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What Is A Plastic Injection Mold? A guide for newcomers and designers

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Finished injection moulds

In our last article, we delved deep into ‘What is injection molding‘. Due to space limitations, we didn’t focus on injection molds. We understand that injection molding and plastic injection molds are inseparable. This article will comprehensively decode ‘what is a plastic injection mold’, packed with valuable information. It’s ideal for newcomers, designers, and industry peers.

If you’re looking to discover more about injection mold manufacturing services and their case studies, click on ‘Injection Mold Making Services‘. There, you’ll find services, quotes, and case studies offered by FirstMold.

Finished injection molds set neatly in the mold assembly shop
Finished injection molds set neatly in the mold assembly shop

What Is a Plastic Injection Mold?

Definition of Plastic Injection Mold

Plastic injection molds are tools used for injection molding, a method utilized in producing industrial plastic products. We have previously discussed injection molding, a process that involves heating plastic (such as plastic pellets or fibers) to a molten state. This molten plastic is then injected into molds using an injection molding machine. After cooling, the material solidifies into the desired shape and size. Put simply, this process melts solid materials into a liquid, places them into a fixed mold, and waits for them to cool and solidify before removal.

Generally, the vast majority of products that require mass production (not limited to plastic items) rely on molds. The inception of industrial development began with mold manufacturing, which is why molds are often heralded as the “mother of industry”!

Cavity and core in an injection mold made by Firstmold
Cavity and core in an injection mold made by Firstmold

Importance of Injection Molds In Manufacturing Processes

The significance of molds cannot be underestimated.

Firstly, molds substantially enhance both productivity and product quality. Compared to manual production, molds can process raw materials into parts or products of specific shapes quickly and accurately. At the same time, molds can expand production lines both horizontally and vertically, thereby boosting productivity.

Secondly, mold manufacturing plays a crucial role in automated and intelligent production processes. Through mold manufacturing, production costs can be effectively reduced, production efficiency can be increased, and the production process can be fully automated.

produce headphones by injection molding
produce headphones by injection molding

Injection Molding and Injection Molding Machines

We’ve mentioned that to mass produce plastic products using molds, the assistance of injection molding machines is needed. For those unfamiliar, you can simply consider an injection molding machine as a tool that injects plastic raw materials into the mold. The collaborative work of the mold and injection molding machine is what we refer to as the injection molding process.

For those interested in learning more about injection molding machines, feel free to click on “Injection Molding Machines” to discover more information about them.

Plastic Injection Mold Basics

Types of Injection Molds

When we discussed injection molding, we noted that what people often refer to as “injection molding” is actually “plastic injection molding” most of the time. The term “plastic” is frequently omitted in conversation, but in reality, injection molding encompasses various types. Plastic injection molding is just one of these types.

The relationship between “plastic injection mold” and “injection mold” is similar. In this article, we primarily focus on topics related to “plastic injection mold.”

Before we delve into the structure of molds, it’s crucial to understand the types of injection molds. Injection molds can be categorized in several ways based on different criteria.

By Product Material:

Plastic injection molds: Standard injection molds, what we talk about in this article.

Metal injection molds: Used for the die-casting process which can produce metal products like aluminum and zinc.

Silicone injection molds: For silicone rubber products, This type is often used in medical and kitchenware.

By Mold Material

Aluminum Molds: Lightweight, cost-effective, and quicker to manufacture. It has a shorter life span and is Ideal for low to medium-volume production.

Steel Molds: Highly durable and wear-resistant, suitable for high-volume production. Steel molds can be more expensive.

Beryllium Copper Molds: Often used in areas of the mold requiring rapid heat removal or in inserts to improve cooling rates. Beryllium copper is the most expensive.

By Production Volume:

Prototype molds: Also known as rapid tooling

Production molds: Also known as production toolings. It’s designed for mass production.

By Runner System:

Hot Runner Molds: Molds that use a heating device to keep the melt in the runner from solidifying.

By Numbers of Cavities

Single-Cavity: Produces one part per cycle, this depends on the mold design.

