Overmolding Material Selection

Overmolding, also known as secondary forming, is a secondary processing technique. Instead of injecting two different plastic materials simultaneously on a single machine, it involves injecting them sequentially. The process unfolds in two distinct stages: First, the base product is molded and ejected from an initial mold. Then, this partially completed product is placed into a second mold for a subsequent injection step. Typically, this process requires two separate molds and does not necessitate a specialized dual-color injection molding machine. This technique is generally employed for applications where a soft rubber material is overmolded onto a hard plastic substrate. The first injected material serves as the base material or substrate, usually a rigid plastic, while the second is the overlay material, typically an elastomer.

Material Selection and Compatibility Considerations

Overmolded products are created using the secondary injection molding method, often referred to as insert molding. Selecting the appropriate resins for this secondary injection requires careful consideration of multiple factors. These choices depend partly on the inherent properties of the base material and partly on the performance requirements of the final product. Key considerations include:

• Chemical Resistance: Ensures the product maintains its integrity and performance when exposed to specific chemical environments.
• Flame Retardancy: The materials must comply with relevant ecological, environmental, and safety standards. An eco-label indicates adherence to these environmental and social standards.
• Wear Resistance: Prevents the overlay layer from degrading, becoming recessed, or peeling off due to abrasion.
• Shore Hardness: Must meet the required tactile feel or other specific physical property specifications.
• Impact Resistance: Must satisfy the structural strength requirements of the product.
• Melting Point: Must be suitable for the operating temperature conditions of the application to prevent softening or deformation during normal use.
• Bonding Mechanism: The way materials bond is crucial. When materials are mismatched, bonding primarily relies on mechanical interlocking, often achieved through undercut structures designed into the mold. However, when materials are well-matched, a stronger chemical bond forms. Typically, compatible materials share similar chemical structures or contain components that can interact. If the base and overlay materials are incompatible, the bond is often limited to mechanical interlocking, lacking strong chemical adhesion.

Challenges and Considerations for Hard Plastic Overmolding onto Hard Plastic

While soft rubber can be readily overmolded onto various rigid base materials, offering flexibility in choice, overmolding hard plastic onto another hard plastic is subject to significant limitations:

• Localized Application: Hard-over-hard overmolding is feasible for localized areas but is generally unsuitable for large-area coverage or forming closed-loop designs.
• Avoiding Identical Materials: Overmolding the same hard plastic material onto itself is generally not recommended.
• Melting Point Similarity: If the two hard plastic materials have identical or very close melting points, “color bleeding” or material mixing can occur during the injection process, severely compromising the product’s appearance quality. Example: If both the base material and the overlay material are PA6-GF30 (30% glass fiber-reinforced Nylon 6), issues like color bleeding, excessive injection pressure, and defects such as flash or base material damage at the interface are highly probable.
• Injection Pressure Differences: Hard plastics typically require injection pressures 30% or more higher than soft rubbers. Excessive pressure can often lead to poor sealing at the bonding interface, causing burrs or flash, and potentially damaging the underlying base material.
• Process Complexity and Cost: Overmolding hard plastics onto other hard plastics demands very high requirements for both mold design and injection molding process control. Various defects are prone to occur, leading to significantly increased production costs. Consequently, hard-over-hard designs are generally discouraged unless absolutely necessary.

Conditions for Successful Hard Plastic Overmolding

If overmolding with different hard plastic materials is absolutely necessary, the following conditions should be met:

• Melting Point Difference: There must be a discernible processing temperature difference (melting point difference) between the two materials. Typically, a difference of at least 30°C is recommended, with the base material’s processing temperature being higher than that of the overlay material.
• Overlay Area Limitation: The overmolding (or color separation) area for the hard overlay material should be kept as small as possible. This minimizes the risk of cracking caused by the material’s inherent large volume shrinkage and potential uneven shrinkage.
• Shrinkage Rate Control: The shrinkage rate of the hard overlay material should be as small as possible. Non-crystalline or semi-crystalline plastics are generally preferred, as they typically exhibit a shrinkage rate between 0.4% and 0.6%.

The Value of Secondary Injection Molding

Secondary injection molding offers significant advantages. It not only endows the product’s surface with a desirable soft tactile feel but also significantly enhances its functionality and overall value. Over the past decade, this technology has indeed revolutionized consumer product aesthetics, design philosophies, and functional expectations. While it is perhaps best known for creating “soft-touch surfaces,” its applications are much broader. It enables ergonomic design, two-tone aesthetics, clear branding, and improved product characteristics. Furthermore, it allows for the integration of various functionalities, such as noise reduction, vibration damping, waterproofing, and impact resistance, thereby substantially increasing the product’s overall value.