Custom Blow Molding Glossary
Here’s a glossary of custom bellows blow molding, to help you identify critical success factors for your application (or to help you win Who Wants to Be a Millionaire, Plastics Engineer Home Version).
REQUIREMENTS
FLEXIBILITY
How much force is required to compress the bellows? Flexibility depends on material and geometry, such as wall thickness and design (size of the radiuses, and whether the corners are sharp or well radiused). Some bellows stay compressed when you compress them – think of the articulated soda straw that stays open when you extend it, and vice versa.
STIFFNESS
How much force is required to change the shape of the bellows? Stiffness is a function of the material, design and wall thickness. The pipe that goes from the sink to drain
For example, after you remodel a kitchen, sometimes the sink drain pipe doesn’t exactly line up with drain in center of sink, and you need a flexible pipe to make the transition. That’s a perfect application for a stiff bellows.
In most applications, you don’t want resistance. Bellows that dispense medicine typically need to be flexible. A bellows that protects the front fork of mountain bike shouldn’t be too stiff – if it is, the shock absorber it’s protecting won’t work properly.
You want a relatively stiff bellows to draw a vacuum (for example, for pulling out transmission fluid for an abrasion test). If you’re suctioning mucus out of an airway, you need the bellows to be stiff enough to function effectively, but not so stiff you can’t get it compressed.
We’re currently working on a project for medical company. In the first iteration, we discovered that the bellows was too stiff for the arthritic hands of the end user. So we decreased wall thickness in our search for the perfect stiffness.
COMPRESSIBILITY
How easy is it to squeeze the bellows from its maximum length to its minimum, and what are those two dimensions? Geometry and wall thickness determine the amount of compression. Compressibility is related to stiffness.
MEMORY
After you compress a bellows, how long does it take to go back to its resting height? Does it make it all the way back?
Generally, we want to minimize memory, so the bellows returns to its original state. For example, we produced a bellows for a high end air mattress. If someone sleeps in one position for a while, the bellows below collapse. When they roll into a different position, the bellows no longer supporting their weight should return to their original height.
We’ve discovered that helical bellows (that look like spirals from the side) are better at minimizing memory than the conventional stack of convolutions. (We’re awaiting the call from the Nobel committee)
TRAVEL
How much distance from the bellows’ extended to compressed state during the course of its functioning?
SOME UNUSUAL FUNCTIONS OF CUSTOM BELLOWS
PUMPING
A pumping bellow performs a suctioning or pushing action, but must cycle rapidly. It needs a valve to maintain its function. When the bellows fills with water, it pushes the water out, returns and refills, and repeats. It does this wonderful trick thanks to a backflow prevention valve.
DAMPING VIBRATION
Bellows can be used to dampen vibration by virtue of a liquid inside bellows to absorb the shock. Think of an old time record album turntable, with vibration dampening bellows under the four corners to keep the record spinning evenly when Rosco the rampaging bulldog bounds across the floor.
CUSHIONING
If the bellows is filled with air instead of liquid, it can cushion and the volume of air in the bellows can be controlled to adjust the level of cushioning as might be needed beneath the driver’s seat in a tractor for an 18 wheeler.
PACKAGING
Visualize the collapsing straw. Grow it to bottle size. Ship it collapsed with margarita concentrate – add tequila when the time is right. Why ship air, or even water. The bellows solution – ship a container collapsed, it could be opened up by consumer. Don’t ship air if you can avoid it.
Now that you know all about custom bellows, feel free to give us a call at 401-723-3000 or fill out the form below to start talking to one of our engineers about your application.
