Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – To produce composite – plural part or multilayered article
Reexamination Certificate
2001-05-08
2003-09-30
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
To produce composite, plural part or multilayered article
C264S328800, C264S328170, C264S349000, C366S078000, C366S083000, C425S130000, C425S204000, C425S587000
Reexamination Certificate
active
06627134
ABSTRACT:
BACKGROUND OF THE INVENTION
The most common commercial form of injection molding machine is known as a “reciprocating screw”. In this type of machine, thermoplastic polymer is melted, mixed, and conveyed by means of a screw having one or more flights rotating within a heated pressure vessel. The screw is also permitted to translate axially to allow for the accumulation of melted material at the end of the screw. When sufficient melt has accumulated, the screw is stopped and translated forward to inject the melted material into a closed mold. In common practice a non-return valve is situated at the downstream end of the screw to prevent back flow into the screw flights during the injection portion of the cycle.
A variation of this process, known as co-injection or sandwich molding, has been commercially practiced for a number of years. In its most common embodiments shown in
FIGS. 1 and 2
, co-injection is achieved by means of a molding machine fitted with two or more plasticizing units, each one containing a reciprocating screw
12
,
14
enclosed in a separate heated barrel
16
,
18
. The output of these plasticizing units is brought together by a system of manifolds
20
which convey the several materials to the point of injection
22
into the mold
24
. According to well known principles of viscous flow, the first material to enter the mold remains substantially on the outside of the molded part, and material injected later remains substantially in the core of the part. The resulting “sandwich” construction yields a number of advantages, the principle ones being: (1) to make a part with a chemically foamed core, gaining the light weight, low pressure, and flat surfaces of a foam part without the characteristic streaky exterior, (2) to use low cost recycled, “off-spec” or uncolored material where it is not visible, and (3) to make a part with different properties on the inside and outside, as for instance the presence or absence of reinforcing fibers or other property-changing additives.
These advantages are offset by the high cost and complexity of a machine requiring two or more independent reciprocating screws, together with the associated controls for simultaneous and/or sequential injection. Numerous attempts have therefore been made to reduce this complexity by having at least the injection function be performed by a single element, building a composite shot containing a plurality of melted materials within a single accumulation space. Examples of these attempts include U.S. Pat. No. 4,978,493 to Kersemakers et al., U.S. Pat. No. 3,966,372 to Yasuike et al., and U.S. Pat. No. 5,443,378 to Jaroschek et al. In all of these examples, at least one secondary extrusion screw and barrel is caused to communicate with the primary barrel by means of a melt-carrying manifold structure through which the secondary portion of the shot is charged. Because of the multiple barrels and screw drives, machines of this type still have disadvantages involving the high initial cost of the required components and associated control capability.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a device and method whereby the sandwich molding process can be performed within the space of a single heated barrel, and furthermore by means of a machine having a single rotational screw drive.
Another object of the present invention is provide a device and method for making possible adaptation of a conventional single-material molding machine to perform the sandwich molding process with relatively minor and low-cost changes.
A further object of the present invention is to provide an injection unit having an externally heated pressure vessel hereafter called the “barrel” enclosing a primary, outer, screw having one or more flights. The screw being movable rotationally and axially to respectively plasticize and inject a metered quantity of melted thermoplastic material. The screw has a hollow cylindrical bore extending through its length which forms the enclosure for a second, inner, screw having a smaller diameter than the primary screw. The secondary screw remains substantially fixed in its axial orientation with respect to the primary screw, but can be rotated within the primary screw to plasticize material. The downstream end of the bore in the primary screw is provided with an opening to allow material from the secondary screw to be extruded into an accumulation space in front of the primary screw. The opening is preferably provided with a non-return valve to keep material from leaking back into the secondary screw.
Another object of the present invention is to provide a clutch mechanism between the primary and secondary screws to transmit rotation to the respective screw elements. Rotational motion is applied by means of a hydraulic or electric motor to the upstream end of the secondary screw. With the clutch closed, relative rotation occurs only between the primary screw and the outside barrel causing primary material to be extruded. With the clutch opened, relative rotation occurs between the two screws, causing secondary material to be extruded.
Another object of the invention is to provide a secondary feed throat to convey granular raw material to the inner screw through openings cut through the wall of the hollow outer screw.
A further object of the invention is to provide a machine for forming objects having: (1) a first rotatable screw; (2) a second rotatable screw; and (3) a single screw drive for selectively causing rotation of the first rotatable screw and the second rotatable screw. This machine can also include: (a) a single barrel containing the first rotatable screw and the second rotatable screw; and/or (b) a clutch for selectively connecting the single screw drive to one of: (1) the first rotatable screw; and (2) both the first rotatable screw and the second rotatable screw. Furthermore, the first rotatable screw and the second rotatable screw can be substantially coaxial. Also, the first rotatable screw can operate on a first material; and the second rotatable screw can operate on a second material such that each of the objects is formed from a combination of the first material and the second material.
An additional object of the invention is to provide a machine for forming objects having: (1) a barrel; (2) a first rotatable screw; (3) a second rotatable screw; and (4) an accumulation space for forming a shot, such that the barrel surrounds at least a portion of each of the first rotatable screw, the second rotatable screw, and the accumulation space. Also, the first rotatable screw can operate on a first material; the second rotatable screw can operate on a second material; and the shot formed in the accumulation space can contain first material and second material. Further, the first material can be a skin material; the second material can be a core material; and at least one of the first rotatable screw and the second rotatable screw can inject the shot into a mold such that skin material encapsulates core material. In addition, the accumulation space can enlarge when at least one of the first rotatable screw and the second rotatable screw move axially.
Another object of the invention is to provide a screw assembly for a machine for forming multi-material objects having: (1) an outer screw for operating on a first material, the outer screw having a hollow inner section; and (2) an inner screw for operating on a second material positioned substantially within the hollow inner section of the outer screw; wherein at least one of the outer screw and the inner screw includes a non-return valve to prevent backflow of material into at least one of the outer screw and the inner screw. In addition, each of the outer screw and the inner screw can include a respective non-return valve. Further, the non-return valve can be selected from the group consisting of: a ball type valve, a ring type valve, and a poppet type valve.
A further object of the present invention is to provide a machine for forming objects having: (1) a hollow barrel defining a longitudinal d
Community Enterprises, LLC
Cooper & Dunham LLP
Heitbrink Jill L.
LandOfFree
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