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Molding corners and end caps to EPDM weatherseals. (Tech Service).

Santoprene thermoplastic rubbers 121-65W233 (65 durometer A) and 121-79W233 (79 durometer A) are specially formulated thermoplastic vulcanizates (TPVs) which bond to vulcanized thermoset EPDM (sulfur and peroxide cured) through physical interaction at elevated temperatures.

Typical applications are:

* Hard corner molding for glass-run channels;

* end caps for beltline seals and hood seals; and

* fixation parts for any type of complex weatherseals. Data are available in the literature (ref. 1). Additional heat aging data and adhesion properties at different temperatures are shown in tables 1 and 2.

Part design

Part design of corners and end caps is very important. The highest bond strength will be developed and maintained by following a few guidelines:

* Avoid severe undercuts--demolding of thermoplastic rubber parts is more critical than for thermoset rubber parts;

* always create the largest surface area to yield the strongest bond;

* maximize overlaps on shear-joints in non-visible areas to increase overall bond strength.

Mold design

The extrusion profile must be clamped to stop it from being pushed out of the tool under injection pressures. This can be achieved by adding stopper/grip pins of 2.0-2.5 mm (0.079 to 0.098") diameter. The height of the stopper pins should be 50 to 60% of the profile thickness. If the stopper pins are too high, they can be ground to the required size. The pins should be located 2-3 mm (0.079 to 0.118") away from the EPDM/ 121-65/79 W233 interface. A schematic is shown in figure 1.

[FIGURE 1 OMITTED]

Design efficient cooling channels in the mold to obtain a short cooling time.

Use a well-designed venting system with a maximum channel depth up to 0.038 mm (0.0015"). Since high injection speed is required to achieve high bond strength, adequate venting is critical for this application to reduce gas trapping. Trapped gas can reduce the bond strength at the EPDM and 121-65/79W233 interface. If there are blind areas, such as the bottom of a hole or at the interface, it is suggested to place a flattened ejector pin in that area. Venting of the runner system is also recommended. A core that is thick, sintered/porous metal is effective in eliminating air entrapment. The porous metal should be located at the EPDM/12165/79W233 interface (ref. 2).

Use a runner system that is as short as possible to fill the corner or end cap. This will ensure low flow resistance. Full round runners are recommended since they have the smallest surface-to-volume ratio and most efficient cross-section for pressure transmission and heat retention. If full round runner systems cannot be used, it is recommended to use modified trapezoidal runners, which consist of a round bottom with a 5[degrees] taper on the walls. This type of geometry has been proven to provide more efficient melt flow. Half-round runners are not recommended.

Shrinkage information for the TPVs is provided in table 3. In most cases, shrinkage is minimum for these grades over EPDM. The TPVs will create a fusion bond to the EPDM, which will constrain the rubber so it cannot shrink.

Preparation of extrusion profiles

To achieve good bond strength between the EPDM and the TPV interface, the following guidelines should be followed:

* Use freshly cut extrusion profiles (1-2 hours maximum) prior to molding. Longer periods will increase the chance of contamination and thus minimize bond strength.

* Extrusion cut should be uniform and not ragged. It is recommended to keep the cutting device sharp, so that the top layer of the extrusion does not curve in and provide a fault-prone point.

* Silicone-based mold release is not recommended since silicone inhibits adhesion. Airborne silicone is detrimental to adhesion, and silicone-based slip coating is also not recommended.

* Vertical miter cuts (perpendicular to the extrusion) have been found to provide more consistent parts than straight cut profiles.

Processing

These TPVs are shear dependent materials that can be processed on conventional thermoplastic equipment for injection molding. The molding equipment should have adequate injection pressure/speed and holding pressure/time control combined with accurate temperature control. Good, solid packing of the part will contribute to maximum bond strength. These two grades should be dried at 82[degrees]C (180[degrees]F) for four hours prior to processing.

Recommended melt temperature for the TPVs is between 220-260[degrees]C (430-500[degrees]F). Using these elevated melt temperatures, polymer degradation may occur if the molding machine is left standing idle for longer than 10-15 minutes. A melt temperature over 230[degrees]C (450[degrees]F) may cause drooling from the nozzle. To compensate for a large barrel, use a lower temperature in the rear zone so the material is not subject to high heat settings during its long residence time. The recommended processing conditions for start up are shown in table 4.

