FINISHING PROCESS FOR EXTRUDED PROFILES

Abstract: Pronounced surface roughness or undulations will sometimes be obtained on the unsized surface of plastic extruded hollow profiles. Particularly is this so for plastic resins containing fibres. In the invention smooth finishes are obtained on both external and internal surfaces of a hollow profile by passing the extruded resin first through or over a cooled sizing unit and then over or through a finishing unit in the form of a plug or a ring which will usually be heated. The heating will allow the asperities or the unsized surface to flow locally to conform to the plug or ring surface without disturbing the basic form of the pipe or tube wall. Subsequent cooling of the smoothed surface will be by heat loss to the interior of the pipe wall downstream of the finishing unit so that smoothed finishes are retained in the cooled pipe or tube.

Description: Finishing process for extruded profiles This invention relates to the problem of obtaining smooth finishes on extruded plastic compositions particularly, although not confined to, thos containing discrete fibres. It has long been known that when thermoplastics are extruded from a die, or more of the surfaces of the extrudate while broadly conforming to the shape of the die aperture may nonetheless be subject to surface irregularities (Fig 1). Such irregularities in the case of extrusion for blown film are often referred to as "sharkskin" or "orange peel", but similar surface imperfections can occur in thicker sections such as those for pipes. Generally, as shown in Fig 2, the problem of surface irregularity on either the outside or the inside of an extrudate issuing from an annular die is solved by the use of a sizing unit which has the triple function of cooling the extrudate, determining its dimensions and smoothing its surface. Under the process conditions customarily employed for the common thermoplastics (HDPE, PVC and PP), the use of either an inside sizing unit or an outside metering unit may provide an adequate finish on the unsized surface, while obtaining a high quality finish on t sized surface. However, pronounced irregularity or undulations on the unsized surface ca be obtained with such single sizing units and inventions have been propos (eg UK 810703) for simultaneous inside and outside sizing to obviate this disadvantage. It has long been appreciated that the addition of glass or other stiff fibres to a thermoplastic in a suitable fashion can yield increased stiffness and strength in the solid state. It is also the case that above certain length such fibres profoundly affect the thermoplastic in its molt state. In particular, the problems of roughness on extruded surfaces are greatly accentuated, to the extent that a single sizing unit will not provide an adequately smooth finish on the unsized surface, even if it provides an adequate finish for the sized surfaces. In published PCT applications WO 8804228 and WO 8808365, means are describ for the production of extrusions with long fibres, typically in the range 5 to 15 mm, as distinct from short fibres (often in the range of 0.3-1.00 mm). It is found that as the fibre lengths and concentrations in the polymer increase so the roughness problems on the unsized surface increase. The form which such roughness typically takes is shown in Fig 3 but the invention is not limited to such forms. Known technology (eg UK 810703) for simultaneous internal and external sizing does not provide general solution because with long fibre-containing thermoplastics the viscosity is such as to cause blockage in the sizing units. The purpose of this invention is to provide means for obtaining a fine finish on both external and internal surfaces of extruded thermoplastic composition, particularly those containing long fibres, in a way which do not interfere with established processing methods. According to a first aspect of the present invention there is provided polymer processing apparatus comprising first an external or internal pip or tube sizing unit, followed respectively by an internal finishing plug external finishing ring in such a position relative to the sizing unit an at such a controlled temperature maintained by heating or cooling means, will cause the asperities or undulations on the unsized surface to flow locally to conform to the plug or ring surface. According to a second aspect of the present invention, there is provided method of polymer processing comprising assing polymer resin, which may contain discrete fibres or fillers, through the apparatus as defined in previous paragraph, thereby obtaining smooth finishes on both inside and outside surfaces of the cooled pipe or tube. The basis of the invention rests on an appreciation of the viscoelastic behaviour of polymer melts as accentuated by the presence of fibres. Immediately on leaving a die aperture (Fig 4) the velocities in a polyme liquid undergo a rapid rearrangement with material E, at or near the cha surfaces constituting the die, being subject to a rapid acceleration in order to catch up with material C, coming from the centre of the die gap Under these conditions the surface molecules E undergo stretching from w they will recover once the acceleration ceases, unless restrained. If external sizing is used, as shown in Fig 4, this restraint takes the for s rapid freezing (Fig 5(a)) on the outside/, while molecules on the inside, Fig 5(b), are relatively free to recover their relaxed (coiled) state, producing