CA1116364A - Structural foam molding process - Google Patents

Abstract

STRUCTURAL FOAM MOLDING PROCESS

ABSTRACT OF THE INVENTION

A process is disclosed for molding a foamed thermoplastic article characterized by a foamed core, a non-foamed exterial shell and a surface that reproducibly and faithfully replicates a predetermined portion of the inner surface of the mold in which the article is made, wherein a foamable mixture comprising molten thermoplastic material and gas foaming agent is introduced through a nozzle into a mold cavity main-tained at a temperature sufficiently low to cause the outer portion of the mixture to form a self-supporting exterior shell in the mold and introduced at a volume sufficient to fill the mold cavity, allowing the outer portion of the charge to cool and form a shelf-supporting exterior shell while maintaining the mold cavity at a pressure above the foaming pressure of the mixture and there releasing the pressure within the mold cavity to provide a temperature and pressure gradient to cause the thermoplastic material therein to contract and gas desolubilization and expansion to produce a foamed core and exterial solid shell.

Description

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The present invention relates to a novel structur-al foam (SF) process which permits molding in a cold mold and produces structural foam parts composed of a dense rel-atively non-foamed integral skin whose surface xeproducibly and faithfully replicates a predetermined portion of the inner surface o~ the mold in which the article is made, and a foamed core whose cell structure is highly unifonm.
The structural foam process, as we know it com-mercially today, was originally invented by Richard G. Angel~
Jr. in the early 1960s. That process was described in U.S.
Patents 3,268,636 and 3,436,446. As early as that time the major shortcoming of structural foam, namely, surface irreg-ularities, was recognized. These surface imperfections are collectively called the "swirl" pattern; it may or may not include blisters, pinholes, pockmarks and streaks. All these bubble-caused defects made it impossible to make a structural foam part whose surface replicates the interior mold surface.
A structural foam technology was later developed by Richard G. ~ngell, Jr. and co-workers which permits molding of s~ructural foam artieles having a surface that reproducibly and faithfully replicates the surface of the mold. That proc~ss was described in U.S. Patent 3,988,403.
While the technology employed is rather ingenious, it does present a process having a few recognizable economic disad-vantages: the cycle time is at least about 50 percent ~J~

