CA1185759A - Method for production of expanded polystyrene panels - Google Patents

Abstract

ABSTRACT:
"Method for production of expanded po1y-styrene panels"
A process for manufacturing expanded polystyrene panels includes the following stages:
a) Preparation of pre-expanded panels of polystyre-ne by extrusion and simultaneous injection if propel-lent agent into an extruder fitted with an extrusion die of suitable type and in particular fitted with means allowing for good mixing of the molten polymer with the propellent agent and means allowing for uni-form cooling to the best value if the temperature of the expandable plastic mass before exit from the die.
b) Conditioning of said panels at room temperature or at whatever temperature higher than room tempera-ture, provided it is such to prevent the panels from undergoing either dimensional variation or deformation due to internal stress or true expansion.
c) Forming of pre-expanded panels into a mould-box by means of heat furnished by convection by a hot fluid the ratio between internal dimensions of the box and measures of pre-expanded panels being such that 60 percent at least of the volumetric increase given to the mass is affected to their average increase in thickness.

Description

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The present invention is directed to a method for the production of expanded polystyrene panels and more specifically, expanded polystyrene panels having a thickness of 20-80 mm which are primarily used for the thermal insulation of walls and flat roofs in varlous buildings. The qualLty of the panels depends upon their longevity, heat transmission factor, density, compressive and flexural strength and dimensional stability.
Expanded polystyrene panels are currently produced by the industry using two basic processes. The first process involves the pre-expansion of expandable polystyrene beads and the subsequent additional expansion of such beads into suitable molds by means of steam. Panels of very low density, for example, 0.010 kg/c.dmc., can be obtained by this process while panels normally accepted for insulation in buildings have a density such as 0.015 0.020 kg/c. dmc. Even though such panels are relatively inexpensive to produce they are used less frequently because they tend to crush after a period of time since the cohesion keeping the single particles of expanded material together, while poor at the time o~ production, has a tendency to diminish still further. With the second process, the panels are made by extruding molten polystyrene containing a blowing agent, for instance, Freon 12 or a mixture of Freon 11 and 12, into the atmosphere where the mass e~pands. Panels manufactured according to the se-*Trademarkcr/~

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cond process generally having a much hlgher denslty, for example ~.03 - 0.045 kg!c. dmc., compared to paneis manufactured in accordance with the first process and are considered more advan~ageous due to longer life a.nd better mechanical properties, chiefly sti.ffness and toughness which are much higher than in the first case.
The present invention is directed to ~a~
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~*~ processes by which pre-expa.nded panels extruded -- from polystyrene and oth~r similar polymers having a densi.ty of 0.030 - 0.045 KgtC. dmc. can successfully be transformed into panels having a densi.ty of 0.016 -0.0~4 kg/c. dmc. and a thickness generally twice as ~reat as the original panels. It is known that an ex-truded pan~l of expanded polystyrene tends to expand ~urthc:r when heated to a temperature higher than its softeni.ng point, for example to 100 - 120C. One of the ~eatures o~ the present invention is the use o:f ap-paratt.l~ and processes allowing -the exploitation of the above tendency in an ef~icient industrial manner and to control such a tendency in the best way possible for obtaining panels having a very low density and at the same time having an exceptional thickness and toughness which was quite impossible prior to the pre-sent invention. As a rnatter of fact, panels manufactur-ed accordi.ng to a process covered by the present inven-tion join the low cost and low density o~ panels ob-tained by steam.molding -to the high mechanical strength and the unlimited life of panels obtained by extrusion.
The process accordi.ng to the p.resen-t invention is characterised in that it i.nc.ludes the ~ollowing 7~

s-tages:
aginy pre-expanded extruded panels of polystyren~ at approximately room temperature for approximately a month, placing each of the pre-expanded aged panels into a mold box having.a length and wid-th slightly larger than the length and width of each panel and a dep-th approxirately 1.6 to 2 times the thickness of each panel, heating each ?anel in the mold box by circulating a heating fluid in a~jacency to the panel, thereby further expanding the panel into rull engagement with the mold box to obtain an expanded panel having a density belo~T 25 KG/C.M. and a-modulus of elasti~ity in compressio above 60 KG/SQ C.M.
The foregoing and other objec_s, ~eatures and advantages of the invention will be apparent from the ,..~,,; .
following more particular description of a preferred embodiment of the invention as illust.rated in accom?aving drawings.
Figure 1 is a perspective vie-.J, partially broken away of an experimental apparatus accord ng ~o the present invention.
Figure 2 is a par-tial section~l view taken along the line II-II in Figure 1.

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Figure ~ is a side elevation view, partially ln section, o~ a L irst commercial embodimen-~ of an ap-paratus aceording to the presen-t invention.
Figure 4 is an end elevation ~Jiew, partly in sect.iorl o~ the apparatus shown in Figure 3.
Fi.gure 5 is a perspective view of the li.gh't fra-me for connecting -the top plate to the ri.gid movabl.e frame.
- Figure 6 is a side elevation view, part'l~ br~oken away of a second commercial embodiment of,an apparatus according to the present invention with the molds all tilt.ed to the left.
Figure 7 is a side elevation view~ partly broken away of the second commercial embodiment of the appa-ratus as shown in Figure 6 with the molds all tilted to the right.
Flgure 8 is an eniarged detail view s~owing the relationship of two adjacent parallel molds from the apparatus in Fig. 7.
Figure 9 is a top plan view of the apparatus shown in Figures 6 and 7 showing two molds tilted in oppositc directi.ons.
Flgure 10 is a side elevation view of the mold element according -to the present i.nvention.
Figure 11 ls a side elevation view o:~ a mechani~
cal asse~bly for pivoting the individual plate elements shown in Figures 6 and 7.
Figure 12 is a plan view of an oven for contain-ing the apparatus shown in Figure 6.
Figures 13, 14 show a variation o-f the oven of ~575~

