May 14, 2024

Comprehensive Guide to Plastic Injection Molding

CNC milling machine finishing cut the injection mold parts with the solid ball end mill tools

Injection molding is a manufacturing technique where a thermoplastic polymer is heated until molten and then injected into a mold that defines the shape of the final product. In this process, the mold is maintained at a temperature below the plastic’s solidification point. This cooling causes the molten plastic to solidify quickly into the desired shape. After a brief period to allow the plastic to set, the mold opens, and the formed part is either automatically ejected or manually removed.

The essential equipment used in injection molding includes the machine itself, the mold, a water heater to control mold temperature, a material dryer for preparing the plastic, and sometimes robotic arms. These robots assist in transferring the finished products from the machine to a designated area and perform additional tasks such as quality inspection, removing excess material, and arranging the products on pallets for distribution.

How Does Plastic Injection Molding Work?

Depending on the project’s requirements, plastic injection molding starts with designing and machining molds from either steel or aluminum. Steel molds are more costly but are durable enough to manufacture millions of parts over their lifespan. In contrast, aluminum molds are less expensive and commonly used for producing smaller batches or prototyping.

Engineers employ specialized machinery to create these molds to meet specific customer specifications. This machinery often includes CNC (Computer Numerical Control) milling machines, EDMs (Electrical Discharge Machines), and surface grinders to achieve the precise dimensions and shapes required.

After a mold is crafted, it undergoes various testing phases to confirm its functionality. Typically, small prototype runs ensure the mold produces parts that meet the customer’s exact specifications. These tests help verify the mold’s accuracy and the quality of the parts before full-scale production begins.

Materials for Injection Molding

resin material for injection molding

Many types of plastic materials can be used in injection molding. Some of the most common include:

  • ABS (Acrylonitrile Butadiene Styrene): Known for its strength, flexibility, and ease of machining. Used in automotive parts, electronic housings, and toys.
  • Polypropylene (PP): Versatile, flexible plastic with high chemical resistance. Used in packaging, auto parts, healthcare products.
  • Polycarbonate (PC): Extremely strong with temperature and impact resistance. Used in medical devices, safety equipment, electronics.
  • Polyethylene (PE): Lightweight and chemical/moisture resistant. Used in packaging films, containers, pipes.
  • Polyamide (Nylon): Highly durable and resistant to wear and abrasion. It is used in mechanical parts and consumer products.

Material Properties and Molding Effects

The properties of the plastic material, such as melt flow index, density, strength, and heat deflection temperature, affect how it flows into the mold, solidifies, and performs in the final part.

  • Flowability: How easily the molten plastic fills intricate mold details. Depends on melt flow index.
  • Shrinkage: How much the plastic shrinks as it cools. Depends on material density. Must be accounted for in mold design.
  • Strength: How the material withstands forces/loads. Depends on tensile/flexural strength. Critical for load-bearing parts.
  • Heat resistance: The max temperature the material can withstand. Depends on heat deflection temperature. It is important for parts to see high heat.

Material Selection Guidelines

Choosing the right plastic involves analyzing application requirements and material properties:

  • Functionality: Strength, flexibility, temperature resistance needed
  • Environment: Chemical exposure, outdoor use factors
  • Aesthetics: Texture, clarity, color, finish requirements
  • Material Candidates: Assess properties and moldability
  • Validate with Prototyping: Test moldability and part performance

Recycled Plastic Considerations

Using recycled plastics can reduce environmental impact. Considerations when using recycled resins:

  • More variability in material performance
  • May need to modify molding parameters
  • Limit recycled content for critical features and dimensions
  • Best for non-appearance-related parts

Mold Design and Fabrication

fabricated mold for injection molding

Proper mold design is critical for successful injection molding. The mold is responsible for forming the plastic into the desired shape and allowing for efficient cooling and part ejection. Essential considerations in mold design include:

Types of Molds

There are two main types of molds used in injection molding:

  • Steel molds – Made from tool steel or stainless steel. They are very durable and capable of producing 500,000+ injection molded parts. However, they are expensive and have long lead times for fabrication. Best suited for long production runs.
  • Aluminum molds – Made from aluminum alloys. They are less expensive and have faster fabrication times than steel molds. Best suited for short prototype or low volume production runs up to 10,000 parts. They wear faster than steel molds.

