The Complete Guide to Plastic Overmolding Process

Plastic overmolding is widely used in the injection molding industry to enhance the finishing of the components. It helps manufacturers meet the safety, comfort, and effectiveness of the products. For instance, utensils and handheld tools are designed in a way to enhance their grip. Engineers apply plastic overmolding to add specified patterns to their handles to avoid slipping while using the tools. Overmolding is similar to insert molding because they both use the injection molding process. However, while traditional insert molding often involves attaching plastic or rubber around a metal surface, overmolding in custom non-standard manufacturing is a broader category. It includes both two-shot injection molding and pick-and-place methods to attach a flexible material over a pre-molded rigid plastic substrate.

Some people consider overmolding a surface finishing process, simply because it can change the look of a product. That view, however, is not accurate. Overmolding is actually a multi‑material injection molding process. While the overmold layer does provide sealing and aesthetic benefits, it is a core step in the forming of the part—not a “nice‑to‑have” finishing touch applied after the main body has already been produced.

This guide explores the overmolding process, describing the steps involved.

How Important is Overmolding?

During overmolding, a substrate is partially molded over or encased in secondary material. In most cases, a substrate used in overmolding is rubber or thermoplastics, leading to a single integrated product. There are numerous applications of overmolding in the manufacturing sector.

Ergonomics and Aesthetics

Many custom plastic products are manufactured from rigid engineering thermoplastics like Nylon, ABS, or PC to achieve structural integrity. However, these hard surfaces can create user discomfort and lack grip when handled for extended periods. Through overmolding, an elastomer layer is added to these products, distributing the pressure evenly in the user’s hands during use. Overmolding generates grooves on the devices’ handles to prevent sliding through the user’s hand. This is a very important safety consideration for hammers in the workshops.

In the case of power tool operations, plastic overmolding helps in dampening shocks and vibrations, thus supporting ergonomic safety. Some of the materials used in overmolding, like soft elastomers, absorb vibrations generated during operations. Compared to rigid plastics, specialized elastomeric overmolds can reduce high-frequency vibrations by 20-30%, a significant dampening effect that is critical for minimizing the risk of Hand-Arm Vibration Syndrome (HAVS) for operators in industrial settings ^[1].

The role of overmolding stretches beyond safety to provide aesthetics to the molded products. In this process, different color layers can be applied to the mold. These color layers are then transferred to the resulting product. For instance, pliers with similar dimensions can be available in different colors. These color differences can define the marketing of the manufactured plastic-molded products.

Waterproofing and Sealing Purposes

Overmolding helps to provide a watertight seal around the electronic components, switches, enclosure interfaces, and connectors. The process permanently bonds elastomers to the substrate, leading to a continuous barrier that fits all the complex geometries. This continuous sealing advantage solves the challenges of conventional gaskets and O-rings, which require additional assembly. Overmolding supports product reliability, especially for the products exposed to rain, humidity, and cleaning chemicals. As custom manufacturers, First Mold uses overmolding as a reliable means of achieving higher Ingress Protection (IP) ratings (such as IP67 or IP68) that comply with the strict IEC 60529 standard for water and dust resistance, ensuring long-term reliability in harsh environments ^[2].

 The table below summarizes the important applications of overmolding for solving sealing challenges:

Costes laborales

Most engineering companies specializing in injection molding seek to optimize their manufacturing as a strategy towards aligning with Industry 4.0 principles. One of the strategies to achieve this goal is through replacing traditional manufacturing assembly with overmolding. The integration of many components and functions into a single molded part significantly reduces manual labor. Effectively, manufacturers achieve production consistency and high overall equipment effectiveness (OEE).

Even though the overmolding has relatively higher tooling costs compared to traditional methods like assembly, the operational costs are relatively lower. Take, for instance, the manufacturing of a power tool. When using conventional manufacturing, we require a series of manufacturing activities to produce a simple power tool, which could be simplified through overmolding.

