Stereolithography and Digital Light Processing

Technology

Photopolymerisation processes open the possibility to go directly from Computer-Aided Design (CAD) to a physical prototype. It can be used to make prototypes or a final product. There are also many advantages of this technology over the traditional processes used to develop a product such as allowing early customer involvement in product development, complex shape generation and also saving time as well as money. As the process uses liquid to form objects, there is no structural support from the material during the build phase so support structures will often need to be added. The SLA process has a high level of accuracy and a good finish compared with traditional manufacturing where multiple machines will have to be used to achieve the same result. For the part to be strong enough for structural use, post curing and structural supports need to be implemented. The process of photo polymerisation can be achieved using a single laser and optics. Blades or recoating blades pass over previous layers to ensure that there are no defects in the resin for the construction of the next layer. This way good surface quality can be achieved.

Typical applications


Photopolymerisation is commonly used in:

  1. Stereolithography (SL, SLA)
  2. Digital Light Processing (DLP)

Digital Light Synthesis (DLS)\nSLA technology is extremely versatile and it can be used when precision is the overall priority and where form, fit and assembly are critical. Considering the level of quality SLA can achieve, it’s particularly useful for creating highly precise 3D printed casting patterns from small to very large scale, quickly and inexpensively. It is often used in resin-based 3D printers ranging from hobbyist tools to professional, industrial machines.
Characteristics\nSLA allows manufacturers to save time on highly precise parts, especially when there is a requirement for a number of functional prototypes or a quick single casting pattern. For low to mid-volume parts normally machined from traditional methods, 3D objects from SLA machining provide comparable characteristics and don’t require slow, expensive retooling for customisation. In addition, SLA allows for lower material costs, as the unused resin stays in the vat for future projects, and it creates parts that are more flawless overall. The product requires no additional machining after the object is manufactured as SLA provides good quality finishes.

Materials

Many different materials can be used for Stereolithography. The Plastics and Polymers commonly used are; UV-curable Photopolymer resins, ABS-like Resin, PPE-like Resin, Glass-Filled Nylon-like Resin.
Sustainability\nThe process requires little to none post processing meaning that the finished product will not need additional machining to remove any material. Subtractive manufacturing like CNC machining removes material off a workpiece to create the final product. A lot of the material that is taken off will not be able to be reused and is wasted. Although it takes longer than most other additive manufacturing processes, it uses less energy and time than other manufacturing processes. Less machining time and resources used will reduce the harm to the environment.\nAlthough it can provide good quality products in a smaller time frame and with less resources than traditional manufacturing, the manufacturing process requires postcuring and structural support for a strong product.

Recycling

There are a number of materials that can be used for this process. Depending on the application, materials can be selected that are more sustainable and have recyclable properties. The unused resin in the vat can be used a number of times which reduces the amount of material sent as landfill. The product is made up of multiple layers that are cured by UV rays, this presents an opportunity to inspect the quality at the various layers so any issue identified can be sorted before the product has been fully completed. Detecting problems early on will reduce the amount of resources used on failed products.

Energy Use

Stereolithography uses virtual designing software so resources, money and energy are not used to produce physical prototypes. The best way to reduce the impact of the manufacturing process is to complete the task once, meaning that the project is scoped, planned and the printer is set up correctly before starting the process. Ensuring that the drawings are correct and that the software has been programmed correctly, materials and tools correctly selected will reduce errors. If the process needs to be repeated the extra energy used and material consumed will increase the environmental impact and that can be avoided with expertise and planning. Minimising energy use will be good for the environment and will reduce production costs.

