CWC Technology Brief

COMMINGLED PLASTIC LUMBER MANUFACTURING PROCESSES

This Technology Brief describes the basic equipment requirements and the several types of processing systems that have been developed specifically for processing commingled plastics. The processes may be roughly categorized into four basic types: compression molding, intrusion (flow) processes based on Klobbie's design, the "Reverzer" process, and continuous extrusion. The information presented here is intended to be a snapshot of the processing systems presently in use. It may provide insight for individuals interested in developing commingled plastic products or those that are presently manufacturing plastic lumber products.

During plastic recycling's first years, the manufacture of plastic lumber products from commingled plastic wastes was considered to be the major market for plastics. More recently, advances in various sorting technologies have led to an emphasis in other recycling techniques and systems. However, because sorting massive volumes of commingled materials, (necessary for full-scale recycling), faces economic barriers, the ability to develop and manufacture products from commingled materials will continue to be an important segment in the plastics recycling industry.

The following sections review the equipment requirements and various manufacturing processes in the plastics lumber industry.

Equipment Requirements

Smith (1) lists the following requirements for a commercially successful secondary recycling operation based on processing commingled plastics:

· The machine has to be capable of subjecting the plastics mixture to a high shear rate at high temperature for a short time period. High shear processing is necessary in order to achieve good dispersion. Short residence time is required in order to avoid material degradation caused by high temperatures.
· The product has to be manufactured in one step. The cost of a two-step process (e.g., homogenizing and pelletizing followed by extrusion) is usually too high to make it economically attractive. The raw material for a given product should have a fairly constant composition. This is required in order to produce products with consistent material properties. In addition, studies have shown that product properties are often dependent upon material feedstock composition.
· Output must be maximized. High production rates are necessary in order to minimize costs such as machine time and overhead.
· Proper selection of the product must be made. The mechanical properties of mixed plastics products are generally low. Because of nonuniform colors and poor surface finishes, the appearance is often poor. This makes it difficult for mixed plastic products to compete with products made from virgin materials. However, they can compete with a variety of wood and concrete products where these properties are acceptable. In many cases the mixed plastic products offer properties that are superior to the original materials.

Process Descriptions

The "Reverzer" Process: Developed by the Mitsubishi Company is one of the earliest methods developed for producing commingled plastic products. The process basically consists of material preparation, extrusion, and forming. Commingled waste plastics are fed into and mixed in an extruder where rapid melting and homogenization occur. A vertical screw plunger discharges the material into a mold. The mold is then cooled in a water bath or in a water spray tunnel. The Reverzer process can manufacture products by three systems: intrusion molding, extrusion, and compression molding. With auxiliary equipment, the Reverzer process is capable of producing a variety of shapes and sizes.

Intrusion (Flow) Process: A number of manufacturers process commingled materials by intrusion processes, many based on the Klobbie process. The Klobbie system consists of an extruder, several rotating molds, and a tank of cooling water. The commingled material is mixed and melted in the extruder and then is forced into one of the molds. After the mold is filled, the carousel rotates allowing another mold to be filled. The filled mold passes through the tank of chilled water allowing the mold to cool, and then is ejected. The process features high throughput and has the ability to produce thick-wall moldings such as pallets, as well as linear profiles such as plastic lumber products. Compression Molding: The Recycloplast system has proven to be the most successful technology based on the compression molding process3. The process mixes commingled plastics with other materials, melting the materials by a friction process. An extruder presses the material into premeasured, roll-shaped loaves. The loaves are then conveyed to a press charging device, which fills a sequence of compression molds. Products are cooled, ejected onto a conveyor and then carried to storage. The process has high throughput and is capable of producing a variety of thick-walled products such as pallets, grates, benches, and composting boxes. However, the plant size is large and the capital investment is high. Continuous Extrusion: Continuous extrusion of mixed plastic waste is very similar to technologies used in continuous pipe manufacturing. The systems have the ability to produce hollow profiles and foamed products. These techniques save materials cost and may provide a weight-saving advantage in certain applications. In addition, the system can produce profiles of varying lengths, such as very long profiles. However, continuous extrusions systems require more space than other systems because they require long, straight cooling sections. The system requires long cooling times in order to avoid temperature gradients which can create voids and profile faces that contract inwards. Therefore, the capacity of this system may be lower than the other molded systems4.

References

1 Smith, H.V. "Some Criteria for the Successful Commercial Recycling of Heterogeneous Plastics Waste," Recycling World Congress, Basel, 1978.
2 Leidner, Jacob. "Secondary Recycling", Plastics Waste: Recovery of Economic Value, pages 169-170, 1981.
3 Van Ness, et al., "Commingled Plastics", Plastics Recycling: Products and Processes, page 195, 1992.
4 E.PA., Evaluation of Recycled Plastics Lumber for Marine Applications, EPA/600/R-93/166, page 10, September 1993.

This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Report Dated: October 1994

Fact Sheet Update: December 1994