Extrusion of flexible PVC weatherseal compounds begins with dry-blend premixing in a turbo-cooler combination operating at 1,500–3,000 min⁻¹ and discharge temperatures between 105 °C and 125 °C, followed by cooling to 40–50 °C under mechanical agitation. The resulting dry blend typically has a bulk density of 0.55–0.75 g/cm³ and is fed into a counter-rotating conical twin-screw extruder with barrel zones controlled at 145–175 °C. Adapter and die-head metal temperatures are held at 170–185 °C to complete plastication without exceeding the PVC degradation threshold. Vented barrels are maintained at −80 to −95 kPa to remove residual moisture, plasticizer volatiles, and trace hydrogen chloride. The operative thermal boundary is defined by the onset of autocatalytic dehydrochlorination, which accelerates markedly above 190 °C in unstabilized PVC but can be shifted to 195–205 °C in adequately stabilized flexible compounds. The processing window is generally no more than ±5 °C around the melt-temperature setpoint because lower temperatures produce unplasticized resin particles that appear as surface dimples at the die lip, while higher temperatures generate yellowing, edge tearing, porosity, and acid-induced corrosion of downstream calibrators. Viscous dissipation across the metering zone rather than barrel heating often controls the upper boundary; an adiabatic temperature increase of 10–20 °C can be observed between the barrel wall and the melt core at screw speeds above 80 min⁻¹ in a 25:1 L/D single-screw retrofit line. This radial temperature gradient is the reason grade changes from low- to high-molecular-weight PVC require screw-speed reductions of 15–25 % before barrel temperature changes are made. Process technicians monitor pressure at the breaker plate; values above 300–350 bar combined with melt temperature above 180 °C indicate excessive shear heating and a potential loss of stabilizer efficacy. The onset of degradation is accompanied by a detectable increase in volatile acidity measured by Congo red paper, as specified in ISO 182-1, and by a drop in melt viscosity of 10–20 % over 30 min due to molecular chain scission. The practical control limit for flexible PVC weatherseal extrusions with high-gloss surfaces is therefore 170–180 °C at the die entry, with die-head metal temperatures not exceeding 185 °C for continuous runs longer than 8 h.

Barrel temperature uniformity across a counter-rotating conical twin-screw extruder is rarely tighter than ±3 °C after thermal equilibration; when the rear barrel zones drift above 175 °C, the dry blend can bridge in the feed throat, causing feed pulsation and a visible variation in profile wall thickness of 0.10–0.25 mm. The onset of feed bridging is not a material decomposition event but creates a process oscillation that exposes a portion of the melt to extended residence time downstream. Residence-time distribution studies using tracer granules show that the mean residence time in a 90 mm conical twin-screw extruder at 180 kg/h can be 90–180 s, but the tail of the distribution can extend beyond 10 min in stagnant regions around the breaker plate and die adapter. This tail is the critical variable for thermal stability because the stabilizer system must protect the melt against the residence-time tail, not only the mean. When flexible PVC weatherseal formulations are run at 170 °C with a static heat stability of 60 min at 200 °C by ASTM D2115, the apparent safety margin is large; however, accumulated stabilizer consumption in high-shear regions and the residence-time tail can reduce the effective dynamic heat stability to 15–25 min. The resulting boundary is observed as a transition from glossy to low-gloss surface, followed by the formation of microporosity in areas where the profile wall thickness exceeds 2.0 mm. For these thicker sections, screw speed is reduced and die temperatures are lowered by 5 °C to reduce both shear input and thermal degradation in slow-moving melt films.

Heat Stabilizer Failure Modes in Flexible PVC Weatherseal Extrusion

The thermal stability boundary of a flexible PVC weatherseal compound is governed by the interaction between stabilizer chemistry, plasticizer volatility, filler acidity, and melt residence-time distribution. Mixed-metal stabilizers based on calcium/zinc, barium/zinc, or tin mercaptide move the HCl-evolution boundary but do not eliminate it; each system imposes different failure modes at the die lip. In Ca/Zn systems, zinc burning often appears above 175 °C as a pink-to-orange color shift and die plate-out on chrome-plated surfaces, especially when the zinc stearate fraction exceeds 0.5 phr. Ba/Zn systems tolerate slightly higher die-head temperatures, typically up to 180–185 °C, but they are sensitive to sulfide staining in closed calibrators and cannot be used in formulations that will be print-ink coated without corona treatment. Tin mercaptide stabilizers provide the widest dynamic thermal boundary, with die-entry melt temperatures of 185–190 °C before surface degradation becomes detectable, but they generate volatile sulfur compounds that require increased vacuum capacity and can tarnish brass calibration mandrels. The differences are not merely additive; they alter the slope of the viscosity-temperature curve and therefore change the adiabatic melt-temperature rise at constant screw speed.

