{"id":7582,"date":"2026-10-26T08:46:31","date_gmt":"2026-10-26T08:46:31","guid":{"rendered":"https:\/\/uplastech.com\/wpc-wood-plastic-composite-extrusion\/"},"modified":"2026-10-26T08:47:13","modified_gmt":"2026-10-26T08:47:13","slug":"wpc-wood-plastic-composite-extrusion","status":"publish","type":"post","link":"https:\/\/uplastech.com\/ru\/wpc-wood-plastic-composite-extrusion\/","title":{"rendered":"WPC Extrusion: Formulation, Drying and Why the Vent Is Not a Dryer"},"content":{"rendered":"<style>\n.upl-blog{font-size:17px;line-height:1.78;color:#1f2937}\n.upl-blog p{margin:0 0 18px}\n.upl-blog h2{font-size:26px;line-height:1.3;color:#0b2b33;margin:46px 0 14px;font-weight:700;letter-spacing:-.015em}\n.upl-blog h3{font-size:19.5px;color:#0d3b47;margin:28px 0 10px;font-weight:700}\n.upl-blog ul,.upl-blog ol{margin:0 0 20px;padding-left:22px}\n.upl-blog li{margin:0 0 9px}\n.upl-blog figure{margin:0 0 34px}\n.upl-blog figure img{width:100%;height:auto;display:block;border-radius:10px}\n.upl-blog figcaption{font-size:13px;color:#6b7280;margin-top:10px;line-height:1.5}\n.upl-blog .toc{background:none;border:0;border-radius:0;padding:0;margin:0 0 34px}\n.upl-blog .toc .lwptoc{margin:0}\n.upl-blog .toc h4{margin:0 0 10px;font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#0d5563}\n.upl-blog .toc ol{margin:0;padding-left:18px;font-size:14.5px}\n.upl-blog .toc li{margin-bottom:6px}\n.upl-blog table{width:100%;border-collapse:collapse;font-size:14.5px;margin:0 0 10px}\n.upl-blog th{background:#0d3b47;color:#fff;text-align:left;padding:11px 13px;font-weight:600;font-size:13.5px;border:0}\n.upl-blog td{padding:11px 13px;border:0;border-bottom:1px solid #e8ebed;vertical-align:top}\n.upl-blog tbody tr:nth-child(even){background:#fafbfc}\n.upl-blog .tablecap{font-size:12.5px;color:#6b7280;margin:0 0 30px}\n.upl-blog .myth{background:#fff8ef;border:1px solid #f6ddbd;border-radius:8px;padding:16px 20px;margin:0 0 26px;font-size:15.5px}\n.upl-blog .myth b{color:#a85c10}\n.upl-blog .callout{background:#f2f8f9;border-radius:8px;padding:18px 22px;margin:0 0 26px;font-size:15.5px;border-left:3px solid #14707f}\n.upl-blog .faq h3{border-top:1px solid #eceff1;padding-top:22px;margin-top:0;font-size:18px}\n.upl-blog .faq h3:first-of-type{border-top:0;padding-top:0}\n.upl-blog .cta{background:linear-gradient(135deg,#0b2b33,#0d5563);color:#fff;border-radius:12px;padding:34px;margin:48px 0 30px}\n.upl-blog .cta h2,.upl-blog .cta .cta-h{color:#fff;margin:0 0 10px;font-size:24px;font-weight:700;line-height:1.3}\n.upl-blog .cta p{color:#c9dee3;margin:0 0 20px;font-size:16px}\n.upl-blog .btn{display:inline-block;background:#e07a1f;color:#fff;text-decoration:none;font-weight:700;padding:13px 26px;border-radius:8px;font-size:15.5px}\n.upl-blog .rel{border-top:1px solid #e8ebed;margin-top:40px;padding-top:26px}\n.upl-blog .rel h4{font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#6b7280;margin:0 0 14px}\n.upl-blog .rel ul{list-style:none;padding:0;margin:0}\n.upl-blog .rel li{padding:11px 0;border-bottom:1px dashed #e8ebed}\n.upl-blog .rel a{text-decoration:none;font-weight:600;font-size:15.5px}\n@media(max-width:640px){.upl-blog h2{font-size:22px}.upl-blog table{font-size:13px}.upl-blog .cta{padding:24px}}\n\/* \u5355\u7bc7\u9875\u9690\u85cf WhatsApp \u81ea\u52a8\u5c55\u5f00\u7684\u95ee\u5019\u5361\uff0c\u53ea\u4fdd\u7559\u5706\u5f62\u6c14\u6ce1\uff0c\u907f\u514d\u906e\u6321\u4fa7\u680f\u8be2\u76d8\u63d0\u4ea4\u6309\u94ae *\/\nbody.single-post .ht_ctc_chat_greetings_box{display:none !important}\n<\/style>\n<div class=\"upl-blog\">\n<figure>\n<img fetchpriority=\"high\" decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/ex-wpc-decking.jpg\" alt=\"Wood plastic composite decking boards on a profile extrusion line, showing the wood grain surface and cut ends\" width=\"1536\" height=\"909\"><figcaption>WPC decking coming off a profile line. The wood grain is embossed after the calibrator, while the surface is still hot enough to take an impression \u2014 the compound itself is uniform, and every visual difference between boards is put there deliberately.