~
reinforcing fibers all contribute a siynii_al-t role in ~ ' ~

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the finished product properties of strength, stiffness and durability.
One embodiment of wrapping material is a prepreg which is an epoxy resin soaked fabric. A prepreg is prepared by immersing a fabri-c into an organic solvent (acetone) solutio,n of epoxy resin, then removing the cloth once impregnated and finally dried. The prepreg which is obtained is tac~;y and semisolid. (Since the epoxy resin contains a curing agent, the prepreg is usually stored at low temperatures in order to prevent premature curing of the resin.) The nature o the curing process of the prepreg material must be such that its curing temperature matches closely with the temperature at which the core or mandrel material expands during the actual molding process.
The preferred embodiment of core wrapping material as far as the TERTM process is concerned is dry fabric because of its lower costs in comparison to prepregs and also because many more ~orms o~ ~abric are available in the dry form than are availahle in the prepreg.
Once the mandrel or core has heen wrapped in the ~; fabric, the wrapped structure is placed withir) a mold cavity. With regard to the placement of the ~ahric wrapped mandrel in t~e ~nold cavity, it is important that the wrapped mandrel be slightly undersized .
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`l ( ( relative to the interior size of the mold cavity such that the structure slightly rattles within the mold cavity. If the wrapped mandrel is oversi~ed or if the mold is undersized such that no rattling of the wrapped mandrel is observed when the mold is shaken, this means that when the liquid epoxy is injected into the mold, it will not wet the entire surface of the wrapped fabricO This in turn means that an article of incomplete, irregular or imperfect surface Eeatures will be obtained which makes the product unacceptable.
The clearance between the wrapped mandrel and the conEining mold surfaces should especially be no lass than about 0.127 ~n nor ~ore than about 0.76 mm.
Once the wrapped mandrel is positioned within the mold, a low viscosity thermosetting resin is injected into the mold such that the fabric and mandral is surrounded and wetted (impregnated and covered) by the thermosetting resin. The thermosetting resin which is used should have a viscosity of about 8000 cp or less at ambient temperatures and preEerably decrease to less than 100 cp at the elevat~d processing temperature.
The resin is preferably an epoxy resill, althougll other thermosetting resins such as polyurethanes or polyesters can also be employed. The thermosettiny resin which is used also should be one whose gel time which is sufficiently lon9 such that ~he Eoam core can ,, . . :, ., :

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Following the injection of the epoxy resin, the mold is heated. Heat is transferred Erom the mold through the thermosetting resin to the foam core. Once the témperature of the core reaches the temperature of expansion of the core material, the foam core expands such that the wrapped surface of the core is forced against the confining surfaces of the mold. The major factor which regulates the temperature oE the molding step is that the temperature must be at least that which is the temperature at which the particular core material being used will expand. The thermosettin~
resin which is injected into the mold must Inaintain its liquid st~te long enough until the foam has fully expanded against the interior mold surfaces and expelled all excess resin from the mold throuyll a vellt provided in the mold. Once the foam core llaS eXP~I)ded, the thermosetting resin should be formulated to rapidly harden and cure. Accordingly, the curiny properties of the thermosettill-~ rcsin m~lst be consistent ~ith this stated profile of the moldin~ step. The mold core is then cooled an~ the object is removed from the mold.

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The thickness of the outer layer oE the cured product laminate which consists of hardened thermoset resin impregnated fabric is determined by the thickness of the wrapped layer. The thickness of the layer may vary widely, but usually ranges from a minimum thickness of abou~ 0.25 ~n to just about any greater thickness desired with a usual maxirnum thickness of up to about one cm.
The temperature whicll is used in the molding process should closely match the expansion temperature of the core or mandrel material as stated. Obviously, if the temperature is not high enough, the foaln core will never expand and the process .~ill not ~ork. For the specific core materials described above, polyimides generally expand at temperatures ranging from about 149 to about 205C, while polyvinyl chloride resins generally expand at temperatures ranging rom about 104 to about 163C and polyurethane resins expand at temperatures ranging from about 65 to about 107C.
An aspect o~ the moldiny process is that the mold portions provide the means of heating selected ~reas o~
the riyid foam core within the mold such that areas oE
the rigid ~oam core can be uniEormly heated to allo~ a - controlled rate of expansion of the riyid foaln core before other areas oE the riyid ~oam core are heated.
~~ In fact, in e~treme cases the selective he;ltiny process . . .

