Multi-component injection molding

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Toothbrush handle, manufactured in a multi-component process

The multi-component injection molding is used for producing molded parts , consisting of two or more different plastics are made. In the simplest case, the plastics only differ in color in order to achieve a certain design . However, different materials and thus different properties can also be combined in a targeted manner.

There are various processes which all have in common that injection molding machines with two or more injection units but only one clamping unit are required. The parts can thus be produced inexpensively with just one tool in one operation. The injection units must work in harmony, but always be controllable independently of one another. The components can be injected through a single special nozzle or introduced into the mold at different points.

Applications

Multicolor injection molding

Different colors of the same material are used in one part. This applies z. B. in multi-colored car taillights.

Multi-raw material injection molding

In this process, different materials are processed in one part in order to e.g. B. to achieve a hard-soft connection. Examples are sealing caps with molded-on soft seals or keyboards where the pressure point is achieved with a molded-on silicone seal.

The plastics should have a certain degree of adhesion to one another, such as PP / PE , PMMA / PS , CA / ABS and PC / ABS, provided that the finished part requires a solid connection between the components (composite injection molding).

Assembly injection molding

Another application is the production of assemblies in which the individual components can move against each other. In this case, the components do not weld together, but rather shrink freely when they cool, so that there is a play between the components (IMA - In Mold Assembling ). Components that do not adhere to one another are preferred for processing. This technology enables the production of injection molded components with a hinge or joint function , for example for air vents in cars or for toy figures ( Playmobil ). The complex assembly of the individual parts is no longer necessary.

Multi-component process with clearly separated components

This results in parts with strictly separated component areas. First, an unfinished pre-molded part is produced, which is then overmolded with another melt in a further step. The process is therefore also called overmolding . The part geometry is usually decisive for the selection of the respective technology.

Transfer technology

After the first injection molding process, the pre-molded part is transferred into a new mold cavity with space for the pre-molded part and the new component with the help of a handling device, robot or operator. Application: viewing panels in device housing

Turning / shifting technology

The tool (usually only half) is rotated or moved into a new position after the first injection process and the pre-molded part is overmolded in the new position with another nozzle. Application: Toothbrushes (rotating or moving), parts with hard backings and soft surfaces. Multicolored lenses from modern vehicles.

Core retraction technology

A core is withdrawn in the tool to make room for the newly added component. Application: device housing with different colored areas.

Multi-component process with components running into one another

If all components are injected without the first being able to solidify beforehand, blurred boundaries arise.

Sandwich process

Sandwich injection molding with a transparent outer component

This process usually results in parts in which the component inside is not visible because it is completely encased by the outer material. Typically, recycling material is often used as an invisible component in this process. The inner material can also be foamable. High-quality material is then used as the outer skin.

In sandwich injection molding, the swelling flow of the masses when flowing into the mold cavity (mold cavity) is used. The melts fill the cavity one after the other from the gate. The molding compound flowing in first is continuously placed against the wall, where it is last pushed by the second component flowing inside. Two injection units work together on one injection head, which, depending on the control by valves or multiple shut-off nozzles, allows the masses from all injection units to flow in as required. The source flow ensures that this complete encasing of the components succeeds perfectly down to the smallest wall thickness. The sprue can be sealed by the first component.

In internal pressure injection molding , a gas , usually nitrogen or water , is used as the internal component . The fluid escapes after the outer component has solidified, so a hollow body is created.

Co-injection

With co-injection, in contrast to the sandwich process described above, the components are not injected one after the other, but simultaneously into the same cavity.

Marbling

Colored artificial flower petals and artificial jewelry are examples of marbled parts. Here, the changing components (mostly the same material but in different colors - but in any case they have to be well-tolerated materials) are also visible on the surface. This is achieved through intermittent injection units.

Individual evidence

  1. a b c d e f g h Walter Michaeli : Introduction to plastics processing . 5th edition. Hanser Verlag, Munich / Vienna 2006, ISBN 978-3-446-40580-6 , chap. 6.3.3. Process variants , p. 129–132 ( limited preview in Google Book search).
  2. a b c d e f g h i Christoph Jaroschek: Injection molding for practitioners . 2nd Edition. Hanser Verlag, Munich 2008, ISBN 978-3-446-40577-6 , chap. 4.5 Multi-component injection molding , p. 115–133 ( limited preview in Google Book search).