Injection Molding Processes: Cavities, Multi-Component, Gas Assist

Single-Shot Molding: The Standard Process

In single-shot molding, molten plastic is injected through a nozzle into a closed mold. It is the most widely used process and the foundation for almost all specialty variations.

Typical applications:

• Enclosures and housings (ABS, PC, PA)
• Technical components (POM, PBT)
• Packaging (PP, PE)
• Brackets and clips (PA6 with glass fiber)

Tooling cost: €3,000–30,000 depending on complexity and cavity count.
Unit cost: €0.05–5 (depends on material, part size, and batch quantity).

Multi-Cavity Molds: Reducing Unit Cost

A multi-cavity mold produces several parts per cycle. Cavity counts range from 2 to over 128, depending on part size and machine capacity.

When does it pay off?

• From ~5,000 units/year, the efficiency gain is significant
• Tooling cost increases by 60–150% per additional cavity
• Unit cost drops by 30–50% versus single-cavity tools
• Cycle time remains nearly the same (for smaller parts)

Example: A plastic clip costs €0.12/unit with a single-cavity mold. With 8 cavities, the price drops to €0.05/unit — with only a €5,000 tooling premium. Break-even: ~50,000 units.

Multi-Component Molding (2K / 3K)

Multi-component molding bonds two or more plastics in a single tool cycle. The classic example: a grip handle with a soft TPE zone over a rigid PA carrier.

Advantages over manual assembly:

• No manual assembly → lower unit cost from ~10,000 units
• Chemical bond between materials → higher durability
• Design freedom: color gradients, functional zones, markings
• Reduced logistics (one part instead of two)

Disadvantages:

• Tooling cost 2–3× higher than single-component tools
• Limited material combinations (bond must be ensured)
• Longer tool development (4–6 weeks instead of 2–4)

Typical pairings: PA+TPE, PC+TPE, ABS+TPE, PP+EVA. Bonding is ensured through primers, plasma pretreatment, or chemical compatibility.

Gas-Assist Injection Molding

In gas-assist molding, nitrogen is injected into the still-molten core after the initial shot. The gas pushes the plastic against the mold wall, reduces internal stresses, and enables thick sections without sink marks.

Applications:

• Thick grip handles (e.g., tool handles, power tool grips)
• Hollow profiles with smooth surfaces
• Parts with large cross-section transitions
• Material savings of 20–40%

Cost factor: The gas-assist module costs ~€15,000–25,000 on top of the molding tool. Only pays off at medium to high volumes (from 10,000 units).

Sandwich Molding: Core-Skin Principle

In sandwich molding, a core of inexpensive material (e.g., recycled PP) is encapsulated with a skin of high-performance plastic (e.g., glass-fiber-reinforced PA). The result: high strength at reduced material cost.

Typical application: Structural parts with a cosmetically demanding surface — e.g., industrial equipment housings, automotive interior parts.

Tooling: Special sandwich nozzle required (+€10,000–20,000). Skin thickness can be precisely controlled.

Overview: Costs and Decision Criteria