Injection molding method

• US 5,246,646 A
• Filed: 04/15/1991
• Issued: 09/21/1993
• Est. Priority Date: 11/02/1988
• Status: Expired due to Term

• Current Assignee: Dean Lamer
• Original Assignee: Dean Lamer
• Inventors: Baxi, Indra R.
• Primary Examiner(s): Tentoni, Leo B.
• Application Number:
  US07/685,519
• Time in Patent Office: 890 Days
• Field of Search: 264/40.3, 264/85, 264/328.8, 264/328.12, 264/328.13, 264/500, 264/572
• US Class Current: 264/40.3
• CPC Class Codes: B29C 45/1704 the fluid being introduced into the interior of the injected material which is still in a molten state, e.g. for producing hollow articles B29C45/1732 and B29C45/1734 take precedence; injection blow-moulding B29C49/06
• CPC Class Codes: B29C 45/1732 Control circuits therefor
• CPC Class Codes: Y10S 425/812 Venting

Abstract

A method and apparatus for making a plastic injection molded part with a smooth surface or skin and a hollow core wherein thermoplastic material is injected as a molten stream into the mold cavity through a sprue bushing fixed in the mold. Simultaneously, an unmeasured quantity of inert gas is introduced through an adapter into the molten stream at the sprue bushing substantially coaxially with the melt stream and at a pressure sufficient to penetrate the thermoplastic material to form a gas cavity in the molten material in the mold. For retrofitting an existing mold, an adapter can be added to the exisiting sprue bushing. During plastic injection and cooling in the mold the gas is maintained at a high pressure, preset, constant and adequate to maintain the plastic against the mold surface until it is self-supporting. The gas is exhausted from the mold, back through the sprue bushing and adapter, before the mold is opened. Adequate preset high pressure is maintained at the gas supply by a large cylinder having a positive displacement member therein that is controlled by a pressure sensor in the gas pressure line to the mold to maintain preset adequate gas pressure within the cavity.

Claims

1. A method of gas assisted injection molding of a plastic part comprising the steps of:

(1) injecting molten plastic into a mold;

(2) introducing gas into the plastic from a source of gas at a preselected pressure;

(3) monitoring the gas pressure during step (2) upstream of said mold; and

(4) using the monitored gas pressure as feedback to control the gas pressure injected into the mold during step (2) to ensure that said monitored gas pressure remains substantially constant and at the preselected gas pressure during step (2).

2. The method as recited in claim 1, wherein the gas source is a gas supply chamber and the gas pressure is controlled by controlling the volume of the gas supply chamber.

3. The method as recited in claim 2, wherein the gas supply chamber includes a piston which is moved to control the volume of the chamber in response to the monitored gas pressure.