{parse block="google_articles"}Resin is fed to the machine through the hopper.   Colorants are usually fed to the machine directly after the hopper.  The resins enter the injection barrel by gravity though the feed throat.  Upon entrance into the barrel, the resin is heated to the appropriate melting temperature.

The resin is injected into the mold by a reciprocating screw or a ram injector.  The reciprocating screw apparatus is shown above.  The reciprocating screw offers the advantage of being able to inject a smaller percentage of the total shot (amount of melted resin in the barrel).  The ram injector must typically inject at least 20% of the total shot while a screw injector can inject as little as 5% of the total shot.  Essentially, the screw injector is better suited for producing smaller parts.

 

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Figure 1: Diagram of Injection Molding Machine	Â	Figure 2: Photo of Injection Molding Machine	Â	Figure 3: Diagram of Heated Screw Conveyor

The mold is the part of the machine that receives the plastic and shapes it appropriately.  The mold is cooled constantly to a temperature that allows the resin to solidify and be cool to the touch.  The mold plates are held together by hydraulic or mechanical force.  The clamping force is defined as the injection pressure multiplied by the total cavity projected area.  Typically molds are overdesigned depending on the resin to be used.  Each resin has a calculated shrinkage value associated with in.

Some Typical Complications

Burned or Scorched Parts:  Melt temperature may be too high.  Polymer may be becoming trapped and degrading in the injection nozzle.   Cycle time may be too long allowing the resin to overheat.

Warpage of Parts:  Uneven surface temperature of the molds.  Non-uniform wall thickness of mold design.

Surface Imperfections:  Melt temperature may be too high causing resin decomposition and gas evolution (bubbles).  Excessive moisture in the resin.  Low pressure causing incomplete filling of mold.

Incomplete Cavity Filling:  Injection stroke may be too small for mold (ie. not enough resin is being injected).  Injection speed may be too slow causing freezing before mold is filled.

Diagrams courtesy of Plastics: Materials and Processing, Prentice Hall by A. Brent Strong

Dilute Phase versus Dense Phase Conveying

The specifying engineer typically has four choice in specifying a pneumatic conveying system.

1. Dilute phase vacuum operation{parse block="google_articles"}
2. Dilute phase pressure operation
3. Dilute phase pressure-vacuum operation
4. Dense phase pressure operation

Vacuum systems allow multiple product inlets through the use of simple diverter valves. However, it becomes costly to have multiple destinations because each must have its own filter receiver with partial vacuum capability. Vacuum systems are also more "distance sensitive" than pressure systems due to the maximum pressure differential of 5.5 to 6.0 psi. Dilute phase pressure systems can easily achieve pressure differentials of 12 psi. Pressure-vacuum operation (utilizing both methods) are sometimes ideal for a given conveying setup. A very common application is the unloading of a standard railcar. Since the cars cannot be pressurized, air is pulled from the outside, through the car (carrying solids with it) to a filter. Then after the filter, a blower can be used to forward the solids to the final receiver.

The choice between dilute and dense phase operation is typically dependent on the solids properties. For example, the lower velocity bulk phase operation is popular ofr highly abrasive products or for those that degrade easily. For example, this method is popular in transporting kaolin clay.

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Figure 1: Dilute Phase Pressure Conveying	Â	Figure 2: Dilute Phase Vacuum Conveying	Â	Figure 3: Dilute Phase Pressure-Vacuum Conveying

Designing a System for Dilute Phase Operation

Considering designing a pneumatic conveying system yourself? Probably not a good idea. There's as much art involved as there is science and such a design should be left to professionals. Consider that even different grades of the same material have been known to convey differently. Testing is a must (as you'll see from the method below). Before you can even make any good judgements from the method presented here, you need to know solid friction factor for your solids (which we'll discuss later) and the minimum gas velocity required to move your particles. So, if you're involved in designing a system from the ground up, seek assistance from reputatable people in the field of conveying. If you're already familiar with your solids, the method below can be used to examine the pressure loss expected in your system. The method presented here is very good and has been stood the test of real systems over time.

The design method presented here is based on the work of Dr. F.A. Zenz and Dr. D.F. Othmer as published in their book "Fluidized and Fluid Particle Systems" published in 1960 (see References). This method was presented by A. T. Agarwal of Pneumatic Conveying Consulting Services LLC in the January/February 2005 issue of Powder Handling and Processing. This method has been widely used and is generally found to be within 10% of measured pressure losses.

