New technique permits fast, ultra-thin-wall molding.

New Technique Permits Fast, Ultra-Thin-Wall Molding

A new manufacturing technology for injection molding ultra-thin-wall containers and cups is being made available by Primtec, Rancho Santa Fe, Calif., an R&D company that licenses its technology to molders. The Flow Filling process is said to enable molding parts as thin as 4 mils as well as very thin barrier layers. Other benefits of the technique are said to be low clamp requirements, automatic core centering, and very high strength-to-weight ratio.

RIBS ARE KEY

One key to the process is designing the part with parallel ribs as flow channels, about 3/4 in. apart, up the sides. The ribs may be located on inner, outer, or both surfaces. Typically, the ribs range from 0.02 to 0.04 in. thick, and the walls between them are 0.004-0.008 in.

During injection, the melt flows first along the ribs and then spreads sideways from the ribs into the side wall, where it instantly cools and solidifies. The melt in the base wall and the lower side wall solidifies long before the mold cavity is fully filled and full injection pressure in the cavity is reached.

This has implications both for core centering and clamp force. The solidified plastic of the side wall between the ribs locks the core in place early in the cycle, preventing the core from moving off center. Meanwhile, the required clamp force is low because most of the base already is solidified before high pressure is reached in the cavity. Only the ribs remain fluid for the whole cycle, and clamp pressure is not required to hold together parts of the mold in which the plastic has solidified.

Because of low mold cavity strain, mold wear is reduced and mold inserts may be small. The combination of small mold inserts and low clamp requirements is said to allow high productivity because many cavities may be placed in the molding machine.

DEMONSTRATED AT NPE 

The Flow Filling process is best suited to high injection speeds. The desired injection ram speed (forward motion of the screw) for a single-cavity mold is about 200 mm/sec and about 1000 mm/sec for a 16 + 16 cavity stack mold.

The technique was developed on injection machines from Netstal-Machinery Inc., Fitchburg, Mass., and was demonstrated on a 90-ton press at the Netstal booth during last June's NPE show in Chicago. There, a flower pot mold was running at a cycle time of 1.9 sec, producing 6-in.-diam. pots of homopolymer PP (70 MI) with walls 6 mils thick and weighing only 15 g - about half as much as conventionally molded pots. Primtec estimates that the new process will make possible the manufacture of 10-oz homopolymer PP drinking cups in a 16 + 16 stack mold on a 350-ton press with a 4-sec cycle. Complete manufacturing costs - including resin and machine time - would be only $15 per 1000.

Parts molded by this process are said to be extremely tough, partly because the flow direction in the ribs is nearly perpendicular to that of the side walls between the ribs, producing a biaxial melt orientation effect. Tests have shown that products made of homopolymer PP "are virtually indestructible to human hands," according to the company. Also, the ribbed flow channels provide extra strength for stacking. And the almost instantaneous cooling of the thin walls is said to achieve high transparency with PP homopolymer.

NEW APPLICATIONS SEEN

The process opens up a number of new possibilities for thin-wall injection molding, according to Primtec president Ole Sorensen. One promising application is multilayer containers. The Flow Fill process can produce barrier layers as thin as 2 mils, reportedly with a higher degree of control than can be achieved by alternative methods such as sheet thermoforming and injection blow molding. Sorensen predicts that multilayering may account for about half of his process' potential uses. He adds that, in addition to achieving good toughness and transparency with inexpensive homopolymer PP, Flow Filling also has had good results with nylon.

Nonexclusive license agreements may be negotiated with Primtec, and exclusive licenses are available for certain niche products. There is a royalty fee of 2.5% of plastic product sales.

PHOTO : This 6-in.-diam. flower pot has a wall thickness of only 6 mils and is molded at a cycle time of 1.9 sec. Parallel ribs make possible mold filling at this speed and consistency.