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longer, the cell structure is worse than in normal rough sur-face structural foam and ~he mold cost is about 25 percent higher due to the necessity of cyclically heating and cooling the mold in order to achieve surface replication.
Furthermore, it is energy intensive.
Because of the economic return which would accrue from savings in cycle time, energy, mold cost and the pos-sible extension of the useful life of the mold whose surface is to be replicated, it is desirable to provide a process which circumvents the attendant problems of the process des-cr ~ed in U.S. Patent 3J988~403.
In prior structural foam molding operations of the type disclosed in U.S. Patent No. 3,268,636, the extruder is operated under a significant positive pressure and the mold is maintained at atmospheric pressure. Because of this, the molten foamable plastic undergoes a spontaneous foam forma-tion on passing from tha high pressure to the atmospheric pressure environment and the resulting foam proceeds to fill the mold. That is, the material state, its temperature, pressure, rheology, thermal contraction, bubble nucleation, growth and coalescence are uncontrolled; consequently, swirl pattern and non-uniform cellular structure result.
It has been found, in the practice of the process of this invention, that faithful and reproducible surface replication with uniform cell structure can be achieved by controlling the material state, its temperature, pressure, rheology, thermal contraction, bubble nucleation, growth and coalescence. By suitable choice of pressure and temperature, one can fix the state of the foamable material, the state of interest here being the single phase with the gas completely dissolved in the plastic material. At this state, the mold-filling process is as if the plastic material does not con-tain any foaming agent,similar in character to mold-filling in injection molding with no attendant surface defects. For example, from all available information, an SF machine op-erated at a nitrogen ~P (the difference between the blowingagent pressure and the extruder barrel pressure) of 400 psi and a temperature of 200C. yields a foamable resin con-taining nitrogen whose weight fraction is < 3.75 2 10 in polystyrene and ~3.75 x 10 3 in high density polyethylene.
From available data which illustrates the actual pressure-composition behavior of polystyrene-nitrogen and high den-sity polyethylene-nitrogen, it can be safely estimated that a back pressure of (400-600) psi in the mold would suffice to prevent foam formation. Assuming, therefore, that a molten foamable plastic is in3ected into a mold whose pres-sure is higher than the foaming pressure, no foaming would occur as the material ills the mold and bubble-caused sur-face defects will be absent on the surface o the part.
That is, bubble formation has been prevented at least for the duration of time when the mold is being filled.
Another poin~ considered in making structural foam by practicing the process of thls invention is the material thermal expansionO When a thermoplastic material is heated at a constant pressure, -the kinetic energy increases and the volume generally increasesO For example, polyethylenes, at ambient temperatures, are in their semi-crystalline solid 10 stateO
As the temperature increases, thermal agitation disr~pts the highly packed crystallites and the volume in-creases0 At the melting range, the lattice pattern is com-pletely disrupted and the polymer forms an essentially amor-phous mass. At a temperature of about 200Co J the density difference between the amorphous melt and the crystalline solid range between ~20% and 30% depending on the original crystallinity of the particular polyethyleneO High density linear polyethylene usually gives the largest density dif-ference (~30%)O
This implies that if a mold whose cavity is 100 ccis filled completely with high density polyethylene at ~00CO~
and the system is cooled to 20C., the total volume of the product will be ~70 ccO The surface~ of course, would be wrinkled owing to the combined effects of crystallization and thermal contractionO
If, however, the same mold is filled with foamable resin, against a back pressure high enough to prevent foaming, and the back pressure is released after the mold is complete-ly filled, the following would result~ The melt will firstassume the contour of the mold without imparting the afore-mentioned bubble-caused irregularities. The skin will solid-ify while the interior of the melt will foam; such foams ~ ~6 3~ ~

would occupy the volume being created by the normal therm~l contraction of the polyethylene on cooling~ For~unately, the interior cools last and the internal pressure of the piece is high enough to allow bubbles to form in the core rather than wrinkles on the surface. For this reason, the level of blowing agent (or QP in the case where nitrogen gas is used as the blowing agent) is also a critical parameter in that it de~ermines the magnitude of the foaming pressure, Pf As the incoming plastic melt pushes back the gas respon-sible for the back pressure, some of this gas will be trappedin the micro-cavities in the moldO In order not to have sur-face micro-heterogeneity on the molded part surface, the foaming pressure must be of the same order of magnitude as the gas pressure in the micro cavities whose value is approx-imately the same as the back pressure. Hence, not only is the back pressure, Pb, the ~P and the Pf important, the dif-ference between the P~ and the Pb has a primary influence on the microgeometry of the structural foam surfaceO These pressures also play an important part in con~rolling the shear veloci~y and the thermal contrac~ion of the foamed plastic as well as the nucleation, growth and coalescence of the bubble, all of which are important in de~ermining the cellular structure and surface ~uality of the finished SF product~
For some thermoplastic material, the density re-duction will not be as substantial on account of the absence of crystallinity. For example, if the foamable polystyrene-is fed into the mold at ~ temperature of 288Go ~ the maximum achievable density reduction is only ~1204%~

The process of the present invention takes advantage of such combined thermal contraction phenomenon together with the pre-backpressure mold technique and leads to the following results:
(a) Since the plastic did not foam on entering the mold, surface irregularities cormmon with the normal SF will be absent even if the mold is cold.
(b) Because of the backpressure and nozzle pressure - the plastic will replicate the mold surface. A
cold mold surface is most beneficial since the enveloping skin rigidifies fast while the interior remains molten and therefore foamable.
(c) On release of the backpressure, the molten foamable plastic in the interior fo~ms.
(d) Because of the contracting phenomena, the cell structure control is considerably improved compared to that employed in present commercial technology.
From the data previously discussed, a bsck pressure of ~ 300 psi in the~mold ifi generally suficient to produce structural foam with reproducible and fàithful surface replication in a cold~mold, it being understood that the following process ~ariables are determinative of the attain-ment of the surface replication: ~
~ i) Level (concentrationj of foaming agent in the plastic material (in the case of chemically generated blowing agent), (ii) The difference between the blowing agent pres-sure and the extruder barrel pressure, aP, determinin~ Pf.
It should be noted that Pf is not tirec~ly measurable; it can only be determined after-the-fact through ~P;
(iii) The duration of time it takes to introduce the foamable plastic charge into the mold, ta;