The values in the preceding table are a compilation of supplier technical bulletin values and production control data for medium-molecular-weight suspension PVC with K-value 65–68; published data for specific resin lot variations is limited, particularly where the base resin ionic content varies by more than 10 ppm. Because the dynamic thermal boundary is a function of the residual thermal stabilizer fraction in high-shear melt films, no single static oven test can certify a compound for continuous extrusion without a companion residence-time study. The failure mode at the die is not always degradative in a chemical sense; some calcium/zinc systems show a reversible plate-out on die surfaces that disappears when the die metal temperature is reduced by 3–5 °C, while others produce irreversible chromium pitting at temperatures above 185 °C after 24 h of continuous operation.

What Sets the Upper Melt Temperature Limit in EPDM Profile Extrusion?

In cold-feed rubber extruders processing EPDM dense profiles, the upper melt temperature limit is set by the interaction between accelerator kinetics, peroxide half-life, and the residence time in the screw channel. EPDM weatherseal compounds are typically extruded on pin-barrel cold-feed machines with screw lengths of L/D 16:1 to 24:1 and pin rows arranged in the plastication zone. Barrel temperatures are maintained at 50–70 °C in the feed section, 70–95 °C in the compression and metering sections, and 90–110 °C at the head and die. The stock temperature measured by needle pyrometer at the screw tip is generally 100–125 °C for sulfur-cured EPDM profiles and 100–120 °C for peroxide-cured profiles. The upper boundary is not the thermal decomposition of the hydrocarbon rubber; EPDM can withstand short excursions to 200 °C in inert conditions, but the cure system begins to generate crosslinks at much lower temperatures. Sulfur-donor systems with dithiocarbamate or thiuram accelerators can develop scorch at 115–125 °C when residence times exceed 5–10 min, while dicumyl peroxide systems have a lower scorch-onset threshold at 105–115 °C because of the half-life of the peroxide and the presence of unsaturated side chains.

The practical processing window for a sulfur-cured EPDM weatherseal is therefore narrow: die-head metal temperature is usually kept at 95–115 °C, and the stock temperature must not exceed 130 °C in any screw-flight location. This is confirmed by Mooney scorch measurements at 130 °C per ASTM D1646, where typical production compounds have a t5 value of 8–15 min. Peroxide-cured EPDM is even more restricted; because the cure reaction is thermally activated and not limited by sulfur-solubility effects, die-head settings above 110 °C produce pre-vulcanized particles that appear as hard lumps in the profile surface. These scorched particles are carried through the die and do not re-disperse, creating a surface defect that is indistinguishable from filler agglomerate unless the profile is tensile-tested after extrusion. The maximum allowable stock temperature for continuous operation is 120 °C for peroxide systems, with the lower boundary of 90 °C set by the need to suppress melt fracture and achieve sufficient green strength for the calibration sleeve. A deviation of ±5 °C is considered critical for profile dimensions because the die swell and green strength change rapidly in this temperature region. Extruder speed is trimmed against a laser micrometer to control the cross-section within ±0.15 mm.

The relationship between die temperature and surface quality in EPDM is nonlinear. Below 90 °C, the surface becomes rough and torn at the die exit due to high elastic energy release; above 120 °C, the surface initially improves but the compound begins to vulcanize inside the die land. The upper thermal boundary is therefore a kinetic boundary, not a thermodynamic decomposition limit. This is why thermal scanning of the die using infrared pyrometry is insufficient; the critical temperature is inside the die land, where wall contact and shear heating can raise the local melt temperature by 10–15 °C above the bulk stock temperature at high screw speeds. A compound that is safe at 30 kg/h can fail catastrophically when the rate is increased to 45 kg/h without any change in barrel setpoints, because the shear heating in the die land pushes the local temperature above the scorch threshold. For this reason, scale-up from laboratory extruders to production lines requires the use of moving-die rheometer cure curves at 110–130 °C per ASTM D5289 to map the minimum Mooney scorch window against expected residence time. Mooney viscosity ML 1+4 at 100 °C is typically 40–90 MU for profile compounds, and compounds above 90 MU generate enough shear heating to reduce the safe screw-speed envelope substantially.

