CA1277818C

CA1277818C - Artificial urethane rock and method for molding

Info

Publication number: CA1277818C
Application number: CA000559972A
Authority: CA
Inventors: Enos Dean Jarboe; Rodney D. Jarboe; Edward M. Barclay
Original assignee: Futura Coatings Inc
Current assignee: Futura Coatings Inc
Priority date: 1987-12-03
Filing date: 1988-02-26
Publication date: 1990-12-18
Anticipated expiration: 2008-02-26
Also published as: US4847026A; GB2213089A; GB8804456D0; AU1206588A

Abstract

ARTIFICIAL URETHANE ROCK AND METHOD FOR MOULDING
ABSTRACT OF THE INVENTION
A method for producing simulated rocks by spraying a high density polyurethane elastomer in a flexible mold. The method comprising reproducing a rock in molded form, not limited in size. A flexible mold is prepared by applying numerous coats of latex rubber upon the rock exterior in combination with a fabric embedded within the rubber latex layers. A supporting cradle for the latex mold is produced by spraying a low density urethane foam composition over the latex mold. The mold and cradle are stripped from the rock and are sprayed with a two component structural urethane system employing an isocyanate component and a polyol component with appropriate catalysts, fillers and additives. The structural urethane elastomer is built up to substantial thickness to provide a strong load bearing polyurethane molded rock simulating the original rock surface with curing taking place in a matter of seconds or minutes.
After cooling, the molded urethane rock is removed and the mold may be reused for repeated molding operations.