Waste Materials

Stereolithography can reduce waste as it uses the exact resources required for the product. This is achieved as the product is manufactured in layers. Moreover, the materials left over from the product can be used again so wastage is minimal. The finish of the product is the most accurate of all additive manufacturing processes. This will reduce the amount of material removed in the post processing stage as the quality is already good.
Disposal of waste material\nThe materials in the Stereolithography process can be used a number of times over its lifecycle. However, there are concerns with its mechanical properties, as they get worse each time the material is reused. There are various effects of reprocessing including reductions in tensile strength by (5.2%), the tensile strength at break by (8.3%) and the impact strength by (20.2%). Concerning the thermal characteristics, results showed a slight decrease of thermal stability of some materials, lowered cold crystallization temperature and a slight reduction of the melting point with increasing number of cycles.

This means that although the material is recyclable to an extent, at a certain point the material deteriorates and is unsafe, unreliable and is destined for landfill. Although eventually the material will go to landfill its prolonged usage justifies its disposal.
Machine Footprint\nStereolithography has a reduced footprint as there is less need for transport of materials and when shipping becomes necessary the product weight is lighter than other manufacturing methods. Less materials are wasted and only one type of machine is required reducing the need for more conventional machines with higher carbon footprints.

Safe Manufacturing Procedures

Stereolithography uses UV radiation to cure the resin on each layer of the object. If your skin is overexposed to the UV radiation it can cause harm to the skin. It is most notorious for causing various forms of skin cancer. Covering up the exposed skin such as using gloves and wearing suitable eye protection will help mitigate against these risks. Reducing interaction will also reduce the impact of the UV rays like leaving the room when machining is in process and periodically checking it.

The part is also put in a UV oven to cure it which will be hot so safety signs and labels should be displayed. Since there are different chemicals used in the manufacturing process appropriate caution must be shown to its disposal and its handling.  Post curing and structural supports need to be used if the object will be used in structural applications. The finished object will need to be tested before it can be used by the consumer to reduce risk of failure.
Complexity they can achieve\nStereolithography has the capability to achieve more complex finished products assuming the appropriate structural support and post curing is used on the object. Making a gear using Stereolithography would require less support than a cup would. It will have to be oven cured to achieve a better finish. It is not the fastest manufacturing means but is the most accurate.

One of the attributes that Stereolithagraphy has is the quality of finish for its components. Even in complex products the finish is consistent. Complex designs would require additional support and would use more time than that of a simple design. The repeatability of the products manufactured are better than any other AM process.

Tolerances

The tolerances on an SLA part are typically less than 0.05 mm, and this technology offers the smoothest surface finish of any additive manufacturing process.

Surface Finish

SLA can create parts that will have a smooth finish right out of the printer, which makes them easier to polish, paint and finish if necessary. Additional machining isn\'t required to achieve a suitable result.

Capacity

The volume of the VAT for the printer gives the maximum size that the object can be in order to be printed. The size of the support available for the object also determines its size.

Accuracy and Achieving Consistency

The predictability of the stereolithography method allows for consistent reproduction of various products with complex shape, size, and other physical requirements.
The process of Stereolithography can achieve high accuracy and therefore consistency. Blades or recoating blades pass over previous layers to ensure that there are no defects in the resin for the construction of the next layer which will ensure that each product is identical. The photo-polymerisation process and support material may cause defects such as air gaps, which need to be filled with resin in order to achieve a high quality model. Although, air gaps may form the machine will trace over the imperfections and fill it with resin to maintain a uniform appearance.

Speed of operation

Stereolithography also brings painstaking accuracy without the painstaking time. Because of SLA’s speed and precision, prototypes are easy to make and faithful to the final design, which means you can identify design flaws, collisions and potential mass-manufacturing hurdles before production begins.

Stereolithography benefiting a Manufacturing Organisation

Stereolithography can benefit a manufacturing organisation as products produced have a high level of accuracy and good finish. The high detail accuracy has a typical tolerance of 0.05. The finish requires little to no post- processing. The process is relatively quick compared to a Lathe. The build areas are large, meaning that the object being manufactured are bigger than that produced by material extrusion. It can produce intricate geometry meaning that complex structures that are normally impossible with traditional manufacturing are able to be made using stereolithography. The material selection is vast with over 30 materials to choose from with a range of properties.