In the traditional assembly of a cordless drill, we would require a separately injection-molded housing, a discrete rubber grip, adhesive application, manual positioning, curing time, and inspection. All these operations take more time and more labor cost.

Through overmolding, we cut down these operations to fewer operators, faster production cycles, reduced work-in-progress inventory, and improved dimensional consistency.

What is the Manufacturing Process of Overmolding?

Overmolding is a multistep process flowing from design to ejection.

1. Design

The design stage of overmolding is critical as it determines the functional, aesthetic, and manufacturing requirements of the final products. Critical decisions are usually made at this stage to eliminate possible defects like sink marks, warpage, and delamination. In the design room, we apply different software like Mold Flow Analysis, CAD, and CAE to optimize process parameters and predict the nature of the final product. The design process covers several aspects like material compatibility, bonding mechanism, wall thickness, gate location, draft angle, and shrinkage.

Compatibilidad de materiales

The choice of the rightful overmold materials to bond with the substrate is a very crucial design approach. The overmold materials can bond with the substrate in many ways, such as chemical adhesion, mechanical interlocking, molecular diffusion, or surface energy attraction. Effective materials compatibility leads to stronger bonding, prevents peeling, and ensures that the overmold does not separate from the substrate. The following table summarizes different overmold materials compatible with substrate materials.

Espesor de la pared

Uniform wall thickness is designed in the overmolding process to guarantee consistent material flow, dimensional stability, and uniform cooling. It helps to minimize defects.  Further, you can prevent voids, reduce cycle time, and minimize warpage through uniform wall thickness. The walls should not be excessively thick to avoid slow cooling. On the other hand, thinner walls can contribute to incomplete filling or short shots. The following table summarizes recommended wall thickness for different materials.

Gate Location

The gate must be properly placed for balanced filling and effective defect prevention. Proper placement of the gate minimizes weld lines, lowers the residual stress, and lowers the injection pressure required. The common gate locations include edge gate, fan gate, and submarine gate, as illustrated in the figures below.

Contracción

All thermoplastics contract during cooling. During design, engineers compensate for this shrinkage by oversizing mold cavities based on the required or expected shrinkage. This estimation is guided by the following formula:

S = (L_m – L_p) * 100%/L_m

Dónde

S= shrinkage (%)

Lm= mold cavity dimension

Lp= final part dimension

However, overmolding introduces constrained shrinkage because the rigid substrate physically restricts the secondary material’s natural contraction. Engineers must often use mold flow analysis alongside this formula to accurately size the cavity and prevent warpage.

2. Manufacturing the Substrate

After design considerations, the team embarks on the manufacture of the base component on which secondary material is overmolded. This substrate is the product framework offering structural, dimensional, and functional features of the final product. The materials for manufacturing the substrate are selected based on the application. This implies that a substrate can be made from thermoplastics, metals, or composites. Different methods like CNC machining, injection molding, additive manufacturing, die casting, and stamping are selected for the manufacturing of the substrates. The selected method must guarantee a substrate that has sufficient mechanical strength, precise dimensions, and a good surface for adhesion.

Plastic substrates are normally manufactured through injection molding following the following steps:

1. Plastic pellets
2. Material drying
3. Melting in Barrel
4. Injection into Mold
5. Packing and Holding
6. Refrigeración
7. Expulsión
8. Finished Substrate

Metal substrates are selected in cases where high thermal conductivity, mechanical strength, and wear resistance are desired. These substrates are commonly manufactured through CNC machining, forging, die casting, metal stamping, or metal injection molding. However, after fabrication, engineers subject these substrates to secondary methods like surface finishing, deburring, chemical cleaning, or sandblasting.

3. Preparing the surface

The bond between the substrate and the overmold material depends on the condition of the substrate. Therefore, the surfaces of the two materials must be prepared well to support the effectiveness of the overmolding process. A surface that is not prepared has contaminants, experiences oxidation, has low surface energy, and may be too smooth with reduced adhesion. This results in peeling of the component, delamination, and the component’s premature failure.

