The 740THz Protocol for Resin Printing - Part One

What you need to know and buy to get started with SLA Printing

Photo of small SLA-Printed gears and support

Foreward

Resin printing provides an exciting process for the production of complex functional parts for a variety of purposes, including proof of concept demonstration, aiding visualisation and inspection of designs for manufacture by other means, and in select applications as a primary manufacturing process. It possesses a number of useful properties, including the independence of printing time from geometric complexity, the ability to efficiently print many parts in parallel, and the capability to reproduce fine detail. However, in order to fully realise these potential benefits, it is vital to adopt a well-defined, controlled, consistent process. The following has been developed over three years of experimentation and practice using commodity hardware and consumables, with the goal of allowing the reproducible and precise printing of intricate mechanical and functional parts including mating features, threaded fastenings, and small module gears.

Equipment

This process was developed using various models of Anycubic Photon 405nm LCD masked SLA printers, including the ‘OG’ Photon and Anycubic Photon Mono SE; however, it should be generalisable to most commodity resin printers. It is advisable to consider the following features when reviewing the suitability of an SLA printer:

The quality of the LCD, including its size (preferably ≥6”), type (mono being superior), and pixel pitch (≤50μm being desirable);
The quality of the UV source, including its power (best expressed as watts/cm2 of screen area) and type (matrix/“parallel” lensed sources being superior);
The rigidity of the mechanical parts, particularly the z-axis (dual Hiwin-style linear rails mounted to a sturdy upright being ideal), base plate, and print bed joint.

Modifications

The only modification presently advisable is the flexible magnetic print plate ('flex bed'), which is inexpensive and has benefits for safety (removing the need to use a sharp and potentially hazardous scraper) as well as productivity, and reducing the possibility of damage to parts and the print bed when removing completed prints.

Accessories

For the purposes of consistency in post-processing it is worthwhile to also obtain a UV curing chamber (preferably heated, with thermostatic control), or a wash-and-cure system. Alternatively, a magnetic stirrer, ultrasonic cleaner, or collection of glass or plastic 'pickle jar' containers watertight sealing lids should be available for cleanup.

Safety Equipment

Single-use nitrile gloves are preferable, and can be found inexpensively in bulk quantities. Applicable standards which should be considered include:

FDA 510(k) ‘Medical Examination Glove’, ASTM D6319, or EN455 approval;
ISO 374-1:2016 approval for chemical resistance (Ideally Type A or B);
ASTM F 1671 or ISO 374-5:2016 for porosity and barrier performance (Ideally Level 1 or 2).

Other national and international standards exist, and information on glove performance and certification should be reviewed.

Lightweight polycarbonate safety glasses or goggles should be available; standards-complaint models are readily available and inexpensive. Applicable standards include:

ANSI Z87 with D3 (splash protection), or
EN 166 with type 3 (splash protection) and class 1 optical performance.

Other national and international standards and certifications exist and should be reviewed.

IPA-soaked single-use microfibre swabs may be bought in bulk quantities, and can be convenient for cleaning up small spills of resin; they are not essential, although some other sort of soft disposable absorbent material should be available for dealing with spills.

Consumables

The principal consumable required in the SLA Printing process are 405nm UV curing resin, and a cleaning solvent - typically IPA. While a wide range of resins are available at a variety of price points, general purpose commodity resins are typically available for $40 / €35 per litre. Commercially pure (99.9%) IPA is around 1/8th to 1/10th of this price. Most occasional printers can expect a litre of resin to for a few months, and a litre of IPA to last longer still.

Resins

Many commodity 405nm resins intended for SLA printing are readily commercially available. The choice may be influenced by cost, properties of the cured resin, and aesthetics, and will need to be made in view of the intended application. However, it is worthwhile to make principal use of either one, or only a small number of resins to allow greater familiarity with the performance and requirements of the specific resin in use.

Compositions vary, but all resins must be handled with care, and their MSDS should be reviewed and followed. 'Water Washable' or 'Eco' Resins should not be considered to be safe or innocuous. All Resins should be considered an irritant, sensitising, and photosensitising substance, and PPE should always be worn when using it. If skin contact does occur, it should be immediately rinsed off in line with safety advice in the MSDS

Solvents

99.9% Isopropyl Alcohol (IPA) is the preferred solvent for resin cleanup. It is inexpensive and presents relatively low hazard in comparison to other alternatives. However, it should be remembered that it is flammable, has a tendency to evaporate and produce vapours, and it can be irritant, intoxicating, and poisonous if consumed. It should be handled with care, used in well ventilated spaces, and and any spills promptly cleared up.

Preferably it should be held in small quantities of up to 5L, and purchased in individual bottles of a smaller volume (500-1000mL), and should be stored securely and out of sight and reach of children. Only one container should be in use at a time, as it will tend to both evaporate and absorb moisture from the air over time.

Lubricants

White Lithium Grease specified for NGLI grade 2 is ideal for lubricating metal-on-metal contacts in leadscrews, ballscrews, and linear rails; it is inexpensive, readily available, and provides a good coating of metal surfaces ensuring lubrication, preventing corrosion, and avoiding it running off vertical surfaces. Only small amounts are needed, and it will not need to be replaced frequently. Around 10-20g should be sufficient to lubricate a printer, and it should be replaced when it becomes significantly discoloured - depending on usage, this may take 6-12 months or longer.