Multi-Cavity: Produces multiple parts per cycle. As with single-cavity molds, they are more common and depend on the mold design.

By Core Mechanism:

Two-Plate Mold: Simple structure and easy maintenance.

Three-Plate Mold: Has two parting levels, allows for more complex parts with multiple gates.

By Special Features:

Family Molds: Produces a set of different parts in the same cycle, typically used for small parts that go together in the final product.

Insert Mold: Allows for the incorporation of metal parts into plastic parts.

Overmold: Involves molding plastic over a previously molded part, useful for creating multi-material products like toothbrushes with a soft grip.

Two-shot Mold: This is a special type of overmold that also makes plastic products in different materials but in a single cycle.

A two-plate mold
A two-plate mold
A production tooling
A production tooling

Structure of Injection Molds

The injection mold system consists of several critical components, each playing a vital role in the molding process. Here, we provide an overview of the basic structure of a mold, emphasizing its complexity. We plan to introduce each mold component’s functions, structure, and considerations in detail later. For advanced mold knowledge, interested readers can visit the “Injection Molds Components” navigation page. Now, let’s briefly discuss the foundational elements of a mold:

Molding System

The molding parts form the mold cavity and include inserts, mold cores, and side cores.

Gating System

The gating system is a transitional pathway leading molten plastic from the injection machine’s nozzle to the closed mold cavity. It includes the main runner, sub-runners, gates, and cold slug wells. Beyond cold runner systems, hot runner systems are also utilized.

Temperature Control System

This system maintains the mold temperature within an optimal range, involving cooling and heating aspects, though cooling is more common. It features cooling water channels, cooling wells, and beryllium copper cooling, with water, oil, beryllium copper, and air as the control mediums.

Ejection and Return System

Also known as the demolding or ejection system, it ensures the safe and damage-free removal of the plastic part from the mold.

Guiding System

This system includes guiding components. Guiding parts mainly consist of guide pins and bushings for mold cavity and mold core, and guide slots for slides. Positioning involves locking blocks and conical positioning structures.

Slide System

For plastic parts with lateral projections or holes, slides (or inserts) must be pulled before ejection. This mechanism involves slider angle pins, sliders, T-slots, hydraulic cylinders, and spring positioning parts.

Venting System

The venting system expels air from the mold cavity as the molten material fills it and allows air in during mold opening to prevent a vacuum. Venting can occur through parting line vents, grooves vents, insert vents, rod vents, vent pins, etc.

Workflow of Injection Mold Making

Videos you often see of mold processing typically showcase techniques like CNC machining, EDM (Electrical Discharge Machining), and wire cutting. However, the real work involves many meticulous steps before and after these processes. Only through these steps can we ensure that the molds won’t exhibit any injection molding flaws during their final use.

Normally, a mold manufacturing company follows these steps from receiving a mold-making order to delivering the finished product:

1Receive mold orders and drawings
2Design for Manufacturability (DFM) review
3Customer approval of DFM
4Mold design review
5Customer approval of mold design
6Order mold materials based on the mold design
7Receive mold materials at the factory
8Deep hole drilling
9CNC machining programming
10Wire EDM machining
11EDM machining
12Assembly of inserts
13Fitting the mold
14Fitting the mold for parting line and draft angle
15Follow-up work on parting line and draft angle
17Mold assembly
18Mold trial

Materials Used in Injection Molds

Materials used in mold manufacturing are incredibly varied, encompassing metals, plastics, inorganic non-metals, and paraffin, each finding its niche based on specific needs and demand.

