How to... carry out quality control in additive manufacturing

Materials World magazine
1 Jul 2018

Peter Morgan, Technical Product Specialist at Elementar UK, discusses how 3D printing works, the role of metals and plastics in the process, and what techniques can be used for quality control. 

Additive manufacturing and 3D printing, is one of the most exciting technological innovations in decades – imagine having the ability to print anything you want to the exact specification you need. 

For instance, you can use computer-aided design tools to design and create prosthesis that’s an exact fit for the recipient’s anatomy. And if you are an astronaut that does not have the right tool to repair a key system on the International Space Station, you can easily print your missing tool with the right schematic.

Despite the tremendous benefits, in particular with plastics and metals, it is imperative to ensure the material used to print with remains at the highest quality. Failure to do so will result in inferior quality results, leading to breakages or prints that fail to bind properly.

How 3D printing works

3D printing works by breaking down an object into small slices and producing the item from the bottom up, section by section. These pieces are glued together to form a solid object.

Depending on the printer, the heated extruder head (the device responsible for distributing molten metal or plastic) moves in three dimensions while the device lays down the filament, layer by layer. Each part of the object that is being printed can be quite complex, meaning 3D printers can integrate moving parts, such as hinges or wheels, onto the same piece.

Materials used in additive manufacturing are paramount to how the item looks and feels, as well as its quality. Many 3D printers use polylactic acid or an acrylonitrile butadiene styrene filament. However, metals, wood, and even biological matter can be used to produce an object. 

The model of the 3D object to be printed is loaded into a computer onto a triple-axis grid. The X-axis represents the side-to-side movement of the print-extruder head, the Y-axis represents the front-to-back motion and the Z-axis indicates the object’s height.

Once the printer has all of this information, it estimates the height of what is being printed and analyses how the extruder and the build platform must move in order to lay down the appropriate filament layers – a process called slicing. The software then converts slicing data into a programming language called G-code that contains all the instructions the printer needs to complete the task.

Plastics in 3D printing

The most common type of plastic 3D printing is fused filament fabrication (FFF). First, a continuous filament of a thermoplastic material is fed through a moving printer extruder head. Molten plastic is then forced out of the print head nozzle to create the item.

FFF can offer many advantages to businesses considering 3D printers. The properties of plastics are extraordinarily diverse, with density playing an instrumental role in the malleability of the material. Polyethylene (PE), for instance, is the most common plastic on Earth, and can be manufactured at different densities for different purposes. For example, ultra-high-molecular weight PE is used for fishing lines and low density PE is used for plastic bags.

When considering using plastic for additive manufacturing applications, the highest quality should be used. This is because an inferior quality plastic will result in a poor 3D print. 

Elemental analysis holds the key to determine the quality of plastics on an industrial scale. Spot checks using elemental analysers, such as the inductar range of cubes, can help identify poor quality raw materials. 

Plastic contains additives to change the properties of the polymer, and, as such, may increase nitrogen and sulphur quantities within the hydrocarbon. These elements may give undesirable physical characteristics, resulting in a lower quality print. A simple elemental analysis will identify impurities in the plastic.

How to check the quality of a finished product

The quality of the powders used in additive manufacturing is important because these powders impact the physical properties of the finished product. For instance, it plays a crucial role in shaping the product’s tensile strength/brittleness, heat tolerance, resistance to corrosion, and impact resistance. 

As the process requires the sintering of different components to form an alloy, a precise understanding of how the chemical composition of the starting materials impact the finished product is needed. 

This is because elements that were present at the beginning of the process can be lost or changed under the heating process. The composition of carbon, sulphur, oxygen, nitrogen, and hydrogen will impact on the physical properties of the final product. 

As such, it is essential to analyse the final product to compare elemental composition. By comparing good and bad samples, you can understand what differences there are between the two.

In addition, the powders used in additive manufacturing can oxidise over time. Therefore, it is important to gain a comprehensive understanding of how quickly this will occur and to find out more about how this oxidisation will affect the finished item. 

How the future of additive manufacturing stacks up remains to be seen. However, the benefits of using powders – especially the removal of physical limitations of products – should see them playing an important role as the industry develops. Similarly, improvements in the quality of the powders used will help to increase the number of products that can be made.

Existing barriers to successful additive manufacturing are the physical properties of the alloys formed. One area of development is to improve their strength to match that of cast/forged metal items. When this technique is refined, it will be possible to produce 3D-printed items far cheaper. Currently, cast/forged items are expensive due to the cost of the moulds that need to be created for the production of the piece. This is common with other items, such as plastics.

With powders being vital to help revolutionise many industries, the importance of using high quality powders is clear. Additive manufacturing could play a crucial role in production in the years ahead and so, it is imperative that the materials used are properly tested and analysed. With the right equipment, any fears that a powder is unsafe or unsuitable can be allayed.