Description

1.2778~8 BACKGROUND OF THE INVENTION
In the past, artificial rocks have been made by a variety of processes.
Polyester fiberglass has been employed in the past for producing artificial rocks but presents a problem in that of requiring a gel coat or barrier coat to be first sprayed in the mold. The polyester resin conventionally employed for structural strength requires fiberglass reinforcing due to the inherent brittleness of the polyester resin. Also low production is encountered limited to one or two parts per mold per day.
Plaster and concrete have also been employed but are of disadvantage in that it is too heavy for producing large specimens. Durability, weathering and resistance to cracking and chipping represent further problems. Production is also generally limited to one or two parts per mold per day.
A particular problem has been presented in molding artificial rocks. Such rocks may be made in solid form of one type of plastic or another but have been limited in variety of design because of the high cost of molding as well as requiring time for production of both the mold and the molded product. For landscaping purposes different sizes and types of artificial rocks are desirable and high mold costs and operation expenses may be limiting factors in the produc-tion of such rocks of different size and appearance to simu-late a natural setting.
A further problem has been in the presentation of arti-ficial rocks of durability, strength and hardness that can withstand the rigors of the environment wear and tear that might be occasioned. Such rocks, if used in an outdoor setting, must be able to withstand weathering over a period of time and substantial physical abuse by the public that may be encountered in their landscape or other type of setting in which they are employed.
SUMMARY OF THE INVENTION
By means of this invention there has been provided a simplified process for manufacturing artificial rocks of varied shapes, surface appearance and sizes. The rocks may be used in landscape settings of one type or another to simulate natural rocks. Their light weight, strength and resistance to weathering provide a wide range of usage.
The artificial rocks are simply prepared by the employ-ment of a two component isocyanate and polyol which cures in minutes to form a process commonly known in the trade by the acronym SIM, "SPRAY-IN-MOLD". The spraying is accomplished with two component proportioning equipment and impingement mix or static mix plural spray gun.
In the process of this invention a flexible mold is first prepared using the rock to be simulated as a model.
The flexible mold is simply and cost efficiently prepared by applying multiple coats of latex to the exterior of the rock. A reinforcing material, such as cheese cloth or the like is employed at intermediate stages of the application of the latex layers to provide integrity to the built up layers of latex which comprise the mold.
In order to provide support for the latex mold, a cradle support is provided by spraying low density polyurethane foam at a substantial thickness on the exterior of the latex coating. The cradle support ensures that the latex mold holds the shape of the rock model when removed.
After the spraying has been completed and allowing a short period of time for cooling of the molded urethane artificial rock, the mold is easily separated. The mold may then be employed for repetitive spray molding operations to ~Z778~8 produce additional artificial rocks as desired.
In the carrying out of the spray molding process a 100% solids system is utilized. As a result there are no solvents and thus the system meets all EPA VOC (volatile organic content) limits and reduces fire hazards due to flammable volatiles.
The urethane system employed in the production of the artificial rock is comprised of an isocyanate component and a polyol component. The polyol component or side employs polyalcohols or polyamines having OH or NH2 functional groups from 2 to 8 that do react. Also usable are unsaturated poly-ester resins with vinyl monomers that provide reactive sites in addition to OH or NH2 sites. Liquid modifiers may be incorporated as plasterizers that do not react as well as unreactive solid modifiers for fillers. Foaming or blowing agents may be used such as low boiling point liquids or low temperature decomposing substances or conventional water blowing methods. Catalysts may be employed or organometals and amines that promote reactions of OH:NCO, H2O:NCO, NH2:NCO
or NCO:NCO or polymerization of unsaturated components.
To provide co]or and varied surfaces to the artificial rock, colorants, dispersion dyes and pigments may be added. To reduce ultra-violet oxidation and enhance weathering anti-oxidants and ultra-violet absorber additives may be utilized.
In the isocyanate component or side various isocyanates, polymeric and monomeric, may be employed with NCO functional groups varying from 2 to 8. Also employable are products of the above isocyanates with polyols and amines to produce prepolymers terminated with NCO functionality. These can further be modified by the unreactants listed above.
The above features are objects of this invention. Further objects will appear in the detailed description which follows ~277~
and will be further apparent to those skilled in the art.
For the purpose of illustration of this invention, a preferred embodiment thereof is shown in the accompanying drawing. It is to be understood that the drawing is for purpose of description only and that the invention is not limited thereto.
IN THE DRAWING
Figure 1 is a view in front elevation showing a rock to be simulated;
Figure 2 is a view similar to Figure 1 showing the flexible latex and reinforcing fabric in place upon the rock;
Figure 3 is a view similar to Figure 2 showing the spraying of a two component, low density urethane foam forming the cradle;
Figure 4 shows the foam cradle and flexible latex mold being stripped from the rock;
Figure 5 is a view in vertical section through the mold and cradle, showing the spraying of the two-component 100% solids urethane system; and Figure 6 is a view similar to Figure 1, partially broken in section showing the molded artificial rock.
DESCRIPTION OF THE INVENTION
In this invention urethane compositions are employed that are 100% solids (solventless) and that avoid solvent evaporation and environmental problems of volatile organic content (VOC) associated therewith. The urethane compositions may be applied by conventional spraying techniques into an open mold process to provide a high structural modulus with substantial load bearing characteristics and high fidelity reproduction.
The structural urethane employed provide a fast cure ~;~7781t3 enabling finished parts to be demolded after cooling in two to fifteen minutes compared to four to eight hours as in the case of a FRP, i.e. fiberglass reinforced polyester or concrete. The properties of the molded urethane product may be varied from flexible to stiff without the requirement of fiberglass reinforcing. The molded product has not only good structural strength but is lightweight and resistant to corrosion and weathering. The structural urethane product has excellent "wet-out" to the mold surface and reproduces in fine detail to provide a high fidelity end product.
The various properties of the structural urethanes have been found to provide a unique advantage in producing artificial rocks of various sizes and configurations which can be utilized in a myriad of usages, such as landscaping J
zoos, amusement parks, gardens of one type or another and the like. With the spray molding process of this invention some four to six rocks, as an example, may be produced per hour instead of one or two a day from a mold. The two components structural urethane compositions can be sprayed directly on a mold without the need of a "gel" or "barrier"
to produce faithfully existing surface detail without voids, pinholes or other surface defects that would occur with a conventional FRP polyester without a barrier coating.
The end product is lightweight compared to plaster or concrete artificial rocks or the fiberglass reinforced polyester.
The mold employed in this invention is unique and can be fabricated to provide a relatively inexpensive mold production which can be utilized without the requirement of expensive and complex molds. In the preparation of the mold the native rock used as a model for the artificial urethane rock to be reproduced, exemplified by the rock 10 in Figure 1, is first cleaned as with soap and water.

