New PET preform machines pack more capacity in less space.

New space-saving, high-capacity systems for injection molding PET bottle preforms were recently introduced by two equipment suppliers. Netstal Machinery Inc., Devers, Mass., and Husky Injection Molding Systems Ltd., Bolton, Ont., both announced new 96-cavity preform machines at the Bev-Pak Americas Conference held in Fort Lauderdale, Fla., in April. Both machines are said to offer unusually high ratios of productivity to floor-space footprint, as well as cycle-time improvements when thick preforms are produced.

Small size, high output

Netstal broadened its PET Line with a new dedicated two-stage injection machine that has double the cavitation of what had been its largest model, the PET Line 48. Yet the new 96-cavity system has the smaller unit's overall dimensions, and it operates at cycle times no more than 0.5-sec longer than the 48-cavity model, says John Scarpucci, North American manager of PET systems.

According to Scarpucci, an 18-g preform for a 0.5-liter non-carbonated mineral-water bottle having a ribbed shape, flat bottom, screw cap, and 2.5-mm (0.098-in.) wall thickness can be molded in approximately 11 sec with the new machine compared to 10.5 sec with the 48-cavity unit. The mold is built by Otto Hofstetter AG in Germany.

Netstal used some unusual approaches in order to keep the dimensions of the new 600-ton, toggle-clamp machine to the same 8.85 x 36.08 ft size of the PET Line 48. Rather than design a whole new screw, Netstal equipped the new machine with the same 110-mm, 23:1, low-shear screw - two of them, in fact, to double the plastication rate. Both screws feed the same central shooting pot, which has a larger volume capacity and larger plunger than on the smaller machine. "Lock-to-lock" time is estimated at 3.5-4 sec.

Space savings also come from use of a top-entry rather than side-entry gripper arm, which has two servo drives in order to retain the same speed (9.8 ft/sec) as the 48-cavity robot with one drive.

For post-mold cooling, the preforms are held by the gripper arm for one full cycle before they are placed into one of three cooling stations above the machine. Preforms remain in the cooling stations for three more complete cycles and are finally released onto a conveyor.

All process variables are controlled by Netstal's Sycap closed-loop process control. Tight process regulation is what allows the PET Line to consistently mold preforms that are within the strict 3-ppm acetaldehyde limit (ground-preform method) of the French Mineral Water Quality Standard, Scarpucci says.

New rotary-platen models

New models are also coming to Husky's unusual Index series of two-platen preform injection machines, according to Michael Urquhart, Husky's v.p. of service and sales for the Americas. The series was introduced last spring (see PT, June '97, p. 19). The novel design, whose two-stage injection unit, servo hydraulics, and GE Fanuc controls are the same as those on Husky's standard G-PET machines, uses a four-station rotary turret on the moving platen to perform simultaneous injection and external cooling without the need for a robot.

The only model available now is a 250-ton model (IN250PET) that handles 32 or 48 cavities with a standard pitch and up to 33-mm thread. It can mold a 47.5-g, 4-mm-thick preform for a 2-liter bottle in 12.8 sec, versus 22 sec for a conventional 600-ton, 96-cavity machine. In other words, it offers the productivity of a 72-cavity system while occupying less floor space.

This model will be joined by a 90-ton model with eight or 16 cavities, which will first be shown at the K'98 show in Dusseldorf this October. A month or two later will see the arrival of a 400-tonner with 64, 72, or 96 cavities. A larger-pitch option for up to 43-mm neck finish offers six or 12 cavities on the 90-ton Index model, 24 or 32 cavities on the 250-tonner, and 48 on the 400-tonner.

At their largest cavitation, output rates for a 600-ml, 28-g, 4-mm-thick preform are estimated at 40 million/yr for the 90-ton, 110 million for the 250-ton, and 205 million units for the 400-ton model. Husky is also experimenting with a two-face in addition to the four-face rotary-platen machine that will reduce tooling costs while maintaining high productivity on thin-wall mineral-water preforms.

Urquhart also reported that there have been several improvements in the Index machine since it was first introduced:

* Faster hydraulics yield shorter dry-cycle times.

* A new screw design reportedly gives improved melt quality while permitting 30-40 [degrees] F lower stock temperatures.

* Improved mold cooling gives better preform quality at the faster speeds. There's also more efficient external preform cooling on the cores. Together, these two enhancements improve cycle times by 20%.

* Improved control screens are more user friendly and provide SPC trend data on more than 20 variables.

Husky reported last year that gate crystallinity is greatly reduced with the Index machine - presumably an effect of immediate external cooling. Now the company is investigating the possibility that improved gate quality may permit lightweighting preforms by up to 0.5 g.