Multi-Component Molding

Two-shot molding, co-injection, and hard-soft combinations of thermoplastics are all variations of Multi-Component molding. This is a multi-step molding process that produces an assembly, comprised of two or more integrated components.  Ticona’s expertise in this area means that we can provide part design recommendations, possible material combinations, tool and machine recommendations, and other useful processing parameters.  We look forward to working together with you.

Multi-component processes enable one to:

• Produce parts not possible with single shot molding
• Produce multi-colored or multi-resin parts and assemblies for functionality
• Produce parts with movable segments or components using “in-mold assembly”
• Eliminate some post-molding assemblies such as snap-fits, ultrasonic welding, adhesives, screws, or bolts
• Eliminate other operations

Typically co-injection or “sandwich” molding is the injection of a skin to partially fill a cavity, followed by the core component to pack out the part.  This process can use two injection units and rotary molds designed for sequential injection, or a robot transferred mold.

Two-shot injection molding allows the first material to cool before the second one is injected.  Critical variables such as draft and mold temperature must be considered when using either of these processes.

Applications possible with two-component molding include: gaskets, parts with molded-in seals, parts with shock-absorbing or soft grip features, acoustic dampening, flexible hinges, multi-color parts, toys with movable parts, and automotive AC or heater louvers (frame, vanes and connecting pieces).

Combining hard polymers with soft materials in a 2-shot molding process can enable new functions:

• Haptic elements on hard surfaces  
• Inject damping elements
• New designs (such as complicated gaskets) are made possible
• Balance tolerances

Hard/Soft combinations of materials currently are used in applications such as: control elements for venting, levers, radio, or lights; conveyor chain links; clips with a gasket or damping element; a sensor housing head lamp regulator; a clamping device for an automotive CD player; linear drive gaskets; a thermos mechanism; a housing with rubber gasket, and a lamp socket for a washing machine.  Our work with soft component suppliers has resulted in new patented hard-soft combinations(with outstanding adhesion) for hard materials such as: Hostaform® /Celcon® POM, Fortron® PPS, Celanex® PBT, and Celstran® LFRT. We can provide testing data and “know-how” for resin combinations to obtain a synergistic and complementary system.

Normal Steps for Multicomponent Molding

1. Resin A is used in Shot #1.
2. Resin B is used in Shot #2.  The molding machine can be constructed to achieve this or the mold can be actuated in some fashion to accommodate Shot#2.  The Shot #2 resin may form an adhesive (or mechanical) bond with the Shot #1 resin. It could also be constructed to move in some fashion in, around or about the part molded from Shot #1.
3. A third (or fourth) shot can be employed to make an even more complex assembly.
4. The assembled part is ejected from the mold.

Processing Considerations

Machine/Mold:

• The injection molding machine will need to have 2, 3, or 4 plastication/injection units (one unit for each different resin).
• The molds are more expensive than standard molds since the cavity blocks may rotate between Shots #1 and #2, or other mold components may need to actuate/articulate in some fashion between shots.  The required control systems may also increase the mold costs.
• The machine’s multiple screws can be arranged in a Vertical position (90 deg.), an L-position, or a “Piggyback” arrangement as shown below.

Tool Designs:
There are also several types of tool designs such as:

• Core Back  - A sliding core is first closed and Melt 1 is injected, then the sliding core is opened and Melt 2 is injected.
• Rotating Plate - This two-station tool rotates in a vertical or horizontal direction for the injection of Melts 1 and 2.
• Index - The mold is physically transferred from one point to another.

In addition to the tool design, one should also consider the wall thickness, the surface structure of the part from Shot #1 (for venting problems), the tool surface and temperature (for demolding), the gating location (for adhesion in dependence of flow path), the kind of contact (flat or overflow), and how the part will be demolded (force in the adhesion area).

Materials:

• Resins A, B, C, etc. should be generally compatible, with no antagonistic effects between resins.
• Where movement of assembly components is desired, resin shrinkage becomes an important factor.
• Code and agency requirements (such as FDA, UL, etc.) should be factored into decisions regarding materials. It is possible that the complete assembly would need to be tested.
• Plans for a multi-shot molded part should be reviewed with all of the material suppliers involved. Information such as the melting point, energy transfer, surface tension, molecular wt, rate of crystallization, mold release, internal lubricants, reinforcement ingredients, pigments, and stabilizers should all be considered.

Process Variables:

• Use the recommended molding conditions for each material at the outset and then refine them if necessary (molding conditions can vary widely between materials).
• The resin having the highest melting point or glass transition temperature should normally be “shot” first  
• It is often desirable to preheat parts to be overmolded to achieve better adhesion
• Coating parts to be overmolded occasionally can be useful to achieve better adhesion.
• NEVER try to use multi-shot regrind unless it has been thoroughly studied (as severe degradation could occur).
• Other processing variables that should be studied are the melt temperatures of the two materials, the tool temperature, the injection speed, the hold pressure, screw retraction, and air traps

Testing:
Multicomponent parts should be tested at temperatures above those expected in end-use and exposed to thermocycling to expose any problems with expansion/contraction.  The tests should also rule out any antagonistic relationships between the two resins.