Overmolding is currently one of the most popular and advanced rapid tooling methods and offers significant potential for production costs, cycle times and new design opportunities for product manufacturing. Thanks to the various design advantages of 2K molded products, it is widely used in mobile phones, pens, cars, household appliances, hand tools and household items, etc.
This article mainly introduces the development and trend of overmolding technology. In particular, it explains how small-batch prototype production is carried out in the product development phase for 2K overmolded parts.
What is overmolding?
Overmolding is an injection molding process in which a single part is created by combining two or more different materials. The first material is generally referred to as support material, parts or all of which are covered by subsequent materials (overmolded materials).
Injection molding was initially used to improve the appearance of consumer products, but is now quite popular. The technology is used in a variety of applications and can even be used to improve product functionality such as improving chemical resistance, providing electrical insulation, providing long-lasting support, providing a softer, smoother feel, etc.
There are essentially two different types of manufacturing processes for overmolded products.
The. Soft material pulverizes hard material : Common materials in multiple injection molding are thermoplastic elastomers (TPE) used in the substrate (such as ABS (acrylonitrile butadiene styrene), PC, PC/ABS, PA and SAN). The most commonly used TPEs are SEBS, TPE-V, TPE-U, TPE-E and TPE-A. These “soft fairing products” have improved features such as insulation, chemical resistance, better ergonomics, better handling, grip and superior aesthetics.
b.Hard material overmolding hard material : Another application is secondary overmolding materials, which are also made of hard plastic. For example, rigid molded plastic with a metal insert or rigid transparent molded plastic with non-transparent parts. Typically, the error rate in the production of 2K injection molded transparent parts is very high, which poses great challenges to the control process.
Overmolding process: 2K injection molding and two-component injection molding
2K (two-color) injection molding (molded from two components) is a two-step process, that is, a material is injected into a product in a 2K injection molding machine and then jumped into the plastic tube from the corresponding mold cavity on the 2K machine by rotating the mold to another part. The part is used as an insert, another material is injected to form a two-material product. The injection molding machine is double cylinder or double cavity, the injection molding process releases two types of materials at the same time. Meanwhile, the manufactured product is used as an insert that is implanted.
Another one is Two-Component Injection Molding which means that one piece of material is made in an injection molding machine and the piece of material is placed as an insert in another mold and then the second material is injected. Insert molding (the industry name is overmolding, secondary injection molding): In the insert molding process, a rigid part is first manufactured (usually a rigid plastic part) and embedded in the mold cavity. TPE is then used to mold this component and finally the complete product is obtained. Conventional injection molding equipment can be used for insert molding. Rigid placement of parts can be done using manual or mechanical arms. Normally coated metal components can only be used in this way.
The adhesion strength of overmolded parts
Physical method: through buckle design, surface lamination, tapping on the surface, and then to achieve the second type of material T The package is formed (covered) directly. The characteristic of material bonding made exclusively by this method is that the physical bond has strong adhesion, while the external physical bond has little adhesion.
Chemical method: Connect the two materials through the molecular affinity between two materials and the bonding strength of chemicals to form a single piece, two or more.
Although fastening and physical bonding methods are often used together in practical applications, it is obvious that bonding between the two materials is a more reliable and flexible method. This strong chemical bond consists of the mutual solubility, penetration, penetration and entanglement of molecules or molecular chains.
Four types of overmolding
There are four common types of overmolding available.
- Plastic on plastic – Both the substrate and the plastic molded onto the substrate are plastic. The material of the plastic components must have the same or similar melting point.
- Plastic on Metal – The substrate is made of metal. A CNC machined, cast or forged metal part is first placed in the mold and then plastic is molded over the metal part. Did you know that you can mold metal inserts into plastic parts?
- Elastomer on Plastic – The substrate must be made of rigid plastic. Elastomers can be soft materials such as rubber and polyurethane.
- Elastomer on Metal – The substrate is metal. A CNC machined, cast or forged metal part is first placed in the mold and then the elastomer is molded over the handle.
However, not all materials can be overmolded. For example, if we only want to make a few prototypes, it is unlikely that we will find transparent elastomers that can be overmolded with the material itself or other types of resin.
How do you make the best overmolding products?
1. Material selection: There are mainly three main factors that affect the bonding of TPE/rigid plastic at the molecular level.
1) The polarity of TPE material is similar to that of rigid plastic (otherwise it would not be mutually soluble, permeable and penetrating when melted);
2) The surface tension of TPE material is lower than that of rigid plastic (otherwise, the TPE melt could not spread on the surface of the rigid plastic insert).
3) When the TPE melt flows along the surface of the hard plastic into the mold cavity, heat is released during the cooling process, which can quickly and effectively melt the surface of the hard plastic and form a thin interpenetrating layer.
2. The 7-Point Guide to Overmolding
1) 2K molding is better than the effect of two-component molding (multi-component molding).
Advantages of 2K form: The first shot of rigid plastic injection molding is still hot and immediately goes into the second cavity of the 2K injection molding machine. Because no moisture is absorbed during the transfer process, on the one hand, the high-temperature melting of TPE on the surface can easily cause ablation, forming an ultra-thin layer, and on the other hand, there is no adsorption of water vapor on the surface.
2) Choose the highest possible feed (melting) temperature within the allowable range for TPE. Otherwise, the heat from the TPE would not be enough to remove the hard plastic material surface.
3) When overmolding the second shot, it is better for the hard plastic molding cavity to adopt a high molding temperature to provide heat energy. When the hard plastic reaches a high temperature, the high-temperature TPE melt cools slowly, providing enough heat and time to corrode the hard plastic and form an ultra-thin, insoluble layer.