Figl.lre 12~
A rn.~ss of pre-expanded polystyrene ~btain~d by ex~rusion by ar.y sui.table extrusion appara~us an~l hav-ing a rectangular shaoe l1 long, 12 w~de and 13 thic~, wherl immersed in a stirred solutj.on at a temperatl~re of 100- 115C, wili irregularly expand in every direc-tion and af-ter a time proportional to the square of the thi.ckness finally reached~ for example after three mi-nutes starting from an initial -thi.ckness of 20 mm, the temperature of the solution being 107C, its volume will double the initial one. Figures 1 and 2 show an experi.mental apparatus or mold box for carrying out the process according to -the present invention.
The mold box 20 is comprised of` an open rectangular 1.5 frame 22 having two long sides, one of which is shown at 24 and two short sides, one of which is shown at 26.
The box 22 is sandwiched between a bottom member 28 and a cover m~mber 30. .Since the two members 28 and 30 are substa.ntiall.y :ident;i.cal, only the cover member will be de5cr:ibed i.n detail. The cover member 30 is comprised of a sheet 32 of rela-tively stiff material having a ~lurality of apertures 34 extending therethrough. The apertures are circular and each have a d~ameter of 10 rnm with the total surface area of the apertures equal.ly approximately 50% of the to-tal surface of the sheet 32.
The surface of the shee-t 32 fasing the box 22 is cove-red with a wire net comprised of stainless s-teel wire 0O4 mm in diarneter formi.ng 80 mesh per sq. cm. A rec-ti].l.near frame 36 haviilg cross-pi.eces 38 and 40 is se-~o cured to the surf`aGe of the sheet 32 opposite the wire ~57~;~

net 42 to pro~Jide ~upport for the sheet and net, Thebottom mem~er 28 and the cover member 30 a.re each pro-vided wi-th a plur~lity of aper-tured tahs 43 and 44, respectively, t~!rough which a nut and bolt assembly 46 may extend for holding the bottom ancl cover members in clamped engagement again~t the box 22. The pre-expan-ded polys-tyrene block or shee-t 48 is shown in Figure

2 withir. the box 22 resting on the wire net 4~ of the bottom member 2~.
The ratio between the inner dimensions of the mold box and the dimensions of the pre-expanded block as described above is as fo].lows:
1 (inner length of the box) = 1.03 1 b (inner width of the box) = 1.03 1.2 .5 1 (inner height of the box) = 1.9 13 If the block 48 of pre-expanded polystyrene is t;hen i.rnrnersed in a stirred solution at a 100 - 115C
l;ernperature after pu-tting it into the box as shown in Figures 1 and 2, after a time approximately propor~io-nal to the square of the final thickness, it.will cx-pand to a double volume thus filling the entire box alld after a l.onger time exerts a pressure on the box gencrally up to a maxirnum of 0.2 - 0.3 kp,/sq. cm. lt was thus found that when immersing the pre-expanding ~lr~
~J mass conta.ined in t;he mold box into the hot solution there was no impedi.men-t to expansion due to the fact that i-t can take place in a singl.e directi.on thus sub-stantially doubling its thickness. It was also found that the expansion -taking place ~.n tl~e box according to the present inventi.on compels the expanded mass to 7~

acqu~re a cellu?ar structure consi.sting of cells the cross-section of which alono planes normal to the two maln faces of rthe pane] are chie:fly elllp~ical, the main axis of the ellipses bei.ng chiefly nor~al to the 5 two main faces of the panel, imparts to the panel thus formed a high modulus of elasticity in compression ap-proY~imately two or three times higher than for panels which were given the same volumetric expansion by di-rect i.mmersion into a heating fluid. At the same time 10 it was found that while the expanded mass as formed in the rnold box tends to maintain, after cooling, ..he shape and dimension O r the mold itself, a mass of even denslty and much lower stif~ness as obtained by free expansion in a hot. liquid will shrink to a sensible 15 ~x~ent after cooling.
In a variatiorl of the foregoing experiment, it was found tha.t, the stif-fness of the formed panel could be brought to excellent values by using a relatively low expansion ternperature and hence a relatively long 20 expansion cycle. I-t was also discovered that the t~/o main plates o~ :the mold blocks consi.sting of the aper-tured sheet covered by a wire net could be varied. The holes dri].led in -the sheet could have a di.ame-ter for instance, o-f 1.5 to 15 mm and preferahly range in pa-rallel rows so that the center dlstance between holes is constan-t. To allow :~or heat transmission by con-vection from the hot solution to the pre-expanded mass in the best possible way, the distance between the hole c~enters should be such that -the bored surface is at 30 l.eas~, 5% of the flll.]. surface. The wire nets which, l~S~5~

along wi-th the bored s~eet, form the two main plates of the box, ean be made from a wire having a diameter equal to or less than 1 ~ ~nd woven in such a manner that the distance between the wires is always equal to or less than 1.5 mm. If the mesh of the net is too large the material of the panel being formecl will infiltrate too much into the in-terstices thus causing troublesome adhesion between the panel and the mold plate, The bored sheet acts as a first supporting struc-ture for the wire net and the finer the wire net the smaller should be the diarneter of the holes drilled in the sheet~ It is also possible to replace the above ar-rangemerlt with two plain bored sheets provided that the bores are so small as to reduce the depth of in--~,5 filtratioll of the expanded mass into the holes to a negligible quanti-ty so that the adhesion between the formed p~lnel and -the bored sheet is practically nill.
It will ihus be possible to use, for example, a bored sheet wlth holes having a diame-ter equal to or ],ess than 2 mm covering the 5% to 50% of the entire surface.
In the experiments described above, the expans-ion oI` the pre-expanded mass was obtained by immersion in a sl;irred fluid which in this case was a liquid.
It was found that the results of tests were practically unchanged if the fluid used for the heat convection is a gaseous fluid such as air. Of course the expansion time ~ith a g!aseous fluid will be much longer than in the case when the pre-expanded mass is expanded by di-rect immersion in a hot liquid, especially in the case when the pre~expanded mass is immersed in the gaseous ~ a8S~

g fluid ~herl put into a mold box having an in'tial tem-perature ~qual to or a little higher than the room tem-pera-ture.
Using t`ne k.now-hol~ and general principles deve-loped by using the experimental apparatus of Figure 1 and 2 several different -!~ypes of industri.al apparatus we:re deve~loped primarily for use with a liquid fluid as the heat convec-tion means~ Such apparatus allows - for an economical and efficient industrlal production of expanded polystyrene panels or of panels from an-other expanded polymer of a similar type being charac~
terized by a low density and excellent quality~ Va-rious apparatus developed allow for the automatic open-ing and closing of the mold wherein the bottom plate is fixecl an.d the top and side plates are movable. As in the e~perimental appa.ratus the two main plates may be either a bored sheet covered by a wire net or a plain bored sheet. In -the indus-trial apparatus it was found that the expansion o~ the pre-expanded mass and the space included between the top and bottom main plates and the four side plates could be effected in the shortest possi.ble ti.me if a solution was thermally regu].ated at 95 - 135C (preferably at 100 - 120C) and was circulated by suitable means as described here~
inafter below the bottom plate and above the t,op plate.
The industrial apparatus are provided with an arrange-ment such that the mold can be automatically and in-stantaneously opened after the expansiorl of the mass in order to release the panel. for easy removal and -the subsequent replacement of a fresh pre~expanded mass 1~357~