Notably, steel is the most utilized material in mold making. Knowledge of mold materials is essential for mold manufacturing plants. Designers and newcomers need only a basic understanding, so here we list some commonly used steel materials for mold making:


  1. 45 — High-quality carbon structural steel, the most commonly used medium carbon quenched and tempered steel.
  2. Q235A (A3 steel) — The most commonly used carbon structural steel.
  3. 40Cr — One of the most extensively used steels, belongs to alloy structural steel.
  4. HT150 — Gray cast iron.
  5. 35 — Common material for various standard parts and fasteners.
  6. 65Mn — Commonly used spring steel.
  7. 0Cr18Ni9 — The most commonly used stainless steel (US steel grade 304, Japanese steel grade SUS304).
  8. Cr12 — Commonly used cold work die steel (US steel grade D3, Japanese steel grade SKD1).
  9. DC53 — Commonly used Japanese cold work die steel.
  10. DCCr12MoV — Wear-resistant chromium steel.
  11. SKD11 — Tough chromium steel.
  12. D2 — High carbon, high chromium cold work steel.
  13. SKD11 (SLD) — Deformation-resistant tough high chromium steel.
  14. DC53 — High toughness, high chromium steel.
  15. SKH-9 — Wear-resistant, high toughness universal high-speed steel.
  16. ASP-23 — Powder metallurgy high-speed steel.
  17. P20 — Generally used for larger plastic molds with moderate requirements.
  18. 718 — Used for high-demand large plastic molds.
  19. Nak80 — Used for high gloss, high precision plastic molds.
  20. S136 — Corrosion-resistant and requires mirror polishing plastic molds.
  21. H13 — Commonly used for die casting molds.
  22. SKD61 — High-end die casting molds.
  23. 8407 — High-end die casting molds.
  24. FDAC — Added sulfur to enhance machinability.


Aluminum alloy is made primarily of aluminum, with a small amount of other metals added as alloying elements. This material is obtained through a series of processing and treatments.

Common aluminum alloys used in molds include aluminum-silicon alloy and aluminum-magnesium alloy. Aluminum-silicon alloy is known for its high hardness and good toughness, while aluminum-magnesium alloy boasts high strength and good heat resistance. Both can be chosen based on different processing needs. Aluminum alloy molds are mainly used in die casting, gravity casting, and low-pressure casting, offering advantages such as high molding precision and good surface quality.

aluminum mold
aluminum mold

Beryllium copper

Beryllium copper plays a significant role in mold making due to its unique properties:

Enhancing Mold Hardness and Wear Resistance: Beryllium copper offers higher hardness and wear resistance compared to general copper alloys. This allows for the creation of molds with high-hardness cores and cavities, which can be used for extended periods without damage.

Improving Mold Thermal Conductivity: Beryllium copper’s thermal conductivity surpasses that of standard copper alloys, making it an effective material for mold heaters. It enhances the efficiency of mold heating and improves the mold’s thermal conductivity. This leads to more uniform heating of the product, thereby increasing production efficiency.

Increasing Mold Precision and Stability: Beryllium copper can be used to produce high-strength and high-hardness mold guide posts and mold plates. This enhances the mold’s stability and precision, ensuring the quality of the product.

Mold Fitting and Polishing

After completing mold processing, two indispensable steps are mold polishing and mold fitting.

Mold polishing serves two purposes. First, it enhances the mold’s smoothness, ensuring the produced items have a clean, attractive, and aesthetically pleasing surface. Second, it facilitates easy demolding, preventing plastic from sticking to the mold. The process typically involves using oilstones, sandpaper, and polishing compounds to sand the mold cavity’s surface, aiming to achieve a mirror-like finish.

Mold cleaning and polishing
Mold cleaning and polishing

Mold fitting, usually done with a specific fitting machine, checks the precision match between the mold core and mold cavity.

An experienced technician can get twice the result with half the effort
An experienced technician can get twice the result with half the effort

Sometimes, the processes of mold polishing and mold fitting are alternated until the final mold is produced.

This approach ensures both the aesthetic quality of the product and the efficiency of the molding process.

Mold Protection, Maintenance and Repair

Protecting molds is crucial due to their specificity, precision, and vulnerability. This is particularly important for mold manufacturers, and also for clients who purchase molds for injection molding in their own factories. The protection measures can be summarized as follows:

Preventing Rust: To avoid rust caused by water leaks, condensation, exposure to rain, or fingerprints on the injection molds.

Preventing Crashes: To prevent damage due to broken ejector pins or pins not retracting properly.

