Currently, more than 95% of the WPC, produced in the world, are produced by extrusion. In recent years other technologies and methods of processing WPC are developing. Let’s see some technological issues related to the WPC production.
Extrusion - a continuous process, consisting in punching a material having a high viscosity in the liquid state, through a forming tool (die head) to obtain a product with a desired cross-sectional shape. In the industry of polymers extrusion is used from the 30-ies of the last century.
By extrusion of WPC we get different molded products for the manufacture of window and door frames, furniture parts, including board materials may receive up to 3 meters, builders' joinery: flooring, paneling, skirting, trim, railings, and so on.
Extrusion is the most studied method for WPC production. This technology had been improved over the years by North American and European equipment manufacturers. Suppliers of extrusion lines have extensive experience and are ready to pick their own line elements and formulas based on the analysis of customer’s raw materials.
Theoretically, at the opening of a new production you can bet on other types of polymer processing to produce various products. However, for the most part they are the know-how of individual companies, with only a narrow range of solutions, and whose production lines can not be scaled in accordance with the wishes of the customer.
Let’s consider the basic steps of the WPC production by extrusion.
- Preparation of raw materials
The ratio of filler and binder during extrusion can vary widely depending on the purpose of the product:
• 40-95% – wood –plant fillers;
• 60-5% – synthetic binders .
When the content of the polymer binder less than 10%, a sharp decline in the strength of products is noticed, and increased content (over 20%) does not lead to significant improvements in physical and mechanical characteristics of the product, but causes an increase in their value.
Preparation of raw materials depends on the type of line and the technology used. There are two basic approaches: extrusion using semi-finished material and "direct extrusion." With direct extrusion individual components (raw wood, polymer and additive) are served through volumetric dosing directly into the extruder. Wood components supplied as the raw material must undergo a pretreatment step to ensure reliable dispensing. Wood wastes split with knife or hammer crusher, crushed to fine grinding with rolling machines (such as rolling mill), with pin mill. The material is then sieved and fractionated. If the composition of the WPC’s finished granules is balanced, the composition of the direct extrusion compound must be constantly monitored by volumetric dosing system. You also need to perform special processing of wood filler as its size, shape, and are of great importance. The most commonly used wood flour (dispersity 0.01-1 mm) and sawdust (from 1 to 8 mm), at least - the chips (10-20 mm) or particles of coercive forms: flake, fiber-like, and others. The finer fractions suitable for the production of profiles without additional surface finish, medium - for finishing films, veneer or colored, and coarse - for technical profiles. European equipment manufacturers recommend to use wood waste fractions of 0.5-3 mm.
The second important point in the direct extrusion technology - the humidity of wood raw material. Optimal humidity, declared by different manufacturers is different. This is due to the design of extruder. For example, for the German equipment moisture content even up to 10% does not adversely affect the performance of the equipment. Low humidity (8-6%) at the same time contributes to its sparing operation. According to other technologies humidity of wood fibers should not exceed 1.3% (but not below this value, because of the loss of their flexibility, thereby deteriorating the properties of the profile), because in this case the process becomes more stable, comparable to the extrusion of pure PVC, and eliminates the need for frequent adjustment of parameters of equipment.
The strength of product made of dry raw material strength is considerably higher as the impact toughness, which is increased by 2-3 times. It is worth noting that semi-granules are also subjected to the final drying in an extruder for reducing the humidity.
Finally, the third feature - the need for selection of additives. When irregularities or proportions of chemical additives or burrs appear on the surface of product due to attachment to the molding tool, or the strength get broken - due to delamination of the wood-polymer mass and internal slip layers. But there are additives, which proportion error does not affect neither the manufacturing process nor the strength or appearance of the finished product, but appears only during the operation. For example, in the formulation of antioxidant is only a few tenths of a percent, but their absence can affect the properties of the product dramatically, especially when applied on the open air: after some time the material starts to oxidize and break down. In the practice of American manufacturers, decking products which have a warranty period of not less than twenty years, met dramatic examples of the vast losses due to claims for materials with wrong dosing of antioxidants.
- The production process
The main element of the WPC production line is extrusion equipment or extruder.
