News

December 19 2002

Technical Paper - Coupled Fluid Injection

December 19, 2002
Technical Paper - Coupled Fluid Injection
COUPLED FLUID INJECTION

Incoe Corporation
The process of injection molding is continually presented with new challenges due to its versatility and large potential to improve other processes.  Plastic injection molding has provided countless solutions for many manufacturing requirements.  Within the family of injection molding there are “specialty” processes, Co-Injection, Low Pressure Molding, Overmolding, Coining, and Gas Assist to name a few.  The subject of this paper is focused on the gas assist process. The original gas assist process was developed to core out thick sections of injection molded articles.  This provided savings in the form of reduced material content per part and cycle time reduction as well as other benefits.  Further developments have resulted in the gas assist process being used for improving the surface quality of flat molded articles such as housings and cabinets used in the business machine and television industry.

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This paper will now review a new process to gas assist molding technology. This is a patent pending process referred to as Coupled Fluid Injection – CFI.

Conventional gas assist molding follows the technique of injecting an amount of plastic into the mold and then following with gas, usually nitrogen, to perform the pack and hold function that is normally provided by the injection molding machine.

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When applying the conventional gas assist process to very long molded articles such as automotive trim and structural components, where the material injection point and the gas entry point are required to be at the end of the part (which is often the case), it is difficult to complete a gas core out the entire length of the molded article. The balance of material and gas is difficult to control in this instance. If there is insufficient material in the cavity the gas will burst the part. Increasing the plastic material results in less room for the gas.  One solution to this condition is a patented process known as “spill over”.  A reservoir is added to the mold cavity thus allowing the plastic to be moved by the gas into the reservoir. This allows the gas to penetrate further toward the end of the molded part.  Although this process does provide a solution it adds cost to the mold, increases the cycle time and often the “spill over plug” of material cannot be recycled.

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Along with the advantages of the gas assist process came several new process problems. Mostly these manifest as cosmetic surface defects. Molders learned various process finesse techniques to overcome these new problems. Success of the process generally depends on the gas and plastic to enter the cavity at controlled rates during the molding cycle. In order to meet these process challenges gas control systems have had to become more sophisticated in order to deliver control pressure and time sequences consistently. Other techniques such as altering melt temperature, injection pressure, and back
pressure of the molding machine, and mold temperature all have been used to produce improved results.

In many instances less gas penetration than originally desired is used in order to produce a cosmetically acceptable part. This of course defeats the original intention.

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A new gas assist process, Coupled Fluid Injection, and a control device specifically designed for that purpose have been developed to improve these and other problems associated with gas assist molding.  The CFI process is unique from conventional gas assist as it insures the simultaneous injection of plastic and gas into the mold cavity.  All other gas processes must have all of the plastic in the mold before the gas enters the cavity. 

The injection of the gas into the plastic stream is controlled to start at any point during the injection of the plastic into the

 

mold.  The gas can be stopped before all the plastic is injected into the mold or just as the injection of the plastic into the mold stops.

The gas injection is powered by the same pressure source used to inject the plastic
into the mold.  Therefore, the pressure of the plastic injection is exactly the same as the gas injection pressure.  Note:  Neither the plastic pressure nor the gas pressure can dominate the other.  Result is a simultaneous injection of plastic and gas or Coupled Fluid Injection.
It is difficult for the conventional gas assist process to produce a uniform wall section.  The CFI process, however, produces a uniform wall section as a result of the simultaneous control of a compressible (gas) and a non-compressible (plastic) fluid.  Unlike conventional gas assist molding the CFI process allows the gas an opportunity to uniformly pressurize the melt while it is
entering the cavity.  This is accomplished by the controlled metering of the desired volume of gas during plastic filling.  The gas pressurizes the melt evenly since they both flow in the plastic part at the same speed.

