Designing and building an injection molded prototype turned out to be a very difficult process.

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After several months the injection mold tooling was finished and we began testing the unit.




 

 

 

 

 

Designing and Building An Injection Molded Prototype


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Designing and building an injection molded prototype and going through the de-bugging process.

Since the engineering firm declined to get involved with the project, I decided I would design the prototype myself.  I didn't get far though.  Our injection molding investor made it clear that he did not want to proceed without a "professional" design engineer to do the design work on the prototype and the injection mold design.

We found a professional engineer, and hired him to design the hot water saver. He wanted $40,000 to do the project.  There went our budget right out the window.

The design he came up with didn't work worth beans.  He designed the cylinder to be injection molded in two halves, with a joint in the center of the tank.  The two halves were to be clamped together with a band clamp. He chose General Electrics new Ultem resin, a strong new high temperature engineering resin. 

His design called for two more pieces injection molded from polycarbonate that would slide down over the upper and lower tank halves to prevent them from bulging or bursting when pressurized with water from the source of water such as the water main feeding a home.  Some homes have water pressure exceeding 100 psi.  At 100 psi, the force pushing the tank halves apart was over 5,000 pounds!

After several months the injection mold tooling was finished and we began testing the unit.  It was a total disaster!  When we pressurized the cylinder, the two tank halves would just pop out of the band clamp at around 40 psi.  We had to totally re-design the joint between the two cylinder halves, clamping them together with 16 one quarter inch bolts and custom made washers.

We had to change the material to glass-filled Ultem for more stiffness and strength.  We also had to re-design the top rod seal gland since the engineer's design used screws to hold the seal in place, and the cylinder failed at the screw bosses. We ended up using a seal retainer that used a bayonet type mounting with the top of the cylinder to hold the seal in. 

The piston wasn't nearly strong enough and had to be re-designed as well, along with a custom designed piston seal. The two cylinder halves had a slight taper.  There had to be a taper to be able to get the cylinder halves off of the core of the tooling.  As plastic cools in the mold it shrinks.  The Ultem would shrink onto the core so tightly it would never come off the core without a taper.

So what we ended up with was a cylinder that had it's widest point right at the center of the cylinder. This caused the piston to stick at the center of the cylinder, hence we came up with the custom designed seal which stopped the piston from getting stuck half way down in its stroke.

We also ended up needing special custom designed brass fittings on the water inlet and outlet, incorporating an o-ring seal against the tank.  We needed it because pipe threads being tapered, produce a huge hoop stress when over tightened, cracking the female threaded pipe fitting in the cylinder.  

By this time General Electric had taken an interest in our project.  They were anxious to find customers for their new Ultem plastic resin, and they provided us with finite element analysis of our design and other engineering assistance.

 They sent a team down to our molders to help mold the cylinders as we tried to obtain a cylinder that would hold a pressure of 250 psi which we needed to obtain in order to have our product listed with IAPMO (International Association of Plumbing and Mechanical Officials), to meet the requirements for the Uniform Plumbing Code. 

General electric even flew myself and our molder to Massachusetts along with the tooling to try molding it in one of their facilities.

Eventually we got all the structural problems worked out. 

The cylinder had to be insulated since it stores hot water drawn from the water heater during periods between hot water usages.  We had tooling built to mold a Styrofoam covering for the cylinders, like an ice chest.  

Eventually we got all the design and engineering problems solved, and were ready to market the Hot Water Saver.

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