Removing Burrs: To avoid burrs caused by improper cleaning, material punching, manual wiping, contact with sprue pliers or knives.

Preventing Missing Parts: To ensure no damage occurs from missing components like rods or washers during use.

Preventing Compression Damage: To avoid damage when the mold closes with products still inside.

Preventing Low-Pressure Damage: To prevent damage from overly high low-pressure protection settings.

Mold Repair

While we won’t delve into the details of mold repair, it’s essential to follow certain principles from a protection standpoint during maintenance:

  1. Avoid damaging or wetting the mold during disassembly. Move it smoothly.
  2. Heat the mold before spraying a small amount of release agent.
  3. Perform a comprehensive inspection and apply rust prevention measures. Carefully dry and remove debris from cavities, cores, ejector mechanisms, and guide rails, then spray with mold rust preventative and apply grease.

Mold Maintenance

During continuous operation, molds require maintenance due to wear and tear on parts, degradation of lubricants, water leaks, and compression damage from plastic materials.

Mold maintenance is generally divided into daily maintenance and downtime maintenance.

Daily Maintenance includes:

  • Regular rust removal (mold base, parting line surfaces, cavities, cores, etc.).
  • Regularly reapplying lubricants (ejector system, lifters, etc.).
  • Regular replacement of wear-prone parts (rods, bolts, etc.).

Downtime Maintenance requires professional technicians to disassemble the mold and conduct professional testing and protection of the mold cavities, ejector pins, etc.

This structured approach ensures the mold’s quality and lifespan, maintaining optimal performance in the manufacturing process.

Plastic Injection Mold Advanced

Mold Design

Mold design is the most critical phase in the mold manufacturing process, directly influencing the final product’s quality. This stage is also of great interest to many of my friends. Let’s start by examining the qualifications needed for a typical mold designer at FirstMold, as outlined in their job posting requirements:

Job Description:

  1. Responsible for designing molds according to the new product development schedule.
  2. Organize and participate in various mold design structure reviews (DFM).
  3. Collaborate to resolve technical and quality issues during mold manufacturing and testing.
  4. Organize and participate in mold testing to ensure smooth acceptance of the mold.
  5. Identify defects in products and molds and improve them.

Position Requirements:

  1. Minimum 5 years of experience in plastics and design, passionate about mold design work.
  2. Proficient in drawing software like AutoCAD, UG, CATIA.
  3. Able to independently produce plastic mold design drawings.
  4. Good communication and coordination skills, obedient to management, with strong execution.
  5. Responsible for the 3D design of plastic part molds.
  6. In charge of creating processing drawings.
  7. Promptly guide, handle, coordinate, and solve technical issues that arise during the production process.

This job posting effectively highlights the complexity of mold design. So, what does the mold design process look like?

Main Process of Professional Mold Design

The client provides the product drawing → Analyze the product → Confirm the type of plastic → Confirm mold material → Convert to engineering drawings → Apply shrinkage for mirror imaging → Perfect the assembly drawing → Order mold base → Order mold core material → Demold design→ 3D mold splitting → Determine runner/gate system → Design structure according to the layout drawing → Disassemble inserts → Correct 3D drawings → Produce engineering drawings (mold core/inserts/mold base/miscellaneous parts) → Create BOM → Proofread and review.

This process also involves the criteria mold designers rely on when designing molds.

Criteria for Mold Design

The primary basis is the product drawings and samples provided by the client. Designers must thoroughly analyze and understand the product and sample, checking all items one by one during the design process.

Accuracy of Dimensions

For plastic products that require a high aesthetic appearance but low dimensional accuracy, such as toys, aside from transferred dimensions, other dimensions just need to match well.

For products with strict appearance and size requirements, consider if the draft angles are reasonable, the uniformity and thickness of the product walls, the type of plastic (choosing mold steel and determining shrinkage rate), surface requirements, and product color. Generally, color has no direct impact on mold design. However, for products with thick walls or large exteriors, uneven colors are more likely; and the darker the color, the more apparent any defects.

