Material Selection:
Neither of these parts have particularly specialist requirements. Any moderately strong polymer will do for the clips and the stiff nose piece. Examples would be polyethylene and polypropylene. The type of nose piece that is adjustable would be made of a more pliable material like silicone.
Manufacturing Techniques:
As both these part are very small and neither require very rigid tolerances they can be very easily and cheaply mass produced using moulding techniques.
Detailed Analysis of One Part of the Product-The Lens
The project briefing for this project required us to examine, in detail, one part of the product. The section before has provided a brief discussion of all the different parts of a pair of swimming goggles but now I will look at one part in greater depth- the lens.
The first thing that comes to mind when you think of lenses, from a design perspective, is that they need to be transparent. This applies to all sorts of lenses. From swimming goggles to eye glasses, from camera lenses to the lenses on the most expensive NASA telescopes, they all need to be transparent.
Transparent materials
The dictionary definition of transparent is “able to be seen through: pellucid: pervious to rays” (Chambers Twentieth Century Dictionary). The most obvious material that is transparent is glass. This is widely used in many applications which require the user to see through the material. However glass is very brittle and is therefore not widely used in eyewear. This is particularly true in the area of sports eyewear, which is effectively what goggles are. For this reason polymers are used.
Some Possible Materials
Many different polymers are, or can be made, transparent. The following list shows a list of the more common ones.
- Polyurethane (PU)
- Polycarbonate
- Poly(methyl methacrylate) (PMMA)
- Cyclic Olefin Copolymers (COCs)
- Polystyrene (PS)
While all of these polymers have good optical qualities some are more suitable for use in swimming goggles. Before we can decide which is the most appropriate we need to look at each one in more detail. Once they have been analysed in depth the pros and cons of each can be determined and a final choice can be made on the optimal material. The following sections take an in depth look at each of the polymers.
Polyurethane (PU)
Polyurethane is a highly versatile polymer consisting of a chain of units joined by (carbamate) links. Polyurethane polymers are formed through by reacting a containing at least two with another monomer containing at least two () groups in the presence of a .
A step-growth process is a fairly uncommon process (the addition process accounts for 70-80% of all polymer processes). It’s a process in which bi functional or multifunctional react to form , , longer and eventually long chain .
[5]
While this briefly explains what PU is and how it’s formed it doesn’t explain why I considered transparent polyurethane when I was looking at possible lens materials. The reason I considered it is because it has a number of beneficial properties. These are outlined below:
- High optical transmittance of light
- Excellent UV-stability
- A wide range of mechanical capabilities. From flexible to hard, all extremely tough
- Good adhesion to a variety of substrates
- Advantages against epoxies like more transparency, non-yellowing
- Refractive index around 1.50
[6]
Of particular benefit are the high optical transmittance of light and the excellent UV-stability. This stability means that the polymer will not change colour or lose its mechanical properties when it is exposed to sunlight (UV-rays) for prolonged periods. This can sometimes be a problem for polymers as when they are left in the sun for too long they can take on a yellowish hue and sometimes become cracked and lose some of their strength. This is obviously a concern in the case of swimming goggles as they are often used outdoors and can be left in the sun when not in use.
Poly(methyl methacrylate) (PMMA)
PMMA is a clear vinyl plastic that is used in many applications where a glass look is required but where actual glass does not meet the physical requirements. Many well known companies use PMMA in their products. For example the chemical company Rohm and Haas makes windows out of it and calls it Plexiglas, while Ineos Acrylics also makes it and calls it Lucite which is used in many common bathroom applications.
PMMA is made by free radical vinyl polymerisation from the monomer methyl methacrylate. Free radical polymerisation is used to make polymers from vinyl monomers, which are small molecules containing carbon-carbon double bonds.
The physical structure of PMMA is shown below.
Figure 2: Polymerisation and Structure of PMMA [7]
Again PMMA has very good qualities when it comes to being used as a lens material. The table below is a selection of some of the data compiled by an American (hence the imperial units that can be seen) acrylic glass company that use PMMA.
Figure 3: Table Showing The Properties of PMMA [8]
A number of useful properties can be found in the table. It can be seen that PMMA has a similar optical refractive index as PU. The thermal properties also improve PMMA’s suitability to be used as a lens material in goggles. The forming temperature is fairly low (<200°C) which means that is will be relatively cheap to produce the lenses. A low forming temperature generally lowers the polymers deflection temperature and maximum recommended temperature. While this is true in the case of PMMA it is not a problem as both temperatures fall just below the 100°C mark. This is much higher than the maximum water temperature that the goggles will be exposed to. So unless the swimmer pans to put his or her head in a boiling kettle the goggles should have no problem handling temperature.
