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December 19 2002

Technical Paper - Co-Injection and Sequential Molding

December 19, 2002
Technical Paper - Co-Injection and Sequential Molding

Co-Injection And Sequential Molding
Process and Control Technology
 
 
 
Incoe Corporation
Co-injection molding has realized a “rebirth” with recent improved technology allowing the processing of engineering resins.  The benefits of this process provide properties which improve part performance that would otherwise not be possible without co-injection.  Additionally, engineering materials are typically used to produce components with structural or cosmetic requirements that could not be successfully molded without the process of sequential molding techniques.  Improved programmable valve gate control technology combined with in-mold co-injection hot runner systems are now available to produce molded parts with superior performance that could not be achieved using a conventional molding process.

Examples are as follows:

Co-Injection Benefits

Molded Parts Can Be Produced Less Expensively

Recycled or off “Spec” material can be used
Minimized use of colorants
Potential cycle reduction

Part Performance Advantages

Elastomeric skin encapsulates rigid core
Brittle material can be strengthened with high impact resin skin
Class “A” surface skin with structural core
Skin material with inherent lubricity over rigid core for molded gears
E.F.I. shielding properties using a core material with metallic content for use in electrical applications
Electric components with conductive skin material over less expensive core material
Gas barrier properties for food and beverage containers and closures

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The advantage realized by utilizing the co-injection process is to encapsulate an inner  material or core by the resin normally used for the molded part as the skin material.  The  benefits of co-injection using a dissimilar core material, is either cost savings, performance improvement, or both in the part to be produced.  As with all molding applications, consistency is critical.  Maintaining the proper skin-to-core material relationship is a process requirement and is the criterion for good quality production using co-injection molding.
Co-injection processing is not a new technology, but has had limited success in the past, particularly with engineering grade materials.  Recently, the process has been dramatically improved with the introduction of a patented material mixing pin.  Skin material is directed to flow across the face of the pin.  This design interrupts the flow path and reconfigures the molecular characteristics to properties similarly produced in the extrusion process.  Uniformity of the skin material’s layer is then achieved and maintained, thus eliminating the tendency for core material “breakout”.  This technology has been made available by Bemis Manufacturing.  Their many years of processing “know how” in both injection and extrusion techniques has produced this innovation.
The co-injection process begins with skin material filling the cavity to approximately 40-50% of the total volume.  Core material is then introduced.  The laws of physics control the direction of the core flow path, which is the center section of the skin material.  Skin material fill rate is then reduced, while core material enters the cavity at normal velocities for the material being processed.  At the completion of core material fill, skin is once again allowed to flow, completing encapsulation of the core material.  Finally, pack and hold pressure is applied by the injection unit controlling the skin material
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In order to produce parts in multiple cavity molds or parts which are multi-gated, a unique hot runner valve gate system incorporates a uniquely designed tandem cylinder which creates three discrete positions.  Skin and core material flow are controlled by this mechanism.  Since the design provides an on-off type condition, the process is extremely accurate and repeatable.

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The process begins by the initial phase of skin fill when the valve gate pin is retracted to the first position created by energizing ports 1 and 3 shown on the far left of the schematic.  After the initial skin material enters the cavity, core material is introduced by de-energizing port 3, while port 1 remains energized (shown in the middle of the schematic).  At the completion of the core filling sequence, skin material is once again allowed to flow, completing encapsulation and final pack pressure by repeating step 1. 
   
Sequential molding techniques are not new.  This technology has been used for a variety of molding applications with many systems in operation for more than 20 years.  The original goal of this process was to remove or predictably position weld lines in parts that require multiple gates.  Later demands, mostly driven by the automotive industry, lead to other applications which require sequential molding techniques.  Producing interior components with a fabric or vinyl material applied during the injection process, generally referred to as “back molding”, is an example where using sequential molding techniques is absolutely necessary for good quality production.

