66.7 LAMINATE OBJECT MODELING (LOM)

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Invented by Michael Feygin 1985, marketed by Helisys as LOM 1015 and LOM 2030. Also manufactured by Paradigm and Sparx AB as HotPlot

uses thin sheets of material (most notably paper and polystyrene) that has a heat activated adhesive on one side. Sheets are piled up one at a time, and heat is used to melt sheets together. A laser then cuts the sheet into thin sections that form the slice.

Slice thickness depends on material and ranges from 0.002" to 0.02". Materials in use are,

- butcher paper

- plastics

- ceramics

- composites

The laser uses the typical x-y and optics systems for the laser

More than one layer can be cut at once, but the accuracy decreases as the number of layers increases.

As material is cut, it is not removed. Material that is to be discarded is cut into "tiles". There are chunks of material that will support the part, and are easily removed to recover the part.

When complete the part is in the middle of a block. Outside there is a "wall" to support the tiles, and in turn the tiles support the parts.

A heated roller compresses the laminate to the other layers. The thickness is harder to control, so the height of the material is measured each time to ensure accuracy.

The final part requires careful removal from the tiles, and is finally sealed to keep moisture out, and prevent layer separation.

***************** Include LOM process photos

The system uses a CO2 laser, and the cuts are done at varied powers and speeds. Note that these cuts are not done in raster lines such as other techniques

The other laser positioning systems operate differently, in this case the mirrors are rotated, which is better suited to drawing vectors, as opposed to rasters.

advantages,

- no chemical changes, and minimal heating, so the shrinkage is trivial, and stress induced deformation is very small.

- shrinkage is compensated for

- no "developing/heating time" is required

- the laser only has to cut the part outline and hatching, not all the internal area.

- no supports needed

- a large variety of materials can be used: butcher paper is $2/lb for 0.004" thickness

- the system is inexpensive to maintain

- non-toxic materials

- these machines are well suited to desktop operation

disadvantages,

- removal of the tiles can be difficult because the laser cuts through the layers, not between them. This requires schemes to weaken material layers that are at a solid/air interface. Cross hatching is used to "burn-out" and weaken materials

- delicate parts can be damaged when removing tiles.

- enclosed volumes will trap the support material

- the material properties change with the direction of the laminate

- a great percentage of the material is wasted

- the surface is rough

- machinability is limited because of delamination

- ventilation is required for fumes when burning

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The machine costs are itemized below,

The laser power is 20-50 W

Accuracy is +/- 0.002" in x-y and +/- 0.001" in z

has been used for,

- concept

- design verification- fit/form

- mold production

time required is hours to days

wax can be used to add fillets

to do sand casting

1. build polyurethane molds from the LOM model

2. build polyurethane, or epoxy pattern equipment

3. produce sand molds

4. cast many metal parts (tolerances were in the range of 1/5 to 3/100")

plaster casting

1. build rubber, epoxy, or polyurethane plastic molds from LOM model

2. pour rubber pattern from mold

3. produce plaster molds

4. cast up to 100 parts (tolerances in the range of +/- .01" to .02")

Investment Casting (one shot only)

1. Apply sealant to LOM model

2. Develop a mold using ceramic slurry

3. place in autoclave to cure ceramic shell

4. Burn out LOM paper in oven and remove ash

Indirect Investment casting

1. make RTV silicone or epoxy mold of LOM part

2. mold wax pattern

3. make ceramic mold out of part

4. use autoclave to melt wax and cure shell

5. put in oven and burn out wax.

6. 6. cast part

making RTV Silicone Rubber Molds

1. Gating is added to the LOM model

2. the model is placed in a box and RTV silicone is poured in to cover the model

3. the silicone is degassed and cured

4. the silicone mold is split, and the part is remove.

5. 10 to 30 polyurethane parts can be cast in the mold

Selected specifications are given for a LOM 1015,

Selected specifications are given for a LOM 2030,