Cyclic Olefin Copolymers (COCs)
COCs are relatively knew type of polymer. They are examples of amorphous polymers. This means that they are not rigid in structure. They comprise a new class of polymeric materials showing properties of high glass-transition temperature, optical clarity, low shrinkage, low moisture absorption, and low birefringence.
They diagram below shows the chemical structure and physical properties of cyclic olefin copolymers:
Figure 4: Polymerisation of COCs [9]
The cyclic part of the name describes the circular shape of the final polymer, which is similar to things like benzene rings, while olefin relates to an unsaturated containing one or more pairs of carbon atoms linked by a double bond.
[10]
Polystyrene (PS)
PS is a very common type of plastic which we come in contact with everyday. It is a thermosetting polymer which means it melts when it is heated. This is a benefit when it comes to designing swimming goggles. This is due to the fact that they wont come into contact with any extreme temperatures during their service life and it allows the polymer to be manufactured at a lower temperature which means production costs are reduced.
Polystyrene is a . Structurally, it is a long hydrocarbon chain, with a phenyl group attached to every other carbon atom. Polystyrene is produced by , from the monomer styrene. This free radical vinyl polymerisation method is similar to that described in the PMMA section above. The only difference is the monomer used. In PMMA methyl methacrylate is used whereas in PS styrene is used. The figure below shows a very brief diagram of how PS is formed.
Figure 5: Polymerisation and Structure of PS [11]
PS is a clear plastic which is available commercially in both pellet and sheet form for a relatively low cost. This makes it a very viable option in many production processes. The table below shows some of it’s relevant properties.
Figure 6: Table Showing Properties of PS [12]
Initially it can be seen that PS has excellent optical properties. Of particular note is the refractive index which, at 1.56-1.6, is better than most types of glass. The processing temperatures are also fairly low which, as explained above, reduces the production costs and makes a pair of goggles more commercially viable. The maximum service temperature is listed as - °C. While 300°C is fine 30°C could well be a temperature that goggles might come in contact with. This mean that PS lenses would require heat treatment or additives which would add to their cost.
Overall, however, they are one of the cheapest types of plastic. So why would it not be used in goggle lenses? There are a number of short problems that could arise if PS was used. The most serious deficiencies are low impact strength, poor weatherability and poor chemical resistance. This would make the goggles unreliable and not particularly durable. The other concern is an environmental one. In today’s world care for the world and its future is a big concern for any designer, and this is an area where PS falls down. It has a recycling number 6. This means that it can only be recycled at very specific locations and is not biodegradable. Polystyrene already contributes to much of the worlds pollution and it is for this reason that it is becoming less and less popular as a manufacturing material.
The Final Choice
While all of the materials described above have many benefits in terms of being used in swimming goggle lenses they are rarely seen. This is because there is a more popular and more suitable choice which is used in almost every design throughout the industry-Polycarbonate.
The following section will now take a detailed look at polycarbonate and explain why it is used ahead of the other materials
Polycarbonate (PC)
Polycarbonate is a tough, dimensionally stable, transparent thermoplastic that has many applications which demand high performance properties. This includes being used in not only swimming goggles but many different types of lenses. From eyeglasses to high performance skiing goggles many make use of polycarbonate.
Polycarbonate can be synthesized from bisphenol A and (carbonyl dichloride, COCl2). The following diagrams and explanations show how this occurs.
The first step in the synthesis of polycarbonate from bisphenol A is treatment of with . This deprotonates the of the bisphenol A .
The deprotonated oxygen reacts with phosgene through carbonyl addition to create a intermediate (not shown here), after which the negatively charged oxygen kicks off a (Cl-) to form a .
The chloroformate is then attacked by another deprotonated bisphenol A, eliminating the remaining chloride ion and forming a dimer of bisphenol A with a carbonate linkage in between.
Repetition of this process yields a polycarbonate with alternating carbonate groups and groups from bisphenol A.
[13]
So now that we know how it is formed the next thing to look at is why it is suitable to be used in the lens industry. The following list shows some of the relevant properties of PC:
Figure 7: Table Showing Properties of PC [14]
Once again the optical properties are one of the most important. PC has a high transparency and a low haze. This along with the fact that it’s refractive index is roughly 1.6 () make it very suitable for any product that requires transparency.