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~2~ 5 can be conducted so as to only apply heat to a given area of the rigid foam core, while other areas of the rigid foam core exparience no heating whatsoever until the aforesaid heating and expansion of rigid foam cora material is accomplished.
The selective heating aspect of the method is provided by the construction of the mold portions which are exemplified in the mold half of Figure 1 and the cross-section of a complete mold sllown in Figure 2.
Vertical baffles 7 are placed in spaced apart relationship within the hollow interior of each mol'd portion between the exterior and interior metal surfaces o~ each mold portion. These baffles provide the mold portions or halves, as the case may be, with strength which prevents deformation of the mold portions or halves during use, while at the same time defining the flow pattern of the heated fluid which is passed into each mold portion which provides the heat required for uniEorm and controlle~ expansion of tha rigid foam core. As the heate(1 Eluid passes into a given mold uortion, the bafflcs define the channelcd flow pattern of the ~eate~ fluid vlitl~ tll* mold interior so that a selected portion of the interior ; mold surface is heated to supply the heat necessary to ~ expand th~ corresponding .selected arca of tlle rigid ,,.
foam core within the mold.~ If a non-salective heatin~

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regimen is desired, enough heated fluid is supplied to each mold portion to fill the entire hollow interior of each mold portion so that the entire mass of rigid foam core is expanded.
Various techniques can be employed to selectively heat a rigid foam core within a mold. Referring to Figure 2, selective heating of a rigid foam core can be achieved by injecting heated fluid through inlet 13 into the hollow interior of the mold portion in an amount and with a channeled flow pattern such that the heated fluid provides the heat necessary for the lower and middle regions of the rigid foam core. Once these areas of the foam core have expanded, additional heated fluid can be passed into the hollow interior of the mold portion such that the remaining top portion of the rigid foam core is heated for expansion. To complete the selective heating process enough heated fluid is passed into the hollow interior of each mold portion Zf~C5-t ~2sll`
until excess ~ escapes from a vent in the top of the mold.
Another alternative mold construction for selective heating would be to so construct ~he mold with Interior baffles in appropriate positions such that two or more isolated interior regions are established within the hollow interior of a given mold portion with each of the interior regions being ., , . - .: ,~

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provided ~ith a fluid inlet and a fluid vent. This technique provides for the selective heating of a given area of the rigid foam core as each individual interior region of a mold portion is filled with heated ~luid.
The present invention also embraces any other mold modification and fluid low pattern which achieves th~
heating of selected areas of the riyid foam core within a mold.
The mold which is used in the process of the present invention can be constructed of most any kind of commonl~ available metals with nickel and aluminum being the preferred metals. Most preEerred of the metals is nickel because of its excellent durability and heat transfer characteristics, and because of economic considerations.
Fiyures 1 and 2 show typical cross-sectional and top views respectively of a mold oE preferred construction which in this case is used in the production of paddles. The mold provides for the fabrication of the blade portion of the paddle. The view of ~iyure 1 shows the internal confininy surEaces 3 of a Inold 1 which ellclosl3 void sL)ace ~ in wllictl cl wrapped mandrel is expanded. The ~nol,~ Ilas external surfaces 5. The internal and e~ternal surfaces within each half of the mold arc separated ~ a hollow interior cosltaininy spacing baffles 7.

