In industries, many applications require different core and surface properties – a single material cannot achieve that. In such cases, we layer one material on top of another through overmolding. But what’s overmolding? Is it flexible with material combinations, and how effectively do these materials integrate?
This article examines the plastic overmolding process, mold design considerations, and the diverse applications it serves. Additionally, it compares overmolding to other similar techniques in injection molding.
What’s Overmolding?
Overmolding is a hybrid injection molding process to manufacture parts from two or more materials, usually thermoplastics and elastomers. It involves layering or encasing one material on top of another. The two polymers may fuse chemically or mechanically to become one solid object. For instance, in power tool handles, overmolding allows us to create a hard plastic core, like ABS, complemented by a soft, tactile rubber grip.
The overmolding process resembles traditional injection molding but involves multiple injections to introduce different materials.
In one type, direct overmolding, the initial material fills the mold and cools to settle. Subsequently, a second material is injected to form an outer layer or envelope around the first.
Whereas, in insert overmolding, a rigid part gets prepared separately and placed into the mold, where another material (elastomer) is injected over it.
Overmolding Process Step-by-Step
The overmolding operation is completed in these steps:
1. Material Selection
Material selection is the most important phase. Choose materials that meet the internal and external physical requirements of the product. Consider the intended use, such as ergonomics, comfort, or vibration dampening, and whether the product needs water or thermal resistance. The overmolding materials themselves need to bond together to prevent separation after molding.
2. Mold Design and Setup
The design of the mold must accommodate the properties and thickness of both materials, which requires different considerations compared to standard molds. The wall thickness should be uniform (no more than 4mm), and it needs to have gates where the wall section is thickest. The flow ratio needs to stay below 150/L.
Just like other injection molds, overmolds are created using CNC machining and are made from durable metals like steel or aluminum to withstand the pressures and temperatures of the injection molding process.
3. Injection Molding Setup
The setup is custom-designed based on the materials and their layering sequence. For dual-material overmolding, setups may use multiple injection units. These units are configured to rotate the mold, such that each injector can precisely deposit its respective material.
Initially, the unit injects a base layer material. It cools to form a rigid overmolded substrate. Then, another injector adds material on top of it. In some cases, the substrate is separately made in one unit and then injected with an elastomer layer.
4. Ejection and Inspection
After the molding process, the part is ejected from the mold. The part undergoes a thorough inspection for defects, such as incomplete bonding, air pockets, or surface imperfections.
5. Post-Processing
The process returns a solid part, which is a combination of all materials injected. However, it may need to undergo some post-processing processes. For instance, trimming for excess material, polishing to improve surface finish, or further curing to enhance material properties. The goal is to achieve the desired aesthetics and functional characteristics in the overmolded part.
Overmolding Design Tips
A lot of variables are involved when it comes to making a mold. These few overmolding design tips will guide you along the way:
Understand Material Properties
The core idea of using the molding technique is to design plastic parts with the desired characteristics. So, the material selection should be based on that. However, do consider their inter-compatibility and physical properties, such as melting temperatures and expansion coefficients.
Since both materials are different, the melting temperatures and expansion coefficients may not be similar. Some of the resins shrink off on settling, causing warpage. To counter that it’s advisable to use a core component (substrate) whose flexural modulus and melting temperature is higher than the secondary material.
Thickness also matters. A thicker layer of elastomer (TPE) will bring on vibration dampening and softness. In contrast, a thin layer feels harder and is a choice for making rib structure.
Optimize Part Geometry and Mold
The mold design must accommodate multiple injections, ensuring even wall thickness between 2 to 4 mm. Avoid deep ribs and sharp corners to reduce stress concentrations and facilitate easier flow of the molten material.
Maximum radii in sharp corners should be less than 0.5 mm. As a general rule, incorporate a draft angle of 1 degree per inch of depth to ease part ejection.
Enhance Bonding
Materials should ideally blend at the molecular level through chemical bonding. For optimal adhesion, the contact temperature should be close to the melting point of the materials.
Where chemical adhesion is not feasible, employ a mechanical approach using interlocks. Another option is texturing the substrate material so that resin fills into the gaps to provide adhesion.
Table 1 shows the bonding compatibility of overmold materials.
| Subtrate MaterialOvermold Material | ABS/PC Cycloy C2950-111 | ABS Lustran | PBT Valox 357-1001 | PC Lexan | Nylon 66 Zytel 103 HSL NC010 | PP Profax 6323 |
| TPU – Texin 983-00000 | *C | C | C | C | **M | M |
| TPV – Santoprene | M | M | M | M | M | C |
| TPE – Santoprene 101-64 | M | M | M | M | M | C |
| LSR – Elastosil | – | – | M | M | M | – |
| TPC – Hytrel 3078 | C | C | C | C | M | M |
| *C indicates a chemical bond recommendation**M indicates a mechanical bond recommendation | ||||||
Design for Manufacturability
In manufacturing, the goal is to streamline the process such as parts production, replication, and assembly is easy. One way is to reduce the number of sub-parts so that the assembly steps are minimized.