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"what\">What WPC actually is<\/h2>\n<p>Wood plastic composite is a thermoplastic \u2014 usually HDPE, sometimes PP or PVC \u2014 loaded with 40 to 70 % wood flour by weight, plus a coupling agent, a lubricant package and a colorant. Nothing about it is exotic. The difficulty is that wood and plastic want opposite things from the process.<\/p>\n<p>Wood flour is hydrophilic: it holds 4 \u2013 8 % moisture as delivered, and that moisture has to be gone before the die or it flashes to steam in the melt and leaves voids, bubbles and a rough surface. Plastic is hydrophobic and melts at 150 \u2013 200 \u00b0C. Wood starts to degrade \u2014 darkening, then volatile release, then char \u2014 at around 200 \u00b0C, and the rate rises steeply above 220 \u00b0C. So you have a melt that must be hot enough to flow and cold enough not to burn the filler inside it.<\/p>\n<div class=\"callout\">\n<p><strong>The governing constraint:<\/strong> the entire WPC process lives in a window of roughly 160 \u2013 190 \u00b0C melt temperature, with moisture below 0.5 % at the die. Every equipment and formulation decision follows from those two numbers. A machine that cannot hold 175 \u00b0C &plusmn; 3 \u00b0C while removing 6 % moisture is not a WPC machine, whatever the brochure says.<\/p>\n<\/div>\n<h2 id=\"formulation\">Formulation: what each component does<\/h2>\n<table>\n<thead>\n<tr>\n<th>\u041a\u043e\u043c\u043f\u043e\u043d\u0435\u043d\u0442<\/th>\n<th>Typical loading<\/th>\n<th>\u0424\u0443\u043d\u043a\u0446\u0438\u044f<\/th>\n<th>If it is wrong<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>HDPE or PP virgin resin<\/td>\n<td>25 \u2013 40 %<\/td>\n<td>Matrix, melt strength, weather resistance<\/td>\n<td>Too little: brittle, water-absorbing board<\/td>\n<\/tr>\n<tr>\n<td>Wood flour, 40 \u2013 100 mesh<\/td>\n<td>40 \u2013 65 %<\/td>\n<td>Stiffness, thermal expansion control, wood appearance<\/td>\n<td>Too coarse: rough surface, weak weld lines. Too fine: feeding and dust problems<\/td>\n<\/tr>\n<tr>\n<td>Maleic anhydride grafted PE (MAPE)<\/td>\n<td>1 \u2013 3 %<\/td>\n<td>Couples wood to matrix \u2014 the single biggest strength lever<\/td>\n<td>Without it, tensile strength drops 30 \u2013 50 %<\/td>\n<\/tr>\n<tr>\n<td>Lubricant (stearate, wax, PE wax)<\/td>\n<td>1 \u2013 4 %<\/td>\n<td>Reduces die pressure, improves surface, controls output<\/td>\n<td>Too much: poor weld strength, plate-out. Too little: high torque, rough surface<\/td>\n<\/tr>\n<tr>\n<td>Calcium carbonate or talc<\/td>\n<td>0 \u2013 10 %<\/td>\n<td>Cost reduction, stiffness, nucleation<\/td>\n<td>Excess: density up, impact down<\/td>\n<\/tr>\n<tr>\n<td>Impact modifier (POE, EPDM)<\/td>\n<td>0 \u2013 5 %<\/td>\n<td>Low-temperature toughness<\/td>\n<td>Omitted in cold climates: brittle failures<\/td>\n<\/tr>\n<tr>\n<td>Colourant \/ UV package<\/td>\n<td>1 \u2013 4 %<\/td>\n<td>Appearance, fade resistance<\/td>\n<td>Omitted: greying within two seasons outdoors<\/td>\n<\/tr>\n<tr>\n<td>Biocide<\/td>\n<td>0.3 \u2013 1 %<\/td>\n<td>Mould and mildew resistance<\/td>\n<td>Omitted in damp service: surface mould<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 A working WPC formulation and what each part contributes. Loadings are weight percent.