followed by trle subsequent automatic and instantaneous closure of the appa~atus to start a new cycle~
A flrst embodlment of a suitable industrial ap-paratus is shown in Figures 3-5 wherein the bottom plate assembly 50 is fix~ed and the top plate assembly ~ 52 may be raised and lowe~l~ed into cooperati,ng re].ation to the lower plate assembly 50.
The bottom plate assembly 50 is provi.ded with ,- a pair o~ upstanding cylindrical legs 82 which are con-nected at their upper ends -to a transverse frame member 840 The top pl.ate assembly 52 includes an upper sup-port frame 100 which is provided with a pair of la-te~
rally extending hollow tubular sleeves 86 which are slidab'ly mounted onto the 'cylindrical .l.egs 82 so as to:
1~ gu:ide the ~lpper su.pport frarne 100 for vertical reci--, procati.ng movement relative to the bot-tom plate as-sembly 50. Such vertical reciprocating movemen-t of the upper support frame 100 is driven by a pair of pneuma- -tic cylirlders 88 supported on -the cross-frame 84 and ~0 havin~ the:ir stems 90 connected to the upper support frarile 100.
The top plate assembly 52 further includes a lower suppo:rt frame 6Q which is supported hy the upper support'frame 100 and is adapted to sli~e vertically relative to the la-tter, as it will be apparent from the followi.ng description.
The lower support frame 60 is connec-ted to the lower end~ of two tubular rods 102 which are slidably rnounted within the upper s~pport frame 100. The rods 102 are provicled with adjustable nuts 106 which engage ~18S75~

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the upper SuppoLt frame 100 when the latter is in the raised positi.on illus~rated irl Figures 3, 4, so that the lower support frame 60 is supported by the upper support frame 100. The rods 102 are guided for verti-5 cal reciprocatory movement in a pair of bearing sleeves 108 carri.ed by the frame r~ember 84. A pair of adjusta-ble rings 110 and 112 are threaded on the upper end of each rod 102 for engagement with the frame 84 and the ~ pivoted latch 114, respectively. Each pivoted latch 10 114 is moved to~ardsand away from a respective rod 102 ,, by means of a doublè acting pneumatic cylinder 116 hav-ing a piston rod 118 connected to latch ~embers 114.
The upper support framé 100 includes four vertical plates 124 provided with stiffening fins 126. By way ~5 of exampl.e, the shortest sides 124 are 600 mm and the longes-t sides 124 are 1250 mm.
The bottom plate assembly 50 and the lower sup-port frame 60 of the top plate assembly 52 are each compri.secl. of w:ire net covered perforated sheets 54 and 56, respcctively~ .s:imilar to the wire net covered per-fora-ted sheets 32 in the experimental apparatus of ~igures 1 and 2. A pai,r of retaining sheets 57 and 58 ~re secu~ed to the $upport frames 59 and 60 of the lower and upper plate assemblies 50 and 52, respecti-ve].Y. Rectangular frames 62 and 64 are sandwiched , ~etween the retai.nlng sheets and the wire net coveredperforated shee-ts of.` the bottom and top pla-te assembli-es 50 and 52, respectively. The frame 64 is shown in detail in ~igure 5 and is comprised of a pair of side 30' strips 66 and a pair of end strips 68 secured together 5~

to define a L ectangular ~ame havirlg a plurality of parallel s~aced apart square sectioned s-trip~ 70 ex-tending bet~een ~'ne sicie s-trips 66 parallel to the end strips ~8. T~e side s~rips 66 are providecl wi~h a plu-rality of holes 72 to allow a flow of fluid into thespace betwecn the strips 70. The frame 62 associated wi.th the bo-,tom plate is .identical to the frame 64 des--cribed a~Jove. The bottom plate assembly 50 is provided wl-th strips 74 and 7G which in conjurlc,tion with the tapered side walls 78 and 80 define a chamber for re-cei~ri.ng t;he pre-expanded polystyrene panel prior to the initiati.on of the subsequent expansion cycle. The tapered wal.ls 78 and 80 help guide the pre-expanded polystyrene panel into place within the rectangular 15 f'xarne d~fineAd by the s-trips 74 and 76A The clearances ~etween the pre-expanded panel and the strips 74 and 76 are c,n the order of the clearances between the pre-expanded parlel 4~ and the rectangular f`rame 24 in the experimentc-ll apparatus described above.
The longer side plates 124 of the upper support frame 100 of top plate assembly 52 bear two hori.~ontal ro~s of holes for ci.rculating a heating or cooling li-quid through the mold, which are generally aligned with the holes 72 i.n the lower support frame 60.
Two pairs of side walls 120 and 122 shown in Figures 3 and 4 define with the upper surface of the lower support 59 a container for circulating the treat ing flui.ds for the panels. In:lets 123 and 125 are pro-vi.ded Wi.tilill the. challlber -for the heating fluid and the coolirlg fluid respectively. Walls 78 are provided wi.th l~S7~;~

