Injection Molding Wood-Plastic Composites

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					             Injection Molding Wood-Plastic Composites
       Injection molders are just becoming acquainted with this new class of molding
materials. It pays to learn some basic processing guidelines before jumping in.

        Wood-fiber/thermoplastic blends have already made a name for themselves in
extruded decking and fencing boards. Now they are moving into injection molding. Some
injection molders are hesitant to experiment with new materials like wood-plastic
composites (WPCs). Molders that have experimented with them have encountered a
number of challenges, including inconsistent quality, inconsistent supply, and generally
greater difficulty in working with WPCs in comparison with more familiar injection
molding materials.

                                    However, recent developments in the manufacture
                           of WPC compounds have significantly improved the quality,
                           consistency, and capability of this environmentally friendly
                           material. In fact, the latest generation of WPCs can be run
                           smoothly through traditional injection molding equipment
                           with minimal adjustments to process settings and no physical
                           hardware modifications. However, these materials do have
                           some processing characteristics that distinguish them from
In their natural color,    familiar molding resins.
WPCs provide an
“organic” look and feel            WPCs can be made with a variety of plastics, such as
with a light brown color    polyethylene, polypropylene, and polystyrene. What’s more,
and consistent grain.       WPCs are just one category of an emerging family of
Birdcage bottom.            materials that can be termed “thermoplastic biocomposites.”
Besides wood, these biocomposites can utilize other natural fibers such as rice hulls,
palm fiber waste, or flax. The molding guidelines outlined below apply specifically to
wood-fiber/PP blends, but the general principles are believed to be generally applicable
to other thermoplastic biocomposites as well.

Why use TP biocomposites?

        There are many compelling reasons to create products
and components out of thermoplastic biocomposites. As they
contain up to 50% organic fibers, these materials offer
injection molders a material option that is more
environmentally friendly than typical petrochemically
derived polymers. In addition to the “green” factors,              Calibration dials
thermoplastic biocomposites reduce a molder’s exposure to increasing petroleum prices,
reduce the energy costs associated with production, and produce an end-product with
great structural rigidity, an aesthetically pleasing finish, and new, highly marketable
performance capabilities.

        Wood/PP biocomposites tend to be lower in cost and weight than unfilled resins
or glass-filled resins. WPCs are competitive with calcium carbonate-filled or talc-filled
PP in cost, performance, and processing. But WPCs have the advantage of lower density,


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which reduces their effective cost and can be beneficial in transportation and other uses
that put a premium on light weight. Applications can include automotive, construction,
sports, toys, and other consumer goods.

                                   Good candidates for injection molding with WPCs
                           are parts with thick walls and ones that would benefit from
                           excellent rigidity and dimensional stability. However, they
                           should not be subjected to excessive impact, since WPCs are
                           less shatter-resistant than some traditional injection molding
                           materials. While WPCs are best suited to parts with thick
walls, processors can compensate for thinner walls by blending WPC with additional neat
polymer.

        WPCs offer the combined properties of wood and plastic—excellent moisture
barrier plus the ability to be screwed or nailed like wood, along with an organic look and
feel.

Materials selection

    To ensure the quality of finished components, it is critical
to utilize high-quality thermoplastic biocomposite pellets.
There are two main areas of the pellet composition to focus
on:

      Dryness. Surface moisture should be less than 1.5% WPCs can be dyed with
       while internal pellet moisture should be less than 1%. excellent results.
       Failure to control moisture content can result in visible splay and increased
       brittleness.
      Proper encapsulation and uniformity. Pellets should be clean and relatively
       consistent in size and shape. There should be no fines, chads, or streamers. In
       addition, there should be no powdery residue, which is a sign of improper
       equipment design or maintenance on the part of the pellet manufacturer.

         One of the benefits of the current generation of WPCs is that they can be blended
very efficiently with additional unfilled PP or other resin. In this manner, molders can
still reduce their material costs and obtain the “green” benefits of the material while
tailoring the wood-fiber level. Through blending, molders can achieve different
performance characteristics—for example, improve the shatter resistance of components
such as car bumpers or increase the structural rigidity of neat resin.

