3D printer combines top & bottom DLP projection

WASP started work on this project three years ago, running into several problems along the way that slowed down progress. A few months ago, inspiration struck: the image of a film immersed in a liquid opened up the WASP designers' minds.

In the TOP DLP the image coming from a high definition projector is directed toward a film that is immersed, on its lower side, in a photosensitive liquid. The print plate is initially laid onto the film and - moving progressively away from it - determines the layered growth of the high definition 3D print.

This mechanism solves the issue of the piece’s detachment from the projection plane, with up to 100% print success rates. Once the operation is complete, a special, watertight lid closes the vat (which can be replaced even during the printing process), thus reducing cleaning requirements and related issues.

A high definition projector that projects a 9cm wide image can reach a precision of 0.08mm. Building parts with such accuracy is the dream of any technical designer but a good quality projector and resin are not enough. This is the premise that the TOP DLP project built upon. The image can be projected both from the top and the bottom: projecting it from the top on a plate that gradually lowers enable simple and affordable prints.

The question remains on why many 3D printer manufacturers continue to project light from the bottom. The answer is on Wikipedia, under 'capillary action'.

Capillary action (sometimes capillarity, capillary motion or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper, in some non-porous materials such as liquefied carbon fibre, or in a cell. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces… in short the capillary action is due to the pressure of cohesion and adhesion which cause the liquid to work against gravity.

The behaviour of fluids on a geometrically complex part generates errors in the shaping of the object when light is projected from the top side. The error is acceptable for amateurs but not in the professional arena, where DLP precision needs to be fully exploited. This is the reason why all manufacturers have been focusing on bottom projections. The image is projected on the bottom of a transparent container after this has been made anti-adherent. The print plate is laid at the bottom and slowly raised, as the part 'grows'.

In truth, even this apparently simple solution is such only at first sight. An anti-adhesive surface is necessary and it must coincide with the bottom of the resin container. The anti-adhesive treatment can be obtained by applying a layer of silicon on the bottom of the vat.

Normally DLP and SLA manufacturers supply replacement vats or silicon and it is the user’s responsibility to change the container and replace the silicon in the printer. This makes the system more complicated to use as the vat needs to be emptied and refilled, removed and cleaned. This means that the details relative to the vat’s replacement determine the printer’s end quality.

The part’s adhesion to the silicon layer is also an issue. In spite of the silicon’s anti-adherence properties, the part tends to remain attached due to the suction effect. With use the vat become more opaque and consumed, thus attaching to the print and causing a higher percentage of print failures, as the print attaches to the projection plane and not to the print plate. Low quality 3D printers can have failure rates as high as 50%.

The solution first developed by WASP three years ago was to use a Teflon film instead of silicon, but this did not prove fully efficient. The film wrinkled up and ripped with use, the liquid oozed out ruining the 3D printer. The difficulty in building a simple, functional and safe vat stopped the project, until the inspiration a few months ago. This allowed WASP to reimagine and design the TOP DLP in order to combine the advantages of affordability and cleanliness of top side projection and the precision of bottom side light projection.

At last, after solving the issues of the part’s detachment from the print plane, and those of vat cleanliness, WASP added a system to filter out the fumes, thus eliminating the unpleasant odour that lingers around photo-polymerising 3D printers.