Gas-assisted two-shot injection molding process

Details Gas-assisted two-shot injection molding process Gas-assisted two-shot injection molding process

2001-06-28

2003-09-23

McDowell, Suzanne E. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

Producing multilayer work or article

C264S572000

Reexamination Certificate

active

06623688

ABSTRACT:

BACKGROUND
The present invention relates to injection molding a composite article including a rigid thermoplastic and a soft thermoplastic.
In many industries, it is desirable to combine the structural characteristics of a rigid thermoplastic with the sealing abilities of a soft and flexible thermoplastic. For example, in the automotive industry, a cowl—the transition trim part between the hood and windshield—typically includes a rigid piece of plastic and a soft, blade-like seal bonded to an edge of the rigid piece. The rigid piece forms the body of the cowl and is secured to a body panel of the automobile. The blade seal engages the hood, providing an interface between the cowl and the hood to prevent water from leaking into the engine compartment and to suppress engine noise.
Conventional rigid/soft thermoplastic composites such as cowls are made with a “two-shot” injection molding process. This process uses a mold that is initially separated by a retractable mold insert into two chambers—a body chamber corresponding to the body of the cowl, and a seal chamber corresponding to the blade seal. In the process, molten structural or “hard” thermoplastic is injected into and fills the body chamber. Next, the mold insert is retracted to expose an edge of the structural thermoplastic to the seal chamber. Then, a molten soft thermoplastic is injected into and fills the seal chamber, and simultaneously bonds to the exposed edge to form a rigid/soft thermoplastic composite cowl.
Although the resultant rigid/soft composite cowl includes the desired structural rigidity and sealing capability, it suffers a significant shortcoming. Due to repeated contact with the hood and exposure to excessive heat from the engine, the blade seal deforms from its original shape, and becomes incapable of satisfactorily engaging the hood to provide the required seal. This loss of sealing ability of the blade seal is common in many other rigid/soft thermoplastic composite applications as well.
In an unrelated field, pressurized gas conventionally is used to add rigidity to hard thermoplastic bodies. In one application, pressurized gas is injected into molten hard thermoplastic resin as the resin is injected into a mold. The gas pushes resin out of its way to form cavities within the hard resin. When the hard resin cools, the completed hard plastic body includes cavities that enhance the rigidity and structural strength of the body. This use of pressurized gas provides a way to increase rigidity of hard plastic bodies; however, many opportunities exist to use pressurized gas in novel applications.
SUMMARY OF THE INVENTION
The aforementioned problems are overcome in the present invention that provides a single mold injection process in which a rigid thermoplastic is bonded to a soft thermoplastic and pressurized assist gas is injected into the soft thermoplastic to establish a cavity and thereby form a compressible bulb seal.
In a preferred embodiment, the present invention generally includes the steps of: injecting a molten rigid thermoplastic into a first mold chamber; injecting a molten soft thermoplastic into a second mold chamber; establishing fluid communication between the first and second mold chambers so that the rigid and soft thermoplastics bond to one another along an edge; and injecting pressurized assist gas or liquid into the soft thermoplastic to establish a cavity in the soft thermoplastic. Preferably, the pressurized gas pushes molten soft plastic out of the gases' way and the space occupied by the gas forms the tube-like cavity through the soft plastic. After the thermoplastics cool, the resultant composite is removed from the mold. With the cavity formed in the soft thermoplastic, the soft thermoplastic portion of the composite forms a compressible and resilient seal with a cross section resembling a light bulb—hence the name “bulb seal.”
The present inventive process provides an efficient and economical way to integrally bond a rigid thermoplastic to a soft thermoplastic that includes a bulb seal in a single mold process. With the process, resilient bulb seals may now be integrated into a variety rigid/soft thermoplastic composite articles including automotive trim pieces, such as cowls, doors and door jambs; household appliance trim; construction materials, such as doorways and windows; and the like. Accordingly, these composite articles offer improved sealing capabilities because the integral bulb seal is less prone to deformation over time due to repeated contact with an item against which it interfaces.
Additionally, the present invention provides a novel use of assist gas. Rather than use assist gas to make hard thermoplastic parts more rigid and strong, the present invention uses assist gas to make soft plastic bodies more supple and resilient.
These and other objects, advantages and features of the invention will be more readily understood and appreciated by reference to the detailed description of the preferred embodiments and the drawings.


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