Two-color/multi-color injection molding technology and material selection

Two-color injection molding is to uses two types of plastic injection for one part.
 
Broadly speaking, two-color injection molding includes over-molding, which is achieved by injection molding twice with a common injection molding machine.
 
Narrowly speaking, two-color injection molding is the use of a two-color injection molding machine, two different plastics in the same machine injection molding to complete the parts, the common is the rotary type.
 
The former does not require high equipment requirements, but the production efficiency is low, basically only applicable to the requirements that are not highly soft. The latter has a good range of application and product quality and high production efficiency, which is the current trend.

Two-color injection molding requirements

The paired materials for two-color injection molding must meet two basic compatibility conditions, bonding compatibility and process compatibility. In addition, the following requirements must be met.

(1) Injection units, parallel and co-directional. Parallel opposite direction. Horizontal and vertical L-shape. Y-shape co-directional single-cylinder injection structure;

(2) Hybrid injection nozzle, pattern. Waves. Flow marks. Gradual layer. Special injection nozzles such as lamination;

(3)Clamping die, standard type. Vertical turntable type. Horizontal turntable type. Rotating shaft type. Manipulator rotation type, etc;

(4) Charging, in addition to the standard hydraulic motor drive, there is also ESD (ElectricScrewDrive) electric charging structure;

(5) Oil circuit, with ACC accumulation pressure and high-speed injection and closed-circuit design.

Two-color injection molding features

Strong performance and high energy saving: After the energy-saving transformation of the injection molding machine, the system can respond quickly, and the injection molding machine can make rapid adjustments of the supply according to its own needs, which can effectively improve the utilization rate of the electric energy of the injection molding machine, thus achieving high energy efficiency.

Good stability: The system is a double closed-loop control of pressure and flow, and the supply of the injection molding machine is decided according to the demand of the injection molding machine. The system can reach a new equilibrium state after a short adjustment process or return to the original equilibrium state under the effect of a given input or external disturbance.

Good fast responsiveness: fast responsiveness is one of the important signs of the dynamic quality of the servo system, due to the short production transition process time, generally within 200ms, in order to achieve the requirements of overshoot, the transition process is required to have a steep frontier, the energy-saving transformation of the rising rate to be large, the injection molding machine to reach 1500 rpm in less than 0.03 seconds.

High accuracy: the accuracy of the transformation refers to the precise degree that the output quantity that can follow the input quantity. The servo motor adopts permanent magnet technology, which is accurate. Quickly, the servo motor adopts PLC technology, the control is more precise, and the allowed deviation is generally between 0.01~0.00lmm.

Energy-saving: Adopt blank taking system to reduce power consumption.

Improve efficiency and reduce cost at the same time: achieve high response, high repeatability, improve speed stability; using 2 kinds of raw materials or 2 kinds of colors can be molded at the same time, greatly reducing the process and manpower thus achieving cost savings; parameter setting, system adjustment for very simple digital operation.

Two-color injection materials

Two-color mold material selection

1. Abrasion resistance. When the blank is deformed in the mold cavity, it flows and slides along the surface of the cavity, which causes violent friction between the surface of the cavity and the blank, resulting in the failure of the two-color mold due to wear and tear. So the wear resistance of two-color mold material is the most basic. It is one of the most important properties. Hardness is the main factor that affects wear resistance. Generally speaking, the higher the hardness of the two-color mold parts, the smaller the wear, and the better the wear resistance. In addition, the wear resistance is also related to the type of carbide in the material. Quantity. Shape. Size and distribution.

2. strong toughness. Most of the working conditions of two-color mold are very harsh, some often bear a large impact load, which leads to brittle fracture. To prevent the two-color mold parts in the work of sudden brittle fracture, two-color mold to have high strength and toughness. Two-color mold toughness mainly depends on the carbon content of the material. Grain size and tissue state. 3.

3. fatigue fracture performance. The two-color mold work process, in the long-term role of cyclic stress, often leads to fatigue fracture. The form of small energy multiple impact fatigue fracture. Tensile fatigue fractures contact fatigue fracture and bending fatigue fracture. The fatigue fracture performance of two-color mold mainly depends on its strength. Toughness. Hardness. And the content of inclusions in the material. 4.

4. high-temperature performance. When the working temperature of the two-color mold is high, it will make the hardness and strength decrease, leading to early wear or plastic deformation and failure of the mold. Therefore, the two-color mold material should have high tempering stability to ensure that the two-color mold is in the working temperature, with high hardness and strength.

