Gas-assisted injection molding of hollow ribbed article

In a process of gas assisted injection molding of the type wherein an article is formed in a mold, the article having a rib with a hollow base and solid upper part, the rib upper part having a width narrower than the base;

• wherein heated molding material is injected into a relatively cool mold to thereby be cooled somewhat, and gas is then injected into the mold, to cause the gas to flow through and along the rib and thereby hollow the base of the rib;
• the improvement which comprises:
• shaping the mold to make a rib having a substantially rectangular cross section base, a substantially rectangular cross section upper part and a stepped transition zone therebetween;
• the base width adjacent the transition zone substantially greater than the upper part width adjacent the transition zone;
• wherein, the shape of the rib causes cooling in the upper part and transition zone sufficient in degree, compared to the base, to cause the gas to flow preferentially through the base and not through the transition zone and upper part;
• to thereby more consistently make the base of the rib hollow and the upper part and transition zone solid.

In a process of gas assisted injection molding of the type wherein heated molding material is injected into a mold to form an article having a rib;

• wherein, following injection of molten molding material, gas is injected into the mold for the purpose of flowing through the rib to hollow the rib;
• and, wherein, the injected gas pushes molten molding material along the length of the rib from opposing directions and into a subsection of the rib, to thereby form the subsection as a solid rib subsection lacking any hollow, while adjacent rib portions are hollow;
• the improvement which comprises:
• narrowing the width of the rib in the vicinity of the subsection which is solid and which lacks any hollow, compared to the width of the adjacent rib portions which are hollow, to lessen distortion which would otherwise occur.