Mold Design Features

Some key design features that enable proper mold filling, cooling, and part ejection:

  • Draft Angles: Angled surfaces that allow the part to be removed cleanly from the mold. Typically 1-5 degrees.
  • Uniform Wall Thickness: Maintaining uniform thickness where possible allows for even cooling and minimizes sinks and warpages.
  • Cooling Channels: Channels cut into the mold to circulate cooling fluid and solidify the plastic evenly. Improper cooling can lead to long cycle times or defects.
  • Ejection System: Compressed air or mechanical pins that eject the finished part out of the mold cavity.

Mold Fabrication Process

The mold fabrication process involves:

  1. Designing the mold in CAD software based on the part design
  2. CNC machining the different mold components from metal blocks
  3. Polishing and finishing the mold surfaces
  4. Assembling the mold and integrating cooling and ejection systems
  5. Testing the mold with short test runs to ensure proper filling and part quality

The role of CAD/CAM technology is essential for precisely designing and machining the mold. Quality inspection at all stages also helps avoid costly errors. Proper mold design, fabrication, and testing ultimately consistently produce good-quality parts.

The Injection Molding Process Detailed

Injection molding is a complex yet efficient manufacturing process that produces various plastic parts. At a high level, the process involves injecting molten plastic materials at high pressure into a mold cavity, where they harden as they cool into the final shape.

1. Clamping

This stage requires applying force to close and secure the mold halves using either a hydraulic system or a toggle clamp. Proper sealing at the parting line ensures that the pressure from the injected molten plastic is effectively countered, preventing any separation of the mold halves during the process.

2. Injection

This stage is broken down into two or three steps. Initially, a screw moves forward, powered by either a hydraulic cylinder or an electric servo motor, to push the plastic through the barrel’s nozzle into the mold. The process begins with controlling the injection speed and transitions to maintaining pressure (“pack and hold”) to compensate for plastic shrinkage. This careful control, decoupling, aims to optimize throughput while ensuring precise control over the molding process.

3. Recovery or Plasticization

During this phase, plastic pellets are melted as they pass through the barrel’s heater bands and are sheared by the screw’s rotation. The molten plastic collects in front of the screw, ready for the next injection cycle. After initial setup, the screw’s mechanical action largely sustains the necessary heat for this process.

4. Cooling

To preserve the quality and dimensions of the molded parts, cooling channels within the mold circulate a cooling agent such as water. This stage is crucial as it often consumes most of the cycle time, emphasizing the need for effective cooling to maintain production efficiency.

5. Demolding

This final stage involves removing the solidified part from the mold. Mechanisms such as slides or lifters retract, followed by the part being ejected mechanically, by compressed air, or manually.

Ready for a new cycle, the mold is installed onto an injection molding machine, and plastic pellets are fed from a hopper into the machine. As these pellets move down the barrel, they are heated and plasticized. This molten plastic is then forced into the mold in a controlled manner to form the part. After cooling, the mold opens, the part is ejected, and the cycle begins anew.

Process Parameters

Parts of automatic injection molding machine - dryer heat hopper tank for drying plastic granules

Several essential process parameters need to be properly set and controlled in injection molding:


The plastic pellets must be heated to a molten state in the barrel of the injection molding machine through a series of temperature zones. Optimizing temperature is critical – too low can make the plastic too viscous to flow properly into the mold, while too high can degrade the material.

Injection Pressure

Sufficient injection pressure, typically measured in PSI, must be applied to force the molten plastic into the mold cavity quickly and completely before it solidifies. Pressure is optimized based on factors like part thickness and flow length. Insufficient pressure causes short shots. Excessive pressure can damage molds.

Packing Pressure

After initial injection, packing pressure is applied to force additional material into the mold to account for material shrinkage as it cools. Ideal packing time/pressure minimizes sinks, voids, and warpages in parts.

Cooling Time

Solidifying the melted plastic before opening the mold requires sufficient cooling time. Cooling times are optimized to maximize production throughput without deforming part geometries or properties.

Part quality and efficiency can be optimized by precisely controlling these parameters through process monitoring and testing. Machine sensors, data acquisition software, and statistical process control techniques are used.

Importance of Process Monitoring/Control

Continuous monitoring and control of process parameters are critical in injection molding. They ensure repeatability and consistency for each production run. Rigorous process control enables early detection of deviations from specifications and corrections before defective parts are created, preventing wasted time and materials. Optimizing process parameters reduces cycle time without sacrificing quality, improving productivity and costs. Automated feedback control via sensors and software is typically used for precise control.

Quality Standards and Compliance

Quality is critical in the injection molding industry, especially when the molded parts are intended for specific applications that require adherence to certain regulatory standards and conditions. Ensuring that parts meet these standards is essential for compliance and maintaining the integrity of the manufacturing process.