An increase in cooling time increases the bond strength between the EPDM and the TPV interface.

Discussion

Processing these TPVS at their highest melt temperature (260[degrees]C [500[degrees]F]) and using a mold temperature of 40[degrees]C (105[degrees]F) will yield a strong bond to the EPDM profile. Cooler temperatures may provide a faster cycle time, but bond strength will be sacrificed.

If bond strength of corners or end caps needs to be increased, the following process modifications can be implemented (these do not apply to profiles less than 5 mm [0.20"] thick):

* If the EPDM profile cross-section thickness is above 5 mm (0.195"), preheat EPDM profile ends in an oven to 80 to 110[degrees]C (175-230[degrees]F), place into the mold at 20-60[degrees]C (70-140[degrees]F) and process immediately. One disadvantage may be that the profile ends are more difficult to insert into the mold. Prewarmed profiles will have expanded dimensions so that insertion may be more difficult and time-consuming

* Place EPDM profile ends into the mold and use an external heat source (quartz spots or heat metal wedges) to only preheat the end parts to 80-110[degrees]C (175-230[degrees]F) and process immediately. This processing system may function well using a rotary table or shuttle press.

* Place EPDM profile ends into a warm mold at 80-110[degrees]C (175-230[degrees]F) and allow 30 seconds to warm up. This method can only be used when corners or end caps are simple, because demolding at 93[degrees]C (200[degrees]F) could easily deform the part.

The preceding recommendations on preheating the EPDM profiles are not ranked as to priority.
Table 1--adhesion properties on compression molded EPDM aging
performance after one week

Test temperature, Hot air
[degrees]C ([degrees]F) 70 (158) 100 (212) 120 (248)

Property

Santoprene rubber 121-65W233
 Modulus 20%, MPa (psi) 0.6 0.8 0.9
 (87) (116) (131)
 Modulus 50%, MPa (psi) 1.1 1.4 1.4
 (160) (203) (203)
 (73) (160) (276)
 Modulus 100%, MPa (psi) 2.1 2.3 2.3
 (305) (334) (334)
 Elongation, % 360 335 470
 Ultimate tensile, MPa (psi) 2.8 2.9 3.4
 (406) (421) (493)
Santoprene rubber 121-79W233
 Modulus 20%, MPa (psi) 1.1 1.5 1.8
 (160) (218) (261)
 Modulus 50%, MPa (psi) 2.1 2.5 2.8
 (305) (363) (406)
 Modulus 100%, MPa (psi) 2.8 3.1 3.3
 (406) (450) (479)
 Elongation, % 150 210 205
 Ultimate tensile, MPa (psi) 3.1 3.6 3.9
 (450) (522) (566)

Test temperature, Cataplasm Fuel B Motor oil
[degrees]C ([degrees]F) 70 (158) 120 (248) 120 (248)

Property

Santoprene rubber 121-65W233
 Modulus 20%, MPa (psi) 0.7 1.0 0.9
 (102) (145) (131)
 Modulus 50%, MPa (psi) 1.3 1.4 1.4
 (189) (203) (203)
 (870)
 Modulus 100%, MPa (psi) 2.2 1.8 1.7
 (319) (261) (247)
 Elongation, % 253 315 250
 Ultimate tensile, MPa (psi) 2.4 2.7 2.3
 (348) (392) (334)
Santoprene rubber 121-79W233
 Modulus 20%, MPa (psi) 0.9 1.5 1.6
 (131) (218) (232)
 Modulus 50%, MPa (psi) 1.9 2.3 2.5
 (267) (334) (363)
 Modulus 100%, MPa (psi) 2.7 2.8 3.0
 (392) (406) (435)
 Elongation, % 115 270 180
 Ultimate tensile, MPa (psi) 2.8 3.6 3.4
 (406) (522) (493)

Notes:

(1.) Testing was performed by overmolding 121-65W233 and 121-79W233
over an EPDM strip. S2 dumbbell-shaped samples were cut from
overmolded plaques (60 mm x 60 mm x 3 mm [2.35" x 2.35" x 0.120"])
and tested per ISO 37. The direction of pull was perpendicular to the
EPDM and 121-65/79W233 interface.

(2.) For Fuel B and motor oil testing, the S2 dumbbell-shaped samples
were exposed per ISO 1817 prior to testing per ISO 37.

