CNC trimming: more affordable for more processes.

They're not just for thermoforming and composites anymore. Lower-cost CNC routers are opening an easy route to automation for blow and injection molders, too.

Picture a high-tech plastics-processing plant. Computers control all the equipment. Robots and conveyor belts move unfinished parts around the plant, while raw materials surge through a vacuum-loading system. Molded or thermoformed parts come hot off machines running under tight SPC control--only to end up on a rickety benchtop where an hourly worker wielding a hand router and scraping knife prepares to operate. Even if all hand-trimming techniques aren't quite so backward, manual secondary operations tend to stick out in an otherwise sophisticated facility like, well, a sore thumb.

Yet many processors still rely on manual labor to put the finishing touches on high-quality parts. For injection and blow molders especially, this lack of automation represents an on-going paradox: "It's kind of silly to automate your molding machines and still have all these manual secondary operations," says Todd Herzog, president of Accu-Router Inc., a maker of CNC routers.

It should be no secret by now that automation improves product quality by imparting accuracy and repeatability that hand operations cannot match. Not only can CNC machines trim better than your average worker, they can trim faster, making short but profitable production runs possible. CNC machines can reduce in-process inventories and speed up delivery times. In short, automation with CNC machines can make you more competitive. "When you do things by hand, you're vulnerable to competition from those who don't," notes Ken Susnjara, president of Thermwood Inc., another manufacturer of CNC routers.

Hand trimming with knives or routers becomes less attractive all the time given the upward spiral of labor costs and the expensive risk of repetitive-stress injuries and accidents. "With some jobs, you just can't be enough on guard against injuries. I can't even begin to put a dollar value on all the accidents we've avoided over the years," says Paul Hickey, president of Auburn Vacuum Formers, a custom thermoformer in Auburn, N.Y., that has all but replaced the use of hand tools and table saws with two Thermwood five-axis CNC routers.

Even thermoforming operations using more productive trimming dies can benefit from more automation. "Trim dies are fast--until you have to move a hole 1/8 in.," notes Susnjara, who guesses it might cost $25,000 to change the die. A CNC program change, by contrast, might cost two minutes worth of engineering labor.

Historically, there have been good reasons why many molders have steered mostly clear of automating secondary operations. Machines and fixturing methods long favored by thermoformers and composites fabricators do not necessarily work on parts made by injection molding, blow molding, rotomolding, or RIM. "Molders have always wanted to automate, but the machines needed to be a lot more flexible with a more scientific approach to fixturing," says Patrick Bollar, president of CNC supplier Motionmaster Inc.

Another drawback--especially for smaller processors--is that CNC has never been cheap. Six-figure price tags were typical, and some high-end five-axis machines for composites weighed in at over $1 million. Over the past year, however, technological improvements have made CNC trimmers and routers more suitable for a wider variety of processes. And some brand-new machines have hit the market with smaller price tags--as low as 870,000 for a five-axis deflashing machine. Before plunging into secondary-operation automation, there are some economic issues that today's prospective CNC user should consider.

ACCOMMODATING VARIABLE PARTS

For CNC routers to fit into more post-molding operations--especially trimming or deflashing--suppliers had to address a fundamental snag rooted in CNC's otherwise-desirable consistency. As Motionmaster's Bollar explains, "CNC will put the cutting head in exactly the same place every time." So long as part-to-part dimensions are equally consistent, CNC works just fine. In fact, all the CNC suppliers report positioning accuracies for their machines of at least [+ or -]0.005 in., with a typical repeatability of [+ or -]0.002 in. Bollar notes, however, that molders of all types typically work with tolerances around [+ or -]0.03 in. "The machines always do better than that," he says.

Unfortunately, molded part dimensions often vary, and a "fixturing problem" arises as users try to apply inflexible fixtures and programs to ever-changing parts. The good news is that CNC technology now seems to have caught up to needs of the molding process.

Suppliers and users alike have devised ways of referencing the machining head to the part itself, rather than the mounting table or fixture. Thermwood, for instance, recently came out with a patent-pending floating-table option that turns its three- and five-axis routers into deflashing units. According to Susnjara, the table rides on a cushion of air that allows the part to be pushed up to 1/2 in. off center. Meanwhile, the tool is fitted with a bearing follower, which maintains contact with the part and determines how far off center the table needs to move as part dimensions change. Floating tables can be retrofitted to several Thermwood models, including a brand-new, beside-the-press flash-removal system. At roughly $70,000, this Model 7 trimmer is--by CNC standards--a low-cost, five-axis model that provides easy entry into CNC automation for blow molding, RIM, and rotomolding. Susnjara adds that injection molders have used the machine, too, since some processors still confront flashing on some parts. This new machine features a full enclosure to trap noise and dust, as well as an integrated scrap-retrieval provision to blow trimmings back into the molding machine's hopper. The Model 7's work envelope measures 30 x 30 x 12 in.

