Overview

HTV solid silicone rubber (High-Temperature Vulcanizing silicone) is the industry standard for compression molding, transfer molding, and extrusion of durable rubber parts. This FAQ addresses technical pre-sales certifications, material properties, and post-sales processing challenges to help engineers and procurement specialists avoid common failures.

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Frequently Asked Questions

Q1: What is the maximum temperature resistance of HTV solid silicone rubber?

The maximum continuous operating temperature of standard HTV solid silicone rubber is 230°C (446°F). Special high-temperature formulations using phenyl or ceramic fillers can withstand intermittent exposure up to 300°C (572°F). Below -50°C, the material maintains flexibility without cracking, making it suitable for thermal cycling applications from -60°C to +250°C for most general-purpose grades.

Q2: Does HTV solid silicone rubber have FDA or LFGB food-contact certifications?

Yes, specific platinum-cured HTV solid silicone rubber grades are FDA 21 CFR 177.2600 compliant and LFGB certified for food contact. Peroxide-cured grades are generally not recommended for food contact due to residual byproducts. Key requirements: post-curing at 200°C for 4 hours to reduce volatiles, third-party migration testing (<10 mg/dm² overall migration in 95% ethanol), and certification documentation from raw material suppliers.

Q3: What is the difference between HTV solid silicone rubber and liquid silicone rubber (LSR)?

HTV solid silicone rubber is supplied as a millable gum or pre-compounded sheet with a putty-like consistency, requiring two-roll milling and compression/transfer molding at 150-180°C. LSR is a pumpable two-part liquid injected at low pressure into hot runners at high speed (cycle times 30-60% faster). HTV suits low-volume, large-part, or thick-section parts (over 5mm), while LSR excels at micro-molding (under 1mm precision) and high-volume automated production.

Q4: What is the correct curing agent ratio for HTV solid silicone rubber?

The standard catalyst ratio is 1.0 to 1.2 parts per hundred rubber (phr) for peroxide-type curing agents such as 2,4-dichlorobenzoyl peroxide (DCBP) or dicumyl peroxide (DCP). Using less than 0.8 phr causes under-cure (tacky surface, low tensile strength). Exceeding 1.5 phr leads to bloom (white powder on surface) and brittle tearing. For platinum catalysts, use 0.5-0.7 phr plus inhibitor adjustment. Always calibrate your two-roll mill dispersion to ensure uniform mixing.

Q5: Why do my molded HTV silicone rubber parts have air bubbles or blisters?

Air bubbles are caused by trapped volatiles or insufficient compression force. The three most common causes: (1) Insufficient pre-press degassing – open mold for 2-3 seconds after first 20% of cure time; (2) Excess peroxide catalyst (>1.3 phr) generating benzoic acid gas; (3) Moisture absorption from uncured compound stored without sealed polyethylene liners. Fix: Increase mold clamping pressure to 150-200 kg/cm², pre-bake the blank at 80°C for 30 minutes, and reduce catalyst to 1.0 phr.

Q6: How do I calculate shrinkage for HTV solid silicone rubber molds?

Post-cure shrinkage of HTV solid silicone rubber ranges from 2.5% to 4.0%, with 3.2% being the industry average for 50-70 Shore A hardness grades. Compensation formula: Mold cavity dimension = Final part dimension × (1 + shrinkage rate/100). Example: For a 100 mm part with 3.2% shrinkage, mold = 103.2 mm. Factors increasing shrinkage: higher filler load (mineral-filled grades >4%), thin walls (<1.5mm), and post-curing at >230°C. Platinum-cured systems shrink 0.5-1.0% less than peroxide systems.

Q7: What is the shelf life of uncured HTV solid silicone rubber, and how should it be stored?

Uncured HTV solid silicone rubber has a shelf life of 6 to 12 months when stored in original sealed polyethylene bags at 5-25°C (41-77°F) and 40-60% relative humidity. Avoid temperatures above 35°C (accelerates crosslinking) and direct sunlight (UV degrades the gum). Indicators of aging: increased Mooney viscosity (harder milling), surface tack loss, and blooming of white powder. Opened material must be used within 2 weeks if stored at 25°C in a resealed bag with desiccant.

Q8: What hardness range is available for HTV solid silicone rubber?

HTV solid silicone rubber is available from 20 Shore A (very soft, similar to a gel insole) to 80 Shore A (rigid, like a car tire tread). The most common industrial grades are 40, 50, and 60 Shore A, balancing tear strength (typically 25-45 kN/m) and elongation (250-600%). For oil seals and high-wear applications, 70-80 Shore A with added PTFE or carbon filler reduces friction. Durometer tolerance per ASTM D2240 is ±5 points.