A primer on coinjection molding

A primer on coinjection molding
By Jack Avery
Avery Plastics Consulting
 
Coinjection molding (also know as sandwich molding) is a sequential process in which two materials, a skin and a core, are sequentially injected through a specially designed nozzle or manifold. The process begins by injecting the skin component to partially fill the mold, followed by filling the cavity with the core material and then finishing it off with another injection of the skin material.

Coinjection molding provides a range of performance options through the selection of different material combinations:

Solid skin/solid core

Solid skin/foam core

Flexible skin/solid core

Foamed skin/solid core

These combinations can provide product features such as soft-touch/rigid core, inherent electromagnetic interference shielding, aesthetic surface with high-strength core and the use of recycled materials in the core.

Processes

A number of processes result in injection molded parts:

Two or more materials are plasticized in separate barrels and injected into the cavity through a common nozzle assembly and feed system. This technology was developed and introduced by Battenfeld.

Monosandwich technology involves a secondary extruder added onto an injection molding machine. Once the skin material is plasticized in the injection molding machine, the secondary extruder is engaged in extruding the core material into position behind the skin. The two materials are injected into the mold cavity as in a normal process. Monosandwich technology is patented and is available through Ferromatik Milacron.

Twinshot uses a special screw with two independent melting zones to produce layers of different materials within a single barrel. During injection, the principal of fountain flow results in a three-layer construction. Twinshot is a patented technology is available on machines through a number of machine manufacturers and can be retrofitted to existing machines exclusively through Spirex.

Materials

A range of materials can be used. Because the materials are used in combination, relative viscosities and adhesion between skin and core are critical considerations in material selection.

In coinjection molding, material enters the cavity first and develops a flow front. This may be called the fountain flow effect. As material contacts the cold wall of the mold, it solidified, forming the skin layer. Before the skin material reaches the end of the cavity, the second material is injected to form the core. The core material develops a second flow front with a different velocity profile than the skin material, and pushed the skin material ahead of it. The relative viscosities of the skin and core material are important. Because the core material pushed the skin material ahead of it, it is desirable for the core material to have a higher viscosity than the skin material. This produces a “pluglike” flow front for the core material. Low-viscosity material in the core may cause the core flow front to travel too fast relative to the skin, resulting in core “breakthrough.”

Material suppliers have information regarding adhesion characteristics of a range of material combinations. However, actual performance must be evaluated in the applications, as process conditions and application use conditions will have a significant impact on performance.

Actual performance must be demonstrated in the application, as molding conditions and environmental use conditions will have a significant impact on performance. Also, not all formulations of the same material provide the same performance.

Design considerations
Designing parts for coinjection is almost the same as designing for injection molding. Designs incorporating louvers, grilles and sections with multiple openings produce flow turbulence during filling. This can produce breakthrough that may result in part defects.

Balance of flow is critical when producing components using coinjection molding to prevent breakthrough.

Gate location is also important to optimize core material content in asymmetric parts with varying wall thickness. For example, side gating a part to be molded by the coinjeciton process is simpler than center gating.

Sufficient wall thickness is required in coinjection molded parts to alow for enough of each material in the part to provide the desired performance. Typically, wall sections are greater than 4 millimeters for foam core/solid skin, 3-4mm for solid core/solid skin and about 2mm thick for a consistent skin/core/skin structure.

It is important to remember that thin sections will be skin material only.

Applications

Coinjection molding is used to produce components for a range of markets including automotive, electronic, industrial, appliance, business machines, agriculture, recreation and leisure. The use of this process is driven by several factors: unique performance requirements not available from a single material, cost and the ability to use regrind/recycle in the core.

The traditional use of coinjection molding has been applications such as handles requiring structural performance and cosmetic appearance, where the cores of structural foam or glass fiber-reinforced materials are encapsulated in unfilled resins. Another combination is the use of flexible skin over a rigid core to produce a “soft” touch. Cost reductions result from several factors including elimination of secondary finishing, utilizing lower performance/lower cost materials only in the skin area.

Coinjection molding applications include:

Automotive handles, arm rests, distributor caps, headlamp reflectors, license plate holders

Bottles

Copier enclosures

Desk accessories

Lawn and patio furniture

Lawn spreader wheels

Housings for mechanical equipment, speakers, printers

Microwave dishes

Toilet seats

Tool handles

ack Avery, who retired from GE Advanced Materials in 2004 after 34 years with the company, is founder and president of Avery Plastics Consulting in Salt Lake City, Utah, USA. E-mail jackaveryconsultantsmold.wiki.

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