Thermwood isn't alone in addressing the dimensional-stability problem faced by molders. Motionmaster also offers floating tables, as well as floating heads on the rotary axis to accommodate shifting part sizes. And because the primary source of the troublesome dimensional changes is ambient temperature, Bollar reports that some customers use temperature sensors that trigger modifications to the CNC program "on the fly."

Bollar also recommends "a more scientific approach to fixturing" that involves a better understanding of how parts change dimensions. As an example, one Motionmaster rotomolding customer identifies the growth/shrinkage regions on each part in order to establish multiple reference points for guiding the machining head.

CNC users can contribute other much-needed advances to the fixturing problem. Fremont Plastic Products Inc. of Fremont, Ohio, an industrial blow molder now well advanced on the CNC learning curve, often experiences dimensional changes in parts. Engineering manager Steve Wagner says color and regrind-level changes are partially at fault, though climate variation also aggravates any mismatch between parts and the CNC program. Fremont solved the problem by storing multiple tool paths--one for each color--and then establishing a range within which CNC operators could make adjustments to the tool paths to offset any remaining variations.

At Auburn Vacuum Formers, Hickey explains that learning to how make high-quality fixtures has eliminated his company's "biggest source of potential error" from CNC operations. Agreeing with Bollar, he points out that the CNC machines themselves inherently perform better than the demands placed on them by most plastics applications. But unless the you can build good fixtures, you won't achieve the levels of accuracy that CNC can provide.

IMPROVING ECONOMICS

Thermwood isn't the only company to have introduced lower-cost CNC automation. Accu-Router, which has mostly supplied large machines for composites and thermoforming applications, offers a new CNC router that provides five-axis functionality for about $50,000 more than the company's three-axis machine and about half as much as a standard five-axis model (see PT, June '94 p. 41). What the company has done is mount a two-axis, servo-driven head on a standard three-axis machine. Its 12-18 in. reach is about half that of a standard five-axis unit, though speed and accuracy are equivalent.

Apart from lower price tags, features that increase machine productivity can also make CNC trimmers more economically attractive than in the past. Motionmaster reports that productivity-enhancing options like dual tables and tool changers have become especially hot sellers in recent months. Bollar says the automatic tool changers, which used to be found only on more expensive machines, significantly speed setups whenever the job calls for multiple machining operations. And dual tables, which all CNC suppliers offer, can keep machines running continuously by leaving one table free for setup while the other holds the current workpiece.

In addition to bringing out its low-cost Model 7, Thermwood has increased its machine speeds in the past year to better suit at-the-press trimming. "You must be able to trim a part in the time it takes to mold it," Susnjara says. The company switched to a-c servo drives, which can accelerate or decelerate faster than the d-c drives they replace. On an identical program, the a-c servo is 35-40% faster than the d-c, Thermwood reports. In thermoforming applications, the increased speed makes CNC a "more realistic alternative to die pressing," Susnjara says.

THE RIGHT CNC FOR THE JOB

No one CNC machine fits into all applications. Some processors may get by with a router having only three instead of five axes--and save a substantial amount of money. Techno/Isel offers three-axis machines with CNC software for as little as $9000. According to vice president Dr. George Klein, the company's new Series III machines have been made more rugged than earlier models--with features such as cast-aluminum gantry supports, built-in way covers, and optional machine enclosures. Table sizes range from 14 x 11.4 x 3 in. to 49 x 41 x 3 in. Klein says the machines are suited to flash removal on relatively flat parts, thanks to the MasterCAM 3-D software included with the machine.

All the other suppliers, however, stress that the majority of plastics applications--anything but flat work--need a five-axis machine. "Very little, work in thermoforming requires straight three-axis," says Susnjara. He cites trimming bottoms and undercuts and drilling top holes as examples of jobs that exceed three-axis capability. "You really need a five-axis machine whenever you have a three-dimensional part and the cutter must be perpendicular to the machined surface," agrees Accu-Router's Herzog. That description fits most molded parts that need trimming in combination with other machining operations.

Among the five-axis machines there is also significant differentiation in price ranges. On the low end of the price spectrum are Motionmaster and Thermwood; both offer five-axis machines that start at roughly $70,000. Accu-Router falls in the middle of the price spectrum with larger five-axis models starting at roughly $300,000 and its modified three-axis machine costing about $150,000. Five-axis Heian machines, imported from Japan by Stiles Machinery Corp., also fall in the middle range at roughly $250,000 and up.