In single-screw thermoplastic vulcanizate lines running at 30–60 kg/h, the thermal boundary is defined by the melting point of the polypropylene matrix and the residual crosslinked EPDM phase rather than by scorch chemistry. TPV grades used in weatherseal profiles are processed at melt temperatures of 190–220 °C, with vented single-screw extruders of 25:1–33:1 L/D and compression ratios of 2.0:1–2.5:1. Barrel zones are set at 200–230 °C, adapter and die at 210–230 °C. The upper boundary is 230 °C for continuous operation; above this temperature, polypropylene oxidation begins to produce acetic and formic acid traces that can accelerate corrosion in the calibration sleeve and cause surface pitting on the profile. The EPDM rubber phase itself is embedded in the PP matrix and does not exhibit classical scorch, but the crosslinked rubber particles can agglomerate if the melt temperature is too high or if the screw shear is excessive. This agglomeration is visible only after the profile is heat-aged at 135 °C for 1,000 h per ASTM D573, when the surface can develop microcracks that are misinterpreted as external oxidation. The lower processing boundary is set by the melt viscosity; below 185 °C, the melt pressure at the die can exceed 250 bar, and the profile edges will not fill, especially in thin lip sections below 0.50 mm. The acceptable melt-temperature window is therefore wide in absolute terms but narrow with respect to drawdown and dimensional stability. Moisture must be removed before processing if the relative humidity exceeds 60 %; pre-drying at 80 °C for 2–4 h is required to prevent die-face splay and internal porosity. Die swell in TPV profiles is lower than in flexible PVC, typically 5–15 %, but the recovered elastic energy can cause edge waviness if the drawdown ratio exceeds 1.20:1.

Calibration of TPV profiles requires closed vacuum calibrators with slot widths compensated for shrinkage of 0.5–1.5 % in the machine direction and 0.3–1.0 % in the cross-machine direction. Water temperature in the first calibration stage is held at 10–25 °C; if it is lowered below 5 °C, surface chill marks appear, while above 30 °C the profile retains heat and can deform after the puller. The resulting operational boundary is not strictly a material degradation limit but a thermally driven dimensional stability limit. Production-scale failures in TPV weatherseal lines are generally associated with the creation of low-molecular-weight PP chains during prolonged high-shear operation above 220 °C, which reduces melt strength and causes an increase in die drool at the exit. The drool is identified by its yellowing upon heat aging and by its lower melt flow rate when tested per ISO 1133-1:2022 at 230 °C/10 kg; the degraded fraction produces MFR values 2–5 g/10 min higher than the virgin pellet. Continuous monitoring of melt pressure and melt temperature at the adapter is more useful than barrel setpoints alone because the thermal boundary is shifted by screw speed, back-pressure, and regrind content.

Die Land, Drawdown, and Melt Fracture Boundaries for Complex Hollow Profiles

Complex hollow profiles with thin lips, internal webs, and folded bulb sections impose a thermal boundary that is specific to the die-land geometry and not solely to the compound. The die land controls pressure drop and wall shear rate; for EPDM profiles with 3–5 mm wall thickness, the apparent shear rate in the die land is typically 50–200 s⁻¹, while for thermoplastic TPV and flexible PVC profiles it ranges from 100–500 s⁻¹. When the local wall shear rate exceeds the critical value for shark-skin or stick-slip, the extrudate surface becomes periodically roughened even if the melt temperature is within the thermal stability window. This melt-fracture boundary is a function of melt temperature, molecular weight, and die-land length. For EPDM, raising the die temperature from 90 °C to 110 °C reduces the apparent viscosity sufficiently to suppress sharkskin at the expense of accelerated cure; the die temperature cannot simply be lowered because the pressure drop increases and the profile surface tears at the die exit. For TPV, the critical shear rate in a 1.5 mm land length can be 500–800 s⁻¹ at 220 °C; above this threshold, the profile surface shows a fine periodic striation that remains visible after cooling. For flexible PVC, the critical threshold is often lower, 200–400 s⁻¹, because the compound has a wider molecular-weight distribution and a tendency toward wall slip. The thermal boundary is thus a process window located at the intersection of melt viscosity, shear heating, and die-land pressure.

