Everything you need to know about overmolding

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Everything you need to know about overmolding

The history of overmolding goes back decades as a manufacturing technique, This process allows two or more materials to be injected into a single product, achieving different colors and tactile appearances.

As a product designer, it is necessary to have a comprehensive understanding of the two-color injection molding process and its related design principles.

What is Plastic Overmolding?

Overmolding also know Double Shot Injection or two-shot Injection, It is the combination of Two or more materials to form a special mold that can produce two plastic materials, which allows two different plastic materials or colors to be integrated into a single product in a single production process.

This technology has many advantages, such as improving product quality and functionality and improving product aesthetics.

How Does It Works?

overmolding

In the overmolding machine operation, the initial step involves injecting the first material into the mold to create the substrate material.

Subsequently, the machine rotates 180 degrees, positioning the second set of molds for injection.

The second material is then injected and fused with the previously molded material, allowing the production of composite products with diverse colors or material properties.

Types of Overmolding Machine

Vertical Overmolding Machine:

Vertical overmolding machines have a vertical injection unit, allowing for efficient overmolding of vertical or perpendicular surfaces. They are suitable for applications where space is limited or when molding onto vertically oriented parts.

Horizontal Overmolding Machine:

Horizontal overmolding machines have a horizontal injection unit and are commonly used for molding onto flat or horizontal surfaces.

Rotary Table Overmolding Machine:

Rotary table overmolding machines use a rotating table to move the mold between stations, facilitating multi-material or multi-component molding.

Multi-Station Rotary Overmolding Machine:

This type of machine features multiple stations on the rotary table, allowing for more complex overmolding processes.

Overmolding Materials and Processing Attention

Overmolding material refers to the combination of 2 kinds of plastic materials with 2 colors or more, which is described as follows.

Types of Overmolding:

#1. Soft Plastic over Hard Plastic.

#2. Hard plastic over hard plastic.

Types of Overmolding Materials:

#1: Soft Plastics: TPU、TPE、TPR、LSR,

#2: Hard Plastics: ABS、PC、PC/ABS、PP、PVC;

Overmolding Processing Attention:

#1: The compatibility of plastics: the materials combined with each other need to have the physical properties that can be compatible.

#2: Two-color injection molding plastic products should choose raw materials with good thermal stability and low melt viscosity to avoid decomposition due to high melt temperature and long residence time in the flow channel. The most widely used plastics are polyolefin resins, polystyrene and ABS materials.

#3: There must be a certain temperature difference between the melting point of the two materials, which is generally recommended to be 60 ° C, and it is recommended to be at least 30 ° C.

The melting point temperature of the first injection material is high, generally the first shooting is PC or PC/ABS, the second injection is TPU or TPE, the PC thickness is 0.6-0.7mm, and the software is above 0.4mm.

Try to widen the contact area of the two parts before and after, add grooves and other ways to increase the adhesive force in the design, or use core withdrawal for the first injection, part of the material for the second injection is injected into the first injection, and the surface of the first injection mold should be as rough as possible.

#4. For two-color injection molding plastic products, higher melting temperatures and injection pressures are necessary due to the longer and more complex mold flow channels, resulting in increased flow resistance of the molten material.

The Benefits and limitations of Overmolding

Benefits of Overmolding:

Enhanced Product Functionality:

Overmolding allows the combination of different materials, providing improved functionality and performance. This is particularly beneficial for creating products with soft-touch grips, ergonomic designs, and enhanced durability.

Reduced Assembly Steps:

By molding multiple materials or components together, overmolding eliminates the need for additional assembly steps. This streamlines the manufacturing process, reduces labor costs, and enhances overall efficiency.

Improved Grip and Comfort:

Overmolding is commonly used to add soft and tactile surfaces to products, enhancing user comfort and grip. This is advantageous in the production of tools, handles, and electronic devices.

Cost Savings:

Cost Savings: Despite initial tooling costs, overmolding can result in overall cost savings by reducing the need for additional parts and assembly processes. It can also lead to lighter and more cost-effective designs.

Limited Of Overmolding:

Higher Tooling Costs:

Overmolding are more intricate than injection molds, requiring the use of two mold sets on the same injection machine. The need for precise dimensional accuracy, as the two sets must match, increases production costs compared to using two separate single injection molds.