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To begin the  process a calculation is made to determine the total volume of the cavity(s). If the desired outcome is to produce a part with a 10% material reduction which would be the gas, then the corresponding plastic volume would be 90% of the total volume. The gas
volume is pre-charged and enters the cavity during the injection of the plastic at
the identical pressure produced by the injection molding machine. Since the flow
of both fluids (gas and plastic) is not interrupted during the filling and packing phase of the cycle, witness marks (hesitation, shadow, and permeation) that can occur during standard gas assist molding are all eliminated.

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The precise volume of gas injected into the flowing plastic dictates the final gas pressure within the cavity.  Using the example of a molded part to be produced containing 10% gas and 90% plastic, the entire volume of plastic must be accumulated in the injection cylinder of
the molding machine.  The remaining 10% of the cavity volume being the N2 gas is pre-pressurized to match the pressure of the injection molding machine’s injection pressure that is required
 to fill the cavity.  This gas is stored in a fixed volume chamber designed for this process.As an example, if 5,000 psi is the required pressure for the injection of plastic in our example of 90% plastic, then the gas pressure of 10% must also be stored at 5,000 psi.  This will result in the pressure within the molded part to be 5,000 psi.

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If it is determined the gas pressure is too high then only 5% of the gas is stored at 5,000 psi.  Since the result is now 95% of the total cavity volume the pressure in the molded part will be less or 2,500 psi.

Correspondingly, the opposite is true, if more than 10% of the gas volume is accumulated, as an example 20%, then resulting pressure in the molded part will
be higher or 10,000 psi since only 10% was made available for gas.

The ability to accurately control the ratio of plastic to gas in a simple and fast manner makes it possible to easily arrive at a ratio of plastic to gas producing high quality molded parts.

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The preferred method of using the CFI process is combined with a patented hot runner nozzle or bushing that controls both the plastic and gas injection at the gate. This eliminates the need for an injection molding machine shut off nozzle and eliminates a sprue as well as the potential for plastic leakage.  The precise opening and closing, or on/off selection of plastic and gas flow is controlled by a uniquely designed valve gate mechanism.

Venting of the gas from the molded part is accomplished through the same valve pin in which the gas entered the part. The gas can then either be exhausted to atmosphere or reclaimed.
 

This combination of the CFI process and the patented gas/plastic hot runner molding system allows the successful operation of multiple cavity molding.

ADVANTAGES

1.    Precise control of plastic to gas ratio
2.    Use same power source for injecting plastic and gas
3.    Ease of process set up
4.    No costly maintenance of gas booster
5.    All functions of gas and plastic are controlled by linear position
6.    Cosmetic surface defects are eliminated, i.e., sink, shadow and permeation marks
7.    Unlike conventional gas assist, the injection of plastic does not stop or hesitate until the cavity is full, due to the simultaneous injection of gas/plastic
8.    No oil in nitrogen as is experienced with piston pumps
 
9.    Precise control of gas volume and pressure.  The amount of gas accumulated at a pre-determined pressure assures a quality molded part, as well as the desired weight reduction
10.    Multiple cavity hot runner systems are now possible
11.    Class “A” surface finish is assured due to the even distribution of the gas pack pressure
12.    Consistency in the part weight reduction is as good as the injection molding machine’s ability to be repeatable since the gas is always at a fixed volume



Incoe is the owner of this technology and has applied for patent protection on the process and apparatus.

About INCOE® Corporation
Since 1958, INCOE® has engineered productivity built hot runner systems starting with their original patented design of the first commercial hot runner nozzle. Today, a wide range of nozzles and manifolds, pre-wired unitized systems, complete hot halves and advanced control technologies provide optimized systems suitable for appliances, automotive, electronics, medical disposables, packaging and technical markets. A network of representatives in over 35 countries are supported by INCOE® facilities located in the United States, Germany, Brazil, China, Hong Kong and Singapore. Wherever your molding operation is, INCOE® can support your business with complete hot runner systems engineered for your application. That's
INCOE® Hot Runner Performance.



For more information:

INCOE® CORPORATION
1740 E. Maple Road
Troy, MI 48083
T (248) 616 0220
F (248) 616 0225
e-mail: info@incoe.com

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