WEEK 15
15. Applications and Implications
These are the question that I need to answer at this stage of the project:
What will it do?
Industrial Rotational Moulding or Casting is a production process used to make hollow parts. A mold made of metal is filled with plastic chips and heated in an owen (sometimes the mold can be self heated). The plastic melt and the mold, which is loaded onto a frame, rotates upon two axes at the same time. The mold keep rotating for a certain amount of time in which the plastic adhere to the internal surface of the mold and, as the mold temperature is dropped down, the plastic hardens.
My Rotocasting Machine will work with silicone rubber mold and polyurethane resin. Therefore it won't need heating as the liquid resin will keep moving and covering the internal walls of the mold dring its pot life, until full cure.
Who's done what beforehand?
There has been many approaches to do it yourself rotational moulding, some are commercial products or toys, others sport a rather backyardish approach, others feature a nice packaging and are easy to fabricate. All of them nevertheless, have the same structure: an outer frame rotating on the x axis, an inner frame connected to the outer and rotating on the y axis, movement transferred from the base to the inner frame with pulleys, angled gears, chains or a second motor.
In 2012 I designed and fabricated a traditional two frame machine featuring 3D printed gears, lasercut pulleys and cnc'ed bearings holder, I presented it at Rome's European Maker Faire, but never completed it with a motor.
My approach
The double squared frame system works pretty well, but it has been done so many times I decided to try something different. With any luck it will even be more efficient to fabricate and operate.
My idea is to make a double ring. The outer is connected on the x axis on a support base and it is connected to the inner ring by a circular array of steel balls so that the inner ring can rotate inside the outer one on the z axis. It is basically a huge Ball Bearing.
What materials and components will be required? How much will it cost? Where will they come from?
Everything on the list might already be in a Fab Lab, or it can be acquired from hardware shops, sports stores or online vendors (for the steel balls).
What parts and systems will be made?
The machine is made of a base with two supports, an outer ring, an inner ring, a DC Motor, two gears to connect the base to the inner ring. The electronic parts are a satshakit, a motor shield with H-Bridge, 12/24volt power supply, possibly a wi-fi/bluetooth module. A desktop application will be used to control motor' speed.
What processes will be used?
I like to give myself some design rule to comply to, as it makes my design process easier, so I decided that the machine has to be "as fabbable as possible", meaning that it should be easy to make in a Fab Lab using machinery and materials already present in the shop and having to buy.
Most of the parts will be made from a single board of wood using a CNC milling machine, with two very common mill tips: a 12.7mm ball-end and a 6,35mm upcut squared-end.
The gears might either be 3d printed or cnc'ed but this aspect needs further investigations.
What tasks need to be completed? What is the schedule?
What questions need to be answered?
The first question is: will the big-bearing design work? One critic aspect lays in how the rotation action is transferrd from the base to the inner ring, depending on the allowances in the rings/balls connection the system might be too loose (making the gear snap) or too tight (getting the gear stuck)
How will it be evaluated?
The project will be a success if: