Industrial facilities, warehouses and food processing factories have a common problem: There are doorways to the outdoor loading areas that need to be sealed against dust, insects, birds or the elements, but workers driving forklifts or pushing pallet jacks need to get through them hundreds of times a day.

Since time is money, conventional doors are out; it would take too long for a worker to hop off of their rig to open and close the door each time. Since money is money, automated doors are out.

What's needed is a door that a pallet jack or forklift can push through without damaging it, and that doesn't cost a fortune. Over the years, three low-tech approaches have been developed.

The first is vinyl strip curtains. These do an acceptable job of providing a seal, yet are easily parted by folks hauling loads.

Image: Ideal Warehouse Innovations, Inc.

Image: Ideal Warehouse Innovations, Inc.

They have the advantage of being transparent, so workers can see if there's an obstruction on the other side.

Image: Bird B Gone

Installation is straightforward, and individual strips can easily be replaced if damaged.

Image: Ideal Warehouse Innovations, Inc.

The second approach uses "impact doors" with bi-swinging hinges. These doors have protective polypropylene sheets that are bolted to the inner edge of the door's face, initially sticking out the wrong way, then folded over and bolted to the other edge of the door. If this description is confusing, the photo below will make it crystal clear.

Image: Traffic Doors and More

This wave profile effectively turns the flat sheet into a flexible bumper. It's a brilliantly simple and cost-effective idea.

Image: Vortex Doors

And of course, it can be scaled up to the height of whatever's slamming into it.

Image: The Australian Trellis Door Co.

One disadvantage to the impact door method is that it doesn't offer the visibility of vinyl strip curtains. So the third approach borrows from the first two: Swinging vinyl doors with impact plates at the bottom.

Image: Traffic Doors and More

With this design, the impact plates can be left flat; they don't need to offer the same level of protection to the flimsy vinyl as they do to a solid door.

Image: Discount Commercial Doors

The fourth approach is the simplest-looking, yet the most complicated from a materials perspective. Here aluminum tubing, rubber extrusions, ABS facing and foam insulation are all combined to make a flexible door:

Image: L.K. Goodwin Co.

That's way too complicated for my tastes, and seems to provide the worst visibility out of all of four approaches. However, this design has apparently been around since 1952, so what the hell do I know.