What You Should Know About Molding Wood-Plastic Composites

While wood-plastic composites (WPCs) broke onto the scene in the 1990s as materials primarily extruded into boards for decking and fencing, optimization of these materials for injection molding since then has greatly diversified their potential applications as durable and sustainable materials. Environmental friendliness is an attractive feature of WPCs. They come with a significantly lower carbon footprint than purely petroleum-based materials and can be formulated using exclusively reclaimed wood fibers.

A wider range of material options for WPC formulations is opening new opportunities for molders. Recycled and biodegradable plastic feedstocks 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 WPCs are a much more versatile material than was once thought.

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WHAT MOLDER SHOULD EXPECT FROM SUPPLIERS
A growing number of compounders are now offering WPCs in pellet form. Injection molders should be discerning when it comes to expectations from compounders in two areas especially: pellet size and moisture content.

Unlike when extruding WPCs for decking and fencing, uniform pellet size for even melting is crucial in molding. Since extruders do not have to worry about filling their WPC 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, and is not overly focused on the earliest and initially most prevalent uses for WPCs.

When pellets are too large they have a tendency to melt unevenly, create additional friction, and result in a structurally inferior final product. The ideal pellet should be about the size of a small BB 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 WPCs should expect the same shape and uniformity they associate with traditional plastic pellets.

Dryness is also an important quality to expect from a compounder’s WPC pellets. Moisture levels in WPCs will increase along 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 lower 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 savings of time and money. Injection molders should consider shopping around for WPC pellets shipped by the manufacturer with moisture levels already below 1%.

FORMULA & TOOLING CONSIDERATIONS 
The ratio of wood to plastic in the formula of a WPC 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 to be removed at a higher temperature where conventional plastics are still too soft to be removed from their molds.

If the product will be manufactured using existing tools, the gate size and general shape of the mold 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. But, given the existing options for different particle sizes, this outcome should be completely avoidable.

PROCESSING WPCs
Processing specifics also have a tendency to fluctuate significantly based on the final formulation of the WPC pellets. While much of processing remains similar to that of traditional plastics, 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. 

WPCs are also compatible with foaming agents, for example. 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 WPCs differ significantly from conventional plastics. WPCs generally process at temperatures around 50° F lower than the same unfilled material. Most wood additives will begin to burn at around 400 F.

Shearing is one of the most common issues to arise when processing WPCs. 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 WPCs at a lower temperature, ensuring the gate size is adequate, and removing any unnecessary turns or right angles along the processing pathway.

Relatively low processing temperatures mean that manufacturers seldom need to achieve temperatures higher than for a traditional polypropylene. This minimizes the difficult task of taking heat out of the manufacturing process. There’s no need for the addition of mechanical cooling equipment, molds specifically designed to reduce heat, or other extraordinary measures. This means further reduced cycle times for manufacturers, on top of already faster cycle times due to the presence of organic fillers.

NOT JUST FOR DECKING
WPCs 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 lawn furniture to pet toys. 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 benefits become better known.

For injection molders, this means a number of variables specific to each formulation 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. 

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