by Denise Sullivan Denise Sullivan No Comments

The Effect Surface Finish Has on PTFE Seals

Surface finish plays an essential role in the effectiveness of PTFE seals. The different finishes provide different degrees of contact between the two components, which affects the seal’s strength and reliability. In this paper, we will discuss the effect of surface finish on PTFE seal performance and suggest ways to improve seal quality.

Surface finish on PTFE

The Influences of Surface Finish on Friction

The surface finish is critical for polymer-based seals. Despite being considered a soft plastic, PTFE is significantly harder than traditional o-ring materials. Because of this, if the mating surface is designed for other materials, it can cause the PTFE seals to leak.

As you can see in the graph below, the improvement in the surface finish has a favorable effect on the frictional force required.

The following specifications were used in the force test that resulted in the above data.

  • Stroke Speed: 4 in/min (102 mm/min)
  • Shaft Diameter: 0.1875in. (4.762mm)
  • Ambient Temperature: 73°F (23°C)
  • Mating Surface Material: 17 -4 PH S.S.
  • Mating Surface Hardness: -40 Rc

Surface Finish Influences on Wear

The finish of the mating surface is one of the main factors in the PTFE seal wear. Wear on the seal is generally proportional to frictional force. In other words, less friction reduces the wear on the seal. 

The following graph shows how a smoother surface finish reduces the PTFE seal wear rate.

To calculate the wear rate in microinches RMS, we used the formula IN3 -MIN/LB-FT-HR -10-9. For microinches Ra, we used the formula CM3 -MIN/KG-M-HR -10-9.

We used the following test parameters during our testing to ensure accuracy for each test.

  • Surface Speed: 55 fpm (17 m/min)
  • Loading Stress: 55 lb./in2 (4 bar)
  • Wear Rate in Air: @ PV 3025 lb./in2 x ft/min (7.5 N/mm x m/min)
  • Mating Surface Hardness: Rc 42
  • Matin Surface Maerial 17 -4 PH Stainless steal
  • Mating Surface Finish: 1.6 to 111.1 microinches RMS (1.4 to 100 microinches RA)
  • Ambient Temperature: 70°F (21°C)
  • Ambient Relative Humidity 75% RH
  • Duration 5 Hours
  • Seal material FP

Influences on Sealing Ability

Finally, we looked at how the surface finish affected PTFE’s sealing ability. In general, the sealing ability of PTFE is proportional to the fluid’s viscosity. If a media, like gas, has a reduced viscosity, it is more difficult to seal. In these situations, having a smoother finish on the mating surface can help ensure fewer leaks.

We’ve found that different mediums require not only different finish ranges between them but also different finish ranges when considering if the surface is dynamic or static. You can see our suggested finish in the table below.

MediaDynamic Surface Static Surface 
Gases and Liquids at Cryogenic Temperatures2 to 4 microinches RMS
(1.8 to 3.6 Microinches Ra)		4 to 8 Microinches RMS
(3.6 to 7.2 Microinches Ra)
Gas at Non-Cryogenic Temperatures6 to 12 Microinches RMS
(5.4 to 10.8 Microinches Ra)		12 to 32 Microinches RMS
(10.8 to 28.8 Microinches Ra)
Liquids8 to 16 Microinches RMS
(7.2 to 14.4 Microinches Ra)		16 to 32 Microinches RMS
(14.4 to 28.8 Microinches Ra)

As you can see, the mating surface finishes can profoundly affect PTFE seals. Because of this, if the mating surface is designed for other materials, it can cause the PTFE seals to leak. Finally, we examined how this affected PTFE’s sealing ability. In general, the sealing ability of PTFE is proportional to the fluid’s viscosity. 

by Denise Sullivan Denise Sullivan No Comments

CNC Machining Basics

cnc machining

Computer Numerical Control (CNC) machining is a manufacturing process that uses pre-programmed computer software to dictate the movement of the tools and machines. It allows devices such as lathes, mills, and grinders to cut three-dimensional objects.

To understand CNC machining, we are going through a basic overview of CNC machining, including terminology, how they work, and the advantages and limitations of a CNC machine.

Terminology

Understanding the terminology associated with CNC machining is the first step to understanding CNC machining. CNC stands for computer numerical control. A computer and CAM software are used to animate, control, and observe the motions of the machine using digital data. 

CAM stands for computer-aided machine or manufacturing. It uses different software programs to generate tool paths and NC code for a CNC-operated machine based on data from 3D computer models. CAM only uses CAD to generate code for the device to follow. 

CNC machines could be robots, sheet metal stamping machines, lasers, grinders, lathes, water jet cutters, or welders. The computer controlling these machines is often an onboard controller. However,  the controller may also be an external PC.

How Do CNC Machines Work

CNC machines cut a desired piece based on what is programmed into the system. Therefore, it begins with designing the component. This step is necessary to establish the dimensions and features of the finished product.

Often the component design takes place in a CAD-CAM package. It allows the flow to continue through the program without going through file translation. Suppose they are not in the same package or software package. Then the CAD models will have to be imported into the CAM software.

Designing the component using CAD software allows you to construct the real piece and see what it looks like on the screen. The program is a code generator for the CNC machine that compiles coordinates to direct the cutting head during the manufacturing process. 

The final step in CNC machining is to choose the proper parameters. These include:

  • Voltage
  • Cutting speed
  • RPMS

The software will choose how the parts are arranged and oriented in relation to the raw materials. The part shape and tooling influence the configuration of the other parameters and equipment used.

Open and Closed Loop CNC Machining

CNC machining takes place in either an open-loop or closed-loop system. Open and closed-loop refer to the control process of the system. The position of the piece is determined by the type of system used.

The signal runs in a single direction between the CNC and the motor in an open-loop system. This system uses stepper motors, but the use of stepper motors doesn’t necessarily indicate an open system.  This system is commonly used when the forces and speeds are minimal.

In an open system, once the motor is in place, the controller has no feedback. That means unless a limit switch is tripped, the controller runs the operation as if there are no errors. 

A closed-loop system allows the controller to receive feedback. Because the controller can receive feedback, the controller can rectify irregularities and correct errors when necessary. There are two types of closed-loop systems. With the first type, the return signal feeds directly into the controller. The other type returns the signal feedback to the PC running the program.

A closed-loop system is necessary to ensure speed and consistency. It is the ideal choice for accuracy in creating a part. However, because there are more working parts, they have more components that can fail and be harder to tune.

CNC Machining Advantages

There are many advantages of using CNC machining for part manufacturing. For one, it is an excellent method for producing multiple parts that are identical to one another. If you need upwards of 100 parts, CNC-machined parts are more cost-effective than 3D printing.

CNC machining creates precision pieces. With a tolerance of +/- 0.127mm, machined parts come down to +/- 0.005mm. The equipment can produce them quickly. 

CNC machines can be used with various materials. Alloys such as zinc and magnesium are standard materials. In addition, aluminum, steel, brass, and titanium are common choices.

Limitations

While CNC machining has many advantages, the process has several limitations. One of them is using older machines to perform this process. CNC machines require a system that can perform demanding tasks. As a result, older computers and devices may not be able to handle the tasks.

Another limitation is that most CNC controllers may be constrained by only comprehending circular arcs and straight-line motions. Archs are often allowed on the main XYZ planes. However, some parts require a five-plane rotation that some controllers can’t comprehend. 

Conclusion

CNC machining is a significant advancement in manufacturing with the aid of computer technology that will give you accurate parts. It is a quicker and more efficient way to produce parts.