Sealing and Shielding

Electrically conductive plastics continue to provide reliable EMI shielding in a wide variety of applications. Specifically, thermoplastics can overtake bulky metal enclosures due to their superior weight, EMI shielding capabilities, and simpler manufacturing process. However, before purchasing any thermoplastic, it is important to consider performance capabilities.

Electrically conductive thermoplastics are typically sold as a pellet blend of two or more components, made up from a variety of base polymers and stainless steel pellets. In small applications and sizes, these blends can generally be effective but are always bound to have consistency issues. In transportation and handling the stainless-steel pellets will settle to the bottom, resulting in an inconsistently shielded final product. This problem becomes more evident when molders run large quantities of parts and when they store the mix in large containers.
 

Selecting a one pellet plastic material, like Parker Chomerics PREMIER PBT 250-FR, eliminates this problem, since there is no mixing of pellets. Instead of having stainless steel pellets as a separate component, stainless steel fibers are pultruded into the plastic pellets to provide shielding. Single pellet thermoplastics can be sold in large quantities, unlike pellet mixes.

Understanding the plastic’s UL 94 rating will help determine how it will perform under fire hazard conditions. In order to receive the UL 94 5VA certification, the plastic must stop burning after 60 seconds on a vertical plaque, with no drips or holes, according to the UL website. This rating, is the highest level of flammability resistance for any thermoplastic. The next rating down, UL 94 V-0, means the plastic can stop burning within 10 seconds on a vertical specimen and the drips are non-flammable. Most thermal plastics that are electrically conductive fall under the UL 94 V-0 rating and require an extra coating to be EMI shielded. But Parker Chomerics PREMIER PBT 250-FR earns a 5VA rating at 2.5 mm thickness without the need of a coating to achieve electrical conductivity.

In addition, plastics can also be more cost effective than metal enclosures. Although metals are typically more cost effective initially, the secondary machining requirements of

many metal components can add significant cost and lead time. For example, many die cast parts require machining operations to drill and tap threads while most thermoplastics can be molded with pre-formed holes and use thread forming screws. Also, plastics are significantly lighter than metal enclosures, helping to better achieve light-weighting goals.

Parker Chomerics PREMIER PBT 250-FR, a single pellet UL 94 5VA electrically conductive thermoplastic, is known for its excellent performance where petrochemical exposure is common. Typical applications include retail fuel dispenser pumps, housings, dispensers and face plates, electronic payment terminal housings, security access points, and more.

This blog contributed by Page Ludl, marketing co-op, Chomerics Division.

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In our July Semiconductor entry, we noted that lowering the cost of ownership is a multi-faceted goal. We discussed how one of the areas for potential improvement is mechanical design and how the Parker EZ-Lok seal is a major solution to mechanical seal failure. In this entry, we’ll investigate a notably different type of cost-reduction opportunity – material selection – and see how Parker’s innovative HiFluor compounds can reduce seal costs to as little as half.

When it comes to the seal industry, the semiconductor market is well known as one where the most premium, chemical-resistant compounds are a necessity. Microelectronic manufacturing processes involve chemistries that push the limits of what elastomeric compounds can withstand in terms of both chemical aggressiveness and variety. The perfluorinated materials (FFKM) capable of withstanding these environments require intricate manufacturing processes regulated by closely-guarded trade secrets and the significant investment of resources.

These factors drive the price of FFKM compounds to the point of being as much as 50 times the cost of any other variety. Cutting just a slice out of this cost can result in significant savings – a chance to take out a quarter or even half the pie would be advantageous indeed. Fabricators should be continually on the lookout for more cost-effective compounds that show equal performance in their pertinent operations.

This is why Parker’s HiFluor compounds offer an opportunity for cost savings that shouldn’t go unnoticed.A unique hybrid of performance between FFKM and the simpler technology of fluorocarbon (FKM) elastomers, HiFluor offers the most superb chemical compatibility in the many semiconductor environments where the high temperature ratings of FFKM aren’t necessary – and at a fraction of the cost.

