Is single cavity net shape molding more efficient than multi-cavity?

Kagan Pittman

Conventional wisdom has it that more is better. Does this method of thinking apply to multi-cavity net shape molding versus single cavity? A growing number of sources are saying no.

For injection molding companies looking for the greatest flexibility in their production process, multi-cavity machines are not efficient. Multi-cavity machines are significantly larger and more expensive than their single cavity counterparts. Multi-cavity machines are also significantly less flexible. Let’s imagine the following scenario: Injection molding company A is competing with company B.

Company A purchases a 200-ton multi-cavity machine to produce a large number of parts simultaneously. Company B, purchases ten single cavity machines. Companies A and B set up their machines in about the same amount of time and begin processing a their molds. Then the worst case scenario occurs: One of company B’s machines break down and so does company A’s only machine. Company B continues to produce parts to meet their customer’s order while staff tend to the broken down single cavity machine. Company A experiences a complete process shutdown.

Multi-cavity machines are limited in their flexibility in other ways as well. “In many cases, net shape molding is not possible with multi-cavity due to part-to-part and cavity-to-cavity variation in the same cycle,” said Roy Schroeder, lead center manager at Freudenberg-NOK Sealing Technologies. “In many cases, a secondary operation is needed like trimming, deflashing or inspection,” Schroeder added. “Tooling in many cases is also costlier upfront and to maintain.”

Roy Schroeder, lead center manager at Freudenberg-NOK in Findlay, stands beside one of the companies 12 single cavity net shape molding machines.

The advantages of single cavity net shape molding
To work around these issues, Freudenberg-NOK is breaking the mold of conventional thinking behind batch manufacturing and proving that the single cavity alternative is more efficient. “Single cavity machines are counterintuitive in the minds of many, but it is the next logical iteration of lean manufacturing,” Schroeder said. “If you think about it, the easiest and most reliable way to make something is to make it one piece at a time.”

Freudenberg-NOK’s cavity approach uses compact, custom-engineered one cavity injection and compression molding machines to produce thermoplastic parts. Employing 12 single cavity machines in their Findlay, Ohio automotive Fluid Power Lead Center (LC), the company has noted improved part quality and reduced scrap and waste. Using single cavity machines, Freudenberg-NOK has experienced a drop in rejected parts per million from 20 or so, to around 1.

Single cavity machines have eliminated prototyping requirements, supplemental production steps and many unpredictable variables. “The same tooling used for prototyping is used for production, so no “redesigns” or new tooling is needed,” Schroeder explained. “The lead time for single cavity tooling is very short as well.”

Findlay facility expansion reaches completion
The same Findlay facility where the single-cavity machines are currently employed has recently completed a 43,000 square foot expansion, consolidating and expanding an existing engineering test lab. The new lab now accounts for approximately 2,000 square feet of the total and represents an investment of approximately $450,000 in equipment and building expenses. Providing new material characterization, failure analysis and mechanical property test capabilities, the project has created 25 new jobs at the facility.

The new lab will perform analysis and testing of 700 high-performance plastic parts, including thrust washers, seals and other components for the automotive industry. A new engineering test rig has been installed to allow measurement of torque loss. Collected data will be used to help improve custom engineered sealing solutions that reduce CO2 emissions and improve fuel economy. The plant currently manufactures 185 million seals and related components annually and anticipates the new addition will increase production by approximately 25 percent per year. Collectively, the plant renovations represent an investment of $8.6 million in construction and equipment upgrades.

Freudenberg-NOK will be adopting their successful single-cavity molding approach at the company’s other global locations.