(iv) The back pressure, Pb;
(v) The tiTne, tbl, when the back pressure is applied (this is usually the zero time of the cycle, i.e., t = o)i (vi~ The time, tb2, when the back pressure is released;
(vii) The difference be~ween Pf (and/or ~P) and Pb;
(viii) The temperature, Tm, at which the mold is maintained;
(ix) The temperature, Tp, of the foamable plastic charge, and (x) The total time, tt, the article spends in the mold.
The values of these variable process parameters must be coordinated, in accordance with one aspect of the invention, to make structural foam with uniform cell struc-ture and reproducible and faithful surface replication.
It is further understood that: Pb, Pf, Pn and ~P
have a primary influence on surface quality of the produc~, skin thickness and cell structure; (Pn-Pb) and (Pb-Pf) have a primary influence on surface microgeometry of the product;
and ~2have a primary influence on surface quality, skin thickness and cell structure; tnl and tn2 have a primary in-fluence on cell structure, skin thickness and surface qual-ity; ta has a primary influence on surface quali~y; T~ must be high enough to permit rapid mold filling, but not too high so as to be able to prevent foam formation at a reason-able level of Pb, Tm must~be low enough for the structural foam product to rigidify within an acceptable time period;
for a given mold temperature, tt must be sufficient for the product to ha~e a Qelf-supporting skin strong enoug'n to prevent any deformation, due to residual Pf, after the part .~ 8 ~h ~ 3'~ ~

is taken out. The in~ection pressure should ~e such that melt fracture is not induced in the single phase foamable plastic material as it fills the mold.
In accordance with the present invention a process is provided for molding a foamed thermoplastic article char-acterized ~y a foamed core, a non-foamed exterior shell, and a surface that faithfully and reproducibly replicates a pre-determined portion of the inner surface of the mold which comprises: feeding as a charge a molten mixture of a thermo-plastic polymer and a soluble gas foaming agent into a moldmaintained at a temperature sufficiently low to cause the outer portion of said mixture to form a self-supporting exterior shell in said mold and at a volume sufficient, in the unfoamed state, to substantially fill said mold cavity; allow-ing the outer portion of the charge to cool in said cavity to form a self-supporting exterior shell while maintaining the mold cavity at a pressure above ~he foamlng pressure of said mixture; thereafter releasing the pressure within the mold cavity to provide a temperature and pressure gradient to cause the thermoplastic material therein to contract and gas desolubilization and expansion so as to balance the volume contraction of said charge which would otherwise have resulted from said gradients, to produce a foamed core and exterior solid shell; and removing the resuItant article from said mold cavity.
The invention employs thermoplastic poly~ers such as high and low density polyethylene, polypropylene, ethyle~e/
vinyl acetate copolymer, ethylene/ethyl acrylate copolymer, and other homopolymers and interpolymers of olefins; poly-styrene and styrene copolymers such as a polymer of acrylo-nitrile, styrene and butadiene; polycarbonates such as 4,4'-bisphenol-A-based polycarbonate; acetal homopolymers and .~
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copolymers; polyamides such as nylon 6 and nylon 6/6; poly-aryl polyhydroxy e~hers (e.g., the high molecular weight, base-ca~alyzed, condensation product of 4,4'-blsph~nol-A
and epichlorohydrin); polysulfones; polyesters such as polyethylene terephthalate; polymethyl methacrylate and other acrylic polymers; ~nd other the~moplastic polymers, which can be employed either singly or in mixtures. Conven-tional additives such as heat and light stabilizers, anti-oxidants, fillers, dyes and other colorants, can be employed in the thermoplastic polymer.
The blowing agent that ls employed in the invention can be a dissolved material that is a gas after the back pressure is released, and at the temperature conditions in the mold. Such materials include nitrogen,carbon dioxide, pentane, methylene chlorid~, trichloromonofluoromethane, dichlorodi-fluoromethane, and trichlorotrifluoroethane.