Material Compatibility Issues:

Ensuring compatibility between different materials used in overmolding is crucial. Mismatched materials may lead to poor adhesion, delamination, or other quality issues. Thorough material testing and selection are essential.

Production Speed:

Overmolding processes may have longer cycle times compared to traditional molding due to the need for multiple injection phases. This can impact production speed and output.

Overmolding Design Guide

The design of products for overmolding should adhere to the following guidelines:

#1. The design of the internal interface

The design of the internal interface

The strength of the bond between the substrate material and the overmold material relies on material compatibility, process temperature, bonding surface characteristics, forming sequence, and the design of mechanical interlock structures on the internal bonding surface.

To enhance bonding strength, it is advisable to maximize the bonding surface area.

If the internal bonding surface is too small, consider increasing roughness through surface biting and incorporating mechanical interlocking structures in the design.

#2. Design of the exterior matching surface

Designing the mating surface appearance for both the first and second injections poses a significant challenge, requiring ample bonding strength to prevent issues such as flash, burr, curling, and peeling.

This challenge is particularly evident in two-color injection molding involving soft and hard plastics. Common design approaches for appearance matching surfaces include two categories: Convex type and Concave type.

#3. Uniform wall thickness and avoid sharp corners

Two-color injection molding falls under the category of injection molding, necessitating adherence to Design for Manufacturability (DFM) guidelines.

This includes maintaining uniform wall thickness, avoiding sharp corners, and ensuring smooth transitions, as recommended in standard injection molding design practices.

#4. Draft Angle

Draft Angle

The draft angle holds significance in overmolding as it dictates the mold to which the part adheres during rotation.

In the initial injection, the part should adhere to the mold in motion, while in the subsequent injection, the part should attach to the ejector side of the mold.

Following the first injection, if the part’s surface directly connects with another stationary die, it’s vital to confirm that the plane possesses a draft angle of sufficient magnitude to prevent scratching when the die is closed.

5 Tips of Overmolding mold structure

The mold structure for overmolding is a critical component in the manufacturing process, as it determines the final shape and quality of the overmolded part.

Overmolding molds are typically more complex than traditional molds due to the need to accommodate multiple materials and components. Here are key elements of an overmolding mold structure:

#1. The two shapes of the Overmold must be different to form a single product, while the two shapes of the overmold must be exactly the same,

#2.The front and back dies of the mold must align perfectly after being rotated 180° to the center.

#3. Designers should meticulously verify parameters such as the maximum thickness, minimum thickness of molded parts, and the distance between KO holes.

#4. To prevent the second injection mold from rubbing against the initially molded products, designers can incorporate a designated gap in the design of the second mold.

#5. During injection molding, allowing for a slightly larger size in the first injection can facilitate a snug fit with another male mold during the second molding, ensuring a seamless and precise connection.

Application of Overmolding

Overmolding finds diverse applications across industries, enhancing product functionality and aesthetics. Common applications include:

Consumer Electronics:

Overmolding is widely used in electronic devices, providing improved grip, durability, and design flexibility for products such as mobile phones, remote controls, and wearable devices.

Automotive Components:

Automotive industry utilizes overmolding for components like handles, knobs, and interior parts, enhancing both functionality and visual appeal.

Medical Devices:

Overmolding is employed in medical equipment, creating ergonomic and comfortable devices. Surgical instruments, handles, and wearable medical devices benefit from this process.

Tool Handles and Grips:

Hand tools and power tools utilize overmolding to improve user comfort, grip, and overall usability.

Sporting Goods:

Overmolding is commonly used in the production of sporting goods like bicycle grips, golf club handles, and equipment handles to improve grip and comfort.

Toy Manufacturing:

Toys often incorporate overmolding for safety, comfort, and design versatility, creating products that are visually appealing and durable.

Aerospace Components:

Overmolding is applied in the aerospace industry for specific components, offering lightweight solutions with improved functionality.

Medical Equipment:

Overmolding is used in the medical equipment industry to create handles, grips, and ergonomic components for devices used in healthcare settings.

Author

Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.

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