Part 2

COMBATING ENGINEERING MINDBLINDNESS SERIES

HIGH PRESSURE

DIE CASTING

What Every Designer Needs to Know

PART TWO

Gating, Porosity, Surface Treatments & Design for Manufacture

Joseph P. McFadden Sr.

Engineering Fellow, Zebra Technologies

Adjunct Professor of Mechanical Engineering, Fairfield University

Collaboratively developed with Claude · May 2026

mcfadden@snet.net  ·  www.MCFADDENCAE.com

If you are joining us here directly, the series is built on a simple premise: engineers who can see beyond the boundaries of their own discipline make fewer mistakes, ask better questions, and design better products. Mindblindness is not a character flaw — it is a gap in exposure. This series closes those gaps.

In Part One we covered what die casting is, the difference between cold and hot chamber machines, the three main alloys and what they mean for your design choices, and the complete journey from furnace to ejected part — told through the lens of your decisions as a designer.

If you have not listened to Part One, I recommend starting there. What we cover in this part builds directly on that foundation.

Welcome back.

In Part One we covered what die casting is, the difference between cold and hot chamber machines, the three main alloys and what they mean for your design, and the complete journey from furnace to ejected part — translated into terms that connect to your decisions as a designer.

Now we go deeper. We are going to talk about the gating system — because understanding where and how metal enters your part is one of the most valuable pieces of knowledge a designer can have. We are going to talk about porosity, what causes it, and how your design decisions either create it or prevent it. And we are going to talk about surface treatments — because the specification you write on your drawing for plating or anodizing or painting is not just a finish decision. It has metallurgical consequences that can cause your part to fail in service, invisibly, long after it left the factory.

All of it connects. That is the holistic approach.

THE GATING SYSTEM — WHAT IT IS AND WHY IT MATTERS TO YOU

When a die cast part is ejected from the machine, it is not just the part. It is the part plus a network of solidified metal attached to it — runners, gates, and overflow wells. That entire assembly gets separated at the trim operation, and what is left are the gate marks on your part.

Most designers think of the gating system as a production detail — something the tooling engineer handles, nothing to do with them. That is a mistake. The gating system is one of the most consequential decisions in the entire die casting process, and your part geometry is what drives it.

Let me explain what it is, and then explain why you should care.

The gating system is the path the metal takes from the shot sleeve — the cylinder where the metal is injected — to your part cavity. It consists of: the sprue or biscuit, which is the initial slug of metal at the injection end; the runners, which are channels that distribute metal toward the cavity; the ingates, which are the final restrictions through which metal enters your part; and the overflows, which are small reservoirs on the far side of the cavity that collect the first, coldest, most contaminated metal.

Each element serves a purpose. The runners deliver metal without losing too much temperature or pressure. The ingates control the velocity and direction of metal entering the cavity. The overflows ensure the first metal through — which carried air and lubricant vapors and oxide films — exits the part rather than being trapped inside it.

WHERE THE GATE GOES AND WHY IT MATTERS

The ingate is the most critical element. Its location on your part determines the entire fill pattern — the path the metal takes as it sweeps through the cavity. And that fill pattern determines everything: where weld lines form, where porosity concentrates, where surface quality is best and worst, and where the mark is left after trimming.

A weld line — sometimes called a cold shut — is where two separate flow fronts meet inside the cavity and must fuse together. If both fronts are hot enough and moving fast enough when they meet, they fuse cleanly. If one front has lost too much temperature before meeting the other, they do not fuse properly. The result is a visible line and a mechanical weakness at that location.

Weld lines are not a production defect in the traditional sense — they are a geometric inevitability for any part that has features which force the metal to split and rejoin. Your design determines where they form. The gating system determines whether they form in a high-stress area or a low-stress one.

As a designer, you should be asking: where is the gate going to be on my part? Where will the metal flow last? Are those last-to-fill areas in a location that matters mechanically or cosmetically? If the answer is yes, the design needs to change — not the tooling.

POROSITY — THE INVISIBLE DEFECT THAT IS PARTLY YOUR FAULT

Porosity is the presence of voids inside the casting — either gas voids or shrinkage voids. It is the most common die casting defect and the one most often blamed entirely on the foundry. In reality, the designer contributes to it in ways that are preventable.

Gas porosity comes from air and gas that gets trapped during fill. Air that does not escape through the vents before the metal arrives. Lubricant vapors that could not exit from deep pockets in the cavity. Hydrogen dissolved in the melt that comes out of solution during solidification.