Chemically-generated blowing agents which ar~
evolved from the thermal decom?osition of solid compounds can al~so be employed either alonP, in mixtures thereof, or in mixtures with a di~solved material. Illustrative solid compositions which decompose to form gaseous blowing agents include azo com~ounds, N-nitroso compounds, sulfonyl hydra-zides, sulfonyl semicarbazides, and salts and esters of azodicarboxylic acid. Specific lllustrative examples includ- azodicarbonamide, azobisisobu~yronitrile, dini~roso pentamethylene tetramine, ~,N'-dinitroso-W-N'-dimethyltere-phthalimide, 4,4'-oxybis (benzenesulfonyl hydrazide), p-tolue~e sulfonyl semicarbazide, p,~'-oxybis (benzene sulf-onyl gemicarbazide), the barium salt of azodicarboxylic acid, and diisopropyl azodicarboxylate.

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The driving gas may compriæe any gas which is not reactive with the thermoplastic material being driven. Gasses such as nitrogen, carbon dioxide, air, gaseous fluo~:ocarbons, argon, helium and the like may be employed.
It is to be understood that, when the charge of molten plastic material-blowing agent mixture is fed to the mold cavity maintained under back pressure conditions in accordance with the invention, the charge substantially fills the mold cavity.
In the drawings:
Fig. 1 is an elevational schematic view, partially in section, of apparatus capable of practicing the process of the present in~ention to produce structural foam articles;
and Figs. 2 through 6 are a series o sectional schematic views of the no~-zle of the ~ype employed in the production of structural foam articles in accordance with the process of the invention, depicting the process step se~uence and pressurizin~ gas and plastic material-P~as mixture flow throu~h the nozzle in the process of the inven-tion to produce novel structural foa~ articles.

Referring specifically to the drawings, appara~us suitable for praç~icing the process o~ the invention is shown ~chematically in ~he embodiment of Fig. 1 wherein press 10, having platens lOa and lOb, supports mold 12 and plastic material-gas mixture is introduced under pressure throu~h manifold 14~o the mold cavitY 16and pressurizin~ ~as . is introduced through h~llow nozzle asse~bly 18 into the plastic material~gas mixture in the mold. The plastic mater-ial is fed from a feeding device 20 which may comprise a high shear melting extruder to an accumulator 22. A plastic mat-~ 3~ ~

erial-gas mixture is formed in the extruder by the injection of gas through inlet means 21. The molten plastic material-gas m~xture 23 is passed under pressure through conduit 24 to accumulator 22.
The plastic material gas mixture 23 is fed from the accumulator 22 through conduit 25 to manifold 14. Pres-surizing fluid is fed ~hrou~h inlet conduit 26 to the inter-ior of hollow nozzle assembly 18.
Pneum~tic actuator means 28 is mechanically con-nected to the inner portion of hollow nozzle assembly 18which is movably positioned 90 as to be raised and lowered within the nozzle assembly housing which is secured to man-ifold member 14.
As shown in Fig. 1, the mold interior has first been back-pressurized by the introduction of gas therein through conduit 27. Plastic conduit 25 and manifold passage 30 have been filled with plastic material. The mold cavity 16 has then been substantially filled with a quantity of plastic material-gas mixture 38. The inner portion of hollow nozzle assembly 18 is then lowered (as shown) by actuation of pneumatic actuator 28 to interrupt the flow of plastic through manifold passagé 30 and annular space 39 and to position the lower end of the hollow nozzle ass~mbly 18 flush with the in~erior of the mold cavity where ~he ~ressurizing ~as ~ro-cess ~ay be co~menced.
Hollow nozzle assembly 18 comprises a stationary outer body psrtion, an inner concentric, hollow, axially-slidable sleeve portion 18a and ~n inner concentric rod ~alve portion 18c, posltioned within the sleeve portion 18a so as to form an annular pressurizln~ ~as condult nassa~e 18b therebetween.

