Die assembly and method for making a wood-thermoplastic composite

Abstract

The invention relates to a method for making a dimensionally stable wood-thermoplastic composite material composed of a wood component and a thermoplastic component comprising the steps of forming a wood-thermoplastic mass at a temperature above the melting temperature of the thermoplastic component. The mass is then passed through a converging die to form a shaped extrudate profile having a cross sectional shape wherein at least a part of the converging die is maintained at a substantially higher temperature than that of the mass. The profile is then fed through a low friction thermally insulating land section, the land section having the substantially the same cross sectional shape as the exit end of the converging die. The profile is then quenched in a non-oxidizing environment to form a substantially dimensionally stable outer shell around the profile, the shell having a temperature below the melting temperature of the thermoplastic component. The invention also relates to a die assembly for making a wood-thermoplastic composite material.

1 . A die assembly for making a wood-thermoplastic composite article comprising: 
 a) a converging die means for forming an extrudate profile of a desired cross sectional shape, the die means having an entrance end and an exit end, the exit end having a cross sectional shape;    b) a low friction insulating land section secured to the exit end of the converging die means and adapted to receive the profile through a land section passage,    wherein the land section passage has a cross sectional shape substantially similar to the cross sectional shape of the converging section exit end and a low friction surface; and    c) an enclosed cooling section secured to the low friction land section comprising: 
 i) a cooling medium inlet section having a passage adapted to receive the profile,  
 ii) at least one cooling medium channel disposed within the inlet section, the cooling medium channel extending radially from the inlet section passage, and  
 iii) an enclosed cooling flume secured to the cooling medium inlet section and having a flume passage therethrough, wherein the flume passage is in communication with the cooling medium inlet section passage.  
   
   
   
       2 . The die assembly of  claim 1  wherein the land section is about 0.375 inch to 1 inch thick.  
   
   
       3 . The die assembly of  claim 2  wherein the cooling medium inlet section further comprises a plurality of cooling medium inlet channels in communication with the cooling medium inlet section passage.  
   
   
       4 . The die assembly of  claim 1  wherein the converging die means is composed of a plurality of stages each of the stages provided with individual temperature control.  
   
   
       5 . The die assembly of  claim 1  wherein the inlet section passage and the flume passage have a symmetrically larger cross-sectional shape than that of the profile exiting the low friction land section.  
   
   
       6 . The die assembly of  claim 1  wherein the inlet section passage and the flume passage have a larger and dissimilar cross-sectional shape than that of the profile exiting the low friction land section.  
   
   
       7 . The die assembly of  claim 1  further comprising means for controlling cooling medium flow through the cooling section.  
   
   
       8 . The die assembly of  claim 1  wherein the converging die means further comprises an exit land section wherein the land section has a width of from about 0.375 inch to about 1 inch.  
   
   
       9 . The method of claim  8 -wherein the quenching step iv) is accomplished by contacting the profile with a cooling medium in a cooling means, the cooling means including: 
 a) a cooling medium inlet section having a passage therethrough, the passage adapted to receive the profile,    b) at least one cooling medium channel disposed within the inlet section, the cooling medium channel extending radially from the inlet section passage,    c) an enclosed flume secured to the cooling medium inlet section and having a flume passage therethrough, the flume passage having an interior surface, wherein the flume passage is in communication with the cooling medium inlet section passage; and    d) a cooling medium annulus defined between the flume passage interior surface and the profile.    
   
   
       10 . The method of  claim 14  wherein step ii) takes place in a converging die means composed of a plurality of stages each of the stages provided with individual temperature control.  
   
   
       11 . The method of  claim 10  wherein the wood-thermoplastic extrudate is provided at a temperature of about 330 to 350 degrees Fahrenheit and at least one stage of the converging die means is maintained at about 450 to 600 degrees Fahrenheit.  
   
   
       12 . The method of  claim 9  wherein the quenching step d) takes place in an inlet section passage and a flume passage each having a larger and substantially similar cross-sectional shape than that of the profile exiting the low friction land section.  
   
   
       13 . The method of  claim 9  wherein the quenching step d) takes place in an inlet section passage and a flume passage each having a larger and dissimilar cross-sectional shape than that of the profile exiting the low friction land section.  
   
   
       14 . A composite material made according to the method of  claim 11 .  
   
   
       15 . A composite material made according to the method of  claim 12 .  
   
   
       16 . A composite material made according to the method of  claim 13 .  
   
   
       17 . In the process of forming an extruded composite product containing cellulosic particles in thermoplastic binder wherein an extrusion mass is forced through a die at elevated extrusion temperature to form a hot shaped extrudate profile and cooled thereafter, the improvement which comprises: 
 a) heating the extrusion mass in a converging die means to a temperature sufficient to decrease the shear stresses generated between the mass and the die means to form a hot shaped extrudate profile;    b) passing the profile from the die through a first temperature zone wherein the extrudate surface is thermally insulated from exterior cooling;    c) quenching the profile in a non-oxidizing environment thereby setting the composite surface.    
   
   
       18 . The process of  claim 17  wherein the quenching step c) in conducted by: 
 a) contacting the profile surface with a cold stream of non-oxidizing quench fluid and    b) flowing the stream of non-oxidizing quench fluid from the quenching step concurrently along the surface of the profile to effect further cooling of the composite product.