Sealing and Shielding

Most thermal pads, also known as thermally conductive gap filler pads, thermal gap pads, or thermal gap filler pads, have many different layer materials or carrier substrate options to choose from. It can be confusing which layer is supposed to stay on the product and which layer gets peeled off and removed before application. In fact, it’s one of our customer’s most asked about questions and can cause a lot of confusion on the manufacturing floor.

So, which layer should you peel off and which should stay on the thermal gap pad? Read on to find out.

Parker Chomerics, like many thermal gap pad vendors, offers several different gap pad layer options that must be peeled away before the gap pad is installed into the application. 

Think of a thermal gap pad as a sandwich of layers -- there is always a blue poly backing that keeps the gap pad together, but there are five additional carrier substrate options which provide the following benefits:

The woven fiberglass carrier option provides reinforcement and a clean break / low tack interface surface, allowing for re-use of the thermal pad if necessary or for prototyping.

As you can see from the diagram, you peel off the liner to expose the woven glass carrier which does not get removed from the thermal gap pad.

Example: THERM-A-GAP HCS10G.

The aluminum foil with PSA carrier’s primary function is to allow a pressure sensitive adhesive on the thermal gap pad to affix the thermal pad in place.

As you can see from the diagram, you peel off the liner to expose the aluminum foil carrier which does not get removed from the thermal gap pad.

Example: THERM-A-GAP A579.

The polyethylenenapthalate (PEN) film carrier permits the thermal gap pad to see a shearing motion and offers a clear, cost-effective dielectric film with fair thermal performance.

As you can see from the image at right, there is no clear film to peel off that exposes the PEN film carrier, which does not get removed from the gap pad.

Example: THERM-A-GAP 579PN.

The thermally enhanced polyimide carrier permits the thermal gap pad to see a shearing motion and offers an excellent dielectric film with enhanced thermal performance. 

As you can see from the image at right, there is no clear film to peel off, the polyimide carrier does not get removed from the gap pad.

Example: THERM-A-GAP 579KT.

The no carrier or “un-reinforced” option allows the thermal gap pad to have high tack surfaces on both sides, allowing for the pad to be highly conformable, but it does make cutting and handling of the product more difficult.

Once the liner is peeled back, there is no additional carrier on the thermal gap pad, the pad is now exposed.

Example: THERM-A-GAP 579.

Lastly, the base carrier liner, shown in blue, is persistent on the bottom of all thermal gap pad options, and must be peeled and removed prior to installation of the thermal gap pad.

This blue carrier is necessary, as it keeps the gap pad intact and more easily to handle prior to installation. We recommend keeping this blue poly carrier layer on just until the gap pad is placed for the final time.

This blog was contributed by Jarrod Cohen, marketing communications manager, Parker Chomerics Division.

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I have had many discussions with customers as to the value of using an ASTM elastomer compound description on their prints to define a specific application or elastomer requirement versus listing an approved Parker compound number.

Specifying a compound using the ASTM callout is a good start - it clearly defines what you want, it sets a minimum bench mark and it is easy for competitive vendors to understand what you are asking for. The ASTM standards also set specific test parameters which make it easier to do an "apples to apples" comparison between two compounds. However, over time here is what my customers have learned:

1) The ASTM standards are very general; so when my customer defined a specific FKM they needed using an ASTM callout, they received a compliant material that just barely met the ASTM specifications but did not meet their actual operating requirements. The supplier provided my customer with their lowest cost material. The quality of the material was poor and inconsistent, but it met the ASTM criteria they requested. This customer saw a 15% increase in assemblies requiring rework plus the number of warranty claims rose due to seal failures. The twenty cents per seal my customer saved for their $48.00 application was offset by the cost of increased product failures which also resulted in unhappy customers.

2) The ASTM standard does not specifically list what actual chemicals the seal has to be compatible with as well as the operating conditions. ASTM tests compatibility based on Standardized Testing Fluids which are Oils, Fuels and Service Liquids. ASTM uses standard oils which are defined by IRM 901 and 903. Again, the ASTM standards are excellent for comparing compounds, but most people do not have their seals operating in the ASTM reference oils and many sealing applications are exposed to multiple fluids.

3) Most of the engineers or purchasing people who reviewed or utilized an older drawing had no idea why the original engineer chose the compound or why they used the ASTM callout  specified. I typically find that most companies do not know exactly what the ASTM standard  is calling out.

So what is the best way to define and specify an elastomer? Most companies go through a technical process to specify, test and confirm that an elastomer is the correct choice for their application. All of the elastomers that were tested and approved for the application should be clearly listed on the drawing. In addition, the drawing should clearly state that  the approved materials listed were tested to confirm their suitability for the application. All substitutes or new elastomers must be tested and approved by engineering prior to use.

If you have questions regarding the suitability of an elastomer for your application,consult and work with your Parker Applications Engineer. We offer a plethora of compounds to suit your application needs. Ask our applications engineers and chemists for guidance; their vast seal design experience spans multiple industries and applications to solve your sealing challenges. 