Pressure losses experienced in pneumatic conveying systems are the result of the following forces:

Friction of the gas on the inside of the pipe, force required to move the solids through the pipe, forces required to support the weight of the solid and the gases in vertical pipe runs, force required to accelerate the solids, and friction between the solids and the inside of the pipe.

Friction losses as the result of the solids being in contact with the inside of the pipe are usually very small and can be neglected when considering dilute phase transport.

Nomenclature

Early is their work, Zenz and Othmer used the following equations to described the pressure losses in horizontal and vertical pipes:

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It was later concluded that the term (f_p V_p/f V_g) should be replaced by the constant K (friction multiplier for the solids conveyed) because the term was dependent upon the physical properties of the solids being conveyed. The value for K must be back-calculated from actual pressure drop data from an existing system or it must be determined experimentally in plant tests. This lead to the following equation for the pressure drop for the solids in the system.

Thus, the equation used for the total system pressure loss (Base Equation) is:

where:

The Base Equation is the basis for the following equations used in the pressure loss calculations.  The pressure loss is expressed in psi or lbs/in² in the following equations.

Eq. (1)

Eq. (2)

     (Modified Zenz Equation)

Eq. (3)

Eq. (4)

Eq. (5)

= Estimated misc. losses from the systemÂ

Eq. (6)

The Fanning Friction Factor

In order to determine the fanning friction factor, f, the Reynolds Number must first be calculated:{parse block="google_articles"}

where ?_g is the gas viscosity in lbs/ft s.

Then, the friction factor is calculated from the following equation derived from pages A-23 and A-24 of Crane's Technical Paper No. 410:

Where ? is the pipe roughness factor which can be estimated as 0.00015 for smooth pipes or 0.0005 for shot-peened pipes.

Pipe Equivalent Length

For straight pipe runs (either horizontal or vertical) use the actual length of the pipe.  For bends and other devices, use the table below as a guide:

Solids Velocity

Solids also move at a velocity lower than the gas velocity due to drag forces.  The difference between these velocities is called the slip factor.  For most course or hard solids, the slip factor is around 0.80. 

For fine powders, the solids velocity can be closer to the gas velocity and a factor of 0.90 may be more appropriate.  Depending on the size of the particles, the slip factor can range from 0.70 to 0.95.

Solids Velocity in Long Radius Bend

For a 90° radius bend, the solids velocity at the exit of the bend (V_p2) is around 0.80 times the solids velocity at the inlet of the bend (V_p1).  This factor can range from 0.60 to 0.90 depending of the properties of the solids.  For bends that are less than 90°, the exit velocity may be expressed as:

After leaving a 90° bend, use a nominal value of 20 pipe diameters for the length of pipe needed for the particles to accelerate back to their original velocity at the inlet of the bend.

Solids to Air Ratio

The solids to air ratio is calculated as:

In the above, m is the the solids mass flow in lb/s and A is the pipe cross sectional area in ft².  Remember that the gas density (and therefore velocity) will change throughout the system (we'll discuss this in more detail later).

When calculating the gas velocity, reduce the gas mass flow rate by 5% for pressure systems to account for gas leakage through a rotary valve if such a valve is used to feed the solids.

Gas Density and Pressure Loss

As the carrier gas moves through the system and losses or gains pressure (depending on whether the system is a pressure or vacuum system), it's density will increases or decrease accordingly thereby changing the velocity of the gas.   The gas pressure loss equation is shown in Equation 2 above.  The ideal gas law may be employed to calculate the density of the gas throughout the system.

Setting Up the Calculation

The method presented here can easily be set up in a spreadsheet to show the performance of your system.  It is recommend the divide the entire system into 5 to 10 ft sections for such a calculation.  The exit conditions of from section become the inlet conditions for the next and so on.  Also, be sure to account for any gas losses that are known and for any extra gas inlet points in the system.  For pressure systems, start from the end of the conveying line and return to the solids inlet point.  For vacuum systems, start from the solids inlet point and end at the blower inlet.

References

1. Zenz and Othmer, "Fluidized and Fluid Particle Systems", Chapters 10 and 11, published by Reinhold Publishing Corp, 1960
2. Agarwal, A.T., "Theory and Design of Dilute Phase Pneumatic Conveying Systems", Powder Handling & Processing, Vol. 17, No. 1, January/February 2005
3. Kimbel, Kirk W., "Troublefree Pneumatic Conveying", Chemical Engineering Magazine, April 1998, page 78