Introduction
Injection molding is one of the most versatile manufacturing processes in the world. It produces everything from tiny medical components to large automotive parts with remarkable speed and precision. But not all injection molding is the same. Different applications require different techniques—each with its own advantages, limitations, and cost structures.
Understanding the types of injection molding is essential for product designers and manufacturers. Choosing the right process affects part quality, production cost, cycle time, and design freedom. This guide provides a comprehensive overview of the major injection molding types, including conventional, gas-assisted, liquid, two-shot, insert, micro, and low-pressure molding. You will learn how each works, where it excels, and how to select the best option for your project.
What Is Conventional Injection Molding?
Conventional injection molding is the most common and widely used form of injection molding. It forms the foundation upon which specialized techniques are built.
How It Works
- Plastic pellets are fed into a heated barrel
- A rotating screw melts and homogenizes the plastic
- The screw moves forward, injecting molten plastic into a closed mold cavity
- The plastic cools and solidifies
- The mold opens and the part is ejected
Typical parameters:
- Melt temperature: 180–300°C (material dependent)
- Injection pressure: 50–200 MPa
- Cycle time: 15–90 seconds
- Mold material: Steel or aluminum
Advantages
| Advantage | Explanation |
|---|---|
| Versatility | Works with most thermoplastics |
| High volume | Cost-effective for large production runs |
| Complexity | Capable of intricate geometries |
| Consistency | Excellent repeatability |
Limitations
| Limitation | Explanation |
|---|---|
| Sink marks | Thick sections may show depressions |
| Internal stress | High pressure can cause residual stress |
| Tooling cost | Molds are expensive ($10,000–$100,000+) |
Applications
- Consumer goods (toys, containers, bottles)
- Automotive parts (housings, trim)
- Electronics enclosures
- Medical device components (non-critical)
How Does Gas-Assisted Injection Molding Work?
Gas-assisted injection molding (GAIM) uses nitrogen gas to create hollow sections within the plastic part. This reduces weight and eliminates sink marks in thick areas.
How It Works
- The mold is partially filled with molten plastic (typically 60–90%)
- Nitrogen gas is injected into the melt
- The gas pushes the plastic to fill the remaining cavity
- The gas creates hollow channels in thick sections
- The part cools and is ejected; gas is vented
Advantages
| Advantage | Explanation |
|---|---|
| Weight reduction | 20–40% lighter than solid parts |
| Eliminates sink marks | Hollow sections prevent depressions |
| Reduced warpage | More uniform cooling |
| Lower clamping force | Gas pressure assists filling |
Limitations
| Limitation | Explanation |
|---|---|
| Complex equipment | Requires gas injection system |
| Higher tooling cost | Specialized molds and nozzles |
| Steeper learning curve | More parameters to control |
Applications
- Automotive door panels and handles
- Large structural components
- Thick-walled consumer products
- Industrial equipment housings
What Is Liquid Injection Molding?
Liquid injection molding (LIM) is used for materials that start as liquids rather than solid pellets. The most common material is liquid silicone rubber (LSR) .
How It Works
- Two liquid components (base and catalyst) are mixed
- The mixture is injected into a heated mold
- The material cures through chemical reaction
- The mold opens and the cured part is ejected
Typical parameters:
- Mold temperature: 150–200°C
- Injection pressure: 10–50 MPa (lower than thermoplastics)
- Curing time: 30–120 seconds
Advantages
| Advantage | Explanation |
|---|---|
| High precision | Excellent detail reproduction |
| Complex geometries | Can mold intricate shapes |
| Material properties | LSR is biocompatible; flexible; heat-resistant |
| No flash | Low pressure reduces flash |
Limitations
| Limitation | Explanation |
|---|---|
| Material cost | LSR and other liquid materials are expensive |
| Curing time | Slower than thermoplastic cooling |
| Material waste | Cannot regrind cured material |
Applications
- Medical devices (catheters, seals, gaskets)
- Baby products (nipples, pacifiers)
- Automotive seals and gaskets
- Consumer products (watch bands, kitchenware)
What Is Two-Shot (Multi-Shot) Injection Molding?
Two-shot injection molding combines two different materials into a single part in one cycle. The result is a seamless, multi-material product.
