Continuous molding: parts handling can be a snap!

Tired of living with bottlenecks in secondary operations? Injection molding small parts in strips will streamline downstream insertion, assembly, and decorating.

With so many recent advances in injection molding technology, it's easy to forget that molding often makes up just one part of a much larger manufacturing process. To truly improve the efficiency of the whole molding operation, a processor must not forget about refining all the post-molding steps. It's surprising, then, progress has been made in the parts handling that leads up to secondary operations.

Once molded, today's small parts are still left as loose pieces as they proceed to successive steps such as assembly, insertion, decorating, and packaging. Too often an operator must manually select, orient, position, and place parts one at a time. In truth, some applications aren't entirely manual: Parts can travel to secondary operations via dedicated vibratory bowls, conveyors, and even sophisticated robotic systems. In all these cases, however, finished parts remain loose throughout the secondary operations, limiting the opportunity for high-throughput automation while increasing the chances of mishandling and quality-control problems.

An alternative approach to improving the efficiency of secondary operations centers on continuous injection molding, which produces long strips of small parts suitable for packaging on a reel. Each shot typically consists of several parts joined by a connective strip of the same material. From the reels, the connected parts can be fed through any number of secondary operations at speeds up to 12,000 parts/hr. By fostering faster, more accurate secondary-operation automation, continuous molding can increase yields and cut both labor and reject costs. Moreover, continuous molding helps distinguish a molder as a value-added supplier, imparting a real competitive advantage.

CONTINUOUS APPLICATIONS

Accomplished with standard injection machines, continuous molding requires only tooling modifications to accommodate the connection of successive shots. Continuous molding works by indexing each shot partially out of the mold and then fusing its connective strip with that of the next shot. Ultimately, this automatic process produces a continuous length of parts that can be wound onto a take-up reel. During secondary processing, the parts are separated and a vacuum system can remove the joining connective material, though some designs join parts directly to one another with little or no connective strip.

Applications for continuous molding and the subsequent automated secondary operations have few limits. As long as the part geometry allows it to fit on a reel (maximum size so far is 24 in. O.D. x 8 in. wide), the part can be a candidate for continuous molding. The following examples illustrate just a few ways in which continuous molding has transformed secondary operations:

* Complex assembly - An electronic "shunt" consisting of a plastic housing with an integral metal spring clip presented perhaps the toughest automation challenges. Originally the shunts were manufactured and delivered to the end-user in loose piece form. These loose pieces went to an electronics assembly operation where an operator would manually position the part on two adjacent pins to create a bridge between electronic terminals. Problems arose whenever an operator put the shunt on the wrong pin positions, causing the product to be rejected and sent to rework.

To prevent this problem, the shunts are now molded in a continuous chain. In one secondary operation, the plastic shunt housing unwinds into an insertion machine, which puts the spring clip in place. The shunts are then rewound onto a reel and shipped to the end-user. In the final assembly stage, the end-user mounts the reels on another insertion machine. This one features a programmable two-axis table that holds printed circuit boards. The shunt chain is fed into the inserter, which cuts individual shunts from the reel and places them on the proper pins. Because the shunts are properly positioned every time, this automatic system has completely eliminated rework at the end-user's facility.

* Insertion - In one recent application, continuous molding significantly quickened the assembly of a picture hanger made from a plastic hook and a light-gauge nail. The continuously molded plastic hook feeds from its reel through the head of a custom-made insertion machine, which automatically sets 10,000-12,000 nails/hr.

* Decorating - A multicolored medallion, which displays the logo for a Big-Three automobile company, is molded in continuous form. These parts are then indexed through a series of four different decorating heads, each stamping a different color on the medallion. Once dry, the medallions are put back on the reel for shipping to the customer.

QUALIFYING APPLICATIONS

Molders can determine the applicability of continuous molding by looking at the part volumes, the type of secondary operation, and the costs of the molding process. In our experience, users with annual volumes of 300,000 to 500,000 parts should consider continuous molding whenever there is a need for relatively simple secondary operations. At these volumes secondary operations will realize a payback from labor savings and increased yields, even considering the cost of specialized inserting and winding machinery. However, continuous molding may be less appropriate when an assembly has multiple parts that each require a different secondary operation - such as you might find with a toy assembly.

When considering implementation of either continuous molding or automated secondary processes, companies should seek a supplier that can offer turnkey expertise. The supplier should be knowledgeable of both the molding and secondary processes and should demonstrate the potential for integration. Ideally, the supplier would also offer technical engineering assistance for both design and manufacturing implementation.