Thin lubricants such as sewing machine oil or silicone oil are a poor choice, and are liable to run off the rails and screw and seep into the electronics underneath the print bed. An NGLI #2 Silicone grease may be an acceptable substitute for white lithium grease, and has the advantage of better compatibility with plastic and rubber, but is more expensive and more likely to be displaced under pressure from metal-on-metal contacts, leading to poorer coating and lubrication performance. If you already own e.g. PTFE dry lube, this has the advantage that it will not attract dust and contaminants, although as the printer should be closed for the majority of the item this may not be as significant a concern as its reduced performance in coating surfaces and preventing corrosion.

For printers utilising plastic-on-metal contacts - e.g. with PTFE or Delrin nuts or linear bearings, silicone grease is preferred.

Regarding improvised lubrication options: cooking oils should never be used; they are liable to polymerise, forming a sticky coating which is difficult to remove. Vaseline may be an acceptable choice, however if specifically formulated lubricants are not immediately available, at the loadings and usage patterns typically experienced in hobbyist 3D printing applications significant wear is unlikely to occur quickly if the printer is operated even completely without lubrication.

Safety Measures

Gloves and safety eyewear should always be worn when handling liquid resin, resin-contaminated materials and parts, and any resin spills.

While many accidental exposures to SLA resins occur without obvious immediate harm, resins should not be allowed to come into contact with skin, eyes, or mucous membranes; there are anecdotal reports of significant injury following resin exposure, including skin and corneal burns. The MSDS for commercial resins should be consulted for advice regarding their safety and actions to be taken in case of exposure.

Contact with solvents should similarly be avoided; while IPA is a common component of alcohol hand rubs and skin sanitisation solutions, it can be very irritating to eyes and mucous membranes, and in 3D printing process use is often contaminated with resin or other printing materials.

SLA printers, process chemicals, and processing operations should be used in well-ventilated areas to avoid concentration of vapors released from solvents and resins, which may be hazardous and flammable.

Printing Environment

Ideally the printer and other process equipment should be sited in a well ventilated space, protected from strong light and direct sun. Ideally, the printer should be protected from light as much as possible when not in use - for instance, it could be set up in a cabinet, or a cover could be placed over the top when not in use. Small amounts of light of suitable wavelengths to cause curing are likely to leak through to the build chamber through both light leaks and imperfect filtering performance in plastic windows, and even if it does not lead to noticeable curing, over time it may lead to consumption of photopolymer components which will impair printing performance. This may explain some reports of limited 'vat life' of resins not stored in their opaque bottles.

The printing environment should have reasonably consistent temperatures no lower than 60f/15C and not regularly above 90f/30C - with some resins, performance has been noted to degrade significantly at lower temperatures, requiring longer settling and curing times with poorer base and layer adhesion, although where temperatures are consistent process changes may be able to correct some of these issues.

Work surfaces should be uncluttered and topped with impermeable, wipe-clean material, such as stainless steel or silicone. Ideally the work surface should have a turned-up lip to contain spills, although where space is limited a small oil drip pan may be used to hold process equipment and active containers of resin and solvent.

Safety equipment including disposable gloves and eyewear should be within easy reach, as should materials for dealing with accidental spills of resin and resin-contaminated materials, and a disposal container.

Only minimal quantities of process chemicals should be stored in the workspace - ideally only one bottle of solvent and one bottle of resin - and they should be kept in their original containers which should have child-safe lids. Supplies not currently in use should be stored in a cool, dry place out of reach and sight of children and pets. So long as only small quantities are held it should not be necessary to store them in a chemical storage cabinet, the most inexpensive of which can cost around the same as a SLA printer; however, they should be stored like to household cleaning chemicals.

Discarding of solvents and resin

Resins and soiled solvents should not be considered safe to be discarded directly in waste streams or into drainage systems; many resins contain components which are toxic to the aquatic environment when uncured.

In commercial environments solvents can be disposed of in chemical waste streams or through solvent recovery processes which may include filtering and distillation; however, these are often not suitable for small-volume operations.

The following methods are commonly used and suggested amongst SLA printing hobbyists, although MSDS information, applicable local regulations, and common-sense safety precautions should all be observed.

Resin should be thoroughly cured prior to disposal, and may then be discarded alongside household solid waste. This can be accomplished by pouring thin layers up to 1/32” or 1mm thick into a wide-bottomed disposable container and exposing it to direct sunlight or a UV curing chamber until it is well hardened, and adding similar successive layers until all unnecessary resin is discarded. Using thicker layers may lead to incomplete curing of resin at the bottom.

Soiled solvents may be disposed of in a similar way, allowing the solvents to evaporate and disposing of any remaining cured solid residue as solid waste. However, care should be taken, including ensuring that the area is well ventilated to prevent the buildup of solvent vapours, ensuring that there are no flammable materials and sources of ignition nearby, and limiting the total volume of exposed solvents.

740THz