Post-processing of the Product

If the product requires surface plating, especially for multi-cavity molds, it’s necessary to consider setting up auxiliary runners to keep products connected until the plating process is completed and then separated.

Volume of the Product

The volume of production is an important basis for mold design. The client must provide a range to decide on the number of cavities, size, material selection for the mold, and its lifespan.

Specifications of the Injection Molding Machine
Other Requirements from the Client

Designers must consider and verify to meet these requirements.

Examples of Good and Bad Mold Designs

Good and bad mold designs can significantly impact the final product’s quality, production efficiency, and cost. Here are some examples of good and bad mold designs:

Good Mold Design
  • The mold design allows for proper cooling and ventilation to ensure even and consistent product quality.
  • The mold is designed with a simple, streamlined shape that allows for the easy release of the finished product.
  • The mold design considers the type of plastic being used and the intended product’s specifications to ensure proper filling and minimal waste.
  • The mold design includes features such as ejector pins and lifters to facilitate easy removal of the finished product.
  • The mold is designed with durability in mind, using high-quality materials that can withstand the high temperatures and pressures involved in the injection molding process.
A good mold cavity
A good mold cavity
Bad Mold Design
  • The mold design does not provide adequate cooling, resulting in uneven or warped products.
  • The mold design is too complex or includes unnecessary features, leading to difficulty in product release and higher production costs.
  • The mold design does not consider the type of plastic being used or the intended product’s specifications, resulting in overfilling, underfilling, or other quality issues.
  • The mold design does not include necessary features such as ejector pins or lifters, leading to difficulty in product removal and longer production times.
  • The mold is designed with low-quality materials that are prone to damage or wear, leading to higher maintenance costs and decreased production efficiency.

If you aspire to become a mold designer, or if you’re currently a product designer aiming to create better products, FirstMold recommends reading our articles about “Mold Design”.

Mold flow analysis

Mold flow analysis is primarily used in the design and manufacturing process of plastic molds. Simulating and analyzing the flow, cooling, and warping of plastic within molds, aids engineers in better understanding the plastic molding process, predicting and resolving potential issues, and optimizing mold design and production process parameters. Specifically, mold flow analysis serves the following purposes:

Optimizing Product Design:

Mold flow analysis can assess the feasibility and reliability of product designs, and predict and solve potential problems such as sink marks, and flow marks, thereby optimizing product design to enhance quality and performance.

Optimizing Mold Design:

It helps engineers understand the flow and cooling processes of plastic in molds, and predict mold temperature, cooling time, and cooling effects, thus optimizing mold design to improve mold quality and lifespan.

Optimizing Production Process Parameters:

Mold flow analysis enables engineers to understand the flow and cooling behaviors of plastic in molds, predicting optimal injection pressure, injection time, and mold temperature among other production process parameters, thereby enhancing production efficiency and product quality.

Reducing the Number of Prototype Tests:

By predicting and solving potential problems before mold manufacturing, mold flow analysis can reduce the number of prototype tests, shorten the R&D cycle, and lower R&D costs.

Improving Product Quality and Performance:

Mold flow analysis assists engineers in understanding the structural and performance characteristics of products, predicting and solving potential issues to improve product quality and performance.

You can consider mold flow analysis as a higher-level DFM (Design for Manufacturing) review. A comprehensive mold flow analysis is usually charged extra, although this depends on the business strategy of different mold or injection molding companies.

Injection Mold flow analysis
Injection Mold flow analysis

Troubleshooting Injection Molding Issues

In our article on injection molding, we also mentioned injection molding defects. Injection molding defects can be influenced by multiple factors such as poor mold design, inadequate injection process control, and material issues. Common injection molding defects include:

Mold manufacturing companies must be highly proficient in dealing with these defects. In a way, one of the measures of a mold manufacturer’s qualification is their ability to effectively address these injection molding defects.


Injection molding is a fascinating craft. A good product designer needs to grasp its basics to create practical products from the perspectives of material, processing cycle, and product design itself. The knowledge about injection molds goes far beyond this. FirstMold is a company that enjoys sharing knowledge. If you have any questions, feel free to contact us at

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