E-News Update
November 08
LEAD FEATURE
COMPANY NEWS

> Mixed bag of results

> Macchi loses a key figure

Materials News
MaCHINERY News
PROCESSOR REPORT
EXHIBITION NEWS
RUBBER JOURNAL ASIA
INJECTION MOULDING ASIA
POLYURETHANES ASIA
 

 

Polyurethanes Asia

Jet-less mixing
Based on its three decades of experience, Italian company Cannon has, in its high pressure mixing head, made a mixing chamber small enough to overcome the damping effects of the laminar shear.

How does it work?
The mixing of a minimum of two fluids is normally performed through turbulence. There are many ways to create or maintain turbulence: static mixers, dynamic mixers, high speed streams and jets, impingement of jets and streams. All of these methods implement a different way to maintain a high level of energy dissipation throughout shear into the fluid. However, static and dynamic mixers require cleaning by proper flushing with solvents. The other two methods can be performed using a proper mixing chamber and a spool piston that cleans the mixing chamber and controls the transients at the start and the end of pouring.

A new family of high pressure L-shaped mixing heads known as JL (Jet-less) has now been introduced. This latest method involves decreasing the size of the mixing chamber enough to generate a flow speed of one-tenth of a metre/second and using the front shape of the spool (the cylindrical piston that seals the mixing chamber) to create high shear restrictions. This mixing head has an L-shape geometry, which allows the reduction of turbulence of the flow that exits from the mixing chamber and pours it in a laminar form.

In the JL, the mixing is performed by the combination of two co-axial chambers: in the first annular chamber the two reactive liquids are delivered by proper metering devices. A large spool with a V shape front fills the annular mixing chamber. Grooves carved on the spool surface re- circulate the liquids back to the tanks when the head is not operating. By hydraulically driving the spool backwards, an annular mixing chamber is created – a  hollow cylinder with V shaped top and bottom faces. The liquids flow into the chamber at the same pressure. The size of the V-shape common chamber and of the related spool can be set as required. 

Chamber size
What is important is the size of the cylindrical mixing chamber, which has the same centre line of the spool and of the related cleaning pin. This has a cylindrical shape and slides free along the centre line of the spool. The pin front – combined with the V-shape faces – creates a double action. The liquid components flow through these restrictions and reach in twin streams in the mixing chamber where they mix at a high speed. The sharp edged shape and the component’s pressure improve the mixing turbulence, which is combined and maintained along the mixing chamber.

The centre pin is hydraulically controlled backward to perform the mixing or forward to clean the mixing chamber at the end of the injection. The front position of the pin can be set manually, using a turning knob featuring a very fine adjusting screw, to define the restrictions and adapt them to the flow rate so as to create and maintain the pressure in the V-shape chamber. When the spool is closed against the corresponding V-shape faces, the mixed blend is squeezed out from the chamber and this is cleaned.
Some features of the new head are:
• the length of the V-shape chamber is long to permit re- circulation and delivery
• the mixing chamber is also long to allow for efficient  mixing
• the mixing chamber tapers into a much wider and long delivery chamber at a 90 degree angle downwards.

Long nose does the job
The JL features a long and wide discharge duct. Its main purposes are to dump the high speed and turbulence of the stream, to perform the final mixing and to smooth down the whirling of the stream. After leaving the head, the stream becomes laminar and can flow out into the mould with the ideal behaviour. Its length enables deep injection points.

The discharge duct and the self-cleaning rod have specific features:
• the duct is formed by a long bushing that can be replaced easily with other models with different length and diameters
•  the self-cleaning rod has a dual diameter: a larger front with a diameter that matches precisely the duct 

The up and down movement of the rod – for opening and closing the duct –removes and pushes out the foam residuals still in the pouring chamber, stripping out the reacted film from the walls. 

In the meantime, the rod acts like a pump for flushing and re-circulating a small quantity of lubricant oil, contained in a small spacer chamber built behind it. The relatively small sliding surface of the rod prevents the surfaces from sticking together and permits the use of a slim cylinder so the size and weight of the mixing head are reduced and it can be ergonomically handled for manual operations.

The three hydraulically-driven movements are controlled by three valves installed directly on the mixing head. This means that the standard mixing heads can be replaced with the JL without any modifications to the control system. The mixing system, mixing head itself and the cleaning system are patented.

First on the market


The first JL head available on the market is the 24/6, featuring a 200 mm long discharge duct, with an internal diameter of 24 mm and a mixing chamber diameter of 6 mm. This head, which has an output of 300 to 2,000 cc/second, works with two components but more streams can be added. A highlight of this equipment is its 70 to 210 bar pressure, allowing for a simplified dosing unit (pumps, filters and hoses). According to Cannon, these heads have been commercially tested with rigid and flexible foam for more than a year and with frequent shots (up to 60,000/month).

 

Advantages of the JL
•  an efficient mixing method: the higher efficiency has been computer-studied and simulated and confirmed by field tests
• the mixing pressure can be reduced to 70 or 80 bar, depending on the materials used. This allows for a simplified metering unit
• a wider range of flow rates can be handled by the same mixing chamber: the maximum output can be five times higher than the minimum
• skilled operators are not required to set the head’s injection conditions: an easy setting of the centre needle suits a variety of flow rates; pre-positioning of different values can be done manually or in an automatic mode
• a very long discharge duct (patented) can enter pouring holes that are difficult to access and allows for a laminar flow at high flow rates, which is suitable for discontinuous insulation panel and for pipe-in-pipe insulation
• the use of the thin and long cleaning rod comes in useful when using very sticky formulations
• the internal geometry allows for a better handling of foams that use high-froth blowing agents like HFCs or liquid CO2

 

 

 
 
 
.Terms & Conditions | Privacy Policy | PRA Home Page | PUA | IMA | RJA | Contact Us