Polymer processing systems including screws

• US 7,318,713 B2
• Filed: 07/18/2002
• Issued: 01/15/2008
• Est. Priority Date: 07/18/2002
• Status: Expired due to Term

• Current Assignee: Trexel Incorporated
• Original Assignee: Trexel Incorporated
• Inventors: Xu, Jingyi, Cardona, Juan C., Kishbaugh, Levi A.
• Primary Examiner(s): Kuhns; Allan R.
• Application Number:
  US10/198,643
• Publication Number:
  US 20040012107A1
• Time in Patent Office: 2,007 Days
• Field of Search: 425/4.C, 425/4.R, 425/817.C, 425/208, 425/382.4
• US Class Current: 425/4.00R
• CPC Class Codes: B29C 2045/1722 injecting fluids containing plastic material
• CPC Class Codes: B29C 44/3446 Feeding the blowing agent
• CPC Class Codes: B29C 48/51 with internal flow passages, e.g. for molten material
• CPC Class Codes: B29C 48/53 having a varying channel depth, e.g. varying the diameter of the longitudinal screw trunk
• CPC Class Codes: B29C 48/535 with thread pitch varying along the longitudinal axis
• CPC Class Codes: B29C 48/575 provided with elements of a generally circular cross-section for shearing the melt, i.e. shear-ring elements
• CPC Class Codes: B29C 48/625 characterised by the ratio of the threaded length of the screw to its outside diameter [L/D ratio]
• CPC Class Codes: Y10T 29/49716 Converting

Abstract

Injection molding and blow molding systems that include screws having low L:D ratios are provided. The systems are capable of producing microcellular polymeric materials. In some cases, the systems may be formed by retrofitting conventional polymer processing systems. Retrofitting may involve changing (e.g., machining or replacing) existing components of the conventional system, as well as, adding new components to the system. For example, retrofitting generally involves replacing the conventional polymer processing screw with a new screw designed to satisfy conditions needed for processing microcellular materials. In other cases, the systems may be newly manufactured. The retrofitted or newly manufactured systems of the invention are considerably less expensive than newly manufactured microcellular processing systems having higher L:D ratios.

Claims

1. A polymer processing system that is operable cyclically to produce microcellular material, comprising:

an extruder including only a single screw, the screw being mounted within a barrel to define a polymer processing space therebetween, the screw being rotatable to convey polymeric material in the polymer processing space in a downstream direction to an extruder outlet within the polymer processing space, the screw having an L;

D ratio of less than or equal to about 24;

1, wherein the screw includes a wiping section having an L;

D ratio of between of between about 0.5;

1 and about 3;

1, and a mixing section having an L;

D ratio of between about 2;

1 and about 6;

1;

a source of blowing agent connected to a blowing agent port associated with the system to permit blowing agent introduction into the polymeric material within the system;

an accumulation region downstream of the screw and fluidly connected to the polymer processing space, the accumulation region designed to have a mixture of polymeric material and blowing agent accumulated therein; and

a mold downstream of the extruder outlet,

wherein the system is designed to move the screw in a downstream direction to inject the mixture of polymeric material and blowing agent from the accumulation region into a cavity of the mold and to form a microcellular material therein and wherein the wiping section includes a screw flight that passes beneath the blowing agent port.

2. The system of claim 1, wherein the screw has an L:

D ratio between about 20;

1 and about 24;

1.

3. The system of claim 1, wherein the mold is an injection mold fluidly connected to the polymer processing space.

4. The system of claim 3, wherein the mold includes a hot runner passageway connected to the cavity of the mold and a valve associated with the hot runner passageway, the valve having an open configuration that permits flow of polymeric material from the hot runner passageway into the cavity and a closed configuration that prevents the flow of polymeric material from the hot runner passageway into the cavity.

5. The system of claim 1, further comprising an accumulator external of the extruder between the extruder outlet and the mold.

6. The system of claim 1, wherein the blowing agent port is formed in the barrel through which blowing agent is introduced to form the mixture of polymeric material and blowing agent in the polymer processing space.

7. The system of claim 1, wherein the screw is rotatable to convey the mixture of polymeric material and blowing agent in the polymer processing space in a downstream direction to the extruder outlet.

8. The system of claim 7, wherein the screw is designed to form a single-phase solution of polymeric material and blowing agent in the polymer processing space, the single-phase solution being injected in to the mold to form the microcellular material.

9. The system of claim 1, wherein the mold is constructed and arranged to receive a single-phase solution of polymeric material and blowing agent and to form a microcellular material in the cavity of the mold.

10. The system of claim 1, wherein the screw is reciprocatable within the barrel.

11. The system of claim 1, wherein the system is a retrofitted system.

12. The system of claim 1, wherein the screw includes a restriction element positioned upstream of a downstream end of the screw, the restriction element being designed to restrict the upstream flow of polymeric material therethrough during at least a portion of an injection or an ejection cycle.

13. The system of claim 12, wherein the screw includes a restriction element positioned upstream of a blowing agent port formed in the barrel.

14. The system of claim 1, wherein the screw includes a tip valve positioned at a downstream end of the screw, the tip valve permitting downstream flow of polymeric material therethrough in an open configuration and restricting upstream flow of polymeric material therethrough in a closed configuration.

15. The system of claim 1, wherein the blowing agent is either carbon dioxide or nitrogen.

16. The system of claim 1, wherein the blowing agent is a physical blowing agent.

17. The system of claim 1, wherein the microcellular material has an average cell size of less than 100 microns.

18. The system of claim 1, wherein the system is designed to nucleate a solution of polymeric material and blowing agent by pressure drop to form the microcellular material, while injecting the solution into the mold.

19. The system of claim 1, wherein the screw has an L:

D ratio of less than 24;

1.

20. The system of claim 1, wherein polymeric material is accumulated in the barrel downstream of the screw having an L:

D ratio of less than or equal to about 24;

1.

21. The system of claim 1, wherein the accumulation region is located downstream of the screw within the barrel.