The International Organization for Standardization (ISO) is a pivotal body that convenes experts to create standards that foster innovation and solve global challenges. The ISO 9000 series is the most widely recognized quality management standard globally. It outlines a framework for quality management and continuous improvement applicable to any organization, regardless of size or sector.

Various other ISO standards focus on different dimensions of quality management. For instance, ISO 14001 targets environmental management systems, ISO 13485 is specific to medical devices, ISO 45001 addresses occupational health and safety, and ISO 27001 pertains to information security management systems.

At MOS Plastics, we comply with the ISO 13485:2016 standards, which are crucial for manufacturing medical devices. This standard requires an organization to demonstrate its capability to consistently produce medical devices and related services that meet customer and regulatory demands.

Additionally, KS Manufacturing is certified under ISO 9001 in its Tijuana and San Leandro facilities. ISO 9001 is unique in the ISO family as it is the only standard that offers certification. It is designed for organizations of any size and in any industry. It is based on several quality management principles, including a strong customer focus, the active engagement of top management, a systematic approach to processes, and a commitment to continual improvement. These certifications underline our commitment to maintaining the highest quality standards in all aspects of our operations.

Applying Quality Standards in Injection Molding

ISO standards like ISO 9001 and ISO 13485 provide frameworks to implement quality management systems focused on consistency, traceability, and continuous improvement. For injection molding, this includes:

  • Validating processes to ensure parts meet specifications
  • Extensive documentation of procedures and parameters
  • Training programs and clear responsibilities for personnel
  • Regular inspection and testing for defects
  • Control of nonconforming product
  • Corrective/preventive action procedures
  • Management review for ongoing improvement

Guidelines for Implementation

Guidelines for implementing quality standards in injection molding:

  • Review design control and production planning procedures to ensure traceability
  • Validate equipment setup, process parameters, inspection methods
  • Document all processes, including material handling, storage, training
  • Implement inspection of parts to AQL levels per sampling plan
  • Record process data for analysis to fix and prevent defects
  • Conduct regular audits and management reviews

Benefits of Adhering to Standards

Benefits of adhering to injection molding quality standards:

  • Ensure consistently high product quality and reliability
  • Prevent defects and reduce scrap/rework costs
  • Gain customer trust and meet regulatory requirements for industries like medical and automotive
  • Continually improve operations over time

How To Apply for ISO Certification

  1. Document the existing quality system or establish a new one
  2. Train employees on procedures and responsibilities
  3. Conduct an internal audit of requirements
  4. Select registrar to verify compliance via external audit
  5. Register and list certification

How to Maintain Compliance

  • Regular internal audits and improvement projects
  • Updating procedures as processes change
  • Ongoing training in new requirements
  • Management review of data to identify improvement areas

Types of Injection Molding

types of injection molding graphic

The injection molding process isn’t isolated to only one type. Here are the most common three types.

Thermoplastic injection molding is a manufacturing process that creates fully functional parts by injecting plastic resin into a pre-made mold. It has several subcategories.

Insert molding is the process of molding or forming plastic parts around other non-plastic parts or inserts. The inserted component is most commonly a simple object, such as a thread or rod, but in some cases, inserts can be as complex as a microchip or battery.

Overmolding is an injection molding process in which one plastic part is molded over the top of another component (often called the substrate). The substrate is usually an injection-molded plastic part but could also be made from various other materials.

Micro Injection molding is a process for plastic components with shot weights of 1 to 0.1 grams and tolerances in the range of 10 to 100 microns. This molding process permits the manufacture of complicated geometries with maximum accuracy and precision.

Applications and Considerations of Injection Molding

Thermoplastic Injection Molding

  • Applications: Consumer products, automotive parts, medical devices, a broad range of industries.
  • Considerations: Thermoplastic properties, production volume requirements, tolerance, and feature complexity needs

Insert Molding

  • Applications: Electrical connectors, medical devices with metal components
  • Considerations: Bond strength between plastic and insert material, insert placement and alignment during molding


  • Applications: Combining rubber and plastic components in products, soft grip handles
  • Considerations: Adhesion between the substrate and overmolded plastic, curing times, stress from a mismatch in properties

Micro Injection Molding

  • Applications: Miniature medical devices, sensors, microfluidics
  • Considerations: Higher precision equipment needed, managing micro-sized features, surface finishing

Cost Considerations in Injection Molding

The main elements that make up total costs in injection molding:

  • Material Costs: The plastic resin price per pound or kilogram. Varies significantly by resin type.
  • Mold Costs: There is an upfront investment for custom mold design and CNC machining. Typically, this is $1,000 – $5,000+ for prototype/low-volume molds and $5,000 – $100,000+ for high-volume production molds. Steel molds are more expensive than aluminum. Complexity impacts costs.
  • Production Costs: These include the costs of operating injection molding machines and labor for production runs. They also include energy, consumables, and quality control testing.
  • Secondary Operations: Value-added steps like decorating, welding, and assembling.
  • Tooling Maintenance/Replacement: Periodic mold maintenance and replacement after 500,000+ cycles.

