Making a good injection mold is a precise engineering job. It is not simple. It is a multi-step process that turns a block of metal into a tool that can mould thousands or millions of plastic parts.
The process includes four main steps. First is Design for manufacturability (DFM). Next comes Material selection. Then the mold is made via Machining and Fabrication. Finally, Testing and Validation.
Learning to make injection molds is a vital part of the process for the success of mass production with plastic injection molding. This guide is going to help you on this entire journey, one step at a time.
Before You Build: The Role of Design
An injection mold is built long before metallic parts are cut. It all starts with the design of the part itself. This stage is called Design for Manufacturability or DFM.
DFM is the phase of blueprints. Its role is to find and repair design problems that could make the molding difficult or impossible. A constructive DFM evaluation prevents errors that cost time and money. It is the starting point for the whole injection mold making process.
Core DFM Principles
These are the core concepts in DFM that ensure a part can be molded correctly:
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Draft Angles: A draft angle is a little angle added to the wall of the part. This slight taper is mandatory. It allows to push the finished part outside of the mold without any trap or damage.
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Uniform Wall Thickness: The thickness of the walls of the plastic should be the same everywhere. If some places are much thicker than others, the part will not be cooled evenly. This can cause ugly defects like sink marks or warping. Strict adherence to uniform wall thickness and other essential design principles is crucial.
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Radii and Fillets: Sharp bent corners create stress and prevent plastic from flowing easily. By radii of bending corners or fillets, the melted plastic gets to the right places as well as this method increases strength of the empty.
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Undercuts: An undercut shapes the part in a way that it cannot be ejected directly out of the mold. A case in point is a battery cover with a snap-fit clip that pushes on the cover. However, these features make the mold more complex and costly. Usually, they need special moving parts like side-actions or lifters.
The Professional Mold Making Process
After the part design is confirmed, the real work of making the mold begins. Below is a comprehensive and detailed description of how a professional injection mold is produced.
Step 1: Final Mold Design and Simulation
The first step a toolmaker has to do is to design the mold itself. Aided by CAD software, they come up with a 3D model that depicts all the parts of the mold. This model involves the core and cavity, which mold the part’s shape. The runner system through which plastic flows, gates through which plastic enters the part, cooling channels, and the ejector system also help to form the mold.
Specialized software is then used to conduct a Mold Flow analysis. This is a simulation of how molten plastic enters the mold. It enables the foreseeing and solving of air traps or weak spots since these problems will be analyzed without actually building the mold.
Step 2: Material Selection for the Mold
The kind of material used for the mold depends on the project requirements. There are three standard materials, each with a unique benefit of cost vs. durability.
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Pre-hardened Tool Steel (P20): This is the most prevalent alternative. It is a really good choice since it provides a compromise between toughness and ease of machining. It can endure the production of hundreds of thousands of parts.
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Hardened Tool Steel (H13, S7): When ultra-high volume production is targeted (1 million parts and over), or when the use of reinforced plastics like glass fiber mandates, a thicker steel is necessary. These tool steels are more resilient and longer-lasting, and hence they are pricier and take longer to machine.
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Aluminum: For prototypes or very low-volume runs (a few hundred to a few thousand parts), aluminum is a good option. It is much faster and cheaper to machine, but it wears out quickly.
Step 3: Machining the Core and Cavity
This step is the heart of how to make injection molds. Here, big blocks of metal get shaped into the core and cavity of the mold. Most of the work is now done through high-precision China CNC machining services.
The process begins with “roughing”, where a CNC mill quickly plows through large amounts of material in order to achieve a basic shape. Afterward, it moves to “finishing”, which uses finer tools to slow down and carefully create the final surfaces and details. A machinist must diligently supervise tool paths, cutting speeds, and feeds as tools may chatter and not achieve the desired surface finish. This process can take hours or even a couple of dozen hours for the complicated mold.
Step 4: Fine-Detailing with EDM
CNC milling operations cannot generate every characteristic. For very sharp inside corners, deep and thin ribs, or complex textures, a different process is needed. Electrical Discharge Machining (EDM) is used for these fine details.
EDM uses a precisely shaped electrode and electrical sparks to erode or “burn” away the metal. This allows for features that are impossible to cut with a spinning tool.
Step 5: Polishing and Surface Finishing
After machining, the mold surfaces retain tiny tool marks. Before polishing is done on the mold surfaces, they first have to be cleaned off any tiny chips. Then, the surfaces are polished to match the desired finish of the final plastic part. The finish can range from a dull matte to a perfect glass mirror polish.
Most of this work is done by hand as skilled polishers use finer and finer abrasive stones and diamond pastes. The cost of the mold is drastically affected by the extent of polishing.
Step 6: Mold Assembly and Fitting
Now all the finished parts are brought together. The core and cavity inserts are fitted into a large, standard frame called a mold base. The ejector system, cooling lines, and any side-actions are all carefully assembled.
