For over 30 years the leading pallet and sawmill magazine in America.
Pallet People Should Become Knowledgeable About Plastic
Plastic Pallet Primer: Wood pallet suppliers can provide a valuable service to customers who are considering plastic pallets by giving them information on the pros and cons of different types and the differences between plastic and wood.
By Rick LeBlanc
Date Posted: 4/1/2001
(Editor’s Note: This is the first of a three-part series of articles on plastic pallets. The second installment, to be published in the May issue of Pallet Enterprise, will report on the role that wood pallet suppliers can play as plastic pallet distributors, including tips and potential pitfalls. The third article, scheduled for the June issue, will review plastic pallet suppliers, their approach with respect to using distributors, and supplier contact information.)
Regardless whether you consider plastic pallets to be friend or foe, sales opportunity or competing product, it is clear that they have carved a small but solid niche among pallet users.
Benefits such as much greater durability, ease of manual handling, nestability, easy cleaning and others have caught the attention of some pallet users. Penetration in applications such as automotive, downstream grocery, U.S. Postal Service and other markets is significant.
And the niche is growing. One estimate projected plastic pallet usage will reach about 20 million units in 2001, up from only 3 –4 million in 1995. A survey by the National Wooden Pallet and Container Association and Cahners placed plastic pallets at 4% of total pallet purchases in 1999, about the same amount as composite wood pallets. By comparison, wood still dominates at 91% of purchases.
Nonetheless, wood pallet sales personnel can provide a valuable service to customers who are considering plastic pallets; pallet suppliers can provide information on the pros and cons of different types of plastic pallets and the general differences between plastic and wood.
Materials of Choice
The two most common resins used in plastic pallets are high density polyethylene (HDPE) and polypropylene. HDPE is the most frequently used resin in North America while polypropylene is the most popular in Europe.
Both are made from thermoplastic resin, which softens when heated. Thermoplastic resins have greater impact resistance than thermosetting resins and also are less expensive and easier to work with as well as to recycle.
Polyethylene is one of the most widely used plastic materials; its notable features include excellent chemical resistance, ease of processing, strength and durability.
Polypropylene also has excellent chemical resistance and lends itself easily to manufacturing processes. It has several advantages over HDPE, including greater stiffness and better performance at extremely high temperatures.
While often extremely durable, plastics typically are less stiff than wood. As Dr. Marshall ("Mark") White, director of the Virginia Tech pallet and container research laboratory, has pointed out in the past, the lack of stiffness can present somewhat of a dilemma with regard to pallet performance. In order for a higher priced plastic pallet to compete with wood, it must be more durable. In making it more resistant to shock and impact, however, stiffness is sacrificed. The two properties are in opposition. Low stiffness has limited the load capacity of plastic pallets in the past, although edge rackable plastic pallets now are available from some manufacturers. Composite pallets address this issue by adding secondary stiffening materials, such as wood and glass fibers, to the durable plastic primary material.
Plastic pallet manufacturing processes include injection molding, structural foam molding, thermoforming, blow molding, compression molding, and profile extrusion. Rotational molding is another process but has largely fallen out of favor.
In injection molding, a high pressure injector system is used to inject the molten resin. After the part solidifies adequately, the mold is opened and the part is ejected. Molds are very expensive, but the process cycle time is very fast, as low as two minutes.
Structural foam molding is a form of low pressure injection molding and is one of the most popular manufacturing processes. The mold cavity is filled with expanding foam, which forces the resin against the surfaces. The tools (molds) are not as costly as for injection molding. The resulting pallet is lighter, stronger and stiffer.
Thermoforming is a two step process. First, a flat plastic sheet is extruded. Then, either one or two sheets are formed against molds as air between the mold and the sheet is evacuated. Tools for thermoforming are substantially cheaper than for injection molding, but cycle times are slower, in the range of six to seven minutes. The resulting pallet is typically very light-weight and durable. Thermoform pallets also are often nestable, which conserves space when stacked empty. These pallets are not very stiff, although steel inserts can be added to improve performance.
In compression molding, a variety of materials or fillers are used. (One new company experimenting with rubber filler and glass in pallets is using a compression molding process.) In the compression molding process, the material is placed onto a two-piece horizontal mold surface. A press then forces the mold closed, conforming the material to the mold surface.
In blow molding, the resin ‘preform’ is dropped between open mold halves which then are closed together. Air pressure then blows the material against the mold walls, creating a light, hollow pallet. The process is used commonly to make small plastic products, such as milk bottles, but it has been more difficult to adapt to larger products, such as pallets. Blow-molded pallets are similar in appearance to twin-sheet thermoform pallets and have similar properties.
Profile extrusion is the method that is most similar to the wood pallet manufacturing process. Plastic lumber is extruded into a profile. Like lumber, it can be cut to size as needed for a particular pallet order. Because expensive molds do not need to be tooled, smaller custom orders become more economical to run. Different profile designs can increase pallet stringer strength, such as an I-beam profile. A variety of fasteners can be used, such as ring-shank nails. Joints may also be glued or "welded" by heat or high frequency vibration to make excellent joints and connections. Plastic lumber is less sensitive to impurities than other processes.
When to Use Plastic
Plastic pallets, and their perceived benefits, at some point may become part of the conversation between wood pallet suppliers and their customers. The customers may have heard about potential benefits of plastic and may wonder if they would suit their application.
Often the answer is no. The initial cost to start a plastic pallet program typically is high because plastic pallets are priced considerably more than wood.
The application of plastic pallets often hinges upon the completion of numerous trips. If a plastic pallet can make a large number of trips, it eventually lowers the cost per use or per trip below that of a much more modestly priced wood pallet. Of course, the ability to use the plastic pallet repeatedly rests on a program of effective recovery and retrieval. If the pallets are not strongly managed and controlled, losses can undermine the pay-back and potential savings. It is also worth noting that effective pallet management programs also have a price tag.
Other issues are more specific to certain types or models of plastic pallets. Like wood pallets, plastic pallets are not a ‘one size fits all’ product. Plastic pallets may vary significantly from one to another in terms of durability, deck stiffness, washability, flammability, slip resistance of decks, and other factors; these performance issues have been addressed very well by some plastic pallet manufacturers and not by others. A given application may be suitable for one kind of plastic pallet but not others.
Plastic pallets have been very successful for some applications. As mentioned above, downstream grocery, automotive, the U.S. Postal Service and others have made a considerable commitment to plastic pallets. They also are used in the printing industry. Applications where sanitation or ‘clean room’ environments are an important issue have generated sales for plastic pallets.
There has recently been some exploration of the idea that extremely durable plastic pallets might provide a suitable vehicle for more powerful tags encoded with radio frequency identification (RFID) technology. RFID technology may be used to track products through the supply chain. Customers that want to use this technology may be willing to invest in plastic pallets as part of the necessary infrastructure to implement it. The cost of the tags could be amortized over a longer pallet life, and hence a lower cost per trip. Pallet management and control would be critical.
When customers talk about plastic pallets, it would be helpful for wood pallet suppliers to be able to discuss where plastic pallets make sense and where they don’t, and if plastic pallets seem like a good fit, what the best type of plastic pallet might be.
In the next installment, we will look at the pros and cons of adding plastic pallets to the wood pallet product line.