The extruder consists of several main components: a power drive unit , material load unit, equipped with heating elements, working body (screw) placed in the housing, and the die, as well as control, measuring devices (system of setting data and temperature control, sensor etc).
The die consists of a heated body, which is attached to the extruder and the forming tool with the hole, for example in the form of tapered towards the center gap (for making sheets, films), or the channel of certain sections (in the manufacture of pipes and profiles). Single-screw and twin-screw machines are used in the production of various types of products. First are easier and cheaper to manufacture, however, the material in these extruders is not always well mixed, and the transition from one raw material to the other is difficult.
Twin-screw machines are more complex design, resulting in higher cost, power consumption and maintenance requirements. But they provide accurate dosing, adaptation to different types of raw materials, the best mixing of the material and a higher quality of the end product. Furthermore, during the process the effect of self-cleaning and the inner surface of the screw body is achieved. By design twin screw extruders are divided into several types augers can rotate in the same direction or in opposite, interlockingly or not. Finally, the shape of the screw may be cylindrical or conical.
Twin-screw conical extruders are now the most widely used in the production of WPC: these machines provide good mixing, uniform heating of the masses and create high pressure at the outlet. The quality of the final product is largely determined by the shape tool. Heads of special shape are designed for wood-polymer material, which not only allow to obtain complex configuration profiles, but in a certain way orient wood fibers, thereby increasing the strength of the end product.
The process of extrusion includes heating of granules of the compound in the extruder cylinder, which is moving along the cylinder and is converted into a plastic mass.
ДFurther, depending on the type of final product, there are three variants of the manufacturing process:
In the production of profiles the plasticized mass is forced through a die to be formed in the primary profile workpiece. Then the workpiece passes through the calibrator, where the profile is cooled under vacuum, obtains the final shape and dimensions. Further cooling is in a water bath or on roller conveyors. The cutting device cuts the product to the desired length, and it is supplied at relieving device, where products can be fastened or packed.
WPC boards intended for construction of decks, before the final cooling is usually treated with special metal brushes that make the surface less slippery and imitate her natural wood pattern.
The end product of the extrusion line can be beads, semi-finished products. In this case, the granulation head is set on the output extruder, from which, by means of blower through a cyclone finished granules, cooled, received in the big bag or a special container. Often a two-stage scheme used in the production in which one line produces pellets and the other - focused on the production profile (Equipment manufacturers distinguish line of extrusion and granulation line). This organization allows for greater throughput and automation of the extrusion process, as when using the finished granule is not necessary to control the dosing of the various components of the composite. In addition, the granules can be considered as a separate product for B2B- supply to the market.
In WPC production co-extrusion method can also be used when the outputs of two or more units of processing various mixtures combined with special head. Thus prepared, for example, profiles of WPC covered with a layer of water-resistant polymer. A similar method can be applied to wood-polymer mass on the metal rods or wires – i.e. during manufacture directly apply the reinforcing elements on them. Features of the extrusion process has a lot of subtleties. If in the output from the extruder the polymer melt temperature is below required, it hardens and wade through a die to form voids and other defects, and if it is slightly overheated, then poured through a die, while still relatively liquid, and acquires the desired shape. Excessive speed of the melt leads to the fact that its temperature rises due to internal friction, with all the consequences of superheated material. Too rapid cooling is resulting product polymer "freezes" in the stressed form, and these strains then show themselves as distortion and lack of strength. The list goes on - scientific works are dedicated to the behavior of the polymer mass at different stages of the passage through the extruder dedicated to. We only note that the introduction to the polymer composition of raw wood does not simplify the process and to obtain highly-wood-polymer compositions requires specialized equipment and precise observance of all the parameters of the production process. It is because of the complexity of process manufacturers are not in a hurry to use such a "capricious" filler, such as wood, despite the fact that the possibility of producing wood-plastic composites thermoplastic binder has been known for several decades.
It is obvious that the WPC production process by extrusion is a very technologically advanced.
Formulation of WPC is handled by specialized companies and research organizations, usually in collaboration with the manufacturers of extrusion equipment.
At present, there are many options for production - choose optimal configuration based on the capabilities of a particular enterprise, planned production volumes and products purposes.