Processing guidelines

       When molded using proper temperatures, speeds and a non-restricted flow path,
WPC parts will exhibit uniform color and dispersion of wood fibers, minimal stress,
smooth surfaces, and no evidence of gassing. The two most important principles to keep
in mind for molding WPCs and other biocomposites are to avoid excessive heat and
shear.




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Molding Case Study

Baytech Plastics ran JER’s WPC
pellets in an unmodified two-
cavity family mold of P20 steel,
designed for ABS/PC electrical
covers. The trial showed that
WPC pellets based on
semicrystalline PP resin mold         WPC parts molded in a
well in tooling designed for an       family tool designed for
amorphous plastic.                           ABS/PC.

The temperature profile was as follows:

Front nozzle: 370 F, instead of 500 F typically
used for the ABS/PC.
Center zone: 355 F instead of 500 F for ABS/PC.
Rear zone: 355 F vs. 500 F.

These pressures were used:
Filling: 1050 psi vs. 1200 psi with ABS/PC.

Pack/hold: 150 psi vs. 200 psi.

The optimum fill time was 2.4 sec for WPC compared with
3.5 sec for ABS/PC.

The total hold time was 4.6 sec instead of the typical 7 sec for
ABS/PC.

The finished component weighed 20% less than the ABS/PC
component (100 g vs. 125 g).

Baytech Plastics also performed a successful blending trial.
TPO was mixed with PP-based WPC pellets at a ratio of 1:2 to
achieve a wood loading of 20% and at a ratio of 2:1 to achieve
a wood loading of 10%.




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                                Parts molded in ABS/PC
                                tool: (top to bottom) 100%
                                PP-based WPC;
                                TPO/WPC blend with 20%
                                wood; TPO/WPC blend
                                with 10% wood.

         While traditional thinking might suggest that the wood fiber in WPCs would act
as a flow inhibitor, the reverse is often true. Wood/PP actually flows very quickly at
relatively low temperatures and pressures (generally similar to mineral-filled PP). As a
result, injection molders can achieve significant energy savings. They can also achieve
shorter cycle times and higher productivity due to reduced filling and cooling times.

        Typical temperatures for molding wood/PP composites are 340 to 370 F (171 to
188 C) for the rear zone, 360-390 F (182-199 C) for the middle zone, 380-410 F (193-210
C) for the front zone, and 390-410 F (199-210 C) for the nozzle tip.

       Molding pressures of course depend on the design of the part as well as the runner
system and gates. That said, injection molding with WPCs generally requires less
pressure than molding with traditional materials.

         Filling speeds for WPC molding deserve attention. While the material tends to
flow very quickly, it is important to avoid excessively short fill times as the material is
shear sensitive. Increased heat due to overly fast fill times typically manifests itself in a
telltale resin-rich streaking on the surface of the component. This streaking is remedied
simply by slowing down the injection rate.

       Given the lower temperatures used for molding WPCs, hold times are often lower
than with regular materials.

       The nozzle tip used in the molding process should have an orifice as close as
possible to the diameter of the sprue to minimize shearing. Smaller tips may cause
increased shear as well as discoloration caused by overheating of material as it enters the
mold.



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        Injection molded WPC components are rather “natural” in appearance, with a
light-brown tone and a uniform grain. However, a high-gloss finish is achievable, and the
material can be dyed in various colors with excellent color uniformity.

Tooling tips

       Typically, runner systems should have a generous flow path with minimal
obstruction in order to minimize shearing of the material. Likewise, injection gates
should be as large as the mold will permit. Gates that are too small will cause excessive
shear and possible discoloration, as well as a resin-rich appearance at the gate region.



               Commercial WPC Pellets

               JER Envirotech uses waste or byproduct natural fibers and
               virgin or recycled thermoplastics to compound molding and
               extrusion pellets and to extrude composite panel boards.
               These products resulted from joint research with the National
               Research Council of Canada. JER offers more than 60 WPC
               pellet formulations (tradenamed JERtech) for injection
               molding and extrusion, including flame-retardant grades.
               Most are based on wood fiber and PP.

               JER’s newest developments are
               focused on injection molding
               grades and have expanded its
               slate    of     wood      polymer
               composites to include four
               polymer bases—polypropylene,
               polyethylene, HIPS, and TPO—
               as well as four fiber choices: pine, oak, maple, and rice hulls.
               Potential applications include automotive, consumer goods,
               toys, and construction.

