Automotive assemblers want to concentrate their time, effort and money on building better vehicles, not on transporting parts. To that end, parts and subassemblies are often shipped in custom-made plastic trays, or dunnage. These trays protect valuable parts during transport and keep them in the right orientation for automated assembly. The trays move seamlessly from suppliers, to the assembly line, and back to the suppliers in a continuous loop.

Traditionally, automakers have used trays made from injection-molded plastic. However, in the past decade, a growing number of automotive assemblers are using thermoformed plastic trays. Once considered a less accurate and low-cost cousin to injection molding, thermoforming is now the process of choice for making trays. High-precision thermoformed trays can be made to tighter tolerances than injection-molded trays, and they can be made faster and less expensively.

A versatile and economical material, plastic is available in a huge range of types and blends to suit any dunnage application. Injection molding and thermoforming are the main methods for creating plastic dunnage.

Injection-molded trays are created by forcing melted plastic into split dies. Once the dies are created, injection molding can produce trays rapidly. The drawbacks of the process are the high cost and long lead time for making the dies, and the cost of maintaining the molding equipment.

Thermoformed trays are created by preheating a flat sheet of plastic and then pressing it down onto a mold or drawing it by vacuum over a mold. Thermoforming takes less time than injection molding, and it can be done at a lower startup cost.

At one time, fewer plastic compounds were available for thermoforming than injection molding. Today, however, the same range of materials is available for both processes. Thermoforming dies can be created from almost anything-aluminum, ceramic or even wood. The forming process requires only moderate heat, just enough to make the plastic sheet pliable. The plastic does not need to come to full melt temperature, so expensive tempered steel dies are not required.

Thermoformed trays are comparable to injection-molded trays in quality and can often capture sharper detail. For fast setup and production of plastic parts, especially large parts in quantities of 10 to 10,000, thermoforming is now preferred over injection molding. Tooling costs are 90 percent lower, and time to market is greatly reduced.

There are two types of thermoforming: standard and high-precision. Compared with the standard process, high-precision thermoforming produces trays with better mold detail and tighter tolerances. The high-precision process distributes material better in the mold and produces less residual stress in the parts. The process produces precise details in materials ranging from very thin sheets to double-layer sheets for large, heavy parts. The high-precision process does not produce ejection and gate marks, which are often seen in injection-molded trays.

If loading and unloading of trays is done with robots or dedicated automation, high-precision trays are a must. Tolerances of ±0.005 inch from part to part may be necessary for the automation to work effectively. At one time, the only way to produce trays with that type of precision was to use injection molding. Tooling costs would range from $30,000 to $60,000, and delivery times would range from 14 to 16 weeks.

Today, however, high-precision thermoformed trays can be produced much faster and at a much lower cost. Precision trays can be formed from such materials as ABS, thermoplastic polyurethane (TPU) and high-molecular-weight polyethylene. As with standard thermoforming, the main cost is primarily for the basic tooling. This tooling costs 10 times less than the tooling for injection-molded trays, and it can usually be produced in 3 to 4 weeks. Lower prototyping costs also allow for more time on final design modifications, if needed.

Trays Fuel Success

The Siemens VDO Fuel Systems Div. plant in Albuquerque, NM, assembles parts such as pumps for many OEMs, including General Motors, Chrysler and Mercedes-Benz. The plant has three assembly lines using high-precision thermoformed dunnage. The plant has been using the trays for 3 years, says packaging engineer José Ochoa.

Made of ABS, the trays are custom-made for each product. The trays are designed so that assemblers can lift and position them easily. Parts are arranged to be accessible for either manual or automated assembly at the OEM's facility. "We can control how parts are fed from the lines, and operators can easily handle the trays," Ochoa says.

Loading the products into trays is the last step in the assembly process. When a tray is full, the operator puts a lid on it and places the tray on a pallet for shipping to the customer. At the customer's plant, the trays head directly to the assembly line. Once empty, the trays are returned to Siemens to be refilled.

Siemens uses other types of trays, Ochoa says, but it is gradually replacing those with high-precision thermoformed trays as new applications arise and older trays need replacement.

"Each product has to meet both my requirements and my customer's requirements," Ochoa says. "These trays present parts in a way that looks more professional to our customers and costs us only a fraction of other technologies to get the same quality. It makes it easier on both ends."

Trays and Transmissions

The GM Powertrain transmission plant in Pontiac, MI, took a hard look at thermoforming vs. injection molding trays. Engineers had some doubts until they researched the precision thermoforming process. Phil Krasny, forward program packaging engineer, says that the ability of thermoforming to hold close tolerances was what first attracted him to the technology. The money saved on tooling compared with injection molding clinched the deal.

The plant assembles a variety of transmissions. Thermoformed shipping trays are molded to fit the parts for each transmission. The trays hold each part in the exact position to facilitate manual or robotic handling on the assembly line. The dunnage is integrated directly into each assembly station.