~ 13 -orific(-,s 7~a -~'or all-~win,~ fluid flow ther~through.
Walls '2Q and 130 are associaced with the lower suppo~t fram~ 6C of the top plate assembly 52 to prevent the Eluids fL~om -;'lowing ovei~ ~he upper surface o~ the sup-port frame 60~ A discharge valve 132 is provided fort,he c~lamber as shown i.n Figure 4 and is opened and clo-sed by means of a pneumatic piston and cylinder assem-bly 13~o Th.e clisch~-lrge valve 132 is provided for hot f`lujds and a si.milar discharge valve (not shown) which is operated by ~ pneumatic pi,ston and cylinder arrange-ment 136 shown in ~'igure 3 i,s provided for the cooling f`luids. In order to control the level of the sol~ltion in t;he chamber defined by t,he wal,ls 120 and 1~, a plate 140 ha-ving a height less -than the walls 120 and '1.5 12~' is provided wi.thin the chamber. After the :Eluid fills the chamber, the excess fluid will overflow the pla-te 1~ and p?SS throllgh an outlet 142 to a recyciing arrarlgerlle]l-t (not shown)O
Th~- apparatus disclosed in ~igures 3--5 is desi--2() 'ned t,o form patlels having a maximum gauge of 80 mm.
'Ln the operation o:C the apparatus a pre-expanded pcly-styrene panel is firstly deposited on the sheet 54 within the chamber defined by wa]ls 78, ~0. Then -the cylin~ers 88 are operated so as to cause a lowering movement of the upper support frame 100 and a corres--ponding rnovement of 'che lower support frame 60 which rest,s onto the fra~me 100 by means of the nuts 106.
~Jhen the adjustable nuts 110 co;ne into engagemen-t with the upper surf,lce ot` the upper suppor-t frame 100, the frame 60 stops w'l.il.e fra,me 100 corl-t,inues to be moved clo~!n~l~;a.rd'~ ~Arcil the ~er-tical plates 124 engage the lower ?l~e assembly closirlg the mold. Then -the heat-ing flu~.d is ~ntroduced via inlet 123 causin~ e~.pa~-sion Ol the pan~l. During such eXpansioQ frame 60 mo ves upwardly re].ative to fra.me 100 un-til adjustable nul,s 112 engage latches 11~. A~ter expansion, the pa-nel is cooled by means o~ the cooling fl.uid irltroduced via i~Let ~25. Firlally, cyli.nders ~ are operated to raise the frame 100 so as to open the mold~ When the latter _omes into engagement with nuts 1.06, it also causes a raisirlg movement of ~rame 60 so as to return i.n 'ch;- position showrl ~n l~igures 3, 4.
,~ comple-te forrlling cycle of a panel takes place in the .~`ol],owi-lg order:
. 15 1. The operator pUtS a pre-expanded panel on the lower maln plate 54 and pushes a bu'cton (not shown) t;o start the cycle.
2. The pneumatic cylinders 8~ displace the frames 100 and 60 to their lower sl~op rJositions whereupon the t.~.~o l~tch merrlbers 114 will be operated by the pneu-matic p:istorl a.nd cylinder assernb~y 116, 118 to engage the upper surfaces of the rings 112 on the rods 102.
~, The deli.very valve for the hot solution is opened and the hot s.olution ,~ills the chamber to over-25 llG`W the p],ate 140 for recircu:lation through the out-let 142. Duri,ng the schedulecl time expansion takes place and the panel ~.s l.`orined.
4, The deli.very valve is cl.osed and the di.schar~e va'lve 132 f`or the hot solution is opened by mearls o}.`
the pneumatic cylinder 134.

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5. When the chamoe~ is emptied, the di~charge val-ve for the hot solution is closed and the delivery va],ve for a cooling liquid is opened and sta~ed open until the chamber has been filled. After a predeter-minated cooling time, the discharge valve forthe cool-ing liquid is opened and the chamber is emptied. The latches 114 are then moved to the open position.
6. Ilnmedlately after the unlatching of the rods - 102, the pneumatic cyli.nders S8 are actua-ted to raise the frames 60 and 100 to their raised positions. The completed panel. may then be removed and a new extruded slab inserted for a subsec~uent cycle. The entire cycle can be automated by a simple circuit which is primari-ly comprised of a set of interconnected timers.
15The described machine i.s an example of a form-.i.ng unit based on the principles and general knowhow whi.ch are the subject of the present i.nvention. In the ! forcgo:ing example, thc mold is opened and closed by the vertlcal displacement of a main upper plate and f`our side plates. In a variation of th~s machine (not illustrated) the four side plate,s 124 are so shaped as to a:~low obtention of panels with molded tongues and grooves along the si~e thereof. Moreover, each of t'he plates 124 is pivotally mounted onto the frame 100 around i.ts u~per edge so as to allow'extraction of the expanded panel from the mol.d.
In thi.s manner it is poss,ible to o~tain panels having fc,r in.stance two adjacent side walls formed with ribs ancl the other two adjacent s:i.de wa'lls formed ,io with grooves.

75g I'he forming 5ime for a panel, that is thc tlme taken by the pre-expanded panel to expand due to heat ta~en .rom the circulating ho~, solution until it has filled the ertire mold, is, under even worlxing condi-tions, directly propor-tional to the square of the thicknec;s of the desired~ finished panel. The forming ti.me for a ~inished panel 40 mm thi.ck can vary, for even weight and thickness of pre-expanded panels, with ,. ~, circulati,on ~.emperature o` 107C for the hot solut~
.0 ion, from 3 to 4 minutes according to the type of polymcr as well as the type and quantity of propellent agents used ~or the extrusion of the pre-expanded panels. The time taXen by the comple-te cycle under the same conditions wi:ll be ~ to 5 minutes.
TQ make the process more productive and the plant more economical, several forming units can be set up in series and fed by a single pump -for the hot solutlon and a single pump for the cooling solution.
lt ~a~ foun-l that the flow of both hearir,g and coolirlg 2t~ liquids through the unit,s can vary -to a. very small cxterlt, even when clue to working requirement, most o`
the units are ldle, prov:ided that centri~ugal horizon-tal pumps are usecl having delivery-lift curves which arè substantially flat.
~5 The abo~re--described devices according to thepre-sent invention were based upon heat convection by a liqui.d flui.d~ No matte~ which heat convection means is used, that is, liquicl or aeriiorm f`lui.ds, for cor-rect formatiorl o~ the panel, hvth -t:he mold and the parlel itself should be hea-t;ed and maintained at a d~-5~5~

