Tuesday, January 1, 2008

Rotomold Machine

This newer machine's parts are not terribly expensive and their use makes fabrication much easier.
Rotomolding Machine
Note: There is a drawing at the bottom of this page which will help with the following description.
The machine consists of a base with two uprights which support an outer frame. The outer frame supports the miter gears and pulleys as well as the inner frame. The inner frame supports threaded rods that tie clamping boards into place against the mold. In the photo the mold is represented by the 4 x 6 block of wood.
The mold should be positioned in the frame so that it is balanced; this will allow a very small motor to turn the machine. The pulley and miter gear assembly add weight; therefore, for the mold to be balanced, it will need to be nearer the bottom of the machine as shown in the picture.
Small PulleyThe pulleys are 15 and 28 teeth XL with 5/16 inch bores. The smaller pulley is clamped to the base's upright with nuts on the carriage bolt axle. This 15 tooth pulley does not turn; its hub, if it has one, is pressed into a recess that is drilled into the side of the base's upright.
A skate bearing is pressed into a recess in the outer frame. The edge of the bearing can be seen to the left of the pulley in the photo on the left.
A 5/16 x 3 inch carriage bolt serves as the axle. The hole in the outer frame, not the upright, is drilled over sized so the carriage bolt can turn freely in its bearing without dragging on the wood.
Skate bearings fit well enough into 7/8 inch holes drilled with Forstner bits.
Bearing behind pulleyThe larger top pulley turns on an axle made of 5/16 threaded rod. Bearings are recessed in the inside of the supporting wood members. Nuts are used to lock the threaded rod into place and to press the bearings into their recesses. Photo below.

The pulleys are 15 and 28 teeth to give an almost 1:2 ratio so the inner frame will rotate somewhat in sync with the outer frame; however a 30 tooth pulley is not used since it would give an exact 1:2 ratio. The smaller 28 tooth pulley is used so that the machine has to turn many times before it repeats the same positions while moving. This slight out of sync movement helps to better distribute the resin. This gear ratio was derived by trial and error and seems to work well; it uses regular sized pulleys that are easily found.
Threaded Rod Axle
Tensioner Bearings on BeltThe belt is 110 teeth, the pulleys have to be properly positioned for the belt's tension to be correct. The axle center distance is 8-13/16 inches.
It is difficult to get the tension exactly right through the pulley axle placement; the wood is soft and the pulleys' support bearings will offset a bit in their recesses causing the belt to slacken if not immediately then over time.

A pair of bearings can be tied to the side of the outer frame to tension the belt if it is a little loose.
The head of the bolt used to hold the pair of tension bearings is ground or cut down so that it will clear the base's upright supports as the machine turns. Washers are placed between the bearings and the outer frame to position the bearings in line with the belt. The 5/16 inch axle bolt is threaded directly into the wood, there is not a nut on the back. A tee nut could be added if the hole in the wood is misplaced or too large to hold the bolt's threads.
Miter gearsBearing for Miter Gears
The miter gears are 30 teeth with 1/4 inch bores, the bores are tapped for the 5/16 inch axles. Nuts are tightened against the gears to hold them into position; this process makes pinning the gears to the axles unnecessary. It also allows the gears to be easily positioned and aligned simply by turning them on the axles' threads.
A bearing is on each miter gear's axle. One is shown on the short upright block above. The other is recessed in the bottom side of the outer frame. The drawing at the bottom of this page shows its position. The placement of the inner frame hides this bearing.
Washers are used as necessary to position the axles and bearings so that the frames will rotate without rattling or colliding as they turn.
Clamps to hold moldThe clamp consists of threaded rods and boards, the rods are held to the inner frame with tee nuts and/or hex nuts. The rods need to be tied with nuts on each side of the inner frame; they may work loose otherwise and allow the mold to shift while the machine is turning.
The clamping boards that ride the rods are positioned with wing nuts. The prototype uses #10 threaded rod, but 1/4 inch would probably be better because it takes forever to move the wing nuts on the finer threads of the small rods.

