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.

Geared rotomolder.
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.

Since heat cannot be used with this wooden machine, 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 can be made with the CNC machines.

Here is a vase that was made with the machine. This is a mundane example.
Vase cast with roto-molder
Vase made with rotomolder.

The potential is unlimited. So many ideas, so little time.
Here is a unit made by David Snellen, a site-visitor, who wanted to create a machine that could be used by his students.
He thought they would find the geared version to be more interesting than the belt driven unit that is shown below.

He drew and built this machine, and has offered the DXF files of the components for others to use.

The files are in the folder linked here.

"I teach middle school and high school Industrial Technology (shop) and the Rotomolder is an intriguing device. In addition to showing casting, it can be used to demonstrate gears and ratios."

These files were obviously a lot of work to put together, and his generosity is greatly appreciated.
Roto Caster
Rotomold Parts A.dxf is one drawing with all the parts. The parts are labeled. The only text engraving is on the base. Also included are 25 gears sized in ¼” increments from 2 x 8 to 8 x 32.

His said his email address could be posted; it is dsnellen AT gmail DOT com.
Solsylva Belt Drive Rotocaster

This newer rotomolding machine 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.
Construction details begin below the photo.

Rotomolding Machine
Rotocaster using pulleys and a belt rather than gears.

Note: There is a drawing at the bottom of this page that will help with the following description.

The machine consists of a base with two uprights that 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 against the mold.
In the photo above, 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, the mold will need to be nearer the bottom of the machine, as shown in the above 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, as shown on the right.

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 moving frame.
The edge of the bearing can be seen on the left of the nut in the photo.
Skate bearings fit in 7/8 inch holes drilled with a Forstner bit.
A 5/16 x 3 inch carriage bolt serves as the axle.
Small Pulley
Small pulley is fixed against frame.

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.
The larger top pulley turns on an axle that is 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.

Bearing behind pulley
Larger top pulley.
Threaded Rod Axle
Nuts lock bearings into place.
The pulleys are 15 and 28 teeth to give an almost 1:2 ratio. This makes the inner frame rotate somewhat in sync with the outer frame.
However, a 30 tooth pulley is not used because 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 position.
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.
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 with the pulley axle placement. The wood is soft and the pulleys' support bearings will offset in their recesses. This will cause the belt to slacken with use.

A pair of bearings can be tied to the side of the outer frame to tension the belt. Right image.

The head of the bolt that is used to hold the pair of tension bearings is ground or cut so it will clear the base's upright support as the machine turns.

Washers are placed between the bearings and the outer frame to position the bearings in line with the belt.
Tensioner Bearings on Belt
Belt tensioned with bearings.

The 5/16 inch axle bolt is threaded directly into the wood. A tee nut could be added if the threaded hole in the wood strips.
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 the gears in 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 they can be tapped 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 on the short upright block. Right image.

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.
Miter gears
Miter gears.

Bearing for Miter Gears
Gear with bearing.
The clamp consists of threaded rods and boards, the rods are held to the inner frame with nuts and washers.
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 are supported by 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.
Clamps to hold mold
Clamping boards held with
threaded rod and wing nuts.
The motor can be a stepper, a rotisserie motor, a microwave turntable motor or similar.
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.
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 flows.

A microwave turntable motor is shown on the right. This turntable motor has an X shaped drive shaft that accepts a 1/4 inch drive socket quite well. A 1/2 inch socket can be used to drive the table by sliding the socket over the main axle nuts. Lower right image.
The torque required to turn the machine is low when the load is balanced, so decking screws tying the motor to the uprights will offer enough support.

A hand crank will also work. However, it is challenging to keep a slow steady pace.
3 rpm drive motor
Microwave oven motor with socket.

Small Motor on Drive Shaft
Motor mounted to end.
A stepper also works well.
One can be attached to the upright with long machine screws that are tied into tee nuts. Right image.
The tee nuts are pressed into the upright.

A 5/16 inch rod coupler with screws tapped into its side makes a passable stepper to axle linkage.

The stepper can be borrowed from an axis of a CNC table.
The stepper's configuration in the software does not have to be altered. Just manually enter a g-code into the MDI box that tells the axis to move 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.
Tee Nuts for Motor Mount
Tee nuts support the stepper screws.

Coupler to Motor
Stepper coupled to rotomolder.
The dimensions are shown in the drawing below.

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.

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 that is used as the motor coupler is light blue.

The miter gears are purple, 30 teeth 1/4 inch bore and tapped for the 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 are 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 1x4 ripped in half.
The darker uprights are 1x4, and the 18.25 inch base is a 1x6.
All wood to wood connections are made with drywall screws and glue.

Note: 1x4 is actually 3/4 x 3-1/2 inches and a 1x6 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. The bolts 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.
Rotomolder dimensions
Dimensions of rotocaster.

Here is a short video.

Here are the parts ordered from

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, Polycarb Timing Pulley With Aluminum Insert

Good luck!