What is Vat Photopolymerization?
A Guide to the process and its most common forms
Vat Photopolymerization a 30+ year old method of 3D printing that is being used to create everything from prototypes to functional industrial parts. It is used in additive manufacturing to create pieces that can have a wide variety of physical and mechanical properties.
The most common types of vat photopolymerization are Stereolithography (SLA), Digital Light Processing (DLP), and Liquid Crystal Display (LCD). Even more are being developed as software, light sources and optical clarity continue to improve. We will review the equipment and chemicals, as well as the benefits and concerns in this feature.
1. What is Vat Photopolymerization?
Vat photopolymerization, often referred to as VPP, is a 30+ year old method of 3D printing that utilizes a light source to selectively harden photoreactive resin. The 3D printing resin is held in a vat either above or below the light source. In top down printing, the build plate remains submerged below in the resin tank and sequentially lowers when each successive layer is cured. In bottom up printing, the build plate is suspended above the vat, and the light is projected through a film, or window, on the bottom.
The printed part is commonly retracted then replaced near the film so that the resin can flow under it to create another layer. The light source, either a laser or some variation of projected light, irradiates only specific areas on the vat of photopolymer. The base then adjusts to maintain a consistent volume of liquid photopolymer between the light source and the printed image/piece, and the process is repeated until the object is completed.
Vat photopolymerization is known for its extremely good resolution and accuracy. The stated resolution on most printers is the resolution in the XY plane. When using a laser-based system, that is the width of the laser itself. When using any of the projected light methods, that is the best possible resolution of the image (similar to resolution ratings for televisions). The layer thickness is the resolution in the Z direction. This is determined by several aspects including the amount of power delivered by the light, the accuracy and control of the mechanical parts among others. Depending on the type of printer, build sizes will be determined by either the size of the vat or the capability of the light projector.
2. Most common types of Vat Photopolymerization
There are several types of Vat Photopolymerization printers. The first was SLA, which has traditionally been considered to have the best resolution. In traditional SLA, the laser traces each individual layer. The very narrow width of the laser allows for better resolution. Because the laser traces every layer, the speed is typically not quite as fast as other methods in this category.
DLP adds a speed advantage due to irradiating each layer at one time. The projector works precisely like a television projector. The mechanics are vastly similar to other Vat Photopolymerization methods regardless of whether it is top down or bottom up. The technology also allows for typically lower hardware costs, and the use of less resin. There is a slight detriment in that the build volume can be limited due to more limited light power being delivered at the edges.
LCD is very similar to DLP. The main difference is the light source itself. LCD uses a single source lamp and the LCD masks where the light is not supposed to hit the resin.
LCD can theoretically scale to larger build volumes as the light intensity stays the same across the entire vat. The issue is that light can slightly bleed around the mask which can reduce the overall resolution of the print.
As the technology has progressed, companies have developed ways to continuously print with VPP. This is done in bottom-up printers and achieved by using a gas permeable film. This lets oxygen continuously flow through it which inhibits the curing of the 3D resin and allows the print to continually be pulled away as it does not adhere to the membrane. These machines tend to be very expensive, but they have allowed a more rapid growth into manufacturing instead of simply prototyping.
A method still in development, but showing notable promise is volumetric printing. This method requires advanced print engines and optimal optics but allows for much faster printing. Thus far, print speeds have been able to be pushed down to under a minute (albeit relatively small parts).
3. VAT photopolymerization applications
Due to their ability to print objects with high accuracy and details, Vat photopolymerization methods are most often applied in the medical and dental industry. Resin requirements are somewhat method dependent, although lower viscosity tends to be better as it allows for faster flow and more uniform recoating for uniform layers. Relatively fast cure and low shrinkage upon cure are also very desirable. One notable drawback is that the vast majority of Vat Photopolymerization applications are single material which can limit the number of applications for which it can be used.
Vat Photopolymerization is a very versatile, and effective means to produce both prototypes and production pieces. As speed and build volume continue to grow, the economic arguments will only get stronger.
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3D Printing Business Development Manager EnergyCuring
RAHN USA Corp.