Boxing in heals?
Although nylon 6/6 is much more commonly used than polyester (PBT) for injection molding applications, you might be surprised that these materials are relatively similar in properties.
There are, however, a few differences that might make one or the other better for a particular application.
Note: This post refers specifically to Nylon 6/6 vs PBT. I will cover other types of nylon and PET in future posts.
The first thing that you will notice when trying to compare mechanical properties of nylon and PBT is that nylon has two sets of mechanical properties, one for dry and one for conditioned. The properties of nylon vary depending on how much moisture the material has absorbed from the atmosphere. I’m not talking about how wet the material is before molding. I’m talking about after the parts are molded. Nylon parts will absorb and give up their moisture depending on the relative humidity of the environment that they are in. The moisture that it absorbs changes the mechanical properties dramatically. PBT also absorbs moisture but at a much lower rate than nylon and the properties are not affected nearly as much.
For instance, unreinforced nylon 6/6 has a tensile stress of 82 Mpa when dry but if the test specimens are conditioned for 24 hours at 50% RH, the tensile strength drops to 55 Mpa. Unreinforced PBT on the other hand has a tensile strength of 58 Mpa independent of RH. Similarly, the izod impact strength of unreinforced nylon 6/6 is 5.5 kJ/m² dry and 12 kJ/m² at 50% RH. The izod impact of PBT is 4.5 kJ/m².
The bottom line is that in normal conditions, the tensile strength of nylon 6/6 and PBT are pretty similar. If the part is going to be used in an extremely dry environment, the nylon will have more tensile strength. On the other hand, nylon 6/6 has significantly better impact strength than pbt unless they are in a very dry environment in which case they will be about even.
The story is much the same if you add glass fiber reinforcement into the equation. The addition of glass fiber reinforcement adds significantly more tensile strength to nylon than it does to PBT when tested dry as molded but add moisture and the nylon gives up much of the advantage. Nylon 6/6 with 30% glass fiber reinforcement should yield about 180 Mpa dry but it drops to about 125 Mpa at 50% RH which is about the same as PBT with the same amount of fiber glass.
There are two ways of looking at heat resistance. Short term and long term.
In the short term, nylon wins. Heat deflection temperature for PBT is 115°C and 50°C at 0.455 Mpa and 1.82 Mpa respectively. Nylon 6/6 comes in at 190°C and 70°C.
On the other hand, in the long term, PBT wins. Long term heat resistance refers to the ability of a material to maintain its properties when exposed to elevated temperatures over the long term. This is best measured using relative thermal index (RTI). The RTI for PBT is 115°C for impact strength retention and 120°C for tensile strength retention. Nylon 6/6 has an RTI of 75°C for impact strength retention and 85°C for tensile strength retention. These relative thermal index temperatures refer to the maximum temperature that the part should be exposed to over the long term in order to maintain its physical properties.
Nylon and PBT will both hold their dimensions pretty evenly through temperature changes but humidity changes will cause more dimensional changes for nylon. Exposure to moist environments will cause nylon to swell. For high moisture environments where dimensions are critical, PBT is a better choice.
Nylon is available with more filler, reinforcement and additive combinations than PBT. However, there are a number of flame retardant PBT grades on the market while flame retardant nylon is not nearly as common. Nylon is available in flame retardant grades but not many manufacturers make it which means that it will be costlier.
PBT has better electrical properties such as surface resistivity and volume resistivity than nylon. This is why PBT is so commonly used for electrical applications, especially high voltage electrical applications.
PBT is typically priced about 10-13% cheaper than Nylon 6/6 if you buy it from a major manufacturer such as DuPont. This makes PBT a bit of a bargain. However there are a lot of small manufacturers of nylon materials that can offer significantly better pricing than the big guys. There are not as many PBT manufacturers. If you do not need to buy a major brand material to satisfy an approved source list, you can probably find better pricing on nylon.
Both PBT and nylon are relatively easy to process. They both have to be dried but PBT needs to be drier than nylon in order to make good parts. The recommended moisture content is 0.04% compared to 0.2% for nylon. Nylon also has or can have lower viscosity which will make it easier to fill small parts. In addition nylon sets up slightly faster which could result in faster cycle times.
Nylon and PBT are pretty evenly matched property wise in most environments.
There are several reasons to run PBT. If you need flame retardant material, PBT is your best option. There are many FR grades to choose from. PTB is also a better choice for electrical applications. If you have an application that has very critical dimensions and needs to hold those dimensions over a wide range of temperatures and humidity, PBT is the best choice.
You might be surprised to know that the advantages that nylon offers are a little less clear cut. It is slightly easier to process than PBT and offers faster cycle times. The biggest advantage that nylon offers is the sheer variety of grades that are available on the market. Endless varieties of impact modified grades, all sorts of different filler and reinforcement combinations, and even fast cycling nucleated grades are readily available.
As far as heat resistance goes, it’s a mixed bag. Short term favors nylon but long term heat resistance favors PBT.
It’s always nice to have options. If you are considering either of these materials for an application, take a moment to consider if the other might fit the bill better.