American GFM sits firmly in the high end of five-axis machines; its typical prices are around $800,000. Though the company's market is mostly aerospace composites, "other processors could also benefit, provided their volumes were sufficient," says Edward Mihalko, director of sales and marketing. GFM has also come out with a new system for RTM that might interest automotive molders. This machine is intended to sit near the mold and ultrasonically trim the preform to net shape before molding. "It's better to trim net before curing--it's softer," says Mihalko.

American GFM's newest high-end five-axis machine is an 8-ft gantry model with tool changer and combination router/ultrasonic cutter. This machine also features a new programmable vacuum-pod system that eliminates the need for fixtures and spoil boards (PT, Feb. '93, p. 71). Like vacuum hold-down systems offered by several suppliers, these pods pull a vacuum underneath the part, securing it to the table. What's unique, according to Mihalko, is that this computer-controlled system employs new software to determine automatically which pods can rise from the table to grip the part without interfering with the tool path. The proprietary software deploys the pods based on part geometry taken from existing CAD/CAM systems.

For applications like RTM that involve multiple stages of trimming and machining, GFM would typically supply several CNC machines for different stages of the process as it goes from preform trimming to final machining. "The benefit is that all the machines would use the same data set," Mihalko notes.

JUSTIFYING THE COST

Despite what seems like a big price differentiation among the various machine lines, prices do overlap when you add options such as tool changers, dual tables, dust-extraction systems, enclosures, multiple spindles and vacuum clamping to the lower-end units. One thing's for sure, though: CNC is never exactly cheap. For that reason, Thermwood's Susnjara predicts that "little shops will continue to do things by hand and the real industrial shops will automate."

Accu-Router's Herzog points out that many processors initially have the wrong idea of how to justify the cost of installing CNC. "Most people look at labor savings as the big benefit, but it [labor] is not the main cost driver." Instead, he says, you have to look carefully at the far bigger costs that come from building up an inventory of fixtures needed to set up single parts at multiple manual processing stations--from hand router to drill press to punch press to band saw. With CNC, all the secondary operations can all be done at a "single-stop, automated work cell" at far faster speeds. Herzog estimates that a typical thermoformed part needing trimming and secondary machining--hole drilling, for example--can be finished off in 1-2 min by CNC. "The general rule of thumb is: If you have the volume to keep the machine busy, it will pay for itself in six to eight months."

Picking a Winner

When evaluating purchase of a CNC machine, the most obvious strategy is to test run some actual parts. All the suppliers mentioned here expressed a willingness to take a crack at trimming your parts either while you watch or in front of a video camera.

The try-before-you-buy approach doesn't address all the nuts-and-bolts concerns that should inform any machinery purchase--namely, the mechanical design alternatives, choice of drives and controls, and after-sales service. There are two particularly important criteria in buying a CNC machine.

* Rigidity: Suppliers take dissimilar approaches to mechanical construction but they all agree that rigidity is a virtue. "Anything that compromises rigidity will compromise machine performance," says Herzog, explaining that rigidity influences accuracy, noise levels, spindle life, and feed-rate capability. Be sure to ask suppliers how their components--like trackways and ball screws--add up to a rigid and rugged machine.

* Growing room: "Be cognizant that this technology is changing rapidly," warns Susnjara, alluding to the nonstop evolution of computers. "Look at what you're trying to achieve and what you want to do in the future--get equipment that allows you to do that," Susnjara says, citing CAD compatibility as an important feature for future growth. Other things to look for in this department include controls that can be upgraded, a spindle that reaches the entire work table, and multiple hold-down systems.

A Case of Do-It-Yourself CNC

Fremont Plastic Products, Inc. of Fremont, Ohio, is one blow molder that has been successfully using a CNC trimmer for nearly four years. Unable to find a suitable commercial CNC touter at first, this company took a do-it-yourself approach to designing and building its own three-axis machine with a non-programmable rotary head that, in essence, adds a fourth axis. To avoid building a tool changer, Fremont even designed specialized tools that can perform all the necessary routing tasks.

Fremont uses the machine on a blow molded interior console for the Ford Bronco. Not only does the router automate trimming, but it also cuts angled holes, puts a radius on inside edges of the holes, and finally drills a set of mounting holes. Engineering manager Steve Wagner cites the usual reasons for automating: high part volume, strict quality demands, and the potential for all kinds of employee injuries. Availability of CNC permitted Fremont to develop a more complex console design than hand routing would permit.

CNC has been so successful at addressing quality needs and employee safety that Fremont now considers automation for all new jobs. "Some jobs that we got rid of in the past, we now might consider putting on CNC," Wagner says.

If the company were to automate today, "stock" machines from one of the CNC suppliers might get more consideration. Wagner believes that the new floating-table designs, for instance, look especially promising for blow molding applications. "As the price and technology get better every year, it makes more sense to buy a stock machine," Wagner says.