Exploring the Types of Injection Molding

Table of Contents

Types of injection molding - mold design
Types of injection molding – mold design


Injection molding is a crucial process in manufacturing, allowing for the efficient and precise production of plastic parts. Understanding the various molding techniques available is key to selecting the right approach for each project. Injection molds are particularly important for mass-producing plastic parts, enabling the injection of molten plastic into a mold, where it cools and solidifies to create the desired part geometry. Depending on the design and requirements of the final product, different types of injection molds may be used. In this post, we are dedicated to presenting these various injection molding techniques, ranging from basic to advanced. We aim to assist you in determining the most suitable technique for your project’s unique needs. So, let’s get started without delay!

Material pouring into the hopper of the injection molding machine
Material pouring into the hopper of the injection molding machine

Section 1: Traditional Injection Molding

The traditional injection molding is currently one of the most popular methods for creating plastic products, allowing for the production of a complete part in a single cycle. It accommodates various materials like elastomers, rubber, thermoplastics, and thermosets.

The process involves injecting hot molten plastic into a custom metal or aluminum mold through an injection screw in an injection molding machine. After cooling, the plastic solidifies to match the mold’s shape. The mold then opens, and the finished items are ejected by an ejection system. This process is used to make a wide range of everyday items such as packaging, tools, cars, toys, and medical equipment.

Starting with heating plastic pellets, they are poured at high pressure into a mold shaped to match the desired component. As the plastic melts, it fills the mold, creating the piece.

Once injected, the plastic cools and solidifies, resulting in some shrinkage. To counteract this, the part remains under pressure while hardening. After solidification, the part is cooled and removed from the mold.

The process cycle involves controlling and measuring time, temperature, and pressure and includes steps like filling, injection, filling pressure, holding pressure, cooling, and demolding.


Section 2: Thin-wall Injection Molding

As technology advances, thin-wall injection molding becomes increasingly vital, especially with the rise of telecommunications and smart electronic devices requiring thinner and lighter plastic housings. This method offers the chance to cut material costs and boost production due to quicker cycle times.

Thin-wall injection molding demands advanced machinery with precise control functions. Machines like the advanced V-line injection machine are often necessary to meet the specifications of thin-wall parts and micro-injection, with specifications sometimes as thin as 0.15mm.

Rising material costs and the need for shorter lead times have fueled the popularity of thin-wall injection molding. This process presents an opportunity to reduce part material costs and increase production through faster cycle times.

With the high running speed of thin-walled injection molds, cycle times can be as short as 4 seconds. Yet, quality mold design and construction are crucial for success.

The demand for irregular shapes and smaller, thinner parts has made designing and building thin-wall molds highly sought-after. Specialized molds are crafted for mass-producing plastic parts that are thin and lightweight, requiring tight tolerances and extensive plastics knowledge.

Overall, thin-wall molding is a specialized form of injection molding aimed at mass-producing thin, lightweight plastic parts without sacrificing structure, leading to material cost savings and shorter cycle times, thus enhancing productivity and reducing costs per part. This specialized technique brings various advantages including

1. Reduced Cycle Time: Achieves a 50% reduction in fill time and 60-70% less injection molding time, resulting in cost savings.

2. Lower Resource Consumption: Decreases material costs, shipping weight, and cycle times, collectively reducing overall resource consumption.

3. Lower Production Material Cost: Thin-walled parts exhibit a high volume-to-material ratio, effectively lowering material costs per part.

4. Ideal for Recycled Plastic: Efficiently utilizes recycled plastic, contributing to a sustainable plastic manufacturing process.

Applications of thin-wall plastic injection molding span various industries, including plastic packaging, food packaging, medical devices, consumer products, electronic boxes, and more. This versatile technique caters to a wide array of manufacturing needs, delivering lightweight and cost-effective solutions.

Insert Molding -Metal thread insert in plastic
Insert Molding -Metal thread insert in plastic

Section 3: Insert Molding 

Insert molding, a crucial technique in injection molding, enhances the structure of products by embedding metal, plastic, or ceramic parts within the plastic. This process is widely used and plays a specialized role in injection molding.

