Idle hands, shop-vacs and centralized resin distribution

There’s nothing more dangerous than a bored night-shift mechanic in a plastics operation. In my case, idle hands (mine) began to think about better ways of moving resin to the 40 ton “short run” press that was molding small quantities of insulators for an automotive OEM application. Hopper loaders weren’t cost effective for machines which rarely ran for more than an hour or two between changeover (or breakdown), so manual loading from gaylord to hopper by pail was the order of the day. My massive industrial “shop-vac” sat behind my bench, and seemed to have plenty of power to draw dirt through about twenty feet of hose. You can see where this is going.

In no time I’d perfected my homespun resin handling system with which the operator could simply switch on and wave the magic wand across the bottom of the gaylord. The only problem was, it didn’t work.

The concept of sucking resin through a hose doesn’t seem to require much engineering know-how at first glance, but in time I learned why centralized resin distribution isn’t a no-brainer. The problem stemmed from my lack of understanding about the difference between pressure and energy.

Create a pressure differential between two points, and air will flow toward the low-pressure region, right? Of course, I reasoned, and the airflow would carry the resin pellets in the air stream. Serrated vacuum hose and irregularities at the joints in rigid tubing would create turbulence in the airflow, keeping the resin well suspended.

In fact, the set-up successfully deposited dust, dirt and fines into the machine hopper, while leaving the resin no further than three feet into the suction hose. The press operator rolled her eyes, and I retreated to my bench to ponder the problem.

The answer would come to me years later, during a first-year university physics course. The vacuum concept was good, but the reality is that moving a pellet of polystyrene requires accelerating it to the necessary speed, lifting it to the height of the hopper, then decelerating it to zero velocity at the screw. All that lifting and speeding requires the system to do work on the pellet, and to do work, you need energy. The energy comes from the air molecules pushing the resin up through the system in their effort to fill in the low-pressure zone at the hopper. The pellet needed to get the energy from collisions with the fast-moving air molecules, which, like the pellets, give up some energy, converting it to heat, in collisions with the tube inner walls. And the greater the surface area, (or “wetted area” to scientists and engineers) the more energy is lost that can’t be used to move material. Joints and bends make the situation even worse. The result in my home-made system was a sort of dynamic “sorter” which allowed the smaller particles, mainly dust, to move to the hopper because their smaller mass (“weight”) takes less energy to move. If I’d opened the tube, I’d probably have seen the resin pellets stratify by mass along the tube length.

And the point to all this? One is that it takes more power and more engineering than you think to move resin into your press, and the other is that overrating your system makes a lot of sense, and not just where you plan to add capacity. Simply moving a machine and adding a longer tubing run can seriously compromise system performance, even if the tubing is glass smooth and straight as an arrow. Buy lots of power if you plan to rearrange your production floor. It’s expensive, yes, but trust me, you can’t use your Hoover.