Two-color injection molding is to realize a part with two kinds of plastic injection molding.
Two-shot injection in a broad sense includes overmolding, which is realized by two injections on an ordinary injection molding machine.
Two-color injection molding in a narrow sense refers to use of two-color injection molding machines to inject two different plastics on same machine to complete parts, and common one is rotary.
The former does not require high equipment, but production efficiency is low. It is basically only suitable for molding of two materials that do not require a high degree of soft and hard rubber. The latter has good application scope, product quality and high production efficiency, which is current trend.

Matching materials for two-shot injection molding must meet two basic compatibility conditions, adhesion compatibility and processing compatibility. In addition, following requirements must also be met:
(1) Injection unit: parallel same direction, parallel opposite direction, horizontal and vertical L-shaped, Y-shaped single cylinder injection structure;
(2) Mixed nozzles: special nozzles such as patterns, waves, flow marks, gradients, and interlayers;
(3) Clamping mold: standard type, vertical turntable type, horizontal turntable type, shaft type, manipulator rotation type and other mechanisms;
(4) Feeding: In addition to standard hydraulic motor drive, there is also an ESD (ElectricScrewDrive) electric feeding structure;
(5) Oil circuit: ACC accumulator high-speed injection and closed circuit design.

(1) High performance and high energy saving: After energy saving transformation of injection molding machine, system can respond quickly, and injection molding machine can quickly adjust supply according to its own needs, which can effectively improve utilization rate of injection molding machine's electrical energy, thereby achieving high efficiency and energy saving.
(2) Good stability: System adopts double closed-loop control of pressure and flow, supply of injection molding machine is determined according to demand of injection molding machine. Under a given input or external interference, system can reach a new equilibrium state or restore to original equilibrium state after a short adjustment process.
(3) Good rapid response: rapid response is one of important signs of dynamic quality of servo system. Due to short production transition process time, generally within 200ms, in order to achieve requirements of overshoot, transition process requires a steep frontier and energy-saving transformation. Post-rising rate is larger, and time for injection molding machine to reach 1500 revolutions is less than 0.03 seconds.
(4) High precision: Precision after transformation refers to accuracy with which output can follow input. Servo motor adopts permanent magnet technology, which is accurate and fast. Servo motor adopts PLC technology for more precise control. Allowable deviation is generally between 0.01 and 0.001mm.
(5) Energy-saving: adopt a blank taking system to reduce power consumption.
(6) Improve efficiency and reduce costs: high response, high repeatability, and speed stability are improved; two kinds of raw materials or two colors can be molded at the same time, which greatly reduces process and manpower to achieve cost savings; parameter setting and system adjustment are very simple digital operations.

 

(1) Wear resistance: When blank is plastically deformed in mold cavity, it flows and slides along surface of cavity, causing violent friction between surface of cavity and blank, which causes two-color mold to fail due to wear. Therefore, wear resistance of two-color mold material is one of the most basic and important properties of mold. Hardness is main factor affecting wear resistance. In general, the higher hardness of two-color mold parts, the smaller amount of wear and the better wear resistance. In addition, wear resistance is also related to type, quantity, shape, size and distribution of carbides in material.
(2) Strength and toughness: Working conditions of two-color molds are mostly very bad, and some of them are often subjected to greater impact load, which leads to brittle fracture. In order to prevent two-color mold parts from suddenly breaking during work, two-color mold must have high strength and toughness. Toughness of two-color mold mainly depends on carbon content, grain size and organization state of material.
(3) Fatigue fracture performance: In working process of two-color mold, under long-term action of cyclic stress, fatigue fracture is often caused. Its forms include low-energy multiple impact fatigue fracture, tensile fatigue fracture, contact fatigue fracture, and bending fatigue fracture. Fatigue fracture performance of two-color mold mainly depends on its strength, toughness, hardness, and content of inclusions in material.
(4) High temperature performance: When working temperature of two-color mold is higher, hardness and strength will decrease, leading to early wear of mold or plastic deformation and failure. Therefore, two-color mold material should have high anti-tempering stability to ensure that two-color mold has high hardness and strength at working temperature.
(5) Resistance to cold and heat fatigue: Some two-color molds are in a state of repeated heating and cooling during working process, which causes surface of cavity to be stretched and pressure to change stress, causing surface cracking and peeling, increasing friction, hindering plastic deformation, reducing dimensional accuracy, and causing failure of two-color mold. Heat and cold fatigue is one of main forms of failure of hot work molds. This type of mold should have high resistance to cold and heat fatigue.
(6) Corrosion resistance: When some two-color molds such as plastic molds are working, due to presence of chlorine, fluorine and other elements in plastic, they will separate and resolve strong corrosive gases such as HCI and HF after heating, which will erode surface of two-color mold cavity, increase its surface roughness and wear Invalidate. Therefore, materials with superior corrosion resistance must be selected in processing of two-color molds.