It also has a high impact strength which makes it safe to be used in the sports industry. It also has a number of machining advantages which will be discussed in the manufacturing project that follows.
Why Choose Polycarbonate?
At this stage it is important to clarify an important point. As polycarbonate is already used by 99% of goggle manufactures I am not trying to propose a new material, rather I am merely investigating and justifying why it is already used by so many people. It is clearly the right material to use, but I would like to investigate why this is the case.
It can be seen by analysing each table of properties data in the above sections that PC is an excellent material choice. However this is a slow and laborious procedure. A much quicker and easier way, and one that was suggested in the project briefing, is to use the material selection charts that are found in M F Ashby’s Material Selection in Mechanical Design Book.
These charts provide the mechanical/manufacturing engineer with an easy to read, graphical representation of which materials best suit their requirements.
While there are many different charts that tell an engineer about a number of different aspects of their chosen materials it would be tedious to look at all of them. That is why I have chosen to look at the 2 that I consider to be the most important.
The first is the Strength vs Density chart which is shown below. The general trend that can be seen is that as density increases so does strength. This presents a challenge in the case of swimming goggle lenses. The problem is that we want a lightweight material so that it doesn’t impede the swimmer but we also need a strong material that won’t break. We know that a polymer should be used as it needs to be transparent. By looking at the location of polycarbonate we see that it is one of the most suitable polymers. This is because it has one of the highest strength but a similar density when compared to the other polymers.
Figure 8: Selection Chart 1 [15]
The second chart that I think is very important is the Fracture Toughness vs Strength one. These are both key safety considerations when designing a sporting product and are especially important in the case of swimming goggles because they will be in close proximity to the human eye.
The general trend that can be seen in this graph is similar to the previous one. Again, as one mechanical property increases so does the other. In this case it can be seen that the greater strength of the material the higher its fracture toughness is. Ideally we want a material that performs well in both areas. This means that nearer the top right hand corner the material is the more desirable it is. In the polymer section we can see that very few polymers are better than polycarbonate. In fact the only one is polyamide (PA).
Figure 9: Selection Chart 2 [15]
These charts along with the physical data presented in the numerous tables in the preceding sections shows why polycarbonate is a highly suitable material for this application.
Another important consideration when deciding what material to use is its formability. This is because the easier a material is to form the cheaper the cost of production for your component are. The next section deals with the manufacturing of polymers generally and polycarbonate specifically.
Manufacturing
One of the most important considerations when it comes to any design project is how the product will be made. This will have effects on both the cost of the final good and the speed with which they can be produced. These are both critical concerns when it comes to product design.
Before taking an in depth look at how polycarbonate lenses would be made it is important to have a basic understanding of some general polymer processing techniques. This will allow an informative choice when it comes to selecting the most appropriate technique to use. The following section contains a brief summary of some of the more common polymer forming techniques.
General Polymer Manufacturing Techniques
In many cases polymer processing techniques are similar to those used in metal work. Things like extrusion and rolling (calendaring in the case of polymers) are common to both. However there are some fundamental differences as well as some general variation in the processes used.
Some common polymer machining techniques are:
- Injection Moulding
- Extrusion
- Blow Moulding
- Vacuum Casting
- Thermoforming
- Calendaring
The following subsections will take a brief look at each of these techniques.
Injection Moulding
Injection moulding uses a rotating screw which both melts polymer pellets and provides the pressure required to quickly inject the melt into a cold mould. The polymer cools in the mould and the part is then removed from the mould once it has set properly. The diagram below shows a simplified schematic of a basic injection moulding machine. It is worth noting that the polymer pellets are kept heated right throughout the process and only cool once they are in the mould.
Figure 10: Schematic of the Injection Moulding Process [16]
Extrusion
This is a similar process to the injection moulding process described above. The primary difference is at the exit. In the case of injection moulding the molten polymer is forced into a mould whose shape it takes. However, in extrusion, instead of a mould there is a die. This means the softened polymer leaves the machine in the form of rod or tube. The best analogy for an extrusion process is to compare it to toothpaste coming out of the tube when it is squeezed. This is in effect what the process involves. The main difference between polymer extrusion and metal extrusion is that the polymer version uses a screw propulsion method whereas it’s metal cousin uses a heated reservoir and a ram. It is also interesting to note that polymer extrusion can make use of a single screw or a double screw design. The diagram below shows the single screw process in a graphical manor.