<\/p>\n<p>The coupling agent deserves its own paragraph because it is where most cost-cutting goes wrong. Wood flour is polar; polyethylene is not. Without a maleic anhydride grafted polyolefin bridging the two, the wood particles sit in the matrix as unbonded inclusions \u2014 the board looks fine, then fails at the screw or in a cold bend because the failure runs along the wood-matrix interface. Two percent MAPE at around \u20ac2.50\/kg adds roughly \u20ac0.05\/kg to a compound that sells for \u20ac0.80\/kg. Removing it to save six percent of material cost cuts tensile strength by a third. Buyers who specify \"cheapest WPC decking\" are specifying exactly this.<\/p>\n<h2 id=\"drying\">Drying: the step that decides whether the line runs<\/h2>\n<p>Wood flour at 6 % moisture contains 60 g of water per kilogram. In a 500 kg\/h line that is 30 kg of water per hour, all of which must leave the melt before the die. There are three places it can go, and only one of them is acceptable.<\/p>\n<table>\n<thead>\n<tr>\n<th>Where the moisture leaves<\/th>\n<th>Result<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>In a pre-dryer, before the extruder<\/td>\n<td>Correct \u2014 controlled, no process disturbance<\/td>\n<\/tr>\n<tr>\n<td>At the extruder vent<\/td>\n<td>Acceptable for the last 1 \u2013 2 %, if the vent is properly designed<\/td>\n<\/tr>\n<tr>\n<td>At the die, as steam<\/td>\n<td>Voids, bubbles, surface blisters, internal porosity, weak weld lines<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 The three exits for moisture. Only the first two are viable.<\/p>\n<p>A vacuum vent on a twin screw can pull the last one or two percent out of the melt, and many WPC lines rely on exactly that with a vent operating at 50 \u2013 200 mbar. But a vent is not a dryer. If the feed arrives at 6 %, the vent cannot remove it fast enough, the melt foams in the vent, material backs up into the vent port, and the line stops to be cleaned. The rule of thumb is that a vent handles 1 \u2013 2 % moisture; anything above that needs a dryer.<\/p>\n<table>\n<thead>\n<tr>\n<th>Dryer type<\/th>\n<th>Inlet moisture<\/th>\n<th>Outlet<\/th>\n<th>Energy<\/th>\n<th>\u041f\u0440\u0438\u043c\u0435\u0447\u0430\u043d\u0438\u044f<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Hot air oven, batch<\/td>\n<td>6 \u2013 10 %<\/td>\n<td>1 \u2013 2 %<\/td>\n<td>\u0412\u044b\u0441\u043e\u043a\u0438\u0439<\/td>\n<td>Cheap; slow; uneven in thick beds<\/td>\n<\/tr>\n<tr>\n<td>Hot air, continuous belt<\/td>\n<td>6 \u2013 10 %<\/td>\n<td>0.5 \u2013 1.5 %<\/td>\n<td>\u0412\u044b\u0441\u043e\u043a\u0438\u0439<\/td>\n<td>Better uniformity; large footprint<\/td>\n<\/tr>\n<tr>\n<td>Vacuum dryer<\/td>\n<td>4 \u2013 8 %<\/td>\n<td>0.3 \u2013 0.8 %<\/td>\n<td>\u0421\u0440\u0435\u0434\u043d\u0438\u0439<\/td>\n<td>Faster, gentler on colour<\/td>\n<\/tr>\n<tr>\n<td>Infrared \/ microwave<\/td>\n<td>4 \u2013 8 %<\/td>\n<td>0.3 \u2013 0.6 %<\/td>\n<td>\u0421\u0440\u0435\u0434\u043d\u0438\u0439<\/td>\n<td>Fast; capital-heavy; good colour control<\/td>\n<\/tr>\n<tr>\n<td>In-line vent only<\/td>\n<td>0.5 \u2013 2 %<\/td>\n<td>0.3 \u2013 0.5 %<\/td>\n<td>\u041d\u0438\u0437\u043a\u0438\u0439<\/td>\n<td>Not a substitute for drying above 2 %<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 3 \u2014 Drying options. Note that all of them produce a colour shift: over-dried wood flour darkens.<\/p>\n<div class=\"myth\">\n<p><b>Myth: \"the vent will take care of the moisture.\"<\/b> A vent removes volatiles from a melt that is already mostly degassed. Feed it 6 % water and it will remove some of it and then choke. Every WPC line that \"runs badly\" that we have been asked to look at had a vent being used as a dryer.