1 ~778~8 After rinsing, the rock is dried. A uniform application of latex rubber (prevulcanized) is then brushed on the rock surface to a wet thickness of 15 to 20 mils. The coating is dried for thirty to sixty minutes until dry to the touch.
A slight tack to the coating is acceptable for further coats.
Such further coating is effected by repeating the above described brush coating to build up to four to six coats.
After the above first stage of coating has been effected a reinforcing fabric, such as cheese cloth or the like, is added to an additional brushed latex coating by embedding after a short dry period of about fifteen minutes. The thickness of the mold is continued to be built up with additional coats of the reinforcing fabric and latex until a thickness of 1/8 to 1/4 inch is achieved with a final coat of latex being brushed upon the last fabric layer.
The recoat time subsequent to embedding the fabric will be approximately eight to twelve hours depending on the desired thickness.
In order to support the latex mold shown by the reference numeral 12 in Figure 2 when it is stripped from the rock model a cradle 14 as shown in Figure 3 is prepared to hold the configuration of the rock model. The cradle is produced by spraying on the exterior of the latex mold two to three pound density polyurethane foam at a thickness of three to four inches with an "Instant Foam" pack or equivalent material. After the preparation of the cradle and latex mold to form a completed composite cradle and mold 16, it is carefully demolded from the rock model as shown in Figure 4 and allowed to cure at 75-100F for twenty four to forty eight hours before use.
The urethane spraying of the interior of the cradle and mold 16 is carried out employing heated plural component ~7781~

equipment for delivering the isocyanate side and polyol side to the spray gun where the reaction is initiated in a mixing chamber inside the gun. An exact ratio of the isocyanate and polyol components is metered to the spray gun which mixes and produces a controlled spray pattern.
The isocyanate and polyol components are generally 1:1 in ratio but may vary from 1:10 to 10:1 of this ratio. Portable, prepackaged pressurized foam packs can also be used for producing the cradle.
Conventional plural equipment, forming no part of this invention, is used to spray the structural urethane elastomer such as that manufactured under the trademarks Gusmer, Glascraft, Graco or Binks. The spray parameters of temperature and pressure employed in the process entail the use of a primary heater at approximately 100-160*F and a hose heat corresponding to the primary heater and a spraying pressure ranging from 500 PSI to 3,000 PSI. The structural urethane elastomer is sprayed into the interior of the latex rubber mold to build up a thickness of 10 mils to 1 inch. Typical thickness are around 1/8 tol/4 inch. The structural urethane produced can vary in density from 20 to 90 PCF, (pounds per cubic foot) with typical density to 50-70 PCF. Conventional reinforcing, such as fiberglass, may be added while spraying. On larger rocks, the high density structural urethane elastomers may be used in combination with low density foam. This composite provides the balance of economy and weight.
Figure 5 shows a typical spraying operation in which the isocyanate and polyol components are introduced through hoses 18 and 20 to a spray gun 22 to form the polyurethane artificial rock 24. The spraying may be carried out with Gusmer (trademark)spray equipment with primary heaters set at 120*F, hose temperatures at 140*F and a spraying pressure of 1,000 1~77818 PSI. The spray molded rock 24 after cooling may be separated from the cradle 14 with the flexible latex mold 12 as shown in Figure 4. In some cases the cradle 14 may be removed from the mold 14 and the rock 24. The mold may then be stripped from the rock and due to its shape retention easily inserted in the cradle for further molding operations.
The finished polyurethane rock 24 after demolding is shown in Figure 6 and provides a high fidelity simulation to the natural rock model 10 in Figure 1.
The spray molded artificial polyurethane rock 24 has a high flexural modulus with the following physical properties when cured:
PROPERTY TEST METHOD RESULTS

Tensile Strength, PSI D-628 6800 + 100 Elongation, % D-628 20% + 2 Flexural Modulus, PSI D-790 265,000 + 1000 Heat Sag, inches D-3769 0.15 + .025 Hardness, Shore D D-2240 77 + 2 Density, PCF D-792 68 + 3 A typical formulation of the polyurethane spray molding composition employed for the SIM spraying of the interior of the mold 16 as shown in Figure 5 is disclosed in the following example:

EXAMPLE
To spray inside the mold:
ISOCYANATE SIDE By Weight Polymeric MDI (Polymethylene polyphenol isocyanate) 100 parts Circa 32.1% NCO
POLYOL SIDE

Polyether Triol 21.8%
250 OH#
Polyether Diol 49.2 28 OH#
Polyether Tetrol 13.1 770 OH#
Glycol Extender 15.8 1810 OH#
DBTDL 0.1 100 . 0%