Drawdown is the ratio of haul-off speed to die-exit velocity; for weatherseal profiles it is normally kept between 1.05:1 and 1.30:1. Above 1.5:1, high-shear edges become thin and lose compression capability, and the internal web can shift laterally by more than 0.2 mm. The maximum drawdown is limited by melt strength, which depends on temperature and molecular weight. In an EPDM compound, green strength is not high enough at the die exit to sustain drawdown above 1.3:1 without tearing; the extrudate must be supported by a conveyor or calibrated directly. For TPV, a drawdown of 1.2:1 is typical, but the cross-machine shrinkage after cooling can reach 0.5–1.0 % over 24 h, requiring a post-cooling dimensional check per ISO 3302-1 for dimensional tolerances. The thermal link is that higher melt temperature lowers melt strength and increases the drawdown-induced deformation, while lower melt temperature increases die pressure and surface tearing. The practical resolution is to use a longer die land rather than higher melt temperature, but this increases pressure drop and shifts the melt-fracture boundary upward. This is one of the most heavily monitored process conflicts in profile extrusion because the die cannot be changed quickly on a production line.

When Dense EPDM Replaces Cellular EPDM in Compression-Loaded Weatherseals

Dense EPDM weatherseals differ from cellular EPDM weatherseals in thermal mass, heat-transfer behaviour, and compression-load deflection. A dense EPDM profile with a hardness of 70±5 Shore A typically has a Mooney viscosity ML 1+4 at 100 °C of 50–80 MU, while a cellular EPDM compound for sponge weatherseals is compounded to 30–50 MU and contains low-viscosity EPDM grades to allow cell expansion. When a production line is switched from cellular to dense EPDM without changing the barrel temperature profile, the stock temperature at the screw tip rises by 8–12 °C because the dense compound generates more frictional heat in the compression zone. The maximum allowable stock temperature for the dense compound is 125–130 °C; the same setpoint that was safe for cellular EPDM can therefore place the dense compound into the scorch region. This is particularly acute in pin-barrel extruders because the pins increase shear heating by 10–20 % compared with smooth-bore machines. The thermal boundary is compounded by the greater heat capacity of the dense compound: the extruded profile retains heat for a longer period after leaving the die, and the centre of a 5 mm thick section can remain above 100 °C for 3–5 min after entering the calibration sleeve. If the water bath is set above 25 °C, the core temperature is slow to fall, and compression set after 22 h at 70 °C per ASTM D395 Method B can exceed 35 % because of micro-vulcanization during cooling rather than actual cure-system deficiency. For this reason, the barrel temperature profile in the compression zone must be reduced by 10 °C relative to cellular EPDM, and the screw speed must be reduced by 5–10 % to maintain the same discharge temperature.

The replacement of cellular EPDM by dense EPDM also changes the die-transit thermal boundary for corner filling. Dense EPDM has higher viscosity and greater die swell; the die needs to be reshaped to reduce the land length by 10–15 % to avoid excessive pressure drop. If the same die is used, the extrusion pressure increases and the melt temperature at the die entry increases further, narrowing the safe processing window to ±3 °C. The dense profile also has a lower surface-to-volume ratio than a cellular profile of the same external dimension, so the cooling stage is more difficult. Failed dense EPDM weatherseals are not always recognised immediately; latent defects can appear as compression-set drift after the part has been in service for 1,000 h at 70–80 °C. The thermal stability boundary in this application is therefore not just the scorch limit during extrusion but includes the microstructural memory of extended heat exposure in the cooling phase.

For silicone rubber gaskets cured by platinum-catalyzed addition chemistry, the extrusion thermal boundary is lower than for EPDM despite the much higher decomposition temperature of the polymer. Silicone weatherseal compounds are processed on low-compression single-screw extruders with L/D 10:1–15:1 and barrel temperatures from 20 °C in the feed zone to 40–60 °C in the metering zone. Head and die-set temperatures are kept at 60–90 °C; if the die temperature exceeds 100 °C, the platinum-catalyzed crosslinking reaction begins inside the die land and produces gel particles in the profile. The platinum catalyst is active at ambient temperature but is inhibited by a two-part addition system until the two parts are mixed; once mixed, the inhibitor is thermally released at 80–100 °C depending on inhibitor concentration. The practical upper boundary for the stock temperature is therefore 90 °C at the die exit, with a tolerance of ±5 °C. The compound then enters a hot-air vulcanization tunnel at 200–250 °C with residence times of 1–5 min depending on profile thickness. This large difference between extrusion temperature and cure temperature is the reverse of EPDM; the danger is not decomposition in the extruder but premature cure if shear heating in the die land raises the temperature above the inhibitor release threshold. Because the viscosity of silicone rubber is relatively low, the shear heating effect is less pronounced than in PVC or EPDM, but it can still be 5–10 °C at high screw speeds. The result is a processing window at the die of 70–90 °C, with the lower boundary set by the need to soften the compound sufficiently for thin-wall filling and the upper boundary set by the inhibition release point.