<\/p>\n<\/div>\n<h2 id=\"machine\">The machine: why a twin screw, and which one<\/h2>\n<p>WPC needs: powder and fibre feeding, dispersive mixing of a very high filler loading, devolatilisation, and a low-shear pump to the die without over-heating. That combination is a co-rotating intermeshing twin screw, usually with a 32:1 to 44:1 L\/D and at least one vent, often two.<\/p>\n<table>\n<thead>\n<tr>\n<th>Configuration<\/th>\n<th>Typical use<\/th>\n<th>\u041f\u0440\u0438\u043c\u0435\u0447\u0430\u043d\u0438\u044f<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Single screw, grooved feed<\/td>\n<td>Pre-compounded WPC pellet only<\/td>\n<td>Works if you buy pellet; cannot handle flour<\/td>\n<\/tr>\n<tr>\n<td>Twin screw, 32:1, one vent<\/td>\n<td>Up to 50 % wood flour<\/td>\n<td>The workhorse<\/td>\n<\/tr>\n<tr>\n<td>Twin screw, 40:1, two vents<\/td>\n<td>55 \u2013 70 % flour, high moisture<\/td>\n<td>Two-stage devolatilisation<\/td>\n<\/tr>\n<tr>\n<td>Twin screw + gear pump<\/td>\n<td>Tight-tolerance profiles<\/td>\n<td>Decouples output from die pressure<\/td>\n<\/tr>\n<tr>\n<td>Two-stage: twin + single<\/td>\n<td>Very high filler, high output<\/td>\n<td>Twin melts and mixes, single pumps<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 4 \u2014 WPC machine configurations. The two-stage arrangement is used where output and pressure stability both matter.<\/p>\n<p>One configuration detail that surprises people: the die and adapter must be designed for a low-viscosity, highly elastic melt at moderate pressure. WPC at 175 \u00b0C flows more like a filled paste than like molten HDPE, and its melt strength is low because the wood particles interrupt the polymer chain network. That means the die land needs to be short, the draw-down low, and the calibrator close to the die.<\/p>\n<h2 id=\"die\">Die and calibration for WPC<\/h2>\n<table>\n<thead>\n<tr>\n<th>\u041f\u0430\u0440\u0430\u043c\u0435\u0442\u0440<\/th>\n<th>WPC typical<\/th>\n<th>Unfilled HDPE for comparison<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Melt temperature at die<\/td>\n<td>165 \u2013 185 \u00b0C<\/td>\n<td>180 \u2013 210 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>Die pressure<\/td>\n<td>60 \u2013 140 bar<\/td>\n<td>80 \u2013 200 bar<\/td>\n<\/tr>\n<tr>\n<td>\u0423\u043c\u0435\u0440\u0435\u0442\u044c \u0441 \u0433\u043e\u043b\u043e\u0434\u0443\u0445\u0438<\/td>\n<td>5 \u2013 15 %<\/td>\n<td>25 \u2013 45 %<\/td>\n<\/tr>\n<tr>\n<td>Land length<\/td>\n<td>8 \u2013 15 &times; gap<\/td>\n<td>15 \u2013 25 &times; gap<\/td>\n<\/tr>\n<tr>\n<td>Draw-down ratio<\/td>\n<td>1.0 \u2013 1.15<\/td>\n<td>1.2 \u2013 2.5<\/td>\n<\/tr>\n<tr>\n<td>Calibrator distance from die<\/td>\n<td>30 \u2013 80 mm<\/td>\n<td>80 \u2013 250 mm<\/td>\n<\/tr>\n<tr>\n<td>Vacuum level<\/td>\n<td>0.2 \u2013 0.5 bar<\/td>\n<td>0.3 \u2013 0.7 bar<\/td>\n<\/tr>\n<tr>\n<td>Cooling tank length<\/td>\n<td>4 \u2013 8 m<\/td>\n<td>3 \u2013 6 m<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 5 \u2014 WPC's low die swell and low draw-down are the two numbers that most affect die design.<\/p>\n<p>Low die swell is a gift. It means the die orifice is close to the profile dimension, which makes the die easier to cut and the start-up quicker. It also means the melt has little elastic recovery to help it fill a calibrator, so the calibrator must be positioned close and the vacuum applied early.<\/p>\n<p>Cooling is the bottleneck on most WPC lines. Wood flour is a poor conductor \u2014 around 0.1 W\/m\u00b7K against 0.4 for HDPE \u2014 so a 25 mm thick decking board cools from the outside in, and the core stays hot long after the surface is rigid. Pull it too fast and the board bows as the core contracts after it leaves the tank.