Prototyping

CNC Prototyping.

Computerized numerical control (CNC) machining has been used for many years and it underwent evolution and application to many different machines gaining a growing efficiency, precision and waste reduction. CNC prototyping does not use a simple milling machine but a tool for cylindrical cutting that can rotate in different directions and move on several axes (from 3 to 5 axes). This tool can make complex operations of engraving, piercing and levelling with millimeter precision.
Computerized numerical control machines produce specific shapes that could not be produced by any manual tool. Moreover, the majority of CNC milling machines have a special device pumping the liquid to the cutting tool during the production process to reduce the friction.
CNC prototyping is the deal choice for companies in need for high precision components.
All you need is a precise software programming.

CNC milling machines use almost all materials that can be pierced or cut. Nevertheless, most CNC machines work with metals, hard plastics and resins. The milling machine is chosen according to the material density.

Sintering.

Sintering is one of the most advanced technologies for prototypes production.
This technique uses small solid atoms (“powders”) that are fused together by thermal and/or mechanical treatment creating one solid piece. The same product could not be obtained with any other molding technology. The choice of powders and alloys composition and dosage has an important impact on the entire process and the result quality. Products obtained by sintering grant extreme hardness, shape precision, stress resistance (when metal powders are used) and low series production costs.
Its widespread use is due mainly to these special features:
- different types of powders can be used;
- hollow shells containing other products can be made, leaving just one hole to empty powders from the prototype (for instance a whistle with a holed hollow body containing a pea can be made simultaneously);
- back drafts (current technology makes it costly to use back drafts in the final product concept, but when sintering is used directly in production, they may open up new horizons in aesthetics and techniques).


Process

Selective Laser Sintering (SLS) entails heating and fusing a thin layer of powder which settles onto a controlled platform on the z-axis.
CAD data are split into 2D cross-sections with set thicknesses. These data guide a laser beam that traces a cross-section on the surface of the powder, which is preheated to a temperature just below its melting point. The energy fuses the powder, which solidifies into the cross-section. The laser beam only fuses the area specified.
A roller is used to deposit a second layer of powder onto the first.
This process is repeated until the prototype is complete.
Uniform prototypes of unlimited complexity can be created with this system.
The powder in the work area supports the prototype so no additional supports are required, which is a huge advantage. The prototypes are then taken from the work chamber and any excess powder is brushed off (the powder is collected and put back into the cycle). The prototypes can then be painted. Generally almost any material can be sintered as long as its optical absorption properties are compatible with the laser’s radiation.

Materials

The most widely used powders are made of polyamide. This plastic material is a middle of the range option for creating products as generally it is not as stiff as ABS and not as soft as polyethylene. Polyamide can also be blended with fibreglass to strengthen prototypes that have to be tested in real-life conditions.
Somos, however, is an elastomer powder that is used to create "rubber" prototypes with excellent elasticity and resistance properties.
Aluminum powders and metal powders are the latest inventions. When prototypes are made of metal powders, an additional phase is normally added to the standard process. A blend of metal powders and plastic resins is sintered to create the prototype, which is then placed into a chamber/oven. Here the plastic material is sublimed and any cavities in the prototype are filled with bronze, thus creating a 100% metal prototype.
This technology is already being used to produce small injection moulds for plastic materials.

Stereolithography.

Stereolithography, which literally means 3D printing, is one of the most widely used technologies in Rapid Prototyping. It takes CAD images and turns them into three-dimensional resin prototypes in no time, thus cutting out the intermediate phase of mould production. Stereolithography has four different facets:

