Vat Photopolymerisation

Vat Photopolymerisation
describes a group of different additive manufacturing technologies.

Ultraviolet (UV) light is used to solidify /cure thin layers of a liquid photo-polymer-resin.
The different Vat Photopolymerisation technologies use different light sources and technical
setups to emit uv-light and cure the resin selectively. Initially only UV-laser with a scanner systems\
to direct the laser beam were used. Lateron, machines with DLP technology and lights directed by fibers optics with micro-shutters entered the market.

(SLA®) is the oldest of this additive manufacturing technologies.
Developed and patented by Charles (Chuck) Hull in the 80s. The shortcut SLA® is
registered by 3D Systems, the company Mr. Hull founded.
Sterelithography allows to create parts with a very high geometric accuracy.


How does Stereolithography work?

The processes
uses a laser emitting ultraviolet light as an energy source.
The laser is usually a solid statelaser  with the power of just a few watts. The beam is directed using a scanner-system. This is an arrangement of two mirrors directing the beam in the XY-plane / on the surface of the resin. A F-Theta / flat-field lens is used to keep the focal length constant in the z-axis. The build platform is usually a grillage, sitting in a tank filled with the photo active resin.

The process works cyclic in three steps:

1. step
The platform drops by one layer thickness just under the surface of the photo-resin.

2. step
The laser scans / projects the cross-section of the part on the thin film of resin.
The material solidifies locally in this area.

3. step
The platform drops again by the thickness of one layer.
A wiper swipes over the surface. This ensures that resin layer with a constant thickness
is created.

The next layer is scanned and the cross-section is solidified on top of the previous layer.
The part grows layer by layer until it is finished. When the final layer has been created the
platform raises  up and the build platform with the part can be removed. d.

What are supports needed for?
Support structures are required to create overhanging sections. This intricate
lattice structures are made of the same as the build material. Supports are build
simultaneously with the part and have to be removed manually afterwards.

Post processing
Parts are usually washed with alcohol to remove and residual resin. Parts might be
post-cured in a UV-cupboard for full solidification of the material.

The Accuracy of Stereolithography is one of the highest to be reached by additive
manufacturing techniques. Manufacturers state the accuracy with up to 4000dpi.
The layer thickness ranges from typically 0,5 to 0.15 mm but there are exemptions.

Materials are all photo-active polymer resins but their material properties can vary.
From very clear to opaque, from 130 degree Celsius heat resistant materials to materials
with low ash content  for casting. A large variety of materials is available.

SLA machines
Build envelopes of pro-machines range from 250x250x250 mm to 1500x750x550 mm (XxYxZ).
Systems starting at approximately USD100k but can raise up to several hundred thousands.
With the core patents for SLA expired, the market opened for small and entry level machines.
For example Form1 from Formlabs was financed through a Kickstarter campaign. The machine
has a build envelope of just 125 x 125 x 165 mm (4.9 x 4.9 x 6.5 inch). The price was brought down
to USD 4000 and so the technology to into small businesses and to pro-sumers.


What are the applications?

Applications range as much as the material properties vary. Stereolithography is
applied in dental, jewellery and hearing aid industry. It is also used to prototype
products in development, to create  casting patterns for casting of metals, the
creation of medical models and patient specific medical devices.


Continuous Liquid Interface Production  

Clip is the latest development regarding VAT Polymerization technologies.
CLIP works different to the common technologies which work always
in steps, curing resin layer-by-layer. It is a photo-polymerization process
but also assisted by a reaction with oxygen.Carbon3D_CLIP

 The build speed is remarkable higher compared to other AM technologies giving CLIP the potential to be disruptive for similar technologies with lower productivity. This might be why AUTODESK has recently backed CARBON3D with $ 10 Mio. spark funding.


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