Post-cure ovens are controlled with air temperatures up to 250 °C for continuous profiles and 300 °C for short excursions, but the profile surface temperature may remain below the air temperature because of evaporative cooling from low-molecular-weight siloxane volatiles. The use of a thermal imaging camera across the tunnel entrance provides the only practical non-contact measurement of surface temperature; the difference between air and surface can exceed 30 °C in the first 30 cm of tunnel length. Failure to account for this difference leads to under-cure of the profile centre, which is then detected as a low crosslink density using solvent swelling or compression-set testing per ASTM D395 Method B after 22 h at 175 °C. The thermal stability boundary of a silicone weatherseal is therefore a two-stage boundary: the die must not exceed 90–100 °C before triggering cure, while the oven must exceed 200 °C after the profile has entered the tunnel to complete the vulcanization reaction. This is a classic process conflict because the operator cannot directly manipulate one without affecting the other; line speed changes alter both extrusion back-pressure and oven residence time.

Multivariate Influence of Batch-to-Batch Carbon Black Dispersion on Heat Build-Up

Carbon black dispersion quality in EPDM weatherseal compounds is not only a mechanical property issue; it is also a thermal-boundary variable because undispersed carbon black agglomerates act as internal stress concentrators and increase viscous dissipation during extrusion. EPDM compounds for automotive weatherseals are commonly loaded with N550 or N650 carbon black at 60–120 phr; the higher loadings are used for dense profiles requiring UV resistance and low compression set. The carbon black is incorporated in a Banbury internal mixer with a drop-door temperature of 120–150 °C, followed by sheet-off on a two-roll mill at 60–80 °C. If the Banbury cycle is shortened by 10–15 %, the dispersion rating measured by ASTM D2663 can drop from the typical production value of 90–95 % to below 85 %. The consequence is a measurable increase in extruder stock temperature at constant screw speed of 3–8 °C because the agglomerates create local shear peaks and increase the compound viscosity. In a pin-barrel extruder running at 25–40 kg/h, this temperature increase can push the discharge stock from 118 °C to 125–130 °C, which is exactly the scorch boundary for sulfur-cured EPDM. The resulting profile contains hard, partially vulcanized carbon-black-rich particles at the surface, which are often misdiagnosed as die accumulation but are actually batch-consistency failures.

The thermal boundary in a carbon-black-loaded EPDM weatherseal is therefore not a single number; it shifts with dispersion quality, carbon black structure, and base EPDM molecular weight. Low-structure carbon black such as N550 produces lower heat build-up than high-structure N650 at equal loading, but the tensile modulus is lower. Reformulation from N550 to N650 without adjusting the extrusion profile can increase the stock temperature by 5–10 °C and narrow the processing window by half. The same compound is then more likely to fail under high ambient temperature during summer production; a rise in plant ambient temperature from 20 °C to 35 °C changes the extruder barrel heat-transfer balance and increases the stock temperature by an additional 3–5 °C. Published data for specific batch-to-batch dispersion effects on heat build-up is limited, but the relationship between carbon-black dispersion and extrudate surface quality is well documented in rubber processing literature. Production-scale lines typically respond to summer conditions by reducing screw speed by 10–15 % and increasing barrel cooling-water flow rather than raising the scorch threshold through accelerators, because accelerator changes can shift the cure curve and create compression-set failures in service.

