Compression stretch-blow molding: new way to make PET containers.

* There's a new process in development for making oriented PET bottles and cans. It doesn't use injection molded preforms. Instead, it starts with a blank or "puck" that is cut from extruded sheet and compression molded into a concave shape before being stretch-blow molded.

There is no cooling or reheating between the preforming and blowing stages. Developers of this "compression stretch-blow molding" (CSBM) process believe it may let molders use less-expensive lower-intrinsic-viscosity (I.V.) or recycled PET to make containers with little or no acetaldehyde generation and no loss of performance.

CSBM is being developed at the newly established Valyi Institute for Plastic Forming (VIPF) at the University of Massachusetts, Lowell. The Institute is named for the late Dr. Emery I. Valyi, who invented and patented CSBM. The patents and licensing rights for the process are now held by VIPF.

CSBM reportedly offers the advantages of processing at lower temperature (around that of boiling water), lower pressure, and lower shear stresses than are encountered in injection molding. Less aggressive processing conditions should reduce PET degradation and subsequent property loss, and may enhance final properties such as clarity. The Institute expects CSBM-made preforms will result in PET products with mechanical properties equal to those made with injection molded preforms.

The process can make narrow-neck or wide-mouth bottles, jars, and cans. Starting with coextruded or laminated sheet, CSBM may offer a simpler and less expensive alternative to coinjection molding for producing multilayer preforms containing barrier and/or recycled layers.

VIPF is concentrating initially on making PET barrier-laminated cans that could compete economically with aluminum. A prototype single-cavity machine is being designed to produce 1200 12-oz cans/hr. Contributing to the machine's development is Herb Rees, former v.p of design at Husky Injection Molding Systems.

Not ready for prime time 

The CSBM process is not yet production ready but every element of its operation has been verified in making 12-32 oz containers, said Dr. Robert Malloy of VIPF in a paper at April's Bev-Pak Americas '98 conference in Fort Lauderdale, Fla.

The Institute has issued two nonexclusive licenses on the process. Belvac Production Machinery Inc., Lynchburg, Va., a supplier of aluminum canmaking equipment, will eventually build the CSBM equipment. Meanwhile, Berry Plastics Corp., Evansville, Ind., an injection molder of thin-wall packaging, plans to develop commercial containers with the process.

Get the most out of PET resin

The high pressure, temperature, and shear encountered in injection molding PET preforms can degrade the resin, reducing its molecular weight and mechanical properties and generating the byproduct acetaldehyde. "This fact pushed the industry to use higher-molecular-weight, high-I.V. polymers to compensate for the drop in performance," says Dan Weissmann, a senior fellow at VIPF and former director of development at Schmalbach-Lubeca Plastic Containers USA Inc. in Manchester, Mich., a major PET bottle maker. Molders expect a 0.02-0.04 I.V. loss when preforms are injection molded, Weissmann says. So the more costly, high-I.V. PET resin that is injection molded into a preform yields the properties of a less expensive resin in the finished bottle.

The CSBM process reportedly sacrifices no I.V. because the resin is formed at a lower temperature. This gives blow molders an opportunity to use a lower I.V. PET resin or to use the same starting material to obtain superior mechanical properties.

VIPF estimates that while some preform injection molding cycles are as short as 14-15 sec, it will take only 3 sec to compression mold a puck. The CSBM process is predicted to run at 1000-1300 bottles/hr per cavity, a rate competitive with reheat stretch-blow molding, Weissmann says.

From puck to preform

The CSBM process uses a fiat or concave puck that can be made by various methods. Die-cutting pucks from extruded sheet is probably the cheapest method, Weissmann says. Another approach is continuous forming or embossing, in which the PET is cast through an extrusion die onto a rotary mold (a wheel with a cavity and a mating wheel with a protruding core).

The puck is heated to a point just above PETs glass-transition temperature. Compression molding takes place at temperatures more than 250 [degrees] F lower than injection molding and at pressures only one-tenth as great.

PETs typical blow molding temperature is about 210 F, similar to that for compression molding. The CSBM preform is transferred while still at its forming temperature from the compression mold to the blow mold. There is no need to cool and then reheat the preform, unlike other PET processes. The result is both energy and time savings, says Weissmann.

CSBM is less expensive than an injection molding preform system, where the injection tool can cost around $10,000 per cavity, Weissmann notes. A typical two-stage or reheat system may need five or 10 preform cavities to feed one blowing cavity. Hence the potential savings on preform tooling for the CSBM machine can be $50,000 to $100,000 per blowing cavity.

The integrated or one-stage machine typically has one injection preform cavity to one blow mold cavity - the same as in CSBM. However, the one-stage machine runs at a much lower output rate, Weissmann says.

The Institute's prototype machine will compression and blow mold in tandem. A combination radio-frequency (RF) and convection oven will keep the puck hot as it travels down a conveyor to the blowing unit. The machine can be fitted with a barrier sleeve inserter to add a barrier layer to the preform just before it is stretched and blown.