2. Injection molding
Wood-polymer composites are well established in the production of extruded products. At the moment they begin to be used in injection molding. However, some companies engaged in injection form, do not want to experiment with new materials. And molders who have already tested the new technology, there were a number of problems, including fluctuations in quality, supply heterogeneity, and, on the whole, a more difficult task when using WPC compared with conventional materials.
However, recent developments in the field of WPC compounding have significantly improved the quality and uniformity of the performance of this material. Moreover, the latest generation of WPC can be processed on conventional injection molding equipment with minimal adjustment of setting and without physical modification.
Several US companies involved in shaping the individual orders, have developed sophisticated technology using co-injection and foaming. However, these materials do have some processing parameters that distinguish them from known molding resins. Here are some molding instructions that apply specifically to work with a mixture of wood fibers and polypropylene, and other thermoplastic biocomposites.
WPC injection molding is well founded in the production of parts with thick walls (tips for boards and complete for columns, plinths, etc.) for which the advantage is high rigidity and dimensional stability. However, it should be articles that are not excessive impact force because WPC is more fragile than the conventional molding materials.
- Preparation of raw material
Most composites intended for injection molding, using a much smaller amount of plant woody fillers (20-60%) than for extrusion. Furthermore, in the compound they use short fibers of plant origin. Big importance in molding, unlike extrusion, is the type of timber that affects its percentage in the composite and some technological characteristics.
To ensure the quality of finished components is crucial to the use of granules of thermoplastic biocomposites. When creating their formulations should pay particular attention to:
dryness: surface moisture should be less than 1.5%, while the moisture inside the granules - less than 1% (no humidity control can provide visible surface distortion and increased fragility);
homogeneity and encapsulation: granules should be clean and relatively uniform in size and shape; should be small particles or pieces of streaks. Products from the WPC, injection molded, more expensive than extruded since forming requires the prior preparation of the compound and drying the material (injection press usually have no ventilation). 4. Oak Flour corrodes the mold because of the presence of tannic acid. As compared to oak, pine lighter colored, but it has lower flexural modulus, and terpenes which are contained in the pine exude resin. The lowest level of release of undesirable gases is maple.
WPC advantage compared with conventional molding materials is that they can be mixed with additional amounts of unfilled polypropylene or other resin to achieve different performance characteristics (e.g., to increase resistance to splitting components such as automobile bumpers, or a net increase in the structural rigidity of the resin ).
- Production Process
Two of the most important principles that should be considered when forming the WPC - is the need to avoid excessive temperature and excessive shifting. When forming a proper temperature, speed and smooth flow of items will be uniform in color and dispersion of wood fibers, the minimum stress, smooth surface with no signs of gas formation.
Typical temperatures for molding wood-polymer composites are:
• 1710-1880 ° C for the rear area;
• 1820-1990 ° C for the middle zone;
• 1930-2100 ° C for the front zone;
• 1990-2100 ° C for the tip injection hole.
Molding pressure values depend on the design details, as well as the system of gates and valves. With this, injection molding WPC typically requires less pressure values than conventional molding materials. Particular attention should be paid to the speed of filling composite. Since the material has a high flow velocity and shear sensitivity, it is necessary to avoid too short time of filling. High temperature occurring due to rapid filling, usually appears in the form of symptomatic bands with a high content of resin on the surface of wood-polymer composite. Molding nozzle tip, which is used for forming must be provided with a hole with a diameter as close as possible to the diameter of the gate opening information for shifting to a minimum.
Using smaller diameter tips can also result in loss of color due to the overheating of the material at the moment of its introduction into the mold.
Features of process
When molding products from WPC must provide ample flow runner systems with a minimum of obstructions in order to minimize shifting of the material. Gates should be as large as possible, as far as the small size of the mold. Components of the material should be sent directly to the thickest part of the piece. In cases where the sprue hole requires supplied to the edge of the product, it is recommended to use the gates of two-thirds the width of the wall thickness of the part. Location runners should avoid providing connections flow fronts and welds that could be sensitive to strain during operation, as the probability of damage is highest precisely on welded joints.