Insert molding is when an insert typically made of metal is placed in the cavity prior to plastic molding around it, bonding with the insert to make one final product. Overmolding is quite similar, although the initial substrate is molded first, then, plastic is molded around it to create a two-shot final product.
There are many factors for mold design engineers to consider when designing or constructing a mold for this type of project. Some of the main considerations include:
- Evaluating Material Bonding and Adhesion
It is necessary for the over-molded plastic to adhere to the original part, making it imperative to evaluate the bonding properties of both parts. If material bonding proves challenging, designers may consider mechanical bonding to provide a means of connecting the two pieces successfully. Mechanical bonding may be a simple feature, such as a through hole with a counterbore or something more sophisticated, like an elaborate runner system on the bottom of the substrate.
Another option for improving adhesion is the use of heat pretreatment systems that raise the surface temperature of the initial substrate, allowing improved adhesion during the overmolding process.
- Maintain Uniform Wall Thickness on Second Shot
There is typically no shrinkage applied to the second shot, considering the molded substrate will not shrink. Maintaining uniform wall thickness in the second shot is necessary, with the same principles applying to injection molding single plastic parts applying to any voids in the substrate. This includes ribs to be 60% of the intersecting wall thickness with generous radii and fillets for optimum flow.
- Support Substrate During Second Shot
Injection molding occurs with pressure against the mold cavity walls. During initial part molding, this pressure fills the cavity to mold the perfect part. During overmolding, this pressure can cause molding issues if the substrate is not supported adequately to hold up under pressure. Proper mold design should include support areas on the substrate to help the second molding process. These support features must have built-in clearance to accommodate for part tolerance deviations. Any ribbing should be kept to a minimum in these areas when possible.
we are seeing many industries shifting toward insert molding for plastic components. One example is automotive manufacturing and trucking. This is due to weight reduction, which means better fuel economy. This has been a huge asset in manufacturers continuing to meet federal fuel-efficiency standards.
Besides replacing non-mechanical parts like side panels, many polymers are being used in engine parts. They can reduce weight without compromising performance in the motor.
The medical field is a common industry where we are seeing more plastic parts with integrated metal components. This includes many surgical tools and devices.
That is why you are seeing more consumer product manufacturing moving towards plastics. Likewise, aerospace and aviation engineers are turning to high-quality molded plastic for aircraft parts. Like in the automotive sector, lower weight is a vital asset to aeronautics manufacturing.
Plastics have become an asset in military and defense equipment manufacturing as well. It allows the production of lighter gear for the troops. It also has afforded many advantages in armed forces vehicle production, including stealth and armor upgrades that would not be possible using metals.
Finally, a niche industry where plastics have replaced metal parts is LED lighting. The plastic housing is much more versatile and does not absorb heat as metal ones.
The only difference in insert molding is that manufacturers can place components in the mold before it closes. This allows the plastic resins to form around it. As it hardens, the result is a customized part that takes less time to make and weighs less than an all-metal alternative, making it a key process to leverage in DFM.
It is possible to insert many different types of materials in the mold. These include magnets, tubing, mounting brackets, screws, and almost any metal component.
This advantage reflects the shift of many industries in replacing metal parts with plastic ones. Plastic offers lower weight and, depending on the part, comparable durability in operation. It also is resistant to corrosion, unlike most metals.
There are many advantages to this availability. Here are seven to consider.
1. Durability
One big advantage of insert molding is that it affords enhanced product design and manufacturing. The precision with which you can place inserts into a mold design cannot be achieved with other molding processes. In short, plastic parts with integrated components are much stronger and more reliable.
Also, since there is no assembly involved in this one-step process, there is less opportunity for error in manufacturing.
2. Weight
Plastic parts can be up to 50 percent lighter than their metal counterparts. This, combined with strength, is one of the primary reasons that industries are turning to plastic molding for parts. From fuel efficiency to better ease-of-use for surgical instruments, lighter weight enhances performance and functionality.
3. Size
The insert molding process allows the production of parts that are much smaller or thinner, without sacrificing durability or performance. A simple example is vehicle components. Thinner door and console parts mean more room in the interior of the car.
Pins can be automatically placed into a plastic mold, like this custom connector, which then hardens to keep them in place.
4. Economical
Insert molding achieves cost savings in two ways. First, they allow the use of much cheaper materials. In general, resins are much more affordable than metals.
Also, molding production processes themselves are cost-effective. Insert molding is a one-step process. Unlike other types of molding, assembly and labor costs can be streamlined.
5. Efficiency
Insert molding eliminates a second assembly step in the manufacturing process. The insertion of metal or other components is integrated with the molding step, so there is no need for a stand-alone task. This saves time and money associated with labor and specialized equipment for these processes.
6. Variety of Materials
Insert molding can use many different types of plastic resins in the process. These including thermoplastics, which are eco-friendly since you can recycle them. The most common thermoplastics are polyethylene, polystyrene, and nylon. These are all lightweight, durable, and ideal for consumer products to heavy-duty industrial ones.
Other common materials are thermosets, such as epoxy, and elastomers. Natural rubber, which is both strong and flexible, is a good example of the latter.
7. Variety of Designs
Similarly, on this jack, two different pins are placed into the plastic mold.
Besides materials, insert molding allows many different types of shapes and designs for its parts. This is often not achievable using metals. It’s obvious how unlimited configurations have expanded the range of possibilities for engineering and design in different industries
Insert molding is a specialized type of injection molding that combines different materials into a single finished part. While 2 shot molding allows multiple types of plastics to be combined, insert molding allows the inclusion of metal components in a plastic part. For example, you could have a plastic cabinet-door knob with a metal fastening screw embedded during the molding process instead of having to assemble it later.
A wide range of metal components can be used in the insert molding process, including screws, limiters, threaded fasteners, magnets, filtering screens, tubes, clips, mounting brackets, and more. These components can be made from stainless steel, brass, copper, aluminum, nickel, or bronze. Your injection molding company will work with you to select the best combination of metal and plastic based on the part’s requirements.
Good design practices include carefully crafting molds to ensure proper retention of the metal in the plastic part. Some inserts may need to be heated prior to being molded to reduce the thermal difference between the plastic and the metal, which could cause quality issues. There are several reasons to choose insert molding:
It reduces the post-molding assembly required with separate metal elements, which can be costly. Separate installation of these components adds motion waste, increases production time, and involves additional equipment and labor. Insert molding saves those assembly and labor costs.
It enables designers to integrate features into plastic parts that will make them stronger, more durable, and more reliable than native plastic areas. For example, installed metal compression limiters will strengthen fastening areas and eliminate creep potential.
It allows enhanced product design by integrating features not possible with plastic alone: for example, a stainless steel screen insert can provide a filtration area in a plastic part.
It is useful in metal to plastic conversion to make parts more efficient. There is increasing demand across industries to produce parts in plastic instead of metal — to lower weight, reduce production costs, and eliminate corrosion. Insert molding allows retention of essential metal elements within a plastic part. Many industries can benefit from insert molding, such as:
Automotive and Trucking: Metal to plastic conversion parts reduce weight and improve fuel economy
Medical: Plastic parts with metal components are used in a wide range of medical and surgical devices
Consumer Products: Insert molding has applications in an array of industries from electronics to HVAC systems
Aerospace: Insert molding is used for parts for aircraft interiors, controls, communications and guidance/navigation.
Insert molding provides you with the best of both worlds: it retains the strength and functionality of metal parts by embedding those elements into a more economical and efficient plastic part.