Not only can HiFluor be used where even FKM is lacking, but its performance in applications with aggressive plasma exposure is spectacular as well. This can be observed by its overall resistance to plasma-induced material degradation. However, Parker has also developed multiple formulations that display extremely low particle generation when most materials would be expected to suffer severe physical and chemical etch.

Need assistance deciphering exactly where this kind of cost-savings can be implemented? Parker O-Ring & Engineering Seals Division has all the resources needed to help their customers identify opportunities to utilize HiFluor seals.

For instance, one major semiconductor fab had several factors (other than their seals) dictating the frequency of their preventative maintenance (PM) intervals. The fab wanted to replace their seals at these intervals as a precautionary measure to limit the chance of them becoming another PM-increasing factor. However, this caused these premium FFKM seals to be a source of inflated cost. Parker engineers assisted with a process evaluation that resulted in over half the seals being replaced with cost-effective HiFluor O-rings, while the tool regions with more intense plasma exposure were reserved for the elite performance of Parker’s FF302.

Another major fab in the microelectronics industry switched from FKM to FFKM seals in their oxide etch process. The tool owner achieved the desired performance improvement, but soon began searching for less expensive options. Based on guidance from Parker engineers, he recognized the plasma resistance and low particulate generation of Parker’s HiFluor compound, HF355. After implementing this change, he retained the performance improvement, but at a fraction of the cost.

Semiconductor tool owners understand that their aggressive processes require the most robust, expensive FFKM seal materials. The price tag on these seals is greater than those from any other compound family. Fortunately, HiFluor is a proven sealing solution that can bridge the gap and provide the same kind of high performance at a much lower cost. To find out if HiFluor is right for your application, visit us at Parker.com/oes and chat with and engineer. 

This article was contributed by Nathaniel Reis, applications engineer, Parker O-Ring & Engineered Seals Division. 

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It's no surprise that electronic enclosures and housings in aerospace and defense applications are built to meet the most stringent of military standards. These include but are not limited to environmental stresses, EMI shielding, and maintenance requirements. Additionally, the advanced technological requirements of the devices requiring these enclosures mean that these devices must be more rugged and more powerful, as well as smaller, lighter, and easier to replace.

This poses challenges for designers, leading them not only to suppliers who can meet these requirements but partners who can assist in the design and supply chain management of these complex units.

For environmentally-sealed and EMI-shielded electronics devices, there is no better full-system solution than an overmolded or vulcanized cover. 

So, what is overmolding/vulcanizing/mold-in-place gasket anyway?

1. Integration and permanent adhesion - Overmolded gaskets are integrated directly into an enclosure and permanently bonded onto to the housing. This adds a significant level of durability and means that the gasket will not experience wear as quickly as traditional gasketing in field operation.
    
2. Simplification of assembly - Because the gasket is already directly bonded onto the housing, there is no additional assembly needed. This means no more having to place a gasket in a groove or requiring an attachment method such as rivets or pressure sensitive adhesive.
    
3. Ease of maintenance and repair - Especially true in shipboard avionics, overmolded gaskets provide the benefit of minimizing maintenance procedure. There is no risk of forgetting to reinstall gaskets during disassembly and reassembly.
    
4. Tighter tolerance controls - Molding uses a precise process and tolerance-controlled tools meaning the gasket can meet tolerance controls of just a few thousandths of an inch.
    
5. Smaller form factors and custom geometries - With the tight tolerance controls of overmolded covers comes the ability to mold smaller form factors that will optimize sealing of covers and enclosures. The custom geometries mean that design factors such as closure force and sealing path are perfectly accounted for.
    
6. Consolidated supply chain - With the gasket provided directly on the cover, there is no need to work with multiple suppliers, each completing a small part of the assembly. Overmolded covers can be directly supplied in a ready-to-use form.
   

