All right, welcome, everyone. This is the introduction to Electrically Conductive Form-in-Place Gaskets from Parker Chomerics. My name is Jarrod Cohen. I'm the marketing communications manager for the division. I see folks are still joining, so I'll give you guys a few more seconds to join up and then we'll get started shortly. All right, see, folks have joined us, so we'll go over just a few housekeeping details before we begin. Please make sure you've set yourself on mute if you're not already and after we run through the slides, we will have time for a live question and answer session at the end.
So if you have any questions during the presentation, please feel free to type in that Q&A box down at the bottom ribbon there you'll see the Q&A button. Go ahead, submit your questions and our experts will get to them at the end. Then finally, don't worry, if you maybe missed part of this presentation or you'd like to see it again, this will be recorded and sent to you shortly after the call.
Hi, everyone. Thank you for joining. My name is Sierra. I am a mechanical engineer and I've been doing sales with Parker for over ten years.
Hi, everyone, my name is Ben Nudelman, market development engineer for Chomerics, and I've been with the team for about three and a half years. Hello, everyone. This is Brian Flaherty. I'm the specialty materials product line manager.
I've been with Parker for over 20 plus years, developing testing and selling a lot of these connective materials that we're going to be talking about today.
We'll start today with a brief introduction about who Parker Chomerics is, and then we'll dive right into the basics of Form-in-Place EMI shielding gaskets. We'll discuss the design considerations and material selections you might consider, as well as how to work with and dispense these materials. Then we'll discuss testing and validation of Form-in-Place EMI shielding gaskets. And finally, we'll end with a few customer case studies and the all important questions and answers at the end of this webinar.
All right, let's jump right in. And don't forget that at any time you can submit your questions using the Q&A feature at the bottom center of your screen, because we will be answering questions live at the end.
We want to start by sharing some of the common properties of Form-in-Place gaskets and their uses. Form-in-place gaskets, also referred as FIP gaskets, consist of a liquid silicone that's filled with electrically conductive particles and is robotically dispensed. They cannot be dispensed by hand and are controlled very accurately using a preprogramed machine path.
Specifically, form-in-place gaskets are used on very narrow walls or ledges in very complex geometries and meant to be a small form factor solution. So this makes them ideal for densely populated electronics boards. And as a matter of fact, they were developed for inner compartmental, isolation or cavity to cavity isolation, where you need to separate signal processing or signal generating functions from interacting with each other.
The last benefit of Form-in-Place gasketing is that they limit the assembly process and provide a ready to install housing and gasket companies. Let's talk about the three main components of an electrically conductive form-in-place gasket. The first component is a base polymer, which is most often a silicone or fluorosilicone. As with many other electrically conductive materials. The key conductive properties come from metallic powder fillers. These are many of the same fillers that are used in sealants, adhesives and other elastomer gaskets. Finally form-in-place gaskets require either the addition of heat or moisture to cure the gasket once it has been dispensed onto a housing.
There are a number of benefits that come from the unique features of form-in-place gaskets. When we mention small form factor, we refer to the fact that these can be dispensed in smaller bead size than most traditional EMI gasket solutions, going down as small as 16 thousandths tall by 20 thousands wide.
They have excellent adhesive properties on a number of substrates, including machined metals, cast housings and electrically conductive plastics. They also provide high shielding effectiveness that can give more than one hundred decibels shielding effectiveness in the two hundred megahertz to 12 gigahertz frequency range.
Because form-in-place EMI gaskets are robotically dispensed, a standard CAD file can be used to program the defense system and quickly map out the dispensing pattern, also leading to high reproducibility and consistency. So the positional tolerance of the gasket can be held within one thousandths and is able to follow very complex geometries, including sharp turns, corners and serpentine patterns.
Finally, FIP gaskets can meet at "T" joints, something that is very difficult for traditional gaskets such as molded or extruded elastomers or metal products. The robotic dispensing system produces reliable junctions between bead paths that provide continuous EMI or EMC shielding and environmental sealing. This is an example of a robotic gantry dispensing a form-in-place material taken by one of our dispense partners. It shows the dispensing of a silver plated aluminum filled with silicone form-in-place EMI gasket. Known as CHOFORM fifty five seventy five, it offers excellent corrosion resistance on aluminum substrates. As you can see, the material is easily and accurately dispensed.