<\/p>\n<h2 id=\"cooling\">Cooling, and why WPC lines are long<\/h2>\n<p>Cooling length can be estimated from the heat to be removed and the achievable heat transfer coefficient:<\/p>\n<p style=\"font-family:ui-monospace,monospace;background:#f5f7f8;padding:14px 18px;border-radius:8px;font-size:15px;margin:0 0 20px\">L = (m&#775; &middot; c<sub>p<\/sub> &middot; &Delta;T) \/ (h &middot; P &middot; &Delta;T<sub>lm<\/sub>)<\/p>\n<p>For a 140 &times; 25 mm decking board at 400 kg\/h, c<sub>p<\/sub> &asymp; 1.6 kJ\/kg\u00b7K, &Delta;T from 180 to 40 \u00b0C, h &asymp; 350 W\/m\u00b2\u00b7K in a spray tank, P &asymp; 0.33 m:<\/p>\n<p style=\"font-family:ui-monospace,monospace;background:#f5f7f8;padding:14px 18px;border-radius:8px;font-size:15px;margin:0 0 20px\">Q = 400 &times; 1.6 &times; 140 \/ 3600 = 24.9 kW<br \/>L &asymp; 24,900 \/ (350 &times; 0.33 &times; 55) &asymp; 3.9 m<\/p>\n<p>That is the theoretical minimum for the surface. In practice a 25 mm thick WPC board needs 6 \u2013 9 m of tank because the core must be below about 70 \u00b0C before the board can be cut without the saw pulling it out of shape. Add the spray tank, a soak section and a second tank, and a WPC line's cooling section is often twice as long as the extruder.<\/p>\n<table>\n<thead>\n<tr>\n<th>Board thickness<\/th>\n<th>Minimum tank length at 300 kg\/h<\/th>\n<th>Practical installed length<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>10 mm<\/td>\n<td>1.8 m<\/td>\n<td>3 \u2013 4 m<\/td>\n<\/tr>\n<tr>\n<td>16 mm<\/td>\n<td>2.6 m<\/td>\n<td>4 \u2013 5 m<\/td>\n<\/tr>\n<tr>\n<td>25 mm<\/td>\n<td>3.9 m<\/td>\n<td>6 \u2013 9 m<\/td>\n<\/tr>\n<tr>\n<td>40 mm (hollow)<\/td>\n<td>4.2 m<\/td>\n<td>7 \u2013 10 m<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 6 \u2014 Cooling length. Hollow sections cool faster per unit mass but need internal support to hold shape.<\/p>\n<h2 id=\"surface\">Surface: embossing, brushing, co-extrusion<\/h2>\n<p>A WPC board out of the calibrator has a smooth, slightly waxy surface that looks nothing like wood. The wood appearance is applied afterwards, in three ways:<\/p>\n<ul>\n<li><strong>Embossing.<\/strong> A textured roller or a moving belt presses a grain into the surface while it is still at 90 \u2013 110 \u00b0C. Depth 0.3 \u2013 1.2 mm. This is the cheapest and most common method.<\/li>\n<li><strong>Brushing.<\/strong> Rotating wire brushes abrade the surface to expose wood fibres, giving a raised, tactile grain. Requires the compound to have enough fibre near the surface.<\/li>\n<li><strong>Co-extrusion.<\/strong> A thin cap layer \u2014 0.3 \u2013 0.8 mm \u2014 of unfilled or lightly filled compound, pigmented and often with a higher UV package, extruded over the core. The cap takes the embossing better and carries the weather resistance, which lets the core be a cheaper, more highly filled formulation.<\/li>\n<\/ul>\n<table>\n<thead>\n<tr>\n<th>\u041c\u0435\u0442\u043e\u0434<\/th>\n<th>\u0421\u0442\u043e\u0438\u043c\u043e\u0441\u0442\u044c<\/th>\n<th>Weather resistance<\/th>\n<th>Realism<\/th>\n<th>\u041b\u0443\u0447\u0448\u0435\u0435 \u0434\u043b\u044f<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Emboss only<\/td>\n<td>\u041d\u0438\u0437\u043a\u0438\u0439<\/td>\n<td>Core-dependent<\/td>\n<td>\u0421\u0440\u0435\u0434\u043d\u0438\u0439<\/td>\n<td>Budget decking, fencing<\/td>\n<\/tr>\n<tr>\n<td>Emboss + brush<\/td>\n<td>Low\u2013medium<\/td>\n<td>Core-dependent<\/td>\n<td>\u0412\u044b\u0441\u043e\u043a\u0438\u0439<\/td>\n<td>Premium decking<\/td>\n<\/tr>\n<tr>\n<td>Co-extruded cap + emboss<\/td>\n<td>Medium\u2013high<\/td>\n<td>\u0412\u044b\u0441\u043e\u043a\u0438\u0439<\/td>\n<td>\u0412\u044b\u0441\u043e\u043a\u0438\u0439<\/td>\n<td>Exterior cladding, fascia<\/td>\n<\/tr>\n<tr>\n<td>Co-extruded cap, no emboss<\/td>\n<td>\u0421\u0440\u0435\u0434\u043d\u0438\u0439<\/td>\n<td>\u0412\u044b\u0441\u043e\u043a\u0438\u0439<\/td>\n<td>\u041d\u0438\u0437\u043a\u0438\u0439<\/td>\n<td>Where colour matters more than grain<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 7 \u2014 Surface options. Co-extrusion is the only one that decouples appearance from core formulation.