what is two color Injection molding process?
YONGJIANG GROUP | 2024-05-22
What is two color injection molding process？
Two color injection molding (2K molding or two shot molding) is a process that combines two plastics (or two different colors of plastics) into one product during a single molding process.
In this article, the first injection material is called the base material, and the second injection material is called the overlay material.
The most common form of two color injection molding is two identical moving molds that correspond to two different fixed mold cavities.
After the first injection of the base material, the moving mold is opened and rotated 180° using the injection machine's rotatable structure. Then, the second injection is carried out using a material that is different in color or different from the first injection material (overlay material).
After the second mold opens, the ejection mold performs the mold release action for the completed two injections.
The first injection of the base material and the second injection of the overlay material are performed simultaneously, requiring the injection machine to have two injection nozzles for injecting different colors or different base and overlay materials.
In this article, the first injection material is called the base material, and the second injection material is called the overlay material.
The most common form of two color injection molding is two identical moving molds that correspond to two different fixed mold cavities.
After the first injection of the base material, the moving mold is opened and rotated 180° using the injection machine's rotatable structure. Then, the second injection is carried out using a material that is different in color or different from the first injection material (overlay material).
After the second mold opens, the ejection mold performs the mold release action for the completed two injections.
The first injection of the base material and the second injection of the overlay material are performed simultaneously, requiring the injection machine to have two injection nozzles for injecting different colors or different base and overlay materials.
Two color injection molding usually refers to the molding of two different colors or types of plastics. Since the two sets of injectors and nozzles of the two color Injection molding machine are independent and separated, the color of its finished product is mostly two-color distinct, without mixing together. Two-color molds can reduce the assembly process and post-processing of molded products, save costs, increase the aesthetic visual effect of products, and enhance the added value of products.
The biggest difference between a two color injection molding machine and a general hydraulic injection moulding machine lies in the design of the configuration of the injection unit. Generally speaking, the double color Injection machine has two independent and separate injection units, while the general plastic injection machine has only one set. The configuration of the two injection units varies with the design of each manufacturer. As for the design of the moveable platen, a rotating mechanism must be provided, usually with a common design such as adding a turntable or a rotating shaft mechanism to provide a 180-degree reciprocating rotation function, so that the mold produces cyclic alternating actions. Some special two-color molds do not require a turntable or rotating shaft mechanism, but slide alternately or rotate horizontally by the mold.
Characteristics of the injection molding process of two-color products
1. The two color injection molding machine is composed of two sets of plastic injection unit with the same structure and specifications. The nozzle should have a special structure according to the production details, or it is equipped with a structure that can rotate and switch positions. The two sets of molding molds should be completely identical. During plasticizing and injection, it is required that the process parameters such as the temperature, injection pressure, and injection volume of the two plasticizing and injection devices should be the same, and the fluctuation difference of the process parameters in the two sets of devices should be minimized as much as possible.
2. Compared with ordinary injection-molded plastic products, the temperature and injection pressure of the melted material during two color injection molding should adopt higher parameter values. The main reason is that the mold runner in two-color injection molding is relatively long, and the structure is more complicated, resulting in greater resistance to molten material flow during injection.
3. Two-color injection molded plastic products should use materials with good thermal stability and low melt viscosity to avoid decomposition due to high melt temperature and long residence time in the runner. The most widely used plastics are polyolefin resins, polystyrene, and ABS materials.
4. When injecting two color Injection products, in order to ensure that the two different colors of melted materials can melt and bond well in the mold during molding, and ensure the molding quality of the injection-molded products, higher melt temperature, mold temperature, injection pressure, and injection rate should be used.
Structure of two color Injection molds：
1. Traditional two color injection molds have two types: "punch translation" and "punch rotation."
2. Encapsulated mold: Two types of plastic materials do not necessarily need to be injection molded on the same plastic injection machine and are molded in two steps. After the product is demolded from one set of molds, it is put into another set of molds for the second injection molding. Therefore, generally, this molding process is usually completed by two sets of molds, and there is no need for a special double color Injection machine. The mold structure is the same as that of a monochrome injection molding mold. The combination point of the two materials is controlled by adjusting the flow rate of the injection parameters.