With more than 40 years of experience in overmolding, the Process Engineering team at Parker Chomerics can assist with full system enclosure design that takes into consideration such factors as ideal surface finish and groove dimensions to meet customer-driven requirements. We will work with customers to determine whether overmolding is the best solution and feasible based on all necessary specifications.

As devices become more complex, so do the associated supply chains. An electronic enclosure can quickly pick up more than 5 or 6 individual suppliers, each needing to meet various requirements of military standards. In addition to the difficulties of sourcing the best suppliers, a complicated web of interdependent timelines starts to appear. This is where Chomerics can provide significant support and logistical relief. Parker has an extensive network of first-class suppliers and can also work with customer-approved suppliers to manage a project from start to finish. Services offered can include: overmolding, machining, painting, dip brazing, embedded fasteners, part marking, etching, and custom packaging.

Overmolded covers provide countless benefits for customers looking for the best solution to durable and reliable EMI shielded, environmentally sealed housings while minimizing a complicated system of suppliers.

This blog post was contributed by Ben Nudelman, market development engineer, Chomerics Division.

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Closure force requirements are an important consideration for sealing applications, and the Applications Engineering team is often asked for guidance as to how to minimize or predict the amount of force it will take to close a properly-designed face seal groove.  

There are several factors to consider when trying to determine compressive load.  The most important of those considerations are:
 

• What is the hardness of the rubber (durometer)?
• How much will the O-Ring be compressed?
• What is the cross-sectional thickness of the O-Ring?
• How wide is the groove?
• What impact does material family play?

It is correct to assume that as the hardness of the rubber increases so too does the compressive load.  Parker’s O-Ring and Engineered Seals Division offers materials that cover a range of 40-95 durometers (Shore A scale).

Below is an example of compressive load for a 0.139” CS O-ring made from NBR (Buna-N) materials at different measures of hardness (60, 70, and 90):
 

Figure 1: Measure of compressive force requirements of a 0.139”CS, NBR material at different hardness levels.

Another intuitive thought is that as cross-section diameter increases, so does the compressive load requirement.  

Below is a plot of compressive load requirements per linear inch for the standard cross sections (0.070”, 0.103”, 0.139”, 0.210”, and 0.275”), and it follows that logic.
 

Figure 2: Compressive load requirements at different cross-sectional thicknesses on a 70 durometer NBR material.

Groove width impacts compressive load because of how it changes the amount of gland-fill in a given application.  If a groove is narrow, the O-ring is likely to make sidewall contact once compressed.  When sidewall contact occurs, it results in higher compressive load requirements because of how the part is being constricted in the groove.  If there is an application that has high gland fill, adding a draft angle can significantly reduce the compressive load requirements, as shown in Figure 3.  Figures 4, 5, and 6 show the impact of groove width and draft angle on gland fill.
 

Figure 3: Compressive force for 0.070" CS 70 Durometer NBR at 99.8% nominal gland fill when fully compressed.  The graph shows the impact of draft angle on compressive load for grooves that have very high gland fill.

Figure 4: This image shows an O-Ring being compressed with no sidewall contact.

Figure 5: This image shows an O-Ring being compressed the same amount as in Figure 4, but with a narrower groove.  The result is very high gland fill.

Figure 6: This is the same amount of compression as in Figure 5, but with a 3-degree draft angle added. The result is lower gland fill and lower stress on the O-Ring. 

A common myth in the sealing industry is that there is a correlation between material family and compressive load requirements, but as seen in the graphic below, that is not the necessarily the case.  The only truly accurate statement is that silicone materials have lower compressive load requirements than that of other materials.  Otherwise, many material families have significant overlap in the amount of compressive load they generate for the same amounts of squeeze.

Below is a plot of compressive load requirements per linear inch for various 70 durometer materials.

Figure 7:  Compressive load requirements by material family with hardness held at 70 durometer.