Figure 11: Schematic of the Extrusion Moulding Process [17]
Blow Moulding
This is a polymer processing technique that is primarily used for making plastic bottles. Perhaps the easiest way to describe it is to show a diagram of the process and explain from that what is happening.
Figure 12: Schematic of the Blow Moulding Process [18]
In step (1) the polymer has come from an extrusion process which makes it into a tube. This tube feeds down into the open mould and is kept partitioned. In step (2) the mould is then closed around the tube. The top is made airtight while the bottom closes around an air jet. Air is then pumped, or “blown” into the tube in step (3). As the tube has been heated it is allowed to flow plastically and expands until it reaches the walls of the mould. Once this has happened the polymer is allowed to cool and then the mould is released to leave the desired shape as seen in step (4).
While it is an ingenious and highly effective method of polymer production it is only really suitable for the production of bottles and does have much use in the swimming goggles industry.
Vacuum Forming
Vacuum forming is a very effective technique when it comes to anything that requires a bowl like shape. The flow diagram below shows the process in a step by step manor. It shows how the mould is placed in the vacuum chamber below a heated sheet of plastic. When the air below the mould is sucked out the plastic is forced to take the shape of the mould beneath it.
Figure 13: Schematic of the Injection Moulding Process [19]
Thermoforming
Thermoforming is one of the easiest production processes to describe. Its basic explanation is that it is done by heating a polymer to soften it and then shaping it once it has softened. In practice the polymer is heated to a temperature where it has more plastic properties but it is not heated to the point of becoming molten. Once this has happened the material is shaped. This often consists of pressing it around a smooth curved surface to create a rounded finished product.
This method could be used if the swimming goggle design being produced demanded curved lenses to improve the swimmers viewing angle. It would add to the cost of production and would require a longer manufacturing period. The use of this technique would depend on the designers desire to have curved lenses.
Calendaring
This process is very similar to the rolling one that is used by manufacturing engineers to roll sheets of metal. In the case of polymers the rollers are heated to allow the material to become more ductile. It then passes through a series of rollers. These rollers would be positioned in such a way that the polymer is forced through a gap between them, called the roll gap. It usually passes through a number of roll gaps of decreasing diameter until the final desired thickness is obtained. At this point the long sheet of polymer is passed through a number of cooling rolls to set it at the correct thickness. The diagram below shows to different roll layouts, however many more are possible.
Figure 14: Different Roll Set Ups in a Calendaring Process [20]
This process is perfectly suited to any production process which requires thin sheets of a material. For example “cling-film” is made in this way. It could well be used in a swimming goggle lens production process.
Manufacturing the Lenses
One of the cheapest polymer forming processes is the calendaring process described above. This is one of the reasons I think that it should be incorporated into the lens production. Another reason is the inherent shape of a typical swimming goggle lens. They are generally just small flat oval shapes. This means that they can be easily produced by taking sheets of polycarbonate and cutting the desired shape out of them.
How the Process Will Work
Once the sheets of polycarbonate have been produced using a calendaring machine (will need to be rolled to about 2/3mm in thickness) the shape of the lens can be easily obtained from this. All that would be required is to cut the shape out. On of the common ways of boing this is to use a buzz saw as it works well with polymers. I think, however, that a cutting process would be very slow and costly. Since the sheets are thin I think that a shearing process would be more efficient.
A shearing process is one where a die is forced onto the sheet of material and it forces out a shape that is the shape of the die used. The question then is how much force would be required for the press. Luckily there is a simple formula for calculating this.
F = (P x A)/2,000
Where F= required force in tonnage of the press, P= shear strength of polycarbonate sheet (in psi) and A= the sectional area to be cut.
In the case of swimming goggles the shear stress is calculated from a polycarbonate sheet of about 2mm thickness which equals 10 000 psi.
[21]
So now we need to calculate the area. The typical size of a swimming goggles lens can be approximated to a disk with a 3cm diameter (Note: the lenses are not perfectly circular and all goggles are different. This is just a typical approximation to show how the calculation works).