Incoe pioneered the sequential molding process by introducing a position positive valve gate system in 1979. By combining a new improved Valve Gate Sequence Control (GSC)™ with co-injection technology,
processors have the ability to attain class “A” surface finishes coupled with enhanced part performance advantages.  The following schematic displays a filling sequence for a multi-ported sequential co-injection molded part.

Skin material fills at gate 1.  When the skin material fills to a point centered between gate 1 and 2, core material is allowed to enter the cavity.  When the flow front of skin material passes gate 2, core material is stopped, and skin material is allowed to flow from gates 1 and 2.  When core material is allowed to flow from gate 2, all material is stopped from gate 1. This sequence is repeated and moves in the direction of the flow path until the part is full and ready for pack pressure to be applied.

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Critical to the success of sequential molding is the exact and reproducible firing of each valve gate pin. Up until now the process sequence has been controlled by timers.  The new linear controlled Valve Gate Sequence Control™ System provides selective on/off programming of each valve gate pin based upon the injection molding machine’s twin screw position.  Linear velocity displacement transducers (LVDT) are placed on each
injection unit for instantaneous material flow information. 

A touch screen operator interface allows users to program the valve gate open/close
sequence, which is stored in the system memory for the next production run.

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The control is designed with a self-contained hydraulic system.  Instantaneous response to the signal for energizing the valve gate pin is critical to consistency and part quality.  The hydraulic system design incorporates an
accumulator for this requirement.

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A variety of molding conditions can be achieved with this design for multiple cavity molds as well. 

By simply controlling the valve gate pin sequencing, the following process combinations are achievable:

Each cavity contains skin and core material (standard concept)

 Some cavities contain either skin or core material

One cavity may have only skin material, the remaining cavity may have skin and core materials (or any combinations or sequence)

Co-Injection and over-molding are possible within the same cycle

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The hot runner manifold conveys both materials from each injection unit keeping them separate until arrival at each bushing where they are independently controlled by the valve gate system.  Flow analysis is used to assist in the design of the flow channels to ensure balance Generally it is recommended that both materials have similar melt temperature conditions; however, specially designed manifolds which isolate the temperature of each flow channel can be used to accommodate two different melt temperatures.
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Sequential co-injection process technology has recently been put to use in an application for reusing post industrial painted automotive parts such as bumpers.  During the manufacture of automotive fascias the finish paint coating on the molded part can produce a result that is not suitable as a finished product.

Since these scrap pieces have been covered in paint they could not be reused and typically went to a landfill.  However today, the co-injection process provides an avenue for the use of this painted plastic material which previously could not be recycled and compliments the goal to produce products that are friendly to our environment.

The requirements aside from the value of reducing the burden on landfills were as follows:

Minimize any costs associated with using the scrap material.

Avoid any costs associated with using the scrap material.

Avoid any reprocessing of the scrap except for the regrinding process.

 The finished product must have the appearance and performance of the original product.
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A series of tests were conducted to validate the performances of the bumpers.  These included side by side comparison in various stages of manufacture with virgin/painted products and those that had used the co-injection process.  Special attention was directed toward the co-injected parts to verify the absence of skin/core delamination as well as color variations that might effect the finished painted product.  Lastly, the molding cycle of the co-injection process must be equivalent to the original process.

In all cases, the co-injected part demonstrated successful results when compared to the original process and part.

The final product yielded a 30% core content.

The proliferation of co-injection use has been limited not only by the absence of this improved technology, but also by the lack of injection molding machines specifically designed for this purpose.  Processors who now own two-barrel injection molding machines which may have been used for two color processing can be converted to co-injection capable with in-mold co-injection technology.  Additionally, it is also possible to convert standard injection molding machines to co-injection capable machines by retrofitting a second injection unit.  In-mold co-injection manifolds can be designed for new or existing molds.  The Valve Gate Sequence ControlTM System can be interfaced with any injection molding machine. Image 15
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These systems provide processors with increased flexibility to take advantage of co-injection and sequential molding technology.

ACKNOWLEDGEMENTS:

Bemis Manufacturing
Evart Products – Textron
Daimler Chrysler

 

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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