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The pressurizing gas is admitted, by actuation of valve means in inlet conduit 26,, to introduce the gas into the plastic material body 38 within the mold cavity 16 through conduit passage 18b. The plastic material upon pressurization assumes the contour of the walls of the mold cavity and produces a hollow article having an outer skin and a continuous hollow center.
The pressure is maintained on the pressurizing gas until the outer skin of the structural article becomes self-supporting. Thereupon, rod valve means 18c is ac~uatedto close and arrest ~he flow of gas by closing interior nozzle passage 18b. The back pressure through conduit 27 is also then released. Pneuma~ic actuator 28 then acts to retract hollow nozzle sleeve 18a, thereby opening venting port means 42. This permits the venting of the pressurized part, throu~,h the retraction of ~ston 44, throurh ~ent port 45 and, in turn, to radial passage 46 and the space between mold retaining blocks 48.
As shown in the apparatus of Fig. 1, solid plas-tic particles o~ pellets may be fed to the hopper 50 o~ aconventional extruder 20 for the plasticating of plastic material. Plastic matérial 23 is then fed through conduit 24 to an acoumulstor 22 having movable piston member 52 to provide a chamber for recelving and storing the plastic m2terial 23 before passage through manifold passage 30 towa~d the holl~w ~ozzle rod assembly 18.
The first sequence~of-operation, as shown in Fig.

2 of the drawings, depicts the nozzle slee~e 18a ~n the fully closed positlon with respect to the flow of both plas-tic material-g~s mixture therethrough and the flow of pres-surizin~ gas throu~h passa~e l~b thereof. The outlet ~as 13.
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~enting means 42 $s also in ~he closed position due to the closed positioning of rod 44 as well as ~he cutoff position-ing of the hollow nozzle assembly sleeve 18b.
The sequence-of-operation shown in Fig. 3 of the drawings depicts ~he relationship of the elements of hollow nozzle assembly 18 for the step of the process of the inven-tion in which plastic material-gas mixture passes through the nozzle assembly, through the annular space 3~ within the nozzle outer housing, in~o the mold. At this time the inner sleeve 18a is retracted and nozzle valve rod 18c is closed to prPvent the flow of pressurizing gas through the annular conduit 18b.
The sequence-of-operation shown in Fig. 4 of the drawings depicts the next step of the process in which the flow of plastic material-gas mixture through annular passage 39 is arrested by the lowering of nozzle ~leevP 18a and the pressurizing gas ~asses through the annular conduit l~b by the o~enin~ of inner valve rod 18c.
The sequence-of-operation shown in Fig. 5 of the drawings depicts the step of the process in which the pres-surizin~ zas flow has béen arrested by the closing of rod valve 18c and the elevation of nozzle sleeve 18a, to a pos-ition (as sh~n) beyond that of ~rifice venting assembly42, permits the flow of ga~ from the part through the lower end of the outer body port~on of nozzle assembly 18, and, in turn, through the passage formed by the re-trac~ion of pi~ton rod 44 thereby exposing venting pas-~ages 45 and 46.
The sequence-o-operation shown in ~ig. 6 of the drawings depicts all fluid inlet conduit~ and passages (i.e., plastic material-gas mixture, ~ressurizing gas and venting) ~4 3~

in the closed position during that step of the process in which internal foaming occurs within ~he article iII the mold to provide a finished struetural foam article.
The following three examples of the process of the invention were carried out with a high impact poly-styrene composition (84% polystyrene, 10% polybutadiene, 5% mineral oil, 170 additive blend (HIPS) having density of 1.04-gm/cc MI - O.g3 gm/10 min. ASTM-D-1238E; Heat Distortion Temp - ~5.3C. ASTM-D-648.) The mold employed was a commercial mold for the production of tote boxes having the dimensions: ~.5" height; 12"
width; 16" leng~h; ~3/8" thiekness; and having ~ top l~p and slightly curved vertical corners. The interior of the tote box is free of internal dividers. Table below sets forth the parameters and eorresponding data for these examples.