This means that the area is = (pi)(r)2
Therefore A = 3.142*152mm
A = 706.5mm2
This means that the required force of the press is
F = (706.5 * 10 000)/2 000 = 3532.5 N/mm2
Important Things to Consider
Polycarbonate sheets need to be “pre-dried” before they go through any forming process. This is because they absorb moisture at a very high rate and if this moisture is allowed to stay in the sheets it can later form into air pockets and create imperfections.
It is also important to note that water based lubricants and coolants should be used in the polymer processing because many of the chemical equivalents can damage polycarbonate.
Costs
Costs are a very important part of product design. However in a project of this type it is very difficult to work out exact costs. For this reason I will be more discussing what is involved when calculating costs and what costs should be considered rather than actually putting numbers on them.
Production Costs
This is probably the biggest area of concern for a design engineer. It covers a wide range of things. Initially any costs from research into the product need to be included. Then there is something called the start up costs. This refers to things like buying machines and a factory as well as employing staff. It is important to remember that if a new product is developed and a patent is applied for this will also add to the cost.
Then there are the actual costs of running the production line. In the case of my proposed manufacturing process this would involve running the calendaring and shearing machines and taking the good from raw materials to a finished product. These costs include a wide range of things. Everything from the power and insurance required for the process to the staff wages and storage space required to dry the product (referred to as “drying costs”) is included in this section.
While the wages may seem like a relatively small area in an engineering process they are quite a big consideration. This is because engineers can cost anywhere in excess of €80 000 a year to have on your book.
While it is difficult to put actual numbers on the cost of production it is easy to see that it would be relatively cheap. Polycarbonate while is by no means the cheapest polymer is a medium range polymer and the production process is fairly simplified. The fact that there are not many small intricate parts helps to greatly reduce the cost.
As I’ve explained above the costs of production are hard to quantify. However if we were to out source the calendaring process we can get a better idea of how expensive things are. For example an average sized sheet of roughly 2000mm * 1200mm cost just below €100. This is based on taking the prices from euro, pound Stirling and dollar dealers. See the following websites to view these quotes.
Taking a sheet this size, leaving small gaps between the pressed lenses would mean that (2000/30) + (1200/30) lenses can be produced from each sheet. That’s equal to roughly 2100 lenses per sheet. It is worth noting that these prices do not include the tax and delivery charges on the polycarbonate or the other costs of production. It does serve, however, to give a brief insight into the level of costs that would be involved in a process of this nature.
Environmental Issues
Swimming goggles in general do not pose a great threat to the environment. They are not like things like plastic bags or bottles, which are often seen lying around, in that they are not a product that will simply be left behind by a consumer.
Polycarbonate is also recyclable. It has a recycling number of 7. This means that it can be recycled by any facility that can recycle other plastics.
Environmental issues do not only confine themselves to pollution and recycling however. It is also important to ensure that the product is not having a harmful impact on humans or animals that it comes into contact with. Recently there has been some controversy surrounding polycarbonate and its uses. The concerns arise over the the addition of Bisphenol A (BPA) which could be harmful to the human body if ingested. In a food or drink application container, studies have shown the migration of BPA into the food from the container. [22]. While this does place a question mark over the use of polycarbonate in general it does not have an impact on its use as a goggles lens material. This is because BPA is in no way harmful unless it is consumer and this is hopefully not something that would happen to a pair of swimming goggles.
The final environmental issue is in the area of the production process used to make the lenses. This should be done in a way to minimise any harmful side effects to the environment. This means that things should be done to reduce the power required and to ensure that no harmful bye-products are produced. The process described in the manufacturing section above is a relatively “clean” process and shouldn’t harm the environment.
Commercial Exploitation
This term appears to have a number of vastly different definitions. I will now briefly look at the main three understandings of the term.
In some cases it refers to exploiting the work force that is producing the good or excessive damage to the environment arising from the production process. However this later point is more environmental exploitation. The exploitation of the work force relates to a number of things ranging from child labour to sexism in the work place.
The second understanding of the term refers to exploiting, or making the most of the market in which the good finds itself. Things like patenting designs and setting up monopolies are examples of this type of exploitation. In effect it means producing a good and at the same time preventing others from producing the same, or a similar, design.
The final way in which the term can be interpreted is in relation to the consumer or user of the product. In this case it is a question of how the consumer is encouraged to buy the product and how this number can be maximised. This understanding of the term is more of a marketing related one as opposed to an engineering one.
In the case of swimming goggles none of these factors play a huge role. So long as a fair and just production process is used and the product is marketed as well as possible there is not a lot more that we as design engineers can do.
