Today I’m going to show you an article to let you understand 2k injection molding in 2022!(2k Injection Molding Factory/Expert/Supplier based on China.)

In this comprehensive guide I’ll cover:

○Introduction to 2k injection molding.

○Advantages and disadvantages of 2 shot injection molding

○Application of double shot injection molding.

○Material selection for double injection molding.

○Design guidelines for 2k injection.

Lots more

So if you want learn and understand well injection molding, you’ll love this updated guide.

Let’s get started.

2k Injection Molding Factory/Expert/Supplier

01 Introduction to 2k injection molding

1.1 The concept of 2k injection moulding

2k injection molding(2shot injection molding or double shot injection molding)is a manufacturing process used when referring to molding two materials/colors into one plastic part. This plastic molding technology blends two materials or two different colours materials into one ending plastic part by using 2k injection molding machine.

In this article, the first injected material is referred to as the base material and the second injected material is referred to as the overlay material.

The most common form of a 2k injection mould is two identical moving moulds to fit two different fixed moulds.

After the first injection of the base material, the mould is opened and then rotated by 180° using the rotatable structure of the injection machine, then closed the mold and start second shot inejction by using a different colour of material or a different material (covering material) than the first injection.

After the second opening of the mould, the moving mould, which has already been injected twice, will ejected the plastic 2k part.

The first injection of the base material and the second injection of the cover material are injected out simultaneously, requiring two injection nozzles on the injection machine, each injecting a different colour or a different base material and cover material.

2k Injection molding Schematic

2k Injection molding Schematic

2k and 3k injection moulding Schematic


2k and 3k injection moulding Schematic

The most typical example of two-colour injection moulding in everyday life is the toothbrush.

A toothbrush is a product that combines hard and soft plastics. A hard plastic is used for the main part of the toothbrush to provide sufficient strength; a soft plastic is used for the part that comes into contact with the hand to ensure a tactile feel; at the same time two-colour injection moulding provides the toothbrush with a colourful appearance.

1.2 The difference between 2k injection moulding and insert/overmold moulding

(1) Insert/over moulding is done using a common injection moulding machine, where the product is taken out from one mould and then put into another mould for a second injection moulding.

Therefore, insert/over moulding is usually done by two different moulds, no need 2k injection moulding machine.

2k injection moulding is the simultaneous injection of two plastics at once in a dedicated two-colour injection moulding machine.

The two plastics are injected on the same injection moulding machine and moulded twice, but the product only comes out once. Therefore, two-colour injection moulding is usually done by one set of moulds and requires a special two-colour injection moulding machine.

2) The plastic is in a molten state throughout the entire process of two-colour injection moulding, with no cooling shrinkage, which means that there is the opportunity to form a strong chemical link between the base material and the overlay material, with high strength and reduced risk of burrs occurring.

3) Compared to insert/overmold injection moulding, 2k injection moulding does not require the insertion of inserts, resulting in shorter and more efficient moulding cycles, as well as higher quality.

02 Advantages and disadvantages of 2 shot injection molding.

2.1 Advantages of 2 shot injection moulding

1) Increased product design flexibility

2 shot injection moulding can integrate multiple functions into one plastic part, such as soft-sensitive surfaces, ergonomic design, two-colour appearance, brand identity, feature improvement, noise reduction, shock absorption, water resistance and impact protection, saving design space and reducing the number of components, etc.

2) Reduced engineering costs

A single injection moulding process can complete a multi-functional plastic part, reducing secondary processing.

3) Different plastics are integrated in one piece through chemical links for higher quality and better strength.

2.2 Disadvantages or limitations of 2 shot injection moulding

1) Compatibility between two plastics needs to be considered

Because of the different chemical properties of different plastics, their compatibility will directly affect the bonding strength and bonding effect of the bonding surface, not all plastics have good compatibility with each other.

2) The difference in shrinkage rate of plastics should be small

Too large difference will affect the bonding strength of the bonding surface and may cause warping of the product.

