When we are doing product structure design, in order to save assembly time of parts, or reduce impact of assembly tolerances of multiple products (which can reduce type and quantity of finished parts), we often use two parts through two-color molding, inserts injection molding is used to integrate it into one piece.
For example, in a motor controller, copper bars and plastic parts are insert-molded into one body.

 

Consumer products usually use two-color injection molding to mold soft rubber and hard rubber, such as toothbrushes, which are very common. Main part of toothbrush is made of hard plastic to provide sufficient strength; parts that come into contact with hand are made of soft plastic to ensure touch; at the same time, two-color injection molding can also provide toothbrush with a colorful appearance.

 

Today, we will first learn about two-color injection molding.

Two-color injection molding process refers to molding process of injecting two different materials into same set of molds to achieve an injection molded product formed by combining two materials. Some of these two different materials have different colors, some have different softness and hardness. By manufacturing products from such different materials, aesthetics and assembly performance of product can be improved to a certain extent.
Two-color injection molding requires one male mold and two female molds.
The first injection molding is in common mold and the first female mold. After product is formed, mold is opened and directly replaced with second master mold without demoulding, then second injection molding is performed. After molding, two-color injection molded product can be obtained.
In actual production process, two groups of male molds are usually used, each group is responsible for one injection molding and rotates 180° vertically to switch between different female molds.
Usually the first injection material is called base material, and second injection material is called cover material.

 

Insert injection molding is process of making a plastic part of this material or color type on a single-color injection molding machine and then manually injecting a second mold without need for an additional dedicated injection molding machine.
Two-color injection molding is completed by a two-color injection molding machine. There are also two injection molding steps, but they are all completed inside two-color injection molding machine. A special two-color injection molding machine is usually used.
During the entire process of two-color injection molding, plastic is in a molten state without cooling and shrinkage. This means that there is an opportunity to form a strong chemical link between base material and covering material, which has high strength and reduces risk of burrs.
Compared with insert injection molding, two-color injection molding does not require insertion process of inserts. Molding cycle is shorter, more efficient, and quality is higher.

▪ Melting point of base material must be greater than melting point of covering material, otherwise plastic of base material will melt away during secondary injection molding. Difference in melting temperatures of the two materials should preferably not exceed 30~60°.
▪ Thermal expansion coefficients CTE of materials should be as close as possible.
▪ Shrinkage rates of materials should be as similar as possible.
▪ Avoid combinations of amorphous and semi-crystalline plastics.
▪ Two materials must be compatible with each other. You can refer to table below and consult material supplier for actual information.

 

(Green: Compatible; Yellow: Moderate; Red: Worst; Gray: Incompatible)

▪ Thickness options:
Generally, glue position as appearance surface is designed to have an average thickness, so that two-color product can meet design requirements.
▪ Bonding properties of two materials:
Materials for two injection moldings must be well bonded together, otherwise they will separate or become unsealed after a long time or after environmental testing. Therefore, it is usually necessary to make structure between overlap of two materials into an interlocking form, as shown below:

 

At the same time, contact area of two materials can also be increased to increase bonding performance.