<\/p>\n<h2 id=\"defects\">Defects specific to WPC<\/h2>\n<table>\n<thead>\n<tr>\n<th>\u0414\u0435\u0444\u0435\u043a\u0442<\/th>\n<th>\u0412\u043d\u0435\u0448\u043d\u0438\u0439 \u0432\u0438\u0434<\/th>\n<th>\u041f\u0440\u0438\u0447\u0438\u043d\u0430<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Surface blistering<\/td>\n<td>Raised bumps, sometimes burst<\/td>\n<td>Moisture above 1 % flashing at the die<\/td>\n<td>Dry the flour; lower melt temperature<\/td>\n<\/tr>\n<tr>\n<td>Internal voids<\/td>\n<td>Visible on a cut end, sponge-like core<\/td>\n<td>Same, plus excessive draw-down<\/td>\n<td>Dry, reduce draw-down, check vent<\/td>\n<\/tr>\n<tr>\n<td>Rough \/ matt surface<\/td>\n<td>No gloss, drag marks<\/td>\n<td>Melt too cold, or lubricant imbalance<\/td>\n<td>Raise melt 5 \u00b0C; check lubricant level<\/td>\n<\/tr>\n<tr>\n<td>Dark streaks<\/td>\n<td>Brown-black lines along the flow<\/td>\n<td>Local overheating, hang-up in the die, scorched material<\/td>\n<td>Strip and clean the die; check for dead spots<\/td>\n<\/tr>\n<tr>\n<td>Poor weld line strength<\/td>\n<td>Split at the weld on bending<\/td>\n<td>Low melt temperature, low coupling agent, or a weld line in a high-stress location<\/td>\n<td>Raise temperature; increase MAPE; move the weld<\/td>\n<\/tr>\n<tr>\n<td>Bowing after cutting<\/td>\n<td>Board curves along its length<\/td>\n<td>Core still hot when cut; asymmetric cooling<\/td>\n<td>Extend cooling; balance the spray; check the saw clamp<\/td>\n<\/tr>\n<tr>\n<td>Dimensional growth in service<\/td>\n<td>Board lengthens over months outdoors<\/td>\n<td>Thermal expansion coefficient of WPC is 2 \u2013 3&times; PVC<\/td>\n<td>Design for it: &alpha; &asymp; 40 \u2013 60 &times; 10\u207b\u2076 \/K<\/td>\n<\/tr>\n<tr>\n<td>Colour fade<\/td>\n<td>Greying within 12 \u2013 24 months<\/td>\n<td>Insufficient UV package, or no cap layer<\/td>\n<td>Add HALS + UV absorber; or co-extrude a cap<\/td>\n<\/tr>\n<tr>\n<td>Mould growth<\/td>\n<td>Black spots on the surface<\/td>\n<td>Wood flour at the surface plus damp service<\/td>\n<td>Biocide; brushing to remove surface fibre; cap layer<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 8 \u2014 WPC defects, all of them traceable to moisture, temperature or formulation.<\/p>\n<h2 id=\"design\">Design rules for WPC profiles<\/h2>\n<ol>\n<li><strong>Keep wall thickness uniform.<\/strong> WPC's low melt strength means thick-to-thin transitions cause sink and internal voids. Aim for a 1:1.3 ratio maximum.<\/li>\n<li><strong>Round every internal corner.<\/strong> Minimum internal radius 1.5 mm; 2.5 mm preferred. Sharp corners are where the material pulls away from the calibrator.<\/li>\n<li><strong>Design for 2 \u2013 3&times; the thermal expansion of PVC.<\/strong> A 4 m WPC board moves about 10 mm over a 40 K temperature swing. Fixings must allow it.<\/li>\n<li><strong>Do not put a weld line where the part will be loaded.<\/strong> In a hollow section, the weld line is at the far side from the die entry. If that is where a screw fixing goes, redesign.<\/li>\n<li><strong>Allow for a cap layer in the die.<\/strong> If co-extrusion is even a possibility, design the die with the cap channel from the start. Retrofitting is a new die.<\/li>\n<li><strong>Specify the embossing depth before the calibrator is built.