While we don't have specific dispense partners, we have an excellent relationships with a number of dispense equipment manufacturers and we work closely with them to optimize the use of our material with their machines.
Now we'll dig down deeper into the factors to consider for your design. As a reminder, we'll be answering questions at the end of the webinar, so please feel free to submit them throughout the presentation.
So there are many different physical property designs to consider when selecting an electrically conductive form-in-place gasket material and the thing you should consider is how many components a material has.
Yeah, most materials are single component, but some are two materials and often require mixing as well as differentiated dispensing equipment.
Another consideration would be your bead size. Different materials have different types of electrically conductive fillers in different sizes, which affect how small or how large a bead can be. So the smallest form-in-place, as I mentioned before, 16 thousandths by 20 thousandths and the largest are around seventy five thousandths tall by 90 thousandths wide.
Then there's hardness and compression to take into account. Hardness, or the durometer, will tell you how hard the material is when fully cured with most materials in the thirty five to 80 Shore A range. The hardness has an impact on the compression forces required to compress the gasket. There may also be compression set that occurs over time as these gaskets are compressed or cycled through compressions.
And lastly, operating temperature ranges. So where will this electronic device be? Is it going to be indoors? Outdoors? What will be its heat load? Many materials have a max use temperature of around eighty five. We do have one that goes up to one twenty five depending on the cure type.
Here's a brief look into bead sizing and gasket deflection. Remember that this is a liquid material being dispensed out of a round nozzle, so the profile will be that of a D -shape. When it comes to bead sizing, we recommend the ideal height to width ratio to be about eighty five percent. Sierra, when it comes to deflection, why do we recommend a minimum deflection of 20 percent?
Great question. A minimum deflection of 20 percent ensures good electrical contact between the surfaces and the electrically conductive filler has enough bite through to make that contact.
Finally, we suggest a maximum deflection of about 40 percent of the nominal gasket bite, which helps to limit the damage the gasket itself. And not only does this maximum deflection help to limit the damage, but it also helps minimize expulsion of any silicone oils in the material during the curing process.
And just to add, many engineers decide to design in compression features to regulate this deflection and to prevent the over compression. So keep that in mind if you have potential issues with tolerance stack up or you don't trust your assemblers to regulate the torque during installation.
So let's consider the housing or physical part design now. Repeatability is important to make sure that the parts are dispensed properly across batches. You will want to minimize any variation in dimensions as well as tolerances.
Flatness is the same, and keeping parts both flat and parallel will dramatically improve bead consistency. Fortunately, this is something that can be planned for and compensated for using programing tricks and motion program planning. If flatness or planarity are too random, the problems would have to be fixed by the machine shop or die cast before dispensing the parts.
Adhesion will vary from material to material, but one commonality is that primers are generally not needed for form-in-place gaskets. The parts should be cleaned and free of oils, dust and other debris and some surface treatments will help very smooth materials to establish a strong bond.
That's a good point, Ben. These surface treatments also have the added benefit of minimizing corrosion as well.
OK, Sierra, let's discuss two typical housing material types, metallic and plastic.
Metal parts, either machined or die cast, typically have surface finishes such as conversion coatings or nickel plating's that are acceptable for established bead adhesion. Most metallic materials lend themselves well to adhesion, with the exception of some gold plated parts. For thermoplastic parts, there will also be paint or plating or vacuum deposited film onto which the gaskets will be dispensed. So once again, for commonly used materials, this wouldn't cause an adhesion problem.
The exception to that rule is actually the Parker Chomerics premier branded EMI shielding thermoplastic, which actually don't need any kind of secondary finishes or plating or vacuum metallization like Sierra mentioned. Once again, form-in-place gaskets will adhere well to these electrically conductive plastic houses. Be sure to check out our previous webinar on electrically connected plastics available on demand or on our website.
