Embedded Files
Aspire to be an independent learner. Click here to learn how.
Click here to learn all about plastics
Thermoforming Plastics
Thermoforming Plastics
Thermoforming is the word given to describing the most common way of shaping plastics into their desired shape. There are various ways of thermoforming. Here are some of the most common.
Injection Moulding
Injection Moulding
Injection moulding is a manufacturing process where small plastic pellets or granules are heated, softened and pushed under pressure into a mould cavity.
Injection moulding is a highly versatile process because there is an enourmous range of shapes that can be produced and many different types of plastic lend themselves to being injection moulded. It is perfect for mass production because the cost per part can be very inexpensive when making in large numbers. An advantage of injection moulding is that the very detailed shapes can be made that are dimensionally accurate (that is every item looks exactly the same).
Advantages of injection moulding
Precision - parts can be made very accurately.
Repeatable - identical parts can be made over and over.
Low cost per part - The more ‘you’ make, the cheaper it becomes.
Fast - Parts can be made in seconds. Many parts can be made at the same time.
Many materials are available.
Low waste - Because only the material that is needed is injected, there is little waste produced.
Disadvantages of injection moulding
High initial cost - The cost of developing and making the custom tooling and machinery required to injection mould makes the process very expensive to invest in.
High lead time - The process requires development and many months of testing before it can be used to produce the ‘first run’ of parts.
Size limitations - The larger the part, the more difficult it is to manufacture.
Careful design considerations - Some features cannot easily be produced (such as undercuts). Much time and experience is needed to design effective tooling and moulds to avoid issues during manufacture.
Blow Moulding
Blow Moulding
Blow moulding, as its title suggests is where air or another inert gas is used to expand a film of plastic against the inside edges of a mould. It is most frequently used in creating cylindrical shapes such as drinks bottles.
A thin extrusion of parent plastic is fed inside a mould cavity. It is heated so that it softens allowing it to expand. A thin plastic tube called a parison is inserted into the cavity. Gas is then pumped into the parison to inflate it. When the plastic film has covered all the edges of the mould cavity, the mould is cooled and the part can be released. This process is then repeated.
Vacuum Forming
Vacuum Forming
Vacuum forming is a manufacturing process whereby a flat sheet of plastic is heated and softened and then pulled over the top of an mould or former using negative air pressure (suction).
It is used to make thin, hollow, half sections. You will remember using it in Year 7 when you created a plastic case for your electronic product (fuse tester, voice recorder, etc).
The picture shown is of the three stages of vacuum forming:
Heating / softening of the sheet
Inflation (stretching)
Vacuum (suction)
Cooling and mould removal.
Adam Savage (Mythbusters TV engineer) has one type of vacuum former.
This is the same vacuum former that we have in school.
Rotational Moulding
Rotational Moulding
Rotational moulding is a manufacturing process whereby plastic is dispersed around the mould cavity by rotating the mould without the need for pressurising the plastic (forcing it into position under pressure). Instead gravity forces are used to create an even thickness coating. It is generally a less expensive process than injection moulding and can be used for producing smaller numbers of components.
Fact. Americans spell 'mold', in the United Kingdom we spell 'mould'.
This picture shows one-half of a tank being made. You can see the metal mould cavity. A hopper is being used to drop plastic granules into the mould. The mould would be joined with the other half and the whole assembly would then be heated and rotated about two axis (that is two pivots). When the plastic is fully dispersed around the mould, it is cooled and then split revealing the final component.
3D Printing / Rapid Prototyping
3D Printing / Rapid Prototyping
3D printing is an additive manufacturing process. The print material is usually forced through a heated nozzle, giving a fine degree of control over what is being printed. Common plastics used include ABS, and PLA.
3D printing enables direct output from 3D CAD (computer aided design) models. This means that we can get exact reproductions of what the original designer intended.
Another method of 3D printing involves using inert powders which have a release agent inside them, preventing the plastic granules from 'sticking' where the designer does not intend for plastic to be (e.g. the hole inside a cup). When the part is removed the powder can then be reused in another 'print'. A popular process method shown below is called Selective Laser Sintering (SLS). This uses lasers to fuse the plastic granules together.
