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:
- 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.
- Be cautious with hot runners. They can mean longer residence times and extended heat history for the wood.
New twist in tooling
More details on rotating mold cores designed to 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. 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.
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.