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TABLE III

PROCESS PARAMETERS E ~PLES
Exam~le No. _ 1 2 3 Plastic material ~IPS HIPS HIPS
Blowing Agent ~2 N2 ~2 Melt temp., F 475 475 475 Mold temp., ~F 140 140 140 Fill ei~e, sec. 12 12.5 13 Extruder pressure, di~charge, psi30~0 3000 30~0 Extruder pressure, barrel, psi 1300 1300 1300 Blowing agent pressure, psi14~0 14~0 1400 ~P psi . 100 100 100 Accumulator pressure, psi 2950 2950 2950 Shot weight, lbs. 4.62 4.69 4.71 Press force, tons 60 60 60 8ack pressure, psi >6~0 8~0 850 Color painted brown black red ~xample No. 4 The process of the present invention was carried out with polypropylene homopolymer (Shell 5024); O.gl olid density, 5.0 Melt Flsw~. The mold employed was a commercial mold for the production of toilet ~ank top articles. The mel~ temperature was 438F. and ~he blowing agent employed was gaseous nitrogen. Other parameters were recorded as follows:

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~P, (Pf), ps~ 150 Accumulated Displacement, in3 70 B~, PB, psi 400 Fill Time sec. 6.9 Time PB on, sec. 0.0 Time PB o~f, sec. 10.0 Total Cycle Time, sec. 160 - Molded Density O . 81 % Density Reduetion ll.0 ExamPle No. 5 The same composition o Example No. 4 was carried out employing nitrogen gas as the blcwiTlg agent, a melt : temperature of 510F., and provided the following parameters:
~P, (Pf~, p~i 10~
~ Accumulated Displacement, in3 70 : ~P, P~, psi~ ~,00 : Fill Time, sec. 10.7 Time PB on, 8 ec. : 0.~0 Time PB off, sec. 20.0 To~al Cycle Timej sec. 160 Molded Density . 0.~80 % De~ ity ReductioTl 12.~1 :

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Example ~o 6 The same composition of Examples Nos. 4 and 5 was carried out employing nitrogen gas as ehe blowing agent, a melt tempera~ure of 510F., and provided the following parameters:
~P, ~Pf), psi 100 Accumulated Displacement, in3 69 BP, PB, psi 500 Fill Time, sec. 17.4 Time PB, on, sec. O.O
Time PB, off, sec. 20.0 To~al Cycle Time, sec. 160 Molded Density 0.80 Z Density Reduction 12.1 18.
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Claims (3)

What is claimed is:

1. A process for molding a foamed thermoplastic arti-cle characterized by a foamed core, a non-foamed exterior shell, and a surface that faithfully and reproducibly replicates a pre-determined portion of the inner surface of the mold which com-prises: feeding as a charge a molten mixture of a thermoplastic polymer and a soluble gas foaming agent into a mold maintained at a temperature sufficiently low to cause the outer portion of said mixture to form a self-supporting exterior shell in said mold and at a volume sufficient, in the unfoamed state, to substantially fill said mold cavity; allowing the outer portion of the charge to cool in said cavity to form a self-supporting exterior shell while maintaining the mold cavity at a pressure above the foaming pressure of said mixture; thereafter releasing the pressure with-in the mold cavity to provide a temperature and pressure gradient to cause the thermoplastic material therein to contract and gas desolubilization and expansion so as to balance the volume contraction of said charge which would otherwise have re-sulted from said gradients, to produce a foamed core and exterior solid shell; and removing the resultant article from said mold cavity.

2. The process in accordance with claim 1, wherein said thermoplastic material is a material selected from the group consisting of polystyrene and polypropylene.

3. The process in accordance with claim 1, wherein said blowing agent is a gas selected from the group consisting of carbon dioxide and nitrogen.

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