Toolmakers then perform a process called “spotting.” They apply a blue dye to one half of the mold and carefully close it. Where the dye transfers shows how well the two halves meet. They make tiny adjustments until the fit is perfect, which prevents plastic from leaking out and creating “flash.”
Step 7: T1 Trial and Debugging
The last step is the first test, or “T1” trial. Again, the mold is now mounted in injection molding machine and plastic is injected first at that time.
Then these first produced components are accurately measured and any defects are detected. It is very rare for the first parts to be perfect. Based on this inspection, the mold is then disassembled to be adjusted or “tuned.” This cycle of testing and tuning continues until the mold produces parts that meet all specifications. This is a very key part of the professional injection mold making process.
Choosing Your Mold Making Path
Not every project needs a million-part steel mold. Depending on your goals, you can choose between professional tooling and more accessible DIY methods. Understanding the differences is key to planning your project.
| Factor | Professional Steel Molds | DIY 3D Printed Molds |
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| Cost | High ($5,000 – $100,000+) | Low ($100 – $500) |
| Lead Time | Long (4-12+ weeks) | Fast (1-3 days) |
| Production Volume | High (100,000 – 1,000,000+ parts) | Very Low (10 – 100 parts) |
| Part Precision | Very High | Low to Medium |
| Material Options | Nearly all thermoplastics | Low-temp plastics only (e.g., PE, PP) |
| Best Use Case | Mass production, high-quality parts | Prototyping, fit/form testing |
Professional Steel Tooling
Professional steel molds are the standard for any serious production. They are built for high-volume manufacturing, offering extreme precision and a long lifespan. While the initial investment is high, the cost per part becomes very low when making thousands of units. This is the only reliable path for bringing a commercial product to market.
DIY Molds with 3D Printing
For hobbyists and early-stage prototypes, there is another option. It is now possible to do DIY injection molding using 3D printed molds. Special high-temperature resins can be 3D printed to create mold inserts. These are then placed in a metal frame on a small desktop injection machine.
The advantage is extremely low cost and fast turnaround. However, these molds have a very short life, often failing after 10 to 100 shots. They also work only with low-temperature plastics and cannot produce highly precise parts.
Key Factors in Mold Cost & Lead Time
People are often surprised by the cost and time needed for the injection mold making process. A simple, single-cavity prototype mold in aluminum might cost $2,000-$5,000. In contrast, a complex, multi-cavity hardened steel production mold can easily exceed $100,000. These costs are driven by several key factors.
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Part Complexity: The more features, sharp corners, and tight tolerances a part has, the more time it takes to machine and finish the mold. Undercuts that require side-actions add significant cost.
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Mold Material: Hardened tool steel is more expensive to buy and much harder to machine than pre-hardened steel or aluminum. This adds to both material and labor costs.
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Number of Cavities: A mold can be built with one cavity or many. A multi-cavity mold makes multiple parts in each cycle, lowering the per-part price. However, the upfront cost to build a 4-cavity mold is much higher than for a 1-cavity mold.
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Surface Finish: A basic, machine-textured finish is standard. If your part needs a gloss or mirror polish, it requires many hours of skilled manual labor, which adds greatly to the cost.
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Mold Lifespan: A mold guaranteed for 1 million cycles needs to be built from the best hardened steel with more robust components than a mold designed for only 50,000 cycles.
Conclusion: Partner in Precision
We’ve seen that the answer to “how to make injection molds” is a journey. It goes from a digital design to a physical, high-precision tool. The core principles are easy to grasp. But the real work requires deep expertise, advanced machines, and strict quality control.
While some can explore DIY methods for basic prototypes, bringing a real product to market requires a professional approach. For companies that need reliable, high-quality parts, working with an experienced manufacturing partner is the surest path to success. A good partner like Mekalite can guide you through the entire process, from design to production.
Frequently Asked Questions (FAQ)
How long does it take to make an injection mold?
It varies greatly. A simple prototype aluminum mold might take 1-2 weeks. A complex, multi-cavity steel production mold can take 8-12 weeks or even longer, depending on its complexity and the shop’s schedule.
Why are injection molds so expensive?
The high cost comes from several factors. This includes expensive tool steels, many hours of precision CNC machining and EDM work, skilled manual labor for assembly and polishing, and the detailed testing and tuning process needed to get it right.
What is the difference between a core and a cavity?
The core and cavity are The two halves of the mold. In general, the cavity is the fixed half that forms the outer surface of the part. The core is the moving half that forms the inner features. When they close, they create the hollow space that the plastic fills.
Can you make an injection mold with a 3D printer?
Yes, but with major limits. You can 3D print small mold inserts using special high-temperature resins. These are only good for very low-volume prototyping (10-100 parts) using plastics that melt at low temperatures. They are not a substitute for steel molds in mass production.
What is a “family mold”?
A family mold is an injection mold with cavities for several different parts. It is used to produce all the plastic components for a single product in one shot. For example, it could make the front case, back case, and all the buttons for a remote control at the same time. This can save money for low-to-medium volume projects.
