Polymer Processes Flashcards
(29 cards)
What is injection moulding?
Injection moulding is an industrial processed used to manufacture complex items made from thermoplastics.
What is the method for injection moulding?
- Thermoplastic granules are poured into the hopper.
- A screw thread (Archimedes screw) is rotated by a motor. This pulls the granules through the chamber and past the electric heaters, while constantly being crushed together by the shearing force.
- This causes the polymers to melt into a liquid.
- When a sufficient volume of polymer has melted and formed at the end of the screw, a hydraulic ram forces the screw thread forward. This injects the polymer into the mould at a high pressure.
- The mould is water cooled, which enables the molten polymer to harden quickly.
- The mould opens and ejector pins push the moulding out.
- Any excess polymer (flash) is trimmed off the moulding. Formers or jigs may be used to maintain the dimensional accuracy of the moulding while it cools and hardens completely.
What are the advantages of injection moulding?
- Mass production or high batch production
- It allows for complex geometries with tight tolerances - it achieves precise parts with tolerances within 0.500mm.
- It is very efficient and rapid process
- High repeatability with strong durable parts- once you have made a mould it can produce thousands of parts before needing to maintain tooling.
- Very little finishing required because the moulds will deliver a brilliant surface finish
- You can reuse material so there is very little waste - It is a closed system
- Wide range of thermoforming polymers can be used.
- Undercuts and overhangs can be created
What are the disadvantages of injection moulding?
- Start up costs are high - custom tooling is very expensive, initial capital investment in mould manufacture is very high and initial capital investment in the injecting moulding machine.
- Time constraint from initial order to receiving the machine or the mould.
- Heat base usage for the product is limited
- Design changes are expensive
- One off or very low number of parts being made can make the end products really costly.
- Size limited due to cooling of polymer in the mould because of the laminae flow. Patio chairs are about the biggest which can be made.
What are the uses of injection moulded parts?
Objects which have been injection moulded are plastic bottles and bottle caps, Lego and many other children’s toys, computers and other customer electronics, kitchenware (plastic plates and utensils, plastic furniture, flower pots and toilet seats. Also medical devices ( such as heart rate and blood pressure monitors) and wardrobe related products.
What is extrusion moulding?
It is used to produce either solid rod, hollow tube, angle, sections such as ‘I’ and ’T’ sections or channels in polymers. This process is used for continuous production of stock material, and is therefore used by specialist manufacturers. Extrusion can only be used with thermo polymers.
What is the method for extrusion moulding?
- Polymer granules are loaded into the hopper.
- The archimedes screw moves the polymer granules past heaters
- The heaters soften the polymer.
- When sufficient polymer has melted, the hydraulic ram pushes the archimedes screw, forcing the polymer through a steel die. The shape of the die determines the shape of the extrusion.
- The extrusion may be supported by rollers as it leaves the die and is cooled by water or air.
- The extrusion is then cut to the desired length.
What are the advantages of extrusion moulding?
-Unlike most the other processes, with extrusion moulding post-extrusion alterations can be made immediately afterwards while the plastic is still formable
- Like injection moulding because of the efficiency of the process they can make lots of products quickly reduce the price.
- Any waste from the process is made back into pellets and reused
- Again like injection moulding plastic extrusion can be used to make a range of shapes in a variety of thicknesses, textures, colours and sizes in a short amount of time.
What are the disadvantages of extrusion moulding?
-Unlike injection moulding once the product is removed from the extruder it is still hot and will expand (die swell)
- Like injection moulding there is a high initial cost of setup
- Only one type of cross section can be made at a time.
What extraction moulded products are used for? 
Cover to wires to secure it against the wall sheafing for cables and coating of electrical wires with polymers such as PVC, pipes. Also window frames, drinking straws, football goal posts and irrigation piping. As well as things like springs and the ink tube to a biro.
What is vacuum moulding?
It is used to make 3D products with thermoplastic sheets such as HIPS but it can only do sheets up to 6mm thickness. A mould in the shape of the product is made - typically from MDF. This must have a small draft angle to allow the mould to be removed from the polymer sheet when formed into the moulding. Vacuum forming is quite a slow process when compared to other methods, it is normally limited to small-scale production. However, if a larger scale of production is required, moulds or formers can be made from Cast and machined aluminium. Such moulds give consistent results with a high quality surface finish.
What is the method for vacuum moulding?
- The mould is placed on the bed of the vacuum forming machine, which is called the ‘platen’. The platen is lowered to the bottom of the machine.
Polymer sheet is clamped over the mould and a heaters pulled over the polymer sheet.
When the polymer sheet has softened, the platen is raised into the polymer and the heat is removed.
The vacuum pump is switched on, which sucks the polymer onto the mould.
Once the polymer has cooled and returned to a solidified state, the platen is lowered and the vacuum switched off.
The mould is removed from the moulding. Excess polymer is then trimmed off.
What are the advantages of vacuum moulding?
- Each individual piece is relatively fast to produce especially compared to rotational moulding.
- Low manufacturing costs as there is as much expensive customer tooling as there is for injection moulding
- Typically consistent shapes from batch to batch, particularly when using the same mould multiple times
- Versatile and customisable mould shapes as they can be easily made out of MDF unlike the the other processes.
- Easy to scale production up or down. It is much cheaper to do lower scale batch production than injection moulding and extrusion.
