Issue 03/2016

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 [03/16] Vol. 11