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

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Seals made of the fluoropolymer PTFE are used where many other sealing materials (such as rubber elastomers, polyurethanes, fabric-reinforced elastomer seals, etc.) reach their limits in terms of requirements such as temperature range, chemical, friction and wear resistance. That is why PTFE is the most frequently used fluoropolymer in challenging sealing applications. Parker Prädifa produces seals made from pure PTFE and numerous modified compounds with diameters of up to 4.5 meters using economical machining techniques.

Polymer materials like PTFE, PEEK, TPU and selected elastomers are suitable for machining such as turning or milling. This makes it possible to economically manufacture both larger and smaller volumes because no additional tooling costs for molds are incurred.
 

Parker Prädifa has been producing complex machined polymer seals with diameters of up to 3 meters for decades. In the light of a growing demand for increasingly large seals Parker Prädifa has continually developed the manufacturing technology of machining further and is now able to offer diameters of up to 4.5 meters at the highest level of quality. The production of even larger diameters is currently in the pipeline.

The production of large seals for challenging applications is not simply a matter of scaling up know-how of traditional seal design and machining. The reason is that XXL sizes not only pose particular handling challenges in the manufacturing process, but do so even earlier, in the design and testing stages.

The evaluation of the performance of large-scale seals under various load and temperature conditions requires sophisticated simulation models. Particularly critical factors to be considered in the design of large seals include thermal shrinkage and expansion. In addition, even relatively low pressures may result in extreme forces acting on the seals, leading to considerable deformations or even seal failure.

As damage caused by seal failure and leakage may be particularly severe in the case of large seals, reliable sealing functionality must be comprehensively validated prior to their utilization in the respective application. Parker Prädifa uses virtual prototyping for validation. Due to the advanced method of virtualization utilizing sophisticated FEA models costly tests with real-world parts can be avoided and development cycles significantly reduced.

Parker Prädifa ensures top quality of XXL sealing solutions using quality assurance technologies developed in-house. Picture: X-ray inpection of large-diameter seals.
 

Case Studies

• Renewable Energy – Sealing Solutions for Bearings in Wind Turbines
• Oil and Gas – Environmental Seals in FPSO Swivel Stacks
• Medical Device Technology – Seals for Magnetic Resonance Imaging (MRI) Systems
   

More information:

• Website: XXL Size Seals and Molded Parts
• Download Whitepaper: Large-Diameter Seals and Moldings - Material and Special Manufacturing Aspects
• Download Brochure: XXL- Size Seals and Molded Parts - Powerful Solutions for Large-Scale Applications

Article contributed by
Karel Kenis, business development manager PTFE
Engineered Materials Group Europe, Prädifa Technology Division

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Combination electromagnetic interference (EMI) shielding and weather gaskets, more commonly known as EMI shielded combo strip gaskets, are an excellent choice for a variety of applications that require a resilient, highly conductive sealing solution of knitted wire mesh with the integration of an elastomer for weather sealing. Typical applications include electronics cabinet doors, telecommunication trailers, wing panel gaskets for the protection against lightning strikes, and EMP specified requirements and sealing of shipboard and EMI.

There are five major features to consider for EMI shielded combo strip gaskets: the elastomers available, the metals available, the various mesh knit densities available, the various profile geometries available and the option of an overmolded gasket.

Elastomers are available in a silicone sponge or solid, or neoprene in sponge form to meet customer needs such as closure force, fluid resistance and NASA outgassing requirements. Elastomers allows for an increase in gasket life and reduces the overall ownership cost.  Three specific design parameters are the most important variables to take into consideration when evaluating elastomer choices. These criteria are fluid exposure, temperature requirements and necessary compression characteristics of the material. Generally, solid elastomers are used in conjuncture with cast or machined surfaces due to their larger force requirements for deflection. Sponge offerings have less force requirements for deflection and are therefore typically used in conjuncture with sheet metal enclosures.

A broad range of metal alloys are offered to meet the requirements of electrical and galvanic corrosion. This also makes it possible for customers to meet budgetary needs by using Monel (Ni/Cu alloy), Ferrex (SnCuFe), aluminum, or stainless steel.

Various mesh knit layers are available to meet with the required electrical performance. Military applications will require multiple layers to ensure maximum protection while in less extreme applications, less layers are needed. These variations reduce gasket replacement schedules and improve their durability, allowing them to be handled during in-field installation. Most critical of these criteria include galvanic compatibility, electrical
performance, overall gasket durability and temperature range requirements. 

There are round, square, or rectangle profile geometries available that allow for design leniency for application in specific performances.  Which geometry you'd choose depends on the criteria necessary to the application, including, but not limited to, gasket deflection percentage, necessary compression characteristics of the material, application load available for gasket deflection and planned gasket affixation method.

These combo strip gaskets are available in both bonded or overmolded version for tiered performance options. If needed, overmolded gasket can be used but only for wing panel applications.

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

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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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