Method and apparatus for preparing multi-cellular foamed board of thermoplastic resin

In a method of preparing a low density foamed board of a thermoplastic resin by extruding a foamable resin from a die mounted on an extruder and passing through a moulding passage, an improvement which comprises forming the foamable resin to have a flat shape in the die, extruding thus flattened resin from the die having a convex foward end extending in the width direction of the flat shape, introducing the extruded resin immediately into the moulding passage wherein the foamable resin is allowed to commence foaming primarily in the thickness direction at a portion adjacent to and extending in the convex forward end, and then allowing the resin to be foamed in both the thickness and width directions in the moulding passage.

An apparatus for preparing a multi-cellular foamed board of thermoplastic resin which comprises an extruder, a die and a moulding passage, the die having at the forward end thereof an orifice having a broad width with respect to a height thereof, the forward end forming a convex surface which protrudes the greatest in the resin advancing direction at a middle portion in the width direction and retreats progressively away from the middle portion, the moulding passage having a resin passageway therein and a concave surface on the inlet side thereof, the resin passageway communicating with the orifice, the concave surface being engageable with the convex surface of the die so that the moulding passage is closely contacted with the convex surface of the die, and the resin pasageway in the passage being enlarged at a portion close to and extending along the concave surface in the thickness direction and in the width direction toward the resin advancing direction.