And finally, if problems with adhesion do happen, they can always be resolved with a flame or flame treatment or corona discharge treatment. These gaskets are not meant to be used for high pressure sealing applications, but it can provide a good seal for dirt, dust, light rain and, of course, EMI shielding. So don't design form-in-place gaskets and expect them to pass your one meter underwater submersion test, but do expect it to protect against moisture and solid particulates.
We've done several webinars on electromagnetic material topics and corrosion is a topic that is always discussed. It's very hard to completely prevent corrosion, whether it be galvanic or electrolytic, but you should always be prepared to see some level of corrosion. We want to minimize galvanic potential as well as minimize the electrolyte presence.
So for material selection, the biggest decision point is the environment of intended use. So, for outdoor or uncontrolled environments where the gasket will see moisture, a nickel aluminum or a silver aluminum film material are probably the best choice. If the device will be inside a controlled environment than a nickel graphite or a silver copper material could be considered as well.
And to further down selecting that list, bead size may suggest either silver plated copper or silver due to the ability for that filler to be dispensed in small beads.
Right, those beads might be small, but silver copper material should not be used in an outside environment.
And finally, for low closure force applications, Shore A hardness and lower deflection forces, you would steer towards a nickel plated aluminum or silver plated copper.
So there are two main cure mechanisms of form-in place-gaskets. One is a thermal cure in an oven or the other is a moisture cure.
Thermal cure, as the name implies, requires the parts to pass through an oven, generally around one hundred fifty degrees Celsius and are ideal for metal substrates as some plastics might not be able to withstand the cooler temperature.
For that reason, thermal cure materials generally offer greater adhesion strength than moisture cure materials and can be ready to work within less than an hour after dispensing.
Moisture cure materials follow a twenty-four hour cure schedule and cure as a result of atmospheric moisture. They're ideal for both plastics and metal substrates. While they do not require an oven to cure, they generally offer lower adhesion strength than heat cured materials, as well as lower tensile and elongation values.
Moisture cure materials can have cure cycles that are more prone to changes in environment, especially ambient moisture. So it's a good to note that some materials can be compressed prior to full cure or in a wet state in certain applications. This means that they will actually cure around both housings and are mated together for a better bond and environmental seal but cannot be taken apart after curing.
We wanted to make you aware that there are non-conductive form-in-place gaskets purely used for environmental sealing.
Non-conductive gaskets are dispensed and worked with in much the same way as a conductive gasket. They are used for applications that require environmental sealing and complex geometries. They tend to have a lower durometer, so they're easier to compress and some are fluorosilicone based, which will also provide that chemical resistance. And finally, these gaskets can be used in low volumes where traditional molding gaskets tooling is not cost effective.
All right, and it's important that we give you some instances where this is not an appropriate place to use form-in-place EMI shielding gaskets. You should not consider this design if you need high pressure sealing or repeated opening and closing of the box.
Other examples where FIP gaskets should not be used are extreme environments with extreme temperatures and where the gasket may possibly see harsh chemicals like cleaning solvents or jet fuels. Also where outgassing is a concern. For high environmental sealing, high cycling and chemical resistance, conductive elastomer gaskets are a better alternative, and Parker Chomerics offers dozens of different combinations of material and filler types in a near endless array of profiles and shapes. Now let's talk about how to dispense form-in-place gaskets.
Three axis dispensing machines have the x axis, left or right, y axis towards you or away from you and a Z or Zed axis, which is up from and down to the part. These machines may have all three axis in a gantry over the part to be dispensed or the table that holds the part may move, supplying one or more of the axis movement. These machines come in all sizes, from table tops to large inline machines, and there are many excellent suppliers of this equipment all over the world.
Greater than three axis machines add rotation around the three primary axes- here, we're going to call A, B and C. These are the pitch, yaw and roll to move the parts through space. By adding a rotation around two axes on the part, you actually can't dispense parts at an angle. By adding C, the third axis of rotation, you can actually use an asymmetrical needle tip, allowing for the shape to be added to the gasket cross section or dual dispensing of the conductive non-conductive gaskets.
