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Manufacturing with polymers
This unit of work addresses aspects of the following syllabus outcomes
The student:
H1.2 differentiates between properties of materials and justifies the selection of materials, components and processes in engineering.
Source: Stage 6 Engineering Studies syllabus 1999. Board of Studies NSW.
1. Introduction
Transport vehicles have one basic, but major, feature in common: they all contain a range of components manufactured from polymers. The properties of these polymers not only determine their use but how they are manufactured. Their final structure influences characteristics such as flexibility, impact strength, transparency and resistance to abrasion, heat, oils and solvents.
The term 'Polymer' simply means 'many parts' and with reference to polymer (plastic) materials they are natural or synthetic organic materials generally non-crystalline (amorphous) in structure.
There are two broad groups of polymeric materials:
- thermosoftening polymers or thermoplastics, and
- thermosetting polymers.
2. Polymer structures
Thermosoftening polymers generally have a 'long chain' molecular structure (Figure 1) formed by the 'addition polymerisation' process which can be strengthened by cross-linking. Cross linking is achieved with the addition of another chemical to the polymer structure eg. using sulphur to vulcanise rubber.
Figure 1. Part of the long chain molecular structure of Polyethylene, a simple combination of one Carbon atom with two Hydrogen atoms.
Thermosetting polymers have a network structure and are formed by the 'condensation polymerisation' process (Figure 2) which produces water as a by-product of the chemical reaction occurring during setting.
Figure 2. Part of the network structure of phenol formaldehyde (Bakelite).
These structures are extremely important as they determine the method by which components will be manufactured from the polymer.
3. Properties of Polymers
In terms of properties the thermosoftening group of polymers tend to be flexible to some degree, show reasonable impact resistance and are usually able to be re-heated and re-shaped, assuming the ‘new’ shape upon cooling. From an environmental point of view, these properties make thermosoftening polymers ideal for recycling.
In comparison, thermosetting polymers are rigid, often brittle and cannot be re-heated for the purpose of re-shaping. They tend to degrade or burn when heated significantly.
4. Manufacturing methods
Some of the most common industrial manufacturing processes for polymers include:
Compression moulding
Transfer moulding
Rotational moulding
Injection moulding
Blow moulding
Vacuum forming (Thermo Pressure Forming)
Extrusion
Pultrusion
Fabrication
Compression moulding is the process by which thermosetting polymers are usually formed (Figure 3).
The compression moulding process involves placing the polymer ‘dough’ into the die cavity. This ‘dough’ is carefully measured to avoid waste and minimise the amount of ‘flashing’ (fine, thin webs attached to the moulding) around the finished article.
Figure 3a. With the dies apart, the prepared polymer ‘dough’ is placed into the cavity.
Figure 3b. With the die closed, the article is formed and the small amount of flashing on each side will be removed later.
When the die is closed, heat and pressure is maintained until the condensation polymerisation process is completed.
The hot compression moulding process is used to form components from phenolic, urea and melamine thermosetting polymers, as well as alkyd resins.
Transfer moulding is different to compression moulding because the plastic is not fed directly into the die cavity. Instead it is fed into a chamber outside the die. Here it is preheated before a piston forces it through a system of runners and gates that allow it into the die cavity. When the polymer is cured through heat and pressure it is ejected from the mould. The advantage of this process is that the runner system allows the simultaneous production of many small, intricate parts and there is no ‘flashing’ around the finished article.
Rotational moulding is a unique process for manufacturing thermosoftening polymers as it produces hollow items eg. Lawnmower grass-catchers, balls and some types of children’s toys. A split mould is filled with exactly the right quantity of polymer which is then tumbled in the mould once it is bolted together. The die is rotated in a heated chamber until the polymer is completely melted. The die is removed from the chamber and continues to rotate until the polymer is set. The die halves are separated and the article removed.
Injection moulding: In this process thermosoftening polymer powder, or granules, are placed into a feed hopper connected to a heating chamber. When the fluid state is attained, due to the heat, a piston or 'screw type' mechanism forces the plastic through a nozzle and into the die. These are often water-cooled to hasten setting such that when the plastic becomes solid again the die is opened and the component is ejected. Any flashing is then removed.
Modern technology and sophisticated metal machining and finishing techniques means that large objects with fine tolerances are easily produced. The images below (Figures. 4, 5, 6, & 7) show a 60 litre 'tub' and the two-part die which produced it to a high degree of dimensional accuracy. Note the highly polished surfaces of the die.
Figure 4. This complete die weighs in excess of 1.5 tonnes. Note the 4 alignment pins and the ejector pins.
Figure 5. Due to the weight of the die, it must be lifted with a crane and placed into the injection machine. Eyebolts for this purpose are attached to the sides.
Figure 6. Note the highly polished surfaces of the mould and the polymer inlet hole in the bottom centre of the mould.
Figure 7. The finished storage 'tub'. The lid is produced inanother mould.
