How Does Plastic Overmolding Work?

Table of Contents

Overmolding is the process in which two different plastic materials are fused into one product through two injection molding processes. Many products development in our life use overmolding manufacturing technology, mainly for beauty equipment, night lights, automotive interior panels, notebook shells, electric toothbrushes, handles, car buttons, sports equipment, rechargeable battery boxes, power tools, humidifiers, waterproof products, and other fields, etc. They are composed of two or more materials that can provide a comfortable experience.

The over-molded products are mainly injection molded with soft rubber and hard plastic. Soft rubber is commonly used in artificial rubber, TPU, TPR, TPE, and other rubber materials, and the hard plastics can be ABS, PC, PP, etc. Hard-coated with soft rubber is mainly used for products touched by hand, and hard-coated with hard plastic is used chiefly for high-precision products.

Do you know Insert Molding? Insert molding is a similar process. Instead, it uses a preformed part—often metal—that is loaded into a mold and then overmolded with a thermoplastic resin to create a final component. It can seem like a special overmolding, though not precisely because it only takes one injection molding process.

Overmolding is famous for industrial manufacturing, and it helps save manufacturing costs for the automotive industry, medical devices manufacturing, consumer electronics production, etc.

Overmolding is a two-step injection molding

The semi-products produced by ordinary injection molding machines are processed manually and then put into another set of molds for injection molding. These two parts fused to be a whole. Finally, get the desired product. Generally, the overmolding process is completed by two molds with the same structure as the mold used for single-color product injection molding. No particular two-color injection molding machine is required. Ordinary injection molding machines can meet the equipment requirements, which is also a common practice in factories. It is mainly by adjusting the flow rate of the injection parameters to control the bonding point of different materials.

Making plastic molds for overmolding manufacturing should consider the following:

  • In the second injection molding, the mold needs to design the clamping position and avoidance position of the semi-product for the second time so that the product can be placed in the designated position faster and more accurately to avoid the occurrence of mold pressing.
  • When designing the mold, the engineer should consider that the product only shrinks during the first injection molding. So the second injection molding will not shrink when the product is taken out and put into the second mold. The accurate tolerance is measured after the first molding, and the size of the second mold is usually the surface size of the soft rubber product. The dimensional tolerance requirements are not very high.

Two-color injection molding is similar to overmolding to a certain extent, and it is injection molding of two or more materials. But not precisely. 2 K injection molding differs much from plastic over molding.

What’s the difference?

2K injection molding is called double injection molding, two shot injection molding. Two sets of molds are simultaneously installed on the double-color injection molding machine, or the soft and hard plastics are synchronously injected in the same mold. Usually, the injection molding machine for 2K injection molding has at least two barrels. When the injection molding is completed, the indexing plate of the core side of the mold rotates 180 degrees. At the same time, secondary injection molding is performed to obtain the desired product.

(The two-color mold can be an integral two-color mold and a split two-color mold, all of which are installed in a double-color injection molding machine. The principle is the same, but there are differences in mold manufacturing processing.)

Split two-color mold: Two sets of molds need to be set up, and the mold bases of the two sets of molds have the same size. The core side of the mold part is the same, and the cavity side mold part is different. When installing, it is necessary to ensure that the two halves of the molds are at the same height and there is no positional deviation.

Integral two-color mold: The split two-color mold is combined on one mold. The gating and ejection systems are set up on the same mold but must work independently. That is to say, the mold core side is the same, and the cavity side is different. The advantage of the integral two-color mold is that only one set of mold needs to be built, the injection molding processing cost is lower than that of the split two-color mold, and the installation will also be less one mold.

Different from overmolding, it is unnecessary to take out the product when converting from one-step to two-step molding. Both sets of molds should consider shrinkage issues during mold design, and the dimensional tolerance is still based on the size of the first mold. Two shot injection molding technology has tolerances prevail.

The number of the two-color injection molding machine in the factory is limited. Because of the enormous cost, most factories don’t invest in two-color injection molding machines. When scheduling production, it is necessary to determine whether the supplier has a double color injection molding machine that meets the molding requirements of the product.

The product structure of the two-color mold is relatively simple, and the system of the lift and the slider should be designed as little as possible to ensure the safe use and stability of the mold during running in the two-color injection molding machine.

The double color injection mold is used as the benchmark. The guide post cannot be like the guide post of the ordinary mold, and the one with the reference angle is offset to the center position by 2.00mm. Otherwise, the guide pole and the guide sleeve collision can not be closed when the two-color injection molding machine runs.

What are the problems that are prone to occur in overmolding?

Size deviation

As there are two-step injection molding, the hard plastic is injected first, and the soft rubber is injected later. In this case, it is necessary to consider the shrinkage issues of the hard plastic part and soft rubber in the mold design. Because the dimensional deviation has a great relationship with the mold design and parts processing; Secondly, the soft rubber part usually has appearance requirements. The size requirements are not very strict. It is more likely that the second injection molding does not meet the criteria after the first injection molding. In this case, it is necessary to debug the first mold product. The size requirements can be obtained correctly by changing the molding parameters and time.