sired temperature for a predeterMinated tim~. The dc-vices dlscussed above are industrially competitlve since by using a liquid cor.vection m~ans, the heating of the mo].d is almost immediate thereby cauc;ing the quick heating of the pre-expanded extrudate. When using an aèriform fluid as the convection medium, the heating ti.me will he relatively long f`or bot~l the mold and -the panel in order to enable the pa.nel to reach ~- the expansion tempera'cure. However 9 i t was found -that L0 unit.s based upon an aeriform flui.d as the heat convec-tion me~ns can be successfully used if several molds are set up parallel to each other at a distance of 10 to 40 mm from eaCh other. In this way, one of the two main plates, of a mold member forms a gap wi-th one of th~ two main plates of the next mold mem~er. The gap is f.Lrst u.sed for the ci.rculation of a heating aeri-form flu:i.d and then fo. the circula-tion of a cooling aèri:J~orm fl~ id.
In the ernrGdiment o. Figures 6~1C, several rnold 2() members have been set up parallel to each other for pi.votal rnovemen-t to allow for the easy inser~ion and remova:l of pane:Ls.
In Figure 6, a p-lurality of mold members 200 are eacn pivoted at 204 to a support member 202 in par~llel relationship to each other. The construction of the individual mold members 200, which a.re identic-al to each other, is best seen in Eigures 8, 9 and 10.
Each mold rnember 200 is cornprised of two flat parallel p:L~tes 20~ .which may be ol` alumirlium or any other sui-table heat conductive materi.al. A plurality of paral-~1~5~59 1~ --lel spaced hollow aluminium tubes 208 having asubstan,.ially square cross-section are secured betwe~n the piates 206 as ~est seen in ~igure ~. Iron bars 2~7 and 209 are secured bet~Jee}l the aluminium sl-leets 206 5 along the edges thereof to form a rigid assembly. To further reinforce the assemb],y, a pair of elongated U-,shaped charlne]s 210 and 212 are secured to the sur--f`aces of the plates 206 along the shorter edges of`
the rectilinear mold assembly. The iron ba.rs 2~9 are 10 machined at their ends to receive the pivots 2.~4 which allow the mo]d members to rotate relative to the sup--port frame 202. A pair of L-shaped aluminium strip.~
21~ and 218 are secured to opposite sides of each mold member 2no by means of bolts, screws or the li~e along lS the long~r edges of each r~ol.d member. A shorter~
L,-~,haped alumillium st:rip 220 is secured to one side c~:~ the mold member along one short edge of the mold rnc~mber adjacent the U-shaped channel member 212 and ~mother 'shorter L-sha,ped s-trip 22G is secured al.ong 20 thc opr~osi.te side of the rnold member along the oppo-si.te shorter edge adJacent the U-shaped channel ~ember 212. The rnold member 200 may be assennbled by means of nuts and bolts or any other suitable means and pro-vides a subst;antially hollow interior for the passage ' 2~ of` a flui.d therethrollgh. The ends of the channels 208 are open at the opposite ends as are the spaces between each of the channels 208 to ~acilitate the passa,ge of the treating ~lu:i,d therethrough~ The faces of -the L-.shaped str-ips 216, 218 and 220 whi,ch face S0 towal~dc;the interior of a mold, form an angle with the _ 1 9 _ alumin-urn sheets 2Q6 slightly greater than 90 to faci~
lit~te the r~mo-~al of the formed panels upon opening of -the ~iolcl.
In Fig~re 8, two adJacent mold members 200 are shown in section and are disposed parallel to each other ~.t an angle ~ with respeet to the hori.zontal plane.
The individual mold members 200 may be rotated from the position shown in Figure 6 wherein they are dis-,.- posed at said an~l.e ~ relative -to the ~lorizontal plane to the position shown in Figure 7 wherein the indi-vidual mold members ~lre disposed a-t said angle ~ re-lative to the horizontal plane. An extruded, pre-cx,pallded panel, is adapted -to be located between each acljacerlt pair of mold members 200. hs viewed in Figure ~, the ex~rude~l panel would be placed between the up-warcl].y projecti.ng L--shaped s-trip 216 on one member 200 and the downwardly e,Ytending L-shaped strip 21~ on the adja,cen-t rnold mernber which are spaced apart a distance 12. This dis-tance 12 is equal to the desired wi.dth of`
the fi,nal expanded pane]..
When usillg the mu:Ltiple rnold members as ill.us-trated ir! this emho~irrlerlt, it wi,ll be possible to vary the gauge or thiclcne,ss of the formed pane:l. by varying the ang].e ~ o-P the i.ndi.viclual mold elements 200 ~.;ith 2'~ respect to the horizontal plane OI`, ].ess pre~erably, varyi.ng the center distance between the hinge pins for the ~nd:ividual molcl rnembers. It has been found that the relationshii~ between the gauge T of the pane~ 3 the d:istance I bel.ween the hinge centers, the thickness S c,~f the rrlold e].emenl,s 200 and the angle c~ formed by ~:~8575~

- 20 ~

the mold eiements 200 wlth Ihe horizontal plane is ~he following:
C~ ~ = arrsin (~ S) I
where I > T + S
In the apparcltus as shown in Figure 6, I is equa] to 130 mm and S is eclual to 23 mm.
The follow-~ng table indicates the various an-gles ~ for the mold elemen-ts in order to obtaln panels 30 to 60 mm thick.

hi kness o _P nel ~

2~.5 ~5 31.5 34.2 36.9 39.7 rrABLE N0.
It will thus be possible to vary the thickness c;~ the panel within a wide range by varying the ang]e ~ of the members 20'0 wlthin a range which is effi-cient for good operation. In the described apparatus the thickness o~ the -formecl panel can be varied by merely changing the angle C~ of the mold members 20Q
and replacing the L--shaped side plates of the mold.
It shou~Lcl be noted -that when varying the angle ~ the width of the parlel as illustrated by ~Ghe dis-tance 12 in I~igure 12 will also change slightly, The fol ~L~L85~

lowing re1ationshiv is found bet.~een the dlmens7Gns l~ and i2:
I

11 = 12 + ~ ~ ?
Where I equals the distance between t,he hinge centers.
The following table gives the values, calculat-ed by the ahove relati.onship to be taken for 11 i.n - order to obtain pane:Ls of a cons-tant width, for ex-amp].e 630 rnm for tne various gauges:
_uge o~ pansl 11 ~0 73~

TABLE N0. 2 The necessity for a s].igh-t var]ation of 11 when the panel gauge must be changed is not de-tri-mental to either cost or efflciency of the plant.
When changes are made wi-th respect to the desired thic~ness of the panel the L-shaped strips must also be changed in order to obtain -the proper values for l1. Alternatively, the securing means for the L-shap-ed strips cou]d be suitably displaced.
The angular slant ~ of the plate elements 200 can be varied by changing the heigh-t of the support column 730 in Figure 6. The illustrate~ device acts

3~ as a multip]e mo:ld hoth in the position of Figure 6 .