Table 2--adhesion properties on compression molded
EPDM at different temperatures

 Test temperature,
 [degrees]C ([degrees]F)

Property 70 (158) 23 (73)

Santoprene rubber 121-65W233 0.3
 Modulus 20%, MPa (psi) (44) 0.6
 0.5 (87)
 Modulus 50%, MPa (psi) (73) 1.1
 (160)
 --
 Modulus 100%, MPa (psi) 1.6
 96 (232)
 Elongation, % 0.6 280
 Ultimate tensile, MPa (psi) (87) 2.4
 (348)
Santoprene rubber 121-79W233 0.4
 Modulus 20%, MPa (psi) (58) 0.7
 0.9 (102)
 Modulus 50%, MPa (psi) (131) 1.4
 1.3 (203)
 Modulus 100%, MPa (psi) (189) 2.3
 153 (334)
 Elongation, % 1.5 170
 Ultimate tensile, MPa (psi) (218) 2.9
 (421)

 Test temperature,
 [degrees]C ([degrees]F)

Property -20 (-4) -40 (-40)

Santoprene rubber 121-65W233
 Modulus 20%, MPa (psi) 1.0 3.7
 (145) (537)
 Modulus 50%, MPa (psi) 1.9 6.0
 (276) (870)

 Modulus 100%, MPa (psi) 4.0 8.2
 (580) (1,189)
 Elongation, % 560 202
 Ultimate tensile, MPa (psi) 8.9 9.1
 (1,291) (1,320)
Santoprene rubber 121-79W233
 Modulus 20%, MPa (psi) 1.0 4.7
 (145) (682)
 Modulus 50%, MPa (psi) 2.3 7.7
 (334) (1,117)
 Modulus 100%, MPa (psi) 4.1 10.3
 (595) (1,494)
 Elongation, % 283 123
 Ultimate tensile, MPa (psi) 6.8 10.5
 (986) (1,523)

Note: Testing was performed by overmolding 121-65W233 and
121-79W233 over an EPDM strip. S2 dumbbell-shaped samples
were cut from overmolded plaques (60 mm x 60 mm x 3 mm [2.35"
x 2.35" x 0.120"]) and tested per ISO 37. The direction of pull was
perpendicular to the EPDM and 121-65/79W233 interface.

Table 3--shrinkage for Santoprene rubber 121-65W233
and 121-79W233

Santoprene Shrinkage Post Shrinkage
rubber grade 24 hrs. 48 hrs.
 23[degrees]C 110[degrees]C
 (73[degrees]F) (230[degrees]F)
121-65W233
 Parallel to flow 1.35% 1.86%
 Perpendicular to flow 1.39% 0.86%

121-79W233
 Parallel to flow 1.45% 1.67%
 Perpendicular to flow 1.38% 1.10%

Notes:

(1.) Shrinkage measurements were taken on a 60 mm x
60 mm x 3 mm (2.35" x 2.35" x 0.120") plaque with a
fan gate.

(2.) Process conditions were fixed according to grade
following our standard recommended conditions as
shown in table 4.

(3.) Test method ISO 2577.

Table 4--recommended processing conditions
for start up

Description Condition

Melt temperature 220 to 260[degrees]C
 (430 to 500[degrees]F)

Injection pressure 4.8 to 6.9 MPa
 (700 to 1,000 psi)
 (as high as possible without
 moving the profile)

Injection time Less than 2 seconds

Holding pressure 2.1 to 4.1 MPa
 300 to 600 psi
 (50% of injection pressure)

Holding time 4 to 10 seconds

Cooling time 25 to 35 seconds
 (wall thickness dependent)

Mold temperature 40 to 55[degrees]C
 (104 to 131[degrees]F)

Drying 82[degrees]C (180[degrees]F)
 3 to 4 hours


References

(1.) Product data sheet 121-65W233 and 121-79W233.

(2.) Venting section of the Injection Molding Guide (AES-31-02).

Shaival Mehta works as a Senior Engineer At Advanced Elastomer Systems. Mehta has been with AES for five years, where his focus areas include design and processing TPVs. Prior to joining AES, he worked for T&M Antennas.
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Author:Mehta, Shaival
Publication:Rubber World
Geographic Code:00WOR
Date:Dec 1, 2002
Words:1986
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