In modern plastic product fabrication, multicomponent molding (MCM) has been widely applied to diversify product design and simplify the assembly process.

Ideally, MCM is a process in which two or more materials, or the same material with different colors, or recycled and raw materials, are injected into the mold to produce the product. The product not only combines multiple colors, but also multiple functions, such as a combination of soft skin/hard core.

However, the MCM process poses many challenges when used in the real world. First, there are a variety of cavity interchangeable mechanisms and multiple plastication unit setups to choose from. Moreover, the general rules for a single material molding cannot directly apply to MCM process.

Due to its complicated nature and the unclear physical mechanism for the MCM process during injection, the conventional trial-and-error method has its limitations in terms of its ability to effectively detect crucial factors that compromise product quality.

Multiple functions in one product: (a) multicolor in cosmetics packaging, (b) in-mold assembled in toys and (c) ear hook.

To get a clear understanding on how MCM works, we can categorize the other diversified MCM processes, as shown in the figure below, into two groups. The first group is the most common process, which produces products with a distinct interface. This includes insert molding, overmolding and sequential multiple-shot molding.

The second group has an uncertain interface between the two materials. This indistinct interface poses a great challenge forpart designers. Part designers must speculate the correct gating location in order to get the desirable material distribution. Usually, this can only be done through molding trials; however, oftentimes, the geometry complexity of the part is limited.

Co-injection (sandwich) molding and bi-injection molding fall into this group. Also, these processes require coherent movement of the injection and nozzle shut-off, which complicates the mold design and injection machine / barrel design.

MCM processes can be divided into two groups: (left) distinct interface systems, and (right) uncertain interface systems.

Therefore, to resolve the complicated nature and the unclear physical mechanism of the MCM processes, CAE mold-filling simulations may become a powerful tool for problem diagnosis and design validation.

For example, a touch pad was originally developed through combining separate injected pieces. This created less warpage, but caused the part to suffer from poor interfacial bonding strength.

Through the use of the overmolding process (Fig. 3a), interfacial strength can be improved. However, a serious warpage problem can occur easily; the warpage is formed due to the heat accumulated on the interface between the two materials (Fig. 3b).

As a result, the original design and process conditions in single-piece injection molding cannot directly apply to the MCM, and the warpage problem is resolved through a new product design and different processing conditions, suggesting by the CAE mold-filling analysis.

A touch pad development using MCM processes: (a) through mold rotation with two materials, a touch pad is fabricated with a significant warpage problem, (b) the warpage is formed due to the heat accumulated along the interface between the two materials.

Furthermore, most skin/core material combines soft-touch skin and a hard core, virgin skin with a recycled core, or unfilled skin with a fiber-reinforced core. It can be found in commodity, automotive and in structural applications using the co-injection molding process.

The main challenge faced today is controlling the material spatial distribution inside the cavity. For example, Fig. 4 shows the earphone hook design. The hook connects the phone and microphone assembly. The design has a flexible soft skin that provides a better feel when it comes in contact with the ear, while it should have enough stiffness to withstand deformation.

The prior art combines PP as the core and TPE for skin, using the overmolding process. The co-injection process was later tested for a simplified workflow and better joining strength between the materials. During the process engineering stage, it is unclear what the skin/core ratio should be.

Fig. 5 shows the CAE results. According to the prediction, the 50 percent skin ratio will create a core breakthrough leading to defects. The 70 percent skin ratio is expected to have a better skin/core ratio, but unfortunately it deepens the level of the warpage problem, which consequently leads to greater functional problems.

As a result, the simulation tool suggests modifying the gate locations as shown in Fig. 6. One design leads to a low core-filled area in the lock region. Harnessing the aforementioned core breakthrough behavior will resolve this issue nicely. That is, while keeping the 40 percent skin ratio, breakthrough happened at 0.07 sec. And after the breakthrough, the hook portion has the harder PP material which provides the better mechanical strength to fit the product specification.

Earphone hook (a) geometry design and (b) use of PP for core and TPE for skin by the overmolding process.

The co-injection process for an ear hook product development shows the 50 percent skin ratio is to expected to have a breakthrough. The 70 percent skin ratio is expected to have a good skin/core ratio, but fails to fit the specification for warpage.

CAE helps to make gate relocation validation.

Reconsider (a) the 40 percent skin ratio: Breakthrough happened at 0.07 sec; (b) after the breakthrough, PP material is used for the hook portion to provide better mechanical strength and to fit the product specification.


Multicomponent molding (MCM) is widely applied in modern product development. However, due to its complicated nature and the unclear physical mechanism, using a conventional trial-and-error method cannot easily detect crucial factors that affect the process.

Thus, in today's manufacturing world, no matter what kind of overmolding system (with a distinct interface), or a co-injection system (with an uncertain interface), CAE has proved to be a useful tool for design validation, such as optimizing the gate location, evaluating the maximum core ratio with or without causing a breakthrough, and estimating shrinkage and warpage concerning the interactions between the materials.