7~

and the ~osition of Figure 7. When the molds defined by the mol~ members 200 o~ Figllr~ 6 are filled with formed panels after the expansion cycle in an oven to be desc.ribed in detail hereinafter~ the latch 232 is opened and the individual mold members 200 are rotated clockwise either manually or by means of mechanical.
assembli.es described hereinafter f`rom the posi-tion shown in Figure 6 to ~he posil,ion shown in Figure " 7. The individual mold members 200 are rotated indi.-vidual:Ly which wil] allow the operator to remove theformed panels and put a fresl-l extruded pre-expanded panel into the mo].d, Two rigid i.ron frames 234 and 236 shown in Figure 6 provide a suitable support for resist:ing -the pre~ssllre generated by the expansion of the ~anel,c, withi.n the individual mold. The individual mold plate 20G is so designed that air may freely cir-cu:Late in the gap between the two aluminium sheets 206 of` each rnold Inernber.
1ll the~ ernbod:i.rnent of Figure 11 mechanical means have beerl provided for rotating the individual mold rnernbers 200 abou-t their pivots 214. A plural.ity of pneumat;:ic or hydraulic cylinders 240 and 242 are con-nccted t.o l.evers 244 which in turn are connected to the pivots 214 for rotating the panels upon energi.zat-ion of the cyli.nders~
: The schema-ti.c plarl view of the oven in which the mold assemblies are located is shown in Figure 12.
The bJ.ower 215 i.s provided f`or ci.rculating the ai.r through the oven 252 in the directi.on of tthe arrows.
The air is heat;ed ~v passing over heating elements , located in the passage member 254 which also inc,Lude means lor adjustir1g durin~ the cycle the temperature of the air according -to the working time in the tem-perature in the mold plate. A pair of ventilation doors 256 and 258 are provided which may be manually or automatically opened and closed. A throttle gate (not shown) which is controlled by the projection 260 is located within the chamber 262 to open or close the passage for the air through said chamber. When a mold assembly such as that shown in Figure lO is located wi-thin the oven -the external dimensions of -the mold assembly are substantially equal to the internal di-mensions of the oven so as to force most of the air through the passages between the plate 206 of the in-divldual mold members 200. Therefore the plates of' the mold membexs are quickly heated to -the desired temperature and the heat is transmitted from the pla-tes to the panels by both connection and radiation.
Complete expansion is achieved ln a period of time from 12 to 25 minu1;es. It has been found that in order to obtain a more uni~orm and nlore rapid expan~-ion it is useful to charlge the di~ection of the air ~low periodica:lly, for instance every 30-60 seconds.
This can be made by the oven shown in Figures 13, 14 which is provided with auxiliary conduits 304, 306 and gate~ 300, 302. The gates 300, 302 are displaced pe-riodica,lly between the positivns shown in Figures 300, 302 to cause the air ~low to be inverted~ When the throttle gate in the chclmber 262 is closed~ dovrs 256 arld 258 are openecl and the heaters in the section 25 ~57~

- 2~ -are switched o~f so that cold air coming from the out-side is then circulated through the molds in order to cool the molds and the formed panels within the molds in a matter of minlltes.
With the illustrated device the working cycle is as follows:
1. Rernoval of formed panels and laying of unform~
ed panels into the mold. During this operation -the unit changes for example from the configuration shown in Figure 6 to the confi.guratiQn shown in Figure 7.
2. The entire mold assembly is lntroduced into the oven where expansion and formation of the panels is achieved by the passage of heated air through the mold mernbers. 'rhe formed panels are subsequently cooled by the passa~,e o~ unheated air.
3. The mold assembly which may be equipped with wheels mounted on rails may then be removed from the oven for t;he rernoval of -the formed panels and insert-ion o~ fresh pre-expanded panels. In this process the ~o mo:Ld pla-tes will move from the posi-tion shown in Fi-gure 7 to the posi.ti.on shown in ~igure 6.
By the use of several comple-te mold assernblies and ovenC the operator can continuously remove formed panel.s and insert unforMed panels into the molds. In this way the required labour for production of a fini~
shed panel; will only involve the tirne taken to rotate an indi.vidual mo].d member~ and remove a formed panel and lay an unformed panel into the mold, in spite of ;-the fact that the expa.nsion cycle i.s.relative].y long and 130 the average and final expans:ion temperature measured .

in the p2.nel are relati~eI~J low (for example the ave-rage temperature is S7 àegrees C and -the final tempe-rature is 95 degrees C). Therefore in a ~lant having a plurality of units of the type described above is possible to obtain expansion of the panels in a rela-tively low temperature ln order to provide panels hav-ing a very high modulus of elasticlty in compression.
The pre-expanded panels suitable for use with ,, the means and processes covered by the pre.sent in-venti,on are generally expanded polystyrene panels pro-duced by extrusl,on and the simultaneous injection of a propellent agent. The extruded panels do not need to comply with strict rules as to regular shape and dimc:nsi,orls since -this will be achieved during an ex-pansion. By the means ~nd processes of the present invent:i,on it will also be possible to use pre-expanded ; pane:Ls o:E a certain gauge sliced from thic~er panels.
When an aeriform fluid is used as the heat con-vecti.on means it is possible to successfully form a s:irl~,lc~ panel from two or more panels the total gauge of wh:i.ch will ~e equal to the desired gauge. In this case 1;he p:lalralj.ty of panels are piled in a single mold and provide a single f`ormed panel since their surface~ wi].l be fully we]..ded together during expans-; 25 ion.
It was found that the pre~expanded panels are suitable for the process according to the present in-ventl.orl if the ext,ruder is fi-t.ted with proper means for the unifo.rm cool:ing of -the expanda~le plastic ma-30- terial to tlle required temperature before it exits ~ .