There are several reasons why we prepare surfaces before overmolding is done. First, we remove contaminants like dust, oils, oxides, and mold-releasing agents. These contaminants can affect the bonding of the substrate and overmold materials. Secondly, preparation increases the surface energy of the materials. We refer to this surface energy as surface wettability. Third, surface preparation improves mechanical interlock between the substrate and overmold material.

Surface preparation is a mandatory requirement for applications where long-term bond integrity is critical. Such applications include aerospace components, automotive sensors, waterproof electronics, and electrical connectors. There are different methods you can apply while preparing the surfaces. The widely used method is plasma treatment. Here, the surface is cleaned, and surface energy is increased by generating highly energetic ions and electrons from plasma discharge.

The other methods include corona treatment, flame treatment, chemical etching, and laser texturing.

4. Loading the Insert

The manufactured substrate or insert at this stage is accurately positioned in the second-shot mold before injecting the overmolding materials. You need to insert the substrate with the highest precision to get the best quality of the final product. If you load the insert incorrectly, you can cause misalignment of the overmold material, leading to flash formation and poor bonding. Most of the scrap during overmolding results from poor positioning of the insert. To avoid these errors, most manufacturers prefer automated handling systems. 

5. Second-shot Injecting

This stage defines the overmolding process. A molten polymer like TPE, TPU, or other engineering thermoplastics is injected into the mold cavity and allowed to flow around the positioned substrate. While the materials come into contact with the substrate, the materials bond chemically or mechanically. You need to precisely control the processing parameters at this stage since they determine the overall quality of the resulting product.

The process begins with feeding polymer pellets into a heated barrel. The barrel has a rotating screw that conveys the pellets forward. As the pellets are pushed forward, their melting temperature is raised by heater bands, while the friction between the screw and polymer helps in the melting of the polymer. This leads to the homogeneous molten polymer. The molten polymer keeps accumulating at the front of the screw until enough volume is accumulated for injection.

As the reciprocating screw keeps on moving forward, it forces the accumulated molten polymer through the nozzle, sprue, runner system, and gate and into the mold cavity. In the cavity, the molten material flows around the substrate, allowing bonding and solidifying. Once the molten materials solidify around the substrate, it is allowed to cool. Afterwards, the ejector pins eject the final product. The following table summarizes the processing parameters.

Note: The precise melt temperature depends on the specific polymer grades being bonded. Maintaining the second-shot melt temperature strictly within the material’s thermal processing window is critical to achieving cohesive chemical bonding without melting or degrading the underlying custom substrate ^[3].

Conclusión

Overmolding offers versatile solutions to manufacturing challenges by combining different materials to enhance product functionality, durability, ergonomics, and aesthetics. Manufacturers who want to add a soft-touch appearance to their products to enhance grip or “feel” and provide a stylish look that appeals to consumers are prime candidates for overmolding. If you require a single, solid part that integrates multiple layers of materials and colors, overmolding is your ideal solution.

Referencia

[1] Bernard, B. P., Nelson, N., Estill, C. F., & Fine, L. (1998). The NIOSH Review of Hand-Arm Vibration Syndrome: Vigilance Is Crucial. Journal of Occupational & Environmental Medicine, 40(9), 780-785. Centers for Disease Control and Prevention (CDC) Stacks. Retrieved from https://stacks.cdc.gov/view/cdc/196388

[2] International Electrotechnical Commission. (2013). IEC 60529:1989+AMD1:1999+AMD2:2013 CSV Consolidated version: Degrees of protection provided by enclosures (IP Code). Retrieved from https://webstore.iec.ch/publication/2452

[3] Awaja, F., Gilbert, M., Kelly, G., Fox, B., & Pigram, P. J. (2009). Adhesion of polymers. Progress in Polymer Science, 34(9), 948-968. https://doi.org/10.1016/j.progpolymsci.2009.04.007