Thermoforming advances boost quality, cut costs.

CAD/CAM programming of five-axis CNC trimmers/routers and a new device for quick release of plug assists without removal of the mold were two interesting developments that attracted a record 688 attendees to the fifth annual SPE Thermoforming Conference in Independence, Ohio, last month. A formable EMI shielding film and advice on better acrylic/PVC sheet forming were other conference highlights.

AUTOMATING 'ART-TO-PART'

In heavy-gauge industrial forming, the demand for tighter part tolerances is generating greater interest in automated routing and trimming, says Roger Kipp, v.p. of sales and marketing for McClarin Plastics Inc., a thermoformer in Hanover, Pa. "Use of CAD/CAM software to program the trimming of formed parts has started to mature in the last three years," adds Bill Elliott, president of Northwood Designs Inc., Antwerp, N.Y., a software developer and CAD/CAM consultant. In the past, processors typically programmed their CNC trimming machines by hand, which was very time-consuming and often difficult, says Elliott. "Prior to off-line programming with a CAM system, many of the more complex parts were virtually impossible to trim to an acceptable quality using a CNC router." While the number of thermoformers using CAD/CAM is still small, he notes, "more companies are becoming curious and making the commitment to the technology for their trimming as well as their designing and pattern making."

Elliott says the recent generation of PC-based CAM software is primarily based on Microsoft Windows. Northwood Designs sells MasterCAM, a Windows-based 3-D CAD/CAM program from CNC Software Inc., Tolland, Conn. Elliott claims that the CAD/CAM package, in conjunction with software developed by Northwood, solves one of the most difficult challenges in automating trimming. When a formed part emerges from a tool and cools, it shrinks and changes shape slightly. The difficulty is that the part no longer precisely matches the original print or CAD model. "It's really a problem of finding the part's current location in space," says Elliott. This is accomplished by "reverse engineering" key locations on the part surface and bringing that data into the CAD/CAM system. This is used in conjunction with known data about the part design to develop the trimming instructions while taking into account any changes in the part shape or size due to cooling.

MC Systems in Lee's Summit, Mo., markets updated software from CamSoft, Lake Elsinore, Calif. Advanced System 3000 is a 3-D multimedia CAD/CAM package that allows simultaneous communication with up to 82 machine tools from a remote location so that many CNC trimmers in a big shop can be controlled from one point. A multitasking feature allows a programmer to view a part from different angles or with different aspects simultaneously. The software can be used to generate a 3-D tool path of a part or mold for trimming or routing. An Art-to-CAM feature lets a programmer create a 3-D tool path from a piece of 2-D artwork. An image such as a photograph or hand drawing is first scanned into the CAD/CAM software. Then the programmer color-codes contour lines on the artwork, assigning a depth to each color. The program then applies z-axis values to raise and lower the tool.

Photographs, job notes, or voice messages/instructions can be added to any job plan or design. An optional technical-support button connects the computer by modem to CamSoft for online, interactive technical assistance.

CamSoft is in the final stages of testing PC software that marries CAD/CAM to the controller of a trimmer/router. From a CAD image of the formed product and cutting tool, the software "talks" directly to the cutting machine and directs its motion. This eliminates the converting of a CAD design to G-code or M-code instruction for machine operation. A real-time image of the tool motion appears on the PC screen. The product is expected to become commercially available this month.

Both MasterCAM and Advanced System 3000 packages have base prices around $12,000.

QUICK-RELEASE TOOLING

Edward D. Segen & Co. Inc., Milford, Conn., displayed at the meeting two new devices that allow a processor to quickly change multiple cavities, cavity inserts, or plug assists. Two new devices are intended for quick mounting and releasing of plug assists and bottom mold inserts. Segen owner Alain Samson says they are the first products of this kind for thermoforming. The MagLock model is actuated by a 24-v d-c power supply. Segen offers a control box that has two 12-position switches, which allow an operator to control 144 units. A manually actuated version of the MagLock, called the PrestoLock, is released by inserting a screwdriver or small probe through a small hole on the exposed side of the plug or insert.

HOW TO HEAT ACRYLIC/PVC

Advice on how to improve quality and reduce rejects in thermoforming Kydex acrylic/PVC heavy-gauge sheet was presented by James Medalie, technical director of Kleerdex Co., Bloomsburg, Pa. The "Medalie Method" looks at oven temperatures and sheet appearance to determine when the sheet is ready to be formed. "Almost everyone uses just oven temperature," he says, adding that most processors tend to heat the sheet too quickly, which results in blistering and uneven heating, with the core of the sheet often not reaching forming temperature. This can create poor part definition or glossy spots. "The objective is to keep the sheet smooth," he says. Medalie recommends a "slow and low" heating process for conditioning the sheet for forming. Sheet surface temperature should not exceed 400 F.

The top-oven temperature should be lower than the bottom temperature to achieve low gloss on the primary surface (assuming downward forming into the tool). Usually the top heater does 30% of the heating of the sheet and the bottom does 70%, he says.

Medalie identifies four stages in heating the sheet. In the first stage, the sheet shows wide undulations as it begins to soften. During the second stage, ripples form, particularly along the edges near the clamp frame. During the third stage, the material starts to smooth out and sag, though Kydex sags less than other materials because of its high melt strength. Most processors form the sheet at this stage, but there are still ripples at the edge of the sheet, indicating the presence of stresses, Medalie warns.

The last stage is reached only when the ripples disappear and the sheet is smooth and sagging. This indicates dissipation of all sheet stress and that the core of the sheet is at forming temperature. "If you achieve optimal conditions, the core temperature of the sheet should be within 10 [degrees] F of the surface temperature," Medalie says. He concedes that his method may result in longer heating times, but processors can still come out ahead by reducing scrap. Also, acrylic/PVC can substitute for higher-cost flame-retardant ABS.

FORMABLE EMI SHIELDING

The Electrical Specialties Div. of 3M Co., Austin, Texas, has a new EMI shielding material in formable sheet form, which can eliminate secondary coating steps. 3M's 6100 shielding is a laminate of tin/bismuth alloy fibers embedded in EVA polymer.

Precut pieces of the laminate are placed on top of the plastic sheet substrate. When heated, the EVA melts and bonds the shielding fibers to the substrate. The shielding stretches with the plastic during sheet forming; however, the metal fibers elongate rather than break, resulting in a conductive "grid." The amount of shielding in a given area is inversely proportional to the drawdown ratio of the sheet. The material is supplied in 2 x 50 ft rolls.