Your design contributes to gas porosity when you create deep blind pockets that are hard to spray and vent. When you create geometries that cause the metal to fold over itself during fill — trapping air inside the fold. When you specify alloys or surface treatments that introduce hydrogen into the metal.

Shrinkage porosity comes from the six percent volume loss that aluminum undergoes as it solidifies. Metal cannot shrink into empty space — something has to fill that void. In a well-designed part with a well-designed gate, the intensification pressure feeds additional metal through the still-open gate to compensate for shrinkage. In a poorly designed part, the gate freezes before the thick sections solidify, and the shrinkage creates voids in the interior.

Your design contributes to shrinkage porosity when you create thick sections — bosses that are too heavy, walls that vary dramatically in thickness, transitions that are too abrupt. The thick section solidifies last. By the time it needs to shrink, the surrounding thinner sections have already frozen and locked the thick section in. The shrinkage has nowhere to go but inward — forming voids.

Porosity is not always visible from the outside. That is what makes it dangerous. A part can look perfect, pass dimensional inspection, and still be full of internal voids that reduce fatigue life, create leak paths, and cause failure under loads that should be well within the design margin.

X-ray inspection can reveal internal porosity. Leak testing can reveal through-connected voids. But the most effective way to manage porosity is to design the part so that it is minimized from the start — uniform walls, no abrupt section changes, no deep pockets, draft on all surfaces.

WHAT GOOD FLOW LOOKS LIKE — AND HOW YOUR DESIGN ENABLES IT

Here is a way to visualize what the tooling engineer is trying to achieve with the gating system, and how your design either helps or fights that goal.

Imagine filling a room with water from a single inlet in one wall. If the room is a simple rectangle with an outlet vent on the opposite wall, the water fills smoothly from one side to the other — a clean, sweeping front.

Now put obstacles in the room. Pillars, dividing walls, raised floors, odd-shaped recesses. The water has to split around obstacles, rejoin on the other side, swirl into recesses, race through narrow passages and spread in open ones. Air gets trapped in corners the water cannot reach until late in the fill. Some areas fill before others.

Your die casting cavity is that room. The metal is the water. The goal of the gating system is to place the inlet — and design the runner system leading to it — so that the fill front sweeps through the cavity as cleanly as possible, pushing air ahead of it toward the vents and overflows.

Features that help: uniform wall thickness so the metal advances at consistent speed everywhere. Smooth transitions between sections. Generous radii so the metal can follow the geometry without separating from the wall. Well-placed overflow wells in the last-to-fill areas so the cold, oxide-laden metal that arrives there exits into the overflow rather than staying in the part.

Features that hurt: abrupt changes in wall thickness that cause the metal to race through one area and struggle in another. Sharp corners that cause turbulence. Deep pockets that trap gas. Complex geometries that force multiple flow fronts to meet in structurally critical locations.

READ THE CASTING BEFORE YOU COVER IT

There is a principle I come back to in nearly every failure investigation I have ever done, and I want to share it with you before we talk about surface treatments.

I call it lipstick on a pig.

No amount of chrome plating, anodizing, powder coat, or paint changes what is underneath. The surface treatment covers the casting — it does not improve it. If the casting has problems, those problems are still there after plating. Often they are worse, because the plating process itself can introduce new defects on top of the existing ones. And the finish covers everything so completely that by the time the part reaches you as a designer, or reaches the field as a product, you cannot see what is actually going on.

This is why I believe every designer who specifies a die cast part should see the as-cast shot before any secondary operations are performed. Not just the part — the complete shot. The part, the runner system, the gate, the overflows, all still attached. The casting in its natural state. Its birthday suit.

That complete assembly tells you things that the finished, plated part will never reveal.

Look at the flow patterns on the surface. Where the metal flowed smoothly, the surface will be even and consistent. Where flow fronts met and struggled to fuse, you will see flow lines — subtle ripples or texture changes that follow the fill pattern. Where turbulence occurred, the surface may show swirl marks or irregular texture. Where gas was trapped and then partly escaped, you may see small depressions or blisters. All of this is visible in the as-cast state. Almost none of it is visible after plating.

Look at the discoloration. Areas that ran hotter, areas where the die spray pooled and burned, areas where the metal velocity was too high — all of these leave thermal signatures on the as-cast surface. A yellowish or brownish tinge in a specific area is the casting telling you something about what happened there during fill. A bright, almost mirror-like area at the gate region tells you the metal arrived hot and fast. A dull, grainy texture in a last-to-fill corner tells you the metal was cold and struggling when it got there.