4. When injecting two color Injection products, in order to ensure that the two different colors of melted materials can melt and bond well in the mold during molding, and ensure the molding quality of the injection-molded products, higher melt temperature, mold temperature, injection pressure, and injection rate should be used.
Structure of two color Injection molds：
1. Traditional two color injection molds have two types: "punch translation" and "punch rotation."
2. Encapsulated mold: Two types of plastic materials do not necessarily need to be injection molded on the same plastic injection machine and are molded in two steps. After the product is demolded from one set of molds, it is put into another set of molds for the second injection molding. Therefore, generally, this molding process is usually completed by two sets of molds, and there is no need for a special double color Injection machine. The mold structure is the same as that of a monochrome injection molding mold. The combination point of the two materials is controlled by adjusting the flow rate of the injection parameters.
the Processing of two color Injection molds:
1. Mold cavity processing: To ensure the accuracy of the centering after the male and female molds rotate 180 degrees and the size of the inserted and broken surfaces, the mold cavity opening frame should be corrected parallel with the GP hole of the reference edge and measured with the GP hole of the reference angle.
2. Accuracy of the mold core: To ensure the interchangeability of the male and female molds, the dimensional tolerance of the core must be within ± 0.01mm, and the insertion and slope of the broken surface are even more important.
1. Mold cavity processing: To ensure the accuracy of the centering after the male and female molds rotate 180 degrees and the size of the inserted and broken surfaces, the mold cavity opening frame should be corrected parallel with the GP hole of the reference edge and measured with the GP hole of the reference angle.
2. Accuracy of the mold core: To ensure the interchangeability of the male and female molds, the dimensional tolerance of the core must be within ± 0.01mm, and the insertion and slope of the broken surface are even more important.
3. Mold structure: To ensure that the mold structure operates smoothly during the trial or production, the mold must be tested for interchanging its structural actions to avoid structural damage during trial or production.
4. Mold mounting:
1) Remove all locking nuts before mold mounting.
2) First, install the side mold on the B side once, without adjusting the mold to generate high pressure, otherwise, the fixed side and the rotating disc will be unevenly stressed, damaging the machine.
3) put all locking nuts with your hands before tightening the nuts.
4) After the mold is opened, install the second side mold on the A side, tighten all locking nuts with your hands until fully seated but not tightened.
5) After the mold is opened, rotate the male mold 180 degrees.
6) Use the female mold on the B side that has already tightened nuts to match the male mold on the A side, and tighten the nuts of the male mold on the B side and the female mold on the A side.
7) After the two sets of molds are assembled, then adjust and test the mold.
4. Mold mounting:
1) Remove all locking nuts before mold mounting.
2) First, install the side mold on the B side once, without adjusting the mold to generate high pressure, otherwise, the fixed side and the rotating disc will be unevenly stressed, damaging the machine.
3) put all locking nuts with your hands before tightening the nuts.
4) After the mold is opened, install the second side mold on the A side, tighten all locking nuts with your hands until fully seated but not tightened.
5) After the mold is opened, rotate the male mold 180 degrees.
6) Use the female mold on the B side that has already tightened nuts to match the male mold on the A side, and tighten the nuts of the male mold on the B side and the female mold on the A side.
7) After the two sets of molds are assembled, then adjust and test the mold.
The advantages and disadvantages of two color injection molding
2.1 Advantages of two-color injection molding processing
1) Increased product design flexibility: Two color injection molding machine can integrate multiple functions into a plastic part, such as soft touch surface, ergonomic design, two-color appearance, brand identity, characteristic improvement, noise reduction, shock absorption, waterproofing, and impact resistance, etc. It can save design space and reduce the number of parts.
1) Increased product design flexibility: Two color injection molding machine can integrate multiple functions into a plastic part, such as soft touch surface, ergonomic design, two-color appearance, brand identity, characteristic improvement, noise reduction, shock absorption, waterproofing, and impact resistance, etc. It can save design space and reduce the number of parts.
2) Reduced engineering numbers to reduce costs: The one-time injection molding process can complete a plastic part with multiple functions, reducing secondary processing. all ordinary plastic Injection machine should improve it as well.
3) Different plastics are integrated into one through chemical bonding, which results in higher quality and better strength.
