referring initially to fig1 , a corrugated medium web 10 made from a paper web impregnated with a b - phase phenolic resin must be heated to a curing temperature sufficient to convert the b - phase to an a - phase in which the web is fully waterproof . the cured web also becomes substantially more stiff and severe bending of the web is thereafter restricted . however , web stiffness is an important characteristic of the corrugated web and the treated liner web to which it is attached , as will as discussed below , for processing in accordance with the method of open core element manufacturing disclosed in the above identified copending application . in the embodiment shown , the corrugated medium web 10 is formed between interengaging upper and lower corrugating conveyors 11 and 12 , respectively . each of the conveyors 11 and 12 comprises a belt of interconnected articulated flights 13 that have flute - forming teeth 14 to provide flutes of a desired depth and pitch . for example , flutes having a pitch of ¾ in . ( 19 mm ) and a depth of ½ in . ( 13 mm ) are satisfactory . the flights , preferably of aluminum , may be heated to minimize heat loss in the treatment bath to be described . a heated bath 16 is positioned to receive the corrugated medium web 10 after it is formed and the lower corrugating conveyor 12 is directed away from the web and downwardly in a return run . the corrugated web 10 is retained on the underside of the upper corrugating conveyor 11 where the web flutes 15 remain in intimate contact with the teeth 14 of the conveyor flights 13 . the bath 16 contains a low melting point metal alloy that is used to heat the web 10 and cure the phenolic resin as it passes through the bath 16 in contact with the molten alloy . one particularly well suited alloy is a 60 / 40 bismuth - tin alloy which is heated to about 400 ° f . ( about 200 ° c .). electric resistance heating may be used to maintain the bath temperature , but other heat sources may also be used . the bath has a generally horizontal bottom wall 17 , enclosed laterally by a pair of side walls 18 defining coplanar upper edges 20 . the upstream end of the bath is defined by an alloy supply header 21 that extends between the side walls 18 and has a horizontal upper edge 22 that is lower than the upper edges 20 of the end walls . the supply header 21 defines a molten metal distribution reservoir 23 for the uniform supply of molten alloy . the downstream end of the bath 16 is defined by a weir dam 24 that has a horizontal upper edge 25 that lies generally coplanar with the upper edge 22 of the upstream supply header 21 . the weir dam 24 defines an open slot 29 for receipt of the molten metal alloy that overflows the weir . the molten metal is circulated through the bath in a closed circuit including a pump 26 receiving molten metal flowing into the slot 29 in the weir dam 24 and returning it to the alloy supply header 21 where it is distributed evenly and uniformly across the upstream end of the bath and downstream of the upstream and wall 19 . in operation , the corrugated medium web 10 is carried by the upper conveyor 11 such that the tips of the flutes 15 slide over the upper edge 22 of the upper end wall and into contact with the molten alloy . the alloy in the bath is forced by pump pressure up into the flutes on the conveyor teeth 14 . pump pressure is adjusted to provide sufficient head to fill the web flutes , preferably with a slight over - pressure to assure the underside of the fluted web 10 is fully contact by the molten alloy . movement of the conveyor causes the flutes to assist in carrying the alloy downstream and over the weir 24 . this action assures that the corrugated medium web 10 carried on the conveyor 11 is fully contacted by the molten alloy . this , in turn , assures that the entire web 10 is heated sufficiently to convert the phenolic to the a - phase . as the upper conveyor 11 and attached corrugated web 10 reach the downstream end of the bath , the flutes 15 engage and slide over the upper edge 25 of the weir dam 24 and the alloy drops into the slot 29 and travels through return passages 28 in the side walls 18 of the bath by operation of the pump 26 . with a medium web 10 saturated with about 15 % by weight of b - phase phenolic , the web is fully cured to the a - phase if it is retained in a bath of alloy at the indicated temperature for about 4 seconds . referring also to fig2 , a liner web 28 , also impregnated with b - phase phenolic , is directed with a liner conveyor 31 through a second bath 30 of molten metal alloy . the liner web 28 is cured in the same manner whereby the phenolic is converted to a - phase and fully cured . a suitable adhesive is supplied to the tips of the flutes 15 by an adhesive roll 12 while the medium web 10 remains carried on the underside of the upper conveyor 11 . one suitable adhesive is a hot melt polyamide . the glued flute tips are joined to the cured liner web 28 on a contact roll 33 to form a fully cured single face web 34 . fig3 and 4 show details of the molten alloy bath 16 . the alloy return lines 27 are connected beneath the bath to a center alloy supply tube 35 connected to the alloy supply header 21 . the supply header includes the distribution reservoir 23 which , as shown best in fig4 , includes an upwardly sloping lower wall 36 and an upper wall 37 that is provided with a pattern of outlet holes 38 that increase in size from the center laterally in both directions . this arrangement assures uniform distribution of the molten metal alloy across the entire width of the bottom wall 17 of the bath . as shown schematically in fig5 , the conveyor flights 13 , which preferably comprise aluminum extrusions , are carried on a plurality of parallel laterally spaced roller chains 40 to which are attached pairs of oppositely extending upper and lower c - shaped attachments , each having horizontal mounting legs 41 and 42 , respectively . the lower mounting legs 42 are secured to the flights 13 and the upper legs 41 are captured in slots 39 in a low friction plastic bearing rail 43 . the bearing rail is preferably made of ptfe . in order to inhibit leakage of the molten alloy between the conveyor 11 and the side walls 18 of the bath , the inner surface along the upper edge of each side wall is provided with a sealing strip 44 against which the opposite ends of the flights 13 of the upper conveyor 11 bear in operation . the sealing strip may be seen in fig1 and 3 - 5 . it is preferable to apply a vacuum to the upper corrugating conveyor 11 to aid in holding the corrugated medium web 10 in intimate contact with the conveyor flights 13 . one means of providing vacuum is to support the conveyor 11 , via the bearing rails 43 , on the underside of a vacuum plenum 45 , as shown schematically in fig2 . the conveyor flights 13 are attached to the carrying roller chains 40 such that the faces of adjacent flights 13 are spaced apart slightly , thereby allowing the vacuum to be applied directly to the corrugated medium web 10 . the sealing strip 44 also assists in sealing against vacuum loss .