Plastic article or earthenware shaping or treating: apparatus – Immersed shaping orifice discharging directly into liquid...
Reexamination Certificate
1998-12-21
2003-03-04
Silbaugh, Jan H. (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Immersed shaping orifice discharging directly into liquid...
C425S070000, C425S378100
Reexamination Certificate
active
06527532
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a die assembly and a method for extruding a wood-thermoplastic composite material with improved dimensional stability.
BACKGROUND OF THE INVENTION
Composite materials consisting of a mixture of wood particles in the form of sawdust and a thermoplastic material have been known for many years. The materials so formed may be used in many of the same applications as an all wood product but offer the advantages of providing high resistance to rot, insects and moisture. These products can have the same workability as wood and are splinter-free. However, these materials do not exhibit the same physical characteristics as wood and therefore may not be used as structural members in some applications. The recent past has seen increased interest in composite material manufacture as a viable outlet for recycled post consumer thermoplastic materials. This interest has been spurred by the prospect of environmental regulations mandating the recycling of these materials. Also valuable, ever-shrinking landfill space may be conserved if both spent sawdust and plastic material are reused rather than disposed.
Forming a wood-polymer composite into a final product has been accomplished using some of the techniques used for forming all polymer products, including extrusion. While the technology for extruding all polymer products is well-developed with fairly predictable results, the extrusion of a wood-thermoplastic composite material using recycled input materials is subject to much wider variance in the molecular makeup and physical characteristics of the input materials, depending on available sources of recycled material. Moreover, a wood-thermoplastic composite has unique melt flow characteristics, which prevent the literal translation of polymer extrusion techniques for use in composite material extrusion. The present invention is concerned with a die assembly and an associated extrusion method for obtaining improved dimensional stability of an extruded wood-thermoplastic profile. A profile is defined in this art as an article fabricated from the composite material, which has a variable length and consistent cross section. As used herein, the term dimensional stability refers to the tendency of a finished profile of a wood-thermoplastic composite material to hold to predetermined cross-sectional dimensions along its entire length.
There are particular problems encountered with wood-thermoplastic profile extrusion that make control of the dimensional stability of the final product difficult. First, it is well known in thermoplastic extrusion that extrusion equipment and operating conditions must be closely matched to the material being extruded. For example, attempting to extrude a given thermoplastic such as polyethylene in equipment designed for a different type of thermoplastic such as polypropylene will give extremely unsatisfactory results. In contrast, the same extruder is used to make a composite product utilizing a wide range of thermoplastic input materials in the practice of the present invention. When fabricating a composite product using recycled thermoplastic input material, the nature of the thermoplastic material may change from one minute to the next requiring changes to process controls. The process can be unstable.
A second problem is the presence of volatiles in the wood component. Volatiles in the wood-thermoplastic mixture are heated in the extruder barrel and attempt to escape as the profile exits the die. This effect is referred to as foaming and can result in severe dimensional distortion (sagging, bending, bunching or stretching) of the profile. The kinds of volatiles that can be found in a wood-thermoplastic mixture typically include ordinary moisture and wood lignin, and terpenes. Another problem arises from the portion of the wood component that is near the surface of the profile exiting the extruder. At the profile's high temperature, the wood component can oxidize as it is exposed to air, creating a burned appearance on the final product.
Underlying and to some extent magnifying these problems is the tendency of any extruded material to experience a phenomenon called die swell. Die swell is the tendency of an extrudate to swell as it leaves the die due to the release of shear stresses built up in the extrudate as it travels along the extruder barrel and die surfaces. Shear stress develops between the extrudate the die wall causing tears and roughness in the finished surface of the profile in addition to die swell. In ordinary polymer extrusion the shape of the opening at the die exit may have to be changed dramatically from the desired final shape in order to account for extrudate swelling.
Calibration processes have been developed for thermoplastics extrusion to ensure that the extrudate emerging from a die maintains a certain shape. Some of these techniques are described in
Polymer Extrusion
, C. Rauwendaal, (2d reprinted ed., 1990) at pp. 448-450. Calibration typically involves the use of some type of cooled sizing die to fix the shape of the extrudate. Calibration of thermoplastic extrudates typically requires exerting tension variable force on the extrudate to literally pull it through the calibration device. This approach is unworkable for the present invention because a wood-thermoplastic melt lacks sufficient melt strength to undergo appropriate tension. The extrudate of the present invention must be pushed through the die assembly and requires a different approach to ensure dimensional stability.
Various techniques are known for extruding wood-thermoplastic composite materials. U.S. Pat. No. 4,968,463 to Levasseur (the '363 patent) discloses a process for producing extruded or molded objects from waste containing plastic materials. The process includes the steps of preliminary physical treatment, drying if the water content of the initial materials is over about 8% by weight, preheating to a temperature of 80° C. to 160° C. (176° F. to 320° F.), mixing or kneading at a temperature of 120° C. to 250° C. (248° F. to 482° F.) to form a paste, and injection molding or extrusion. The process also contemplates the addition of coloring and/or filling agents such as compost refuse, incineration plant cinders, old paper, ground rags or fines from household waste after the drying step. Levasseur also discloses the use of a water-cooled casing immediately after the die to cool the profile.
U.S. Pat. No. 5,217,655 to Schmidt discloses a process for preparing a composite product from an intake material incorporating commercial, municipal or industrial waste containing organic, plastic and fibrous material. The intake material is first granulated and sized and is then heated in three consecutive stages. The material is continuously mixed during the heating stages and reaches a final temperature of about 160 to 204° C. (320 to 400° F.). The heating and mixing steps are carried out under very high pressures to ensure encapsulation of the filler material by the plastic fraction. These high pressures are stated to range from about 141 to about 492 kg/cm
2
(2,000-7,000 psi). The material may then be co-extruded with a reinforcement structure and formed into a final product using vacuum profile dies or compression molding. The patent discloses mist spray cooling tanks as a preferred method of cooling the final product.
U.S. Pat. No. 5,082,605 to Brooks et al. discloses a method for making a composite material comprising a discontinuous phase of cellulosic fiber encapsulated in a polymeric matrix. The method requires that cellulosic fiber and polymeric material be mixed while simultaneously being heated to an encapsulation point. The mixture consists of a discontinuous phase of cellulosic fibers encapsulated in a polymeric continuous phase. The mixture is then reduced mechanically into smaller golf ball sized particles having a maximum dimension of about 3.8 cm (1.5 in). The resulting particles are then extruded into a homogenous mass while maintaining the particle temperature within the encapsulation range.
Muller John J.
Wittenberg Roger A.
Yarmoska Bruce S.
,MacCord Mason PLLC
Leyson Joseph
Silbaugh Jan H.
Trex Company, L.L.C.
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