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Injection Molding Terms and Definitions


If you’re new to injection molding, you may find that the sheer amount of terminology can be overwhelming. With so many technical terms and definitions, it can be difficult to ensure that you have a full understanding of the injection molding process and its intricacies.

Injection Molding Terms and Definitions

In this comprehensive guide, we’ll cover everything you need to know about injection molding terms and definitions, from the most basic to the most complex. Whether you’re new to the industry or a seasoned veteran, this article is an indispensable tool for optimizing your processes and maximizing your results.

Injection Molding definition

 injection molding is a manufacturing process for producing parts by injecting molten material into a mold. Injection moulding can be performed with a host of materials mainly including metals (for which the process is called die-casting), glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed (using a helical screw), and injected into a mould cavity, where it cools and hardens to the configuration of the cavity.

injection molding terms

1.Plastic Resins

Resin is one of the most commonly used plastic injection molding terms. The raw materials (plastic polymers) used in the process are known as plastic resins. There are hundreds of resins available in the market. With different properties and features like hardness, flexibility, weight, impact strength, tensile strength, elasticity, chemical resistance, heat deflection, water absorption, each resin has its applicability for individual plastic products. 

Additionally, plastic resins also vary in terms of cost. Choosing the ideal resin or a mixture of resins for a particular product requires expertise. Some of the most commonly used resins for plastic injection molding are:

  • Acrylonitrile Butadiene Styrene (ABS)
  • Low Density Polyethylene (LDPE) and High Density Polyethylene (HDPE) 
  • Polycarbonate (PC)
  • Polyamide (Nylon)
  • High Impact Polystyrene (HIPS)
  • Polypropylene (PP)
  • Acrylic (PMMA)
  • Polyetheretherketone (PEEK)
  • Acetal/ Polyoxymethylene (POM)
  • Thermoplastic Polyurethane (TPU)
injection molding materials

2.injection molding machine

injection molding machines

Mold:You can’t go through a list of plastic injection molding terms without discussing molds. Molds are hollow shapes in which plastic resins are injected to create the desired part. Molds are commonly made out of stainless steel. 

Designing of molds is often critical since the accuracy of the mold determines the precision of the product created. Molds are hence significantly expensive to design and create.

Injection molding mold

Computer aided designs (CAD) are used to create molds that have tight tolerance and high levels of precision in order for them to last for several years of production. 

Mold Cavity:The hole or cavity within the mold designed in the shape of the final plastic part is known as the mold cavity. This is the space where a plastic resin is injected. The higher the level of cavitation in the mold, the more is the productivity rate. 

Molders/manufacturers often prefer creating one mold per part rather than creating a common mold with different cavities for each part to get higher accuracy in final products. 

Barrel: The chamber in the injection molding machine where the molten material is heated and melted.

Clamping Unit: The part of the injection molding machine that closes the two halves of the mold, ensuring that it remains securely in place during the injection process.

Gate: The opening in the mold through which the molten material is injected.

Runner: A channel that distributes the molten material from the gate to the cavities of the mold.

3. Colorant

These are coloring or pigmentation components that are available in pelletized form or as liquids. The colorants are mixed with plastic resins to create the desired shade for a product. Specific color concentrates are developed using automated machines through a color matching process. 

Different colors/shades are marked with different numbers which give the engineers an idea of the desired hue while creating the mix. Additionally, it is also important to carefully select the polymers being used before formulating the color concentrate. 


Fillers are substances that are added to the resins during the plastic injection molding process to enhance the properties of the material. 

Sometimes, the requirements of a product are difficult to meet with the properties exhibited by the chosen resins. In such instances, fillers are added to supplement the properties of the polymers being used. 

Various commonly used fillers include silica, mica, glass, clay, calcium carbonate, alumina trihydrate, etc. 

