The advancements in polymer technology have resulted in myriad different plastics, each offering its own unique property. While some polymers are preferred for their strength, others may have specific resistance to certain chemicals, making it the material of choice in an industry where such chemicals are prevalent. A case in point of this is PPS (Polyphenylsulfone). PPS is a relatively brittle polymer, which usually requires a glass fibre reinforcement in order to maintain its structural stability. However, PPS is especially resistant to the chemicals used in pulp and paper industries, requiring that components used in the equipment for pulp and paper be made from this polymer.

While many properties have been tested and improved continually, it has always been the durability of the polymer that has remained a mainstay of its preference. Traditionally, OEMs and equipment manufacturers have always prided themselves on making products that can stand the test of time. Be it a car or a pair of shoes, quality and durability have always played the most prominent role in defining the industry leaders. However, this is suddenly changing.

Shortening product life cycles have started placing questions around the importance of durability. What use is a car that can stay fuel efficient and low maintenance over 10 years, if the consumer is going to buy a new car within 3 years anyway? Nowhere is this phenomenon more pronounced than in the electronics industry. Mobile phones, smart watches and even laptops are no longer needed for more than 18-24 months, before the consumer upgrades to a new variant.

The shortening of the life cycle has an impact on the input materials used. Devices no longer need high-performance materials, which are expensive and drive up the final cost of the equipment. Furthermore, with the ever-changing designs, a component needs to be made with as low an upfront cost as possible, since any one-time tooling and development charges cannot be amortized easily over volumes over time.

In the polymer space, this has created both a dilemma and an opportunity. Polymer components are typically of two variants:

1. Injection moulded parts – where the part is made by injecting molten polymer into a die that has the final shape of the component needed
2. Machined parts – where the part is turned or milled from a rod or sheet

Injection moulded parts are traditionally cheaper, when you only consider the marginal cost of making the part. Not only does the injection moulding use only as much polymer as needed, it also creates complex shapes within seconds and a single die may be capable of housing multiple cavities, so the process is high in productivity.

Machining is expensive, as you are effectively removing excess material from a rod or sheet. Furthermore, it is a length process that only gets longer as the complexity of the part increases.

Machining, however, wins against injection moulding on two accounts. For one, the cost of making the die can be both expensive and time consuming. Making a good die can often take over a month and cost upwards of US$1500 for a single die. In addition to this, the tolerances achievable on injection moulding are not always close. Once the die is made, if the part tolerances are not as per the drawing, either the OEM needs to accept the new dimensions as they are, or the component manufacturer must re-make the die, which is again expensive and time consuming. Machining, on the other hand, can easily attain tolerances of within 20 microns, if done properly. Furthermore, any change in drawing or revision only requires a simple modification of the programme on the CNC machine – which is free of cost and can be done in minutes.

So, even though machining yields a more expensive per-part cost, device manufacturers are slowly accepting that machined components are a better option for short product life cycle parts. Considering the mobile phone market operates on barely an 8-12-month product life, there is little time to invest in making a mould, testing the parts and moving to production. It is simpler for the OEM to utilise the expertise of a good machine shop, where both the polymer and the final component can be changed as required.

While machining is certainly taking off as a preferred option for making polymer parts, the grade of polymer has also been reviewed and revised accordingly. Polymers like PEEK, PCTFE and Polyimides are highly durable, but come at a steep price. For comparison, the price of PEEK would be nearly 20-40 times the price of cheaper polymers, such as Nylons and Polyacetal (Delrin). Considering that high-performance polymers offer, amongst other things, durability as a key property, there is a shift away from the more expensive plastics to those that offer lesser durability at a fraction of the price.

In our own experience, we have seen that while one part is running for an existing generation of equipment, another set of parts is already under development for the next generation. This pipeline allows the OEM the time needed to test and modify the parts as required, so that the time to market for the next generation of devices is shortened as much as possible. The ability to adapt and cater to this changing dynamic is based on a company’s capability to understand and work with many different polymers. Being able to take on a new polymer – based on what the client specifies – and immediately develop techniques to achieve the desired close tolerances on the same is a skill that is slowly acquired. By working with nearly 30 different polymers, Poly Fluoro Ltd. has developed a wealth of understanding on how each family of plastics behaves. This puts us in an ideal position to capitalise on the changing approach of today’s industries.

