High-precision silicone gasket

01

Control flash and post-cure the parts

Wacker's processing guidelines for solid and liquid silicone rubber give 150 to 200 °C as the processing range for ELASTOSIL liquid silicone rubber, note that cured silicone can stick to the mould until, "After multiple cycles, a release layer resulting from the silicone rubber forms on the mold surface", and describe demoulding with ejector pins and brush or blower units. Its troubleshooting table lists, as possible causes of overpacking or flash, mould temperature too low, a defective mould, clamping force too low, injection too fast, holding pressure too high or too long, and an incorrect switchover point to holding pressure. [1]

Wacker explains that post-curing improves mechanical properties such as compression set and removes volatiles, and is recommended for platinum-cured grades in food-contact or medical use, where FDA, BfR or European Pharmacopoeia rules apply. Post-cure in a circulating-air oven with fresh air of 100–120 l/min per kg of silicone to avoid deflagration; Wacker gives 4 hours at 200 °C for sheets 2 mm thick, thicker parts a slower start, and never more than 225 °C or 8 hours, to avoid thermal ageing and embrittlement. [1]

Editorial calculation: a post-cure batch of 20 kg of gaskets would need about 2,000–2,400 litres per minute of fresh air (2–2.4 m³/min) by Wacker's rate. [1]

Give the supplier the drawing with critical dimensions and tolerances, hardness and colour, service temperature and media, compression in the joint, compliance required (FDA, BfR, medical), whether post-curing is required, flash and parting-line limits, inspection method, annual volume and packaging.

02

Prove compression set and food-contact compliance

ERIKS's O-ring handbook lists its standard 70 IRHD silicone compound at −60 to +220 °C with compression set of at most 40% after 22 hours at 200 °C under 25% squeeze, against 18–19% for its FKM compounds at the same temperature, and defines compression set as the share of deflection the elastomer fails to recover. It gives about 0.175 mm as the minimum squeeze for O-rings, since very light squeeze leads almost all elastomers quickly to 100% compression set. [2]

Under US 21 CFR 177.2600, silicone elastomers are among the rubbers permitted for repeated food contact. For aqueous food the finished food-contact surface, extracted with distilled water at reflux, may yield no more than 20 mg/in² in the first 7 hours and 1 mg/in² in the next 2 hours; for fatty food, n-hexane extraction allows 175 mg/in² then 4 mg/in². Articles must be cleansed before first use with food. [3]

Editorial calculation: those limits are about 3.1 and 0.16 mg/cm² for aqueous food and 27.1 and 0.62 mg/cm² for fatty food. [3] Wacker's note that post-curing can be essential to meet food-contact rules is a reason to test extraction on post-cured production parts. [1]

03

Read a compression set test on moulded LSR

Marl et al. (Materials, 2024) injection-moulded two 40 Shore A liquid silicone rubbers, Momentive Silopren LSR 2040 and ShinEtsu KEG-2000-40, at a mould temperature of 180 °C, and post-cured half of the LSR samples for 4 h at 200 °C, "since post-curing achieves complete cross-linking". They measured compression set to DIN ISO 815-1: three specimens each compressed by 25%, held at 175 °C for 22 h, then relaxed for 0.5 h before the recovered thickness was measured, with compression set = (h0 − h1) ÷ (h0 − hs) × 100. [4]

The authors report that for both LSRs the non-post-cured samples had a significantly higher compression set than the post-cured samples, which they attribute to cross-linking sites left after moulding that then form in the deformed state during the 175 °C test. Over about 200 days of storage they found no significant change in compression set for the Momentive grade. [4] Editorial note: this supports asking for compression set results measured on post-cured parts, and comparing grades only when squeeze, temperature and time match, since ERIKS quotes its figure at 200 °C and 25% squeeze while this study used 175 °C. [2]

04

LSR housing seals and compression-moulded parts in the listings

Listings include LSR injection-moulded housing seals and gaskets, and compression-moulded silicone seals among other rubber parts.

05

Grade, tool, post-cure and squeeze behind a gasket's set

Post-cure and squeeze in the joint are the two rows that move compression set, per Wacker and ERIKS. [1] [2]

06

Does the compression set figure match your joint's temperature?

The published figures here are for the ERIKS 71477 compound; ask for the same properties measured on your post-cured part. [2]

07

Send the drawing, use and compliance data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Part: [drawing; critical dimensions; tolerances]
Material: [LSR or solid; hardness; colour]
Use: [temperature; media; compression]
Process: [post-cure; flash limit; inspection]
Compliance: [FDA 21 CFR 177.2600; BfR; medical]
Supply: [volume; packaging]

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