The intricacies involved in designing and developing a plastic part are often hard to imagine. The technique of plastic injection molding has made it possible to use plastic resins in various industries and applications.
However, with some plastic parts, the geometric complexities and the need for additional features like aesthetics, soft-grip surface, etc make it necessary to consider new technologies like plastic overmolding.
The ability to use various overmold combinations like plastic over plastic, rubber over plastic, plastic over metal or rubber over metal has made it possible to enhance the properties of a part.
The sections below provide all the necessary information that you’d need to understand the process of overmolding.
What is overmolding?
Plastic overmolding is a process using the injection molding technique where one material is applied on top of a second material. The procedure uses heat and pressure to create a bond between the two plastic materials. The molding of one thermoplastic on top of another creates a stronger part that has the desired properties of both plastic resins being used.
It is hence the use of two separate plastic injection molds to create parts that have unique textural, visual appeals as well as applications.
The use of plastic overmolding technique has given an edge to the plastic injection molding technique by increasing the functional purposes of the parts being produced. Multiple industries like electronics, agriculture-based sectors, automotive industries, medical units, etc make use of plastic overmolding techniques.
The diversity in the types of plastic resins available has widened the scope of overmolding. The technique works well with both, soft plastics like urethanes and TPE (thermoplastic elastomers) and hard resins like polycarbonate, nylon, etc.
Various overmold combinations are used by manufacturers to enhance the strength of the parts being produced. These include:
- Plastic over plastic- Molding of a rigid plastic substrate on top of a previously molded part.
- Rubber over plastic- Adding of rubber or Thermoplastic elastomer on the top of a molded plastic substrate.
- Plastic over metal- Molding of a plastic substrate on top of a metal cast/part.
- Rubber over metal- Molding of a flexible rubber substrate on top of a metal cast/part.
So how does the process of overmolding actually work?
Here’s a list of the key steps that are included in plastic overmolding:
STEP 1: The plastic resin/substrate is added to the injection molding tool.
STEP 2: The overmolding material is injected around the substrate
STEP 3: The material is left for curing (solidification)
A joined product (with two materials joined as one) is finally produced as the final part.
Following two types of plastic injection molding processes are used for substrate molding and manufacturing of products:
Insert plastic overmolding
This is a commonly used procedure when considering overmolding. In this process, a pre-molded insert is placed inside the metal mold and the chosen thermoplastic is injected over it.
Insert Plastic overmolding technique is considered as a convenient process since it does not require additional machines. Conventional injection molding machines are commonly used for the technique. Additionally, the process has low tooling costs which makes it a more cost-efficient solution as compared to the other techniques.
Multiple Material Plastic overmolding
As the name suggests, this technique makes use of multiple barrels to inject multiple materials at the same time within a common mold. Since both plastic materials are injected at the same time, this overmolding process has a shorter cycle time and is used when faster production rates are needed.
Multiple material plastic overmolding also gives superior part quality which is an added advantage.
Advantages of Plastic overmolding
The technique of overmolding has been used in the past couple of decades to create parts that are high in quality and durability. Additionally, the reduction in labor and assembly costs is a big factor in cutting down the overall production cost.
Following are some of the key advantages of overmolding design:
- Parts produced with overmolding techniques have improved strength and structure.
- Overmolding offers enhanced design flexibility as well as the easy use of multiple materials/resins through injection molding techniques.
- Overmolding process eliminates the need for bonding step during the manufacturing process.
- The parts produced by plastic overmolding have better alignment. This means that there is no loosening of plastic resin which gives better resistance to pressure, shock, and vibration.
- Overmolding presents aesthetically pleasing shapes and styles along with more color options.
- The technique has low assembly and labor costs.
- Faster production rates can be achieved by multiple material plastic overmolding techniques.
Material selection for overmolding
Different plastic resins have different properties in terms of hardness, flexibility, etc. Various plastic applications require specific materials for the overmolding process. An overmolded part could be hard, soft or even a combination of both. The type of touch required in the product usually determines the materials selected for overmolding.
Following are the key factors used to select the material type that is most suitable for the part:
When in need of thermoplastics for making handle grips, it is often necessary to look for very soft textured plastic resins. However, the thinner the overmold for a thermoplastic elastomer, the harder it feels. This is often incorporated by adding ribs that are placed in succession to give the feel of thick material, even when a small amount of material is being used.
Hardness and Flexibility
The hardness of a material is not necessarily related to its flexibility and elasticity. While people are often confused between the two terms, hardness and flexibility are two separate features and can often be seen in one plastic resin. The figures on the durometer are hence not a depiction of a material’s flexibility.
Flexural modulus is a better indicator of flexibility. This is a measure of a material’s flexibility or in other words, it’s resistance to bend. A higher flexural modulus means that the material is stiff with minimal abilities to bend.
Plastic parts that need to have a softer feel often need resins that have low flexural modulus for the overmolding process. These types of plastic resins are often suitable for handles, medical equipment, electrical appliances, etc.
Coefficient of friction
Friction is the resistance that occurs between two surfaces when rubbed against each other. Similarly, the Coefficient of Friction is the amount of force that is required to move one material on top of another.
Thermoplastic elastomers are usually rubbery in texture. This means that the TPEs being soft in nature offer a higher Coefficient of friction. However, this is not true in the case of all plastic resins.
Applications of overmolding
Overmolded plastics find a wide range of applications in multiple industries.
Some of the most common applications of overmolding include:
- Plastic or metal parts with a soft grip handle
- Medical devices
- Knobs for appliances
- Window reveal molding
- Encapsulated electronic devices
- Electrical components
- Automotive parts
Final words: As discussed above, mold design, as well as the type of material selected for overmolding, are two important criteria for custom overmolding techniques. An appropriately selected material helps in maintaining part quality in terms of tolerances and tooling reliability.
Automation techniques are now employed in various manufacturing sectors for creating precise overmolded parts. The reduction in labor costs, faster assembly time, and improved quality and reliability of the products make overmolding a very important process in the world of plastic injection molding.