Thermoplastic Wood Composite Injection Molding
Traditionally, wood composite materials have their largest usage in extrusion. Most of these applications use direct extrusion where dried wood flour and resin are mixed in a twin screw extruder and made into deck boards or sheets. By pre-pelletizing the wood flour and resin, the material can be injection molded.
Advantages:
- Increased heat distortion temperature
- Flex modulus
- Better dimensional stability.
Wood-filled materials' biggest advantage is their weight. At a 40% filler level, wood-filled olefins are about 20% lighter than CaCO3 or talc. The wood also transforms the plastic and gives it a richer, “wood-like” look. Whether the wood-filled product is pigmented and used for its structural value or used simply for its “wood-like” appearance, it will bring a new opportunity to the plastics industry.
Temperature Control
- Wood is more sensitive to temperature and shear than the resin with other fillers.
- Wood will burn at 400F and will degrade when held for prolonged periods over 350F.
- The barrel should be purged with virgin unfilled resin if a machine is going to be down for more than 10 to 15 minutes.
- When sizing a mold for a machine, extra care must be taken to insure that the material does not see excessive residence time in the barrel.
- Burning and degrading the wood will reduce the physical properties and create an unpleasant odor.
Drying
- All wood-filled materials are hygroscopic in nature. The higher the wood content, the higher the moisture content will be.
- When working with wood-filled olefins, the moisture problem is complicated by the barrier properties of the olefins.
- Once dried, the wood-filled olefin should be used immediately or kept in a closed hopper to prevent rehydration.
- PP is easier to dry than HDPE because higher temperature dryer settings can be used.
- HDPE should be dried at no higher than 220F, while PP can be dried at 250F.
- The relative humidity of the area will determine how long the product will have to be dried. At production, the product will be dry. The length of time after manufacture will determine the moisture level and the required drying time. It may be necessary to experiment with your drying equipment to determine drying time.
Tooling
- The addition of the wood filler reduces the viscosity of the material.
- Gates for wood composites will need to be larger than normal.
- Because of the reduced viscosity, the tool will have to be maintained at a higher than normal temperature (100-150F).
PRODUCT SELECTION
Reedy International has developed a unique series of SAFTEC® wood additives for foam systems. These additives include stabilizers and lubricants for use in a variety of resins. They are highly compatible with wood and foam systems and will dramatically improve the cellular structure of the finished product.
SAFTEC® AS (Acid Scavenger) is a unique proprietary wood and other cellulosic fiber stabilizer that:
- Promotes processing by lubricating the wood and preventing it from oxidizing or burning at higher temperatures.
- Allows for better mixing and homogenization without degradation or discoloration of the composite materials.
- Improves flow characteristics and enhances the surface features of the wood when applied to the wood at low levels (around 1%).
- Is ideal for all "foamed" systems.
SAFTEC® WLB (Wood Lubricant Blend) is a unique proprietary external lubricant for wood that is:
- Used in processing foamed plastic wood to prevent "edge tear" and alligator skin.
- Very unique in that it is "foam friendly" and does not contain any stearates or other defoamers.
|