By Design: An expensive over-design

' 2001 report entitled "Injection Molding: Emerging Strategies for Growth, Development - and Survival," there were 359 injection molding companies engaged in the packaging, closure, and container business. The years since have brought consolidation to the industry but in April 2010 trade weekly Plastics News reported that there are still 320 injection molders involved in those markets.

Through good engineering, copying a competitor, trial and error, or luck most OEMs sooner or later wind up producing their products with the best material and manufacturing process. Fortunately, injection molded thermoplastics are the best combination of material and process for many rigid packaging applications. The injection molding process provides a unique combination of attributes that are important to the packaging industry.

Packaging Attributes

1. Injection molded parts contribute to the important attention-getting

point-of-sale, impulse-buying decisions via their seductive shapes and reliably

attractive surfaces that can be transparent, metalized, or of any color, with a

lustrous polish or textures that replicate other materials.

2. This process' ability to produce simple, open containers and complex, precision

components allows an endless variety of snap-fit or threaded childproof, tamper

evident, push-pull, flip-top, hinged, and liner-less dispensing closures, aerosol

valves, and mechanical pumps capable of delivering a stream, a mist, or foam,

adding greatly to customer convenience and sales.

3. The inherent efficiency of injection molding, coupled with the use of multiple-

cavity molds, allows the process to provide all of the above attributes while

producing very large quantities of relatively thin-walled parts at a cost that

is acceptable for single use packaging components.

All of these attributes contribute to injection molding's success in packaging. The most important of these attributes and the one that attracts OEM buyers is the ability to produce large quantities of low-cost parts. These low costs can only be achieved by those who understand the capabilities of the process. It is the failure to understand and use those capabilities that provides us with the following case history.

A Major Case of Over-Design

The subject of this story is the dispensing and metering closure shown threaded onto the neck of a bottle in Figure 1. During shipping and storage this assembly is provided with a friction-fit over-cap that is not shown. In this view the outlet is sealed by the spring seating the upper cone-shaped rubber seal against the threaded closure.

In use, the assembly is inserted into a chemical processing machine. A plunger pushes on the lower cone-shaped rubber seal and compresses the spring until the lower seal seats against the threaded closure, as shown in Figure 2. This creates an open space between the two cone-shaped rubber seals. A specific volume of chemical reagent flows into this open space. The plunger then descends and the spring unseats the lower cone-shaped rubber seal, as shown in Figure 1. The chemical reagent then flows into the processing tray. This process is repeated whenever additional reagent is needed.

My first encounter with this project was a request to determine the feasibility of automating the assembly of the dispensing and metering closure. The assembly, as shown in Figure 2, is made up of nine parts: (1) the threaded closure, (2) a paper gasket, (3) a stamped and formed corrosion-resistant stainless steel (ss) support for a, (4) ss spring and, (5) a screw machined ss spindle, (6) a ss washer, (7) the upper cone-shaped rubber seal, (8) the lower cone-shaped rubber seal, and (9) a ss E-type retaining ring. It quickly became apparent that this assembly had not been designed for automatic assembly.

Discussion with the customer revealed that they had reached the same conclusion and were only seeking a confirming opinion. It was also revealed that the cost of the components and the assembly of the unit were such that the customer would consider redesigning and retooling the whole assembly. The redesigned assembly must, however, fit and function with the existing chemical processing machines, which could not be changed.

The goals of the redesign were to simplify assembly and reduce cost while retaining the original function. One way of simplifying assembly and minimizing part cost would be to reduce the number of components in the unit. There were several design iterations. The best of these is shown in Figure 3. The redesigned unit achieves both goals by reducing the part count from nine to four: (1) the threaded closure, (2) the spring, (3) the closure liner or gasket, and (4) the one-piece seal that replaced the upper and lower cone-shaped rubber seals.

All four parts were injection molded. The closure was polypropylene, the spring was acetal, and the gasket and seal were an olefin-based thermoplastic elastomer.

Assembly consisted of (1) pushing the spring stem into the seal, (2) the three spring elements extending beyond the flange were engaged to locate and retain the gasket, and (3) this sub-assembly was then pushed into the closure. One detail that made this possible was that the seal was designed thin enough to deform and pass through the small outlet opening on the closure.

The Secret of Success

The secret to the success of this part's consolidation project was the molded spring (Figure 4) of the type pioneered by the Dab-O-Matic Corporation of Mt. Vernon, NY. This one-piece part also incorporates the mounting stem for the seal, the flange for sealing against the bottle, and the three extending spring elements that locate and hold the gasket.

Not everyone is familiar with these molded springs and what they can do. Many of those who are aware of them will be discouraged by the apparent difficulty of molding a part of this shape. Such concerns are unfounded, as these springs do not require unscrewing or complex side acting molds. Properly designed these springs will unscrew themselves from a core pin with a simple stripper plate mold.

The original dispensing and metering closure was probably designed by an engineer who was more familiar with metal and rubber than plastics manufacturing processes. The failure to utilize the molded springs, which were available at the time, resulted in a costly over-design.