a more-or-less flat surface. However this recovery process ta a certain time, dependent on the viscosity divided by the natural elasti of the molecules. Should the temperature of the polymer leaving the die (T.) be sufficiently low, this recovery will be incomplete before solidification on the inside has set in and the effect shown in Fig 5(c) will be obtained. The presence of fibres in the melt accentuates the effect shown in Fig 5 by (i) greatly increasing the viscosity (and hence the recovery time) an (ii) interfering with the process of velocity rearrangement (Fig 4) thro the fibres being "hung up" on the die lips before being pulled away into surface of the extrudate. In some cases the fibres (particularly the lo ones above 6 mm) will not be completely pulled into the extrudate surfa but remain sticking out of the solidified surface. One approach to obtaining a smooth surface is thus for a forming process i which the molecules are immediately frozen into position. This was the intention behind the simultaneous cooling and forming both inside and outside by means of cooled sizing units as disclosed in GB 810703. As mentioned this approach is not well adapted to fibre containing compositio because the high viscosities found with these compositions causes blockage in the sizing units when cooling is applied to them. For this reason the first basic principle of the present invention is that sizing on the inner and outer surfaces shall take place in two forming units, consecutively placed with either outside sizing followed by inside, or vice versa. Having formed the first surface S. by pressing and cooling in the normal way, the pipe or tube (not necessarily of circular section) passed to the second forming unit. This second unit will be positioned sufficiently far downstream of the die mouth so that most of the pipe or tube will be below the melting point (T ) of the polymer matrix. If the surface S Z which is to be finished in the second forming ~~ unit U 2 is below or only slightly above T , which is the preferred arrangement, then according to the invention heat is provided to the pipe by U . The purpos of this heat is to melt the polymer to a depth broadly equal tc tnat cf th asperities (Fig 5(c)) (typically 50-150 microns) and not to disturb the f of the main part of the pipe already established by the first forming uni U,. When the material of the pipe leaves the second forming unit U 2 , surface S ? is smooth and is frozen in that form υy neat loss to tne interi of the pipe wall, not it will be noted to U 2 . If surface S 2 arrives at U ? at a temperature very considerably above T , then U 2 may be used to cool S-, not below T , but to a temperature somewh above T , thus establishing a surface condition similar to that described the paragraph above. The material of the pipe will then leave U 2 and S 2 i frozen into its smooth form by heat loss to the interior of the pipe wall described in the paragraph above. The second basic principle of the invention is thus that while the first formed surface S, is frozen by heat loss to the first forming unit U,, 1 wall 2 surface S ? is frozen by heat loss to the interior of the pipe^itself. The invention will now be described by reference to a system in which S j i the outside pipe surface, U. a standard cooled cylinder, S 2 is the inside pipe surface and U 2 is in the form of a heated plug of a diameter corresponding to the bore of the pipe. (The precise diameter of the plug will depend on the shrinkage characteristics of the polymer-fibre composition.) Figure 6 shows one embodiment of the principles of the invention. This comprises die mouth D, from which issues the extruded pipe P, the materia of which may be polymer resin or polymer resin containing fillers among which may be reinforcing fibres. On emerging from D the liquid material P is pressed by air pressure in the space A against the inner surface of first forming unit U. to form surface S. . U. will be cooled to around ro temperature T by cooling water as shown. On leaving U. at E the materia of the pipe will have a temperature profile through its thickness, whereb the outside surface S 1, will be at or near Tw while the inside surface S-, will still be at a much higher temperature, but below the temperature at die T D . In an advantageous arrangement of the invention, S„ will be at or just be T as shown in the plug detail in Fig 7. As tne pipe material passes alo the smooth land L of U 2 , the effect of controlled heating of U„ -j s to cre a molten region M to a depth about equal to that of the asperities. On leaving the plug at face F the region M immediately starts to reduce in si so that the emergent surface S 2 is frozen into the required smooth shape defined by the land L. It is clear that to produce an optimum surface, the melting zone M must no significantly extend/further than the depth of the asperities, otherwise the subsequent freezing process immediately downstream of U 2 will take so long that the asperity-forming process described in Fig 5 (c) will start again. In general the required control over the penetration of heat from U 2 into the pipe is achieved by a combination of high U 2 temperatures and short land lengths L. Where, as is often the case, the length of the unit U„ is several times th internal diameter of the pipe, starting up the extrusion process in tnis embodiment can be difficult without an additional feature of the invention The reason is that the extrudate must be pressed