GUIDE TO EXTRUDED PLASTIC SHEET PRODUCTS

The SPI Sheet Producers Division’s publication entitled, Guide to Extruded Plastic Sheet Products, is intended to provide information on major segments of the plastics industry; specifically, sheet extrusion and thermoforming.The guide is intended to provide the reader with some measure of perspective in understanding the versatility inherent in the family of plastics and in the many different uses to which they may be put.

The guide includes sections on:

1. Extruded/Thermoformed Plastic Products
2. Major Thermoplastic Resins
3. Material Descrptions
   1. Tests and Their Significance
   2. Property Comparison Charts
   3. Performance Summary
4. Sheet Extrusion
5. Thermoforming
6. The Future of Plastic Sheet Extrusion & Thermoforming
7. Trade Names & Proprietary Information
8. Glossary of Plastic Terminology

Excerpt from Section I: Extruded / Thermoformed Plastic Products:

Sheet extrusion is the process of converting plastic pellets or powder into cut sheets or rolls of plastic. This sheet can be further processed into parts via thermoforming. In thermoforming, the sheets are re-softened in heated ovens and then forced into contact with a mold surface by air pressure or vacuum to define the part’s inside or outside shape. The mold only contacts the softened plastic on one side. In general, the appearance side of the part is the non-mold contact side.

There are six major plastic fabrications methods:

• sheet extrusion/thermoforming
• pipe & profile extrusions
• film extrusion
• injection molding
• blow molding
• rotational molding

Each of these processes offers unique capabilities. There are definite reasons for choosing a particular processing method depending on the specific needs of a project. In addition to choosing the correct materials of construction, an experienced designer will simultaneously determine the correct fabrication method.

The sheet extrusion process can yield sheet products with thicknesses ranging from less than .010 in. (film) up to and exceeding 2.0 inch, with widths as great as 30 ft. Due to the capability of producing large thick sheets, one of the advantages of extrusion/thermoforming is the ability to produce extremely large parts on an economical basis. Boat hulls, camper tops, spas, hot tubs, and recreational vehicle parts are examples. The cost to produce these same parts via injection molding would be prohibitive due to mold and injection molding machine costs.

Another unique ability for sheet products is that different polymers can be co-extruded to produce multi layer structures. Two or more extruders are utilized together to add the different materials together in a feed block or manifold which keeps the layers distinct as they emerge from the die used mostly on inline. With this capability, for example, the fabricator can put together polymers to add functionality or improve the cost performance ratio of the final product.

Below are some examples:

• High-gloss expensive layer over tough non-glossy less expensive substrate
• Thin weatherable (usually expensive) layer over a tough non-weatherable substrate
• Low gloss (may be expensive) layer over a tough substrate
• A thin expensive decorative layer such as a marble or wood grain over a less expensive, but tough substrate
• A vapor barrier extruded over a substrate
• A chemically resistive material extruded over a less resistive substrate
• A soft touch material over a cost-effective, tough substrate
• Correctly color matched cap over a non-colored, less expensive substrate
• Ground up recycled parts may be re-extruded and utilized in a core or substrate layer of a co-extruded sheet if the overall sheet physical properties can meet the toughness specifications for the final product. In fact, the sheet extrusion/thermoforming industry has long ago demonstrated the benefits of recyclability. Due to the high molecular weights of polymers used in extrusion vs injection molding, repeated processing is possible without substantial property loss.

Some of these same benefits can be obtained by laminating films onto either co-extruded or monolayer sheet as it is produced. Decorative films and foils as well as weatherable cap films can be laminated to protect the underlying polymer from sunlight degradation.

In terms of materials, as mentioned, higher molecular weight (high viscosity) polymers are used in sheet extrusion, in comparison to injection molding which uses lower molecular weight resins with the benefit that thinner parts or tougher parts can be produced. The lower molecular weight (lower viscosity) polymers are needed in injection molding because lower viscosity is necessary to be able to fill the molds.

Recent advantages in thermoforming have pushed the process envelope higher. Pressure forming, in which additional air pressure up to 100 psi and greater, results in part appearances approaching that of injection molding by pushing the softened plastic sheet into the mold surface, thus giving even greater part detail. The appearance of many pressure formed parts is so good that it is hard to tell whether the part was injection molded or thermoformed.

Twin sheet forming is another variation of thermoforming in which two heated sheets are simultaneously formed into a top and bottom mold. The two halves are joined by pressing the edges into each other while both plastic sheets are still in the softened condition. Plastics with good weld characteristics need to be used in this process. The benefit is that large hollow parts can be formed by this process. Hollow parts are often produced by blow molding and were limited by blow mold size and cost constraints. Some example parts of twin sheet thermoforming are dunnage products, pallets, and hollow, foam-filled, thermoplastic doors.

Another reason to choose extrusion/thermoforming over injection molding is quick turnaround time. Thermoforming molds can be machined or cast faster because they do not have the pressure requirements, runner systems or ejector systems of an injection mold. Therefore, quicker turnaround is possible. For this reason and because thermoforming molds are less expensive, some parts that eventually become injection molded are first prototyped as a thermoformed part.

A product designer needs to consider cost of molds and processing when determining what plastic fabrication technique to pick. Many times this issue involves understanding how many parts will be produced in a yearly time period. Molds for thermoforming are less costly than those designed for injection molding and as a result add less to the piece part cost. On the other hand, injection molding can generally produce more parts pr given time period, thus lowering piece part costs based on machine time. A careful analysis comparing these two costs should be conducted for each project. In general, extrusion/thermoforming wins out in part production runs of up to 10,000 units per year and injection molding wins out at over 100,000 units per yr. The middle ground is determined by other factors as discussed above.

In summary, the versatility of the extrusion/thermoforming processes, potentially high quality part appearance, and material combining options in co-extrusion and lamination have resulted in a process that is still in the growth stage of technology with new product applications being continually explored.

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