How It Works
- The first injection unit injects the primary material
- The mold rotates or slides to reposition the first-shot part
- The second injection unit injects the secondary material onto or around the first
- The materials bond chemically or mechanically
- The finished part is ejected
Advantages
| Advantage | Explanation |
|---|---|
| No assembly | Finished part in one cycle |
| Permanent bond | Chemical adhesion; stronger than adhesives |
| Design freedom | Soft-touch grips; integrated seals; multi-color |
| Consistent quality | Automated; no assembly variation |
Limitations
| Limitation | Explanation |
|---|---|
| Higher equipment cost | Two injection units; specialized machine |
| Complex tooling | Rotating or sliding molds |
| Material compatibility | Must bond effectively |
Applications
- Toothbrushes (rigid handle + soft grip)
- Power tools (hard core + soft grip)
- Automotive interiors (rigid + soft-touch)
- Electronics (hard shell + soft edges)
What Is Insert Molding?
Insert molding embeds pre-formed components (inserts) into the plastic part during molding. Inserts are typically metal but can be other materials.
How It Works
- Inserts are placed into the mold cavity (manually or robotically)
- The mold closes
- Molten plastic is injected around the inserts
- The plastic cools and bonds with the inserts
- The finished composite part is ejected
Advantages
| Advantage | Explanation |
|---|---|
| Function integration | Adds threads, electrical contacts, strength |
| No assembly | Inserts are molded in place |
| Strong bond | Plastic encapsulates insert |
Limitations
| Limitation | Explanation |
|---|---|
| Insert placement | Requires precise positioning |
| Cycle time | Insert loading adds time |
| Tooling complexity | Must hold inserts securely |
Applications
- Electrical connectors (metal contacts)
- Medical devices (metal components)
- Automotive (threaded inserts)
- Consumer electronics (metal frames)
What Is Micro Injection Molding?
Micro injection molding produces extremely small parts with dimensions in the sub-millimeter to millimeter range. It requires specialized equipment and precision tooling.
How It Works
Similar to conventional injection molding but with:
- Ultra-precise injection volume control
- High-speed injection to fill tiny cavities
- Specialized screws for small shot sizes
- Precision molds with micron-level tolerances
Typical part weight: Milligrams to a few grams
Typical tolerances: ±0.005–0.01 mm
Advantages
| Advantage | Explanation |
|---|---|
| Extreme precision | Tolerances in microns |
| Fine details | Features as small as 0.05 mm |
| Material efficiency | Minimal waste for expensive materials |
Limitations
| Limitation | Explanation |
|---|---|
| Equipment cost | Specialized, high-precision machines |
| Tooling cost | Molds require advanced machining (EDM, micro-milling) |
| Process control | Very sensitive to parameter changes |
Applications
- Micro-connectors for electronics
- Micro-fluidic chips for medical diagnostics
- Watch components
- Miniature gears and mechanical parts
What Is Low-Pressure Injection Molding?
Low-pressure injection molding operates at lower pressures than conventional molding. It is used for encapsulating sensitive components and for materials that cannot withstand high pressure.
How It Works
Similar to conventional molding but with:
- Injection pressure: 5–50 MPa (vs. 50–200 MPa conventional)
- Lower viscosity materials (often hot melts or reactive resins)
- Aluminum or softer steel molds (lower tooling cost)
Advantages
| Advantage | Explanation |
|---|---|
| Gentle on components | Protects sensitive electronics |
| Reduced mold wear | Lower pressure extends tool life |
| Lower internal stress | Better dimensional stability |
| Lower tooling cost | Aluminum molds acceptable |
Limitations
| Limitation | Explanation |
|---|---|
| Material limited | Only low-viscosity materials |
| Slower cycles | May have longer cooling/curing |
| Lower strength | Materials may have lower mechanical properties |
Applications
- Electronic component encapsulation
- Cable overmolding
- Sensor housing
- Soft plastic products (gaskets, seals)
How Do You Choose the Right Type?
Selecting the appropriate injection molding type depends on several factors.