against U, in order for t pipe material to be conveyed to the plug end of U 2 without collapsing on t its stem S (in Fig 6) . Figure 8 shows an embodiment of the basic principle of this additional feature. A shroud G, which may conveniently be in steel, consists of a hollow cylinder C, closed at one end H where it is connected to a rod or tube R which passes through the haul-off unit. The other end of G is open with teeth T milled or ground on to its outer surface. For start-up the shroud G is placed over U 2 in such a way that teeth T ar inside the die mouth D. Air pressure is delivered to A. On starting the extruder, poϊymer advances over the teeth T where it freezes (because G i at room temperature}. Wnen polymer is visible at the die mouth D tne haul-off is started and shroud G slides over U 2 and the air A presses the extruded polymer on to U, as previously described. If the cooled forming unit U, were arranged as a plug (Fig 2(b)), so that the inner surface is formed first, the heated unit U 2 would then take the form of a ring. All remarks made above about the melting and subsequent freezing by heat loss to the interior of the pipe would then apply to thi embodiment (Fig 9). It is clear that just as in the aforesaid published PCT applications WO 8804228 and WO 8808365, this invention imposes no limitation on the sh of the tube cross-section. Squares, ovals, hollow Us for example may all formed by the method of the invention. Furthermore, although the invention's principal field of application is likely to be thermoplastics containing fibres or other fillers, tubes of virgin thermoplastics may b advantageously formed using the invention. Typically mirror finishes on inside exceeding those conventionally obtained on the outside with exter finishing units can be obtained with virgin thermoplastics. Under some conditions, the speed of haul-off will be such that in the finishing plug variant (Fig 6) the surface S 2 will not have cooled sufficiently from the die exit temperature (T.) and S ? will be too far a the melting temperature T . In these circumstances the rigid stem S (Fi may be replaced by a flexible tube carrying heating or cooling means to plug U 2 , such that when stretched out under the influence of the extrude pipe, the flexible tube holds the plug U 2 at the correct axial distance D to allow the heating and cooling mechanism to function as described in basic principles. This is mechanically the conventional floating plug configuration, the difference from normal practice being that the plug i heated and positioned to confirm with the two basic principles of the invention described above. There may be .circumstances where more than one finishing unit U 2 (with a plug or ring) may advantageously be used and this arrangement also falls within the scope of the invention.

Claims: Claims 1 Polymer processing apparatus comprising either an external pipe sizing un followed by an internal plug finishing unit or an internal pipe sizing un followed by an external ring finishing unit, the plug or ring finishing u being of such dimensions and being placed in such a position relative to external or internal sizing unit as the case may be and at such a control temperature as will cause any asperities on the surface of the pipe or tu passing over the plug or inside the ring to flow locally to conform with said plug or ring surface without disturbing the basic form of the pipe o tube which need not be of circular cross section. 2 Apparatus as claimed in claim 1 where the internal plug finishing unit or the external ring finishing unit provides heat to the surface of the pipe tube. 3 Apparatus as claimed in claim 1 where the internal plug finishing unit o the external ring finishing unit removes heat from the surface of the pi or tube. 4 Apparatus as claimed in any of claims 1 to 3 in which more than one plug ring finishing unit is used. 5 Apparatus as claimed in any of claims 1 to 3 where the internal plug finishing unit is rigidly attached by a stem to the mandrel of the die. 6 Apparatus as claimed in any of claims 1 to 4 in which an internal plug finishing unit is flexibly attached to another such unit or to the mandr of the die. A process for the production of plastic pipe or tube of circular or non-circular cross-section comprising passing polymer resin either throug an external sizing unit followed by an internal plug finishing unit, or a internal sizing unit followed by an external ring finishing unit, the plu or ring finishing unit being of such dimensions and being placed in such position relative to the external or internal sizing unit as the case may as will cause any asperities or undulations on the surface of the pipe or tube passing over the plug or inside the ring to flow locally to conform the said plug or ring surfaces without disturbing the basic form of the p or tube, thereby creating a smooth finish. A process as claimed in claim 7 where the polymer resin contains fibres. A process as.claimed in 8 whereby the fibres have a length of 3 to 15 mm. A process as claimed in either of claims 8 or 9 wherein the fibres are monofilaments. A process as claimed in any of claims 8 to 10 wherein the fibres are of glass, ceramic, metal or organic material. A process as claimed in 7 where the polymer resin contains non-fibrous fillers.

Inventors: BUSH STEPHEN FREDERICK, BUSH, STEPHEN, FREDERICK
Applicants: PROSYMA RES LTD, PROSYMA RESEARCH LIMITED
Application year: 1990
Main classification: B29C47
IPC codes: B29C47