The cost of manufacturing with this method is more than that of material extrusion. The post processing time and removal of the object from the resin can be lengthy. The objects produced often require support structures and post curing for parts to be strong enough for structural use. The durability of materials are lower because the photopolymers weaken over time especially when exposed to sunlight. Any hollow cavities need to be perforated to drain unused material.

Safe Working Procedures for Stereolithography and material extrusion


The Control of Substances Hazardous to Health Regulations (COSHH 2002)\nSince the two processes use substances which might cause harm to health, the law requires that the organisation control these risks. The term control can apply to actions taken, to processes, or to safety equipment, used to minimise employee’s exposure to hazardous substances. In the Stereolithography process using protective clothing to minimise exposure to rays and staying a safe distance from the machine will reduce risk to employees. In material extrusion handling the filament which could have potential harmful chemicals should be done appropriately and stored in a safe way with clear labelling. Employers have legal duties under the Health and Safety at Work Act to ensure the health, safety, and welfare at work of all their employees. This responsibility extends to other persons, such as pupils, students and visitors who may be affected by activities undertaken in the workplace. The Control of Substances Hazardous to Health Regulations (COSHH 2002) requires employers to ensure that exposure is prevented or adequately controlled. This can be achieved by controlling the exposure of substances that are hazardous to health, as set out in COSHH.

CE Marking for the machines

Prior to purchasing a 3D printer that has either Stereolithography or Material extrusion and there associated materials, there are a number of things to consider. It is recommended that only 3D printer equipment which carries a CE mark is purchased for use in schools and colleges. The CE mark ensures measures have been put in place to reduce risk to the user. Provision and Use of Work Equipment Regulations 1998 (PUWER)

These Regulations, often abbreviated to PUWER, place duties on people and companies who own, operate or have control over work equipment. PUWER also places responsibilities on businesses and organisations whose employees use work equipment, whether owned by them or not.\nPUWER requires that equipment provided for use at work is:

  • suitable for the intended use
  • safe for use, maintained in a safe condition and inspected to ensure it is correctly installed and does not subsequently deteriorate
  • used only by people who have received adequate information, instruction and training
  • accompanied by suitable health and safety measures, such as protective devices and controls. These will normally include emergency stop devices, adequate means of isolation from sources of energy, clearly visible markings, guards and warning devices

PUWER applies to both processes as they must be able to function properly and be safe when in operation. The machine must be maintained regularly and inspected to mitigate against any faults. The equipment is dangerous so only individuals who have the correct knowledge and instruction should use it. The machines should have various safety labels, emergency stop buttons, guards to prevent the user accidentally harming themselves and indicators to show its in operation.

Personal Protective Equipment (PPE)

Personal protective equipment (PPE) must be worn before working with any piece of equipment. PPE is equipment you wear to create a barrier between you and something that could cause harm. Below is a list of safety equipment relevant for both processes.

  • Wear gloves when handling uncured materials.
  • Wear eye protection around liquid materials that can splash.
  • Wear a protective P100 respirator dust mask when accessing the stage area.
  • Proper storage containers for materials.
  • Materials need to be handled properly before they are moved to the AM equipment. Safety includes the testing and validation of materials before you even start using them.
  • Manage waste properly. Testing on materials and checking on how they should be properly disposed is a good preparation for this.
  • Wearing grounding straps.
  • Wearing overalls to prevent exposure to any loose particles.


General safety hazards associated with Material extrusion and Stereolithography

  • Consult the Safety Data Sheets (SDSs) of printing materials
  • Provide OSHA safety training to individuals that work with hazardous chemicals
  • Provide training in the correct and safe operation of the machines
  • Use equipment in a well-ventilated area
  • When using metal materials keep workspaces free of any static electricity
  • Do not open the printer covers once a print job is underway
  • Equip the workshop with fire extinguishers and train on its proper use
  • Use solvent-absorbent pads for spills of printing material
  • Keep model and support materials away from areas where food and drink is stored, prepared, or consumed