Product pressing

Mold pressing is the last thing a factory wants to see and usually happens during injection overmolding. When placing the first injection-molded semi-product into the second injection molding mold, the product is not placed in the designated position, or the product position is shifted, which is a common cause of mold pressing. Therefore, the production efficiency and qualified product rate are low, and we can only strengthen the production awareness of employees or transfer to two-color injection molding production.

Soft rubber can’t wrap the plastic part during overmolding

To make soft rubber and hard plastic fused tightly after overmolding, it is better to use a unique two-color mold material, and the part surface’s roughness is also one factor. When debugging the product, in addition to the start-up debugging to prevent the molded part from sticking to the mold cavity and plugging the material, avoid the use of a mold release agent at other times.

In production, if the first batch of semi-finished products is not immediately transferred to the secondary molding process, preheating treatment under 80℃-100℃ is necessary. It can have better encapsulation and bonding effects.

Short shot in secondary molding

In primary molding, it is necessary to ensure that there is no short shot in the product to improve the qualified rate in the secondary molding. In secondary molding, the soft rubber of TPE material is usually used for encapsulation. Due to the poor fluidity of the material, the finished product is not tightly packaged and lacks edges. It is necessary to re-adjust the molding parameters and improve the injection time and molding cycle.

Forming deformation of the semi-part

The temperature range and material selection of the barrel of the secondary molding should ensure that it is lower the melting temperature of the surface of the primary molding product and the temperature difference is about 20 °C. Secondly, try to reduce the holding pressure parameter or not use the holding pressure during the secondary molding. Otherwise, the shape dislocation of the joint surface or the warpage of the semi-product will quickly occur.

Long molding cycle time

Long molding cycle time is a common problem with overmolding. The main reason for the slowness is the two-step injection molding and semi-part locating required in the actual practice and operation process of overmolding. The working efficiency of the integral two-color mold is higher than that of the split two-color mold, which is higher than that of injection overmolding.

Sticky with mold cavity

The problem of sticking to the mold cavity is easy to occur when the TPE soft rubber is injected into the secondary molding. When designing the mold, the draft angle should be more significant, and the surface should be rougher.

The logo or engraving text is not clear after overmolding

Overmolded mold with logo or engraving text, the text or the logo is usually unclear, mainly because the font’s height is too low, which brings great trouble to the debugging process. The standard “preload” value is 0.02~0.05mm higher than the theoretical value. The font of the formed product will be clear.


Plastic overmolding offers various advantages, including:

· Improving part strength and structure
· Reducing secondary operation, assembly, and labor costs (Molded as one assembly)
· Eliminating bonding step in the manufacturing process (Improved component reliability)
· Plastic over-molded components ensure proper alignment, and prevent loosening, and the plastic resin can provide improved resistance to vibration and shock
· Enhancing design flexibility and multi-material components (Custom plastic over-molding allows for the production of parts molded of multiple thermoplastic materials)
· Plastic overmolding improves people’s experience.
· Reduce the cost of production, reduce the cost and waste associated with assembly defects, and in turn, shorten the molding cycle and improve work efficiency.


Now, double injection molding, injection overmolding processing, and multi-color injection molding processing have been widely used in automobiles, medical products, packaging, electronic products, home appliances, toys, home furnishing, power tools, daily necessities, portable wearable products, and other fields, and have developed rapidly. Various innovative technologies for overmolding, insert molding, two-color, and multi-color molding continue to emerge, especially two-color/multi-color injection molding technology has become the preferred direction for the processing of many high-end products. Multi-color injection molding has gradually developed into a mainstream technology in the injection processing market. More high-quality and high-value-added products have begun using multi-color and multi-material molding processes.

Customized over-molded plastics provide competitive advantages in a wide range of applications. We summarize the overmolding industry, including:

· Medical devices and instruments
· Automobiles
· Parts with soft grip handles
· Window reveal moldings
· Knobs for appliances, controls, and assemblies
· Packaging
· Encapsulated electronic devices and electrical components
· Home appliances
· Toys

Overmolding, insert molding, 2K injection molding, or multi-color injection molding are common in the plastic injection molding industry. Molding forms will be horizontal injection molding and vertical injection molding. This depends on product structure and actual practice.
Plastic injection molding is essential in the modern production of plastic products, and the injection molding industry has seen many changes in recent years. Because of the significant investment in plastic mold manufacturing equipment, much plastic product development customers would like to work with tooling partners to decrease production growing time.

TDL plastic mold will be your trustworthy tool partner.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

Contact Our Experts

Send us a Email, we will feedback to you ASAP!