Compact or Grenade: which pneumatic vent for your polyurethane mold?

A pneumatic vent, also known as a pneumatic degassing valve, is the cylinder that releases the gas trapped in the foam at the moment of injection. This guide covers the criteria actually used in our workshop to point a foam-molding supplier toward the right reference, including when the mold has never been fitted with one.

Compact or Grenade: what really changes between the two?

Overall size, first and foremost. A CF (short for carré facétisé, faceted square) and an NG (nouvelle génération, new generation) do exactly the same degassing job. The NG was created precisely to solve the space problems caused by the round body of the earlier generations. Two dimensions drive the choice: the bore, i.e. the cylinder’s internal diameter, and the stroke, i.e. the distance the rod travels on each cycle.

The typical case we see at customers’ plants: a mold topped with a cover. The available height comes down to a few centimeters, and the 19 mm difference between the two ranges is enough to settle the matter. If the Compact pneumatic vent range won’t fit, the Grenade pneumatic vents will, and they deliver the same degassing result.

Which diameter for which defect?

The diameter is determined by the volume of gas to be evacuated, not by the size of the part. The foam is injected in liquid form, expands, then sets: the defect appears in that final phase. The bigger the defect, the greater the volume of gas to evacuate, and the larger the diameter needs to be.
You don’t put a Ø6 in front of a 10 cm³ bubble. Conversely, the small diameters, Ø6 and Ø8, are designed for hard-to-reach areas: typically the corners of a seat cushion, where a bulkier body simply wouldn’t fit.

0.10 or 0.20 mm flats: how do you decide?

The flats are the machined flat surfaces on the rod (7, 8 or 12 of them depending on the reference) that let the gas escape without letting the foam through. Their depth sets the degassing speed.

• 0.10 mm: more gradual degassing, limited risk of foam leakage. Suited to the most fluid foams.
• 0.20 mm: faster, more intensive degassing, at the cost of a higher risk of foam escaping through the cylinder.

The right setting depends on two things: the fluidity of the injected foam and the amount of gas to be evacuated within the allotted cycle time. That principle is what gave rise to the Event Pilote Grenade Ø6.00 NG degassing valve, whose flats were deepened by a tenth of a millimeter at a customer’s request. It is also the question we hear most often from suppliers discovering automated degassing.

What actually happens when the vent isn’t the right one?

Two scenarios, mirror images of each other, and both visible on the part:

• Degassing too slow: the defect stays in the foam. It is smaller, but it is still there, and it will show through the fabric.
• Degassing too fast: the foam escapes through the cylinder.

The sweet spot is not found in a catalog. It is found when the chemist and the production operator agree on the degassing speed to aim for relative to how fast the foam sets.
This is also what separates a selection problem from a maintenance problem: an unsuitable vent drives the scrap rate up from the very first production runs, whereas a fouled vent degrades gradually, as detailed in our advice on maintaining degassing valves for polyurethane molds.

Why can a well-chosen vent become unsuitable?

Because PU foam chemistry evolves. With a supplier that is already equipped, there is rarely a genuine selection error: the references have been validated for years. The real cause of drift is a change in formulation.
The more fluid the foam is when injected, the more it tends to escape through the cylinders with identical degassing. This phenomenon can only be detected in real conditions, on the line: the degassing valve manufacturer does not have the foam’s formula and has no visibility into it.
If the foam changes and defects appear, flat depth is the first parameter to reconsider, before the degassing system itself.

How do you validate a choice on a mold that has never been equipped?

By handling the cylinder on the mold in question. For a new supplier, or for a mold of a type never produced before, JMP Assemblage can send a cylinder to the production technician. A recent case in point: a railway seat manufacturer whose molds had not been designed for automated degassing. The cylinder we sent made it possible to place it on the mold, handle it and decide on the layout, something no drawing on a screen allows.
As for mounting, our degassing valves for polyurethane molds are fitted using a sleeve: a hole is tapped in the mold, and the cylinder screws in and locks into place. This is the second question that comes up systematically with a new customer, along with the one about flats.

• The clearance height above the mold, cover included.
• The thread and stroke of the vent currently in place.
• A photo of the mounting area.

These three elements can be checked directly against the dimensions published in our degassing valve catalog.

FAQ: choosing the right pneumatic vent

Does a Grenade degas less effectively than a Compact?

No. Both ranges perform identically. The Grenade was developed for molds where space is tight, particularly those fitted with a cover, with no loss of degassing performance.

How much space do you save with a Grenade?

Around 19 mm at equal stroke: 141 mm in overall length versus 160 mm for a Compact on a 50 mm stroke. The body cross-section also drops from 41 to 36 mm depending on the reference.

What do the abbreviations CF and NG stand for?

CF stands for carré facétisé, French for “faceted square”: a square cylinder body paired with a flat-sided rod. NG stands for nouvelle génération, meaning “new generation”: the range that came after the round bodies to solve space problems inside molds.

What is the difference between 0.10 and 0.20 mm flats?

A 0.20 mm flat degasses faster and more intensively. It carries a higher risk of foam leaking through the cylinder. The choice depends on the foam’s fluidity and setting time.

Can the vents withstand the mold’s temperature?

Yes. Molds operate at around 70 to 80°C. The cylinders are fitted with nitrile or Therban seals depending on the version, elastomers chosen to withstand these temperatures in 24/7 operation.

How are the vents fixed into the mold?

Using a sleeve. A hole is tapped in the mold; the vent screws in and locks into place. Thread dimensions are published for every reference in the catalog.

Can you test a vent before ordering a batch?

It is possible for a new customer, or for a mold of a type that has never been equipped. Sending a cylinder allows the production technician to position it on the mold and validate the layout. This option is assessed on a case-by-case basis.

The right vent is the one that fits your mold and keeps up with your foam

There is no single best pneumatic vent in absolute terms. There is the clearance height above your polyurethane mold, the volume of gas to be evacuated, and the fluidity of your foam. Those three inputs determine the range, the diameter and the flat depth. The rest, assembly and packaging, is done in our Colmar workshop, whose floor space was quadrupled in October 2022 to keep pace with demand.
Hesitating between two references, or dealing with a foam whose formula has changed? Describe your mold and your problem to JMP Assemblage on +33 (0)3 89 73 83 83 or +33 (0)6 89 33 61 43, or via the contact form. Your quote goes out within 48 hours once the reference has been determined.