Thinwall Goes Mainstream
High-end applications of so-called thinwall molding, where the flow length to thickness (L/D) ratio is in the range of 100 to 200, are breaking new ground thanks to advances in the concepts of the thi...
High-end applications of so-called thinwall molding, where the flow length to thickness (L/D) ratio is in the range of 100 to 200, are breaking new ground thanks to advances in the concepts of the thinwall process and the technology to successfully achieve it. These technology advances include innovations in machinery, hot runner systems, materials and mold design that are resulting in increasing acceptance and use of thinwall molding in markets such as consumer electronics and automotive.
“We can see a definite swing toward thinwall molding in general injection molding applications,” says Liam Burns, sales engineering manager, thinwall applications, Sandretto USA, Inc. “It’s a very competitive market and everyone’s trying to offer their customer something extra. If you can offer thinwall molding, that’s one more advantage you have over your competition.”
Burns reports that Sandretto USA recently sold six of the company’s Mega TE machines to an automotive supplier which intends to use them to mold a variety of thinwall components, including a novel under-the-hood application. The machines range from 610 to 1100 tonnes in clamping force. When used for thinwall molding, Burns notes, the machines will allow the supplier to reduce material usage and therefore cost, and decrease part weight, two top concerns for companies in the automotive market.
Sandretto also sold a 1760 ton Mega HCS machine, optimized for thinwall applications, to GE Plastics earlier this year. GE is using the machine at its Plastics Process Development Center in Pittsfield, MA to help expand thinwall molding from primarily smaller applications such as cell phones and notebook computers to larger components such as computer monitor housings. The machine, which Burns describes as a “one-off”, is specially designed to accommodate larger thinwall applications, providing injection pressures of up to 38,000 psi and injection rates of up to 57 ounces per second, depending on the material.
RUNNING AT THE MAX
In general, higher injection pressures and rates are needed to fill thinwall part sections in which the width of the flow channel for core material can be as much as ten times smaller in comparison to a flow channel for a standard part.
To achieve these high injection pressures and rates, an injection unit equipped with an accumulator is a must. An accumulator positioned close to the injection cylinder provides quick delivery of a large volume of oil to the cylinder during fill, minimizing pressure drop and enhancing responsiveness.
Burns notes, however, that an accumulator by itself is not sufficient to run thinwall. “You have to look at the whole package, screw and barrel, injection unit, hydraulic system and mold design, when specifying a machine for thinwall molding.” Sandretto uses specially designed screws and barrels manufactured to withstand higher pressures required for thinwall molding, Burns says.
Proper machine control is also a concern when making thinwall parts, says Franz Strohmaier, vice-president, engineering, Engel Canada. Engel’s ES line of high-speed injection presses are capable of injection speeds of up to 1000 mm/s (40 in./s) and feature a 64-bit, RISC-based analog closed-loop control which will detect and correct small, random process variations, such as fluctuations in melt flow index of material or mold temperature. RISC, which stands for reduced instruction set control, is a standard control system which allows a machine to execute certain tasks at much faster speeds. At the extreme end of thinwall molding conditions, however, even a RISC-based, closed-loop system may not be sufficient for the precise, stable control of molding parameters.
“Once you get below 0.2 seconds in fill time, you have to control in an open-loop condition, because the controller can’t pick up on changes in the process,” says Strohmaier. While this generally means having to make non-real-time adjustments to the drive and hydraulics responsible for controlling the screw, at these very fast fill times the Engel control system is equipped to run an adaptive, closed-loop control for one cycle. After the cycle, the controller will make automatic adjustments to the machine to take into account any process changes, such as fill time variations.
Nissei Plastic Industrial Co. makes a number of high-speed injection presses suitable for thinwall molding. The presses are equipped with Nissei’s Triplemelt plunger injection system, consisting of plasticating screw unit, uniform plasticating accelerating zone and injection unit plunger. The Triplemelt design reportedly provides greater stabilization of pressure, temperature and injection rate, enhancing control and precision of thinwall molding applications.
MAKING THINWALL MOLDING AFFORDABLE
For many smaller and medium-size molders, purchasing a machine modified to specifically run thinwall parts remains cost prohibitive, especially if the component being molded is not particularly high-volume. Studies have shown, however, that with good control of mold and melt temperature, it is possible to mold high quality thinwall parts on standard injection machines using injection pressures of approximately 20,000 psi and injection velocities less than 250 mm/s.
A hot runner system with sequential valve gating can play a key role in successful thinwall molding on conventional equipment, as well as equipment especially designed for thinwall molding.
“A hot runner system with sequential valve gating does several things for you,” says Martin Baumann, marketing manager, hot runners, Husky Injection Molding Systems Ltd. “It allows you to physically avoid flow lines, which is important for both strength and appearance. It also allows you to push material further because in effect it reduces the L/D ratio.”
One crucial function hot runners perform in thinwall molding is to reduce the pressure drop in the system so that most of the available pressure is used to push the melt into the cavity, notes Baumann. This is accomplished by maximizing the channel diameter. Husky’s 750 and 1250 series of hot runners are commonly used for thinwall applications and, depending on the application, may be modified with higher strength materials for the manifold and nozzles.
NEW MATERIALS PUSH ENVELOPE
GE Plastics has recently made commercially available a new grade of its CYCOLOY PC/ABS resin specially designed for thinwall molding of large (17 in. and above) flame-compliant, computer monitor housings. The material, CYCOLOY CU 6800, has high flow needed for thinwall, but also processes at a lower melt temperature than other resin grades, says Kurt Weiss, GE’s thinwall program leader. Additionally, the resin is able to pass the impact and shock drop testing required of computer housing equipment at wall thicknesses of 2.5 mm and below. Large computer monitors are a potentially huge application for thinwall molding, notes Wiess, as the market is seeing a trend toward larger monitor sizes.
Weiss says thinwall molding can play an important part in the meeting the contrasting market demands for lower costs, on the one hand, and eco-compliance and improved product aesthetics on the other.
A new material with potential thinwall applications is Trexell Inc.’s Mucell microcellular foam process, says Engel’s Strohmaier. In the Mucell process, foaming in the material of choice is achieved by the introduction of liquid carbon dioxide or liquid nitrogen. The liquid gas is introduced in a section of the screw into the melt under certain conditions of pressure and temperature so that the gas remains in solution, creating a single phase solution with a much lower viscosity. The net result, says Strohmaier, is that a molder can reduce the processing temperature of a given material by as much as 100F, and still maintain high flow. Because the micro-cellular bubbles are only five to ten microns in diameter, finished part integrity and properties are maintained, even in thinwall sections.
Strohmaier reports that the process can be used to make part sections as thin as 0.0001 in. thick. While the process
produces swirling effects in the surface finish, in components with appearance requirements this could be overcome by in-mold labeling, Strohmaier says. Potential applications for the Mucell process are cell phones and under-the-hood automotive. Engel entered an agreement with Trexell earlier this year for the purpose of developing new applications and processing methods for the Mucell process.CPL