   - lasers
   - optics
   - chemistry
   - computing

The first step creates a 3D file where the image is positioned on a platform. Supports are generated to hold up the model while it is being created and the image is then sliced into X and Y layers, which are the construction planes. Afterwards the CAD 3D image is converted into STL format so that the management software installed on the Rapid Prototyping machines can read it.
The construction phase is a Solid Free Form Fabrication process (SFFF) which means that the prototype is built by adding particles or layers until the required shape has been obtained (additive process) as opposed to a subtractive process, which cuts away material from a solid block.
During the construction phase, an optics system focuses the laser beam on the surface of photosensitive liquid resin in a vat. The laser beam triggers a polymerization reaction and creates a thin solid layer: a cross-section of the prototype. The elevator lowers the same distance again and a precision sweeper system coats the solidified cross-section with a thin layer of resin. The laser then traces the second layer on top of the first. This process continues until the prototype is complete. Finally, the elevator lifts it out of the vat.
As polymerization is a lengthy process, the laser does not solidify the cross-section completely. Instead, it cures the profile and a certain number of lines that join the internal and external perimeters. By the end of this phase, the prototype is solidified on the outside, but there is still some liquid on the inside. As its physical consistency is not yet acceptable, the prototype undergoes post-processing in order to complete polymerization. Post-processing involves exposing the prototype to an ultra-violet light; the length of exposure depends on the prototype's dimensions, the complexity of its shape and the type of resin. By the end, any liquid resin trapped inside the piece has been polymerized.
After post-processing, any supports are broken off and the prototype undergoes finishing (painting, sanding or other finishes).
Stereolithography affords high-precision modelling and creates complex shapes with thin walls and good surface quality.
The prototype's high definition means that it can be sent for technical, functional and design analysis. However, a stereolithography prototype is very fragile and should not be put through mechanical tests.

VCS - Vacuum Casting System.

Vacuum Casting, or silicone tooling, is one of the most widespread techniques for the rapid construction of flexible moulds that are used to produce a limited number of prototypes (1 - 25) in a material with similar physical, mechanical and aesthetic properties to the end material. 

Process aim

This process builds a preproduction version of a new model that is suitable for a range of essential functional tests that would be used on the end product. The lead times are short and there is no need to make the end moulds.

How to create a silicone resin mold

A stereolithography master version is suspended in a container. After breathers and pouring channels have been fitted, it is coated with degassed liquid silicone. At the end of this process, the mold is placed in a special oven and left to harden. The mold is then cut into two or more parts with irregular profiles that can be clamped together at a later stage. This tooling can be used to make prototypes with vacuum casting.

Molds made with this process are used to:

- create an almost exact replica of the final version
- conduct functional tests
- carry out marketing tests
- bring forward product approval (in some cases).

Functional tests

Prototypes are not built to withstand a full range of functional tests and so only some of their features should be subjected to physical and chemical trials.
It is therefore important to establish which product features are to be tested with the prototype before a preproduction version is made.
Elastic module . is the most widely simulated property. Materials with the elastic module of thermoplastics (ABS, PA, PP etc.) can be created by using readily available resins (for the commonest thermoplastics) or by making polyurethane especially. The latter option is preferred when other properties have to be simulated, for instance heat resistance, tensile and torsional strength, resistance to some aggressive chemicals, plus self-extinguishing and sound absorption properties.
Only some mechanical and physical properties can be simulated at the same time. Therefore, it is important to establish a priori which tests should be given priority in order to determine what type of polyurethane would best achieve the objectives.
Polyurethanes, however, have proved to be of limited use in destructive tests whenever accurate data regarding breaking points and structural limits are required. In these situations, the prototyping phase should either combine silicone molds and rapid tooling, or simply employ the latter so that the prototypes are made in the end material as per the final production process.
The quality of a prototype is determined by how well it simulates  aesthetic features, i.e. shape, color, transparency and surface finishes. Thus, top-quality silicone preproduction prototypes can be used for presentations and marketing tests, to optimize packaging, for advertisement photographs, press conferences, etc.
Hereunder are  some factors that influence the aesthetic, dimensional and functional qualities of preproduction prototypes:  
- Silicone mold wear: the molds produce between 5 and 25 good quality casts depending on the prototype material and shape
- Mold deterioration: even when the molds are not used the silicone becomes stiffer and frays as time goes by. Consequently, molds have a maximum lifespan of 6 months.
- Limits of polyurethane: the properties of polyurethane change as time goes by. A prototype is created for immediate use because its technical and functional properties are at full strength. As time passes, however, these properties deteriorate on account of polyurethane’s hygroscopic properties, the cross-linking process and its sensitivity to UV rays. Consequently, the material deforms, and becomes stiffer and weaker.

Technical data

Precision: ±0.2% with 0.1% repeatability (max: ±0.2 mm).
Minimum wall thickness: a 0.5 mm thick wall ensures that the mold fills well. A 1.5 mm thick wall will ensure optimum results.
Maximum dimensions: mold dimensions are limited by machine size and prototype volume (max 10 liters).
Casting materials: polyurethane and other materials available in all standard RAL colors.
Applications: inserts, prototypes made of two different materials.