The principle of laser rapid prototyping technology

Most of the traditional industrial forming technologies also follow this method, such as turning, milling, drilling, grinding, and planing; others use molds for forming, such as casting and stamping. However, laser rapid prototyping uses a new forming principle-layered processing and superimposed forming. For those who have studied advanced mathematics, this principle is not unfamiliar. In the application of definite integral, it has been mentioned that the generatrix is the calculation of the volume of a rotating body with a known curve and the calculation of the volume of a solid with a known parallel section, both of which are cross sections with a thickness of △Xi, which are approximately regarded as flat cylinders Or curved side cylinder, and then superimposed:

When ‖△x‖→O, the volume of the known solid of the rotating body or the parallel section is obtained respectively.

The topography is composed of complex curved surfaces. However, as long as we survey and map contours of different heights, we can cut out curved planes of different sizes and shapes on the flat plate (of course, the thickness of the plate and the adjacent two The height difference between the contour lines should conform to the scale of the topographic map), and then layer them together. As long as the contour lines are dense enough, a realistic terrain model can be made.

At present, there are many kinds of laser rapid prototyping machines developed according to the principle of this kind of layered processing and superposition forming. Here are the main ones for a brief introduction:

1. Selective curing of liquid photosensitive polymer (SLA: Strreolithographypparatus three-dimensional lithography equipment)

In 1902, a U.S. patent proposed the principle of using photosensitive polymers to make plastic parts. In 1986, another U.S. patent proposed a rapid prototyping scheme for layered production of three-dimensional entities with laser irradiated liquid photosensitive resin. The U.S. 3DSYSTEMS company based on this In 1988, the first laser rapid prototyping machine SLA-250 was produced.

The configuration material used in this laser rapid prototyping machine is a kind of liquid photosensitive polymer, which will undergo polymerization and curing reaction under the irradiation of ultraviolet light, and change from liquid to solid. Its advantage is that it can directly obtain plastic-like resin parts. , And the surface roughness is small.

The disadvantages are:

(1) The chemical and physical changes in the forming process make it difficult to guarantee the dimensional accuracy, and creep will occur;

(2) Scanning and curing of the entire section is required, the forming time is longer, and further curing is required after forming;

(3) Since the part that is not irradiated by the laser beam is still liquid, the overhanging part must be designed in advance to support it, and then removed after curing;

(4) After curing, the photosensitive resin is brittle, easy to break, and has poor workability. The working temperature should not exceed 100°C. It will absorb moisture and expand, has low corrosion resistance, and is expensive (140-2404/kg);

(5) The life of the laser tube that generates the ultraviolet laser is about 2000 hours.

2. Selective cutting of thin materials (LOM: Laminatedbjeetanufacturing manufacturing of layered objects)

The configuration material used in this laser rapid prototyping is paper pre-coated with hot melt glue. The forming process is similar to the process of making terrain models described above.

The advantages are:

(1) Higher dimensional accuracy;

(2) Only the contour line needs to be cut, and the production efficiency is high;

(3) No need for design support;

(4) The finished sample has a hardness similar to that of wood products, and can be used in an environment below 200 ℃ after a little treatment, and can be cut to a certain extent;

(5) The life of the carbon dioxide laser used is up to 20,000 hours;

(6) The structure material is cheap (8/kg).

The disadvantages are:

(1) Plastic parts cannot be made directly;

(2) The surface roughness is high, and the surface of the workpiece has obvious step patterns, which should be polished after forming;

(3) It is easy to absorb moisture and expand, and the surface should be treated with moisture-proof as soon as possible after forming;

(4) The workpiece lacks flexibility.

3. Selective melting of filamentous materials (FDM: Fused Deposition Deling)

The configuration material used in this rapid prototyping machine is a filamentous thermoplastic material, and its working principle is similar to that of making a label cake. The filamentous material is fed into the nozzle by the wire feeding mechanism, and is heated to a molten state in the nozzle, according to the cross-sectional shape Coated on the workbench and quickly cooled and solidified. After one layer is completed, the nozzle rises one layer height, and then the next layer is coated. The advantages are:

(1) Can directly make ABS plastic;

(2) Higher dimensional accuracy;

(3) The material utilization rate is high.

The disadvantages are: 

(1) The surface roughness is high, and post-treatment is required;

(2) Long forming time;

(3) The material is expensive (250-458/kg);

(4) Supports should be provided at the cantilever structure, but the new FDM rapid prototyping machine is equipped with two nozzles, one sprays the forming material and the other sprays the support material, and the support material can be removed by water, reducing the post-processing time.

4. Selective sintering of powder materials (SLS: Selected aserintering)

The working principle of this rapid prototyping machine is similar to that of SLA, but the molding material used is not a liquid photosensitive resin, but a powdered polymer material, a mixture of metal or ceramics and a binder, etc. The particle diameter is 50-125 靘When forming, first lay a layer of powder material on the worktable and heat it to slightly lower than the melting temperature, then the laser beam scans according to the cross-sectional shape, and the scanned part of the material is melted and bonded to form, and the powder material that is not scanned It is still in the form of powder and granular as the support of the workpiece. After the first layer is formed, the worktable is lowered by one layer height, and then the next layer is laid and sintered. Its advantages are:

(1) Plastic, ceramic or metal parts can be directly obtained, with good workability; 

(2) No design support is required.

The disadvantages are:

(1) The structure of the formed part is loose and porous, and the surface roughness is relatively high;

(2) The forming efficiency is not high;

(3) The obtained plastic, ceramic or metal parts are far inferior to the workpieces of the same material obtained by the traditional forming method, and post-processing such as copper infiltration is required, but it is difficult to ensure the dimensional accuracy of the parts in the post-processing.

Through the introduction of the above four kinds of rapid prototyping machines, we can see that the laser rapid prototyping technology is an integration of a variety of advanced manufacturing technologies.

Since different rapid prototyping machines have different characteristics, it is necessary to make appropriate selections according to different usage requirements. The selection should comprehensively consider the size of the formed part, the accuracy requirements of the formed part, the purpose of the formed part, the shape of the formed part, And the material requirements of the formed parts, etc., and the production cost must be weighed, for example: 

Large parts and entities are suitable for LOM production;

Small parts and thin-wall parts are suitable for SLA, FDM, SLS production;

Plastic parts can be made directly with SLA and FDM, while LOM must be made through silicone molds and reactive injection molding;

Metal parts can be made directly by SLS, and other methods must be recast by casting;

SLA and LOM can be made with high surface roughness requirements;

Available LOM production in higher temperature environment, etc.