Controlled casting of a shrinkable material

A method and a device are disclosed wherein a shrinkable polymer material is formed in situ in a mold without defects and with no internal stresses. A monomer or polymer solution is injected into the mold and solidified sequentially through the mold by exposure to an agent such as ultraviolet radiation, with simultaneous addition of monomer or polymer solution into the area of the mold not yet exposed to the solidifying agent. By controlling the rate at which the solidifying agent is moved across the mold and the monomer or polymer solution is injected into the mold, the resulting product completely fills the mold and is stressfree.

TECHNICAL FIELD 
This invention relates to controlled solidification of a pre-polymer 
mixture in a precisely dimensioned confined space. In particular, it 
relates to the manufacture in a confined space of precisely dimensioned 
stress free articles made of polymers either by in situ polymerization or 
by devolatilization or coagulation of a dissolved polymer mixture 
BACKGROUND OF THE INVENTION 
The manufacture of molded plastic parts made of polymers can be broadly 
stated as taking place in two distinct steps. The monomeric material used 
to make the polymer is usually polymerized separately prior to the shaping 
stage of the plastic part. There are, of course, exceptions; however 
generally speaking the polymer, once made, is ground into relatively fine 
particles or pelletized and then heated to liquefy the polymer or 
alternatively a solvent is added to dissolve the polymer. Subsequently, 
the liquefied polymer is either injected into a mold, extruded, spun or in 
some instances blow molding takes place. There are situations where direct 
casting is performed; however, in casting, and for that matter in 
injection molding, there exists an unacceptable degree of shrinkage and 
flow-induced molecular orientation for the most precisely dimensioned 
parts. When a solvent is used to liquefy a polymer for later solvent 
devolatilization in an attempt to obtain stress free geometric shapes, 
concomitant shrinkage caused by solvent evaporation results in cracks and 
unacceptable stresses in the hardened polymer. 
Similarly, if in situ polymerization is attempted, the pre-polymer reaction 
mixture or raw material that will make up the polymer may shrink upon 
polymerization up to 20 percent. Thus, neither in situ polymerization or 
molding of an already formed polymer has proved successful in manufacture 
of precision stress free plastic parts. To compound the problem, when the 
molded part exceeds certain thicknesses, cavitation due to shrinkage will 
frequently leave unacceptable bubbles in the part. These bubbles are 
indicative of internal stresses. 
Three exemplary fields exist where polymers are appropriate for use; 
however, the limitations set forth above, that is shrinkage of polymers at 
mold time or at polymerization time, limits usage of polymers for 
precision articles. Similarly shrinkage of a dissolved polymer upon 
devolatilization also limits use of parts formed in this method. 
When the article is to be used in an optical application, e.g., a large 
lens, a second requirement, in addition to precise dimensions, is present. 
That is, there can exist no internal stresses in the article as internal 
stresses will result in birefringence. Such is the case in injection 
molding of large plastic lenses and injection molding of optical or 
magnetic data storage discs currently used in compact disc recorders and 
personal computers and anticipated as being used as storage media for data 
in other computer systems. Both these products are now injection molded; 
however, both with the above restrictions. In the case of the lenses, it 
is common to utilize a glass blank of a size somewhat smaller, but 
generally conforming to the lens curvature desired in the final product. 
In this instance the monomer is polymerized about the glass blank in a 
relatively thin film. However, blanket exposure of the entire lens leaves 
bubbles and internal stresses that would occur anywhere but more 
frequently in narrow spaces. These defects appear often in the center or 
the thickest part of the lenses if the lenses were made entirely of the 
plastic material. The present invention addresses such problems in both 
types of construction. 
In the case of the disc, injection molding generally results in some 
flow-induced and thermoplastic internal stresses. Such stresses cause 
birefringence and thus the storage capacity and data detection reliability 
of the disc are limited. 
In the third example, a liquefied polymer sheet, when cast onto a flat or 
curved support will warp or crack if allowed to devolatilize or coagulate 
over the entire surface simultaneously. Such warping or cracking is caused 
by shrinkage due to solvent loss. 