5.Types Of Injection Molding Machines

There are several different types of injection molding machines, each with its own unique features and advantages. Here are some of the most common types:

types of injection molding machines
Hydraulic Injection Molding MachinesElectric Injection Molding MachinesHybrid Injection Molding Machines
All Hydraulic Injection Molding Machines is the only type of plastic injeciton molding machine before 1980s. until the year of 1983, a Japanese company introduced the first all-electric injection molding machine in the world.Electric Injection Molding Machines is available in the 1980s and quickly became popular in high precision plastic parts manufacturing industry. The full electric servo drive injection molding machine`s positioning accuracy is extremely high (of ± 0.0001 in) in nowadays but with very expensive price.Hybrid Injection Molding Machines combines hydraulics and electric motor drives into a single variable-power AC drive power source that operates on-demand. It have both benefits of two types of injeciton moulding machines: Electric Injection Molding Machines and Hybrid Injection Molding Machines.
Advantages of Hydraulic Injection Machines:Advantages of Electric Injection MachinesAdvantages of Hybrid Injection Machines
Lower initial price: The base price of these machines are significantly less than its alternatives.Energy efficiency: Electric injection molding machines only use energy when in action—meaning the motor output is only equal to that of the load requirements.Diversity: the possible combinations for hybrid injection molding machines open up new degrees of design flexibility.
Cheaper parts: If a part on the machine needs to be replaced due to damage or the parts life-span reached its limit, the replacement parts are lower cost and easier to find.Less-down time: There is no chance of leakage from oil. Also, no oil use means no filters to be replaced, which means consumable expenses are less.Best of both: Hybrid molding machines combine the energy savings and accuracy of electric molders with the force-generating capacity of a hydraulic unit.
Stronger parts: The hydraulic parts of the machine are more resistant to wear and tear—they are built to last.Faster process: These machines are digitally controlled, meaning the process is entirely repeatable and needing no supervision.Less downtime: These require less maintenance and experience less downtime than an all-hydraulic or all-electric molding machines.
Power: Hydraulic molding machines have exceptional clamping force.Clean operation: Since there is no use of oil for this machine, it is safe for use if you are manufacturing medical and food packing parts. Typically, these parts should be created in a clean-room area.Price point: Falls nicely between all-hydraulic and all-electric molding machines, making it an affordable option. Especially for medical device manufacturers.

6.Injection Molding Defects

Are you experiencing defects in your injection molding works process? If so, you’re not alone. It’s a common problem that can cost your company time and money.

Here are some common injection molding defects with its descriptions and causes.

common injection molding defects
Moulding defectsDescriptionsCauses
Burn marksBlack or brown burnt areas on the part located at furthest points from gate or where air is trappedTool lacks venting, injection speed is too high.
FlashExcess material in thin layer exceeding normal part geometryMould is over packed or parting line on the tool is damaged, too much injection speed/material injected, clamping force too low.
Flow linesDirectionally “off tone” wavy lines or patternsInjection speeds too slow (the plastic has cooled down too much during injection)
JettingJetting is a snake-like stream which occurs when polymer melt is pushed at a high velocity through restrictive areas.Poor tool design, gate position or runner. Injection speed set too high.
Weld linesSmall lines on the backside of core pins or windows in parts that look like just lines.Caused by the melt-front flowing around an object standing proud in a plastic part as well as at the end of fill where the melt-front comes together again.
Sink marksLocalised depression (In thicker zones)Holding time/pressure too low, cooling time too short.
Short shotPartial partLack of material, injection speed or pressure too low, mould too cold, lack of gas vents.
Weld lineDiscoloured line where two flow fronts meetMould or material temperatures set too low (the material is cold when they meet, so they don’t bond).
WarpingDistorted partCooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures.

7.Injection Molding Cycle Time 

injection molding cycle time
  • Cooling Time: Once the mold cavity is filled with molten plastic, the material needs time to cool and solidify. The cooling time is a critical part of the cycle as it affects the part’s dimensional stability and quality. The cooling time is influenced by the type of material used, the thickness of the part, and the efficiency of the mold cooling system.
  • Dwelling Time: After the cooling phase, there is a dwelling time during which the material remains in the mold to ensure it solidifies fully and reduces the risk of warping or distortion.
  • Ejection Time: Once the cooling and dwelling stages are complete, the finished part is ejected from the mold using ejector pins or other mechanisms. The ejection time is the duration required to remove the part from the mold.
  • Mold Opening/Closing Time: The time it takes to open and close the mold between cycles is also part of the overall cycle time. This time can vary based on the complexity and size of the mold.

Why Knowing These Terms Matters

Having a firm grasp of injection molding terminology will allow you to communicate more effectively with colleagues, customers, and suppliers. It will also help you to understand the nuances of injection molding and make informed decisions about process parameters, material selection, and equipment purchasing.

In addition, knowledge of these terms can also help to reduce potential problems and errors in the injection molding process. By knowing and understanding injection molding terms, it becomes easier to identify potential issues before they become significant problems, and therefore, address them proactively.

