Building a Better Molding System for a Better Mousetrap

Manufacturers continuously strive to develop better mousetraps for their production needs. For Bell Labs, this isn't just a figure of speech - it's literal. The Madison, WI, company has been supplying rodent traps to the professional pest control market for more than 25 years. Bell Labs' two latest `better' mousetraps are the result of a fast track project that drove its engineering department to develop an equally innovative injection molding solution.

Dan Johnson, Bell Labs' engineering manager, was on a tight production schedule and needed inventory posthaste on the new three- and 10-component traps. However, trap assembly couldn't start until he had batches of all 13 components. 

The shop already had three Milacron 550-ton injection molding machines (IMMs) with hot runner systems pumping out parts for other trap models. Johnson purchased a Milacron 120-ton IMM for the new traps and wanted to use hot runner technology on that machine to keep the shop "sprue-picker-less."

Realizing family molds for each trap were not the answer, Johnson's moldmaker suggested combining D-M-E standardized hot runner technology with a Master Unit Die (MUD) molding system for a quick-change, hot runner molding setup. Companion multi-cavity mold inserts can be changed in minutes, not hours, without removing the mold frame or hot runner manifold. 

This flexible hot runner setup facilitates short runs on each component, allowing Johnson to build stock in all and begin assembly. It also eliminates the hassles associated with sprue pickers, regrind and part sorting.

Unique Processing of Unique Traps

One of the two new traps is based on the classic snap trap design and uses three injection molded components with a spring. The other trap is more intricate in design, having 10 molded plastic parts. This model is a repeating mousetrap that can collect multiple live mice. Drawn by the scent of bait, the mouse enters the trap and actuates a trigger that flips the mouse into the trap's isolated collecting area. The trap is immediately reset to catch another mouse.

Eleven of the 13 components are run on the new machine. Trap components include gears, flippers, cases and lids that are molded from virgin polystyrene, K-resin and ABS. Critical dimensions on articulating components are held to q0.005".

Johnson approached Mike Westfahl at MJP Tool to develop a system that could not only accurately produce the trap components, but could be built within three weeks. He left the project somewhat open-ended for Westfahl, counting on the moldmaker's 21 years of experience to develop the most appropriate system for this application. "This was definitely a fast track project," says Johnson. "Mold design actually started before all of the part designs were completed."

Westfahl knew the only way he could meet the time constraints was by purchasing off-the-shelf components. "A custom manifold normally takes five to six weeks for delivery," says Westfahl. "And since mold design actually started before product design was finalized, we couldn't order a custom manifold because the nozzle port locations weren't finalized." 

After performing research and receiving advice from D-M-E and MUD, Westfahl decided to go with D-M-E's Flow-Mate four-drop hot runner manifold and a MUD standard quick-change frame system. Both Johnson and Westfahl had successfully worked with D-M-E in the past and knew they could count on quick delivery. "In fact, the manifold was delivered the next day after ordering," says Westfahl.

The challenge for Westfahl was to locate the nozzle ports on the manifold so that they were common to each of the 11 mold units. Likewise, cavity layout was somewhat challenging because variations in part depths make it necessary to sometimes inject at different points on a part. Mold units varied between two and eight cavities. 

Westfahl located nozzle ports and mated the Flow-Mate manifold to MUD frame within the three-week timeframe, despite never previously assembling a system such as this. "The Flow-Mate design was easy to work with," says Westfahl. "D-M-E had various sizes available, so I picked the one that matched the size and locations I was trying to hit for the four nozzle ports."

The Flow-Mate manifold is pre-engineered and externally heated. Two- and four-drop "in-line" and "X" versions are available in a variety of sizes. The manifolds have pre-machined, 10mm-diameter flow channels with factory-installed, replaceable tubular heaters. Nozzle port locations are machined by the customer, providing a high degree of flexibility.

MUD Frame Flexibility Pays Off

This was Johnson's first experience with the MUD quick-change system. "Molds may stay on our other machines for months at a time, so quick mold change was never really an issue," says Johnson. "But since we had 11 parts for one machine with relatively short runs, we needed to have something that allowed simpler, faster changeovers."

Johnson admits his shop crew might not be in the same class of some custom molders, but if they are changing only mold units, and not material, changeover on the MUD system takes only 30 minutes. "And this is with guys that have only done it a couple of times," explains Johnson, "so they are sure to get faster." Two mold changes per day are typical.

The new machine with D-M-E hot runner/MUD frame setup has been successfully running 24/5 production since November 1999, allowing the company to ship on time. "This is a great setup for shops looking to run numerous multiple-cavity molds through one machine without dealing with cold runners," says Westfahl. "In this case, one manifold satisfied 11 different parts."