Canadian Plastics

INJECTION MOLDING: Do More with Less

For smaller-sized shops, mounting a growth-oriented strategy based on innovation may be difficult, as resources for investments in new machine technology can often be scarce. These molders may be over...

September 1, 2001   By Michael Legault



For smaller-sized shops, mounting a growth-oriented strategy based on innovation may be difficult, as resources for investments in new machine technology can often be scarce. These molders may be overlooking using or implementing technologies that require nominal cost, and add significant value.

NEW WAYS TO “SKIN” A CAT

Ube Machinery Inc.’s DIEPREST technology is a skin-insert molding technology which allows molders to inject plastic to a fabric, vinyl foam, sheet film or pre-formed skin insert without using an adhesive or special mold. While the technology has been commercialized for only about four years, automotive companies, including Toyota and Nissan, are utilizing it for a number of automotive interior applications, according to Taku Tawarada, marketing and sales manager, Ube.

One of the main advantages of DIEPREST is that it can be retrofitted to a standard electric machine, with a high-level control system, Tawarada says. The machine can still be used for other types of molding, not only skin insert molding, thus giving an operation greater flexibility. Electric machines are required, says Tawarda, because the technology requires highly accurate and repeatable clamp movements, with less than fractions of a millimeter variation.

The heart of the technology is a multi-stage clamp control software which precisely controls clamp movement through a servo-valve controlled, closed-loop clamp unit. In the process, a vinyl or TPO skin is inserted into the mold cavity and the plastic is injected behind it. Ordinarily, gluing or other low pressure technologies requiring special machinery are used to bind plastic to skin, because high injection pressures in the cavity will destroy the grain of these skins. DIEPREST overcomes this limitation by controlling clamp movement.

In one configuration, DIEPREST C, skin is inserted into the mold cavity and the clamp is closed. Plastic is then shot behind the skin, while the clamp, as a result of the injection pressure, is allowed to open. The clamp is then closed to form the part, then opened quickly before damage can be done to the skin.

Tawarda says the process does not add to cycle time, as clamp opening and closing occurs during cooling time. There are several other DIEPREST configurations specific for particular processes or materials. In addition to automotive, the technology is being targeted for appliances, furniture, medical equipment and consumer products.

In-mold film technology has been used for some time to add decoration and other printed effects to the surface of molded parts. More recently it is seeing increasing use as a replacement for painting on a variety of plastic parts. Using in-mold surfacing film, in conjunction with gas-assist molding, enhances the benefits even further, says Karl Berdan, technical sales representative, Bauer Plastics Technology Group.

Berdan is currently helping an automotive OEM set up an in-mold film production line for body side moldings to be used on a popular sport utility vehicle for the next model year. The production module consists of four injection molding machines ranging from 125 to 500 tons in clamping force. The film used on the moldings is non-preformed or flat film, which, compared to pre-formed film, requires less handling and is less expensive. Film is indexed from a roll, where it is molded onto a TPO substrate. After molding the part is laser trimmed, treated with an adhesion promoter and strung with double-sided tape (See diagram).

“The film eliminates the cost of painting, which is enormous,” says Berdan. “Also, the gas-assist takes out about 15 seconds of cycle time and the hollow part is more rigid, which makes it easier to attach the double-sided tape.”

ADDING PRESSURE — MORE OPTIONS

A twist on conventional gas-assist molding, in which nitrogen gas under pressure is injected through a channel or pin into the plastic, Incoe’s external gas molding (EGM) applies gas onto the surface or selected surface of plastic flowing into the mold. Much like regular gas-assist, EGM can be used on long flat or contoured parts to eliminate sink marks and surface blemishes, reducing required clamping tonnage requirements in the process. However, according to John Blundy, Incoe’s vice president of business development, the similarity ends there.

“Conventional gas-assist is often used to pack out ribs and bosses, but it’s not without problems, such as permeation of gas into thin-wall sections of the part,” says Blundy.

EGM requires a nitrogen source and a simple gas delivery device. Gas is injected at pressures typically below 1000 psi through the core side of the mold. Blundy says EGM is ideal for any part that has a rib structure on the core side, and as well is targeted for parts with thin-wall sections, such as cell phones, computer housings, business machine covers and automotive instrument panels. In the case of the latter parts, EGM may be a more economical substitute for conventional gas assist molding.