The evolution in polymer technologies has ensured that there is virtually no application where polymers do not provide some benefit. Polymers are increasingly being turned to in areas that require high-stress resistance combined with light weight. As such, metal replacement is being done across industries, facilitating the need for extensive testing in order to prove the polymer can suitably sustain itself over the long term.

The rise of 3D prototyping has had many believe that conventional methods of sample development would soon be phased out. In fact, many who do not fully understand the limitations of 3D printing have even suggested that conventional production methods could soon be done away with, as the speed of 3D printing is augmented to match the throughput of existing processes.

While 3D printing is no doubt a useful tool, it suffers from limitations. For one, the layering process – while very effective in making certain kinds of shapes – does not lend itself of creating a polymer structure with adequate tensile properties. Especially in the direction of the layering, the part will always be inferior to a polymer that was either cast whole from its molten form or compressed at high loads and then cured under heat. Another issue is that many polymers – such as PEEK – are subjected to internal stresses that do not fully release until the heat on the part crosses the glass crystallinity point. Conventional machining processes make allowance for these stresses. Parts are often annealed mid-way through the machining operation, allowing the stresses to be removed so that the part itself does not suffer deformation while under application.

Another limitation of 3D printing relates to the rise of 3D software technologies. While earlier it was assumed that 3D prototyping allows a better understanding of the fitment of the part into the assembly where it would be used, we can now use 3D software to get the same idea. 3D software not only allows for the fitment to be assessed to a very high degree of tolerance, but also allows the stress and load parameters to be applied, giving a full understanding of whether or not the part will deform during use. This, in effect, negates the need for 3D printing as a whole and allows for a part to be fully proven before moving into the sample development and bulk production phases.

Given that the conventional production processes are here to stay, it is worth exploring them further to understand the strengths and weaknesses of each. Primarily, polymer parts can be made either by machining or by injection moulding.

Machining involves

As we will see, each has its benefits and limitations. We will compare these processes along some of the key parameters.

1. Complexity
While machining does allow for some degree of complexity, more often than not, very complex parts need to be injection moulded. Typically, contours, fine radii and a high-gloss finish can only be attained using injection moulding. While machining programmes employing CAM software can be used to achieve intricate dimensions, the equipment needed and the extra time it would take to attain this would make the process costly.

2. Cycle time
In general, the cycle time needed during machining will increase along with part complexity. More dimensions imply more operations and parts can sometime take in excess of an hour to machine, if the dimensions are varied enough.

In comparison, injection moulded parts would rarely vary in cycle time as complexity increases. Usually, an injection moulded component would take only under a minute to mould, and this time would only increase if the part weight was increased.

3. Volume
While machining can be done at high volumes, truly large-scale parts usually rely on injection moulding. As described above, a cycle time of only a minute may be needed for an injection moulded part, as compared with machining that can take as little as under a minute but go all the way up to over an hour.  As such, the cost differential between injection moulding and machining can be as much as 5-10X – considering the cost per minute to run the machines are somewhat comparable.

4. Tolerance
Tolerance is one of the areas where machining truly gains an advantage. Machining tolerances go down to as close as 10 microns and ensure that this tolerance is held over a large batch of components. In comparison, injection moulding would usually not commit to such close tolerances. One key difference between the processes is that with machining, the drawing is King. Any dimensions given on the drawing (within reason) are set in stone and must be achieved if the part is to be approved. In contrast, injection moulding will try to come to as close as possible to the drawing dimensions. Any minor deviations are then submitted for review and the drawing is re-created accordingly. Usually, this is because the cost of re-making/ re-working the tool may be high and/or infeasible. So the end-user takes a call based on fitment and the part can be approved.

5. Polymer
Other than cycle time, the key cost comparison between moulding and matching rests on the amount of polymer used. Machining is a process that starts with a stock shape (a rod or a sheet) and removes that which is unnecessary to make a final part. In contrast, moulding is additive. We create an empty mould and fill it with just the right amount of molten polymer needed to make the part.

In the case of more expensive polymers, this cost saving from lower consumption can be quite significant – especially for high-volume parts.

However, there are some polymers that simply cannot be injection moulded. PTFE and UHMWPE – to name just two such polymers – can only be made from compression moulding. These polymers have no melt-flow, meaning that even at their melting point, they do not become liquids, so that putting them into a mould would be infeasible.

6. Development

While injection moulding is usually the cheapest way to make a part (owing to a lower cycle time and a more efficient use of raw materials), where it is expensive is in the development.

