Silicone Transfer Moulding: Better for Complex Parts and Inserts

How transfer moulding works

Transfer moulding adds one part to the compression process: a pot. The uncured compound is placed in a heated chamber above the cavity, the mould closes completely, and a plunger then forces the compound through a small sprue into the cavity that is already sealed.

That single change in the sequence fixes three problems at once. The cavity is closed before any material enters it, so the compound cannot escape past the parting line as flash. The material enters under pressure and fills from the bottom, so air is pushed ahead of it and out through the parting line rather than being trapped. And the insert, if there is one, is clamped in position by the closed mould before the compound arrives, so it cannot move.

The cost is the pot, the plunger, a sprue that has to be trimmed, and a tool that is more complex and more expensive than a compression tool. For a simple part at low volume that is not worth it. For a part with an insert, a tight flash requirement, or a geometry that traps air, it usually is.

Waste: the number that decides the volume

In compression moulding the blank has to be heavier than the part, because the cavity has to be overfilled for the compound to reach the edges. That excess becomes flash. In practice the flash and runner allowance runs about 18 percent of the compound for compression, 6 percent for transfer and 2 percent for liquid silicone injection.

Silicone compound is the most expensive input in the part, so an 18 percent allowance is not a rounding error. On a part that uses 50 grams of compound, compression wastes 9 grams and transfer wastes 3, and the difference is often larger than the difference in tooling cost amortised over the first year.

Table 1 — Compound loss by process, and why none of it comes back.

The re-use question matters because people assume cured silicone flash can be reground. It cannot be re-compounded into new material: the crosslinks are permanent, and ground cured silicone is a filler at best. That is why the waste percentage is a direct cost and not a recoverable one.

Preheating buys about a third of the cycle

The pot sits in the heated platen, so the compound is at mould temperature before it enters the cavity. That is a real thermal advantage: in compression moulding the blank goes in cold and the first part of the cycle is spent heating it, while in transfer moulding it is already hot when it arrives.

Measured on a 4 mm section, compression cures in 4.2 minutes at 160 °C, 2.6 at 180 and 1.6 at 200. Transfer at the same temperatures takes 3.0, 1.9 and 1.2. That is a consistent saving of about 30 percent, and it comes from two places: the preheat, and the fact that the closed cavity holds pressure on the part for the whole cure rather than relying on the press to stay closed against a bleeding flash.

There is a second-order benefit that shows up in the scrap rate. A compression part that is slightly under-cured backrinds at the flash line, and the fix is to lengthen the cycle for the whole tool. A transfer part has no flash line to backrind, so the cure window is wider and the tool is more forgiving of a compound that is a little fast or a little slow.

Inserts are the strongest argument

A silicone part with a metal insert is where transfer moulding earns its price. In compression moulding the insert sits in an open cavity while the blank is loaded around it, and when the mould closes the flowing compound pushes on the insert. A small insert, a thin insert or an insert with an asymmetric shape will move, and the part will have the insert in the wrong place.

In transfer moulding the mould closes on the insert first and clamps it, and then the compound flows in. The insert cannot move because the steel is holding it. That is why transfer is the standard process for silicone parts with threaded inserts, sensor inserts, electrodes and any overmoulded hardware.

• Design the insert so the mould can clamp it directly, on a surface that does not have to be covered in silicone.
• Allow 0.05 to 0.1 mm of clearance around the insert where silicone has to flow, and none where it must not.
• Keep the insert away from the sprue entry, because the incoming compound is at its hottest and fastest there.
• Preheat metal inserts to mould temperature, or the compound will cure against a cold surface and bond poorly.
• Check the insert position after moulding, not just visually: a 0.2 mm shift is invisible and out of tolerance.

Tooling: the pot, the sprue and the cull

A transfer tool has three features a compression tool does not: the pot, the sprue channel and the cull. The pot has to hold enough compound for all the cavities plus the sprue and the cull, and its diameter is set by the volume rather than by the part. The sprue has to be short and generously tapered, because silicone is viscous and a long thin sprue will not fill.

Table 2 — The tool features that decide whether a transfer tool works.

The cull is the piece of cured compound left in the pot and sprue, and it is the visible cost of the process. It is also a useful diagnostic: a cull that is fully cured and hard means the pot was hot enough; a cull that is soft and tacky means the compound was not up to temperature and the parts in the cavities will be under-cured even if they look fine.

What goes wrong

Table 3 — Transfer moulding defects and their causes.

Short shots on the far cavities of a multi-cavity tool are almost always a runner balance problem rather than a pressure problem. Raising the transfer pressure fills them, and it also over-packs the near cavities and moves the inserts. Balance the sprue before turning the pressure up.

Choosing between compression, transfer and LSR

The three processes overlap, and the decision is usually made on volume. But volume is only one of four criteria, and the other three can override it.

1. Does the part have an insert that must stay in position? If yes, transfer or LSR, not compression.
2. Does the flash line have to be minimal or absent? If yes, transfer or LSR.
3. Is the geometry one that traps air, such as a deep blind pocket? If yes, transfer with a vent, because compression will trap it.
4. Is the annual volume above about 50,000? If yes, LSR injection, because the cycle is seconds and the labour is zero.

Below 5,000 parts a year, compression is almost always the cheapest route even with the extra waste, because the tool is so much cheaper. Between 5,000 and 50,000, transfer is usually the sweet spot: better parts, less waste, a moderate tool, and a cycle that does not need a dedicated press. Above 50,000, LSR injection wins on cycle time and on the absence of manual handling, and the tooling cost is amortised quickly.