GM sends digital drawings of each part to the thermoforming vendor. The vendor is appraised of how the part should be held in the tray and how it will be handled on the line. Once the design is approved, the trays are manufactured and sent to the supplier, which packs the trays and sends them to GM. When the trays are empty, they return to the supplier to be refilled.

"We're using the high-precision thermoforming process because...standard thermoforming may not be able to hold the tight tolerances that we need," Krasny says.

If the parts are assembled robotically, they must be spaced far enough apart in the trays for the end-effector to access them easily, Krasny explains. If the parts are handled manually, the trays must provide enough clearance for hands and fingers to get under the parts. The plant uses trays to handle parts of various sizes, ranging from a valve that's 1 inch tall and 0.25 inch wide to a component that's more than 12 inches in diameter and more than 15 pounds.

"To decide whether to use injection molding or high-precision thermoforming for trays, we analyze the cost difference between the processes for each new job," says Krasny. "We [let the] total...savings make our decision. Right now, we've saved over $200,000 just in tooling alone using precision thermoformed trays instead of injection molding.

"For instance, we've got a project coming up for a new hybrid transmission. The part weighs 74 pounds. The tooling for precision thermoformed trays runs around $10,000. With injection molding, the cost to set up would be [approximately] $55,000."

In the automotive industry, every part for a new program must go into a returnable container. No expendable packaging is allowed. Precision machined parts need to be in protective packaging. Many parts have areas that simply can't be touched because of their finely machined surfaces. Such parts require specialized, reusable containers.

"We use from 1,500 to 5,000 trays at a time," Krasny says. "We purchase them [according to] a 15-day routing supply, meaning 15 days between the supplier and the plant. That covers transportation and in-house use for each location."

GM orders new trays when it needs to handle new or redesigned parts, when parts need extra protection, or when ergonomic problems arise on the line. "These trays are very durable. They can be reused for 3 to 10 years," says Krasny.

Handling Engine Parts

Setting up material handling systems is a lot of work, and equipment budgets are surprisingly small even for major automotive manufacturers. They simply don't have enough people to manage entire programs, so they outsource the work to consultants. Ghafari ESG LLC (Dearborn, MI) is one such consulting firm. The firm provides turnkey service, including setup, dunnage and deliveries.

"The alliance between Chrysler, Mitsubishi and Hyundai is producing one very nice engine, and we are managing the material handling [for] that program," says Cesar De La Garza, project engineer for Ghafari. "This encompasses all the packaging that needs to be developed to interface with the automation in the plant."

High-precision thermoformed trays were used for the project because most of the engine plant's processes are fully automated, says De La Garza. Injection-molded trays were simply too expensive.

"We started developing some new tray concepts with our thermoplastic vendor, and we came out with five different trays that interface directly with the automation. I told them my exact specifications and tolerances, how I wanted the parts to be packed in that tray-upside down, on the side. I let them know what kind of grippers we were going to use so they could develop the tray to accommodate them. They came back with five different designs for five different parts, [and all] are working perfectly," De La Garza says.

The thermoplastic vendor manufactures the trays and ships them to the supplier. The supplier makes the parts, fills the trays, and sends them to the engine plant. Empty trays are sent back to the supplier.

"This is a very tough tray. The material is ABS. This [material] gives us the precision that we need," De La Garza says.

Ghafari started using thermoformed trays about a year ago. Before that, Ghafari was using injection-molded trays. De La Garza says thermoformed trays are just as durable as injection-molded trays, but the tooling for injection molding runs approximately 300 percent higher than precision thermoforming.

Even though Ghafari is satisfied with thermoformed trays, it hasn't completely given up on injection-molded trays. "We didn't want to do everything at once," says De La Garza. "We are still using some injection molding, particularly for extremely large trays, over 48 inches. We're talking with our thermoforming vendor to develop and test precision thermoformed trays with very large geometries. Right now, we are using 21 inch by 24 inch [thermoformed] trays. The large ones are the next step."

For more information about thermoformed plastic trays, call IPR Automation-Sohner Plastics at 734-222-4847 or visit www.iprautomation.com.

Special Trays for Special Applications

Twinsheet trays were developed for extra durability. These extra-rigid trays are thermoformed from two sheets of plastic: The top layer holds the parts, while the bottom layer is flat, which helps the tray traverse conveyor rollers smoothly. These trays are useful for heavy parts or to reduce vibration during shipping.

Parts with sharp edges, such as engine blocks, cylinder heads and transmission rings, are problematic for all types of plastic shipping trays, whether thermoformed or injection molded. The sharp edges typically shave slivers of plastic from the trays. These slivers attach to the parts, which then have to be cleaned and inspected before assembly. This negates the benefit of shipping the parts in protective trays.

To solve this problem, a new type of plastic sheet was developed for thermoformed trays. The sheet consists of a rigid base layer of ABS and a cut-resistant cover of TPU. The sheet requires special thermoforming tooling and techniques, but the material cannot be cut by sharp edges, eliminating slivers.