A machined component can be developed for practically nothing more than the time and effort it takes to programme the part into a CNC machine. Injection moulding requires a die, which is expensive to make and can sometimes take weeks to complete. It is mainly for this reason that most prototype applications prefer to get the parts machined and only look towards moulding when the volumes increase. Even with increased volumes, the cost of the mould needs to be amortized over the expected volumes the mould can produce. This is then added to the part cost to compare whether or not there is any saving from moulding the component.

Once all these factors are weight against one another, an informed decision can be taken on the process to follow.

PTFE sliding bearings have become a mainstay for any application that requires a combination of loads and movement. The key feature of the bearing is that it can accommodate high vertical loads, while offering a low coefficient of friction. As such, the standard model of PTFE against polished Stainless Steel allows for coefficients of friction as low as 0.03 (near rolling friction) while being able to sustain vertical loads of up to 400Bar.

The applications of this sliding arrangement include pipe support systems, boiler plants, bridges, and walkways. The plain PTFE sliding bearing does not lend itself to loads exceeding 150 Tonnes, hence the application does not extend to areas such as flyovers and railway bridges, where the loads are in excess of this value.

Design

Designing a plain PTFE sliding bearing involves an understanding of the various loads involved and ensuring that the PTFE does not succumb to these loads, even under their most extreme applications. For the most part, while PTFE is rated to a compressive load of up to 400Bar, we apply a 50-70% safety factor in design, keeping the design load within 120-200Bar.

Other loads such as lateral loads, uplift loads, and even rotation, can be factored into the bearing using metal bolts that can accommodate these loads. Care, however, must be taken in ensuring that PTFE is used in any area where there would be sliding. So, if metal bolts are used to prevent movement in one of the lateral directions, a PTFE element is employed around the metal bolt, such that the bolt itself does not rub directly against the sliding plate thereby increasing the friction.

One of the main design arguments that result with PTFE bearing is around the thickness of the PTFE pad employed. The thickness of the pad has long been specified as being between 3mm and 5mm as per the BS 5400 standard – which remains one of the only standards that pertain specifically to the design of PTFE sliding bearings. The standard recommends that the PTFE sheet should be 5mm in the care of a recessed bearing (a bearing where the PTFE pad is bonded within a recess on the metal plate) and 3mm in the case of an un-recessed bearing. The case for making the recess is quite sound. Bonding the PTFE within a pocket milled onto the surface of the metal plate ensures that the PTFE does not come loose of the metal over time. Barring cases where the shear load on the PTFE is likely to be low, a recessed option is generally preferred by designers.

Nonetheless, a lot of debate does arise as to why the values of 3mm and 5mm are found acceptable.

Shear Loads

Recently, we were asked to supply 8mm thick polypropylene sheets for a bearing project. Among the technical requirements was a shear load confirmation. Against a given horizontal load, we needed to confirm that the sheets would not deform, causing failure.

Given the Shear Modulus (G) of the material, the above formula can be used to calculate the value of F needed to displace the sheet. Since this was an infrastructure project, we assumed Δx to be 0.1mm (a very small displacement) and given the other properties, the force was calculated.

In our particular example, a force of 750Tonnes would be needed to displace the polypropylene sheet by 0.1mm. Considering the actual loads expected were within 10 Tonnes, this safely allowed us to state that the bearing sheet would not be affected by the shear load.

The more interesting aspect of this, however, is the function that l (the thickness of the sheet), plays in the calculation. For the same shear strain, a higher value of l would imply a higher value of Δx. In other words, increasing the thickness increases the risk of deformity.

Thus, the sheet thickness – whether it is polypropylene or PTFE – needs to be minimized in order to minimize the effects of shear. However, too thin a sheet would risk damage from tearing and/or wear out. Furthermore, grit that finds its way between the bearing plates would be more likely to cause damage. PTFE normally swallows the grit, allowing it to stay embedded just below the sliding surface, so that it does not compromise the sliding movement. If the sheet was under 3mm thick, larger pieces of grit would cause problems.

Because PTFE has such a low coefficient of friction, shear loads are not normally considered an issue. Whatever the thickness of the PTFE, it would require a tremendous load to deform the sheet due to shearing. Nonetheless, the shear load is worth keeping in mind, especially when a design engineer argues that 3mm or 5mm is too thin and that a higher thickness of sheet is needed to accommodate the load.