Co-injection molding (sandwich molding, multi-material molding)

Co-injection molding is the term used to describe the process of feeding several melts, one after the other, into one mold during the injection molding process. The melts should make contact with each other but should not flow into one another. The resultant composites may be permanent or it may be possible to move the individual components separately. In-mold decoration and back-molding processes are sometimes also included in this category. Here, a substrate, e.g. in the form of a fabric or film, is inserted into the mold before the melt is injected in. According to this definition, co-injection molding includes the following processes:

• Two-material co-injection molding / two-color co-injection molding, overmolding
• three-color co-injection molding / three-material co-injection molding / multi-color co-injection molding
• Moving composites in injection molding
• Multi-material molding, sandwich molding
• 3D MID (Molded Interconnect Devices)
• Back-molding of film and fabrics

However, in this article, co-injection molding covers only with the first three items on the list. For the others, please refer to the special articles.

Two-material co-injection molding:

By two-material co-injection molding we mean the injection of two different melts into the mold, one after the other. They may comprise the same material in different colors or two different materials. Other terms to describe this process are two-color co-injection molding or two-component sandwich molding. The principle is basically the same, although there are essentially five different methods of carrying out the co-injection molding process. We can divide them first of all into rotating and non-rotating mold systems.

• Rotary plate
• Rotating mold
• Turnstiles and rotating cores
• Transfer technique
• Core-back technique
• Stack-mold technique

Reference is made here to the respective mold chapter.
The arrangement of the two units is more or less independent of the mold technology but is governed essentially by the geometry of the molded part. The units can be arranged either parallel or perpendicular to each other.

The use of the two-material co-injection molding process makes it possible to produce parts that satisfy stringent specifications. These may be multi-part products for everyday use such as food packaging, toys, components for electrical equipment or other engineering components such as rubber bearings, rollers, and sealing elements or various kinds of damping elements and housings with integrally molded seals.

The parts can be produced economically by co-injection molding in one step. Only one mold and thus only one machine is needed to manufacture the moldings. There is no need to pre-treat the metal as is usual today, for example, by degreasing with solvents. There are many reasons in favor of using co-injected structures. For example, the two-material co-injection molding method allows several functions to be integrated in one part. It also improves quality and minimizes costs, e.g. by reducing assembly work.

Furthermore, it is possible to obtain or improve certain performance properties such as functional, tactile or design features. Composites of rigid and flexible materials open up a wide range of new possibilities. These composites have a variety of functions, with the flexible component exhibiting the typical springy and elastic properties to provide resilience and a cushioning effect, or non-slip characteristics for good grip, while the rigid component contributes its strength and stiffness to prevent distortion where loads are applied.

Three-color and three-material co-injection molding:

To manufacture, for example, car rear lights, three or more different components are needed. For this and similar cases of application, the three-color or three-material co-injection molding method is used. This process is comparable to the previously described two-material co-injection molding process as far as the mold technology, processing and machinery are concerned. All that is needed in addition is a third injection unit and a suitable mold with a gating system for the three components, which of course is correspondingly more complex.

Continuing this principle, several plastics can be combined together to create one part (multi-color co-injection molding).
The latest development involves 5-color rear lights in which the metallizing of the reflector surfaces is integrated inline into the production process. With hygroscopic plastics (e.g. PMMA, PC), this prevents moisture absorption and produces a high-grade coating.

Bond strength in sandwich molding:

(based on a paper from Dr.-Ing. Karl Kuhmann, KT University of Erlangen, "Decorative surfaces of injection molded parts", Seminar Carl Hanser Verlag, Wiesbaden, April 1999).

When several plastic parts are joined by sandwich or co-injection molding, the aim is to achieve a positive, permanent bond of the plastics components. The attainable bond strength of adhesion-compatible plastic combinations under mechanical load and on exposure to certain media is of particular interest. As far as the adhesion and manufacturing precision are concerned, advantages are generally obtained through the simultaneous production of pre-moldings and encapsulated finished components, as well as sequential two-step production in one mold without intermediate demolding. The first step involves injecting a pre-molding from one of the components, then encapsulating certain areas of the partly cooled pre-molding with one or several plastic components.

Factors influencing the bond strength:
The bond strength can be influenced among other things by the process parameters during injection molding. The extent depends, however, on the material combination and thus on the specific prevailing composite mechanism. In addition, attention must be paid to the local rheological and thermodynamic conditions (also with regard to time) at the interface of real two-dimensional parts [94]. These influences were reproduced on a modular specimen mold. It is only possible to apply the results to practical parts on a qualitative basis.

Observations on the formation of a permanent material bond during co-injection or sandwich molding make use of adhesion theories, including the diffusion theory. A reliable theoretical prediction of the bond strength is, however, not possible with this theory, nor is it possible to predict the qualitative influence of the injection molding parameters, if, for example, interactions exist between the injection molding parameters and if the boundary conditions for the model do not fully apply to the physical processes actually occurring in the sandwich molding for a material/part combination. A positive bond in the sense of "injection welding" necessitates adhesive bonding of the components through sufficient wetting of the partially cooled pre-molding through the melt heat of the newly injected component and a "rapprochement" of the molecules from both components. Fig. 21 shows different adhesion mechanisms that (depending on the material combination) overlap and can contribute towards the "technical" bonding strength. The formation of adhesive forces is based among other things, on intermolecular interactions (dipolar, dispersion and induction forces).

Apart from the material combination, the injection conditions in the real molded part also influence the bond formation. As the distance from the gate varies, so the flow and cooling conditions vary, and this has differing effects on the bond. In addition, the conditions for the second component may change through different gating geometries, flow cross-sections and flow directions compared with the pre-molding. Even with constant flow cross-sections, the gating conditions change in different areas of the molding, for example through the later cooling of the melt, the time between the wetting and pressure build-up or the relaxation of orientations.
Finally, for good adhesion compatibility, there are other criteria to take into account, especially the so-called property compatibility. For example, depending on the component geometry, excessive differences in shrinkage or thermal expansion can lead not only to distortion but also, if combined with different stiffness of the components in relation to temperature, to failure of the material composite through the overlapping of internal stresses. The various influences described here involve the processes taking place during co-injection molding and they must be taken into account when developing and evaluating a molded part.

Basically, any material that is suitable for injection molding can also be used for co-injection or sandwich molding. The only proviso is that the adhesion of the materials to one another has to be good.