We build molds and run injection molding production in our own Xiamen factory, so we see the same design mistakes again and again. Some of them add cost to the tool. Others cause sink marks, warpage or stuck parts once production starts. This guide covers the rules we check first, and explains the reasons behind them so you can apply them to your own part.
The rules below are starting points. The right values depend on your resin, surface finish and mold design, so treat them as a checklist to review with your molder rather than final specifications.
Keep Wall Thickness Uniform
If you only remember one rule, make it this one. Plastic cools and shrinks as it solidifies, so thick sections cool slowly and shrink more than thin ones. When one area is thicker than its neighbors, the surface sinks or voids form inside, and the part can warp as the thin and thick areas pull against each other.
Uniform walls also fill more evenly and shorten cycle time, because cooling time rises steeply with thickness. For many general-purpose resins, walls in the range of roughly 1.2 to 3 mm work well, but the right number depends on the material and the part size. Check the recommended range for your resin before you fix a value.
When thickness has to change, taper it gradually. A common guideline is a transition of at least three times the thickness difference. Sharp steps create flow restriction and stress concentration.
If a section must be thicker for stiffness, add a rib instead of building up the wall. Ribs give strength with much less material and do not create a heavy, slow-cooling zone.
Add Draft to Every Vertical Surface
Draft is a slight taper on walls that run parallel to the direction the mold opens. Without it, the part grips the core as it shrinks onto it. The result can be scratches, distortion, or in the worst case, damage to the mold.
A common starting point is about 1 degree per side for smooth surfaces, with 2 degrees preferred where possible. Ribs and bosses can use less, often around 0.5 to 1 degree, but they should still have draft. Textured surfaces need more draft than polished ones, because the texture acts like many tiny undercuts. Heavier textures can require several degrees.
Plan draft early. Adding it late often means reworking walls that were already set, which changes wall thickness and can trigger another round of design review.
Design Ribs Instead of Thick Walls
Ribs stiffen a part without adding the thick section that causes sink. The guidance below comes from common industry practice, and the exact figures vary by source and material:
| Feature | Common guideline | Why it matters |
|---|---|---|
| Rib base thickness | About 50–60% of the nominal wall (structural parts may go higher) | Thicker ribs cause sink marks on the opposite face |
| Rib height | Up to about 3× the nominal wall | Taller ribs are harder to fill and eject |
| Rib spacing | At least 2× the nominal wall | Allows plastic to flow and cool without problems |
| Rib base radius | About 0.25× the wall or more | Reduces stress concentration at the root |
| Rib draft | About 0.5–1.5° per side | Lets the rib release cleanly from the core |
On cosmetic faces, keep ribs thinner. A high-polish surface shows sink more easily than a textured one, so the rib base should be kept at the low end of the range or moved to the non-visible side of the part.
Handle Bosses With Care
Bosses hold screws and inserts, and they are one of the most common sources of sink. The usual approach is to make the boss outer diameter about 2 to 2.5 times the screw or insert diameter, make the boss wall about 60% of the nominal wall or less, and support the boss with ribs rather than making it solid.
Bosses should be avoided on polished or optical surfaces. Even a well-designed boss can leave a visible mark on a high-gloss face. If the boss must sit on a cosmetic surface, move it to the inside of the part or redesign the mounting.
Note on threads: Molding threads directly into a part is possible but often adds mold cost and reduces thread quality. For frequent assembly, consider threaded inserts or self-tapping screw bosses instead.
Round the Corners
Sharp internal corners are hard for molten plastic to fill and concentrate stress when the part is loaded. Design the inside radius at about half the wall thickness or more. Design the outside radius as the inside radius plus the wall thickness. Starting both radii from the same center keeps the wall uniform around the bend.
Rounding corners is one of the cheapest design changes you can make. It improves flow, reduces stress cracking, and rarely adds cost.
Avoid Undercuts Where You Can
An undercut is any feature that blocks the mold from opening in a straight line. Undercuts usually need side actions, lifters or collapsible cores. Each one adds tooling cost and maintenance, and it slows the cycle.
If an undercut is needed for function, such as a snap fit, a side action is often the right answer. Before you commit, check whether the feature can be moved, simplified or split across the parting line. A part that can be molded with a straight pull is usually much cheaper to tool.
Plan the Gate and Parting Line Early
Gate location controls how plastic flows into the part. It affects weld lines, flow marks, gate vestige and how the part fills. Choosing the gate position early, before the wall and rib geometry is locked, gives your molder more room to avoid problems.
The parting line is where the two halves of the mold meet. It affects draft direction, flash risk, and the visible seam on the part. Plan it together with the gate, and keep it away from critical cosmetic and sealing surfaces where you can.
Common Mistakes That Raise Cost
- Uneven walls. Causes sink, warpage and longer cycle times, and often needs a mold revision to fix.
- Missing draft. Causes sticking, scratches and mold wear. Adding draft late means redesign.
- Overly thick ribs or bosses. Create sink on the visible side and slow the cycle.
- Tight tolerances everywhere. Drives up inspection and tooling time. Specify tight tolerances only on functional features.
- Unnecessary undercuts. Add side actions and maintenance to the mold.
- Late gate and parting line decisions. Can force wall or rib changes after the design is frozen.
Before You Send the Design
A quick self-check catches most of these issues before a quote is requested:
- Is the wall thickness uniform, and is any change tapered gradually?
- Does every vertical face have draft, with more on textured surfaces?
- Are ribs and bosses sized relative to the nominal wall, and kept off cosmetic faces where possible?
- Are internal and external corners rounded?
- Are undercuts necessary, and can any be removed or moved?
- Have you thought about gate location and parting line?
- Are tolerances tight only where the function needs them?
If you answered no to any of these, the design can usually be improved before any steel is cut.
How RpMolds Can Help
Because we make molds and run production in the same factory, we review designs with both the tool and the process in mind. We look at wall thickness, draft, ribs, gating and parting line together, and point out changes that reduce tooling cost, cycle time or defect risk. Our team has supported clients in Europe and North America through this kind of review for many years.
Send us your 3D file and a short description of the application, and we will return a DFM review along with a quote.
Get a DFM review on your design
Share your STEP file, material and expected volume. We will flag design issues and give you a line-item quote.
Request a quoteSummary
- Keep wall thickness uniform, and taper any change gradually.
- Add draft to every vertical face, with more for textured surfaces.
- Use ribs for stiffness instead of thick walls, sized relative to the nominal wall.
- Size bosses carefully and keep them off polished surfaces.
- Round internal and external corners.
- Minimize undercuts, and plan the gate and parting line early.
Good design makes molding faster, cheaper and more repeatable. When you are ready, send us your files for a review.