Look at the overflows. The overflow wells collected the first, coldest, most contaminated metal that entered the cavity. If those overflows look dark, grainy, or show evidence of oxide inclusions, that tells you about the melt quality going into the part. If the overflows are clean and consistent, that is a good sign. If they are irregular or show evidence of turbulence, that turbulence happened in your part too before the metal reached the overflows.

Look at the gate area. The gate is where the metal entered at the highest velocity and temperature. It is often the cleanest area of the casting for that reason. But if you see erosion marks, or a rough, torn surface at the gate, that tells you the velocity was too high and the gate is eroding the die steel — a problem that will worsen over the life of the tool.

Look at the parting line. The flash — the thin fin of metal that forms where the die halves meet — tells you about clamping force and die fit. Excessive flash means the die is opening under injection pressure, or the die faces are wearing. A slight, consistent flash is normal and trimmable. An irregular or heavy flash is a warning.

All of these blemishes, discolorations, and surface patterns are preludes. They are the casting telling you what happened during its formation — and what may happen to it in service. A flow line in a cosmetically irrelevant area may be acceptable. The same flow line across a stress-critical feature is a failure waiting to happen. You cannot make that judgment if you have never seen the part in its natural state.

Now — and this is the equally important second half — the secondary operations you specify can introduce their own defects on top of whatever the casting arrived with.

The acid pickling step in electroplating attacks the surface. If there are pores or micro-cracks at the surface, the acid enters them, widens them, and the plating then seals them in with whatever contamination the acid left behind. The anodizing process creates a hard oxide layer more brittle than the base aluminum — in a part with residual surface tension, that brittle layer can crack. Shot blasting for pre-treatment work-hardens the surface and can drive existing surface defects deeper.

So you have two separate phenomena to understand: the defects the casting brings to the finishing line, and the defects the finishing process adds. The as-cast part shows you the first set clearly. The finished part hides both sets behind its surface. This is why failure analysis on plated castings is so difficult — you are often peeling back layer after layer of processing history before you find the original source of the problem.

Ask to see the as-cast shot. Ask your foundry to show you the part before it goes to finishing. Make it a standard part of your first article review. The five minutes you spend looking at the naked casting may save you months of failure investigation later.

SURFACE TREATMENTS — THE DECISIONS THAT FOLLOW YOUR CASTING

Electroplating — chrome, nickel, copper — requires the part to go through an acid pickling bath before the metal can be deposited. That acid bath cleans the surface so the coating will adhere. It does that job. But in doing that job, it also generates hydrogen at the metal surface. Atomic hydrogen. Small enough to slip between the atoms of the metal lattice and diffuse inward.

Once inside the metal, that hydrogen migrates. It finds grain boundaries, pores, and areas of high stress — exactly where it does the most damage. Then the plating goes on, and it seals the hydrogen in.

The result is a part that looks perfect. That measures perfect. That passes inspection. And that, days or weeks or months later, fractures. Brittle. Sudden. Without the kind of plastic deformation you would expect before failure.

This is hydrogen embrittlement. And it disproportionately affects die castings because of the residual tension in the core we talked about earlier, combined with the porosity that provides pathways for hydrogen to migrate, combined with the thin walls that have very little material to absorb the effect before it reaches a critical location.

The bake-out treatment — heating parts to around one hundred and ninety degrees Celsius for several hours immediately after plating — drives out mobile hydrogen before it has time to concentrate at grain boundaries. The word immediately matters here. The window for effective bake-out narrows rapidly after plating. Hydrogen that has had time to migrate to grain boundaries and form molecular hydrogen in micro-voids cannot be removed by baking.

As a designer, if you are specifying electroplating on a die cast aluminum or zinc part, you need to understand this mechanism and build the bake-out requirement into your specification. And you need to understand that parts with internal porosity — which provides additional hydrogen migration pathways — are at significantly higher risk. This loops back to wall thickness, section design, and everything else we discussed.

Anodizing is an alternative surface treatment for aluminum that does not involve the same degree of hydrogen risk. It is an electrochemical oxidation process that builds up a protective oxide layer on the surface. However, not all aluminum die casting alloys anodize equally well — the high silicon content of alloys like A380 produces a mottled or gray finish. If you want a consistent, attractive anodized finish, your material specification matters.

Powder coat and liquid paint are much more forgiving of surface condition and are not associated with hydrogen embrittlement. If your plating specification is driven by aesthetics rather than by a functional requirement — conductivity, solderability, wear resistance — paint or powder coat may be the right answer, and the risk profile is completely different.

Conversion coatings — chromate or non-chromate — provide basic corrosion protection for aluminum and magnesium with minimal processing risk. They are often used as a base coat under paint or powder coat.