<\/strong> Embossing changes the outer dimension by up to 1 mm and the calibrator has to be sized for the embossed surface, not the smooth one.<\/li>\n<\/ol>\n<div class=\"callout\">\n<p><strong>The number to remember:<\/strong> 40 \u2013 60 &times; 10\u207b\u2076 \/K. That is WPC's coefficient of linear thermal expansion, against 60 \u2013 80 for PVC and 12 for aluminium. A 3 m WPC board grows 5 \u2013 7 mm between winter and summer. Every fixing detail, every joint tolerance and every installation instruction has to account for it, and the boards that \"buckle for no reason\" on a deck are boards installed tight.<\/p>\n<\/div>\n<h2 id=\"faq\">Frequently asked questions<\/h2>\n<div class=\"faq\">\n<h3>What wood species and mesh size should the flour be?<\/h3>\n<p>Pine or oak, 40 \u2013 80 mesh for most profiles; 100 mesh for thin walls and smooth surfaces. Hardwood flours give better stiffness; softwood flours feed and disperse more easily. Avoid flour with bark content above 5 % \u2014 it does not wet out and it shows as dark specks.<\/p>\n<h3>Can WPC be recycled?<\/h3>\n<p>Yes, and it is one of its advantages. Production scrap and end-of-life board can be reground and re-extruded at up to 30 % in the core, with a small loss of strength and a colour shift. Closed-loop regrind of the same formulation is routine; post-consumer mixed WPC is harder because the wood content and the polymer type vary.<\/p>\n<h3>Why does my WPC smell like burnt wood?<\/h3>\n<p>Because it is burning. Local overheating in a dead spot, or a melt temperature above 200 \u00b0C, or a screw that has been stopped hot with material on it. The smell is a leading indicator \u2014 the visible dark streaks come later.<\/p>\n<h3>What is the density of WPC?<\/h3>\n<p>1.15 \u2013 1.35 g\/cm\u00b3 for solid profiles, against 0.95 for unfilled HDPE. Foamed WPC runs 0.6 \u2013 0.9 g\/cm\u00b3 and is used where weight matters more than stiffness. The density is what makes WPC heavier than the timber it replaces \u2014 a point worth raising early with a customer who expects it to feel like wood.<\/p>\n<h3>Do I need a gear pump on a WPC line?<\/h3>\n<p>On tight-tolerance profiles, yes. WPC's pressure sensitivity is high and its melt is compressible, so a gear pump between the extruder and the die stabilises both output and dimensions. On a decking board with &plusmn;0.5 mm tolerance, it is optional; on a window profile with &plusmn;0.15 mm, it is not.<\/p>\n<h3>Can WPC be foamed?<\/h3>\n<p>Yes, with a chemical blowing agent or CO\u2082 injected at the barrel. Foaming cuts density by 30 \u2013 40 % and improves thermal insulation, at the cost of surface quality \u2014 foamed WPC has a swirl pattern that has to be covered by a cap layer or accepted as a design feature.<\/p>\n<h3>How much does a WPC die cost compared with a PVC die?<\/h3>\n<p>Typically 1.3 \u2013 1.8&times; an equivalent PVC profile die, because the flow channels must be designed for a low-viscosity filled melt and the die needs more wear-resistant internals. A decking die runs \u20ac15,000 \u2013 \u20ac35,000; a co-extrusion decking die with a cap channel, \u20ac30,000 \u2013 \u20ac55,000.<\/p>\n<h3>What is the biggest cause of WPC line downtime?<\/h3>\n<p>In our experience: moisture. Either the dryer underperforming, or a batch of flour arriving wetter than specified, or the dryer being bypassed \"just for this run\". The second biggest is die fouling from degraded material building up in a dead spot.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Developing a WPC profile? Send us the section and the service conditions<\/p>\n<p>We will come back with a formulation starting point, a die concept, the drying requirement and a realistic line speed \u2014 before you commit to tooling.