Rotomold Machine
This rotomolding machine was
made as an experiment with gears and low
cost molding techniques. It is made of MDF and powered by a BBQ grill
rotisserie motor. Resin is poured into a mold that is clamped inside the machine. The assembly rotates on two axes and the tumbling motion distributes the resin over the entire inside of the mold as the resin cures.  Since heat
cannot be used with this model, fiberglass resin is used. It
cures quickly so the machine only needs to rotate a few minutes per
casting. The inside of the mold is coated with Vaseline before the resin is poured into it; the Vaseline serves as a release agent. The molds as well as the rotational molder can be made with the CNC machines. Here is a vase that was made with the machine. This is a mundane example; the potential is unlimited; so many ideas, so little time. Here is a link (Vac Offset) to another molding project that used fiberglass resin. The mold was carved as a model over which the fiberglass was laid, rather than as a hollow casting mold as used for the vase. ......................
A newer rotomolding machine was made that uses standard pulleys and miter gears rather than the homemade gears that are shown above. This newer machine's parts are not terribly expensive and their use makes fabrication much easier.  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 inch 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. |
 The 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 (a 608 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 oversized 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. |
 The 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. Photos on right and 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 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 standard sized pulleys that are easily found. |
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 The belt has 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 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 is too large to hold the bolt's threads. |
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| 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 aligned by turning them on the axles' threads. The miter gears' bores will probably have to be drilled to 17/64 inch before tapping them for the 5/16 inch axles. (Take care. The gears can cut like saw blades when turned.) A bearing is on each miter gear's axle. One is shown above on the short upright block. The other bearing 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 the frames will rotate without rattling or colliding as they turn. |
 The 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 and stiffen the entire assembly; they are worth the construction time. |
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| 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. 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. |
  A stepper also works well. One can be attached to the upright with long machine screws that are tied into tee nuts. The tee nuts are pressed into the upright. Right image. A 5/16 inch rod coupler with screws tapped 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. You know how I know ;-) Here is a short video. ..............................
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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 (608 bearings) are red and in 7/8 inch recesses. The pair for the belt tensioner is not shown. A cheap source is thrift store roller blades. They can also be found at sports shops; any ABEC value is good enough. 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 in the inner and outer frames 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. Note: 1 x 4 is actually 3/4 x 3-1/2 inches and a 1 x 6 is 5-1/2 inches wide. Holes for carriage bolts are enlarged in the wood with the bearings so 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. |
| Here
are the parts ordered from
sdp-si.com A 1M 4-Y24030 30 Teeth, 24DP / Commercial Miter Gear 2 A 6G 3-110037 (XL) Pitch, 110 Teeth, 3/8" Wide, Urethane Belt A 6Z 3-15DF03710 (XL) Pitch, 15 Teeth, Polycarb Timing Pulley With Aluminum Insert A 6Z 3-28DF03710 (XL) Pitch, 28 Teeth, Polycarbonate Timing Pulley With Aluminum Insert Good luck! |
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