Low-stress molding process makes 'impossible' parts.
The process, which is available for licensing, has evolved considerably since then. Recent developments include use of post-consumer recycle, multi-layer parts, foam/solid sandwiches, and embedding of metal fasteners, metal screens for EMI/RFI shielding, or Kevlar fabrics for structural reinforcement. Maximum part size has increased dramatically - to 9 x 5 x 6 ft. Perhaps most important are increases in productivity. The first dual-mold machine, which has just been put into service, reportedly matches the output of an industrial thermoformer. A four-mold system now in development will double output again.
In two decades, VIM has established credibility with customers such as Pratt & Whitney, General Electric, and IBM, for which B&R makes material-handling trays. Also, auto makers use intricate dunnage made by VIM, and hospitals use VIM-molded medical-waste carts. B&R builds around 20 new molds a month and consumes close to 1 million lb/yr of resin in making VIM parts. B&R has one licensee in Israel that uses a variant of the process to make ballistic armor with reinforcing inserts. In the U.S., the only other current user of VIM is C.R. Daniels, Inc., Ellicott City, Md., which molds tote boxes and bins.
TREATS PLASTIC GENTLY
The VIM process uses a single mold surface fabricated out of sheet aluminum and mounted on a wood-and-steel base. Molds can be male, female, or a combination of both. The tool is enclosed in a box of wood or other material so that the molding surface can be entirely covered with loose plastic powder or granules. The mold is then heated with electricity or gas and vibrated mechanically so that plastic fuses all over its surface in what is basically a sintering process. Fusion takes place only at the mold surface, and part thickness is determined by the length of time the mold is in contact with the material. After fusion, the mold is inverted and excess resin is shaken off the part. Heat and vibration continue for a time to smooth out the non-mold surface. The mold is then cooled by an air blower, and the part is demolded. There is generally no waste to trim.
Parts are said to be molded with extremely low stress and no flow orientation because no shearing or pressure are applied to the material. Parts are also allowed to cool slowly on the mold at ambient temperature, with only a fan blowing over them.
Fried notes that the molding temperature is right around the glass-transition point of the material, but below its melting temperature. Thus much less energy is required than for other molding processes - and less cooling, too. Degradation of heat-sensitive materials such as PVC is also minimized, Fried says.
WIDE DESIGN FREEDOM
The process is said to allow broad latitude in part design and material composition. Large parts are no more difficult or time-consuming to mold than small ones. The largest part made to date is a prototype of a 4-cu-yd (800-gal) LLDPE waste bin that measures 65 x 48 x 48 in. and weighs 185 lb. (The largest waste bin ever injection molded is 2 cu yd or 400 gal.)
Part thickness can be from 0.015 to 1.25 in. Thickness can be selectively increased by raising the temperature of certain areas of the mold.
Multi-layered structures are produced by fusing additional layers of different materials over the first. A layer of recycled resin can be sandwiched between virgin layers, or a foam layer (made with a chemical blowing agent) can be placed between solid layers. Reinforcing fabrics can also be placed between layers, even on selected areas of the part.
Because VIM does not pass the material over a screw or through an orifice, heavily contaminated regrinds can be used - for example, chopped wire scrap containing 8% metal.
Multiple small molds - not necessarily identical - can be processed simultaneously. Because resin will not fuse on the wood sections of the molds, the parts do not need to be trimmed apart. Holes can be molded into parts by placing a PTFE insert on the mold surface (no resin will fuse on the insert). Metal inserts can be embedded in parts by first injection molding resin around the base of the insert and then placing the insert into a pocket in the freshly molded VIM part while it is still hot.
Although B&R most often uses LLDPE, other suitable materials include HDPE, PP, nylon, polycarbonate, and acetal. Rotomolding-grade materials in powder or 0.020-in. micro-pellet form work well, as does reground flake. Standard-size pellets can also be used, Fried says, but they leave a rougher surface on the non-mold side of the part.
The process produces only one molded surface, which is smooth enough to suit medical applications that require rigorous washing. The non-mold surface is usually fairly smooth, as well, and it can be improved if necessary by light application of a flame torch.
VIM can mold many shapes that are difficult or impossible for other processes. Zero-draft capability is one example. A rectangular LLDPE bin liner made for Eastman Kodak measures 44 x 38 in. and 30 in. deep and has zero draft. Ejecting a zero-draft part from a male mold is no problem: Compressed air at only 2 psi is blown under the part as it cools. "The part just walks off the mold," Fried declares.
Parts with multiple deep draws are another case of "impossible" shapes that VIM handles with ease. One example is a special carrier for automobile suspension parts. The LLDPE carrier has 10 pockets of 9-in. diam. and 21 in. deep, with less than 1/8-in. separation between the pockets. Fried doubts that any other method could produce this part.
Slow cooling on the mold helps VIM hold close tolerances. On large parts, such as a 54-in.-long medical-waste cart with a tight-fitting lid, VIM consistently holds [+ or -] 1/32 in. across the part, Fried claims.
Although they're sintered without pressure, VIM parts are strong: An LLDPE tray made for Pillsbury holds a 250-lb bag of cheese. The 3/16-in.-thick tray is designed for stacking five high.
Low-cost tooling is a major strength of the VIM process. Pinhole-free, aircraft-quality aluminum sheet must be used to prevent parts from sticking. Molds can cost as little as $150 or up to a few thousand dollars for the most complex multicavity tools. Most can be built in less than a week. The mold for the 29-cu-ft Kodak bin liner cost well under $1000 and was built in less than a day, Fried says. The largest VIM mold ever built - for the 4-cu-yd waste bin - took three weeks to build and cost $58,000 (versus an estimated $2-3 million for a comparable injection mold).
VIM parts, regardless of size, generally take 15-20 min to mold. Multiple cavities or multiple small molds ganged together can increase productivity. It's also very easy to change molds: They're simply lifted off the trolley.
B&R's newest machine shuttles two molds in and out of a central molding station, so that one mold is cooling while the other is making parts, effectively doubling the output. Fried says the Pillsbury cheese tray will be molded in four-cavity molds with two molds ganged together. The shuttle machine will thus produce eight parts every 15-20 min, which is reportedly competitive with thermoforming cycle times. The machine is more automated than earlier versions: It requires an operator's attention for just 5 minutes twice an hour to unload parts. For further productivity gains, B&R is planning to build a four-station system that could be of either linear or rotary design.
Fried says a licensee could get started in VIM with an investment of $100,000 to $500,000, depending on project scope and machine size, plus a 5% royalty payment. B&R would supply the equipment components for the customer to assemble on site.
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|Author:||Naitove, Matthew H.|
|Date:||Jun 1, 1996|
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