Industry Application

Silicone Rubber Kneader Machine

Sigma Kneaders for HTV Silicone Rubber Compounding

High-temperature vulcanisation (HTV) silicone rubber is produced by compounding a high-molecular-weight polydimethylsiloxane base polymer with reinforcing fillers, processing aids and vulcanising agents. The sigma kneader — also known as a sigma blade mixer or kneader mixer — is the industry-standard equipment for this process. Its counter-rotating Z-shaped blades generate the intense shear, compression and folding action required to achieve complete fumed silica dispersion and uniform compound quality within a sealed, temperature-controlled mixing chamber.

Why Silicone Rubber Compounding Requires a Kneader

Silicone rubber base polymer (polydimethylsiloxane, PDMS) is an extremely high-viscosity material — far beyond the capability of conventional paddle mixers or dispersers. Its molecular weight can reach 600,000–800,000 g/mol, giving it an elastic, dough-like consistency that demands the multi-directional shearing and kneading action of a sigma blade mixer. At the same time, the fumed silica reinforcing filler — which may constitute 20–40 phr (parts per hundred rubber) of the compound — has a specific surface area of 150–400 m²/g and a strong tendency to agglomerate, requiring intense and sustained mechanical energy to fully wet out and disperse into the polymer matrix.

Three process requirements define equipment selection for silicone rubber compounding: high-torque mixing capability for the base polymer, precise temperature control to prevent premature vulcanisation (scorch), and vacuum degassing capability to remove air entrapped during fumed silica incorporation. LRMIX sigma kneaders address all three requirements in a single machine.

Silicone Rubber Products Made Using Sigma Kneaders

Industrial Silicone Seals & Gaskets HTV compound for moulded and extruded sealing components — automotive, aerospace, industrial process equipment
Silicone Tubing & Hose Medical-grade and industrial silicone tubing — requires bubble-free compound and consistent Shore A hardness throughout
Thermal Interface Materials Highly filled silicone compounds incorporating aluminium oxide or boron nitride fillers for thermal conductivity in electronics
Flame-Retardant Silicone Compounds incorporating aluminium hydroxide or platinum-based flame retardants for wire and cable, building materials
Conductive Silicone Carbon black or silver-filled silicone compounds for EMI shielding, keypads, conductive gaskets and sensor applications
Silicone Colour Masterbatch Pigment and dye dispersion in silicone gum base — requires uniform colour distribution with no streaking or undispersed particles

Standard Silicone Rubber Compounding Process

Correct sequence and temperature management at each stage of compounding directly determines the quality and consistency of the finished silicone rubber compound. The following process is standard for HTV silicone rubber production using a sigma kneader:

1
Load base polymer + structure-control agent HTV polydimethylsiloxane gum is charged into the kneader and low-speed mixing begins. Hydroxy silicone oil (structure-control agent) is added early to coat the polymer surface, preventing the "structuring" effect — an irreversible hardening phenomenon caused by fumed silica interacting with the base polymer during storage.
2
Fumed silica — staged addition (critical step) Fumed silica must be added in 3–4 batches, not all at once. Each addition requires sufficient mixing time for complete wetting before the next batch. Typical loading: 20–45 phr. Surface area of fumed silica significantly affects compound viscosity — grades with BET surface area of 200–300 m²/g (such as Evonik AEROSIL® 200 or 300) are most commonly used. Cooling water must be active throughout this stage to keep compound temperature below 50°C.
3
Vacuum degassing After fumed silica is fully incorporated, the kneader is sealed and evacuated to ≥ –0.09 MPa for 10–20 minutes. This removes air entrapped during the high-surface-area silica incorporation stage. Vacuum degassing is essential for products requiring a void-free compound — tubing, sheeting, moulded precision parts and medical applications.
4
Functional fillers and additives Heat stabilisers, flame retardants, pigments, conductivity fillers and other additives are incorporated after the base compound is fully homogenised. These materials are typically added at lower loadings and disperse more easily than fumed silica.
5
Vulcanising agent — added last, temperature critical Peroxide vulcanising agents (DCP, DBPH, etc.) are added only after compound temperature has been confirmed below 40°C. Peroxides begin to activate above 60–70°C — premature activation (scorch) destroys the batch. Low-speed mixing for 3–5 minutes ensures uniform distribution without generating excess heat. Compound is then discharged immediately via hydraulic tilt.
⚠️ Scorch Prevention: The Most Critical Process Control Point

Peroxide vulcanising agents such as DCP (dicumyl peroxide) begin thermal decomposition above 60°C and activate rapidly above 100°C. In a kneader without adequate cooling, frictional heat from fumed silica incorporation can push compound temperature above this threshold before the vulcanising agent is even added. A cooling water jacket that actively controls compound temperature throughout the entire mixing cycle is not optional for silicone rubber production — it is essential.