2.2 Disadvantages or limitations of two-color injection molding
1) Compatibility between two plastics needs to be considered: Because the chemical properties of different plastics are different, their compatibility will directly affect the bond strength and bond effect of the bonding surface, and not all plastics have good compatibility with each other. standard hydraulic Injection moulding machine keep studying and optimizing latest technology.
1) Compatibility between two plastics needs to be considered: Because the chemical properties of different plastics are different, their compatibility will directly affect the bond strength and bond effect of the bonding surface, and not all plastics have good compatibility with each other. standard hydraulic Injection moulding machine keep studying and optimizing latest technology.
2) The difference in shrinkage rate of the plastics should be small as a large difference can affect the bond strength of the bonding surface and may cause product warping.
3) The finished product shape should not be too complicated.
Applications of two color injection molding machine：
1) Applied to power switches, mobile phone keys, car switches, etc. to meet the requirements of partial light guide or transparent.
1) Applied to power switches, mobile phone keys, car switches, etc. to meet the requirements of partial light guide or transparent.
2) Applied to handheld products such as walkie-talkie shells, toothbrush handles, electric tool handles, wrenches, and thermos cups, to improve the feel.the ordinary plastic Injection machine is also widely used on it
3) Applied to products with multiple color appearance requirements, such as keyboards, using two-color injection molding, making them not only beautiful but also resistant to friction during use.
4) Applied to products that require local electroplating, such as function mobile phone buttons, and so on.
Only ABS and polysulfone (PSU) surfaces in the plastic family can be electroplated. If the product's local area or one side needs electroplating, the electroplating area should be injected with ABS, and the area that does not need electroplating should be injected with other materials such as PC.hydraulic Injection moulding machine is also widely used on it.
5) Applied to products that require waterproofing, meeting waterproofing requirements.
Material selection for double color injection machine
As different plastics have different chemical properties, their compatibility will directly affect the bond strength and bond effect of two color injection molding.
Therefore, when designing two color injection molding products, the most important thing is to choose the most suitable base material and covering material.
1) Choosing compatible base material and covering material, the compatibility of common plastics is shown in the diagram below.
The above figure is only a general selection guide. When selecting specific material models, it is better to contact the plastic supplier and seek more professional opinions.
For example, different types of TPE models from the RTP company, although all belong to the TPE series, have significant differences in compatibility with plastics (X indicates good compatibility).
2) The difference in melting temperatures between the base material and the covering material should not exceed 30-60°. Common plastic melting temperatures are shown in the figure below:
3) The coefficient of thermal expansion (CTE) for the base material and covering material should be as close as possible.
4) The shrinkage rate for the base material and covering material should be as close as possible.
5) Avoid combining amorphous plastics with semi-crystalline plastics.
6) The HDT of the base material should be higher.
Design guidelines for two color injection molding machine
The product design for double color injection machine needs to follow the following design guidelines:
1) Design of internal bonding surfaces
The bond strength between the base material and the covering material depends on the compatibility of the materials, the process temperature, the bonding surface, the molding sequence, and the design of the mechanical interlocking structure of the internal bonding surface.
First, try to increase the area of the bonding surface to increase the bond strength.
When the internal bonding surface is too small, on the one hand, roughness can be increased through mulling on the bonding surface, and on the other hand, mechanical interlocking structures can be designed.
Other mechanical interlocking structures are shown below:
2) Design of appearance matching surface
The design of the appearance matching surface during the first and second injections is very challenging, and it must provide sufficient bond strength; otherwise, it is easy to produce flying edges, burrs, curls, and peeling, especially for soft and hard plastic two color injection. Common appearance matching surface designs include step-type and groove-type designs:
3) Uniform wall thickness and avoidance of sharp corners
Two color injection molding is a form of injection molding, so the DFM design guidelines for injection molding, such as uniform wall thickness, avoiding sharp corners, and uniform transitions, must also be followed for two-color injection molding.
4) Demolding draft angle
The demolding draft angle is essential for two color injection molding because it determines which mold the part adheres to during rotation.
During the first injection, the part should adhere to the moving mold; during the second injection, the part should adhere to the mold ejection side.
After the first injection, if the part surface matches directly with another fixed mold, the plane must have a sufficiently large demolding draft angle to avoid scratching during the mold closing process.
5) Weight difference
The weight difference between the base material and the covering material should not be too large, otherwise, it will affect the molding cycle and injection pressure.
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