In insert molding, a part is carefully placed in a mold cavity, and specific plastic material is injected directly into it. This results in a unified part where the insert(s) are encapsulated by the material. The use of engineering plastics in this process enhances wear resistance, and tensile strength, and facilitates weight reduction.

Insert molding proves to be an efficient alternative to assembling parts using solder, connectors, fasteners, or adhesives. It expands plastic capabilities, reducing the final product cost by minimizing the amount of expensive metal needed. Conventional injection machines are employed, leading to lower process and mold costs compared to bi-component injection processing.

Here are the major benefits of Insert injection Molding

1. Reduced assembly and labor costs: Insert molding minimizes assembly and labor by joining components with thermoplastic.

2. Reduced Size and Weight: Eliminating fasteners and connectors, and combining resin and metal strength, results in smaller, lighter components.

3. Increased reliability: Firmly securing each part to thermoplastic prevents issues like loosening, misalignment, and enhances shock and vibration resistance.

4. Greater Design Flexibility:   Insert molding allows designers to create virtually unlimited configurations, offering flexibility in design.


1. Aerospace Industry: Utilized in aircraft seats, stowage latches, lavatories, door handles, and user interface switches.

2. Automotive Industry: Replaces metal parts with durable plastic, producing lightweight automotive components for improved fuel economy.

3. Medical Devices: From simple to complex and sensitive devices, including sutures and implants.

4. Consumer Electronics: Eliminates fasteners and solders, streamlining the assembly process.

5. Military Equipment: Produces economical, efficient, and lightweight precision military equipment, such as portable communications devices.

Overmolded plastic parts
Overmolded plastic parts

Section 4: Overmolding 

Overmolding blends two or more materials to form a single part. Typically, the initial material (the substrate) is either partially or completely covered by a subsequent material (the overmolded material) during manufacturing. For instance, in medical devices, overmolding creates soft-touch handles for surgical instruments, enhancing comfort and function. This technique also allows molding of multiple colors for aesthetic or branding purposes, eliminating the need for additional assembly steps and reducing costs. We have highlighted the types of overmolding commonly encountered in the injection molding industry.

  • Plastic on plastic: Enhances strength or aesthetics by adding a second layer of plastic to the central section.
  • Rubber over plastic: Improves adhesion or creates watertight seals by adding rubber to the core.
  • Plastic on Metal: Boosts durability or aesthetics by applying plastic to the metal core.
  • Rubber on metal: Enhances durability or grip by adding rubber to the metal core.

Essentially, overmolding involves covering the substrate with a subsequent material mixed with additives like dyes and fillers. These materials, including plastics and rubber, are heated to the melting point and poured into molds. However, compatibility issues may arise when overmolding plastics with different materials and manufacturers need to do a detailed compatibility analysis before choosing overmolding materials. Nevertheless, overmolding is applied in various manufacturing scenarios including:

  • Toy Products: Creates toy parts with various colors and perfect designs, simplifying assembly for baby products.
  • Auto Industry: Reinforces plastic parts or alters texture to enhance adhesion in automobiles.
  • Electronic Equipment: Insulates electronic components to improve safety and durability, achieving both insulation and encapsulation.
  • Medical Equipment: Ensures sterility, durability, and operational safety by molding medical devices with plastic, and forming tightly encapsulated structures.
Micro Injection Molding Parts
Micro Injection Molding Parts

Section 5: Micro Injection Molding 

Micro injection molding, a specialized subset of injection molding, is dedicated to producing extremely small components with precision and accuracy. This process employs micro-structured steel molds and advanced machining techniques to create parts weighing less than one gram and with dimensions measured in millimeters.

For starters, micro molding begins with the preparation of a mold containing small cavities molded to the specifications of the desired component. Then, Liquid Silicone Rubber (LSR) is injected into the heated cavity, undergoing vulcanization to form the final part. Nevertheless, some designs incorporate channels to guide material into cavities, while others inject material directly into the part, minimizing waste.

