What is double injection molding?

Double Injection molding

Double injection molding is a component with two plastic injections to achieve. The broad definition of double molding includes overmolding with double injection molding by an ordinary molding machine.

In the narrow sense, double injection molding refers to using a two-color injection molding machine, two different plastics in the same machine to complete the injection parts. The common is rotary.

The former does not require high equipment but low production efficiency. It is only applicable to the molding of soft and hard glue materials. At the same time, the latter has a good scope of application and product quality and high production efficiency, which is the current trend.

What Is Double Injection Molding?

double injection molding parts

Two-color injection molding is a plastic injection molding technique that uses two different colors or materials in the same mold. Usually, the two colors or materials are different, but sometimes it can be the same color or different stages of the material. Two-color injection molding technology is usually used to manufacture multi-color or multi-material parts, in order to increase the appearance, texture, function and other aspects of the product value.

How Dose Double Injection Molding Works?

In two-color injection molding, it is usually necessary to use a special mold to injection the two materials separately. One side of the mold is used to inject the first material and the other side is used to inject the second material. During injection, the mold is usually rotated or moved to a different position to ensure that the two materials can be properly combined to form an integral two-color injection molded part.

Double Color Injection Molding Requirements

double injection molding products

The pairing material of two-color injection molding must meet two basic compatibility conditions, adhesion compatibility, and processing compatibility. In addition, the following requirements must be met: 

(1) Injection unit, parallel in the same direction, parallel to the direction, horizontal and vertical L type, Y type in the same direction single-cylinder injection structure; 

(2) Mixed nozzle, pattern, wave, flow mark, gradual layer, interlayer, and another special nozzle; 

(3) clamping die, standard type, vertical rotary plate type, horizontal rotary plate type, rotary shaft type, manipulator rotation mechanism; 

(4) feeding, in addition to the standard hydraulic motor drive, there are ESD(Electric Screw Drive) electric feeding structure; 

(5) Oil circuit, ACC accumulator high-speed injection, and closed-circuit design.

Double Injection Molding Characteristics

All about plastic: advantages and disadvantages

Strong performance and high energy saving: 

After energy-saving transformation of the injection molding machine, the system can respond quickly, the injection molding machine can adjust the supply soon according to its own needs, and can effectively improve the utilization rate of electric energy of the injection molding machine, to achieve high efficiency and energy saving.

Quick Response

Quick response 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, to achieve the requirements of overshoot, it is required that the leading edge of the transition process is steep, and the rising rate after energy-saving transformation is large. The time for the injection molding machine to reach 1500 revolutions is less than 0.03 seconds.

Good stability

The system is pressure and flows double closed-loop control, according to the demand of injection molding machine to determine the supply of injection molding machine. Under the given input or external interference, the system can reach a new equilibrium state after a short adjustment process or return to the original equilibrium state.

High precision

The precision after transformation refers to the precision degree that the output that can follow the input. Servo motor adopts permanent magnet technology, accurate and fast, and servo motor adopts PLC technology, more accurate control, the allowable deviation is generally between 0.01 ~ 0.00 LMM.

Energy-saving

Adopt the green system to reduce power consumption.

Improve efficiency and reduce cost

Achieve high response, high repeatability, improve speed stability; Using two kinds of raw materials or two kinds of colors can be formed at the same time, greatly reducing the process and workforce to achieve cost savings; Parameter setting, system adjustment for very simple digital operation.

Principle of double injection molding

double injection molding01

The most common form of double injection molding mold is that two same moving dies should correspond to two different cavities of the fixed model. The cavity volume of the second color shell product is usually larger than that of the first color matrix product. The mold should be opened first after the first matrix product is injected. Then the dynamic mold rotates 180°by using the rotating structure of the injection machine, and then the mold is closed. The second injection is made with different colored raw materials or materials for the first injection.

After the second opening, the punch with two injections should be released. Raw materials for the first time injection and the raw material of the second injection were conducted simultaneously. The injection machine has two injection nozzles, injection of different colors or different raw materials, and the dynamic model of the fixed plate with can rotate 180 °rotating devices. For most of the matching material two-color injection fixed plate rotation dynamic model can be used to shape. 

