Blow-Peel and Vat-Lift Technology

One of the biggest challenges to overcome in LCD SLA 3D printing is the separation of each printed layer from the base of the resin vat. In a representative printer, the freshly cured layer will always be in direct contact with the base of the vat which is, in turn, in direct contact with the LCD. To introduce fresh resin under the print, ready for the next curing cycle, the part must be lifted and separated from the base of the vat.

The vat material is chosen for its non-stick properties, so there is no chemical adhesion between the cured resin and the vat. There is still a problem, however. There is a perfect fit between two faces and pulling them apart will mean introducing resin into the gap. Even in air, this can be a tricky problem. If you have worked with finely polished machined components, you may be familiar with the apparent bond created when two smooth, flat surfaces come into close contact. When dealing with water, the problem is multiplied. Anybody who has picked up a glass of chilled drink and taken the coaster with it will be familiar.

Most 3D printing resins are more viscous than water, so the problem is magnified again. In the case where the base of the vat is a flexible membrane such as FEP film, the same logic can be applied to the interface between the film, and the LCD. The fluid in question here is usually air, but the two objects are not inclined to separate.

Concept of LCD 3D Printing

For now, consider the vat base to be part of the LCD. If we imagine that both the LCD and printed part are completely rigid, and we want to separate them by moving one perpendicular to the plane of contact, we are left with an almost impossible problem. To fill the space created, the resin would have to travel instantaneously from the outer margin of the printed layer to its centre, breaking some laws of physics in the process. The resin cannot travel fast enough, and instead, a partial vacuum is created in the unoccupied space and the resin will vaporise to fill it. This ‘cavitation’ leads to bubbles in the resin, but more importantly will apply huge forces to your LCD and printed part. Possible real-world outcomes are the print being pulled off the build platform, or worse, the LCD being pulled out of the printer.

Luckily, we work with materials that flex and deform when exposed to force. A green printed part will have some compliance and an LCD has a degree of flexibility, even when mounted to a thicker glass plate. The structure of the printer itself, including the print plate arm, will not be rigid.  If we separate the part from the vat very slowly, we can allow the resin to be introduced to the gap gradually. Separation will occur at the edge of the printed layer and progress towards the centre. Obviously, this still has several drawbacks, but it is a viable solution for smaller format printers.

With industrial-grade, large format LCD printers, such as LC Opus or LC Magna, a better solution is required. Bigger parts lead to bigger problems. We do not want to compromise on the lifetime of the LCD by exposing it to excessive force and we do not want to compromise on build rate by using extremely slow retract speeds. Photocentric has used a different, novel approach in each case and both are included in one of our patents. LC Opus uses the ‘Vat-Lift’ method and LC Magna uses the ‘Blow-Peel’ method.

Vat-Lift

The Vat-Lift system involves a second stepper motor and lead screw, driving a second carriage mounted on the z-axis rail.  A horizontal pin attached to the back of the resin vat clips into a slot in the carriage. This creates a free-moving hinge. At the end of a layer exposure, the vat lift carriage and the print plate lift simultaneously. As soon as the rear edge of the resin vat separates from the screen plate, air is introduced between the vat film and the LCD. This allows the vat film to behave properly, as a flexible membrane. This gives us a much more favourable situation at the interface between the printed part and the vat film. The film can initially detach at the margins of the part and peel away smoothly as liquid resin is introduced. A wave-front of resin will close across the underside of the part until complete separation occurs. The vat can then be lowered into its default position, ready for the next curing cycle.

3) Vat returns to its original position, ready for the next layer.

Blow-Peel

The Blow-Peel system uses a different means to achieve the same end. There are four holes in the screen plate adjacent to the four corners of the LCD. These sit within the margins of the resin vat, directly underneath the vat film. Air pipes connect these holes to a pump system that can generate positive and negative pressure. A foam gasket is clamped in place under the walls of the vat to maintain an air-tight seal. Just before the print plate lifts, the air pump starts to blow air under the vat, separating the vat film from the LCD.

As before, this allows the vat film to behave as a flexible membrane. The air will naturally be drawn to areas where it is most needed. Even a very large printed part can peel away gently from the vat film as the print plate lifts.  Once complete separation is achieved, the pump system will start to draw air out of the pocket between the vat film and the screen. This ensures that the two are back in direct contact ready for the next curing cycle. The LC Magna essentially breathes in and out every time it prints a layer.

3) Air is drawn out of the pocket between the vat film and the screen, ready for the next layer.