3) The shape of the finished product should not be too complex

03Application of double shot injection molding.

1) Used in power switches, mobile phone keys, car switches, etc. to meet the requirements of partial light guidance or light transmission.

Injection Overmolding lighting
Injection Overmolding lighting
Injection Overmolding plastic switch

2)Application to hand-held products such as walkie-talkie housings, toothbrush handles, power tool handles, spanners and thermoses to improve feel.


3)Products that require a multicoloured appearance, such as keyboards, the use of two-colour injection moulding makes them beautiful without the risk of rubbing off during use.

2k injection molding manufacturer

4)This is used for products that require partial plating, such as the keys of feature phones.

In the plastics family, only ABS and polysulfone (PSU) surfaces can be plated. If a partial area or a single side of the product requires plating, the plated area should be injected with ABS and the area that does not require plating should be injected with other materials such as PC.

5)Applied to products that need to be waterproof to achieve the requirement of water resistance.

04 Material selection for double injection molding

Plastics are different, their chemical properties are different, and their compatibility will directly affect the bonding strength and bonding effect of2 K injection moulding/double injection molding.

Therefore, when designing products for two-colour injection moulding,The first and most important thing is to choose the most suitable substrate material and covering material。

1)Choose compatible substrate materials and coverings,The compatibility of common plastics is shown in the diagram.

The chart above can be used as a general selection guide only. It is always best to contact your plastics supplier for a more professional opinion when selecting a specific material type.
You can contact us too!

The different TPE models from RTP, as pictured, differ considerably in their compatibility with plastics, even though they all belong to the TPE range (X indicates good compatibility).

2)The difference between the melting temperature of the base material and the covering material does not exceed 30-60°. The melting temperatures of common plastics are shown in the following chart.

3) The coefficient of thermal expansion CTE of the base material and the covering material are as similar as possible.

4) The shrinkage rates of the base material and the covering material are as similar as possible.

5) Avoid the combination of amorphous and semi-crystalline plastics.

6) The HDT of the base material should be higher.

05 Design guidelines for  2K injection Molding

The design of a product for 2k injection moulding needs to follow the following design guidelines.

1) Design of the internal bonding surface

The strength of the bond between the base material and the cover material depends on the compatibility between the materials, the process temperature, the bonding surface, the moulding sequence and the design of the internal bonding surface mechanical interlocking structure.

Firstly, the area of the bonding surface should be maximised to increase the bonding strength.

When the internal bonding surface is too small, the roughness can be increased by nibbling on the bonding surface on the one hand, and mechanical interlocking structures can be designed on the other.

2) Design at the Cosmetic mating surface

The design of the Cosmetic mating surface at the first and second injection is very challenging and must provide sufficient bonding strength, otherwise flying edges, burrs, rolled edges and flaking can easily occur, especially in 2k injection moulding of hard and soft plastics. Common Cosmetic mating surface designs include stepped and fluted.

3) Uniform wall thickness and avoidance of sharp corners, etc.

2k injection moulding is a type of injection moulding, so DFM design guidelines for injection moulding, such as uniform wall thickness, avoidance of sharp corners and even transitions, must also be observed for 2k injection moulding.

4) Draft Angle

The Draft Angle is important for two-colour injection moulding because it determines which mould the part is attached to during the rotation process.

On the first injection, the part should be attached to the moving mould; on the second injection, the part should be attached to the ejector side of the mould.

After the 1st injection, if the surface of the part fits directly into another fixed mould, it should be ensured that the plane has a sufficiently large release slope to avoid scratching when the mould is closed.

5) Weight difference

The weight difference between the base material and the cover material should not be too big different, otherwise it will affect the moulding cycle and injection pressure etc.

I hope you enjoyed this updated guide to 2k injection molding in 2022.

Now I’d like to hear from you:

Let me know by leaving a quick comment below right now.

Or contact us
You best 2k injection molding expert.