To achieve excellent bonding strength in multi-color injection molding (including two-color injection molding, overmolding, etc.), the key lies in achieving effective fusion of two materials at the interface. This is not merely a matter of process parameter settings, but a comprehensive system engineering project encompassing material selection, mold design, and process control.

 

Below, I will elaborate on how to ensure compatibility and bonding strength of multi-color injection molded parts from four dimensions: material selection, injection molding process, mold design, and production management.

Materials are fundamental; if materials themselves are incompatible, even the best process cannot salvage situation.
1. Chemical Compatibility (Core Principle)
Whether two materials can bond essentially depends on whether their molecules can diffuse and entangle with each other.
"Like dissolves like" principle: This is golden rule. Polymer chains with similar chemical structures (especially similar polarities) are more likely to diffuse with each other.
Solubility parameter (SP value): This is a quantitative indicator. The closer SP values of two materials are, the better their compatibility and the stronger their bonding force.
Ideal: SP value difference < 0.5 cal¹/²/cm³/².
Acceptable: SP value difference between 0.5 and 1.0.
Incompatible: SP value difference > 1.0, usually indicating very poor bonding force.
Practical recommendation: When selecting materials, always request SP values from material supplier and compare them. If not available, refer to common material compatibility tables.

 

2. Types of Interfacial Compatibility
Based on chemical compatibility, material combinations can be divided into three categories:
Chemically bonded type:
Mechanism: The first material (substrate) contains active functional groups (such as -OH, -NH₂), and second material (covering material) can chemically react with these functional groups at high temperatures to form strong covalent bonds.
Typical Examples:
TPU Overmolding on PA6, PA66: -NCO groups in TPU react with -NH₂ groups in PA to form extremely strong amide bonds. This is one of the strongest bonding methods.
Some functional adhesive resins.
Characteristics: Strongest bonding force, but requires specific materials.
Diffusion-Mixed Type:
Mechanism: Relies on diffusion and entanglement of molecular chain segments at interface when material is in molten state, forming an interpenetrating network. This is the most common case.
Typical Examples:
PC/ABS with PC/ABS (homogeneous materials).
PC and PMMA (similar solubility parameters).
Different colors/hardness grades of same substrate.
Characteristics: Good bonding force, basis for most multi-color injection molding applications.
Mechanical Interlocking Type:
Mechanism: When chemical compatibility is poor, holes, grooves, or other structures are created in substrate through mold design, allowing the second material to form "anchor points" after injection, achieving physical fixation.
Typical Examples:
PP and TPE (chemically incompatible, but TPE can encapsulate PP for sealing).
Any chemically incompatible material combination.
Characteristics: Weaker bonding strength, but sufficient for fixing and sealing certain non-structural components.
3. Material Processing Temperature (Matching)
Substrate surface must be in a molten or elastic state: Injection temperature of second injection material must be higher than glass transition temperature (Tg) or melting point (Tm) of the first injection material. If substrate surface temperature is too low, molecular chains are "frozen" and cannot diffuse with second injection material.
Ideal Temperature Window: Melt temperature of second injection should be close to or slightly higher than processing temperature of the first injection material to ensure reactivation of substrate surface.
4. Material Shrinkage Rate (Matching)
Problem: If shrinkage rates of two materials differ too much, huge internal stress will be generated during cooling, leading to product warping, cracking, or even separation at interface.
Goal: Select materials with similar shrinkage rates whenever possible. If this cannot be avoided, compensation must be made through molds and processes.