13L~S~S~

- 2~ -from the ~-xtrusion die. As a matter cf fact it ~r~as found that low density and very stiff panels in accor-dance with the present inven-tion cannot be obtained when the temperc..ture of the expandable mass before e~-it from the die is higher than a predetermined valuewhi.ch i.s primari.ly depended on the ~uali-ty and quanti-ty of the propeLlent agent and the rnolecular weight of the ~olystyrene resin used. It was also found that both s-ti.ffrless and dimensional stability of a panel formed 10 by the a:i.r of the presen-t invention increase to a pre-cepti.bl.e degree if the pre-expanded panel has been given a ~ertain conditioning immediately after ex'crus-ion. It was a].so found -that -the longer the condition-in~ time the lower will be the minimum density obtained 15 for a certain type of pre-expanded panel. This also holds true in the case of equal conditioning time pe--rlods, having a hi.gher conditioning -temperature Table No. 3 shows the results of forming tests connec:ted wil;}l the apparatus covered by the present in-vention on a number of pre-expanded panels of the same type. Th- thickness and the density of formed panels as well as the condition.i.ng time and temperature of un-formed panels will va.ry and proper mea.sures and re-rna.rks were noted. The data in Table NoO 3 shows that 25 the longer the conditioning time and in the case of equal time, the higher the conditioning temperature, the modulus of elasticlty i.n compression and t~e dimen-si.ona:L st;abili.t.y o:f the fo-rmed panel. tend t,o i.ncrease up to a li.mit shown by the table. It i.s also evident 30 that very low density (for example 18-20 Kg/c.m~), -' _ _ _ _ _ _ tl __ _tl ___ h _ _ _ _ _ i~i ~ h ~ h ,-~ h ' h h ~ h ~ ~ ~ h h h i~ -1 ¦ o l o .- .~I'C l .- I ,_ h ¦ h td h ti) h ~ jl~ ~ ~ ~1 ~

E3 ~ ~ ~ l 'æ ~ ~ l ~ ~s C ~ ~ ~ ~ ~ ~ ~
1a~ ~ ~ r-~ ~ T
~ E ~ ~o l ~ ~ ~ I ~s 1~ C ~ ~ O _ ~ ~ ~ Lr ~

,. ~ r ~ e' e _ e e e--, E e e' E ~ e e e E E e a ~
b1 b~ ~ ~ ~ ~ ~ ~,~ ~
E E~ l ~ E E . I E E E~ E E E E E E b v E

a ~ b O O O E C a C C C a C O C ; C Es E ~ ~ O t O O O O C O ¦ O ro O O O ¦ O C O ~3 a . a 3 _ _ __ _ _ _ _ _ __ _ ~--____ ~ _ ~, c o to C C C C o C ~ I o~ C~ '.' o a _~ o C C D V ----U o O C __ C

h ~ h h Y~ h Y h Y h O E O ~ m 3 C
. t~. ~. ~ ~t ~ d r~' C C ~ C C~ t~ t~ tC ~ ~ . ~. ~ ~ ~ __ ~ ~ ~ N _ ~ _ _ ~ _ C ~ ~ ~ Lr ~ ~ O _ ~s~
especially if accompanied by good compressive strenyth and dimensional stability, can be obtained only in that case where the unformed panels were properly conditioned during a time depending upon the conditioning temperature. The pre-expanded panels used for the test under Table 3 were preparea using high molecular weight~ heat resistant polystyrene resin of the ~TAL~ type produced by Montedison.
It was found that when using a polystyrene resin having a lower molecular weight and a lower heat resistance the results became worse. Table No. 3 gives evidence that stiffness and dimensional stability are closely connected and that high values of modulus of elasticity in compression correspond to satisfactory dimensional stability or a low values correspond to shrinking and deformation in the finished panel. When cooling after expansion and forming, the pressure inthe cells of the panels tends to decrease and to hecome lower than atmospheric pressure. Therefore shrinking and deEormation takes place in the formed panel when the cellular structure is squeezed due to a pressure dif~erenc~ which is established between the external atmosphere and the gaseous mass contained in the individual cells.
Examples Pre~expanded panels 1240 mm long, 590 mm wide, 25 to 60 mm thick are obtained using the Twin-Screw RC.41/E built by LMP. The extruder is fitted with an extrusion die suitable for production of panels 590-600 mm wide, and of all re-- ~8 -* Trademark cr/.~

, ~357'5~

_ 29 -quired means 7 such as expanding plates with adjustable gap~ roller haul-offs for hauling and cooling, side cutters for -,stablishi,ng sharp corners, cutting devices etc.~ for produc-tion o~ panels of said size. The ex-truder is fed with a mixture having the following com-position, at- a rate of 200 Kg. per hour:-Ma-terial Parts (based on weight) TAL polys-tyretle resin lnO
(by Montedison) ,~o Citri,c Acid 0.12 Sodium Bicarbonate 0.220 Talc 0-300 Pen-thabromoc'illorocycloesane 2 ~,5As a propell.ent agentl a m:ix-tu.re of 20 par-t,s by wei~ht of Freon II and 80 par-ts of Freon 12 is in;jected :i.nto the ex-truder at a ra-te of` 28-32 l~g~ per hour;
theref`ore -the prope].len-t agent forms approx. 12.2 to 13.'7 perc~en-t by ~ei.ght of' the expandable mass be:ing 20 forrned :in-l;o the barrel o~ the extruder~ The screw speed is approx. 20 rev. per minute; -the tempera-ture of - the barrel ranges from 220C (-top and cen-ter) to 100C
(end). The expandable rnass enters -the cooling device included in the plan-t a-t a temperature of` 148C and 25 flows ir~to the extruslon die after i.ts cool:ing a,t a telnpera-ture of 135C. At the exit f`rom the die the n-la-teria.:L iis given a~ controlled expansion; between two . teflon coated plates and the panel thus obtai.ned is subsequen-tly gauged and cooled by a set of` roller 30 hau].-off`s. The density of pre-e~panded panels thus 57S~

obtained is 3,' to 39 Kg/c.m..

Exc-lmple No. 1 A pre-expanded panel 1240 mm long, ~90 mm wide, 24 mm thick obtained according to the above process, having a density of 38 ~g/c.m. and condltionecl over one month at room tempera-ture is formed using the uni-t il-lus-trated in Flgo 1 and 2 properly adJusted. The tern---- perature of the heatirlg solution (a saturated solutlon - 10 of Sodium ChloIide :in water) circulating through the unit at a flow o~ about 200 litres per mi.nute is 107C.
The forming -time of the panel is ~'10"; the total time taken by the cycle is approx. 5'.
The dimensiors of the formed pa.nel are 1250 x 15 600 x 45 mm and do not v~ry when agin~; the densit~ is 20 Kg/c.m.; the modu].us of elasl-icity in compression, accordi.ng to ASllM Dl621 -- ~3, is 88 Kg/sq. cm..
Separ~tely a piece cut from a panel of the same type as above and conditioned too for one month time at 7-0 room ~,emperature, i.s immersed for 2'50" in a saturated solutiorl of Sodi.um Chloride 107C. After the ensuing free exprmsion -to all directions~ the density becomes approx~ 20 Kg/c.m.; .the modulus of elasticity in com-pression. results to be 3S Kg/sq. cl~.. Shrinking is 25 shown i.n the expanded mass just 15' after expansion.

