An Overview of Dual-Color Injection Molding Technology

Dual-color injection molding is a process where two distinct plastic materials are injected into the same machine, forming the product in two stages before it’s ejected just once from the mold. This technique is also commonly referred to as two-material injection molding and is typically executed using a single mold, though it usually requires a specialized dual-color injection molding machine.

The principle behind this process involves using a single machine equipped with two or more injection units. By alternately injecting different colored or material plastics, the product gains two distinct characteristics simultaneously during the molding process.

Products made using dual-color molds benefit from fast forming and processing speeds, stable quality, good structural strength, excellent durability, and minimal fitting gaps, which contribute to a superior appearance. Furthermore, this method enhances efficiency and helps reduce costs.

Material Requirements
For dual-color injection molding, the paired materials must satisfy two fundamental compatibility conditions: adhesion compatibility and processing compatibility. This ensures the materials bond well and can be processed effectively together.

Process Requirements

• Design for Hard and Soft Rubber Dual Injection:
  The two materials need a significant temperature difference in their melting points—a general recommendation is 60°C, with a minimum suggested difference of at least 30°C. The first injection material, typically PC or PC/ABS, should have the higher melting point. The second injection usually employs TPU or TPE. The PC layer thickness is generally 0.6-0.7mm, while the soft material layer should be 0.4mm or more.
  To maximize adhesion, maximize the contact area. Incorporating features like grooves can help, or a core-pulling mechanism can be used for the first shot, allowing the second shot material to partially inject into the first. The surface of the mold for the first shot should ideally be made rougher to enhance bonding.
• Design for Transparent and Non-Transparent Dual Injection:
  • Small Lens Applications: The first shot should be non-transparent, and the second shot forms the lens. PC, chosen for its high melting point, is preferred for the first shot, while PMMA is used for the second shot.
  • Decorative Applications: The first shot uses non-transparent material, and the second shot uses transparent material. High-temperature PC is commonly chosen for the non-transparent first shot. PMMA or PC are typical choices for the second shot’s transparent material. If PC is used, it generally requires a UV coating for protection. PMMA offers options like UV coating or surface reinforcement; UV coating is mandatory if surface characters are present.

Mold Requirements

• The core mold (female mold) must feature two different shapes, each forming a part of the product, whereas the cavity mold (male mold) must have two identical shapes.
• When the front and rear mold halves are rotated 180 degrees around the center, they must align perfectly.
• Critical specifications like maximum and minimum mold clamp thickness allowances and KO pin distances must be carefully verified.
• For three-plate molds, designing the runner system (sprue/buret) for automatic demolding is highly recommended.
• When designing the core mold for the second injection, include clearance (undercut) to prevent the second core mold from damaging the already formed product surface (gating area) from the first shot. However, this must be balanced with careful consideration of the strength required in every sealing area.
• Slightly oversized dimensions for the product formed in the first shot can ensure it fits more tightly against the cavity during the second shot, improving the seal.
• Carefully evaluate whether the plastic flow during the second injection might disturb the first-formed product, potentially deforming its surface features.
• Before the mold halves (A and B plates) close, ensure that front mold sliders or lifters do not reset prematurely and damage the product.
• The cooling channel layout for both core and cavity molds should be comprehensive and balanced for optimal performance.
• In the vast majority of cases, the hard plastic part is injection molded first, followed by the soft part. This sequence is preferred because soft materials are more prone to deformation during the second injection cycle.

This versatile dual-color injection molding process is now widely applied across nearly all plastic product domains, including electronics, power tools, medical devices, and household appliances.