Why Choose Injection Molding

Now that we’ve covered the basics of injection molding and some of the terminology involved, let’s talk about why it’s a great option for manufacturing.

Injection molding offers a number of benefits over other manufacturing processes. Here are just a few:

Cost-efficiency: Injection molding is an efficient manufacturing process that can produce a large number of parts quickly and accurately. This makes it a cost-effective option for large-scale production runs.

Design flexibility: Injection molding allows for a wide variety of part designs, including complex shapes and intricate features. This makes it a great option for producing parts with unique requirements.

Quality control: Injection molding offers consistent quality control, as the process is highly repeatable and precise. This makes it an ideal choice for producing parts that require strict specifications.

Material options: Injection molding can be used with a wide range of materials, including plastics, metals, and ceramics. This makes it a versatile option for a variety of manufacturing needs.

In conclusion, understanding injection molding terms and definitions is an essential part of mastering the manufacturing process. With the knowledge gained from this article, you’ll be able to communicate more effectively with injection molding professionals and make informed decisions about your manufacturing needs.

How Injection Molding Terms Impact Your Bottom Line

While understanding injection molding terms and definitions is essential for optimizing your processes, it also has a direct impact on your bottom line. Failing to use the correct terminology or properly communicate with your team can lead to costly mistakes and setbacks.

Having a shared vocabulary and understanding of the key concepts can ensure your team is on the same page and working towards a common goal. Being conversant with injection molding terms also helps you to better communicate with suppliers and clients, who are themselves likely to be seeking professional partners who are well versed in the technology. This can help to grow your business and expand the range of clients and projects you take on.

Having a solid understanding of injection molding terms and definitions is vital for those in the business as it empowers better teamwork, both internally as well as with clients and suppliers. There are many factors involved in successful injection molding, and by mastering the vernacular, you can ensure optimal results and profitable growth for your business.


As a professional in the injection molding industry, it’s critical to have a solid understanding of the fundamental terms and definitions related to this field. From understanding the basics of the molding process to mastering the various components involved, having a thorough grasp of injection molding terminology is essential to ensuring top-notch results.


How Does Plastic Injection Molding Work?

The manufacturing process for molded parts is complex. Before your parts are even molded, we’re there to help you with comprehensive manufacturability feedback and consultative design services, so you can avoid mold design pitfalls before the resin hits the mold. From there, our production process is documented and optimized using scientific molding so you get high-quality parts consistently and reliably with every production run.

Critical-to-quality (CTQ) dimensions you call out on models let us know what areas are most crucial to the success of your parts. Once the parts start molding, the process and samples are qualified using automated inline CMM technology, generating a detailed FAI and process capability study with no additional lead time.

What is injection molding used for?

  • Bottles and packaging:Billions of plastic bottles are made by injection molding each year, making them the most widely injection molded product of all. Most water and soft drink bottles are made from polyethylene terephthalate (PET).
  • Household items:Simple plastic objects such as containers, garbage bins, kitchenware, electric air fresheners and even toilet seats are some of the many household goods made using injection molding.
  • Automotive components:Not all injection molded parts are cheap or disposable. Car parts like bumpers, dashboards and cup holders are often made with injection molding, and can therefore be made in massive quantities to fulfill the needs of automotive suppliers.
  • Healthcare products:Essential medical products like syringes, trays and tubes can be injection molded, since a range of medically safe plastics can be used in the manufacturing process.
  • Construction items:Most buildings aren’t made of plastic, but injection molding can be used to fabricate many construction parts, including vents, gaskets, conduits, insulators and flooring panels.
  • Electronic components:Many thermoplastics are excellent electrical insulators, making them ideal for molded housings used in electronic items.
  • Toys:Most plastic toys are made using injection molding. The simplest of these toys may not even require painting or tinting, since raw plastic pellets can be acquired in a variety of bright colors.

What Is The Difference Between 3D Printing And Injection Molding?

Below is a list of comparisons between 3D Printing and Injection molding using key metrics:

ParameterInjection Molding3D printing
MaterialsThermoplastics, some specialty materials in newer machinesThermoplastics, thermosets, metals, ceramic, impregnated materials
CostHigh initial investment cost, low cost per-partMedium to high initial investment cost, medium to high cost per part
Part sizeHighly variable (both very small and large parts possible)Restricted by build space, parts are generally no bigger than a cubic meter
WasteLow/no waste, and waste can be reground back into stockHigh waste output
Design changesDifficult, changes are slowVery easy, rapid changes possible


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