Here is a basic introduction to the manufacturing processes commonly associated with plastic pallets, including rotational molding, injection molding, structural foam, compression molding, and profile extrusion.
High-Pressure Injection Molding
An injection molder includes a heated barrel with a rotating screw. Virgin plastic and colorant are melted together in the barrel. The melted plastic is then injected into a mold that is clamped together, and pressure is applied. The injection and clamping forces are quite high.
Copolymer polypropylene or high-density polyethylene resin often is used for injection molded pallets. The cost of high-pressure injection molds is quite high, but the process has a fast cycle time. The high pressure also enables the manufacture of pallets or other products with detailed features.
Structural Foam Molding
Structural foam molding is a form of low-pressure injection molding. The mold cavity is injected with polyethylene or polypropylene plastic pellets and nitrogen gas. Heat activates the blowing agents, which in turn create a cellular core with a solid outer skin. This process requires less raw material and also creates a plastic with a higher strength-to-weight ratio. The mold itself is less expensive than those for high-pressure injection molding.
The most common thermoformed pallet anyone is likely to see is a U.S. Postal Service pallet, but they are used in many supermarkets, too. Thermoforming is a two-step process. It begins with heating plastic pellets, typically polyethylene, to create an extruded plastic sheet of the desired thickness. Then, either one or two sheets are formed against molds as the air between the mold and the sheet is evacuated. Thermoforming molds are substantially cheaper than injection molds, but cycle times are slower.
A thermoformed plastic pallet is typically very lightweight and durable. Twin-sheet thermoforms pallets are hollow, which makes them susceptible to water retention if punctured and exposed to rain. Normally this is not an issue because they usually are kept undercover.
Compression molding is another tried and true method of making plastic pallets and molded pallets made of other material, such as wood chips and resin. The two-piece molds are loaded with material manually, robotically, or through extruders. Once loaded, a press forces the mold closed, and pressure forces the material to fill the mold cavity.
In the compression molding process, many different types of recycled materials are used, including various plastics, rubber, synthetic resins, and other recycled waste products.
The rotational molding process dates back to the early 1970s. It begins with granular or powdered plastic material loaded into a shell-like mold. The mold is heated and rotated in two planes perpendicular to each other. This helps distribute and fuse the material to the mold.
The molds for rotational molding are usually cheaper than for other processes. The materials most commonly used to manufacture rotationally molded pallets are linear low-density polyethylene (LLDPE) and cross-linked polyethylene.
Profile extruded pallets look very similar to wooden pallets and are constructed similarly. The raw material is melted and then forced through an extruder. After cooling, the extruded material is cut to size like lumber and fastened with nails, screws, glue or heat welds.
Industrial re-grind and post-consumer recycled material can be used to keep costs down, and the addition of wood fiber or other material to the extruded profile can add stiffness and help reduce.