               JER Envirotech utilizes unique compounding techniques that
               reduce the internal moisture levels in pellets well below 1%.
               This allows JER to create injection molding formulas with
               fiber loadings between 30% and 50% and masterbatch
               formulas with up to 60% fiber loadings. Through proper
               encapsulation of the bio-fiber and thorough blending of the
               fibers throughout the composite, JERtech pellets mold easily
               with standard tooling and dies and offer minimal shrinkage
               and thermal-expansion coefficients.
       Ideally, parts should be gated directly into a thick section of the part. In cases
where an edge gate is required, we recommend a gate width of at least two-thirds the
thickness of the part wall.


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        Gates should be located so as to avoid joining of flow fronts and weld lines at
points that may be susceptible to stress in use. Breakage is more likely to occur at such
weld lines.

Molder training program

        Since WPCs are so new to most injection molders, JER Envirotech has developed
a molder training and accreditation program designed to qualify and educate customers
on the proper use of its biocomposites. Through accreditation, injection molders (and
extruders) gain the knowledge to obtain the best results from these materials. The end
result is a consistently high-quality product that accurately reflects the skills and
experience of the molder as well as the inherent quality of the biocomposite compounds.

              Close Up on Technology: Injection Molding

 Molding Conference Highlights Wood Fibers, Spinning Mold
        Processing guidelines for molders interested in exploring the burgeoning field of
wood-plastic composites was one hot topic at the Molding 2006 International Conference
and Exhibition, held recently in Las Vegas by Executive Conference Management. Other
innovative technologies presented in the papers include rotating-core tooling that erases
weld lines, and a new approach to press-side automation that is truly flexible and cost-
effective.

Molding with wood fibers

        Injection molding of wood-plastic composites (WPCs) is still in its infancy, but it
will grow up fast, predicted consultant Michael D. Burgoyne of Burgoyne Associates. In
his paper, he noted the runaway success of WPCs in extrusion and cited market research
by Principia Partners that projects demand for injection molded WPCs growing 70%
annually to $350 million by 2008. Most WPC’s are based on either polyethylene for use
in exterior building components or on polypropylene for automotive applications.
Injection molded WPCs include end caps for deck boards, fence posts, and railings;
cabinet components; door thresholds; stackable storage containers; tote bins; and hammer
handles.

        Burgoyne said standard equipment is sufficient for molding WPCs, provided that
certain precautions are taken. The biggest challenges are control of moisture and excess
heat. Wood fibers usually contain considerable moisture, and the challenge is to remove
it without damaging the wood. Flash drying, used in particleboard production, can reduce
the moisture content of green wood from a starting point of at least 40% by weight down
to about 1%. Fairly low melt temperatures are also required, which will mean higher melt
viscosities.

      Shear heating can cause the wood to lose strength and even degrade. Burgoyne
recommended several measures to avoid over-shearing WPCs:




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      Screws should have a short mixing section and
       impart low shear.
      Mixing screws can break up the fibers, so use g-p
       screws with 20:1 to 24:1 L/D and 2:1 to 2.5:1
       compression ratio.
      Keep screw speeds low. It helps to use a barrel
       big enough to hold three or four shots.
      Inject at low speed.
      Avoid tunnel or valve gates and shutoff nozzles.
      Use multiple gates when feasible and try to keep
       gates no smaller than 2.5 mm.
      Keep backpressure low.
      Set barrel temperatures as high as possible while
       keeping the melt temperature below 392 F (200
       C). Melt the polymer as much as possible with
       conductive rather than shear heating.
      Keep nozzles short with a wide flow path almost
       equal to the sprue diameter.
      Make sprues and runners as large as possible to
       permit slow injection.                                xFlex is a new modular press-
      Be cautious with hot runners. They can mean           side automation concept from
       longer residence times and extended heat history      Actus. Secondary operations
       for the wood.                                         are provided by “plug-in”
                                                             modules that are docked to the
New twist in tooling                                         main robot module, which
                                                             supplies utilities and control
        More details on rotating mold cores designed to functions.
minimize weld lines in circular parts were presented by Solvay Advanced Polymers,
which offers its patented technology for licensing. First introduced in 2004, this approach
can be especially promising with filled or reinforced polymers, where weld lines can
sacrifice up to 75% of the material’s original strength, said Greg Warkoski, process
technology manager.

