Silicone foam gaskets and cushioning pads are readily die or water-jet cut.
Silicone foam gaskets and cushioning pads are readily die or water-jet cut.
 
Molded synthetic rubber, such as silicone, can be molded with undercuts and other design features without draft angles and ejector pins.
Molded synthetic rubber, such as silicone, can be molded with undercuts and other design features without draft angles and ejector pins.

Steve Hughes
Applications Engineer
Stockwell Rubber Co. Inc.
Philadelphia, Pa.

Despite advances in CAD and FEA software, it is still difficult getting molded housings and hardware to fit as planned, especially in handheld electronic devices. It's also not uncommon for performance requirements to evolve during the design. A changing market may dictate whether the device will need waterproofing, survive a more demanding drop test, or operate at higher temperatures. This may leave designers no option but to specify gaskets and mechanical cushioning pads towards the end of the project.

Gasket design

A good rule of thumb is to first identify what the gasket needs to seal out and the environment it will encounter. IP codes from the International Electrotechnical Commission (IEC) are useful references. These codes help define typical seal requirements for portable electronics that can range from keeping out dust to sealing from moisture during temporary water immersion. The temperature range and the extent of outdoor exposure are also key factors that must be defined.

As portable electronics evolve, they continue to decrease in size. With the smaller size of housings designers are often forced to get an effective seal with less compressive force. As in any design, specifying quality materials is important. Generic gasket materials tend to have a wide range of performance properties. Specifying these, may result in a gasket that doesn't perform as well as expected in long-term situations. Similarly if a designer specifies the material too broadly, performance may be less than desirable. For example, spelling out just "closed-cell neoprene" or "closed-cell sponge rubber" on a gasket drawing may bring disappointing results.

Choosing the right material for various environmental conditions can be tricky. Material manufacturers or soft goods fabricators can give advice on ways of customizing seals and gaskets to meet specific needs. For example, they might suggest compounds such as solid silicone that are readily available in sheet or molded gasket configurations in durometers as low as 10 Shore A. There are even softer elastomerics available such as silicone foams and ultrasoft Poron microcellular urethanes that readily deflect with minimal closure force.

Molded parts

For rugged handheld devices, gaskets of solid rubber may be best. Answering a few simple questions will refine the material selection process and help avoid costly retooling:

  • What type of environment does the device need to withstand?
  • What fluids must be sealed out?
  • Will the configuration require molding because of its profile or complexity?
  • Is the cross section rectangular to make possible die or water-jet cutting?
  • Does the enclosure design need a certain durometer range or color?
  • What is the required time frame for prototypes and initial production quantities?

The answers to these questions will help fabricators pick the best method for production, provide the lowest unit and tooling costs, and give an idea of lead times.

Material versus production-cycle-time is another important metric. A product proven to be quite versatile for gaskets and seals is liquid silicone rubber (LSR). LSRs are resilient over a wide temperature range (-80 through 450°F, are chemically inert, have a wide range of durometers (10 to 70 Shore A), and resist degradation from sunlight and ozone. LSRs are, however, often thought of as an expensive elastomer compared to neoprene, EPDM, and other organic rubbers. But they cure more quickly, reducing cycle times. In many cases, the reduced cycle times of LSRs make them the least costly alternative for small enclosure gaskets.

There are three types of rubber molds: compression, transfer, and injection. Injection molds provide the lowest unit part cost because of rapid curing. Injection molding of synthetic rubbers differs from the more familiar process where hot plastic injects and cures in a water-cooled mold. In contrast, rubber at room temperature injects into a mold that is hot. Molding tools for rubber are not as complex as those for thermoplastics and also cost less. Additionally, because rubber is resilient when fully cured, the injection molds don't need draft angles. And rubber parts with deep undercuts can be demolded without ejector pins.

Rapid curing also keeps tooling economical and shrinks lead time because production can get by with fewer mold cavities. For example, a one or two-cavity LSR injection mold with an O-ring cross section or rectangular cross section can be tooled up in one to two weeks. The tool will support production of 1,000 to 2,000 gaskets within the following week. This fast turnaround can get a wayward project on track quickly when gaskets resolve a minor misalignment.

Rapid prototypes

Water-jet and laser cutting systems can quickly turn out prototype gaskets and seals. They also provide better accuracy and more design freedom than conventional steel-rule cutting dies. These processes also provide cut-to-size samples for immediate testing. In many cases they can give a one-day turnaround if the pattern is on a CAD file.

Water-jet cutting provides a clean-cut edge on thick rubber. Nonabrasive water-jet cutting machines can cut 2-in.-thick sponge rubber and solid rubber in excess of 1-in.-thick to tolerances within ±0.010 in. In many cases water-jet cut prototypes may serve as proof-of-design before building steel rule dies or molds.

Laser cutting, in contrast, is used to pattern films, foils, and thin elastomers. A down side to laser cutting is that it tends to char the edges of elastomeric materials such as flame-retardant silicone foams.

Gaskets or cushioning pads typically are either die or water-jet cut depending on the material and the degree of dimensional precision. Of the two, water jet typically holds the tightest tolerances. These processes are most feasible for components with rectangular profiles and uniform thickness. Parts with nonuniform cross sections or rounded edges will most likely need to be molded. It also may be possible to get prototypes from custom-molded sheets if standard sheet stock is unavailable or if the job needs an odd thickness or unique compound.

Soft materials and different manufacturing processes give rubber parts different tolerances than on metal parts. The RMA (Rubber Manufacturer's Association) has published tolerance tables that can serve as a general guide.