This invention discloses a method for in situ formation of precisely 
dimensioned precision parts made of polymers either by in situ 
polymerization or differential devolatilization or coagulation. 
It is an object of this invention to provide a method that permits in situ 
molding of parts formed of polymers without internal stresses. 
It is also an object of this invention to provide a method that permits 
differential polymerization in a precision mold thereby eliminating voids 
caused by shrinkage. 
It is a further object of this invention to provide a method of casting a 
polymer sheet by devolatilization or coagulation that avoids surface 
cracking of the film. 
It is also an object of this invention to provide a method that permits in 
situ polymerization of relatively thick and large precision parts without 
internal bubbles. 
It is still another object of this invention to provide a method that 
permits casting of precision polymer parts with widely varying 
thicknesses. 
It is a object of this invention to provide an method for differential 
polymerization of variable thickness precision parts. 
SUMMARY OF THE INVENTION 
This invention is a method for forming an article of precise dimensions by 
in situ solidification in a precisely dimensioned mold of a liquefied 
material that upon exposure to a solidifier shrinks. 
The method comprises the steps of providing a mold body having a first 
closed end and a second open end and defining an internal cavity 
corresponding to the precise dimensions of the finished article. The mold 
body is so formed that it may be differentially exposed to a solidifier 
starting at the first closed end and moving in a controlled manner to the 
second open end. The method also includes the step of providing a source 
of the solidifier for imposition upon the liquefied material within the 
mold body. It also includes the step of providing a constant source of 
liquefied material at the open end of the mold body. Finally, it includes 
the step of differentially exposing the liquefied material to the 
solidifier starting at the closed end and proceeding to the open end while 
continuously supplying liquefied material to the open end. The invention 
also includes a device for forming an article of a material that upon 
exposure to a solidifier shrinks The device comprises a mold body defining 
an internal cavity having a portion of the mold body so formed that the 
internal cavity is differentially exposeable to a solidifier, the mold 
body having a first end and a second end with the internal cavity 
conforming to the desired outer dimensions of the article, the mold body 
further includes a gate at the second end of the cavity, the gate 
communicating with the cavity. The device also includes a source for the 
solidifier and means for focusing the solidifier upon a selected area of 
the material such that it is imposed differentially upon the material in 
the internal cavity. Means are also provided for moving the solidifier 
imposed upon the material in the internal cavity relative the internal 
cavity from the first end to the second end at a controlled velocity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 a mold body 10 is shown in cross-section. The mold body 
as shown in FIG. 1 is designed specifically for a lens shaped precision 
article such as a projection lens for a television receiver. The device 10 
shown in FIG. 1 is formed of at least two parts 12 and 14, brought 
together to form a cavity 16. Cavity 16 is formed having the shape of the 
precision body that is desired to be molded. As is usual with a mold, a 
gate 18 provides access to the mold body 10 when the first and second part 
are engaged. Communicating with gate 18 is a reservoir 20 which is 
utilized to feed raw material to cavity 16 through gate 18. Reservoir 20 
is represented in FIG. 1 as a hopper-like device. A vent 21 (see FIG. 2) 
may also be included to facilitate the filling of cavity 16. It should be 
understood that other means for providing raw material to cavity 16 
through gate 18 may be advantageously used. For example, it may be 
appropriate to provide raw material to cavity 16 under pressure. 
Mold body 10, as can be seen in FIG. 1, necessarily has one part, in the 
case illustrated, part 14, that is transparent to a source of energy. A 
source of energy 22 is movable relative to mold body 10 and includes a 
focusing means such as gate 24. The source of energy 22 may be drawn 
across the second part 14 by means of a two-way motor 26. Source of energy 
22 is selected according to the material to be molded. For example, if the 
monomers (sometimes referred to as the reaction mixture or polymer 
precursor) provided to the mold cavity 16 from reservoir 20 are to be 
polymerized by heat, then source of energy 22 is appropriately a heat 
source which is focused through an opening 23 in focusing gate 24. Opening 
23 is preferably designed to focus a plane of energy on second part 14. 