While a bit more technical, we did want to briefly touch on the technology used in the dispense equipment. So most material is provided in cartridges or syringes and is dispensed using a piston controlled by an electric motor or a plunger that is controlled using compressed air. More complex dispensing equipment uses a two pump or plunger system. The first pump is used to feed the material to a second pump, often called a metering pump, which offers more precise control of the compound flow. This allows more cross-sectional control and greater control for starting and stopping the flow. Gears and screw pumps are common types of metering pumps and are often needed when large containers of material are used instead of smaller syringes.
Needles come in many shapes and sizes and are often made from many different materials and can come from many different suppliers. By choosing the diameter of the needle tip opening or orifice in conjunction with the speed of the dispense head movement and the force of the pump, you can control the gasket bead cross section in just about every dimension. Some needles do not have a round opening, but rather are shaped like a triangle or an oval, which allows for some customization of the gasket cross section.
Here we have the various features of the form-in-place bead on a house. The first is the bead start or where the gasket makes contact with the housing. This is where the bead will first establish a tackiness the surface and the rest of the bead path will follow from this point. The point at which the bead ends is known as the bead stop. This point is where the machine will end the dispensing of a specific path and will usually have a quick back and forth motion to separate the bead from the rest of the material in the dispense tip.
It's important to note that parts may often have many different starts and stops, depending on how complex of a part it is. The point at which to intersect is known as a T-junction, as you can see here on the slide.
And all of these features, there can sometimes be additional tolerances on bead height or location that we can get into more detailed on the next slide. But to combat this, bead's can sometimes be trimmed to make sure there are no points, like the tail, that are preventing an effective environmental seal.
As we mentioned, Sierra and I wanted to touch on a few limitations of dispensing. The dispense needle, the point from which the material is dispensed, needs to avoid all cover features and locating or registration pins and using cover holes.
We call this the avoidance zone, and generally it can be about ten thousandths in from the wall or a Z axis obstruction. There are also initiation gap tolerances and termination gap tolerances to watch out for. The initiation gap is the distance between where a bead starts and an interrupting feature, while the termination zone is the distance between where the bead ends and any bolt hole, housing wall or material stand off. Beyond these limits, gasket design bead path is simple. If you can get there, you can dispense there.
When Z or Zed obstructions appear, you can use a longer needle to work around some features.
But on the other hand, the cost of changing needles or obstructions may lead to slower flow and longer dispensary's adding some cost. A simple housing or cover design will often not cause these issues.
Especially because form-in-place beads are used in small applications with limited room for error, it is important to note the tolerances on the bead. Tolerances can often depend on the quality of the dispense equipment, but at the end of the day, the beads are in a liquid state and cure to a hardened elastomer. They will not be able to hold as tight of a tolerance as machine parts can. Oftentimes, beads of different sizes will have different tolerance ranges, with the larger bead needing a larger tolerance.
In most cases, beads will have a single measurement be the leading dimension and the other dimension be a reference relative to that eighty five percent ratio we mentioned.
We did want to point out that while the bead tolerances may be relatively larger or the size gaskets, the center line positioning is held to a plus or minus one hundred thousandths tolerance because of the robotic dispensing and the repeatability of the CAD file. Much of the cost incurred for form-in-place gasketing is the material and the time to program and run the parts. Most dispense machine programmers will determine the most effective way to run the parts while avoiding constant starts and stops and added time.
Internal cavities and wall segments are often run first, with perimeter beads run at the end of the dispensing process. As you can see in the top image, some housings are more complex and require additional starts, stops and T-junctions. Alternatively, the lower image shows a bead dispensed in a single path with only one start and one stop location, simplifying the process, reducing the time and lowering the cost.
Logistics of these parts need to be considered up front to reduce cost of ownership. Where is your design? Are you designing on the West Coast in California? OK, but are you dispensing in Germany and doing your final assembly in Taiwan? You know, you can understand where I'm going with this. Your goal should be to have the housing machined and dispensed as close in proximity as possible to each other in order to reduce time in transit and also eliminate issues with damage during shipping.
