New approaches to injection molding reduce costs by saving material, eliminating secondary operations, and allowing use of smaller presses. These developments were revealed at the Molding 2004 annual conference sponsored by Executive Conference Management in New Orleans in February. They show how to reduce part thickness over ribs without risking sinks, how to apply and cure coatings on multiple parts while they are still in the mold, and how to mold large multi-material parts without requiring oversized presses.
Gas drives part design
External gas molding (EGM), a technique originally developed to eliminate sink marks, shows potential for reducing part weight and material cost by allowing thinner walls over ribs without causing sink marks. “The advent of gas-assisted molding has begun to change the traditional design rules relative to rib design and sink marks,” says Steve Ham, application development engineer at Cinpres Gas Injection Inc. in Ann Arbor, Mich. “We are finding that EGM can influence design rules for rib aspect ratios,” Ham explains. “The rib pattern replaces wall thickness as the core design platform, and this will require part designers to change the order of the design sequence.”
EGM is based on injecting gas between the plastic and mold surface after the mold is filled. The gas applies uniform packing pressure to the plastic, forcing it against the opposite mold surface as it cools and shrinks.
In traditional injection molded part design, the nominal wall section is laid out first, followed by a build-up of projections, ribs, and bosses. “Our analysis shows that most of the resin, the most costly part of the molded component, is used in the skin or nominal wall,” says Ham. “At the same time most of the strength of the part is provided by the rib structure. But if the concept of structural ribbing is to increase the strength with the least amount of weight, there is no purpose to the nominal wall other than appearance and to fill the space between the structural ribs.” Going further, he says rib depth determines strength, but rib width traditionally dictates wall thickness. Rib width is typically determined by ease of filling and ejection. Typical guidelines for solid injection molding recommend an aspect ratio of rib width to nominal wall thickness ranging from 0.5 to 0.75. For gas assist, the rib/wall aspect-ratio guideline is 3.5 to 8.0. With EGM, the allowable aspect-ratio range is 0.5 to 4.0.
Asahi Chemical in Japan, which owns EGM patents along with CGI, has employed the new design approach to produce a fax machine housing. The mold for the part was retrofitted with EGM in isolated problem areas. The part originally had a 2.5-mm wall and 1.5-mm rib (0.6 aspect ratio). In the high-visibility surfaces on the part, the EGM design employed 2-mm walls backed by 3-mm ribs (1.5 aspect ratio). The EGM surfaces were sink-free.
In-mold uv coating
In-mold painting is a new patented process technology recently developed for the Spin Stack molding concept created by Gram Technology, a Danish company in the process of setting up offices in Scottsdale, Ariz. Gram’s Spin Stack is a unique stack-molding concept in which the stack’s center “cube” is divided into columns of rotating cores. Each of the core columns can be indexed 90° by a central motor, giving each column four sides for production. The design creates a stack mold that is lighter and faster-acting than other stack-molding concepts with a rotating center platen, says Jes Gram, president.
Spin Stack molds can be used for multi-component molding or insert molding or can provide additional cooling stations. In-mold painting is the latest advancement of the technology. It combines four steps into one cycle: injection molding, spray painting the part, uv curing the paint, part takeout, and packaging. Together with AGA Linde, a Swedish industrial-gas supplier, Gram has developed commercial systems that are already running in China. The process is producing a window component for a major telecommunications application. In this case, the “paint” is a clear coat that provides scratch resistance (up to 9H pencil hardness). The window is molded in a two-cavity tool on a 200-ton press in a cycle time of 20 to 25 sec.
The keys to the process are the development of the paint-delivery system and the uv-curing technology. Gram says the paint is applied with a specially designed double-piston spray gun that is mounted atop each mold column. A single motor controls the operation of the spray guns. The volume of paint applied is adjustable and overspray is said to be limited. “There is a little overspray, but that is returned to a reclamation box. It does not escape or contaminate the process,” Gram says. For uv curing, the tool is covered with a chamber that evacuates air (which can disrupt transmission of uv rays) and fills the chamber with nitrogen. UV light is applied for “a fraction of a second,” says Gram. The parts are removed by a bottom-entry takeout robot and immediately packaged.
The in-mold process provides a number of benefits for part performance. Bonding between the paint and the part is excellent, as the paint is applied to a just-molded part. Overall yield for the process is greater than 90%. Gram says paint thickness affects cycle time the most. So far the company has only used polycarbonate and PC blends as substrates for in-mold painting, but Gram expects success with polyethylene as well.
Two shots, no mold break
Producing a large multi-color, multi-material part economically can be difficult to achieve, but Canadian moldmaker Hallmark Technologies has developed an approach to make a large automotive part without mold break. “One of the problems with making such a part is the production equipment to run them. Even a part the size of an automotive taillamp runs in a fairly large press, normally 1000 to 1500 tons. This is mostly to accommodate the platen size not the tonnage requirement. So if a window seal has to run in a 3000-ton rotary machine, think what would be needed to run a larger, more complex part such as an automotive door panel or an instrument panel,” says Peter Elford, Hallmark’s v.p. of sales/marketing. Hallmark’s answer was to develop a system that did not require a bulky rotating mold.
Hallmark performed tests on a two-shot stationary mold for a 30-in. stop sign that is produced with a decorative film insert. The first sign was produced with a conventional hydraulic wedge system to pull the blades surrounding the letters and borders. These borders were sealed to the cavity. Clear PC injected in the first shot filled the letters and the border. Then the blades were pulled back and red PC filled the rest of the sign. However, in production, the movements of the blades, wedges, and cores caused problems. The blades occasionally cocked and jammed; flash around the blades scored the mold; and the wedge drivers showed extreme wear after only a few shifts. In addition, the hydraulic wedge system for this large tool required extremely accurate guiding and had to be robust enough to demold the part from the first shot. Large shut height was required to accommodate the hydraulic systems, and there were cooling problems with the blades as well as the rest of the tool due to the core-outs required for the blades.
Hallmark focused on the blade movement and cooling problems. It came up with a new design that utilizes the clamping ram on the press to drive mold actions. Hallmark first removed the wedges and the hydraulics, then used the ram movement to move the core while leaving the blade stationary. This allowed for the blades to be cooled at their base, while movement of the core based on ram position allowed for better control of the process. “To allow the core to move, the ram itself must move back. And it is imperative that we maintain a closed parting line,” says Elford.
Elford says the core becomes a three-plate structure with springs to force the parting line shut while the back moves to open space for the second shot. This opening allows for the placement of spacers under the plate to act as pillar supports. The thickness of the pillars determines the thickness of the second shot. “The result is that we are asking the press to do an injection compression cycle in reverse. Most modern presses handle this well. We have also been able to deliver tonnage at two different shut heights,” says Elford. Hallmark is applying for a patent.