Exqmple No. 2 l'he experiment o:f example No. 1 is repeated with a tempe.rature of circula.t:i.rlg so].ution (aqueous solu-tion 30 of Calciurrl Chloride in suitable concentration) therrnal-- 1~L8~759 ly regula-ted at 125~C; the forminO time of panel ls about 2'45"; t`ne to~al time taken by the cycle is ap-prox. ~'35"; the density of the formed panel is 20 Kg/
c.m.; the modulus of elasticity in compression is 48 Kg/sq.cm..
When increasing the temperature of the heating liquid the expansion -time will. shorten in respect to example No. 1 bu-t the modulus of e].ast:i,cit;y of the form-,. ed panel will decrease a good whlle.
Fxamp_e No. 3 The experiment of Example No. 1 is repeated but the unit is adjusted for a -thickness of 50 mm of the formed pane]
The forming time of the panel becomes 5'; the total tirne taken by the cycle 6'. The final density of the panel is found to be 18 ~Cg/c.m F.,xamp e No. ~
rrwenty panels 1270 mm long. 600 mm wide, 25 mm thick, havirlg a clensity of 39 Kg/c.m. obtained accord~
ing to the above process andconditionecl over one month att room tempera-ture are formed using the unit illustrated in Fig. 8 after adjusting it for a final tthickness of 25 panel of ~5 mm. By means of the oven schema-tically shown in Fi.g. 16 hot air is circulated in the gaps of the multiple mold during a time of 16'. During this period of time the temperature of circulating air, of mold plates and of the inside of' t;he panel vary as a 30 funct:i,on of -time according to the foll.owing table:-1185~5~3 Temperature Temperature of Temperature ~easured Into Tinle Circulatin~, Alr of ?lates ~he Panel 0' 19 19 19 2' 100 56~ ~0 ~' 120 82 55 5 6' 135 100 66~
8' 125 110 75 10' ~10 ~.~,0 8~
12' 110 1~.0 93~
0 1~.0 97 TABL~ N0. 4 TABLE N0. 4 - The heating cycle is followed by a cooling cycle, conducted in -the illustrated manner, lasting about 3'. The den.sity of the formed panels, which are 1280 mm long, 630 mm wide, 45 rnm thick~ is found to be 15 20-4 Kg/c.m.. The modulus of elasticity in compres-s:ion, according to ASTM DI621 - '73, is found to be 1,12 ~Cg/sq. Cnl~ . The compression strcngth at 10 percent de:r::Lectiorl according to A,STM DI621 - 73 is 2.3 Kg/sq.
cm.
Factor ~ o:E' thermal conductivity is 0.025 Koal/
sq. m./hour/C/m.. The modulus of elastici-ty in com-pression ~or panels o~ even density obtained by- steam 'mouldi.ng of pre-expanded beads is generally 18-22 Kg~
sq.cm.. Pan~ls directly extruded which are marketed 25 have a compressi.on strength comparahle to that of pa-ne].s obtained as per this Example No. 4 if their densi-ty is not lower than 33-34 Kg/c.m,.
:
P,XQ pl~-~ No 5 ~ T'he experiment of Example NOO 4 is repeated 3L~85759 star~lr~ ith the ~iates at a temperature of 70-80C
(that is the ~emperature at, the end of the anteeedent cooling cycle1. The duration Gf the heating cycle ne-cessary for forming o~ the panels is reduced from 16 minutes to 12 minutes.

Example ~o. 6 The experiment of Examp]e No. 4 is repeated after adjustirlg the multiple mold for a thi,ckness of 40 mm for the formed panel and an heating cycle a bit shorter. The c3ensity of formed panels is ifound to be 23 Kg/c.m., while the modulus of elasticity in compres-sion :is 116 Kg/sq.crn. and the compression strengt}l at 10 percerlt deflection is ~.5 Kg/sq,cm.

Exarnple No. 7 The experiment o:E Exarnple Mo. 4 is repeated but the length and width o the pre~expanded panels are broug~Jt to resp, 1260 mm (from 1270) and 570 (from 600 mrn). The final density is fo~nd to be 19 Kg/c.m.;
the modulus o~ elasticity in compression 108 Kg./sq~cm;
the compression streng-th at 10 percent de1ectiorl 1.9 Kg/sq.cm~.

Example No. 8 _ _. ______ Twenty pre-expanded panels 1216 mrn long, 698 mm wide, 17 mm thick cut from ex-truded panels 0.037 densi-ty 51 mm thick, conditioned for one mon-th time at room ternperature are i'ornled as for C,xample No. 4 bu-t af-ter 30 aclJusting the unit for a fina], thickness oE form.c-d 1~8s759 .

panel of 30 mm.
The density of formed panels is found to be 19 Kg/c.m,; th~ modlllus of elasticity in compression, ac-cordi.ng to AS'rM ~I621 - 73, is 12~ Kg/sq.cm..

EY.a,rnple No. 9 __ __ _ The experiments of Examples 4-8 inclus:ive are repeated with pre expanded panels extruded by using .'" only Freon 12 instea,d of a mixture of 20 pa,rts by 19 wei,gh-t of Freon 11 and 80 parts of Freon 12 and con-ditioned over one month a-t room temperature.
The results continue -to be substantially the same of the foregoing Examples, i:~ -the circulating air and the plates of the mo].ds are regulated at a ternpera-~,5 ture higher tha.ll about 8-10C.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for obtaining an expanded polystyrene panel comprising aging pre-expanded extruded panels of polystyrene at approximately room temperature for approximately a month, placing each of said pre-expanded aged panels into a mold box having a length and width slightly larger than the length and width of each panel and a depth approximately 1.6 to 2 times the thickness of each panel, heating each panel in said mold box by circulating a heated fluid in adjacency to said panel, thereby further expanding said panel into full engagement with said mold box to obtain an expanded panel having a density below 25 KG/C.M. and a modulus of elasticity in compression above 60 KG/SQ C.M.

2. A process according to Claim 1 further comprising increasing the compression strength of the formed panel by using expansion temperatures not greater than 97°C
and expansion cycles of at least about 18 minutes so as to obtain a panel having a value of the modulus of elasticity in compression which is almost 5.5 times the value of the density of the panel.