Another way is the use of computational tools to refine mold design and process parameters before manufacturing begins. You can use simulations to identify uneven material distribution, which can lead to structural weaknesses.
The choice of materials is another crucial aspect. Choose materials that not only meet the functional requirements but also bond well together. For instance, for a power tool handle that requires a rigid inner structure with a soft grip. The design can have a hard plastic core of ABS, for structural integrity, surrounded by a softer thermoplastic elastomer (TPE), for comfort and slip-resistance.
Plan for Post-Processing
Prepare for necessary post-processing steps such as trimming, polishing, or painting to achieve desired surface finishes. Based on application, add secondary processes like UV stabilization or flame retardant treatments.
If you’re interested in learning more, don’t miss our detailed overmolding design guide.
Types of Overmolding Materials
Overmold materials fall into two categories: thermoplastics and elastomers.
Thermoplastics
Thermoplastics are materials that become moldable on heating and harden upon cooling. You can reuse these materials, melting and remolding them repeatedly without any chemical change. Some popular options are:
Polycarbonate (PC)
PC has an extraordinary impact resistance and optical clarity. Its common applications include bulletproof windows and protective gear. Besides being sturdy, it also resists discoloration. However, PC is prone to scratching and can degrade under prolonged UV exposure, turning yellow.
Polyethylene (PE)
PE finds applications in everything from plastic bags to high-strength containers. In overmolding, its variants, HDPE and LDPE, offer multiple options ranging from rigid structures to flexible parts.
HDPE has high strength, durability, and resistance to chemicals. It is common for making storage bottles, toys, and any item where rigidity and durability are requirements. However, it’s flammable and has poor UV resistance.
LDPE is softer and more flexible than HDPE, suitable for applications requiring flexibility such as squeezable bottles and packaging films. Its resistance to acids, bases, and vegetable oils makes it an ideal choice for food packaging. The main challenges with LDPE include its lower temperature resistance and susceptibility to puncture.
Polypropylene (PP)
PP also has excellent chemical resistance and mechanical properties. The common applications include automotive parts, consumer goods, and living hinges, which require repeated flexing. Its chemical resistivity makes it best for hygiene applications. However, PP’s resistance to UV light is moderate and requires additives for stabilization in outdoor applications.
Acrylonitrile Butadiene Styrene (ABS)
ABS is a tough, versatile thermoplastic used across various industries. It offers excellent impact resistance, good heat stability, and a smooth surface finish. ABS is easy to mold and paint – a material best for aesthetic applications. However, it’s not resistant to chemicals.
Elastomers
Elastomers’ key characteristic is their elasticity and soft texture – which is why they often form the top layer of rigid plastic products: These three elastomers are common for overmolding:
Thermoplastic Polyurethane (TPU)
TPU is a versatile material that possesses the best attributes of rubber and plastic. It is durable, flexible, and resistant to abrasion, making it ideal for applications that require a soft touch like phone cases, seals, and gaskets. It’s oil-resistant and bonds well with plastics like PC and ABS. However, it’s expensive.
Thermoplastic Elastomer (TPE)
TPEs are a class of copolymers (a blend of polymers, usually plastic and rubber) that combine the mechanical and thermal properties of thermoplastics with the elasticity of elastomers. They are useful in overmolding applications requiring a soft grip, like toothbrush handles, tool grips, and personal care products.
Silicone Rubber
Silicone has excellent heat resistance, flexibility, and insulating properties. In overmolding, silicone is primarily used to create waterproof seals, insulating cables, and protective cases for electronics. It adheres well to metals and certain plastics – and is ideal for medical products and cookware where high temperature and sterilization capabilities are required.
Material Compatibility
When choosing materials for overmolding, keep in view the chemical compatibility of the materials. Some combinations of materials naturally blend and form chemical bonds, whereas some require mechanical interlocking to fuse. Chart 1 shows the compatibility of different material pairs.
Moreover, it’s important to consider other mechanical properties, like co-efficient of expansion, friction co-efficient, and flexural modulus. The properties need to complement one another. For instance, if the substrate (core) is a material with high flexural modules, the top resin/elastomer has to have a slightly lower value.
Applications of Overmolding
Overmolding is a popular injection molding process in which TPE is a commonly used overmold. The substrate used in the process can be any material ranging from plastic to metals. This makes the overmolding injection molding process an important industrial process with a lot of applications highlighted below.
| Industry | Specific Applications | Characteristics Achieved |
| Consumer Electronics | Electrical outlet covers | Increased durability and safety |
| Gaskets | Superior sealing | |
| Overmolded phone cases | Better grip, shock absorption | |
| Electronic housings | Protection against environmental factors | |
| Automotive Industry | Interior components (dashboard, knobs) | Aesthetic appeal, tactile comfort |
| Exterior components (bumpers, trim) | Weather resistance | |
| Medical Devices | Handheld overmolded devices (scanners, controllers) | Ergonomic design |
| Wearable medical devices | Comfort, durability | |
| Syringes, patient monitors | Safety and functionality | |
| Lab consumables, needles, catheters, dilators | User comfort, safety | |
| Soft Touch Buttons | Easier Operation | |
| Industrial Equipment | Tool handles and grips | Enhanced grip, reduced vibration |
| Protective enclosures | Mechanical and environmental protection | |
| Consumer Goods | Household appliances (handles, knobs) | Usability and design enhancements |
| Sporting goods (bicycle handles, fitness equipment grips) | Grip comfort, durability | |
| Telecommunications | Overmolded cables, Overmolded connectors | Durability, insulation, strain relief |
Benefits of Overmolding
Manufacturers with a need for design, grip, and stylish appearance all love what the overmolding process gives its product. The process is also not that costly, yet it has improved over the years leading to top-notch customer satisfaction. Below are the advantages of using overmolding.