At this time, the moving mold is not pushed out, and then the mold is closed for injection molding of the second material. After thermal insulation and cooling, the fixed and moving mold are opened, and the products on the moving mold side are pushed out.

As products become more complex. Tricolor or even four-color molding demand began to appear. Usually, there are two types of tricolor machines, namely two station tricolor machines (commonly known as “false tricolor”) and three station tricolor machines (commonly known as “true tricolor”). The difference between the two is not true or false, but according to the different product structure design, the use of different turntable control. Similarly, the four-color machine can also be divided into “two stations” and “multi-station” models.

In terms of technology, the control accuracy requirements of multi-station turntables are higher than that of two stations, and the manufacturing cost of the machine is relatively high. 

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

The structure type of double injection molding

Mold Making Services

1. Core rotary double color injection structure

First of all, the first part of the two-color parts is produced by injection molding, and then the mold is opened and closed. Then the first molding product is transferred into a large cavity to become an insert. The injection device injects another color of plastic into the large cavity, and the plastic insert is wrapped, and the two-color parts can be formed. 

At the same time, the injection device to inject the first plastic into the small cavity, forming the next plastic insert, to be solidified molding products after opening the mold, rolled out two-color plastic parts, moving the mold rotation, closing the mold that is, to complete an injection molding cycle.

This technology can greatly improve product design freedom, so it is often used to process automobile adjusting wheel toothbrushes and a disposable razor.

2. Shrinkage mold core type dual-color injection structure

Shrinkage mold core double color injection molding technology mainly uses the hydraulic device to compress the mold operation.

First, under the control of hydraulic equipment, will be able to like the piston is pushed up the core pressure rising to the top position. The plastic material injection, wait until the first raw material after curing, the activity of the core control, will be another kind of plastic materials for injection, and rising hydraulic control device to make cores, for its curing.

3. Strip plate rotating double injection molding structure

First close the mold, in the first cavity injection of a plastic, open mode moving mold part back, because the shear gate is set in the fixed mold, so when parting shear sprue and core layer products cut off separation, but the core layer products are still in the moving mold part of the strip.

The moving mold continues to retreat, push and fall off the main sprue condensate from the cold hole in the rotating shaft through the pushrod and the pull rod, and then push out the strip plate through the connecting rod and the rotating shaft.  

4. Dual-color injection structure with sliding core

A core to be moved to the mold cavity parts, mold, the first kind of plastic injection, and then cooling mold, installed at the side of the mold of gear drive one core and two cores of sliding, remove the secondary core to mold parts, mold, the second kind of plastic injection, cooling, mold opening, out products and complete a shape. Core sliding is used for forming large plastic parts.

5. Seesaw structure

After injection molding, the mold is opened, and the seesaw rotates around the rotation axis by the broken needle under the action of the push rod. The seesaw will be pushed up by the broken top plate so that the needle moves up, and a broken hole breaks out the boundary of the closed area of the product. Then the secondary injection molding, in the broken plate, returns pin action by the broken needle down to move and let out a round hole so that the second material can be drilled from the hole closed area.

FAQs

Q: What are the advantages of double injection molding?

A: Two-color injection molding can produce more abundant products in appearance, material and color, and improve the value of the product in terms of beauty, texture and function. It can also reduce production costs and speed up production because it can complete the injection process of two materials in one mold.

Q: What are the material choices for double injection molding?

A: Common two color injection molding materials include ABS/PC, PP/TPE, PA/TPU and so on. These materials need to be compatible with each other and not fused with each other during injection molding.

Q: What problems should we pay attention to in the design of double injection molding mold?

A: It is necessary to consider the fluidity and fusion of the two materials, and ensure that the two materials can be correctly combined. At the same time, you need to design two parts of the mold, each part is used to inject a different color of plastic material.

Q: What problems should we pay attention to in quality control of double injection molding?

A: It is necessary to ensure that the two colors are injected in equal quantities and that no bubbles or defects are produced during the injection process. In addition, parameters such as mold temperature, pressure and injection speed also need to be strictly controlled to ensure the quality of two-color injection products.

Author

Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.

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