- Excess plastic can be recycled into more sheets for future production needs
- Supports rapid prototyping process unlike the other methods.
What are the disadvantages of vacuum moulding?
- Only one part or product can be made at a time ( best for small-scale production). It is not nearly as efficient as injection moulding.
- There may be additional costs or materials needed to finish components.
- Works best for thinner sheets of plastic as thick plastic sheets can warp during vacuuming
- Designs have to be relatively simple, as there’s a limit to how much detail you can achieve from a mould unlike most of the other processes.
- Like the other processes if the mould isn’t properly made the final product will be flawed.
- Excess moisture can cause unwanted bubbles to form. There isn’t this problem with processes like injection moulding or extrusion.
- Clear or light coloured parts can show debris from the mould.
- Draft angle creates angle limitations
What are the uses of vacuum moulding?
Examples of products made using vacuum forming are lightweight trays, box inserts and liners, coffee cups and yogurt pots.
What is rotational moulding?
It is used to produce heavy duty, seamless hollow objects that need a large wall thickness. Typical polymers that are used include HDPE and PP. It is generally used to make much bigger polymer products compared with there process for example injection moulding.
What is the method for rotational moulding?
- First you need to put the releasing agent on the surface to allow the product to come off ( can be oil )
- Polymer powder or granules are loaded into a mould, which is clamped and sealed.
- The mould is transferred to an oven where it is heated to 260°C - 370°C, depending upon the polymer used. The mould is rotated slowly ( fewer than 20 rotations per minute ) around two axes, and as the polymer is heated, it coats the inside of the mould.
- It is all then transported away from the heater while still being rotated. Then, once the polymer has achieved the correct thickness, the mould is cooled. Usually a fan and/or water is used to cool the polymer.
- When the polymer has solidified, the part will shrink slightly, allowing it to be removed.
What are the advantages of rotational moulding?
- Low cost tooling, low pressures allow materials such as aluminium to be used unlike injection moulding or extrusion.
- Consistent wall thickness - constant rotation of the mould coats the walls evenly
- Complex double-walled open containers can be produced without secondary processing
- Unlike injection moulding or extrusion the material isn’t exposed to external pressure, increasing its stability and reducing the risk of defects to the finished part.
- Unlike vacuum forming the corners are thicker rather than thinner reducing the risk of failure in stress-concentration points
- It can easily accommodate surface finishes such as detailed textures, logos, symbols and lettering unlike vacuum forming.
- It can make much larger products than the other methods.
- It is best for large scale batch or mass production due to the high setup costs.
What are the disadvantages of rotational moulding?
- At fewer than 20 rotations per minute there can be very high cycle times compared with injection moulding or extrusion
- Raw materials used must be readily converted from granules to a fine powder and must have high thermal stability. This can then make the cost of the polymer much higher
- Low repeatability as moulds are made out of soft metals like aluminium meaning they wear out much quicker and would need to be replaced.
- It is still quite labour intensive compared to injection moulding or extrusion creating high labour costs.
- Surface finish is usually not as good as injection moulding
What are the uses of rotational moulding?
Examples of products are traffic cones, kayaks, water tanks and children’s equipment.
What is thermoforming?
Thermoforming is typically used in industrial production It is quite a slow process, but fine detail such as embossed logos and lettering can be achieved. Thermoforming is a very similar process to vacuum forming, the only difference in the method is the underlined part on step 4.
What is the method for thermoforming?
- The mould is placed on the bed of the machine, which is called the ‘platen’. The platen is lowered to the bottom of the machine.
- Polymer sheet is clamped over the mould and a heaters pulled over the polymer sheet.
- When the polymer sheet has softened, the platen is raised into the polymer and the heat is removed.
- The vacuum pump is switched on, which sucks the polymer onto the mould. At the same time there is an additional mould that is pressed onto the surface of the polymer sheet. The two moulds trap the softened polymer in-between them, giving extra detail to the surface of the moulding.
- Once the polymer has cooled and returned to a solidified state, the platen is lowered and the vacuum switched off.
- The mould is removed from the moulding. Excess polymer is then trimmed off.
What are the advantages of thermoforming?
- It has a fast turnout time and is very adaptable to customer arrangements.
- It has much lower tooling costs than some of the other methods because it works at a much lower pressure. This also makes the initial capital investment much lower.
- You can have detailed in prints and textures unlike vacuum forming
- Like most the other processes thermoforming plastics can be recycled or reused
- Unlike most the other methods thermoforming can make big or small parts
- Smaller volume runs / batch production are more cost effective than like injection moulding. This is because there is a lower initial capital investment and changing the tools is much more flexible and easy with thermoforming than the other methods.
- Light weight products increase sustainability and efficiency.
What are the disadvantages of thermoforming?
- Process is limited to thin walled parts
- It can only form relatively simpshapes unlike injection moulding or extrusion
- Each piece requires trimming unlike injection moulding or extrusion which can be very time consuming
- It is less accurate than other manufacturing processes such as injection moulding. This may make it unsuitable for parts requiring high precision and tolerances.
- It is limited to relatively simple parts with flat and curved surfaces. Therefore it is unsuitable for parts with intricate details.
- It is a relatively slow process unlike injection moulding or extrusion. This may make it unsuitable for mass production.
