Bulk material is stuff handled ‘in bulk’. One LEGO piece is a brick but 1,000 poured into a bag is bulk material. Corn starch, sand, flour, powder-coat powder, gravel, cat food, Cap’n Crunch, coins, screws, Styrofoam beads, lead shot, and gummy worms are bulk materials.

Applications abound where you need to move stuff in bulk. Selective sintering 3D printers, animal feeders, DIY injection molders, toner based PCB makers, home powder coating, automatic LEGO/domino/whatever sorters or assemblers, automated gardeners, airsoft accessories – handling bulk material is part of hacking. College science classes cover solids and liquids, but rarely bulk materials.

Most hackers just pray it works and tap the bin when it doesn’t. Industry does better, but the slang term “bin rash”, the long term result of tapping a 300 ton bin with sledgehammers (video),  shows they don’t get it right all the time either. At the same time, it’s a fun area you can experiment with using kitchen items. So come along with us for a short series on the basics of bulk material handling.

Arching, Ratholes, and Slurping

In a typical bin there is a conical or pyramidal funnel at the bottom with a cylindrical or rectangular vertical section atop it. The joint is called the hip. The funnel ends in an opening, often with a control gate.

Bulk material has little cohesive strength compared to a solid, but of course this varies across materials. For instance, lead shot has little cohesive strength and flows easily. Flour has a higher cohesive strength, and is hard to pour. A material only flows when the forces across a section of the material are greater than the cohesive strength of the material.

Imagine the bin contains only a small amount of wet sand, right above the gate, and we open the gate. Will the sand fall out? The sand is now bridging the opening, and the forces are the same as with any other bridge. We want the bridge to fail reliably.

Usually the first little bit will fail and fall out. But as it does so, it can leave behind a curved surface. If the arched material can bridge the opening, it won’t flow out at all. A little falls out, leaving a cavity with a rounded roof, and flow stops. The material has arched.

If the bin is smooth and the funnel has a steep angle, the material flows by mass flow. All material is flowing, and in the cylindrical section the flow is only vertical. The material top surface will descend without rearranging until it reaches the hip. For many materials, a smooth inner wall and a cone angle greater than 60 degrees is a good design rule for mass flow.

Rather than slipping at the wall-material interface, if the funnel is too blunt or the walls too rough, the material can cohere somewhere internally. This is funnel flow. The material forms it’s own, sharper funnel inside, and only the material inside the funnel flows. Funnel flow isn’t inherently bad, but has more potential problems. (useful page of flow problems here).

It might rathole – material flows, but makes a nearly vertical hole over the opening. Most of the material isn’t moving, and when the hole reaches the surface, flow stops, a you’ve got a stable rathole. Even if the rathole isn’t stable, the material on the outside isn’t moving so you might as well have made a smaller bin. Worse, the trapped material might rot or decompose, or compact into a solid mass.

A couple design tips. The corners of rectangular bins have more wall to ‘support’ the material, so hang ups often occur in the corners. Use a round bin if you have a choice. Also, the rathole’s diameter will be somewhat larger than the diagonal of the opening. Slit openings sometimes perform better than round holes.

The Deluge

When material is dumped it can fluidize, becoming not chunks but independent particles. Dump a teaspoon of flour from chest height. It turns to a cloud of dust instead of falling as a pile.

Feeding into a rathole can lead to fluidization as the new stock falls straight through. Worse, it can knock loose some of the rathole wall. It all crashes out at once, at high speed, called flooding – the new material and falls down the hole as a fluid and discharges at high speed.

This can break the gate or whatever the bin feeds, and many powders present a respiratory hazard or even an explosion risk when they mix too finely with air. And it can lead to segregation.

Segregation

In most bulk material there are varying sized particles, and often different materials: bottom gravel, mixed nuts, a jar of mixed screws. The Brazil nut effect, where a jar of mixed nuts ends up with the Brazil nuts on the top and the peanuts on the bottom, is one form of segregation.

Mix salt and dried peas and pour the mix through a funnel onto a plate. The mix makes a conical pile. The peas roll down the pile and end up at the edges of the plate. Feeding from a small central pipe or chute makes the larger material segregate near the walls. If the hopper is then emptied by funnel flow, the end material is mostly salt. The peas stay behind.

One cure is to have a tall cylinder section with mass flow and keep the material surface above the hip. Then material is removed in the same order it came in, whether it’s from the wall or the center. Inserts can also help exit segregation issues. An upward pointing cone on standoffs inside the funnel forces material from the center to mix with that from was deposited, and a baffle plate below the inlet can change the inlet character from a central point to many smaller feeds. Sometimes just lowering the inlet might help.

Other Issues

Dropping potato chips into an empty two meter bin guarantees breakage, but even relatively light abuse can break particles, as the dust at the bottom of the Cap’n Crunch box shows. Presumably our morning cereal was loaded into the bag at the factory without a couple centimeters of tan colored powder — it must have abraded in transit.

Some materials develop a static charge when they move, and this can interfere with movement. Acrylic or polycarbonate laser-cut bins are often a culprit . A simple experiment – rub a balloon in your hair, then hold it against the material you’d like to move in bulk. If it picks up the material, a grounded metal bin is a fix.

Some materials are abrasive. Sandpaper is sand glued to paper after all. Besides tougher bin materials, deliberately inducing funnel flow can protect the walls. A big funnel just blunt enough to funnel flow makes a good hopper for an abrasive.

TL;DR

Next week, we’ll look in more depth at bulk material properties, and the week after, we’ll delve into machinery to move bulk solids around. This has been a whirlwind tour of some of the terminology and problems that simply putting bulk material in a bin can present.  Hopefully, it’s at least helped to put names to some of the lessons you’ve learned from playing around in the kitchen or the sandbox.

If it’s possible to summarize, the TL;DR for most bin designs is:

Use a round bin with smooth internal walls.
Keep the funnel section walls above 60 degree incline.
The bigger the opening, the simpler your life is. Slits can be good.
If the surface cones downward internally, it’s a warning sign.
Watch for segregation.

See you next week!