Coex process for PET bottles tightens layer control, cuts tool cost.

As an alternative to coinjection molding, a novel extrusion-based approach to blowing multi-layer PET containers is said to offer superior layer-thickness control and inexpensive tooling. The new machine, which is being developed for two customers by Sabel Plastechs Inc., Moscow, Ohio, is currently being applied to making small unoriented bottles of under 8 fl oz (see PT, Dec. '96, p. 30). Biaxial orientation will be incorporated in future models, according to the patented-machine's inventor, Sam Belcher. He plans to have a working model of the machine at NPE in Chicago this June.

HOW IT WORKS

Belcher's machine consists of a 1.5-in. extruder, 32-in. vacuum trough of cooling water, and twin blowing stations. It works by extruding a horizontal pipe of amorphous PET (APET) into the vacuum sizing bath. Two blow molds - one on either side of the extruded pipe - operate in a hand-over-hand motion, alternately closing over a section of the pipe and separating a length of parison from the continuous tube. The alternating operation of the molds acts as a puller. After the mold closes over the parison, the tool indexes to the clamp at the side, where blowing takes place.

The machine is driven entirely by electric motors. Each mold station has a 3-sec cycle, giving the machine a total production rate of about 2400 bottles/hr. Belcher notes that outputs could be increased by extruding multiple tubes simultaneously.

So far, Belcher has focused his efforts on bottles for pharmaceutical, medical, hair-care, and toiletries markets. These bottles are oriented only in the hoop direction. Biaxial orientation can be accomplished by using the mold to pull on the tube in the longitudinal direction. This feature will be included in future models.

TIGHT LAYER CONTROL

Extruder speed is coordinated with the mold movements by means of closed-loop microprocessor control. "We can adjust the speed very quickly. That gives you the control, SO you don't have the tube sagging or are not pulling too hard on it," says Belcher.

For added control over wall thickness, the machine can extrude "bump" tubing, so as to achieve lightweighting or put a little extra material in a certain area of the parison. Belcher says an advantage of extruding PET is that it has only about 2-3% die swell, which makes wall-thickness control that much easier.

Belcher says his machine is well suited for producing multi-layer containers because the simple extrusion technique provides close control over thin layers. "You can put 1-2 mils of an item in there," he says. Belcher is now coextruding three layers, but he believes five layers are feasible. Coextrusion makes it possible to put an adhesive polymer between layers to prevent delamination of the bottle.

Scrap rates for this process are said to be relatively low, at 3-5%. Cost of tooling - which includes two extrusion blow molds and an extrusion die - totals only about $20,000. Approximate price of the machine with tooling is around $250,000.