Still, three critical aspects define micro injection molding – micro-tolerance requirements, microfeatures within larger components, and the size of the part, necessitating precision in part measurements and tight injection molding tolerances.

Successful micro injection molding relies on factors such as tooling, mold design, and manufacturing, requiring cutting-edge machining resources and experienced professionals capable of achieving sub-micrometer tooling accuracy.

For materials, micro-injection molding typically employs high-performance polymers such as liquid crystal polymers, known for their high-temperature resistance and exceptional mechanical properties. In contrast, traditional injection molding uses a wider range of materials, including elastomers, thermoplastics, and thermosets. Nevertheless, micro-injection molding offers a plethora of advantages which include:

1. Precision and Complexity: Micromolding achieves high precision and complex geometries, allowing the production of extremely small components with tight tolerances.

2. Material Selection and Compatibility: Micro-injection molding supports multiple material options, offering excellent mechanical properties, chemical resistance, and thermal stability.

3. Cost-benefit and Efficiency: Micro-injection molding offers significant cost advantages due to lower tooling costs, reduced machine requirements, and shorter cycle times.

4. Multi-step Integration: Micromolding can integrate multiple steps, such as assembly or overmolding, into a single production cycle, increasing efficiency and reducing overall manufacturing time.

Also, micro-injection molding is essential across various industries, including healthcare, electronics, automotive, and aerospace, where precision and small-scale production are critical. It is used in manufacturing medical devices, electronic components, automotive parts, and aerospace components.

However, this injection molding method presents challenges such as tooling complexity, material flow control, quality control, and scalability.  All these challenges require meticulous attention to mold design, injection parameters, and quality assurance throughout the production process.


Section 6: Gas-assisted Injection Molding 

Gas-assisted injection molding is a technique used in the plastics industry to enhance injection molding efficiency and part quality. This process injects pressurized gas into the mold during plastic injection, creating parts with specific characteristics.

The process initiates with injecting plastic into the mold, akin to traditional injection molding. However, gas is introduced into the mold through specific channels during this step, creating internal pressure that directs the plastic to the intended areas of the mold.

The gas, which can be nitrogen, carbon dioxide, or another inert gas, is injected at a specific stage in the process, typically when the plastic has partially filled the mold.

Advantages of Gas-Assisted Injection:

Gas-assisted injection presents several advantages over conventional molding. A primary benefit is reduced part weight, achieved as pressurized gas ensures a more uniform distribution of the plastic within the mold.

Additionally, this technique facilitates the production of parts with thinner walls, leading to decreased raw material consumption and cost savings. Gas-assisted injection also enhances surface quality, minimizing flow marks and imperfections.

Applications of Gas-Assisted Injection:

Widely used across industries like automotive, electronics, packaging, and toys, gas-assisted injection proves particularly valuable in crafting parts with intricate geometries, including automobile bumpers, instrument panels, and packaging boxes.

Moreover, it is applicable to various plastics like polypropylene, polystyrene, polyamide, and polyurethane, showcasing its versatility across diverse applications.

While it offers numerous benefits, gas-assisted injection presents challenges. One significant challenge is the requirement for a more intricate mold design. This design often features appropriate gas channels and ventilation and this complexity can elevate production costs and extend development time.

operating foam injection molding machine
operating foam injection molding machine

Section 7: Structural Foam Molding 

Foam injection molding showcases the versatility of injection molding technology by introducing a cellular foam structure into molded parts. This innovative approach not only reduces material usage but also enhances the structural integrity of the final product.

A specific type of foam injection molding is structural foam injection molding, involving chemical and physical foam injection. This method produces parts with a dense outer layer and a cellular core, utilizing a chemical foaming agent. Though similar to standard chemical foam, structural foam molding operates at lower injection pressures.

The lower pressure ensures a solid, foam-free outer layer, while the core becomes foamy due to gas expansion. This results in parts with an increased stiffness-to-weight ratio, combining structural strength from the dense outer shell and reduced weight from the foam core.

