Preventing Flash at High Clamping Forces in Compression Molding

Turning the press up until the flash disappears is how you print the parting line into the steel and still flash. Compression molding of millable silicone (HCR) needs clamp to close the land on a viscous preform. It also needs a mold that stays flat while you do that. When the plates crown, the center can kiss while the corners leak, or the reverse. You then add more force, the crown grows, and the flash moves.

This is HCR compression (and transfer when the pot still dumps into a compression-style cavity). It is not LSR injection flash, which is a different land and a different pressure source.

What flash is in a compression tool

Flash is rubber that left the cavity through a gap that should have been a land. Causes, in the order a technician should check:

1. Charge too heavy. Extra gum has to go somewhere. If the overflow wells are small, it goes into the land and out.
2. Land dirty, nicked, or too narrow. A 0.1 mm ding is a runner.
3. Preform in the wrong place. A thick biscuit on one side opens the other side.
4. Scorch too fast or flow too late. The skin sets, the core still wants to move, and it lifts the plate for a moment. That moment is a film.
5. Plate deflection under clamp. The high-force case. The press is doing what you asked. The mold is bending.

Only item 5 is fixed by a different clamp *if* you were under-clamped on a stiff tool. Items 1-4 get worse if you only add bar.

High clamp as a cause, not a cure

A compression mold is two plates (or a book) with cavities and a land. Clamp force is meant to keep that land closed while the gum flows and while it vulcanizes and shrinks. If the plate is thin, the cavity layout is a big open area in the center, or the press platens themselves are worn and out of parallel, the stack becomes a spring.

Under a high load:

• The plate dishes. Center land over-crushes (you start to coin a step into the steel). Edges open. Edge flash.
• Or the press rams on the center and the corners of a large plate never see load. Corner flash.
• Overflow that should have been a designed well becomes a thin film that vulcanizes into a skirt you cannot destage without tearing the part.

You will also see thickness variation: thick in the open zone, thin where the land crushed. That variation is a compression-set and seal problem later. It is a clue now.

A transfer pot adds a second force: transfer pressure. If transfer is high and clamp is marginally able to hold the cavity plate, you flash at the cavity while the pot looks fine. Raising clamp blindly without looking at transfer is the same error.

Land and overflow that make high clamp useful

Land width and finish. Wide enough to take the load without cutting the plate, flat enough to seal, vented only where you intend. A knife land flashes and dies. A wide, dirty land flashes and looks like you need more force.

Overflow wells / gutters. Give the extra charge a place that is still in the mold. Wells should fill *after* the cavity. If the well fills first, you designed a short. If there is no well, you designed a flash skirt.

Stops or load buttons. Hard stops outside the rubber land can take excess clamp so the land sees a designed pressure. Without stops, every extra bar goes into coining rubber and steel.

Plate thickness and support. Rib the back. Pillar the center of a large multi-cavity plate. A pretty thin book mold that worked at 50 t may not work at 150 t. The tryout that “needed more clamp” is often a stiffness report.

Parallelism. Check platens with feeler or carbon paper on a clean land, at the clamp you actually use, not at the bench. A 0.05 mm taper across a gasket cavity is a flash machine.

Charge and cure window

Weigh preforms. A “handful” of HCR is not a process. Heavy charges at high clamp still flash; they just flash later in the cycle and bake into a thinner film that is harder to see until destage.

Preheat and placement so flow happens before scorch. If you need extreme clamp to hide a late-flow film, you are late, not under-clamped.

Vent air. Trapped air can hold a land off for the same instant that looks like flash. Vents to wells, not to the operator.

A tryout that does not ruin the land

1. Clean the land. No leftover flash as a “gasket.”
2. Start with the calculated charge and the clamp the tool was designed for, not the press maximum.
3. If flash is even all around, reduce charge or open wells. Do not add 20 t.
4. If flash is on one edge, check preform placement and platen parallel. Then check that edge’s land for a nick.
5. If flash is at corners only under high clamp, the plate is bending. Add support or reduce the area you are asking the plate to hold, or cut true stops. Do not keep climbing.
6. If the part is thick in the middle and the land shows a coined shine, you already over-clamped. Stop. That shine is damaged steel.

Destage will not save a coined land. The next lot will flash with less rubber because the steel now has a path.

What to put on the compression RFQ

Process HCR compression (or transfer), number of cavities, press tonnage range, land and overflow in the design, charge weight as a controlled dimension, and a ban on “flash free via clamp only” as a tryout plan. Ask for a first-article thickness map, not only a pretty edge photo. High clamping force is a tool that has to land on a stiff, clean, designed land. It is not a substitute for one.

FAQ

Why doesn't turning the press up until flash disappears fix HCR compression?

More clamp on a bending plate coins the center land and opens the corners, or the reverse. You then add force, the crown grows, and the flash moves. This is millable silicone compression (and transfer into a compression-style cavity), not LSR injection flash.

What should a technician check before blaming clamp?

Charge too heavy, nicked or dirty land, preform on one side, scorch that skins while the core still lifts the plate, then plate deflection. Only true under-clamp on a stiff tool is fixed by more force. Items above get worse if you only add bar.

How do overflow wells and load stops make high clamp useful?

Wells give extra charge a place after the cavity fills; no well is a flash skirt. Hard stops outside the rubber land take excess clamp so the land sees a designed pressure. Thin book molds that worked at modest tonnage may not work at high tonnage; the tryout that "needed more clamp" is often a stiffness report.

Why is a handful of HCR not a process at high clamp?

Weigh preforms. Heavy charges still flash; they flash later as a thinner baked film. Preheat and placement so flow happens before scorch. If you need extreme clamp to hide a late-flow film, you are late, not under-clamped. Check platen parallel at the clamp you actually use.

What tryout clue means you already damaged the steel?

The part thick in the middle and a coined shine on the land. Stop. Destage will not save that path; the next lot flashes with less rubber. Even flash all around: reduce charge or open wells, do not add tens of tonnes. Corner-only flash under high clamp: the plate is bending.