Why are thermoplastics (engineering plastics) replacing metals and becoming a popular option for machined parts? To answer this question we’ve got a few blog posts that look at different aspects of why people are saying yes to engineering plastics. Pumps and valves have been around for about as long as humans have been constructing things to make life easier. Today pumps and valves occupy places in nearly every industry from medical, laboratory and testing equipment, to oil and gas, agriculture, transportation, buildings and more.

Designing pump and valve components from thermoplastics has the benefit of being made from materials that are corrosion resistant. But, even plastics withstand varying physical elements in different ways so it’s important to understand how plastics can also be affected by the physical elements they will be exposed to. The chart below looks at some of the most common plastic resins and gives a general guideline for how they stand up to potential corrosive elements.

Chart of Chemical Resistance for Common Polymer Resins / Thermoplastics

A Deeper Look at Corrosion and What it Is
Corrosion is the deterioration of a material and its physical properties, Corrosion of a material occurs because of an undesirable reaction with its surrounding environment. In valve applications chemicals may attack the exterior as well as the interior surfaces. As you can see from the chart above both acids and alkalis will attack some materials. Corrosion begins with pitting that is not even visible to the eye. But once it begins corrosion continues to grow and eventually it leads to part failure. But even before creating a leak, pits increase turbulence which affects performance.

Corrosion is caused by more than just hazardous chemicals. As you can see from the chart of common thermoplastics above, sometimes an apparently benign fluid can react, as when sea water flows over brass.

How Can Corrosion of Machined Parts Be Stopped?

The best and most cost-effective way of controlling corrosion is preventing it. Studies have shown that an overall cost savings of 40% can be achieved when corrosion is prevented rather than treated. Prevention entails selecting an engineering plastic that will work best with the media being transported through the device. Whether you are using metals or thermoplastics, all environmental factors should be considered, including cleaning agents and things that might not be thought of as highly corrosive. In some cases a sacrificial layer could be used but these will have a finite life, and as the name implies the sacrificial material needs to be closely monitored and it will still require downtime to apply a new sacrificial layer.

Many customers that replace metal valves and gaskets with engineered plastics often note a number of positive benefits even if the initial part costs more.

• Reduced maintenance
• Reduced Downtime 
• Reduced incidence of part failure
• Longer lasting parts
• Overall cost savings
• Smooth surfaces allow for increased velocity and precision control of flow

Customers with ultra-high purity such as medical device, food processing, or water treatment applications to name a few, require very smooth interior surfaces with absolutely no place for contamination to lurk and with no risk of particles breaking free and joining the fluid. Even microscopic pitting can be cause for parts to be replaced because the pitting can allow for bacteria contamination. Once pitting occurs it is virtually impossible to clean a valve or gasket to the high standards required for high purity applications.

The highly smooth surfaces that can be achieved with machined thermoplastic materials can also reduce turbulence in fluids being transported. This allows for fluids to flow at higher velocities and allows for precision control of pumps.

Beginning with careful material selection, research and talking to experienced plastic professionals can lead to designing pump and valve components that can improve performance and increase life from day-to-day to your most demanding applications.

Do you have questions about material selection for seals and valve gaskets like:

What are the lower cost and lighter weight options to metal-to-metal sealing?

We are having thermal degradation issues with plastics in valve components, can we improve this?

Our seals and gaskets have to withstand higher and higher operating pressure. What  materials have higher compressive capabilities and creep resistance?

These are all questions I am able to assist you with. Feel free to contact me at the information below.

Kendall Montague
Industry Segment Manager

thyssenkrupp Materials NA
AIN Plastics Division

Kendall Montague is a veteran of the plastics industry with 16+ years experience working with OEM and MRO engineers assisting in developing thermoplastics material selection as well as custom design and fabrication using CNC equipment.