We have our own dispensing equipment across the globe, several authorized distributors in the United States and globally that can perform this dispensing operation. So determine where your machined or die cast housing is made, who is dispensing it and where it's being assembled to save yourself time and money up front.
You make a valid point, Sierra. Be smart about the logistics. Not only can Chomerics dispense form-in-place gasketing, we can be a full supply chain lead and source housing's, complete secondary machining or assembly and provide integrated electrical assemblies ready for installation. No matter which logistical strategy you choose, make sure your housings are packaged up properly to avoid damage in transit. You've seen how boxes can be treated, so always assume worst case scenario and use robust dividers, bubble wrap and anything to protect the precious cargo.
Our last section before application examples is testing and validation of material. Electrical properties such as volume resistivity and shielding effectiveness are measured to confirm that the materials will properly shield the cavities they are meant to isolate. Physical testing, such as heat aging, adhesion and compression versus deflection data, gives insights into planning for the use of form-in-place gaskets into assemblies and their long term reliability. Each property has a different test procedure and often an industry spec associated with it, such as IEEE, ASTM or military specifications.
EMI shielding testing for form-in-place gaskets is very similar to that of cured conductive elastomers. The form-in-place gasket is dispensed onto a twenty four by twenty four inch aluminum test plate and then cured per proper instructions. The test panel is then mounted into a brass frame per the standard IEEE 299 test and is then the shielding effectiveness is measured across the frequency spectrum. In these tests, the beads had a width of one millimeter or 40 thousandths, and the proper deflection was controlled with compression stops.
This is the same test setup that was used to test the gaskets after urban gas exposure where the gaskets were exposed to gases such as hydrogen sulfide, nitrogen dioxide and chlorine gas. Form-in-place gaskets can also be measured for shielding effectiveness after heat aging, which is a thousand hours at one hundred twenty five degrees Celsius, high temperature and high humidity testing, which is eighty five degrees Celsius and eighty five percent relative humidity for a thousand hours, as well as numerous cycles of compression and decompression to verify the gaskets will shield well in real life scenarios.
Form-in-place gaskets are not the most robust because of their size, but their adhesion properties can still be tested for and optimized. Adhesion testing uses a set up and tests the gaskets in shear to make sure that they will withstand handling in assembly, as well as several cycles of opening and closing. Compression versus deflection is another common physical property that is tested for to understand what kind of forces are required to achieve the recommended deflection ranges that we mentioned earlier in the webinar. The same compression force setups are used to test for material through resistance relative to gasket deflection.
This testing confirms that a 20 percent deflection is recommended for proper electricals.
Just as with other EMI gasket solutions, form-in-place gaskets will exhibit some galvanic corrosion in uncontrolled environments. Beads of gaskets are dispensed onto an aluminum test panel and run through harsh environmental testing that encourages galvanic corrosion, specifically salt fog. The panels are then cleaned and weighed to determine how much weight loss occurred as a result of the dissimilar metals between the aluminum and the gasket conductive filler. Many form-in-place gaskets can actually utilize conductive fillers such as nickel plated aluminum or silver plated aluminum to minimize the voltage potential and reduce damage to the parts caused by that galvanic corrosion.
Additionally, using a non-conductive bead of material external to the conductive form plates can protect the electrical remaining surfaces from salt, fog and moisture that is needed for that corrosion to occur.
OK, let's take a look at some examples of how form-in-place gaskets have been used in the past.
One of our first and still most common applications for form-in-place gasketing is on handheld devices. If you ask any of the season veterans at Chomerics, they'll tell you that these materials were basically formulated for cell phones because of crosstalk between the transmit and receive functions and as a solution for sealing complex geometric areas. Chances are the cell phone in your pocket uses some kind of Chomerics form-in-place gasketing inside of it. The programmability and automation of form-in-place gaskets means it's especially well suited for high volume applications.
And although it was developed for telecommunication applications, it has become useful in many other industries.
Moving beyond cell phones and into missile applications, this material shows its diverse range of use. With the complexity of missiles and munitions these days, you have a lot of electrical components causing interference, such as GPS, antenna and radar. Not only would this application see harsh environments, most missile housings have a special coating on the aluminum substrate that need that best shielding combined with the best adhesion properties. Failure is not an option in missiles and the use of precise bead and shielding properties of the CHOFORM 5513 stepped up to the challenge here.