3D printing is often mixed up with the phrase rapid prototyping. Rapid prototyping is really any manufacturing process whereby designers can quickly create functional scale models of their design ideas for testing so that they can make quick iterative design changes and 'turnaround' prototypes quickly without the need for specialist tooling.
Rapid prototyping often makes use of 3D printers as these are able to 'nest' components inside of each other at the point of manufacture without the need for further assembly.
See the video blow for a demonstration of a rapid prototyped gear assembly.
3D printing is a rapidly evolving technology and it is now possible to 3D print in all sorts of materials, e.g. concrete, metal, even food! Watch the video above to see how it is changing the world.
Additive manufacturing is a process where individual layers of material are positioned one at a time with the print head moving upwards allowing the component to be made taller and taller.
Watch how 3D printing concrete is enabling architects design and builders construct innovative buildings around the world!
Disadvantages of 3D printing
Disadvantages of 3D printing
Limited range of materials (presently but getting larger all the time).
Build volume (length x width x height) is limited by the machine.
Post processing required with some materials resistant to finishing e.g. sanding or polishing. PLA notably cannot be finished.
Expensive and large volumes do not bring the cost down further.
Some parts need support structures to stop the build from collapsing.
Some printers have low tolerances reducing the build quality (think like picture resolution. The smaller the print head the more accurate the print resolution, the better the finish).
Digital files can be copied and so other peoples work can be copied without permission (copyright theft) leading to counterfeit goods.
Amazing fact - The International Space Station even has a 3D printer enabling engineers on the ground to create custom tools to solve problems as they are discovered. They can then be sent digitally to space and made on-site saving the need for a 'space truck' to have to be launched on a rocket on a supply mission.
Laser Cutting
Laser Cutting
Laser cutters are used to cut or engrave thin sheets of material. It is usually used on flat, two-dimensional surfaces however can also be used to cut or engrave cylindrical shapes too.
Essentially, the laser produces extremely high, but controlled, localised amounts of heat. This heat is used to vaporize a specific depth of material, removing it from the parent object. This way we can either engrave (removing some of the material) or cut (removing the full thickness of material).
This process can only generally be done on surfaces which do not have surface deviations (that is they are not rough or 'hilly'). The reason for this is that the distance between the laser lens and the surface of the material (called the focal distance) must remain constant to avoid a reduction in power transmission.
In plain speak, if you've ever tried to burn something with the sun and a magnifying glass, you'll know that if you lose the beam focus, the object won't burn as well!
What does LASER stand for?
Light Amplification by Stimulated Emission of Radiation, basically creating a tiny columnated (narrow) beam of high intensity light which carries a very high energy wave capable of producing intense heat. It is this heat that does the 'work' in a laser cutter.
This is called a 'living hinge'. It is used to help rigid flat sheets bend around corners. This is done using a laser cutter.
Disadvantages of laser cutting
Disadvantages of laser cutting
Generally only possible on thin flat sheets.
Cannot be done on rough surfaces.
Thickness limited by machine power.
Cannot produce undercuts or out-of-plane profiles (think rounded edges).
Laser cutting is sometimes confused with plasma cutting (like above). Plasma cutting is achieved by igniting flammable gases to create a superheated gas called 'plasma'. If we did this to plastic there'd be nothing left over!
Fun Fact! Plasma is what is found around the outside of black holes when stars are pulled apart by the massive gravitational forces.
Line Bending
Line Bending
Line bending (or strip heating) is a process used to make straight bends across thin sheet thermoplastic. It is usually applied to acrylic which lends itself very well to line bending because it is very 'forgiving' of mistakes. It is easy to re-heat and re-bend as necessary.
Drape Forming / Moulding
Drape Forming / Moulding
Drape forming involves heating the entirety of a sheet of thermoplastic such as acrylic, foamex, or forex.
A former is then used which the plastic is 'draped' over. The plastic sheet then takes on the characteristic shape of the former. It is allowed to cool and then removed, retaining its new shape.
Page updated
Report abuse