1~7781~

The isocyanate and polyol sides are mixed 1:1 by volume using proportioning equipment manufactured and sold under the trademark Gusmer (or similar equipment) to spray the material into a mold. The cured artificial rock 24 is ready to pull in 5 minutes or less.
In the above example the isocyanate and polyol components have typical properties as follows:
PROPERTY (WET~ SPECIFICATION RESULTS

Weight (lbs/gal): ASTM D- 1475 Component Isocyanate 10.5 + 0.2 Component Polyol 9.0 ~ 0.2 Solids by Weight: ASTM D-1353 100%

Mix Ratio: Isocyanate:Polyol by volume 1:1 Viscosity: LV4/60RPM
Component Isocyanate 110 CPS
Component Polyol 800 CPS
In the isocyanate component various isocyanates, polymeric and monomeric, may be employed. The NCO functional groups vary from two to eight. Products of the above isocyanate may also be employed with polyols and amines to produce prepolymers having terminal groups with NCO
functionality. The polyol component employs polyalcohols or amines having OH or NH2 functional groups from 2 to 8 that react with isocyanates. Also usable are unsaturated polyester resins with vinyl monomers that provide reactive sites for hybrid reactions in addition to OH sites. Liquid modifiers may be incorporated as plasticizers that do not react as well as unreactive solid modifiers for fillers.
Foaming or blowing agents may be used such as low boiling point liquids or low temperature decomposing substances.

Catalysts may be employed or organometals and amines that promote reactions of OH:NCO, H2O:NCO, NH2:NCO or NCO:NCO
or polymerization of unsaturated components. To provide color and varied surfaces to the artificial rock, colorants, ~.277~
dispersion dyes and pigments may be added. To reduce ultra-violet oxidation and enhance weathering anti-oxidation and ultraviolet adsorber additives may be utilized.
In this invention various conventional isocyanates may be employed such as the organic aromatic and aliphatic polyisocyanates or mixtures thereof. Organic aromatic polyisocyanates for example may be those having two or more NCO groups, such as 2,4-toluenediisocyanate, 2,6-toluenedi-isocyanate, 4,4'-diphenylmethane diisocyanate, p-phenylene diisocyanate, naphthalene diisocyanate, polymethylene poly-phenyl isocyanates or mixtures thereof, aromatic polyisocya-nates and the prepolymers prepared from such isocyanates and compounds having 2 or more active hydrogen atoms.
Organic aliphatic polyisocyanates include in addition to the hydrogenated derivations of the above aromatic polyiso-cyanates, 1,6-hexamethyleene diisocyanate, 1,4-cyclohexyl diisocyanate, 1,4-bis isocyanoctomethyl-cyclohexane and mixtures thereof.
The polyols may be of a wide range. Suitable compounds include any difunctional or polyfunctional hydroxyl compounds having a molecular weight below about 1800 such as, 1,2-and 1,3-propylene glycol; 1,4- and 2,3-butylene glycol;
1,6-hexane diol; 1,8-octane diol; neopentyl glycol; cyclo-hexane dimethanol (1,4-bis-hydroxy-methyl cyclohexane);