The most common form of two-color product molds is that two identical movable molds correspond to two different fixed mold cavities. Cavity volume of second-color shell product is often larger than that of the first-color matrix product. After injection, mold is opened first, then movable mold is rotated by 180° using rotatable structure of injection machine, mold is closed and second injection is performed with a different color or different raw material from the first injection.
After second mold opening, punches that have completed two injections are demolded. The first raw material injection and second raw material injection are carried out at the same time. Two injection nozzles are required on injection machine to inject different colors or different raw materials respectively. At the same time, movable mold fixed plate should be attached with a rotating device that can rotate 180° , For most of matching materials, two-color injection molding can be formed by rotating fixed plate of movable mold. At this time, movable mold is not ejected, then mold is closed second material is injected. After heat preservation and cooling, fixed and movable molds are opened, product on movable mold side is ejected.

 

With increasing complexity of products, three-color or even four-color molding needs begin to appear. Generally, there are two types of three-color machines, namely, two-station three-color machines (commonly known as "fake three-color") and three-station three-color machines (commonly known as "true three-color") ). In fact, difference between the two is not true and false, but different turntable control methods are adopted according to different product structure design. Similarly, four-color machines can also be divided into "two-station" and "multi-station" models.
As far as technology is concerned, control accuracy of multi-station turntable is significantly higher than that of two-station, and manufacturing cost of machine is relatively high. Therefore, it is not necessary to blindly pursue a multi-station multi-color machine, but to choose the most appropriate solution according to product structure.

 

Injection analysis chart of two-color/multi-color products

First, the first part of two-color part is molded by injection, then mold is opened and closed. The first molded product is transferred into a large cavity to become an insert. Injection device injects another color of plastic into large cavity and encapsulates plastic insert to form a two-color part. At the same time, injection device injects the first plastic into small cavity to form next plastic insert. After product is solidified, mold is opened, two-color plastic part is pushed out, movable mold is rotated, and mold is closed, which completes an injection molding cycle. .
Using this technology can greatly increase degree of freedom of product design, so it is often used in processing of adjustable-wheel toothbrushes and disposable razors for automobiles.

 

Shrink mold core two-color injection molding technology mainly uses a hydraulic device to compress mold. First, under control of hydraulic device, core that can move up and down is pushed to top ascending position like a piston, and plastic material is injected. After the first material is solidified, movable core is controlled to drop, another plastic material is injected, hydraulic device is controlled to make core rise and press, then it is solidified and formed.

 

Clamp mold first, inject a kind of plastic into the first cavity, moving mold part in open mode retreat. Because shear gate is set in fixed mold, shear runner and core part are cut and separated during parting, but core part is still on the part of movable mold. Movable mold continues to retreat, main runner condensate is pushed out of cold material cavity in rotating shaft through ejector rod and material pulling rod, then ejector plate is pushed out through connecting rod and rotating shaft.

 

First move primary core to mold cavity, close mold, inject the first plastic, then cool and open mold. Transmission device installed on one side of mold drives sliding of primary core and secondary core, moves secondary core to cavity part, closes mold, injects second plastic, cools, opens mold, ejects product, and completes primary forming. Core sliding is used to form larger plastic parts.

 

After one injection, mold is opened, broken needle moves downward under action of push rod, so that seesaw rotates around rotation axis. A hole is broken at boundary of enclosed area of product. When second injection molding is performed and mold is closed, return pin of broken plate moves down by broken needle to make a round hole, so that secondary material can be drilled into enclosed area from broken hole.