Silicone Injection Moulding: Cold Runner, Hot Mould, Fast Cure

What makes LSR different

Liquid silicone rubber is a two-component, platinum-catalysed system with a viscosity low enough to be pumped. It arrives in drums as part A and part B, it is mixed at the machine, and it cures in the mould in seconds rather than minutes because the catalyst is fast and the mould is hot.

That combination changes the whole machine architecture. A conventional injection moulding machine has a heated barrel and a cold mould. An LSR machine has a cold barrel, a cold runner, and a mould at 180 to 200 °C. The material must not cure before it reaches the cavity, so everything upstream of the cavity is kept at around 20 °C.

The result is a process that looks like injection moulding and behaves like nothing else in the plant. The cycle is measured in seconds, the scrap rate is low because there is no flash by design, and the labour content is near zero. The capital cost is high and the tooling is the most expensive of the three silicone processes.

Dosing, mixing and the ratio that must not drift

The two components are mixed at a ratio that is usually 1:1 but can be 10:1 or other values depending on the system, and the ratio is critical. Platinum-catalysed addition cure is not forgiving: too little catalyst and the part is under-cured and tacky, too much and the cure is fast but the physical properties drop and the pot life collapses.

A static mixer is used for most LSR, which means the mixing happens in a disposable tube with no moving parts. That is cheap and reliable, and it also means the mixer is a consumable that has to be replaced on a schedule, because a partially cured mixer delivers an unmixed slug into the cavity and produces a part with a soft, tacky region.

Table 1 — The dosing variables that decide whether an LSR part is good.

Degassing is the item that separates a medical or optical supplier from a general one. LSR entrains air during drum handling and pumping, and a bubble in a 2 mm silicone lens is a reject. Vacuum degassing of both components before mixing is standard in that segment and often absent elsewhere.

The cold runner block

The cold runner is the feature that makes LSR economical. It is a water-cooled block between the machine nozzle and the mould, held at 20 to 25 °C, which keeps the material in the runner liquid while the mould is at 190. Without it, every shot would leave a cured runner that has to be thrown away, and the waste would be 30 to 50 percent instead of the 2 percent that a cold runner achieves.

A cold runner can be open or valve-gated. An open cold runner is simpler and cheaper and relies on the material's viscosity and the shot size to balance the flow. A valve-gated cold runner uses needles to open and close each drop, which gives positive shut-off, better balance and the ability to run sequential filling.

The valve gate is what makes a large multi-cavity LSR tool practical. With 32 or 64 cavities, an open runner will not balance and the outer cavities will be short. A valve gate closes each drop after it has filled, so every cavity sees the same packing and the same cure.

Cure in seconds, and what still costs time

The cure curve is steep. At a mould temperature of 120 °C an LSR part takes 25 seconds in the mould, at 140 it takes 14, at 160 it takes 8, at 180 it takes 5 and at 200 it takes 3.2. That is a factor of eight across a range that a hot-oil or cartridge-heated mould can hold comfortably.

Thickness still costs time, but in seconds rather than minutes. At 160 °C, a 2 mm section cures in 5 seconds, a 4 mm in 8, a 6 mm in 14 and a 10 mm in 26. Compared with compression moulding, where a 10 mm section takes 21 minutes, that is a different order of magnitude, and it is why LSR wins on any part with a substantial section.

Table 2 — Cure and output against mould temperature for a 2 mm part in a 16-cavity tool.

Those output numbers are theoretical and they are also the reason LSR tooling pays back so quickly at volume. A 16-cavity tool at 180 °C produces 6,400 parts an hour in principle, and even at 60 percent overall efficiency that is nearly 4,000. No compression process gets close, which is why the crossover volume is lower than people expect.

Tooling and tolerances

An LSR tool is a precision tool. The mould runs at 180 to 200 °C, so the steel has to be dimensionally stable at that temperature and the thermal expansion has to be allowed for in the cavity dimensions. The shut-offs have to be tight enough to hold low-viscosity material, which at 100 to 500 pascal-seconds will find any gap above a few microns.

• Steel selection: hardened stainless or a tool steel with good thermal stability, because the mould runs hot continuously.
• Shut-offs: metal-to-metal contact with no more than 0.005 mm clearance, or the low-viscosity material will flash.
• Venting: 0.002 to 0.005 mm, much finer than for thermoplastics, because the material is so fluid.
• Ejection: silicone is flexible, so stripper plates and air ejection work better than pins on a soft part.
• Heating: hot oil or cartridge heaters with zones, because a 5 °C variation across the mould shows as a cure variation.

The tolerance that LSR can hold is good but not thermoplastic-good. Plus or minus 0.05 mm on a dimension under 10 mm is achievable, and plus or minus 0.2 percent of the dimension above that, but the shrinkage varies with section thickness and with cure temperature, so a part with mixed sections will not hold a single tolerance band everywhere.

The defects that are specific to LSR

Table 3 — LSR-specific defects and what causes them.

The uncured patch is the one to worry about, because it is a functional failure rather than a cosmetic one. A part with a soft patch will fail a compression set test and, in a medical application, will leach unreacted material. The cause is almost always the mixer or the ratio, and the check is a daily shot into a cup to confirm the mix is homogeneous.

When LSR is the right answer

LSR is the right answer when volume is high, when the part must have no flash, when handling is a contamination risk, or when the section is thick enough that compression moulding becomes slow. It is the wrong answer for a large part, a low volume, or a part with a heavy insert that a cold runner cannot accommodate.

1. Volume above about 50,000 parts a year, where the tooling amortises quickly.
2. Medical and infant-contact parts, where manual handling and flash removal are contamination risks.
3. Optical parts, where clarity requires a moulded surface with no post-processing.
4. Parts with a thick section, where the cure time advantage is largest.
5. Parts with fine detail or thin walls, where the low viscosity fills features that HCR cannot.

The exception worth knowing is large parts. LSR machines are built around small shot sizes and high pressures, and a part weighing 500 grams is usually a compression or transfer job regardless of volume, because the dosing and the mould heating become impractical. The boundary is roughly where the shot size exceeds what the machine can dose in a few seconds.