The plane of energy is substantially normal to the movement of focusing 
gate 24. Alternatively if the monomers utilized in cavity 16 are 
polymerized by an ultra violet source or other light source, then source 
of energy 22 may be a light of the proper wave length. Again, second part 
14 is of necessity transparent to the wave length of light utilized in 
source of energy 22 in the event polymerization takes place under the 
imposition of a light source. In the event that polymerization takes place 
as a result of the imposition of heat, second part 14 is appropriately 
thin and made of material that has little or no insulative qualities. It 
may also include passages 34 for cooling. These passages, as will be seen, 
may be selectively used so that a time-dependent temperature gradient will 
be maintained. Similar passages may be symmetrically located in part 12 
(not shown). 
Important to the invention is the fact that solidification of the liquefied 
material in cavity 16, be it monomers that are to be polymerized or a 
polymer which has been liquefied by a solvent is the differential exposure 
of the material in cavity 16 to the solidifier. In the case of the 
polymerization, the solidifier is the source of energy 22, while in the 
case of devolatilization or coagulation of a dissolved liquefied polymer 
mixture, best illustrated in FIG. 5, the ambient-controlled atmosphere or 
a non-solvent extraction bath is the solidifier with exposure of the 
material in cavity 16' to the atmosphere done by removal of the second 
part 14' from the first part 12' of the mold body 10'. More will be said 
about the alternate embodiment in the ensuing discussion. 
Returning now to FIG. 1, movement of focusing gate 24 relative to mold body 
10 is controlled so that source of energy 22 scans across the mold body 10 
starting at the closed end 26 of cavity 16 and moving toward gate 18. 
Should cavity 16 be exposed to a source of energy 22 across the entire 
surface, which is the normal process in polymerization, there would be 
severe shrinkage in the mold body and a strong likelihood that bubbles 
would form in the formed polymer especially in highly stressed portions. 
Reference is made to FIG. 4 wherein an example is shown of what would 
occur with simultaneous polymerization within the mold body. It is to be 
understood that second part 12 has been removed in FIG. 4 and the molded 
form 28 is shown with a void 30 at its upper end adjacent to gate 18. 
Additionally, bubbles 32 formed by cavitation during polymerization also 
may very likely occur in the molded body 28. The voids 30 and bubbles 32 
are attributable to the chemical linkage formation during polymerization. 
In order to avoid these problems, it has been found that providing a 
continuous source of monomer or reaction mixture to be polymerized at 
reservoir 20 and in turn to gate 18 will avoid the formation of 
unacceptable bubbles and voids. Since monomers generally have a relatively 
low viscosity, they will flow easily through gate 18 into cavity 16 to 
fill the volume lost to shrinkage of the reacting mixture. 
In the present invention the reaction mixture which is contained in 
reservoir 20 is constantly resupplied to cavity 16 through gate 18 thus as 
polymerization occurs at the lower end or closed end 26 of mold 10 the 
shrinkage that occurs and would eventually appear as a void 30 as shown in 
FIG. 4 is immediately replenished by the reaction mixture or mixture of 
polymers contained in reservoir 20. It is of course understood that the 
reaction mixture is highly mobile and flows readily to fill the volume 
lost due to shrinkage of the part of the mixture that has already 
undergone reaction. The instantaneous replacement of the space formed by 
shrinkage by unreacted material ensures a final piece that is defect free 
and distortionless. The movement of the energy source 22 relative to the 
mold body 10 must, of necessity, start with opening 23 in focusing gate 24 
moving from closed end 26 to gate 18 in a manner such that polymerization 
takes place at a steady rate from the closed end to the gate end. 
In the event the source of energy 22 is by the nature of the monomer a heat 
source, movement of the focusing gate across the mold body 10 must be at a 
rate that does not permit heat transmission through second part 14 at a 
rate faster than polymerization is taking place. That is, as the heat 
source of energy 22 moves upwardly, second part 14 will absorb heat and 
conduct that heat inwardly to cavity 16 where polymerization takes place. 
The portion of second part 14 above opening 23 must be kept cool which may 
be accomplished by circulating a cooling fluid through passages 34 so that 
the upper portion of part 14 remains cool in relation to the lower portion 
of part 14 thereby providing differential heating of mold cavity 16. 
