The Importance of Proper Orifice Sizing

A change of just 0.2mm can make all the difference.

Overview

Orifice Sizing plays a key role in determining mix quality of the 2 part polyurethane foam in a Reaction Injection Molding operation. Multiple factors contribute to the overall Injection Pressure of your system, and the orifice is the last stop before mixing.

Each polyurethane chemical supplier provides a datasheet that should provide the ideal injection pressure for your chemistry. Other important considerations would be chemical temperature and mold temperature.

The factors that contribute to Injection Pressure are:

• Chemistry Properties

  • Viscosity at Temperature

  • Density

• Desired Chemical Mass Flow Rate

• Orifice Size

A Word of Caution - False Pressure Readings

On most RIM Systems, the chemical pressure is monitored upstream from the mixing head. It is important to note that the pressure sensor is monitoring the total system pressure. This includes everything upstream from the sensor, such as hoses, valves, flowmeters, fittings, and filters. Any restriction upstream from the sensor will create a back pressure all its own. It's important to remember this when observing your measured injection pressures, as any undesired restrictions will give you a false injection pressure reading.

One way to verify you have very little or no undesired back pressure is to remove the orifice from the orifice assembly in your mix head, and replace the orifice assembly. Once you switch the machine to high pressure circulation, you will be able to see if you have back pressure created by something in your system. If the pressures are the same as, or close to the pressures when in low pressure circulation, then you know the system is not causing false readings, and you are getting the true pressure reading at the orifice.

Determining Proper Orifice Sizing

In order to calculate the ideal orifice size, we will need to use the Bernoulli-based orifice sizing equation. This equation is somewhat complex.

The inputs for this equation are:

• Q = Desired flow rate (grams/sec, lbs/min)

• Cd = Discharge coefficient, incorporating Viscosity

• p = Fluid Density (Grams/CC)

• P = Desired Injection Pressure (PSI)

Using these variables, the correct orifice diameter can be calculated.

Since there are a few nuances that can influence the final calculated amount, MHR has developed an Orifice Calculator that can quickly accept the inputs listed above and output the required orifice diameter. Contact us today with your process inputs, and we’ll provide the correct orifice sizing for your application.

Signs your Orifice is the wrong size

1. Pressure Setting is Difficult to Maintain

   An improperly sized orifice can make it difficult to maintain your desired injection pressures. You may find that the pressure setting is "touchy," where even slight adjustments to the adjuster screw causes large swings in the injection pressure readings. You should be able to adjust your injection pressures to within 10-15 psi of your desired target without much trouble.

   
   

2. Foam Quality/Mix Issues

   There are several signs an improperly sized orifice is causing mix issues. Generally speaking, if you are having trouble diagnosing foam quality, it's always best practice to evaluate your orifice sizing.

   
   

   A few examples would be:

   • "Streaking" or "Swirls" where the components are not mixed properly.

   • Soft Spots

   • Large or Uneven Bubbles/Uneven cell structure

   • Voids/Unfilled Areas

   • Surface Defects

   
   

3. Pressure Setting Cannot be Achieved

   Perhaps you cannot achieved a low enough pressure. This would indicate an orifice that is too small. On the other hand, if you cannot achieve a high enough pressure, your orifice may be too big.

Final Thoughts

Getting your RIM Process dialed in can at times be frustrating. Even slight changes to chemistry, the environment, and process equipment can change the quality of foam and reliability of your equipment. Verifying your orifice size is just one item that can make all the difference in a reliable, high quality operation.