Benefits of Foam Injection Molding:

  • Weight Reduction: Especially beneficial in the automotive and aerospace industries for improved fuel efficiency and usability.
  • Material Efficiency: Less material is needed due to the internal foam structure, reducing consumption and production costs.
  • Improved Structural Strength: Provides a high strength-to-weight ratio, crucial for parts facing loads or impacts.
  • Thermal and Acoustic Insulation: Excellent insulation properties for applications requiring resistance to heat transfer and sound absorption.
  • Design Flexibility: Allows for intricate details and complex shapes that might be challenging with solid materials.
  • Cost-Effectiveness: Decreases cycle times, material heating, and clamping forces, contributing to overall cost savings.

Used across various industries, foam injection molding finds applications in automotive, aerospace, medical, and consumer products as highlighted below.

1. Automotive:

   – Dashboard and instrument panel supports.

   – Door panels with enhanced acoustics.

   – Lightweight bumper cores and HVAC boxes.

2. Aerospace:

   – Cabin dividers for privacy and noise reduction.

   – Components for weight-sensitive storage.

   – Comfortable and lightweight seat structures.

3. Medical:

   – Surgical instrument boxes.

   – Housings for medical imaging devices.

   – Customized trays for sterile processing.

4. Consumer Products:

   – Protective sports equipment.

   – Lightweight luggage components.

   – Durable toy components and furniture pieces.

Foam Method vs. Standard Injection Molding:

The foaming method differs from standard injection molding by introducing a blowing agent into the plastic, creating a cellular foam structure. This alters the composition and properties of the final product, resulting in lightweight parts with improved insulation and shock absorption characteristics. Standard injection molding produces solid, dense products, making foam injection molding ideal for applications requiring a higher strength-to-weight ratio and unique thermal and acoustic properties.

Automotive application of RIM
Automotive application of RIM

Section 8: Reaction Injection Molding (RIM) 

Reaction injection molding (RIM) is a process for making plastic parts, where polyolefin and isocyanides, along with other ingredients, are mixed and injected as a liquid mass into a mold. The liquid mass solidifies as it reacts with the heated mold surface.

RIM requires low clamping force and allows for long material paths, under low-pressure conditions. Aluminum molds can be quickly and cost-effectively manufactured. Parts made with RIM compete with those from injection molding, thermoforming, and fiberglass setting.

There are a lot of reasons manufacturers often go for Reactive injection molding (RIM). However, one important reason is its use of thermosetting materials, with the curing reaction occurring simultaneously as the resin flows during molding. Other advantages of RIM includes:

1. Strength: RIM produces high-strength plastic parts due to cross-linking in polymeric structures.

2. Cost Benefit: RIM is more economical for low annual volumes due to the use of economical materials.

3. Versatility: Allows for creativity with varying wall thicknesses and sizes without distortion

Applications of RIM:

RIM molds find applications across industries like medical, maritime, and transportation. They produce polyurethane housings for medical equipment and scientific instruments, as well as rugged parts for electric vehicle charging units, personal diving units, and solar canopies.

RIM molds play a vital role where reliability, durability, and compliance with regulatory standards are essential. They provide precision parts that offer structure and confidence in various demanding environments.

Challenges of RIM:

Thermosetting polymers are difficult to recycle, and issues like flashing, burning, and longer cycle times pose challenges. The interaction between chemical reaction, fluid flow, and heat transfer complicates process control and optimization for RIM.


In a nutshell Injection molding has become a well-established manufacturing technique playing a vital role in producing plastic parts. However, selecting the right type of injection molding depends on project needs which can include part shape, finish, and assembly. Traditional, thin-wall, insert, overmolding, micro-injection, gas-assisted, structural foam, and RIM techniques which have been highlighted in this post offer solutions for various challenges. Consequently, manufacturers can efficiently produce top-notch parts at lower costs by selecting the appropriate injection mold.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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