        He noted that another problem in molding circular, fiber-reinforced parts is
anisotropic shrinkage due to nonuniform fiber orientation. Hence, obtaining precise
roundness is difficult, and dimensions can change if the part is exposed to elevated use
temperatures.

        Previous solutions included thickening the wall in the weld-line area to boost part
strength—at the cost of increased material usage and cycle time. Adding an overflow tab
provides only marginal improvement, requires secondary steps for tab removal, and
leaves a vestige flaw on the part, said Warkoski. Multiple gates can exacerbate the
problem by creating more weld lines.

       Solvay’s rotating-core technology reportedly achieves thinner, stronger, rounder
parts without a weak spot at the weld line. Driven by a servo motor, the cylindrical core
turns during injection and packing. This rotation causes a turbulent shift of the fill pattern
and distributes the knit line around the circumference of the part, Warkoski explained.


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Reinforcing fibers orient uniformly in the direction of rotation, reducing anisotropic
shrinkage and dramatically improving roundness.

      Benefits of the rotating core were demonstrated in molding trials of an 80-mm-
diam. automotive throttle body in cooperation with Delphi Energy and Engine
Management and Century Tool and Mold, both in Rochester, N.Y. Made from Solvay’s
Amodel A-1565 HS, a 65%-mineral/glass PPA, the throttle body requires exceptional
roundness and excellent thermal stability over a wide range of temperatures.

       Thirty parts were molded to observe the effect of changing five parameters: onset
and duration of core rotation, peak rotational velocity (180 rpm in this case), and speed of
acceleration and deceleration of core rotation. Optimum values were specific to the tool
and material, but some general trends were observed:

      Pressure required to achieve the desired injection velocity was reduced about 30%
       with optimized rotational parameters. More intense rotation reduced pressure the
       most.
      Part roundness improved significantly with rotation. Standard deviation of
       roundness was 0.035 mm without rotation vs. 0.025 mm with rotation. The range
       of measurement also decreased from 0.17 to 0.05 mm.
      Izod impact tests were performed on a 0.5-in circular band cut from the part so as
       to include both the gate location and the weld-line area opposite. With rotation,
       impact strength increased from 3.91 ft-lb to 6.61 ft-lb at the weld area, although it
       dropped slightly from 6.66 to 6.11 ft-lb at the gate area. All non-rotated samples
       broke at the clearly visible weld line, while parts made with the rotating core
       broke in a random fashion.
      Hydrostatic burst pressure of the samples increased from 437 to 660 psig with
       rotation.




                  Solvay’s patented rotating-core technology spins the
                  mold core during filling to “erase” weld lines and
                  anisotropic shrinkage. Micrographs show
                  reorientation of glass fibers in the direction of
                  rotation, which improves roundness of a cylindrical
                  part.




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Non-dedicated automation

       Automation systems that are modular and flexible enough to be used for different
jobs are the concept behind the xFlex system from a new company, Actus Automation.
Explained president Paul Gelardi, xFlex consists of a master module, which is the base
for a six-axis articulating-arm robot, plus satellite modules dedicated to specific
secondary functions, which can be added or removed as needed.

        The system consists of reusable components in the form of standard modules. The
master or “manager” module acts as a docking station for the satellite modules and
delivers power, air, controls, and safety monitoring to the satellite modules. The robot
also moves the part from the mold to the satellite modules. The central module
downloads operating programs to the satellites.

        “Plug-in” satellite modules are pre-engineered and pre-tested and come with a
wedge-shaped base frame and safety guarding. Caster-mounted modules can be inserted
or removed to reconfigure the cell in minutes, Gelardi said. A cell can incorporate up to
six satellite modules.

        Actus currently offers 25 types of modules in three sizes. Satellite modules are
available for welding, vision inspection, parts feeding, conveying, packing, assembly, hot
stamping, printing, labeling, cleaning, carton erecting, electrical testing, and pressure
testing. “This concept is more flexible than dedicated automation. It enhances speed to
market, reduces capital costs, and minimizes investment risk through its ability to be used
for one or many jobs,” Gelardi said.




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