Impact of Design Choices in the Cost of Injection Molding

  • Material Selection: Engineering resins are more expensive than commodities. The cost difference can be 10x or more.
  • Mold Design: Larger part sizes and high-complexity molds cost more to produce, while simpler geometries reduce machining time.
  • Production Volume: Set-up costs are high, but the incremental part cost is very low. Higher volumes provide economies of scale.

Cost Optimization Strategies

Strategies to optimize costs include:

  • Design parts for easier moldability with standard materials
  • Consolidate multiple parts into the single mold when feasible
  • Use lower-cost rapid prototyping methods for testing before committed production
  • Produce molds with expected lifetime output to avoid frequent replacement
  • Adjust injection molding process parameters to minimize rejects and material waste
  • Consider secondary operations upfront to enable automation

How Injection Molding is Used Across Various Industries

Injection molding produces plastic parts for many applications across many industries. Some examples include:

  • Automotive – Bumpers, panels, lights, fluid reservoirs
  • Consumer/Electronics – Enclosures, cases, connectors, controls
  • Medical – Diagnostic devices, disposable syringes, sample cups
  • Industrial – Gears, fittings, material handling equipment

Troubleshooting and Optimization

Some common issues faced in injection molding and potential solutions:


  • Cause: Uneven cooling or pressures.
  • Solutions: Balance flow paths, increase vents, and optimize cool time.

Sink Marks

  • Cause: Untimely solidification.
  • Solutions: Increase mold temperature and optimize packing pressure.

Short Shots

  • Cause: Low injection pressure or speed.
  • Solutions: Increase injection velocity and pressure to ensure plasticization.


  • Cause: Clamping force too low.
  • Solutions: Increase clamp force and check for mold damage.


  • Cause: Degraded plastic or foreign debris.
  • Solutions: Filter materials and maintain mold cleanliness.

Process Optimization Tips

Ways to optimize the injection molding process:

  • Monitor and analyze key process data to identify improvement areas
  • Perform design of experiments with varying factors like temperature and pressure.
  • Simulate mold filling and cooling to predict and resolve issues digitally first
  • Account for material property changes from shear heating and moisture.
  • Upgrade to real-time closed-loop process control rather than fixed parameter settings
  • Schedule preventive maintenance to avoid unplanned downtime
  • Leverage automation in secondary operations for improved consistency

Continuous process optimization via data-driven decisions and the latest technology enhances product quality and productivity over time. Injection molding requires vigilance, even with established processes for changing conditions.

Why Choose Injection Molding?

White Medical plastic bottle caps. Made from injection molding machine.

Injection molding is a favored method for manufacturing plastic parts due to several key advantages:


Injection molding offers high repeatability, allowing for the production of thousands of units that are virtually identical. This uniformity is crucial for maintaining brand reputation and reliability, especially in large-volume production.


Once the injection molding process is set up and calibrated, the machines can produce large quantities of parts per hour with impressive energy efficiency. This makes it ideal for mass production.


The process allows for significant customization. Manufacturers can select from a variety of mold designs and specify the type of thermoplastic used, accommodating even the most intricate designs.

Lower Scrap Rates

Compared to CNC machining, which often results in considerable material wastage, injection molding is much more efficient in its use of materials, resulting in lower scrap rates.


Although the initial cost of creating molds can be high, the subsequent cost per unit is relatively low when parts are produced in large volumes. This makes injection molding a cost-effective solution for long-term production.


Injection molding can consistently produce parts that meet high standards of quality, whether the requirement is for strength, flexibility, or intricate detailing. This capability ensures that each component reliably meets the manufacturer’s specifications.

These benefits make injection molding a compelling choice for manufacturers seeking efficient, reliable, and high-quality production of plastic parts.

The scalability of injection molding makes it a strategic process for prototypes to high-volume production. For low volumes, aluminum molds allow low-cost testing of new designs.