In summary, here are some ways to mitigate high compressive loads without sacrificing the amount of squeeze applied to the seal:
•    Use a softer (lower durometer) material.
•    Use a thinner cross-sectional diameter.
•    If gland fill is very high, widen the groove.  If that is not possible, consider adding a draft angle.
•    If appropriate for the application, switch to a silicone seal material.
•    If the application is conducive, consider using a hollow cross section from our extruded product line.
 

This article was contributed by William Pomeroy, applications engineer, Parker O-Ring & Engineered Seals Division. 

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I had a brief career in construction. We used an old hydraulic manlift for elevated projects, and it had a problem – we would start the day eye-level with our work, and after an hour would realize we were balancing on our toes to reach the same level. Cylinder drift was to blame. The lift cylinder was slowly retracting while the machine was off. In a manlift, it’s an annoyance requiring you to raise yourself every half hour, but the drift is dangerous if lifts are used to support heavy loads with the possibility that people or equipment may be under them. This is one reason why when doing automotive repairs and working underneath a car, you should always use solid jack stands or blocks instead of the hydraulic jack.

A guy on my crew told me the lift slowly sank because the piston seals needed to be replaced. I wasn’t a seal engineer at the time, so it sounded reasonable. Now I know drift is more complicated, and it’s critical to understand if you’re responsible for cylinder design. Hydraulic cylinders have two main components: the piston, which is acted upon by pressurized fluid to create force and motion, and the rod, which transfers force and motion to the machinery (in my case, the lift platform) (Fig. 1). Located elsewhere are valves that open and close, controlling fluid flow into the cylinder.

Figure 1. Typical hydraulic cylinder

Let’s remove the piston completely. We now have just a rod in a bore, which is known as a ram-style cylinder (Fig. 2). Assume we have perfect, leak-free valves and rod seals. If we shut both valves, zero oil can enter or leave the cylinder.

Figure 2. Piston removed -- ram style cylinder

Since moving the rod changes the fluid volume inside the cylinder (the rod takes up space), fluid MUST flow into or out of the cylinder for it to move.

Since this can’t happen while our perfect valves are closed, the rod can’t move. This is called hydraulic lock.

As you can see with hydraulic lock, bad piston seals wouldn’t cause drift in our manlift. Volume loss or escape from the cylinder is what caused us to slowly droop back to the floor. In my situation, either the valves were leaking, slowly reducing the volume of oil in the cylinder, or leaky rod seals (easier to spot) were allowing the fluid to escape the system.

There are a few caveats to this scenario. We assume oil is incompressible, which is not entirely true. Because the oil does squish and stretch a little, the rod will move a small amount with large load changes (read up on ‘bulk modulus of hydraulic oil’). This is not drift, since the oil quickly reaches equilibrium and the rod will not continue to move.

Single-acting cylinders (Fig. 3) are an exception, because oil leaking across the piston is leaving the system. This is similar to when the rod seals leak in double-acting cylinders – drift occurs. Double-ended cylinders (Fig. 4) are also an exception because the fluid volume in the cylinder does not change as the rods move.  Both of these systems require low leak piston seals to prevent drift.

Figure 3.  Single-acting piston
 

Figure 4.  Double-ended cylinder

Leakage across the piston doesn’t cause drift but can cause a number of other complications. Rod retraction relies entirely on the piston seal blocking pressure from crossing the piston; I’ve already described how simply pumping fluid into one side of a cylinder without a piston seal will only cause the rod to extend. Using fluid pressure to retract the rod is not possible without a piston seal.

When extending the rod or holding a load (valves open, no hydraulic lock applied), leakage across the piston seal slowly allows pressure to equalize on both sides of the cylinder. Once this happens, effective piston diameter drops to only the diameter of the rod (Fig. 5). Pushing or supporting the same load now requires higher fluid pressure. This can raise pressures higher than the system was designed to see, cracking pressure relief valves.
 

Figure 5.  Reduction in effective piston diameter

In summary, a leaky piston seal won’t cause drift, but it’s bad for efficiency and could damage the system.

