Precise Timing, Drives Thermoformer

To thermoform plastic product packaging, cups, lids, foam plates, and berry box products used in supermarkets requires highly precise, synchronized motion. And with inline, continuous thermoformers, performance requirements go even higher, demanding precise timing and repeatable sub-millisecond-level synchronization between multiple axes.

Brown Machine (http://rbi.ims.ca/3850-528) has improved the performance of its thermoformers by coordinating motion with a series of high-speed programmable limit switches (PLSs) that govern motion based on the position of the thermoformer's main drive. Brown Machine turned to a PC-based motion controller from ORMEC Systems (http://rbi.ims.ca/3850-529) and high-speed PLS outputs on the system's axis modules to increase performance and reliability.

Continuous feed

With a continuous thermoformer, the material is delivered as a sheet from roll stock, or it is fed in directly from an extruder. The sheet goes through an oven where it is brought to its forming temperature, and then into a form station where vacuum and pressure shape the material in a mold into a "skeletal web."

This web is usually moved to a separate trim station where the product is trimmed from the web by matched metal punches and dies or steel-ruled dies. Any scrap is regranulated and put back into the system.

On the form station, servos drive the upper and lower platens, which hold the mold segments. In most applications, there is a process called "plug-assist" which is used to help distribute the material properly in the mold. With a plastic cup, for example, the plug-assist moves down into the mold cavity to help form the cup and optimally distribute the plastic material.

With so many actions needed for proper machine operation, timing, speed, and movement of the mold, as well as platen position where various form functions are turned on and off, all become critically important. Maintaining product quality and optimizing machine performance is difficult because, as the platen is traveling up or down, precisely synchronized process actions need to be coordinated. For another example, a B-Line hot sheet thermoformer, the typical configuration is up to nine-axis. With a trim press, servos are used on the ejectors and the feed system. A vertical trim press is typically a five-axis system.

Continuous form and trim

At one point, Brown Machine experienced some feed problems on a redesign of its vertical trim presses. The system didn't have sufficient accuracy and the servo loops weren't tight enough to maintain position properly—which caused other problems downstream with counters, baggers, and other processes.

"The majority of system improvements related to achieving higher speeds and accelerations, how fast the system could get to proper position, and overall system accuracy," says Matt Salgat, an electrical engineer at Brown Machine. Salgat notes that the main drive needs to precisely initiate motions on other axes, and the group together performs a camming function.

The revised system uses programmable limit switches on the DSP axis module of an ORMEC ORION motion controller to produce closely coordinated motion on the trim presses. The axis module provides three optically isolated PLS outputs that respond at the position-loop update. Each PLS can be independently driven with actual or commanded position of an axis, or by MotionData communications that provide tightly coordinated, multi-axis electronic gearing.

"We also noticed a performance improvement on the forming stations where the PLSs fire the plug-assist," Salgat says. "On most machines, the mold would come in first and, based on mold position, the plug-assist motion would be initiated. When the plug is moving toward the sheet, you are also firing a variety of valves based on plug position. All of that closely coordinated, synchronized motion is done through using the programmable limit switches," he adds.

The PLSs dramatically improved the accuracy of the system and repeatable performance. Brown Machine notes that the vertical trim presses now achieve a running speed of 160 shots per minute. Previously, Brown had a machine running at 140 shots per minute but could not achieve reliable accuracies.

While customers benefit from increased speeds, more commonly they set the machine to run at a 3:1 or 4:1 ratio between the form station and trim press to achieve maximum machine throughput and product quality.

In a previous attempt to implement a similar system, Brown Machine tested a board-level motion controller. But unlike the robust performance and reliability of a standalone, integrated motion controller, Brown experienced problems with system reliability, the power supply, and inconsistent software performance. Once the PLSs had been implemented properly in the ORMEC system, the performance and reliability did improve dramatically, evidenced by fewer service calls.

Another advantage to a PLS system is consistent performance. "The process has to be the limitation on productivity and throughput rather than the machine itself," say Jim Robbins, VP at Brown Machine. "As materials and technology have gotten better, increasing the capabilities of the machines to maximize the process of forming the material is always the goal."

Salgat adds that the new system gives customers the option to heat the sheet up faster, increase the speed of the former, and change process parameters on the mold—bearing in mind that cooling time is critical to consistently high quality parts. Customers run parts more confidently, as long as they maintain a proper ratio between the forming station and trim press. There is also less scrap and less downtime.

Remote support

Brown Machine has used modem communications and PCAnywhere software to perform remote support, diagnostics, and maintenance on its machines for three years. Engineering and service personnel dial-up directly into machines. They use development software provided by vendors to support the Lookout HMI, Allen-Bradley PLC, and ORMEC motion control software used on the machine. Personnel can also modify and support control software through the modem.

"That's been saving us time and money, but has also contributed to machine uptime," Robbins said. He added that the ability to diagnose problems via modem dramatically improves service response—typically faster than service personnel could travel to the airport.

High Output: Brown Machine Thermoformer outputs  a stream of plastic products.