Decision Factors
| Factor | Considerations |
|---|---|
| Part size | Micro molding for tiny parts; conventional for standard; gas-assist for large |
| Part complexity | Conventional for moderate; two-shot for multi-material; micro for fine details |
| Volume | Conventional for high volume; insert/low-pressure for lower volume |
| Material | Liquid molding for LSR; conventional for thermoplastics |
| Function | Insert for embedded components; gas-assist for weight reduction |
| Budget | Conventional lowest per-part; two-shot higher tooling but no assembly |
Selection Matrix
| Requirement | Recommended Type |
|---|---|
| Simple, high-volume parts | Conventional |
| Weight reduction needed | Gas-assisted |
| Multi-material; no assembly | Two-shot |
| Embedded components | Insert |
| Very small, precise parts | Micro |
| Sensitive electronics | Low-pressure |
| Liquid silicone rubber | Liquid injection |
Comparison of Injection Molding Types
| Type | Key Characteristics | Cost | Cycle Time | Precision |
|---|---|---|---|---|
| Conventional | Versatile; simple process; wide material range | Medium-Low | Short | Moderate-High |
| Gas-assisted | Hollow sections; weight reduction; minimal sink marks | High | Medium | High |
| Liquid injection | Liquid materials; high precision; good surface finish | High | Medium | High |
| Two-shot | Multiple materials; no assembly | High | Long | High |
| Insert | Embedded components; added functionality | Medium-High | Medium | High |
| Micro | Very small parts; extreme precision | High | Long | Very High |
| Low-pressure | Gentle on components; reduced mold wear | Medium | Medium | Moderate |
Conclusion
Injection molding encompasses a family of processes, each suited to specific applications. Conventional injection molding remains the workhorse for high-volume plastic parts. Gas-assisted adds weight reduction and eliminates sink marks. Liquid injection serves silicone and other liquid materials. Two-shot combines materials without assembly. Insert molding embeds functional components. Micro molding produces tiny, precise parts. Low-pressure protects sensitive electronics.
Choosing the right type requires evaluating part geometry, volume, material, function, and budget. By understanding the strengths and limitations of each, you can select the process that delivers the best combination of quality, cost, and performance for your project.
Frequently Asked Questions (FAQ)
What is the most common type of injection molding?
Conventional injection molding is the most common. It is versatile, cost-effective for high volumes, and works with a wide range of thermoplastics. It produces everything from consumer goods to automotive parts and electronics housings.
Which injection molding type is best for weight reduction?
Gas-assisted injection molding is best for weight reduction. It creates hollow sections within the part, reducing weight by 20–40% while maintaining structural integrity. It is widely used in automotive and large structural components.
What is the difference between two-shot molding and overmolding?
Two-shot molding uses a single machine with two injection units in one continuous cycle. Overmolding typically involves two separate steps—molding the first part, then placing it in a second mold for the second material. Two-shot is faster and more automated but has higher capital cost.
Can insert molding be automated?
Yes. High-volume insert molding often uses robotic placement with vision verification to achieve placement tolerances of ±0.02–0.05 mm. Automation reduces cycle time, improves consistency, and eliminates manual handling errors.
What materials are used in micro injection molding?
Micro injection molding uses engineering thermoplastics such as LCP (liquid crystal polymer), PEEK, polycarbonate, and nylon. These materials offer the strength, dimensional stability, and flow properties needed for tiny, high-precision parts. Material selection depends on the application's mechanical, thermal, and chemical requirements.
Contact Yigu Technology for Custom Manufacturing
At Yigu Technology, we offer a full range of injection molding services to meet diverse customer needs. Our expertise spans conventional, gas-assisted, two-shot, insert, micro, and low-pressure molding. We help clients select the right process for their application and deliver high-quality parts.
Our capabilities include:
- Process selection guidance – Matching the right method to your requirements
- Precision tooling – In-house mold design and manufacturing
- Material expertise – Thermoplastics; liquid silicone; engineered resins
- Quality assurance – Dimensional inspection; mechanical testing
- Volume flexibility – Prototypes to high-volume production
We serve automotive, medical, electronics, and consumer goods industries with custom injection-molded components. Whether you need weight reduction, multi-material integration, or micro-scale precision, we have the expertise to deliver.
Contact us today to discuss your injection molding project. Let our experience help you choose the right process for superior results.