Investing in durable steel molds results in consistency and economic scale benefits for scaled-up production. Automation also provides flexibility–a small floor footprint cell with a robot can produce millions of parts.

The ability to meet demands from pilot builds to just-in-time supply for large OEMs enables injection molders to grow overall project life cycles. With informed mold and machine selection, the incremental costs stay relatively linear versus exponential savings in CNC machining or other subtractive techniques. This scalability empowers agility in responding to market demands.

Getting Started with Your Injection Molding Project

  1. Develop detailed 3D CAD models of the part design
  2. Select a plastic material based on mechanical requirements, aesthetics, etc.
  3. Partner with an experienced injection molder early and solicit their feedback
  4. Optimize the design for manufacturability and simulate the molding process
  5. Build prototype molds and samples before committing to production
  6. Qualify the process by testing parts to specifications
  7. Plan for scalable production molds and equipment

Choosing Manufacturing Partners

  1. Look for injection molding partners that:
  2. Have expertise in your application or industry
  3. Offer prototyping through high-volume production
  4. Have a quality system certified to ISO standard
  5. Employ advanced equipment and simulation tools
  6. Focus on transparency and continuous improvement

Getting the most from your injection molding partner yields optimal, cost-effective parts.

Contact MosPlastics Today to Learn More About Injection Molding

Injection molding is an excellent choice for large-scale production and an invaluable tool for prototyping and product testing. While traditionally more cost-effective at higher volumes, aluminum tooling has made even the prototyping phase economically viable.

The technique is renowned for several key benefits that make it a preferred method across various industries, including consumer goods, medical equipment, and automotive manufacturing. Some of these advantages include:

  • Repeatability: The ability to consistently produce identical parts ensures uniform quality across all product batches.
  • Exceptional Surface Finish: Injection molding can achieve high-quality surface finishes, reducing the need for additional processing.
  • Complex Part Production: The process can handle complex designs that might be challenging to create using other manufacturing methods.
  • Wide Material Selection: A broad range of materials are available, each offering different characteristics to meet the specific needs of products.

For those interested in learning more about plastic injection molding’s capabilities, especially in the creation of plastic medical devices, experts at MOS Plastics are ready to provide comprehensive insights and support. Contact us to discover how injection molding can enhance product development and manufacturing processes.


  • Acrylonitrile Butadiene Styrene (ABS): A common thermoplastic polymer known for its impact resistance and toughness.
  • Cavity: The hollow space within a mold into which molten plastic is injected to form the part.
  • Clamping: The process of applying force to hold the two halves of the mold together during injection.
  • Cooling channels: Hollow passages in the mold through which coolant flows to regulate mold temperature.
  • Cycle time: The time required to complete one full molding cycle, from mold closing to part ejection.
  • Draft: The degree of taper on vertical walls of a part to facilitate ejection from the mold.
  • Ejector pins: Rods built into the mold to push the solidified part out of the mold cavity.
  • Fill pressure: The pressure under which the plastic is injected into the mold cavity.
  • Gate: A small opening in the mold through which hot plastic enters the cavity.
  • Holding pressure: The pressure applied after initial cavity fill to pack more material and compensate for shrinkage.
  • Hot runner: A heated channel that carries the molten plastic from the machine nozzle to the cavity gates.
  • Injection unit: The part of the molding machine that heats and injects the plastic material.
  • Melt temperature: The temperature to which the plastic must be heated to flow properly into the mold.
  • Mold: The tooling used to produce the desired part shape, typically in two halves.
  • Packing: The filling of the mold cavity with additional material after initial injection.
  • Parting line: The line on a part formed where the two mold halves meet.
  • Polyethylene (PE): A widely used plastic available in low and high-density grades.
  • Polypropylene (PP): A versatile plastic used in many industries for its strength and chemical resistance.
  • Runner: The channel is machined into the mold to carry plastic from the sprue to the part cavity.
  • Shrinkage: The contraction of a molded part as it cools.
  • Shot: The complete cycle of injecting plastic into the mold to form a part.
  • Sprue: The passageway through which hot plastic flows from the nozzle to the mold cavity.
  • Thermoplastic: A plastic that melts to a liquid when heated and returns to a solid when cooled.
  • Tonnage: Clamp force that holds the mold closed during injection, expressed in tons.
  • Vent: A shallow channel cut into the parting surface of the mold to allow trapped air to escape.
  • Warpage: Distortion in a molded part after cooling caused by uneven shrinkage or stress.
  • Weld line: A visible line on a part where two flow fronts meet during cavity fill.