4) In the second step of overmolding, the TPE melt should be ejected as quickly as possible at a high injection speed, as long as no TPE fly edge is formed.
The. At high injection speed, the TPE melt has a short spreading time on the hard plastic surface and a longer time to wear away the hard plastic surface during the molding cycle.
B. The friction between the TPE and the hard plastic surface leads to friction and heat generation. This means that the temperature of the TPE melt also cools slowly, with a longer time elapsed before contact and ablation of the hard plastic surface occurs.
w. Due to the high injection speed, the viscosity of most TPE melts decreases when diluted, which leads to a reduction in surface tension and promotes spreading on the surface of the hard plastic.
5) Avoid moisture absorption or staining on the hard plastic surface, especially with two-component castings. In certain cases this can happen more easily through the transfer process.
6) Nylon 66, moisture absorption from the surface leads to the formation and adsorption of water molecules that form hydrogen bonds. TPE cannot form intermolecular bonds with molecules on the surface of the rigid plastic and the natural bond strength decreases. TPE coated polar hard plastic is certainly also a polar formula, it does not pay attention to moisture resistance, and at the same time it affects the cohesion effect.
7) Pay attention to the washing cylinder problem in the early stage of the testing machine
The. Previously coated with PVC
B. Used to be a machine that produced hard rubber
Three advantages of overmolding
There are three important reasons to use overmolding in your project.
Better aesthetics
If you want to produce a plastic part with multiple colors or different surface finishes, overmolding is the ideal process. Different colored or structured components can be molded together and processed into a part with perfectly connected colors and surfaces. For example, a clear resin part can be overmolded with a black resin part to form a car lens; and metal inserts overmolded with plastic look better than inserts manually inserted into plastic parts.
Better assembly
Two or more components molded together fit together better than if two components were manufactured separately and then assembled together because in the overmolding process, the plastic applied to the substrate is melted and then cured into the substrate, creating a more stable assembly.
Friendlier
Using elastomers like rubber or polyurethane over a hard plastic or metal part makes it more comfortable for the end user. Metal and hard plastic parts are not very comfortable for the human hand. Adding a soft coat to hard plastic or metal parts can soften the texture of the parts, making them easier to use.
Three Methods for Implementing an Overmolding Product Prototype
Due to the complex structure of 2K injection molds, the product development process requires a longer cycle and is more expensive. If we only produce one or more sets of prototypes in the product development phase, do we need to use 2K injection molding or two-component injection molding? Below are commonly used guidance approaches in our rapid prototyping service:
1.3D printing technology
Two-color FDM 3D printers have been around for a long time. Generally, the 3D printer is equipped with two sprinklers to print materials of different colors, thus realizing the two-color mixing model. However, there are still major limitations when it comes to material selection. Although some 3D printers can print multicolored transparent parts, they have rough construction, low resolution, poor texture, and can even be described as “pixelated.” Therefore, there is still a long way to go until 3D printing with polychromatic characteristics and appearance prototypes.
2. Silicone tool overmolding
Urethane casting is a common method for low-volume prototyping. And it offers a decisive advantage in the production of overmolded prototypes.
Silicone tool castings can meet customer specifications exactly, regardless of color, texture and glossy finish, inlay and overmolding, or even optical clarity. It is ideal for trial production of parts with complex process structures that can come directly from the mold. This allows the part to be easily removed from the mold without having to design the release angle. It offers a standard machining tolerance of ±0.15mm/100mm, the highest precision can reach ±0.05mm. The complex structure can be removed directly from the mold.
Furthermore, the choice of materials is also large, as different materials facilitate the production of urethane castings. Materials with certain properties are similar to production plastics such as ABS, PMMA, PC, PP, PA and soft rubber and are impact resistant, high temperature resistant (120°C) or fire resistant (UL94-V0).
Mold making: The silicone tool overmolding process mainly includes master pattern making, silicone mold making and vacuum casting. Using silicone molds, we can produce various types of overmolding products, such as: B. rigid plastic overmolded with rubber, rigid plastic overmolded with rigid plastic, and plastic overmolded with transparent and non-transparent plastic.
Process control: The position of the sprue and the design of the vent hole of the silicone mold pose a great difficulty in the production of the silicone forming mold. Firstly, the inlet channel must be positioned away from the visible surface of the product and sufficient fluidity is required. for the material to enter the mold, you can fill it. Secondly, the position and quantity of openings often need to be adjusted several times, especially when producing the transparent part, because bad air causes obvious visual defects. Finally, during secondary curing, both materials shrink and deform.
3. CNC machining + surface treatment
When CNC machining 2K prototypes, extensive post-processing is often required to achieve the combination of two colors or multiple materials.
Its characteristics are high speed, high dimensional accuracy and appearance quality. The selection of overmolding materials is large; Almost all technical plastics and metal materials can be processed. After polishing and other surface treatments, the production effect can be comparable to or even superior to that of the injection molded product.
Currently, the production of prototypes for overmolding products is still dominated by CNC. Of course, silicone mold has also made great progress. Typically, overmolding prototype processing is carried out by combining CNC overmolding methods and silicone tools.
New advances in overmolding technology
By observing important international fairs in recent years, European and American manufacturers in the field of multi-color injection molding have begun to present several basic injection molding technologies “in combination”. In addition to traditional multi-color products such as car headlights, air conditioning panels, television frames, etc., new applications such as two-color molded interior labels (IML), two-color molding within mold composition (IMA), StackMold, 2K+IML+IMA+StackMold, two-color molding and interlayer injection molding can be carried out with just one injection molding machine.
As a result, multi-color injection molding technology is receiving more and more attention. We not only need to adopt multicolor technology, but also combine it with other technologies to produce fruits more efficiently. This is the objective that the industry must address in the next phase.