The surface finish decision is not just about appearance. It is a materials decision with mechanical consequences. Make it with full knowledge of what it does to the metal underneath.

DESIGNING FOR SECONDARY OPERATIONS

Most die castings need at least some secondary work before they are finished. Understanding what those operations are and how to design for them saves time and cost.

Machining. Features that require tighter tolerances than the casting process can hold — precision bores, sealing surfaces, mating faces — are machined after casting. The casting provides a near-net-shape blank that the machining operation finishes. Design these features with adequate stock for the machining pass. Typically one to one and a half millimeters of stock on diameter for a bored hole. Allow fixturing pads — flat surfaces where the machining fixture can clamp the part without marring a finished surface.

Tapping and threading. Die cast aluminum is soft enough to accept self-tapping screws in many applications — particularly for light, non-structural assemblies. For structural connections, thread-forming taps into a properly sized cast hole work well. For high-load applications with repeated assembly and disassembly, threaded inserts — installed by pressing or casting-in — are the right answer. Specifying a thread directly into a thin-walled boss that will see fastener torques is how warranty claims get generated.

Trimming and deburring. The gate, runner, and overflow marks must be removed. Depending on the geometry, this is done in a trim die, by hand grinding, or in a tumbling operation. Design the gate location so the trim operation does not leave witness marks in cosmetically critical or mechanically critical areas. Allow clearance around the gate location for the trim tooling.

TALKING TO THE FOUNDRY — WHAT EVERY DESIGNER SHOULD DO

Here is a piece of advice that goes beyond the technical content of this guide.

Talk to the foundry early.

Before the design is released. Before the die is ordered. Ideally, before the design is finalized.

The people on the production floor — the tooling engineer, the process engineer, the quality engineer — have knowledge that no design guideline document captures. They know which features in your design are going to be difficult to fill. They know where the parting line will have to go and what that means for your tolerances. They know whether the draft on a particular feature is adequate or whether it is going to cause them problems in ten thousand cycles.

Most designers never visit the production floor. They release drawings, receive first articles, and then deal with problems. The designers who prevent problems — the ones who build a reputation for parts that run cleanly and do not generate warranty claims — are the ones who understand the process well enough to have a real conversation with the foundry.

This guide is a starting point for that conversation. It is not a substitute for it.

THE COMPLETE PICTURE

Let me bring it all together for you.

Your die cast part begins as a liquid metal bath in a furnace, where its hydrogen content is being managed — or mismanaged. It is delivered as a precisely measured shot into a cold shot sleeve, where the clock starts. It is pushed through the die in two phases: a slow, controlled advance to avoid air entrapment, and a fast, high-velocity fill that takes milliseconds. It is pressurized after filling to feed shrinkage. It solidifies with stresses locked in from the outside in. It is ejected, degated, and inspected. Then it goes through surface treatment that can introduce new failure mechanisms if not specified and controlled correctly.

Every step of that journey was influenced by decisions you made on your drawing.

Wall thickness. Draft angles. Rib geometry. Boss design. Parting line placement. Surface finish specification. Material selection. Tolerance call-outs.

None of these are isolated decisions. They are all connected. They all affect each other. And they all trace back to physics — the behavior of liquid metal moving at high speed through a precision steel cavity under enormous pressure, transitioning from liquid to solid in a fraction of a second.

When you understand that physics — even at the level we have covered here — your design decisions change. You stop asking "can we make an exception to the draft requirement" because you understand why it is not an arbitrary rule. You stop specifying electroplating on thin-walled aluminum parts without thinking about hydrogen embrittlement because you understand the mechanism. You stop designing thick sections without realizing they are creating porosity hot spots.

Understanding the complete journey is not optional. It is the whole point.

I have said that twice now — once at the start and once here at the end. I mean it both times.

Every failure tells a story. With what you have learned here, you are better equipped to write a different kind of story from the start. A part designed with the process in mind. A casting that arrives at the field with a good history inside it — not one full of voids and residual hydrogen and stress concentrators waiting for the right moment to cause a problem.

I would rather help you write the positive story. That is why this guide exists.

Thank you for listening.

Joseph P. McFadden Sr. Engineering Fellow, Zebra Technologies. Adjunct Professor of Mechanical Engineering, Fairfield University.

You can reach me at mcfadden@snet.net. My blog is at www dot MCFADDENCAE dot com.

My companion guide — Die Casting Metallurgy, A Comprehensive Guide — covers hydrogen embrittlement, plating effects, stress corrosion cracking, and materials science fundamentals in depth. It is available free at the same address.