Chloroprene weatherseal extrusion at production rates above 25 kg/h is constrained by the narrow gap between the melt temperature needed for acceptable surface smoothness and the temperature at which chloroprene begins to crosslink exothermically. Masterbatches based on sulfur-modified or mercaptan-modified chloroprene with MgO and ZnO cure systems are extruded on cold-feed screw machines with barrel temperatures of 50–70 °C in the feed section, 60–80 °C in the compression zone, 80–100 °C at the head, and 90–110 °C at the die. The upper stock-temperature boundary is 115 °C for continuous operation because the exothermic vulcanization reaction can cause a self-accelerating temperature rise above 120 °C if the residence time exceeds 3–5 min. Chloroprene is more sensitive than EPDM to thermal history because the polymer contains reactive allylic chlorine and the cure-system chemistry generates zinc chloride, which can catalyse further dehydrochlorination. The lower temperature boundary is 80 °C at the die; below this temperature, the compound has high elastic energy and the profile surface exhibits pronounced nerviness and edge raggedness. The process window is therefore approximately 30 °C wide, but operational control is maintained within ±5 °C because the wall-thickness variation of thin sealing lips increases rapidly outside this range. A chloroprene weatherseal for marine or automotive applications is typically filled with carbon black and plasticizer but is not as highly extended as EPDM; the resulting specific heat capacity is lower, and the extrudate cools faster, which is beneficial for dimensional stability but means the die must be heated to prevent premature solidification at the edges.

The substitution of thiourea-based accelerators with safer thiadiazole systems in response to REACH restrictions has changed the thermal boundary. Thiadiazole-cured chloroprene compounds generally show a higher scorch time at 121 °C by ASTM D1646, often 10–20 min, compared with 5–10 min for older ethylene thiourea-based systems. This allows a 5 °C increase in die-head temperature and improves surface quality, but the tear strength after heat aging may be lower. The processing window is therefore not directly transferable from historic line settings; a chloroprene compound reformulated with thiadiazole can be run at a die temperature of 95–105 °C instead of 90–100 °C, but the extruder speed must still be limited to avoid excessive shear heating. When the die-head temperature is raised above 110 °C, the extrudate can develop a granular surface due to the formation of crosslinked microgel particles. The material boundary is detected by the appearance of a fine powder at the die exit, which is not dust from the cutting station but results from crosslinked chloroprene particles being pulled from the die wall.

Melt Temperature Boundaries During Azodicarbonamide Foam Expansion

The expansion of cellular weatherseal profiles using azodicarbonamide (ADC) as blowing agent imposes a thermal boundary that extends from the extruder die to the first hot-air or microwave oven zone. ADC-based blowing agent systems are selected with an activation onset of 160–200 °C depending on particle size and carrier composition; the gas yield is typically 200–220 mL/g under standard conditions. In a cellular EPDM or chloroprene weatherseal formulation, the extruder barrel temperatures are kept between 50 °C and 75 °C, and the die temperature is held at 70–90 °C. The die temperature must remain below the ADC activation onset to prevent pre-blow in the barrel or die land; pre-blow appears as internal voids with collapsed cell walls, poor skin formation, and uncontrolled profile expansion before the calibration stage. The melt-temperature boundary at the die exit is therefore 90–95 °C for ADC systems with activation onset near 160 °C, but lower activation grades may require die temperatures below 85 °C. Some producers use a split die with a cooled front plate to maintain a surface temperature of 70–80 °C while the core remains at 85–90 °C, ensuring that the blowing agent remains unreacted until the profile exits the die.

After the die, the profile enters a microwave or hot-air expansion tunnel operating at 180–210 °C for EPDM sponge; the residence time is 2–8 min depending on cross-section and line speed. The thermal boundary in the tunnel is not simply the activation temperature of the blowing agent but the synchronisation between gas generation and the vulcanization cure curve. If the oven temperature is too high, the surface cures too quickly and traps gas inside the core, causing closed-cell collapse or internal voids; if the oven temperature is too low, the blowing agent decomposes slowly while the cure reaction lags, and the profile can collapse before the cell walls gain permanent set. The acceptable temperature range in the first meter of the tunnel is often ±10 °C, with a surface temperature not exceeding 220 °C for EPDM-based cellular compounds. For chloroprene-based cellular weatherseals, the tunnel temperature is lower, usually 160–190 °C, because the chloroprene cure system is exothermic and can overheat the cell walls. The final cell structure is controlled by measuring compression-load deflection per ASTM D1056 classification and by microscopic cell-size analysis; published data for specific ADC grades in production-scale weatherseal lines is limited because cell formation depends on die geometry, line speed, and oven air velocity. The overriding thermal rule is that the blowing agent must remain inert through the die and begin decomposition only after the extrudate has left the shaping tool, while the rubber cure reaction must reach a sufficient crosslink density before the gas pressure decays.