CNC Milling and Injection Mold Making: A Perfect Match for Precision

Procision Manufacturing

Writer

Plastic injection molding remains the gold standard for producing large volumes of identical, high-quality plastic parts. Its speed, repeatability, and scalability are unmatched, but none of that’s possible without a critical partner: precision-machined mold tooling.

Behind every molded part is a tool, and behind every tool is a blend of CNC milling and EDM machining. In this post, we’ll explore why CNC and EDM are essential to mold making, what their respective strengths and limitations are, and how they shape the future of plastic injection molding.

What’s the Role of a Mold Tool?

To mold a part, you need a mold. This mold — sometimes called a “tool” or “die” — defines the exact geometry of the plastic part and enables mass production.

Molds typically consist of two halves: a core side and a cavity side, both starting as solid blocks of aluminum or steel. These blocks are precisely machined to form a negative of the final part, along with complex internal features such as:

Gates and runners to direct molten plastic
Cooling channels to regulate temperature and cycle times
Ejector systems to remove the molded part cleanly

Making these molds requires extreme precision and robust materials — and that’s where CNC machining and EDM machining step in.

How Are Mold Tools Made in 2025?

Modern mold tools are fabricated using two complementary digital manufacturing methods:

CNC Milling: High-speed rotating cutters remove bulk material with tight tolerances.
EDM (Electrical Discharge Machining): Uses electrical discharges to erode metal, ideal for intricate and inaccessible features.

These methods work together to create tools that are accurate, repeatable, and optimized for production.

What Are the Limitations of CNC Milling?

CNC milling process is the workhorse of toolmaking, handling 90–95% of the mold making job. But it’s not a silver bullet. Here’s where it falls short:

Line-of-sight limitation: Undercuts and hidden features are inaccessible to milling tools.
Limited depth reach: Deep pockets or bores may exceed spindle travel or cutter length.
Rounded internal corners: End mills impart a radius; they can’t make true sharp corners.
Post-processing: Surface tool marks often require hand polishing for finish quality.

That’s why EDM isn’t just nice to have — it’s essential.

How Does EDM Complement CNC Milling?

EDM excels where CNC hits a wall. It uses custom-shaped electrodes to erode complex geometries with zero cutting force. Key advantages include:

Sharp internal corners with high dimensional accuracy
Excellent surface finish straight from the machine
Deep, narrow features like ribs or vents with minimal deflection
Compatibility with hardened steels, such as H13, S7, and other high-performance alloys used in injection mold making

While EDM is slower than CNC milling, it’s the go-to solution for precision details and hard-to-reach features — typically handling the final 5–10% of toolmaking.

Are There Alternatives to CNC Milling and EDM?

A few niche alternatives exist, such as 3D-printed molds using high-temperature photopolymers or sintered metals. But these are best reserved for rapid prototyping or short runs manufacturing.

For real-world production, CNC + EDM remains the most efficient, scalable, and reliable method.

What About Manual Toolmaking?

Manual mold making — using bridgeport mills and hand finishing — was once the industry standard. But it’s labor-intensive, inconsistent, and hard to scale.

Digital fabrication replaces tribal knowledge with repeatability. Today’s CNC machining and EDM systems offer better precision, faster turnaround, and consistent results — all critical for modern production timelines and quality demands.

An All-in-one Solution For Streamlined Production

Combining CNC milling and EDM machining together with plastic injection molding under one roof is still the best way to manage costs, control quality, and lower production lead times. Procision has you covered with a full range of machining and molding services for projects of any size or complexity.