Which Sigma Kneader Configuration Is Right for Your Production?

ConfigurationKey FeaturesBest Suited For
Standard Hydraulic Tilt KneaderCooling jacket, hydraulic 130° tilt discharge, ZLY hard-tooth reducerGeneral HTV silicone rubber compounding, industrial seals, tubing, colour masterbatch
Vacuum Hydraulic Tilt KneaderAll of the above + vacuum system ≥ –0.09 MPaMedical-grade silicone, food-contact products, thermal interface materials — bubble-free compound required
Screw Extrusion KneaderIntegrated screw extruder discharges compound continuously as strip or pelletHigh-volume production, feeding downstream calenders or extruders directly — eliminates manual discharge step

LRMIX Sigma Kneaders for Silicone Rubber

LRMIX hydraulic tilt sigma kneader machine for silicone rubber compounding

Hydraulic Tilt Sigma Kneader

Heavy-duty sigma kneader with hydraulic 130° tilt discharge and cooling water jacket. ZLY hard-tooth gear reducer for sustained high-torque operation in silicone rubber compounding.

  • Hydraulic tilt discharge — 3-minute discharge cycle
  • Cooling water jacket — temperature control throughout
  • ZLY hard-tooth reducer — HRC 58–62 gear surface
  • Vacuum option available
View Details
LRMIX 100L screw extrusion sigma kneader for silicone rubber production

100L Screw Extrusion Kneader

Sigma kneader with integrated screw extruder for continuous strip or pellet discharge. Eliminates manual discharge and feeds downstream processing equipment directly.

  • Screw extrusion discharge — continuous output
  • Suitable for high-volume silicone compound production
  • Cooling jacket as standard
  • Reduces labour and batch-to-batch handling time
View Details
LRMIX sigma kneader machine range for silicone rubber and high viscosity mixing

LRMIX Sigma Kneader Range

Full sigma kneader range from 5L laboratory to 3,000L production capacity. Standard tilt, screw discharge, vacuum, heating and explosion-proof configurations available.

  • 5L to 3,000L — lab to full production scale
  • Multiple discharge options
  • Heating / cooling / vacuum configurations
  • SS304 or carbon steel wetted parts
View All Models

Why Silicone Rubber Manufacturers Choose LRMIX

ZLY Hard-Tooth Gear Reducer Carburised and quenched gears at HRC 58–62 sustain the extreme torque demand of silicone gum mixing without fatigue failure — 40% higher load capacity than soft-tooth reducers
Active Cooling Throughout Mixing Cooling water jacket maintains compound temperature below 50°C during fumed silica incorporation — the essential condition for preventing premature vulcanisation and batch loss
Vacuum to ≥ –0.09 MPa Integrated vacuum system removes entrapped air during and after silica incorporation — required for medical, food and optical silicone applications where bubble-free compound is non-negotiable
Hydraulic Tilt Discharge 130° hydraulic tilt discharges fully compounded silicone rubber in under 3 minutes with minimal residue — compared to 15–20 minutes for manual discharge methods
5L to 3,000L Scale Range Matching lab, pilot and production kneaders allow compound development at small scale and direct scale-up to full production without process parameter changes
Factory Direct, 40+ Countries LRMIX sigma kneaders are used by silicone rubber manufacturers across Southeast Asia, the Middle East, South Asia, Europe and the Americas — supplied factory direct

Need a Sigma Kneader for Silicone Rubber Compounding?

Tell us your batch size, vacuum requirement and discharge preference.
LRMIX will configure the right kneader for your production — factory direct.

WhatsApp +86 198 1875 7839
Email info@lrmixer.com
Contact Form lrmixer.com/contact
Request a Free Quote
Scroll to Top