One of our highest volume applications over the last several years was an automotive sensor application that required tough standards such as zero defects, high shielding and low cost. If you think about the advanced features in modern cars such as collision detection, adaptive cruise control, blind spot detection and autonomous driving, each system has an individual control and sensor housing. With Chomerics large material portfolio and integrated assembly capabilities, we were able to mold the plastic housing, spray it with the conductive copper paint, add a dispense thermal interface material and then dispense a form-in-place gasket in two places as you can see here.
We completed the main housing and shipped the box to the customer where they simply inserted their circuit card and in turn ship to their end customer for final installation on the vehicle.
Now, this application was for a lifesaving medical defibrillator, pretty much the same one you would see on the wall at the airport. The small form factor size of the form-in-place gasket was one of the key attributes that sold the customer on this technology. They had very thin walls along their ledge because originally it was a machined metal board shield and they had machined away all their metal to save weight. So it needed to be lightweight and re-workable. Those were their, their requirements.
So we convinced them to convert their metal board shield into a conductive plastic housing. And then we dispensed the form-in-place gasket along the outside perimeter and through a cross member in the center for that cavity to cavity isolation. It was then attached with a few screws to hold it in place because of the re-workability, rather than trying to glue it down permanently with a conductive adhesive or even soldering a can in place. And as we end the presentation, we wanted to highlight a list of Parker Chomerics form-in-place materials. Please note the last two materials are not strictly conductive, but there are various materials to choose from.
So, Ben, I've listened to this webinar and I'm thinking I have an application where form-in-place is going to work well. What are my next steps?
The first step is to run the design by our application engineering department. And once that's blessed, we can arrange some prototypes to be dispensed through your engineering evaluation. Chomerics has capabilities to run high volumes, but we also have smaller machines for low volume runs, as well as several authorized distributors located around the United States, Europe and Asia who can support programs of various sizes.
Well, everyone hope you enjoyed the mini boot camp today. And just before we go into our question and answer portion, as always, if you'd like to send your questions privately by email, feel free to do so. And in addition, we would love to give this presentation or any of our others related to EMI shielding to your engineering team. So just let us know if you're interested. All right, Ben, why don't you start us off?
All right, well, thank you, everyone, for joining us for this webinar, and we will get right into the questions. I did want to introduce another guest that we have answering questions today. Brian Flaherty, who's a member of the Chomerics applications team, will be providing additional insights. So the first question we have comes from Joe. He asks, "What FIP material can one use to create a 16 mill by 20 mill bead?" And Joe, that's a good question.
A number of our materials are actually meant for use in very small bead sizing. The material that we recommend based on your needs would actually be our fifty five thirty eight material, which is a moisture cured material. Additionally, if you're looking for a thermal cured material, the fifty five thirteen can be dispensed in very small bead sizes, anywhere from about eighteen thousandths tall by about 22 thousandths wide.
OK, and this is Sierra, so we had a question from Will, "is FIP always in silicone? Why not urethanes or other chemistries?" So basically, as I mentioned, that this material was designed for cell phones, so silicone was good enough at the time. We actually have been doing some work in the lab to develop other type of binder chemistries, one of those being a floor silicone form-in-place material. We do not have that yet. But if you do have a need for that, reach out and let us know. We'd love to have more reason to work on it faster. Thank you.
The next question we have is for Brian. The question is, "what are the typical environmental conditions needed for moisture cure materials?"
Well, moisture cure systems at Parker Chomerics typically are established to be in the 50 percent relative humidity range. Our literature will document different conditions for cure schedules. Material can be tack-free in a certain amount of time, it can be handled after a longer amount of time and then it can have a full cure after even longer values. Of particular note is the CHOFORM 5538 that Ben just mentioned. That system is designed to be full-cured within four hours, while other systems may require a full day.
Thank you, Brian. OK, we've got another question, "if one joins the mated surfaces while the form-in-place gasket is not fully cured, how is the shielding effectiveness affected?" Brian, maybe you can answer that question.