5. cold and hot fatigue resistance. Some two-color mold in the work process in the repeated heating and cooling state, so that the surface of the cavity by the tensile. The role of pressure stress, causing surface cracking and spalling increasing friction, hindering plastic deformation, reducing the dimensional accuracy, thus leading to two-color mold failure. Hot and cold fatigue is one of the main forms of hot mold failure, help this kind of mold should have a high resistance to hot and cold fatigue performance.

6. corrosion resistance. Some two-color molds such as plastic mold in the work, are due to the presence of chlorine in plastic. Fluorine and other elements, heat decomposition precipitation HCI. HF and other strong aggressive gas, erosion of two-color mold cavity surface, increase its surface roughness, intensify the wear loss effect. Therefore, it is necessary to choose the material with superior corrosion resistance in the processing of two-color molds.

Two-color injection molding principle

The most common form of two-color product mold is two identical dynamic molds to correspond to two different cavities, where the second color shell product cavity volume is often larger than the first color base product cavity volume, after the first injection of the base product first open the mold, then the dynamic mold using the injection machine can rotate the structure of 180 °, and then close the mold and use a different color from the first injection of raw materials or different raw materials for the second injection.

After the second mold opening, the convex mold, which has already completed two injections, is demolded. The first injection of raw materials and the second injection of raw materials are carried out at the same time, requiring the injection machine to have two injection nozzles, respectively, injecting different colors or different raw materials, while its movable die fixing plate should be accompanied by a rotary device that can be rotated 180°, for most matching materials two-color injection can be molded by rotating the movable die fixing plate. At this time, the moving mold is not ejected, and then the mold is closed for the second material injection, after the heat preservation and cooling, the fixed. After holding and cooling, the fixed mold is opened and the product is ejected from the moving mold.
 

Two-color injection molding machine principle diagram

With the increasing complexity of products. Usually, there are two types of three-color machines, namely two-station three-color machines (commonly known as "false three-color") and three-station three-color machines (commonly known as "true three-color"). In fact, the difference between the two does not lie in the true and false, but in the different designs of the product structure, the use of different turntable control methods. Similarly, the four-color machine can also be distinguished into "two-station" and "multi-station" models.

Therefore, it is not necessary to blindly pursue the multi-station multi-color machine but to choose the most appropriate solution according to the product structure.
 

Structure types of two-color molds

1. Core rotation type two-color injection structure

Firstly, the first part of the two-color part is molded by injection, then the mold is opened and closed, and the first molded product is transferred into the large cavity to become the insert, and the injection unit injects the plastic of another color into the large cavity to encapsulate the plastic insert, and then the two-color part is molded. At the same time, the injection device injects the first kind of plastic into the small cavity to form the next plastic insert and then opens the mold after the product is cured and formed, pushing out the two-color plastic parts, rotating the mold, closing the mold, that is, completing one injection molding cycle.

With this technology, the freedom of product design can be greatly improved, so it is often used in the processing of toothbrushes and disposable razors for automobiles.


2. Shrink mold core type two-color injection structure

The shrink mold core type two-color injection technology mainly uses a hydraulic device to compress the mold. First of all, under the control of the hydraulic device, the core that can move up and down is pushed and pressed to the top rising position like a piston, and the plastic material is injected, and after the first material is cured, the movable core is controlled to fall down, and another plastic material is injected, and then the hydraulic device is controlled to make the core rise and pressed, and then it is cured and molded.
 
3. Rotating two-color injection structure of release plate

Close the mold first. Inject one kind of plastic in the first cavity, open the mold and move the mold part backward, because the shear gate is located in the fixed mold, so the shear gate is cut off and separated from the core part during the parting, but the core part is still on the mold part. The moving mold continues to retreat through the top bar. The main sprue will be pushed out from the cold material cavity in the rotating shaft by pulling the material rod and then pushing out the release plate by the connecting rod and rotating shaft.
 
4. Core sliding-type two-color injection structure
 
Firstly, the primary core is moved to the cavity part of the mold and the mold is closed. The first plastic is injected, then the mold is cooled and opened, and the transmission device installed on the side of the mold drives the sliding of the primary core and the secondary core to move the secondary core to the cavity part, and the mold is closed. Injection of the second plastic, cooling. Open the mold. The product is released and the primary molding is completed. The core sliding is used for molding large size plastic parts.

5. Seesaw structure

After the primary injection, the mold is opened and the broken needle is pushed downward by the pushrod, which makes the seesaw rotate around the rotation axis and pushes the broken ejector plate upward by the seesaw. Then, when the secondary injection molding is closed, the broken pin moves downward under the action of the return pin of the broken plate to make a round hole, so that the second material can be drilled into the closed area through the broken hole.