Cushioning pads

Cushioning pads often end up being one of the last components specified on a job. Cellular urethanes or silicone foams such as Poron from Rogers Corp., Rogers, Conn., are often materials of choice because they are more cost effective and may be readily fabricated into any size and shape. These materials exhibit good compression set and load retention, important for the tough mechanical cushioning needs of portable electronics.

Acrylics comprise the vast majority of pressure sensitive adhesives supplied on gaskets and cushioning pads. Acrylic adhesives generally withstand from -40 to 250°F. Some are formulated for slightly higher operating temperatures and can adhere to low-surface-energy surfaces such as powder coating or certain plastics. Pressure-sensitive adhesives are generally available in thicknesses from 0.002 to 0.005 in. and come on a treated paper or plastic release liner.

Mylar or PET film-supported adhesives help ease assembly of fine-detail gaskets. The films boost dimensional stability during removal of the release liner. On gaskets that see shear forces during installation, there may be a separation of the adhesive and film carrier over time. A high quality transfer film may be a better choice in the presence of long-term shear forces.

Make contact:


Rogers Corp., (860)774-9605, www.rogerscorporation.com
Stockwell Rubber Co. Inc., (800) 523-0123, www.stockwellrubber.com



General gasket guide for portable electronics

ConditionsIndoor, 32 to 150°FOutdoor, -40 to 185°FReference
Dusttight/
No water
contact
Die-cut microcellular urethane,
high-grade closed-cell
neoprene or EPDM sponge
Die-cut silicone foam, closed-cell
silicone sponge, or high-grade
closed-cell neoprene or EPDM sponge
IP60
Dusttight/
Water spray up
to 60° angle
Die-cut microcellular urethane,
high-grade closed-cell
neoprene or EPDM sponge
Die-cut silicone foam, closed-cell
silicone sponge, or high-grade
closed-cell neoprene or EPDM sponge
IP63
Dusttight/
Water and
splashproof
Die-cut closed-cell
silicone sponge
Die-cut closed-cell
silicone sponge or
molded solid-silicone rubber
IP64
Dusttight/
Temporary
water immersion
Die-cut or molded
low-durometer
solid-silicone rubber
Die-cut or molded
low-durometer
solid-silicone rubber
IP67
Die cut also includes water-jet cutting.



Property comparisons of cellular gasket materials

MaterialGradeDeflection
force, psi
Compression
set (73°F)
Compression
set (158°F)
Suggested
temperature (°F)
Relative load
retention
Comments
CELLULAR URETHANES
4701-30-20125-04Very soft3 to 8<2<100 to 194ExcellentMicrocellular, low outgassing
4701-40-20125-04Soft/medium7 to 13<2<100 to 194ExcellentMicrocellular, low outgassing
4701-50-20125-04Firm13 to 23<2<100 to 194ExcellentMicrocellular, low outgassing
CLOSED-CELL SPONGE
R411-N neopreneSoft2 to 5<25N/A-40 to 200FairCuring agents may outgas
R431-N neopreneMedium9 to 13<25N/A-40 to 200FairCuring agents may outgas
R495-T EPDMSoft2 to 5<25N/A-40 to 200FairCuring agents may outgas
V-740 VinylSoft2 to 5<10N/A-15 to 160PoorPlasticizer migration
CELLULAR SILICONES
HT870Soft2 to 7<5<5-67 to 392GoodOpen cell, UL94VO flame rating
HT800Medium6 to 14<5<5-67 to 392ExcellentClosed cell, UL94VO flame rating
R10470-MMedium6 to 14<5<10-100 to 500GoodClosed cell, mechanically durable
Deflection force test per ASTM D1056, 73°F
Compression set test per ASTM D1056, 50% for 22 hr



Property comparisons of solid elastomers

 Durometer range
(Shore A)
Tensile
strength (psi)
Compression
set (% @ 158°F)
Temperature
range (°F)
Comments
Neoprene30 to 80600 to 1,80015 to 25-40 to 275May be required
for oily conditions
EPDM40 to 80800 to 1,60015 to 30-40 to 325For NBC (nuclear, biological,
and chemical contamination)
Gum-based silicone20 to 80500 to 1,2005 to 10-100 to 500Traditional silicone
compound
Liquid silicone rubber10 to 70800 to 1,2005 to 20-80 to 450Faster cure cycles,
lower unit cost



Dimensional tolerances of die and water-jet cut sponge components

 Tolerance, ±
Drawing designationWater jet*Die cut*RMA BL1**RMA BL2**
For thickness up to 0.125 in.
Under 0.5 in.0.010.0150.0250.032
0.5 to 0.99 in.0.0150.020.0250.032
1 to 6.3 in.0.150.0250.0320.04
over 6.3 in. multiply by0.00250.0040.00630.01
For thickness over 1.25 to 0.25 in.
Under 0.5 in.0.010.020.0250.032
0.5 to 0.99 in.0.0150.0250.0250.032
1 to 6.3 in.0.0150.0320.0320.04
over 6.3 in. multiply by0.00250.0050.00630.01
For thickness over 0.25 to 0.5 in.
Under 0.5 in.0.010.020.0320.04
0.5 to 0.99 in.0.0150.0250.0320.04
1 to 6.3 in.0.0150.0320.040.05
over 6.3 in. multiply by0.00250.0050.00630.01
For thickness over 0.5 in.
Under 0.5 in.0.150.0320.040.05
0.5 to 0.99 in.0.0150.040.040.05
1 to 6.3 in.0.020.050.050.063
over 6.3 in. multiply by0.0030.00630.00630.01
*Stockwell Rubber Co.'s capability for Water-jet and die-cutting adhesive-backed, cellular elastomers.
**From the Rubber Manufacturers Association Table 35, for designing components from cellular elastomers.