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Injection moulding Injection molding of wood-plastic composites While everyone knows wood-plastic composites (WPC) e. g. for decking and fencing, now a wider range of material options for WPC formulations are opening new opportunities for molders. Recycled, biodegradable and biobased plastic feedstock can further enhance the sustainability of these materials. There are an increasing number of aesthetic options, which can be manipulated by varying the wood species and wood particle size in the composite. In short, optimization for injection molding and the growing list of options available to compounders mean wood-plastic composites are a much more versatile material than what was once thought. What injection molders should expect from suppliers A growing number of compounders are now offering wood-plastic composite pellets. Injection molders should be discerning when it comes to what to expect from compounders in two areas especially: pellet size and moisture content. Unlike when extruding wood-plastic composites for decking and fencing, uniform pellet size for even melting is crucial. Since extruders do not have to worry about fitting their wood-plastic composite into a mold, the need for uniform pellet size is not as great. Hence, it’s important to verify that a compounder has the needs of injection molders in mind specifically, and is not overly focused on the earliest and initially most prevalent uses for wood-plastic composites. When pellets are too large they have a tendency to melt unevenly, create additional friction and settle into a structurally inferior final product. The ideal pellet should be 4 – 5mm in diameter and rounded to achieve an ideal surface to volume ratio. These dimensions facilitate drying and help to ensure a smooth flow throughout the production process. Injection molders working with wood-plastic composites should expect the same shape and uniformity they associate with traditional plastic pellets. Dryness, too, is an important quality to expect from a compounder’s wood-plastic composite pellets. Moisture levels in wood-plastic composites will increase with the amount of wood filler in the composite. While both extruding and injection molding require low-moisture content for best results, recommended moisture levels are slightly less for injection molding than for extrusion. So again, it’s important to verify that a compounder has considered injection molders during manufacturing. For injection molding, moisture levels should be below 1 % for optimal results. When suppliers take it upon themselves to deliver a product already containing acceptable levels of moisture, injection molders spend less time drying the pellets themselves, which can lead to substantial saving of time and money. Injection molders should consider shopping around for wood-plastic composite pellets shipped by the manufacturer with moisture levels already below 1 %. Formula and tooling considerations for woodplastic composites The ratio of wood to plastic in the chosen formula of a wood-plastic composite will have some effect on its behavior as it goes through the production process. The percentage of wood present in the composite will have an effect on the melt flow index (MFI), for example. As a rule, the more wood that is added to the composite, the lower the MFI. The percentage of wood will also have a bearing on the strength and stiffness of the product. Generally speaking, the more wood that’s added, the stiffer the product becomes. Wood can make up as much as 70 % of the total wood-plastic composite, but the resulting stiffness comes at the expense of the ductility of the final product, to the point where it may even risk becoming brittle. Higher concentrations of wood also shorten machine cycle times by adding an element of dimensional stability to the wood-plastic composite as it cools in the mold. This structural reinforcement allows the plastic part to be removed at a higher temperature than it would if using an unfilled polymer. At temperatures where unfilled resins are still too soft to be removed from their molds, composites made with wood can successfully be ejected. If the product will be manufactured using existing tools, the gate size and general shape of the molding should factor into the discussion of optimal wood particle size. A smaller particle will likely better serve tooling with small gates and narrow extensions. If other factors have already led designers to settle on a larger wood particle size, then it may be beneficial to redesign the existing tooling accordingly. Processing wood-plastic composites Processing parameters also have a tendency to fluctuate significantly based on the final formulation of the wood-plastic composite pellets. While many of the parameters remains similar to that of conventional plastics such as PE or PP, specific wood-to-plastic ratios and other additives meant to achieve some desired look, feel or performance characteristic may need to be accounted for in processing. 20 bioplastics MAGAZINE [<strong>03</strong>/16] Vol. 11
Injection moulding molding By: Mike Parker Product Development Manager GreenDot Cottonwood Falls, Kansas, USA Wood-plastic composites are also compatible with foaming agents, for example. The addition of these foaming agents can create a balsa-like material. This is a useful property when the finished product needs to be especially lightweight or buoyant. For the purpose of the injection molder though, this is yet another example of how the diversifying composition of wood-plastic composites may lead to there being more to consider than when these materials first came to market. Processing temperatures are one area where woodplastic composites differ significantly from conventional plastics. Wood-plastic composites generally process in temperatures around 10 K lower than the same, unfilled material. Most wood additives will begin to burn at around 200 °C. Shearing is one of the most common issues to arise when processing wood-plastic composites. When pushing a material that’s too hot through too small a gate, the increased friction has a tendency to burn the wood and leads to telltale streaking and can ultimately degrade the plastic. This problem can be avoided by running wood-plastic composites at a lower temperature, ensuring the gate size is adequate and removing any unnecessary turns or right angles along the processing pathway. An injection molding standard Wood-plastic composites aren’t just for decking anymore. They are being optimized for injection molding, which is opening them up to a vast array of new product applications, from furniture to car parts. The wide range of formulations now available can enhance the benefits of these materials in terms of sustainability, aesthetic diversity and features such as buoyancy or rigidity. Demand for these materials will only increase as these perks become better known. For injection molders, this means a number of variables specific to each formulation that must be accounted for. But it also means molders should expect a product that’s better suited to injection molding than feedstock that was designated primarily to be extruded into boards. As these materials continue to develop, injection molders should raise their standards for the characteristics they expect to see in the composite materials delivered by their suppliers. www.greendotpure.com bioplastics MAGAZINE [<strong>03</strong>/16] Vol. 11 21
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