It appears that the inner frame is over built since all that is necessary is an axle support for each of the bases of the threaded rods; the clamp boards would hold everything in place.
This leaner version was tried but it was difficult to load and unload the mold with the play that existed between the ends. Also the bearings rattled since there is only one bearing on each side of the frame. The extra boards of the inner frame hold everything together well and stiffen the entire assembly; they are worth the construction time.
3 rpm drive motorSmall Motor on Drive Shaft
The motor can be a stepper, a rotisserie motor or a microwave turntable motor among others.
A speed of around 4 rpm seems to work well, though this will depend on the shape of the mold and the type of resin used. The rotisserie motor turns at 5 rpm and the turntable motor turns at 3 rpm.
The machine can be turned faster, but care has to be taken to not move the resin so quickly that it forms bubbles as it tumbles.

A hand crank will work as well, since the curing time is rapid for some plastics it will not be that much trouble to turn it by hand. However it is challenging to keep a slow steady pace.

A microwave turntable motor is shown above. Micromark.com sells a motor that looks just like this but I do not know if they are the same.
This turntable motor has an X shaped drive shaft that accepts a 1/4 drive socket quite well. A 1/2 inch socket can be used to drive the table by sliding it over the main axle nuts. Above right picture. The torque required to turn the machine is very low when the load is balanced, so decking screws tying the motor to the uprights will offer enough support.
Coupler to MotorTee Nuts for Motor MountA stepper also works well. One can be attached to the upright with long machine screws tied into tee nuts that are in the upright. Right image.
A 5/16 inch rod coupler with screws threaded into the side of it makes a passable stepper to axle linkage. Left image.

The stepper can be borrowed from an axis of a CNC table.
The stepper's set-up in the software does not have to be altered. Just manually enter a g-code into the MDI box that tells it to go some long distance at a slow rate. This should be done carefully with very slow feed rates. It is easy to tell the stepper to go WAY too fast by confusing feed-rate with rpm ;-)
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The dimensions are shown below in the drawing.
Parts were purchased from both mcmaster.com and sdp-si.com; if lucky all parts will be in stock at the same time from the same supplier ;-)

The pulleys are green, 15 and 28 teeth. The small pulley's hub is in a recess in the upright; the pulley is clamped into the upright with the two nuts on the carriage bolt.

Skate bearings are red and in 7/8 inch recesses. The pair for the belt tensioner is not shown.

Carriage bolts are yellow 5/16-18. There are three at 3 inches, and the one at the bottom is 2 inches.

Nuts are dark blue 5/16 inch; they are used to clamp the carriage bolts into the frames and to hold the threaded rod into position.

Threaded rod is dark brown, 5/16 inch x 8.25 inch

The rod coupler used as motor coupler is light blue.

The miter gears are purple, 30 teeth 1/4 inch bore and tapped for 5/16 bolt and rod. Nuts lock the miter gears onto the threads. It will probably be necessary to drill the bore a little larger for it to tap easily; the plastic may split otherwise.

Clamping boards are not shown in the drawing. They are 13.5 inches long and tied to threaded rods that extend through the inner frame near the carriage bolts.

The light colored wood is made of a 1 x 4 ripped in half. The darker uprights are 1 x 4 and the 18.25 inch base is a 1 x 6.
All wood to wood connections are made with drywall screws and glue.

Holes for carriage bolts are enlarged in the wood with the bearings so that the carriage bolts can freewheel. The carriage bolts are tightly clamped into the wood that does not have bearings recessed into it. They are clamped so tightly that the nuts are pulled into the wood. Washers are used as necessary to remove play in the frame's bearing connections.

The outer frame freewheels on the bearing on the pulley side; therefore the bolt hole in the outer frame (not the upright) will have to be enlarged for the frame to turn freely around the carriage bolt.

A 30 second video is here.(1.2 MB wmv file)
Rotomolder dimensions

1 comment:

Adam said...

The images no longer work on this post. It looks like a neat project though.