Similar cooling passages may be appropriate in the upper portion of part 
12 (not shown). 
Referring specifically to FIGS. 1 and 2, the exemplary molded body 36 shown 
therein is a convex-convex lens. 
Referring to the alternate embodiment in FIG. 3, wherein the energy source 
122 is fixed and the mold body 110 is movable relative to the source of 
energy, a different type of exemplary cavity 116 is illustrated. The 
cavity 116 is a flat, disc-like cavity which is appropriate for optical or 
magnetic discs utilized currently in recording of music and the like and 
for storage of data in computerized systems. Currently, these optical or 
magnetic discs are injection molded which imparts residual stresses 
yielding a product that may warp with time. Birefringence, which occurs in 
injection molded optical data recording substrate is also highly 
undesirable. Here again, the reaction mixture is provided to cavity 116 
through gate 118 as a continuous source avoiding the stresses inherent in 
injection molding. (In injection molding processes, the material provided 
to the mold cavity is generally thermoplastic with a high distortion 
temperature that has already been polymerized then pelletized and melted. 
The injection molding takes place under extremely high pressure, followed 
by relatively fast cooling with almost an assurance of inherently 
unacceptable residual stress for the manufacture of optical or magnetic 
discs.) 
In FIG. 3, the mold body 110 with its first part 112 and second part 114 
transparent to source of energy 122, is moved relative to source of energy 
122 with polymerization occurring first at the closed end 126 and moving 
across cavity 116 to gate 118 by the relative movement of the source of 
energy and the mold body. Here again, second part 114 is transparent to 
the source of energy be it heat or a particular type of light. In situ 
polymerization in this instance may be more adaptable to heat triggered 
reaction as the control of the incidence of heat upon the cavity 116 where 
the molded body 136 is of a uniform thickness is more readily controlled. 
Here again the unreacted mixture contained in reservoir 120 communicates 
with cavity 116 through gate 118 to fill the volume lost due to reaction 
shrinkage thereby compensating for any shrinkage in the disc. The final 
product is a perfectly shaped disc with all portions in a stress free 
state and free of any internal voids. It lacks birefringence and residual 
distortion and is dimensionally exact to the degree provided in cavity 
116. 
This method of preparation of precision molded polymeric parts applies to 
all monomers, monomer mixtures, and monomer/cross linker mixtures. As 
result, this method permits the broadest selection of the reaction 
chemistry to achieve precision parts with the required mechanical, 
thermal, optical, tribological, magnetic, moisture sensitivity and 
dielectrical properties. The list of monomers, monomer mixtures and 
monomer/crosslinkers thus would embrace all such materials known or new 
monomers to be synthesized. 
Alternatively, as already noted, cavity 116 can be in any shape capable of 
being used as a mold. The advantage to differential polymerization is that 
one obtains precision parts that are stress free and flawless. 
Referring now to FIG. 5 a device for solidification of a liquefied polymer 
mixture by the application of a solidifier is shown. As in the embodiment 
just discussed a mold structure 10' is utilized in this embodiment. Mold 
structure 10' has a first part 12, and a second part 14' with second part 
14' being slidably removable from the first part 12'. In this embodiment a 
liquefied polymer mixture, that consists of a solid polymer and perhaps 
other fillers that have been dissolved in a solvent is contained in 
reservoir 20, which is in communication with cavity 16'. Cavity 16' is 
filled with the liquefied polymer when second part 14' fully closes cavity 
16'. In the mold structure shown in FIG. 5 a flat plate like structure or 
sheet will be the result of the casting process. Should drying or 
devolatilization take place simultaneously over the entire flat plate or 
sheet, the sheet will warp or crack as shrinkage occurs due to solvent 
evaporation. Equivalently, when a nonsolvent extractant is used to leach 
out the solvent as in coagulation, a certain degree of shrinkage will 
result depending on the relative rates of solvent leaving the mixture and 
nonsolvent entering the mixture. If, however, second part 14' is withdrawn 
slowly from the mold body 10' so that devolatilization and/or coagulation 
can occur in a differential manner such as described above with the 
polymerization process, then the dissolved polymer feed contained in 
reservoir 20 can flow into the mold cavity 16' to fill the spaces that 
occur because of the shrinkage. While shrinkage may not be as great in the 
devolatilization or coagulation of a liquefied polymer as in 
polymerization, it is sufficiently significant that cracking will occur in 
the finished sheet. Should it be necessary to evacuate the space above 
mold cavity 16' or to use some atmosphere other than ambient air or to use 
a liquid nonsolvent or nonsolvent vapor, a vacuum pump or source of 
solidifier 22' may be affixed to a chamber that surrounds the mold body 
10'. 
Referring now to FIG. 7, a schematic is shown for a device used for 
devolatilization of a liquid polymer to form a curved part. In particular 
mold body 10" is formed with a first part 12" and second part 14" (See 
FIG. 8) forming a curved cavity 16". In this particular embodiment second 
part 14" swings from an axle or pivot point 52 to differentially expose 
mold cavity 16" to an evaporating atmosphere. The reservoir 20" is filled 
with a liquefied polymer and communicates through a conduit 54 with a gate 
56 to ensure that mold cavity 16" is continuously filled with liquefied 
polymer. The liquefied polymer may be subsequently devolatilized by the 
withdrawal of second part 14" in a sequential or differential manner as 
described above. 
It should be understood that application of this invention to any shape or 
mold to form precision parts by either in situ polymerization or 
devolatilization or coagulation of a pre-solidified polymer feed is 
limited only to the extent of the capacity of the mold maker. For example, 
it is specifically addressed toward but not restricted to precision parts 
such as lenses and compact discs, and lightweight structural parts that 
require precision molding and must be stress free. 
OPERATION OF THE EMBODIMENTS 
Operation of the aforedescribed invention should be clear to those skilled 
in the art, however, a brief review is offered for consideration. 
Referring to FIG. 1, the mold 10 is clamped together in a conventional 
manner with reservoir 20 in the position shown. Reservoir 20 is filled 
with the reacting mixture in this case a monomer, a mixture of monomers or 
a monomer/crosslinker mixture loaded with an initiator and/or other 
catalysts, such that the material will easily flow into cavity 16. It is 
important to ensure that cavity 16 is fully filled with the reacting 
mixture before polymerization is attempted. Accordingly it may be 
appropriate to provide a vent 21 to the mold cavity 16. In the event a 
vent is employed, it should be closed and plugged before polymerization 
takes place. Closing the vent will assist in drawing additional reaction 
mixture into cavity 16 during polymerization rather than permitting air to 
enter the mold. 
Once mold cavity 16 is filled, the source of energy 22 may be activated and 
focusing gate 24 moved relative to mold body 10 thereby imposing either 
heat or light, as appropriate, to the mold body in a differential manner. 
Should heat be the source of energy, then it may be appropriate to 
activate cooling passages 34 at the upper end of the mold body to ensure 
that heat conduction through the mold body will not initiate 
polymerization in the upper portion of the mold before the focusing gate 
24 traverses the entire face of the mold. 
Once focusing gate 24 has completed its passage and polymerization is 
complete in the mold body 10, then the mold structure can be taken apart 
and the molded precision part removed. 
Operation of the embodiment shown in FIG. 3 follows the same pattern as 
that described above and will not further be described herewith. 
In the devolatilization or coagulation differential casting process shown 
in FIGS. 5, 6, 7, and 8, the liquefied polymer mixture contained in 
reservoir 20' or 20" is allowed to flow into the mold space 16' or 16" as 
appropriate and completely fill the mold cavity. Once the mold cavity is 
completely filled, then withdrawal of the second part 14' to expose the 
liquefied polymer differentially to the solidifying atmosphere or 
nonsolvent may be accomplished. The rate of removal of the second part 14' 
is dependent upon the liquefied polymer to be solidified. This of course 
will vary with the different materials utilized and in part may be 
dependent upon the thickness of the material. In FIGS. 7 and 8, the same 
procedure is followed except that the second part 14" is swung away 
sequentially to form a curved shape as indicated. 
While this invention has been described in relation to certain embodiments, 
it is not to be so limited, rather, it is limited only to the extent of 
the appended claims.