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/ru\/%d1%80%d0%b5%d1%88%d0%b5%d0%bd%d0%b8%d1%8f-%d0%b4%d0%bb%d1%8f-%d1%8d%d0%ba%d1%81%d1%82%d1%80%d1%83%d0%b7%d0%b8%d0%b8-%d0%bf%d0%bb%d0%b0%d1%81%d1%82%d0%bc%d0%b0%d1%81%d1%81\/\">Request a WPC feasibility review<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/ru\/twin-screw-vs-single-screw-extrusion\/\">Twin Screw vs Single Screw Extrusion: Which to Specify<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ru\/co-extrusion-profiles\/\">Co-Extrusion Profiles: Cap Layers, Bonding and Dies<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ru\/profile-calibration-cooling\/\">Vacuum Calibration and Cooling in Profile Extrusion<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ru\/%d0%b4%d0%b5%d1%84%d0%b5%d0%ba%d1%82%d1%8b-%d1%8d%d0%ba%d1%81%d1%82%d1%80%d1%83%d0%b7%d0%b8%d0%b8-%d0%bf%d0%bb%d0%b0%d1%81%d1%82%d0%bc%d0%b0%d1%81%d1%81\/\">Plastic Extrusion Defects: 12 Problems, Causes and Fixes<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ru\/custom-plastic-extrusion-profiles-guide\/\">Custom Plastic Extrusion Profiles: Materials, Dies &amp; Tolerances<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Wood plastic composite extrusion in practice: formulation and coupling agents, moisture and drying, machine configuration, die and calibration, surface finishing and WPC defects.<\/p>","protected":false},"author":1,"featured_media":7572,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"WPC Extrusion: Formulation, Drying and Venting","_seopress_titles_desc":"Wood plastic composite extrusion in practice: formulation, coupling agents, moisture and drying, machine configuration, die and calibration, surface finishing and WPC defects.","_seopress_robots_index":"1","_seopress_robots_follow":"1","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"36","_seopress_robots_breadcrumbs":"","_seopress_robots_freeze_modified_date":"","_seopress_robots_custom_modified_date":"","_seopress_robots_canonical":"https:\/\/uplastech.com\/wpc-wood-plastic-composite-extrusion\/","_seopress_social_fb_title":"","_seopress_social_fb_desc":"","_seopress_social_fb_img":"","_seopress_social_fb_img_attachment_id":0,"_seopress_social_fb_img_width":0,"_seopress_social_fb_img_height":0,"_seopress_social_twitter_title":"","_seopress_social_twitter_desc":"","_seopress_social_twitter_img":"","_seopress_social_twitter_img_attachment_id":0,"_seopress_social_twitter_img_width":0,"_seopress_social_twitter_img_height":0,"_seopress_redirections_value":"","_seopress_redirections_enabled":"","_seopress_redirections_enabled_regex":"","_seopress_redirections_logged_status":"","_seopress_redirections_param":"","_seopress_redirections_type":0,"_seopress_analysis_target_kw":"wpc wood plastic composite extrusion","footnotes":""},"categories":[17,36],"tags":[],"class_list":["post-7582","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-plastic-extrusion-process","category-profile-extrusion"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/posts\/7582","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/comments?post=7582"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/posts\/7582\/revisions"}],"predecessor-version":[{"id":7592,"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/posts\/7582\/revisions\/7592"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/media\/7572"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/media?parent=7582"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/categories?post=7582"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/ru\/wp-json\/wp\/v2\/tags?post=7582"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}