When we talk about plastic fabrication, we refer to the design, manufacture or assembly of plastic products through a manufacturing process. The properties of plastic determine the method of manufacturing and the processing parameters. There are plenty of technologies to fabricate plastic parts, and the choice is based on the requirements of the final application, the number of parts to be produced and the available budget.
Plastic is one of the most versatile and inexpensive materials available for consumer products and manufacturing industries. This whitepaper contextualizes the fabrication of parts and components using plastic through a variety of technological processes.
The necessity and the goal of a business indicates which technology better suits the business needs. Lead time, budget, final product properties and product use will determine the plastic and fabrication process used for the manufacturing.
Conventional Manufacturing Technologies
These type of processes involve changing the shape of the raw material to create the model, using moulds or hard tools to mechanize the material.
- Moulding: The technology that produces plastic pieces with the form of a mould. Some Moulding technologies are casting, injection, thermoforming, rorational, compression, and blow moulding.
- Extrusion: A process in which plastic is gradually melted and mixed inside an extruder. Some turning screws help to heat up the material and push it through a die that gives the shape of the section.
- Calendering: A mass of plastic material is forced to pass through two counter-rotating rolls who transform it into thin film.
- Subtractive Manufacturing: A process which cuts material away from a solid block of raw material to construct a 3D model. Although it can be done manually, it is most typically done with a CNC Machine.
Additive Manufacturing Technologies
While traditional methods involve shaping or carving plastic in order to obtain the final object, additive manufacturing creates models by adding thin layers of material. There is a wide range of 3D Printing Technologies -another name for additive manufacturing- and each one of them has its own variety of applications, like prototyping, end-use parts, design iteration, etc.
Extrusion – FFF
FFF is the most spread additive manufacturing process. It builds objects by depositing melted material layer-by-layer, using thermoplastic polymers as raw material.
The extrusion path is pre-calculated to design all the layers of a model. Then, the printer uses a filament made of a thermoplastic material and pushes it through the print head using gears. The print head will melt the filament at an adequate temperature and will depose it through a nozzle along the XY-plane to create one layer. Once the layer has cooled down, the platform moves down the height of one layer and deposits the path of material to build the second layer. In some printers, the platform remains fixed always at the same height, and the moving part along the Z-axis is the print head.
The process continues until the full model is completed.
Vat Polymerization – SLA
SLA is an additive manufacturing process that selectively photo-polymerizes a liquid curable resin from a vat to create solid models.
The UV laser path is pre-programmed to design all the layers of a model and the build platform lowers down to contact the bottom of the vat. Photopolymers are sensitive to UV light, so the first layer of these materials will solidify when the UV light draws the first path of the design. The first layer will be the one contacting the bottom of the vat.
Vat Polymerization – DLP
DLP is an additive manufacturing process that selectively photo-polymerizes a liquid curable resin from a vat to create solid models.
This technology is similar to SLA, as they both use curable resins as raw material. The main difference between them is the type of light source used to cure the photopolymer. While in SLA the light comes from a UV laser, in DLP it comes from a digital light projector screen, creating only a single image per layer.
Powder Bed Fusion – SLS
SLS is an additive manufacturing process that selectively sinters powdered plastic materials with high power lasers.
As other 3D printing technologies, SLS requires a previous stage of calculating the laser path all over the layers of the model. Inside the printer, there is a tank full of plastic powder and a build platform. The platform lowers down the height of one layer and a roller takes the raw material from the tank and coats all the printing surface. Then, a laser impacts on the layer of material following a pre-calculated path to create the model. The platform moves down again and the roller fills the space with new plastic powder. The process continues to complete the full part.
Material Jetting is an additive manufacturing process that deposits and cures droplets of material using photopolymers sensitive to UV light.
The first step is to pre-calculate the deposition and cure path. The build platform is placed at a height of one layer from the nozzle that drops material. The print head deposits the droplets onto the platform following the deposition path to create a layer of one model. The material is only dispensed when needed, not following a continuous path.
Using thermal or piezoelectric actuators, the print head changes the pressure within the nozzle and the material falls down. Then a UV light attached to the print head cures and solidifies the material. The build platform moves down the height of one layer and the further layers are built up on top following the same method.
Binder Jetting is an additive manufacturing process that uses a liquid bonding material to bind parts of a powder bed.
As it is the case in other technologies, the printhead path is pre-calculated. The printer leaves a space of one layer between the print head and the build platform. This space is then filled with a powdered material to create a homogeneous surface. After that, the print head draws the path and deposits the bonding agent which binds the powdered parts.
It is now possible to create full-coloured parts using the same technology as a traditional inkjet printer. The Binder Jetting printer can add and combine colour pigments to the bonding agent.
Once finished, the object is left inside the non-bonded powder and is cured. The obtained part is in “green state”, it means the model has high porosity and low mechanical properties. There are some post-processing stages to improve mechanical properties.
This paper has brushed through the plastic manufacturing industry, describing both conventional and additive technologies. Lead time, price and accuracy are important elements to take into account when deciding which technology should be incorporated into a manufacturing workflow.
While traditional manufacturing offers a large scale production at low cost, the costs are high if the aim is to produce low quantities. Scaling up through a traditional manufacturing process also means longer wait times and production times, as moulds are based on prototyping and the process can be prone to error and delays in obtaining a final mould. In opposition to this, additive manufacturing will render inexpensive low batch productions and short lead times but will not be as cost effective when scaling up production.
Similarly, additive manufacturing offers the ability to produce complex geometry tailored to every single produced piece, while traditional manufacturing will require more simple geometries.
Additive manufacturing enables to produce a great range of shapes that would not be possible to manufacture in a single print using traditional technologies. When the requirements are high accuracy and low amounts, the most suitable technology is additive manufacturing. If, however, the need is to obtain a deliverable that must be mass produced, then the traditional manufacturing technologies would be better suited for this job.
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