Seal Strip Profiles: Flexible and Rigid in One Extrusion

Two materials, one die

A seal strip is a co-extruded profile with a rigid carrier and a flexible sealing leg. The carrier is usually rigid PVC or a glass-filled PP, and its job is to hold the strip in a groove or against a frame. The leg is a thermoplastic elastomer or a flexible PVC, and its job is to deflect and seal.

The two materials leave the die together and bond while they are still molten, so the joint is a weld rather than an adhesive bond. That is what makes the profile possible: a strip that was assembled from two parts would come apart under repeated compression, and one that was made from a single material could not be both stiff and soft.

The die has two flow channels that meet at the exit, and the interface position is controlled by the relative melt viscosities and the channel geometry. If the viscosities are too far apart, the interface wanders, and the strip has a soft patch where the carrier should be or a hard spot in the sealing leg.

Sealing force comes from the soft leg

The sealing force is what the leg pushes back with when it is compressed, and it is set by the hardness and the geometry. Measured as force per 100 mm of strip at increasing deflection, a 55 Shore A leg gives 3 N at 10 percent deflection, 7 at 20, 12 at 30, 19 at 40 and 28 at 50.

The same geometry in 70 Shore A gives 5, 11, 19, 30 and 44 N, and in 85 Shore A 8, 17, 29, 45 and 65. The curves are not proportional: the harder compounds stiffen faster as the deflection rises, so the difference between 55 and 85 Shore A is a factor of two at 10 percent and a factor of 2.3 at 50.

The design question is how much force the seal needs. Enough to close the gap against the service pressure and to take up the tolerance in the frame, and not so much that the frame distorts or the user cannot close the door. A door seal on a domestic appliance typically wants 10 to 20 N per 100 mm at its working deflection; a weather seal on a window wants 5 to 10.

Table 1 — Compression force per 100 mm of strip, by hardness and deflection.

Retention comes from the hard leg

The carrier has a different job: it has to stay in its groove under the pull of the seal, under thermal cycling and under the installation force. That is a stiffness and a geometry problem, not a hardness one, and it is solved with barbs, a dovetail and a wall thickness that resists collapse.

Pull-out force rises with insertion depth and with the number of barbs. A single-barb carrier inserted 8 mm holds about 70 N per 100 mm; at 12 mm it holds 160 N, and at 16 mm 220 N. Two barbs roughly double those figures at the same depth, because each one has to shear through the groove wall separately.

The carrier also has to resist compression set in its own right. A rigid PVC carrier that is squeezed into an undersized groove will creep and lose its grip over months, and the strip will work its way out. That failure looks like a bonding failure but is actually a carrier stiffness problem.

Compression set is what ends the seal

A seal fails when the leg stops pushing back, and that happens through compression set rather than through wear. Set is the permanent deformation left after the load is removed, measured after 22 hours at temperature, and it rises steeply with temperature: 12 percent at 23 C, 18 at 50, 26 at 70, 38 at 90 and 52 at 110.

A 52 percent set means the leg has lost half its recovery, so the sealing force at the working deflection is roughly halved. That is the point at which a seal that passed its initial leak test starts to weep. The temperature that matters is not the ambient but the temperature at the seal, which on a door frame in sun or next to a heater can be 30 C above ambient.

Material choice sets the curve. A standard TPE will hold a seal to about 80 C; a cross-linked TPE or a silicone leg holds to 120 C or beyond, at a higher cost. The specification should quote the set at the service temperature, not at room temperature, because a compound that passes at 23 C can fail at 70.

Bonding the two materials

The bond between a rigid PVC carrier and a flexible PVC leg is a weld between compatible materials, and it is strong. The bond between a rigid PP carrier and a TPE leg is between incompatible materials, and it needs a tie layer or a compatibiliser in the TPE.

The test is a peel test on a cut section, and the failure mode matters more than the number. A cohesive failure inside the soft leg means the bond is stronger than the material. An adhesive failure at the interface means the bond is the weak point and will fail in service, whatever the peel number says.

Interface position is the other variable. If the two melts meet too far inside the die, the interface is diffuse and the carrier loses stiffness; too far outside and the bond is thin. The position is set by the die geometry and by the melt pump rates, and it has to be checked on a section rather than assumed from the settings.

Table 2 — Material combinations and how the bond is achieved.

What to specify

A seal strip is specified by what it has to do, and the drawing should carry the numbers that decide whether it does it.

• Sealing force at the working deflection, in newtons per 100 mm, with a tolerance.
• Compression set after 22 hours at the service temperature, quoted as recovery.
• Pull-out force from the groove, and the groove dimensions it is designed for.
• Hardness of the sealing leg and of the carrier, separately.
• Bond strength and the required failure mode, cohesive rather than adhesive.
• Colour and UV stability if the strip is exposed.

The sealing force and the compression set are the two that decide whether the product leaks, and both are missing from most seal strip drawings. A strip quoted only by hardness and colour is a strip that will be supplied to whatever the extruder finds convenient.