These materials are designed in mind with a certain minimum amount of compression, typically a 10 percent minimum compression is necessary to eliminate the electrical contact resistance between the gasket material and its interfaces. There are options to consider low, low values, but they're going to be application specific. If you use a wet, CHOFORM application where you dispense the material wet and then assemble them together, those products will also be electrically conductive and you'll still be able to maintain sufficient contact that the electrical contact resistance is still not a factor.
Adding in a little bit of detail to Brian's answer, so, yeah, when you think about the wet dispensing process, one of the things to consider is that when the parts come together, the parts actually we'll see a lot of damage to the gasket if they're taken apart. So wet dispensing is for a very specific application where you won't need additional to reach any additional components and finally, when you have conversion coated parts or parts that have thicker coatings, the wet dispensing may not give you necessarily that bite through all the way when you get that minimum 20 percent compression on a fully cured part.
OK, so we have a question from Mark. "Is it possible to dispense microwave absorber material in a similar fashion as FIP?" And so I'll take this one. Yes, it is. We do not have a current dispensable absorber material. We have it in pad form, but we and also paintable sprayable form, but we're working on a FIP dispensable microwave absorber. So stay tuned.
Another question we have is, "what are the risks for going too low on the compression or too high?" Brian, if you want to talk to that?
Sure, as I alluded to earlier, if you do not get enough squeeze on the gasket, a minimum compression rating, then you may not bite through those oxide layers that Ben was talking about. And that means that your electrical contact resistance is going to be unusually high. And when that's high, you don't shield very well. In addition, if you're worried about weather resistance, a low compression force could also mean a partial leak. On the flip side, if you take the same CHOFORM material and you compress it very high, let's say 60 percent, all that extra pressure is going to possibly cause some of the silicone oils that are present in the material to be driven out of the part. It's kind of like taking a sponge, a damp sponge and squeezing it too hard. So these are general limits of 20 to 40 percent compression. And if you elect to go beyond them, there is some risk and they are application specific, but we can certainly consider them.
Hey, and we've got a question from Ali, "So how large will the bulge of the dispensed gasket be where the profile starts and stops compared to the nominal size of the gasket?" So I believe that is in reference to the tail, which we discussed usually needs to be trimmed off. Brian, do you have an idea of how big that bulb, that tail is? I'm not actually sure.
We talk about that a little bit in the form-in-place selector guide. There is some some documentation in that where we explain that starts and stops both looking down on the part from the Z axis will have a larger bulb and also down in the side view. In figure two and three of our dispensing guide, they talk about the size of that zone and how long it is, but it doesn't actually tell specifically how much larger the start and stops would be. It's not terribly significant, but it's a factor in dispensing the product that we're not going to be able to to get away from.
When you do start, there's going to be a little bit of that bulge on the part. If you'd like we can take that as an action item and follow up with details afterwards. Thank you.
Providing a little bit more details on that. What we say is that typically the gasket height, depending on the size of the gaskets to a certain limit, has a tolerance of plus or minus four thousandths. Larger beads have a high tolerance of about plus or minus six thousandths. At the initiation zone and at the termination zone, there's actually an additional tolerance of that plus or minus six thousandths. But again, with smaller beads, we're typically able to keep a tighter tolerance at where the beads and at where the beads start and where the beads end.
Another question we have is, I guess, "how did the compression versus deflection values of form-in-place gaskets compare to traditional EMI gaskets, either solid or hollow?" In general, they compare pretty closely. Hollow gaskets that are made of a fully cured conductive elastomer will have very low compression forces. But small gaskets that are fully cured relative to a form-in-place gasket will have very similar compression forces.
And with that, I think that was all the questions that people had. There was one private question. To that person, we will be answering that offline. So thank you guys again for joining our webinar. Hope you learned something today.
Last thing we wanted to point out is that when you all receive an email with the recording of the webinar, you'll actually receive a link to the form-in-place selector guide that has information on the different materials, as well as tips and tricks to dispensing and some of the tolerances and key values that we mentioned today. Thank you all very much.