In systems that are quickly cycling, a slow piston leak may go unnoticed as a tiny efficiency loss – the pressure reverses before a significant amount of fluid can leak past the seal to cause problems. On the other hand, cylinders that move slowly or must hold a position for extended periods benefit from no-leak piston seals.

At Parker, we see a wide variety of applications, and we manufacturer piston seals in many styles and materials to cover all of them. Softer durometer materials like those used for our PSP and T-seals are better for tight sealing. Harder durometer materials like our BP and PTFE cap seals are more extrusion resistant and wear longer in fast-stroking applications.

We also offer hybrid designs like our CQ profile. The CQ design utilizes glass-filled PTFE for low friction and long-wearing but also features a rubber insert to reduce leakage.

Cylinder drift is a concern in many hydraulic applications. It’s commonly mistaken as piston seal failure but is usually a combination of factors involving the valves. Understanding cylinder mechanics is vital to identifying the root causes of failure and for designing systems that are resistant to drifting.

Recommendations on application design and material selection are based on available technical data. They are offered as suggestions only. Each user should make his own tests to determine the suitability for his own particular use. Parker offers no express or implied warranties concerning the form, fit, or function of a product in any application. 

 
This article was contributed by Nathan Wells, application engineer, Engineered Polymer Systems Division.  

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You just spent 6 months testing, stretching, aging and exposing your new seal design to 12 different chemicals. Finally, you are done, so what does a good technical drawing for a seal include? For most companies, the drawing is simple. For an O-ring, we draw a generic circle and show an ID and width with some sort of material call out. But now fast forward 20 years, someone consults the drawing, how do they know the criteria you used to select the seal specified?

Just last week I asked my customer who was having seal failure issues on their engine sensor, “Was the original seal specified to be compatible with biodiesel?” The engineer consulted their drawing, but besides the generic circle it lacked any background on what material compatibility was considered when the seal material was selected. The ASTM description on the drawing did not include a reference to or indicate compatibility with biodiesel.

Over time, the operating parameters of a system or product can change so it is important to know what parameters were used for the original seal selection. The goal of the drawing is to assure that the engineers and procurement team understand what performance is required from the seal and why the specific elastomer was chosen.

So how do you make your drawing more valuable to your company?

1. Define and list on your drawing all the operating conditions you anticipate the seal will see such as temperature, pressure and any other application specific operating conditions.

2. Prepare a list of fluids as well as the concentrations of each fluid that your seal will be exposed to and again add these on your drawing. In addition, make sure you consider fluids that could come into contact with the seal indirectly, through failure of other systems that are part of the product or even by cleaning the product.

3. List on the drawing the selected compound and manufacturer. Define clearly what testing the compound was put through or what testing is required for approval.

4. If you select a compound that was resistant to compression set, high temperatures or low temperatures as well as explosive decompression, this should be clearly stated on the drawing.

5. List clearly the industry standards the seal is required to meet, such as UL, FDA or NSF.

Time and time again, I see seal quality and performance failures when a new supplier is selected and the real requirements for the seal were either forgotten or not clearly defined. Clearly defining these parameters and making them transparent will allow your purchasing and technical team to understand, select and evaluate the correct compound that meets your products sealing requirements.

Once you select a compound for your specific application, it is important to test and validate that the compound chosen is compatible with the fluids you are using. Parker can typically supply small compound samples for soak testing in the fluids your seal is exposed to. If you choose to list an alternate compound on your drawing, that compound must also be tested and validated for compatibility.

Parker offers design assistance for all of our sealing products so before you even design the seal, define the space or groove the seal fits into. Call us, the earlier in the design process the better. Parker will assist you with selecting the proper seal, defining the elastomer requirements, and designing the mating groove; we can provide a cost-effective solution whether it is off the shelf or a custom manufactured solution

Remember when developing drawings standards, assure yourself that if someone consults a drawing that is 2 years old, 5 years old or even 20 years old, they will know what the original seal design intent was.

Fred Fisher, technical sales engineer, Parker Hannifin Engineered Materials Group

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