2-methyl-1,3-propane diol; glycerol; trimethylol propane;
1,2,6-hexane triol; 1,2,4-butane triol; trimethylol ethane;
pentaerythritol; quinitol; mannitol and sorbitol; methyl glycoside; diethylene glycol;triethylene glycol; tetra-ethylene glycol; polyethylene glycol; dipropylene glycol;
dibutylene glycol and polybutylene glycols. It is of course also possible to use polyols containing further hetero atoms such as nitrogen or sulphur,or double bonds. These include, ~277~
N-methyl diethanolamine; N-ethyl diethanol amine; di-@-hydroxy ethyl aniline; N-cyclohexyl diethanolamine; triethanolamine7 di-~-hydroxy ethyl sulphide; 2-butene-1,4-diol; di-~-hydroxy ethyl urea; and di-~-hydroxy ethyl urethane.
Chain extenders may also be employed with the polyol component. Suitable hydroxyl-containing chain extenders which are free of aliphatic amine hydrogen atoms include, for example, ethylene glycol, propylene glycol, trimethylol propane 1,4-butane diol, diethylene glycol, dipropylene glycol, bisphenols, hydroquinone, catechol, resorcinol, triethylene glycol, tetraethylene glycol, dicyclopentadiene-diethanol, glycerine, low molecular weight ethylene and/or propylene oxide derivatives of glycerine, ethylene diamine, diethylenetriamine, mixture thereof and the like.
Suitable aliphatic amine-containing chain extenders having at least one primary amine group which can be employed herein include, for example, ethylene diamine, 1,3-diamino-propane, 1, 4-diaminobutane, isophoronediamine, diethylene-triamine, ethanolamine, aminoethylethanolamine, diaminocyclo-hexane, hexamethylenediamine, methyliminobispropylamine, iminobispropylamine, bis(aminopropyl)piperazine, aminoethyl piperazine, 1,2-diaminocyclohexane, polyoxyalkyleneamines, bis (p-aminocyclohexyl) methane, triethylenetetramine, tetra-ethylenepentamine, mixture thereof and the like.
Also suitable are the aminated polyoxypropylene glycols having an average amino hydrogen equivalent weight of from about 60 to about 110.
Various catalysts may be employed in the production of the polyurethane and the RIM process. Amine containing polyols employed may or may not require a catalyst but those polyols not containing a nitrogen atom normally require a catalyst. Such catalysts may be, for example, organometal -- 1~ --1277~113 compounds, tertiary amines, alkali metal oxides and the like.
Organo-metal catalysts include, for example, organo-metal compounds of tin, zinc, lead, mercury, cadmium, bismuth, antimony, iron, manganese, cobalt, copper vanadium and the like such as,for e~ample, metal salts of a carboxylic acid having from about 2 to about 20 carbon atoms including, for example, stannous octoate, dimethyltin dilaurate, dibutyltin dilaurate, dibutlytin diacetate, ferric acetyl acetonate, lead octoate, lead oleate, phenylmercuric propri-onate, lead naphthenate, manganese naphthenate, copper naph-thenate, vanadyl naphthanate, cobalt octoate, cobalt acetate, copper oleate, vanadium pento~ide, mi~tures thereoi and the like.
Amine catalysts include, for example, triethylenediamine, triethylamine, tetramethylbutanediamine, N,N-dimethylethanol-amine, N-ethylmorpholine, bis(2-dimethylaminoetbyl)ether, N-methylmorpholine, N-ethylpiperidine, 1,3-bis-(dimethylamino)-2-propanol, N,N,N',N'-tetramethylethylenediamine, mi~tures thereoi and the like.
Alkali metal alko~ides ~hich can be employed as catalysts ior urethane ~ormation include, ior e~ample, sodium eth~ide and potassium etho~ide, In general the conventional various isocyanates, polyols, chain e~tenders, catalysts, iillers, blowing agents and other additives are well described in U.S. Patents 4,065,410;
4,076,679; 4,125,487; 4,218,543 and 4,269,945 .

Various changes and modiiications may be made ~ithin the invention as ~ill be apparent to those skilled in the art. Such changes and modiiications are ~ithin the scope and teaching oi this invention as deiined ln the claims appendsd hereto.

.....

Claims

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

1.
A method for molding an artificial urethane rock which comprises selecting a rock to be reproduced, creating a flexible mold and supporting cradle having a mold cavity conforming to the exterior of said rock, said mold having a substantial degree of flexibility and being created by applying an uncured liquid rubber forming material to said rock, curing said uncured liquid rubber forming material to produce said mold, said cradle being formed by applying by spraying a low density urethane foam forming mixture to the exterior of the rubber mold, spraying a two sided urethane composition into the mold cavity, said urethane composition comprising an isocyanate side and a polyol side and being mixed in a spray gun in a spray-in-mold process, carrying out said spraying to build up the molded artificial urethane rock to a desired thickness and stripping said cradle and mold from the molded artificial urethane rock after curing.

2.
The method of claim 1 in which the cradle mold is created by applying said uncured liquid rubber forming latex material on said rock in a multiplicity of layers and a fabric is embedded between said layers.

3.
The method of claim l in which selected pigments and fillers are added in the polyol side.

4.
The method of claim 1 in which the artificial rock and rubber mold are removed from the cradle, the mold is then removed from the artificial rock and the mold is repositioned in the cradle.

5.

The method of claim 1 in which the low density urethane foam is of two to three pound per cubic foot and is applied by spraying to a substantial thickness.

6.
The method of claim 1 in which the two sides of the urethane composition are solvent free and are introduced separately into a reaction chamber of a spray gun to provide a 100% solids polyurethane artificial rock mold composition.

7.
The method of claim 6 in which the isocyanate and polyol sides are introduced to the spray gun in a volume ratio of isocyanate to polyol of 1:1 to 1:8.

