Use of co-injection molding to produce composite parts...

Details Use of co-injection molding to produce composite parts... Use of co-injection molding to produce composite parts...

1999-06-15

2001-10-30

Kuhns, Allan R. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

Producing multilayer work or article

C264S259000, C264S261000, C264S278000

Reexamination Certificate

active

06309586

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to products of and methods for producing complex shapes of composite molded articles, including a snowboard with metal edges, by the process of co-injection molding.
BACKGROUND OF THE INVENTION
None of the following is admitted to be prior art to the present invention.
Injection molded, including co-injection molded, articles could be vastly improved if there was a method for producing complex shapes of composite molded articles, in a single molding operation; thus optimizing the value of injection molding and maintaining or improving the characteristics expected of similar non-molded composite articles. For example, a problem that has long plagued the art of snowboard manufacturing has been the time and cost associated with manufacturing multiple layer laminated snowboards with metal edges to meet specific aesthetic and performance requirements. Injection molding has been sought after as a means of reducing such time and costs of manufacturing, but to date, has been unable to achieve all of the aesthetic, cost and performance attributes the public has come to expect of modem laminated snowboards. What is needed is a method for producing complex shapes of composite molded articles that meet or exceed the aesthetic and performance requirements expected of similar non-molded composite articles, preferably at reduced costs.
Injection Molding
Injection molding is where thermoplastic polymers are gravity-fed from a hopper into a barrel, melted by a reciprocating screw and/or electric heat and are propelled forward by a ram (piston, plunger) or the screw (used as a plunger) into mating steel or aluminum molds, which are cooled to below the heat-distortion temperature of the resin. The injected plastic material contracts as it cools (mold shrinkage) and shrinks. When cool enough to retain its shape, the plastic part is ejected from the mold.
Typically, good part design requires adequate taper (draft) of side walls, radii at inside corners, minimal variations in wall cross-sections, use of ribs 60% or less of outer wall thickness for stiffness, strength and minimal sink marks. Thermosetting polymers can also be injection-molded. For these materials, the barrels on the injection-molding machine are heated by hot water to a point safely below cross-linking temperature; the polymer is then propelled by ram or screw feed into heated molds. After they cure, the parts can be ejected while still hot because they have already thermally set or cross-linked.
Injection molding of polymers has revolutionized many industries. Most products today contain some form of plastic molded parts in them or consist entirely of plastic molded parts. Such products include toys, automobile parts, computer covers, phones, liquid containers and many, many other articles, too numerous to recite. However, modern molding operations have not been able to produce complex shapes of composite molded articles, in a single molding operation, that meet or exceed the aesthetic and performance requirements expected of similar non-molded composite articles. For certain products, aesthetic or performance requirements dictate that non-molded components, such as metals or ceramics, be utilized. However, incorporating polymer materials and non-molding materials in the single molding step has not been able to achieve the complex geometries required of some products, including molded snowboards with metal edges. For example, round washers have been embedded into molded parts as the base legs of appliances, computers and other similar articles to lift them off of the floor. However, such components do not require the metal and molded polymer to retain a bent, curved or complex shape whereby the shape of the article presents forces that separate the polymer and non-molded component.
Co-Injection Molding
Co-injection molding takes advantage of a characteristic of injection molding called fountain flow. That is, as the cavity is filled, the plastic at the melt front moves from the center line of the stream to the cavity walls. Because the walls are below the transition temperature (freeze temperature) of the melt, the material that touches the walls cools rapidly and freezes in place. This provides insulating layers on each wall, through which new melt makes its way to the melt front.
Sequential co-injection processes have two barrels and one nozzle in an injection molding machine. The skin polymer is injected into the mold first, then the core polymer is injected. The skin polymer is the material that is expected to be deposited on the cavity wall over the entire surface of the part. The core polymer displaces the skin polymer at the hot core, pushing it to fill the rest of the cavity. The end product is a sandwich-like structure, with the core polymer in the middle and the skin polymer on the surfaces of the part.
As two materials are processed, two hopper/screw/barrel assemblies are required for co-injection. A special co-injection nozzle allows the operator to alternate between the two materials with the speed, timing and accuracy necessary to optimize the co-injection application.
The advantages of this process are: the combination of two material properties into one part, and the maximization of the overall performance/cost ratio. One good example of co-injection is the use of polymer re-grind as the core material, while maintaining surface finish quality by using virgin polymer as the skin material. Other applications include using thermally more stable polymer as the core material to increase the thermal resistance of a part, or using a high melt-flow index polymer as the core material, to reduce the overall clamp force. Still other applications include using a core material that is lighter and exhibits more flexibility than the skin material to combine strength and flexibility to the desired part, while keeping the weight down. One combination may include a fiberglass loaded core material to provide a structural component to the part and a different skin material to maintain a smooth, consistent surface.
Numerous articles on the co-injection process have been authored. Some articles include,
Co
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Injection Molding ′
95
: Market Outlook
, by Thomas W. Betts of Battlefield of America, Inc;
Molded
-
In Shielding Using the Coinjection Process
, by Thomas W. Nash and Ralph J. McDonald, IBM Application Business Systems, Rochester, Minn. 55901
; Coinjection Molding with Automotive Polyolefins
, by Bruce R. Denison, D & S Plastics International;
Recycling—Why Not Use Co
-
Injection Molding
?, by Joseph McRoskey of Co-Mack Technology, Inc. and Thomas W. Nash of Thomas W. Nash & Associates; and
Co-Injection Molding: Current Applications
, by Joseph McRoskey of Co-Mack Technology, Inc. (May 31-Jun. 2, 1998). The foregoing articles are incorporated herein by reference, including any drawings.
Injection and co-injection molding operations that incorporate additional non-molded components, such as metal edges, comprise additional technical hurdles to overcome, especially when such non-molded components are incorporated into a single molding operation step. Such hurdles include the need to adjust for shrinkage of the polymer as it cools in the mold; securing the non-molded components within the mold and eliminating or minimizing surface lines from inserting components into the mold. Previous attempts to address such technical matters have not succeeded in producing a snowboard that meet the expectations of an industry dominated by laminated snowboards.
To date, efforts to produce molded snowboards have focused on producing low cost snowboards, causing quality, performance and aesthetics to suffer. For example, a molded snowboard is available under the name ESP and is distributed by the EMSCO Group of Girard, Pennsylvania, USA. The board is a low grade plastic material with metal edges. The snowboard lacks smooth continuous curves and surfaces, lacks a proper camber, is warped, contains bumps and surface irregularities from inserting plugs into holes made for bin

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