Blow moulding: Thermoplastic polymer articles, such a soft drink bottles made from PET (polyethyleneteraphthalate), are formed by blow moulding. A hot, thin extruded tube (Figure 8a) of the polymer is gripped in a die as an internal blast of air forces it out against the sides of the mould (Figure 8b). It is held in the die until it cools and is then released (Figure 8c). The polymer assumes the shape of the die (Figure 8d).
With the die closed and the tube pinched, the air pressure, which has blown the bottle to shape, is maintained until the plastic is cooled to room temperature.
Vacuum forming relies upon air pressure to form a shape. A heated sheet of polymer is clamped above a mould and the air in the mould is evacuated leaving a partial vacuum. The air pressure above the mould forces the plastic sheet down into the mould to form the shape which is removed when cooled. This process is often used for transparent canopies and covers over lit signs such as those seen in service stations or fast food outlets.
For a very comprehensive coverage of the vacuum forming process with many sequenced sectional diagrams, investigate this site presented by smi; SPECIALITY MANUFACTURING INC. at: http://www.marketingtech.com/designguide/dgtherma.htm
Extrusion (Figure 9) involves the use of powder or granules, mixed with dyes as required, which are placed into a feed hopper connected to a heating chamber.
When the fluid state is attained due to the heat, a piston or 'screw type' mechanism forces the plastic through a nozzle and into the mould or die. The dies are often water-cooled to hasten setting such that when the plastic becomes solid again the die is opened and the component is ejected. Any flashing is then removed.
Pultrusion is similar to extrusion except that thermosetting polymers are used and the composition of the composite polymer requires a greater force to move it through the die. For this reason, a pulling force is incorporated into the forming process to overcome resistance during manufacture of the article.
Fabrication processes may also be used to manufacture complex polymer components. This is due to the ability of some polymers to be ‘bent’ and 'welded'.
You should further investigate the manufacturing processes described above by attempting the following activities.
Activity 1
The ‘Extrusion’ process manufactures an amazing array of polymer products and custom designed profiles are produced to meet individual customer requirements.
- Briefly describe the profile extrusion process.
- Discuss the versatility of the process in terms of specific plastic and mechanical property enhancement.
Web site research reference: http://www.craneplasticsmfg.com. 
Click on extrusion to see information about extrusion.
Activity 2
'Pultruded' polymer products are manufactured in a similar way
to 'Extruded' products. To answer the following questions, go to this site to use as a research reference:
When you enter the site, click on the FAQs option: http://creativepultrusions.com/product.html
- What is 'Pultrusion'?
- What are the most typical resins used in the Pultrusion process?
- What are the most typical reinforcements used in the Pultrusion process?
Activity 3
Compression Moulding is a manufacturing process which involves the use of pressure to form the polymer into the desired shape. It is used for producing prototypes and for low volume production of rubber components
Rubber is a versatile polymer and the simplest method of forming it is by compression moulding.
- Briefly describe this process.
- List some of the disadvantages and restrictions associated with this process.
- Compare and contrast Transfer moulding with Injection
moulding.
Web site research reference: http://www.rubberatkins.co.uk/manufacturing/processes.html - Getting the right combination of ingredients and
properties is essential to delivering the exact quality of
product demanded by the customer.
Click for Drop and Drag Activity
Web site research reference: http://www.bpf.co.uk/bpfindustry/plastics_materials.cfm
Activity 4
Rotational moulding dates back hundreds of years to when the Swiss first invented the process to produce hollow chocolate ‘eggs’. Answer the following questions as they apply to today’s use in the manufacture of polymer products.
Look at this interesting site first: http://www.rotomould.in , where you will find out about the benefits of rotational moulding and applications, then go to http://www.plastigi.be and answer the following four questions (note the informative animation). Note that you must first select 'ENGLISH' then from the left options choose 'Production Process'
- List the four stages in the rotational moulding process.
- How is the mould cooled when the process has completed forming the process?
- In the heating environment, the mould rotates in both a vertical and horizontal plane. What term is used to describe this 'forming process motion'?
- There are more than twenty major industrial applications for the rotational moulding process. List what you consider to be the top six.
Manufacturing with Polymers
COPYRIGHT related to diagrams.
Copyright ownership of the following diagrams is a listed below.
Note: Figs. 4, 5, 6, & 7:
Dies photographed at Bell Dies; Sydney. Images processed and prepared by R. Metcalfe
(Metcalfe Resources) and reproduced courtesy of Bell Dies;
Sydney.
| Long_Chain_Polymer.jpg | Fig_7_60_Litre_Tub.jpg |
| Network_Structure.jpg | Blow_Moulding_A.jpg |
| Compression_Moulding.jpg | Blow_Moulding_B.jpg |
| Fig_4_60_Litre_Tub_Dies.jpg | Completion_Table.jpg |
| Fig_5_60_Litre_Tub_And_Dies.jpg | Completion_Table.jpg |
| Fig_6_60_Litre_Tub_Die_Cavity.jpg | Completion_Table_Answer.jpg |
© R. Metcalfe; Metcalfe Resources 2003.
Permission is granted for reproduction
on the Charles Sturt HSC Online Web site from the beginning of 2004.
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