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:: - , :-Figure 2 is a top view of a mold half having external surface 5 under which is palced a series of baffles 7 within the hollow interior of the mold half~ Bolt holes 9 are shown around the peri~hery of the mold which provides sites by which the mold halves can be secured in.position against each other around sealing groove 8. A flexible elastomer or rubber cord such as a silicone rubber cord is placed within the groove such that when the two halves of the mold are bolted together, the cord is co!npressed and seals against any leakage of the therlnosetting resin after it has been injected into the cavity. Injection port 11 in communication with the interior of the mold through channel 10 i5 shown where low viscosity liquid thermosetting resin is injected into the mandrel fil~ed void space of the mold. Inlet port 13 provides an opening into the void space between the outer and inner surface of the mold which contains the supporting baffles through which a liquid can be injected in order to provide heat for the moldin~ process.
The preferred nickel mold of the ~rcsent invention is prepared by electrodepositing a layer oE metal on the conforminy surface of a master to form the inner surface of one half of the mold. Normally, the meta1 ; is deposited to a thickness oE ahout one quarter of an ~ inch. ThereaEter, st~el baEEles 7 as shown in Fi~ures ~ .
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.. .,:. :... . . ~ -~$~L~5 1 and 2 are placed in position over the electrodeposited metal layer and are ~elded into position on the metal surface. The baffles normally are about one hal inch tall although the height factor may be varied as required. -The void spaces between the baffles are then ~illed with a wax and then the fabrication is immersed into a nickel electroplating bath where another nickel layer is deposited over the wax and baffles to form the exterior housing of the mold half. The fabrication is removed from the electroplating bath and then is warmed in order to melt the wax for its removal fro~ the interior of the mold half. The other half of the mold can ~hen he fabricated in the same manner as described above. The baffles within the mold halves serve the important functions of being elements of construction which add strength to the mold halves and control the flow path of the heating 1uid which can be the likes of steam or oil, throu~h the mold half.
Having now fully described this invention it will be apparent to one of ordinary skill in the art that many changes and modi~ications can be Inade thereto without depa~tiny from the spirit oE the inv ntioll as set forth herein.

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Claims (16)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1) A method of forming a composite sandwich core molded article, comprising:
a) forming a thermo-elastic rigid foam core into a desired shape;
b) wrapping the pre-formed foam core with a fabric;
c) placing said wrapped pre-formed foam core in a mold whose inner confining surfaces form the shape of the final article and which provides the capability of heating selected areas of said pre-formed foam core;
d) injecting a liquid thermosetting resin into the mold such that the thermosetting resin surrounds and wets the fabric wrapped about the pre-formed foam core;
e) selectively heating desired areas of the mold to a temperature sufficient to expand the rigid foam core under the areas of the mold being heated to compress the fabric wrapped surface of the foam core against the inner confining surfaces of the mold and completing the heating of the rigid foam core to complete the molding process; and f) cooling the mold and removing the article.

2. The method of Claim 1, wherein said thermo-elastic rigid foam core is shaped in step (a) by direct molding with heat.

3. The method of Claim 1, wherein said thermo-elastic rigid foam core is shaped by compression molding with heat.

4. The method of Claim 1, wherein said mold provides said selective heating capability by virtue of its construction in which a series of vertical baffles are placed in spaced apart relationship within the hollow interior between the exterior and interior metal surfaces of a mold portion; and wherein said selective heating of said rigid foam core is achieved by passing a fluid heated to the desired temperature into each mold portion so that the fluid flows into the hollow interior of each mold portion in a pattern which is defined by the said baffles in order to provide a uniform distribution of heat to effect a controlled rate of expansion of the selected area of said rigid foam core being heated.

5. The method of Claim 4, wherein the molding process is completed by allowing said heated fluid to flow through all regions of the hollow interior of each mold portion to provide the heat necessary for the complete expansion of said rigid foam core.

6, The method of Claim 4, wherein the composite molded article being formed is the blade of a paddle, and wherein the molding of the blade is effected by initially selectively heating the interior confining -21- .

surfaces of each mold portion which define the broad surface area of the blade and then completing the molding of the blade by heating that portion of the foam core which is joined to the shaft of the paddle.

7. The method of Claim 4, wherein the mold of step (c) is made of nickel.

8. The method of Claim 1, wherein the pressure of compression exerted by the expanding foam core is as high as 100 psi.

9. The method of Claim 1, wherein the shape of the confining surfaces of the mold define the structure of the blade of a paddle.

10. The method of Claim 1, wherein the heat expandable material of the rigid foam core is polyvinyl chloride, a polyurethane or a polyimide.

11. The method of Claim 1, wherein said thermosetting resin is a low viscosity epoxy resin formulated with a gel time which is sufficiently long so as not to interfere with the expansion of the rigid foam core.

12. The method of Claim 1, wherein the thermosetting resin has a viscosity of less than 8000 cp.

13. The method of Claim 1, wherein the fabric wrapping of step (b) is a dry fabric, a prepreg or combination thereof.

14. The method of Claim 1, wherein the dry fabric wrapping is made of kevlar, fiberglass, graphite or polyester.

15. The method of Claim 1, wherein the fabric wrapping is a prepreg material.

16. The method of Claim 1, wherein during the expansion of the foam core in step (e), excess thermosetting resin is expelled from the interior of the mold.