“You don’t have to design in extra thick ribs to create a gas channel with EGM,” notes Blundy. Additional benefits of EGM include up to a 60 percent reduction of required clamping tonnage, and production of a stress-free part. EGM can also save material costs when molding materials with high shrinkage rates, such as PC/ABS. The gas is used to pack out the plastic, taking up the volumetric shrinkage of the material in the mold. Incoe is licensing the technology to molders in North and South America.

Battenfeld will introduce its new water injection technology, Aquamold, at this year’s K Show. Aquamold allows the production of very thick-walled parts such as pipes and handles in much shorter cycle times. As the process cools both the interior and exterior of the part simultaneously, cycle time reductions of up to 70 percent, compared to gas-assist, have been achieved in trial runs. Injection of water also creates an interior with extremely smooth interior walls.

“There’s going to be some definite applications for it,” says Bruno Lamont, regional manager, Battenfeld Canada. At the K Show Aquamold will be used on a Battenfeld TM 2100/100 injection molding machine to mold a multi-colored beach ball racket.

Aquamold requires pressure control modules that are specially designed for the requirements of injecting water under pressure. The modules are stand-alone units and are moveable from machine to machine, according to Lamont. Additionally, the process needs a specific entry point for water into the mold, as well as a drainage point. Battenfeld will work with the molder to ensure proper design and set-up of a particular application, says Lamont.

TINY BUBBLES, BIG SAVINGS

MuCell is a microcellular foam process which, when used in injection molding, provides a reduction in part weight and material usage, decreased cycle time and other benefits. It has been on the market for several years, but many molders are just beginning to understand the unique processing requirements that make it different from any other injection molding technology.

Developed and licensed by Trexel Inc., MuCell employs gas, in the form of a supercritical fluid (SCF), to create microcellular bubbles, or cells, ranging from five to 50 microns in diameter within the plastic. While it is not suitable for parts requiring a glossy, Class A surface finish, MuCell is being targeted for automotive under-the-hood applications, HVAC components, electrical/electronic parts such as fuse boxes and connectors, and business equipment. Molders purchase MuCell through injection molding machine manufacturers. At present, ten injection machine OEMs offer MuCell technology with their machines. These OEMs are Arburg, Battenfeld, Engel, Ferromatik Milacron, Husky, Italtech, JSW, Krauss Maffei, Toyo and Van Dorn Demag.

While MuCell can technically be used with all thermoplastics, its particular niche is engineered, filled resins used in high-volume applications, where the payback is quickest, says Dan Szczurko, Trexel vice president of business development.

“One of the main benefits offered by MuCell is a reduct
ion of cavity pressure, which in turn reduces molded-in stress and improves part dimensions,” says Szczurko. Because MuCell makes the plastic less viscous, it also reduces requirements for machine clamping tonnage.

Engel recently sold a 600-ton tiebarless machine equipped with the MuCell process. The customer is using the machine to mold an HVAC component previously molded on a 1500-ton machine, according to Kai Jacobsen manager of machine sales and technology. Jacobsen says Engel has also sold MuCell-equipped 300-ton and 400-ton tiebarless machines in the last four months into the automotive market.

“We’re seeing a lot of interest in the combination of MuCell and tiebarless technology,” says Jacobsen. He says Engel has sold about 40 MuCell-equipped machines worldwide, and estimates that better than two-thirds of those machines have been tiebarless. Engel has a test machine available for MuCell development work at its technical centre in Guelph, ON.

As well, Husky Injection Molding Systems offers molders MuCell application development and support through its Detroit Technical Center in Novi, MI, where it has installed a 175-ton MuCell-capable S-Series machine. Husky is offering MuCell on machines ranging from 65 to 8000 tons.

Szczurko estimates there are now several dozen commercial MuCell applications worldwide. “A positive sign is that we are getting repeat orders. The technology is not only being used, but it is being deployed.”

No doubt, this is also a sign that molders are learning that expanding their arsenal of molding technology is in the long-term benefit of themselves and their customers.


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