8.
The method of claim 6 in which the isocyanate side comprises a polymeric 4-4'diphenylmethane diisocyanate having a functionality of free NCO of 28-34%.

9.
The method of claim 6 in which the polyol is a hydroxy containing polyester or polyether.

10.
The method of claim 9 in which the polyol comprises a polyether triol, polyetherdiol and a polyethertetrol.

11.
The method of claim 9 in which the polyol comprises substantially 22% polyethertriol having a hydroxy number of 250, 49% of a polyetherdiol having a hydroxy number of 28, 13% of polyethertetrol having a hydroxy number of 770, a glycol extender and a catalyst of dibutyltin dilaurate.

12.
The method of claim 6 in which the isocyanate side comprises a polymeric 4-4'diphenylmethane diisocyanate having a functionality of free NCO of 28-34% and the polyol is a hydroxy containing polyester or polyether.

13.

The method of claim 12 in which the polyol comprises substantially 22% polyethertriol having a hydroxy number of 250, 49% of a polyetherdiol having a hydroxy number of 28,

13% of polyethertetrol having a hydroxy number of 770, a glycol extender and a catalyst dibutyltin dilaurate.

14.
A molded artificial urethane rock, said rock being self-supporting and having a high structural load bearing strength, said rock being produced by a method for molding the artificial urethane rock which comprises selecting a rock to be reproduced, creating a flexible rubber mold and supporting cradle having a mold cavity conforming to the exterior of said rock, said mold having a substantial degree of flexibility and being created by applying a liquid uncured rubber forming material to said rock, curing said uncured liquid rubber forming material to produce said rubber mold, said cradle being formed by applying by spraying a low density urethane foam forming mixture to the exterior of the rubber mold to provide a self-supporting cradle, spraying a two sided urethane composition into the mold cavity, said urethane composition comprising an isocyanate side and a polyol side and being mixed in a spray gun in a spray-in-mold process, carrying out said spraying to build up the molded artificial urethane rock to a desired thickness and stripping said cradle and mold from the molded artificial urethane rock after curing.

15.
The molded artificial urethane rock of claim 14 in which said rock is produced in the form of a shell having a density of about fifty to seventy pounds per cubic foot.

16.
A molded artificial urethane rock, said rock being comprised of a urethane molded shell having a substantial structural thickness with an exterior surface simulating the exterior of a natural rock, said urethane molded shell being in the form of an open shell supportable on the ground, said urethane molded shell of the rock being self-supporting and having a substantial structural load bearing strength, said urethane molded shell of the rock having physical characteristics of tensile strength of about 6800 pounds per square inch, elongation of about 20 percent, flexural modulus of about 265,000 pounds per square inch, heat sag of about 0.15 inches, hardness of about 77 Shore D, and density of about 20 to 90 pounds per cubic foot.

17.
The molded artificial urethane rock of claim 16 in which said rock has a density of about 68 pounds per cubic foot.

18.
The molded artificial urethane rock of claim 16 in which said shell has a density of about fifty to seventy pounds per cubic foot.

19.
A cradle and mold for producing an artificial molded urethane rock, said mold being comprised of a mold liner of flexible rubber and having an internal mold cavity surface conforming to the exterior of the artificial molded urethane rock to be molded, said cradle being comprised of a low density semi-rigid self-supporting urethane foam material, said cradle being separable from said mold liner and having an internal surface conforming to and receiving in mating relation said mold liner, said foam material being applied in an effective thickness to be self-supporting by spraying to the exterior of said mold liner.

20.
The cradle and mold of claim 19 in which the mold liner and artificial molded urethane rock produced therein are readily removable as a unit from the cradle and said mold liner may easily be stripped subsequently from said rock and reinserted in said cradle for repeated molding operations.

21.
The cradle and mold of claim 19 in which the cradle has a slight degree of flexibility to facilitate stripping of the molded urethane rock.

22.
The cradle and mold of claim 19 in which the cradle has a density of two to three pounds per cubic foot.

23.
The cradle and mold of claim 19 in which the mold liner and artificial molded urethane rock produced therein are readily removable from the cradle and said mold liner may easily be stripped from said rock and reinserted in said cradle for repeated molding operations and the cradle has a slight degree of flexibility to facilitate stripping of the molded urethane rock.

24.
The cradle and mold of claim 23 in which the cradle has a density of two to three pounds per cubic foot.