Additive manufacturing

Additive manufacturing

The first application was the rapid prototyping because the target was producing the same shape as the 3D model. Subsequently, it was used for making rapid tooling, because the produced parts were able to fit with themselves. In the last few years, this technique is used to make a final product because the finished model has better mechanical properties than the older models.

Pro

• Design freedom: It is possible to produce a very complicated shape. In fact, this kind of manufacturing becomes convenient when the shape is complicated.
• Cost: It is possible to produce an entire assembly in one step. This leads to reduced cost and time. Moreover, simplifying the logistics about the supply chain (less space in the warehouse, the company can send one file instead of a lot of parts...).

There are two main parameters that affect the model's cost:

• Material: The material used is very expensive compared to the material that is being used in conventional manufacturing (10-20 times more).
• Machine: The building process time affects a lot the cost of the product because the speed of the machine is low.

Why is it used if it is slow? Because the time between the first design and the final product is lower compared to the other kinds of manufacturing. AM results are expensive but in some applications can lead to a reduction of future costs (e.g., plane, satellite, save weight means save dollar). Therefore, it is a good option when a few pieces are produced.

The position of break-even point depends on:

• The size of the product
• What kind of CM is compared to AM
• …

Cons

• Process and material properties are more coupled compared to other CM.
• The final part is a lot sensitive to the machine parameters. Since it is difficult to control them during operation, it is difficult to have the same quality in every produced part.
• Standardization is limited, which means that we are unable to establish if that machine is better than another.
• The many destructible inspections, to control quality, are useless in this case because only one part is produced.

Slicer post

3D CAD .STL FILE BUILD SOFTWARE PROCESSING.STL: It works by removing model construction data and approximates the surface of the model with a triangular mesh. Every triangle is described by:

• Coordinates of vertices
• Direction cosine of the face normal that tells the machine where the material must be applied.

Accuracy

It's represented by the maximum offset between the surface of the 3D CAD model and the STL approximated geometry. It affects the part quality. Rule: The maximum size of the triangle may be smaller than the machine resolution to avoid the loss of quality.

STL errors

The STL can have the following errors:

• Inverted normal: The surface normal points in the wrong direction. The machine doesn’t understand where to put the material.
• Bad edges: They can be due to the near bad edges or holes.
• Shells errors: A shell is a collection of triangles connected to each other. Normally a part has only one shell because every triangle of the part is connected to every other triangle.
• Noise shells: Shells composed of few triangles that could result from the STL conversion.
• Intersecting shells: Different shells overlapping. This issue leads to a double laser exposure → more heat → distortion.
• Disconnected shells
• Triangles errors:
• Intersecting triangles: Pierced faces
• Overlapping triangles: They can be fixed by deleting and creating new triangles in place of the overlapping ones or by moving parts point.
• Unnecessary triangles: The same error of inverted normal.

Fixing

• Automatic (80%)
• Manual (20%)

In order to remember this notion, it's possible to remember this sentence: "We need a watertight STL geometry".

Orientation

Different orientation affects:

• Building time and therefore cost: Build more layers is more expensive than building a larger section. In order to minimize the building time, we must minimize the number of layers. Minimum layer thickness depends on the machine, materials, etc...
• Mechanical properties: Strength in Z direction is lower than X, Y directions. Sometimes minimizing the production cost leads to a reduction of a part strength in the interested direction.
• Appearance: One of the main effects is the Staircase effect. The worst condition is reached when the surface is almost horizontal.
• Support material usage: It's necessary when there is an OVERHANG.

Overhang

A surface with a certain slope with an angle over the self-support angle.

Support structure

It's material placed underneath the overhang:

• It increases the building time and material usage.
• Require subsequent removal operation.
• Have an impact on surface finish.

Placement

In some cases, it is possible to nest parts together in order to maximize the machine volume usage. This solution leads to a machining cost reduction and sometimes a support number reduction.

Slicing

Software operation which cuts the model into layers with the same thickness. At the same condition, thicker layers lead to faster build but reduce the surface quality (more staircase effect).

Support removal

Support can be removed:

• Manually: with scissors or nippers (tronchesino)
• Dissolution: The machine uses a different material for support

Post processing

There are different ways to improve surface quality:

• Grinding (macinazione)
• Polishing (lucidatura)
• Machining (lavorazione a macchina)
• Electrochemical finishing
• Shot peening (pallinatura)
• Coating (rivestimento)

Other process

Other processes can be used:

• Thermal treatment (acciaio)
• Post-curing (polimeri)
• Infiltration

Process types

Additive manufacturing process can be subdivided into two main sections:

• Selective deposition (SD): The material in fluid form is added layer by layer and consolidated to generate a solid model.
• Selective consolidation (SC): The material still be in the tank and a source of energy consolidates the material into a solid model.

Vat photopolymerization (SC)

The used material is a thermoset polymer (more brittle than thermoplastic). AM process in which liquid photopolymer in a tank is selectively cured by a source of energy (laser, digital projector).

Material extrusion (SD)

The used material is a thermoplastic polymer (more ductile than thermoset, dissolving in a solvent) or a composite. AM process in which material is selectively dispensed through a nozzle.

Material jetting

Thermoplastic polymer (The poly is cured after the deposition).

Material jetting drop on demand

Wax.

Binder jetting (SD)

The used material is sand or metal. AM process in which a liquid glue is selectively deposited to join powder materials. The produced model is a composite material in which the matrix is the glue and the fibers are the powder materials.

Powder bed fusion (SC)

• Multi jet fusion (thermoset polymer)
• Selective laser sintering (thermoset polymer, ceramic, glass, metal)

Powder bed fusion (SC)

• Direct metal laser sintering/Selective laser melting (metal)
• Electron beam melting (metal)

Direct energy deposition (SD)

AM process in which focused thermal energy is used to melt the material as they are being deposited.

Powder bed fusion

In powder bed fusion, a heat source melts/sinters powder particles to join with each other and generate a solid layer. Once a layer has been completed, the build platform is lowered by the thickness of the layer and a new layer of powder is spread upon it by the recoater. When the build is completed, the loose powder is recycled and the objects are separated from the building platform through wire cut.

Wire EDM

Electrical Discharge Machining (elettroerosione) is the process in which a thin metal wire, usually brass, is carried through material submerged in a tank of dielectric fluid.