Aluminum Epoxy Tooling Project - A Case Study

What do you do when a company calls you out of the blue and says that they need two new molds and a load of injection molded parts in a month? Well, if you're a manufacturer that wants the business you have to be prepared to move fast - sending price quotes and hoping your design and build processes come together without a hitch. Many choices need to be made during this process and with every choice is the possibility its outcome may affect the well-being of the tooling, the process and the manufacturer.

In one such project, designers at Line By Line Design (East Aurora, NY) - an injection mold tooling manufacturer - found themselves in that spot when fielding a telephone inquiry from Keilhauer - a Canadian-based furniture company looking to outsource some work. The parts Keilhauer needed developed were two office chair frames - one in glass-filled rynite and one in polypropylene. The parts were needed for a new chair that was being launched at Neocon - a furniture trade show scheduled for the following month.

The size of the project was daunting. LBLD had to acquire special handling equipment in order to move the molds around the shop, which each weighed 1,500 pounds - 10 times the size of anything LBLD had ever handled. The parts were quite large as well, measuring 2' x 2'. But on the plus side, both companies ran with the same Pro/Engineer software system, which helped to streamline the project.

Before signing on the dotted line, Keilhauer had two concerns:

1. Did LBLD have the capabilities to produce such large rapid prototype molds?

2. Could its tools run 30 percent glass-filled rynite, which is known to be fairly abrasive?

Having the capabilities to work with these materials would turn out to be essential in the tooling development process.

"We had never used Rynite on a project before and because of the importance of the trade show, we needed to be confident that LBLD could successfully produce the parts in the specified time frame," says Ron Blais, a product designer at Keilhauer. "The type of design we were attempting required an awful lot of testing. We were working with an entirely new chair."

After LBLD indicated it could do the job, Klein and his team immediately set about creating a mold design and building stereolithography (SLA) masters of the parts. All of the surfaces on the SLA model were then smoothed to a fine finish. Measuring larger than the SLA's limits, the parts had to be built piecemeal - together with the tolerances of the part and heat treated to about 300ºF. Completing the molds and heating them to about 500ºF, LBLD designers ran them in a 450-ton press, running 10 parts of each. Low pressures were used with a high fill rate, so they could get as much material into the tool at the lowest possible pressure as fast as they could. Melt temperature for the rynite was about 550ºF and for the polypropylene it was 420ºF.

Knowing when to heat the rynite was important because once it cools down it is hard to remelt again. The LBLD team also had to run 30 percent glass material, which many places don't like to run partly because it can be very abrasive.

"Because cycle times were not critical here, we could experiment with the mold temperatures a little bit," says Joe Cosenza, mold designer at LBLD. "Knowing when to ramp up to it for short shots then moving to fill the part out completely."

The life expectancies of filled epoxy in prototyping material for unfilled polymers is about 50 to 200 pieces - with a one-half inch minimum core diameter and a 1:1 height-to-diameter ratio. Like silicone rubber tooling, epoxy tooling requires a master pattern. The pattern is created using an RP process. When the pattern is completed it is then embedded in a parting line block to create the mold's parting line. Metal inserts are placed in areas where epoxy is unlikely to withstand pressures of the injection molding process.

Epoxy is then cast against the pattern and parting line block combination to create the first side of the tool. Once the epoxy has cured, the assembly is inverted and the parting line is removed - leaving a pattern embedded in the first side of the tool. The second side of the tool is then cast against the first.

"With epoxy tooling you have to address the critical areas, like do we need aluminum inserts or have to modify the geometry without affecting the engineering criteria," says Jim Klein, project designer at LBLD. "You also have to identify the mold layout; where you're going to put your water lines and how you're going in with your ejector pins. Here, patience is everything."

After taking the parts back to Toronto for assembly and testing, Keilhauer realized that the rynite part did not perform as well as intended. The rynite's characteristics were key to the part's workability. The tools were returned to LBLD for design modifications. Klein said his team immediately found that the tool's spring section had to be changed from about .300 to .500, running 10 parts using 30 percent glass-reinforced rynite and another 10 with 45 percent glass-reinforced rynite.

After testing it was determined that using the 30 percent glass fill worked perfectly. Klein said his company learned some valuable information about gating and cooling from working with the prototype epoxy tooling. From gating LBLD learned that the gate size had to be increased in order to fill better and with cooling the company found that the sections toward the top geometry of the parts were thicker and therefore needed increased cooling cycle time. So, Klein said that both the gating and cooling had to be revised in order to make a better tool.

"The back had to be flexible, yet structurally sound. The material, molding, shrinkage, cooling and structural strengths had to all come together," says Blais. "Using the rapid prototype process allowed us to test the parts' performance, which allowed us to make modifications prior to proceeding with the more expensive permanent tools."