Enhanced Product Durability
Overmolding introduces a secondary material that shields the primary component from mechanical stress and environmental factors. One example is over-molded seals on waterproof cameras which protect the inner components from dirt and water.
Improved Aesthetics and Ergonomics
It can enhance the ergonomic design of products by adding contours and soft grips tailored to user interaction. Kitchen appliances, for instance, blenders, feature overmolded handles that improve grip and comfort.
Material Efficiency
The fusion of two materials in one mold creates a single part. Separately designing each one would have required more material and machining further would have caused more waste.
Functional Integration
Remote controls utilize overmolding to integrate soft buttons that are responsive and comfortable to press. Similarly, power tools feature rugged plastic holding areas which makes it easier to hold and use these tools.
Enhanced Performance
Overmolding can combine materials to create composite parts that perform better than their single-material counterparts. The interior components such as dashboard controls are overmolded with soft plastics that not only enhance tactile feedback but also improve wear resistance.
Limitations of Overmolding
The process shines in many aspects but there are a few limitations as well:
Material Combability Issues
Not all materials are compatible with bonding. If joined, it leads to inadequate bonds between layers. The final product may fail under stress due to weak adhesion.
Increased Manufacturing Complexity
Overmolding involves multiple injection steps and possibly different materials, requiring intricate tool designs. This complexity leads to longer cycle times and the need for specialized machinery.
Higher Initial Costs
The initial costs for the process are higher due to the need for multiple material inventories and specialized overmold tooling for each material. Consequently, the production cost per unit also gets high.
Design Constraints
Overmolding imposes limitations on part geometry and material choices; certain complex shapes or combinations of materials may not be feasible due to differences in melting temperatures and mechanical properties.
Insert Molding vs Overmolding
While overmolding and insert molding share some similarities with hybrid molding processes, they are fundamentally different techniques.
Both processes allow for the creation of parts with varied plastic combinations, enhancing the mechanical, aesthetic, and tactile properties of products. As a result, their applications often overlap in industries such as consumer goods, automotive, and electronics.
Overmolding is a two-step process where the first material is injected into a mold, followed by a second material layered on top. Insert molding, on the other hand, involves molding plastic over a prefabricated substrate (insert), such as a metal part, screw, or pin.
One key difference lies in the preparation: Insert molding requires separate setups for the two material parts, which increases both time and cost. Overmolding, however, utilizes the same equipment for both the base preparation and the layering process, making it more time- and cost-efficient.
Additionally, overmolding typically involves plastics and elastomers, while insert molding usually incorporates metal inserts.
For a deeper comparison, check out our comprehensive overmolding vs insert molding guide.
2K Molding vs Overmolding
Two-shot molding involves the injection of two different materials into the same mold during a single molding cycle. The process begins with the injection of the first material, which forms part of the product. The mold then rotates to a new position for the second material to be injected.
In contrast, overmolding typically requires two separate molding cycles. The first material is molded into a basic shape, which, after cooling and hardening, is placed into a new mold where the second material is injected over or around it.
A key distinction between the two processes lies in material usage and design flexibility. In overmolding, the substrate or base material is usually rigid, with a softer material applied as the second layer. Two-shot molding, however, doesn’t require a hard substrate and can combine any two plastic materials.
Overmolding is often used for creating products with both functional and aesthetic enhancements by molding multiple materials. Two-shot molding, on the other hand, is specifically designed for combining exactly two materials, making it ideal for adding dual tones or functional layers to the final product.
To understand the distinctions between these methods, take a look at our overmolding vs two-shot molding guide.
Co-molding vs Overmolding
Co-molding is a subset method of overmolding, having the same characteristics and applications. The process requires that the materials have compatible melting temperatures and bonding properties since they are molded together at the same time.
Usually, the combination is a hard substrate of plastic and over a layer of rubber. However, two similar materials can also be co-molded.
Is Overmolding Right for Your Project?
If you require a single, solid part that integrates multiple layers of materials and colors, overmolding is your ideal solution.
RapidDirect provides exceptional overmolding services that cover a wide range of materials. Whether you need plastic over plastic, rubber over plastic, or plastic over metal, we can accommodate all possible combinations. Our parts are precision-engineered to meet stringent industrial tolerances of ±0.0025mm
Contact us now for a free quote and expert guidance from our industry professionals.
