DFM Services: Injection Molding Design Guidelines for Wall Thickness, Draft, Gates, and Sink Marks

Here’s something every engineer learns eventually: a part that looks perfect in CAD doesn’t always mold perfectly. Maybe a thick section sinks. Maybe a wall drags coming out of the tool. Maybe a knit line shows up right where the part needs to be strongest. That’s what DFM services are for. They catch those problems while they’re still easy to fix, back when the part is just a drawing and not a block of hardened plastic.

At Metro Plastics, Design for Manufacturability (DFM) is the first step on every project. We’ve already covered how manufacturing design decisions drive down scrap and cost at a high level. This time, we want to get practical and walk through what our engineers actually look for when they review a part.

What DFM Services Cover (and Why Timing Is Everything)

A DFM review is a conversation between your design and the molding process. Can this part fill completely? Will it cool evenly? Will it come out of the mold cleanly, every time, for hundreds of thousands of cycles?

To answer those questions, our engineers look at wall thickness, draft, ribs and bosses, gate location, undercuts, material, and tolerances. None of these live in isolation. Change the resin and the shrink changes. Move the gate and the knit line moves with it. That’s why we look at the whole part together.

Timing matters just as much as the review itself. Early on, a design change costs some engineering time and a CAD revision. Wait until the tool is cut, and that same change could mean rework, welding, downtime, and a pushed launch date. We dig into this in our post on early supplier involvement. In other words, the earlier we see your part, the more money we can save you.

Wall Thickness: Thin and Even Wins

If there’s one thing to get right, it’s wall thickness. Nearly every common molding defect traces back to it.

Think about how a part cools. The outside freezes first, and the inside follows. In a thin wall, that happens quickly and evenly. In a thick wall, the core stays molten longer and keeps shrinking after the surface has already set. The result? Sink marks, voids, and internal stress. And if some walls are thick while others are thin, they cool at different rates and the part warps.

So what should you aim for? Most thermoplastic parts land somewhere between 0.040 inches and 0.150 inches (1 mm to 4 mm), depending on the resin. More important than the exact number, though, is consistency. Pick one nominal wall and stick with it as much as you can. When the thickness has to change, taper it gradually, over a length about three times the change, rather than stepping it abruptly. And if a section is getting bulky, hollow it out and add ribs for strength instead.

There’s a bonus here, too. Thinner walls cool faster, and cooling is usually the longest part of the molding cycle. Shorter cycles mean a lower part price.

Draft Angles and Ribs: Helping Your Part Let Go

Picture pulling a straight-sided cake out of a pan. It fights you. Now picture a pan with slightly sloped sides. The cake slides right out. That slope is draft, and your molded part needs it too.

Without draft, parts scrape against the mold on the way out, leaving drag marks or sticking entirely. A good starting point is at least 0.5 degrees per side, though 1 to 2 degrees is even better when the design allows. Textured surfaces need extra, roughly 1 to 1.5 degrees for every 0.001 inch of texture depth, because texture grips the steel. Don’t forget to draft ribs and bosses as well. They’re easy to overlook, and they’re often exactly where parts stick.

Speaking of ribs, they’re your best friend when a part needs to be stiffer. It’s tempting to just thicken the wall, but that brings back all the cooling problems above. A rib adds strength without the bulk. Keep ribs at 50 to 60 percent of the wall thickness and no more than three times the wall in height. For bosses, core them out and connect them to nearby walls with ribs or gussets so they don’t turn into solid lumps.

Ejection is something we take seriously. Metro’s internal Tooling Standard lists over 30 considerations for how a part releases from the mold, and good draft and rib design make every one of them easier.

Gate Placement: Where the Plastic Gets In

The gate is the doorway where molten plastic enters the mold. Where you put it shapes how the whole part fills.

The general rule is to gate into the thickest section so the plastic flows from thick to thin. Gate into a thin area instead, and the plastic can freeze off before the thicker sections are fully packed. You’ll also want to keep the gate off cosmetic surfaces, since it leaves a small mark behind.

Gate location also decides where knit lines form. When plastic splits around a hole and meets up again on the other side, it creates a seam that’s weaker than the surrounding material. You can’t always avoid knit lines, but a well-placed gate can move them somewhere they won’t matter. Just as important, the last spot to fill needs venting, or trapped air can cause burn marks and short shots.

As for gate types, it depends on the part. Edge gates are simple and versatile. Submarine gates snap off automatically when the part ejects. Hot tip and valve gates cut down on runner scrap for high-volume molds.

How to Avoid Sink Marks in Injection Molding

Sink marks are those small dimples you sometimes see on the surface of a part, usually right across from a rib, boss, or thick section. They’re one of the most common reasons parts get rejected for cosmetics. The good news is that they’re almost always preventable at the design stage.

Most of the fixes come straight from what we’ve already covered. Keep ribs at or below 60 percent of the wall. Core out thick areas and bosses. Gate into the thicker sections so they pack out properly. And keep material in mind: semi-crystalline resins like polypropylene and nylon shrink more than ABS or polycarbonate, so they’re more prone to sink.

How Metro Plastics Delivers DFM Services

Our engineering team brings a combined 100+ years of experience to every project, and DFM isn’t an add-on for us. It’s simply how we work.

We get involved early, reviewing material selection, mold analysis, and manufacturability before any tool is cut. Every mold we build or source follows our 20-page Tooling Standard. When a design needs to be tested in the real world, prototype tooling lets you hold actual parts before committing to production steel. And because design changes are a normal part of development, we quote engineering changes at cost. The right fix should never be held up by a markup.

Once production starts, our in-house tool room and on-site Metrology Lab keep things on track, from first article through every run after. We also choose materials based on how parts actually perform once molded, not just what a datasheet says. You can read more about our plastic engineering services here.

Partner With a Molder That Offers Real DFM Services

The best time to improve a part is before the mold exists. A good DFM review can trim material, shorten cycle times, and save you from expensive tool rework, all before the first shot is ever fired.

If you have a part on the drawing board, or one that’s giving you trouble in production, we’d love to take a look. Your project deserves the Metro Advantage.

Contact Metro Plastics to schedule a DFM review for your next project.