Heat and sink
A thick mass stays hot and can pull the opposite face. Mark show faces behind bosses and islands.
Thick section design
Thick Section Design for Injection Molding: Core the Mass Before It Sinks
A thick island stays hot and can sink the opposite face or void in the core. Core the mass, add ribs if stiffness must stay, and mark show faces. DFM review comments on your section views before quoting.
Design guideline
Core the mass before it sinks the opposite face
A thick section is a local mass that stays hot after the nearby skin has frozen. That extra heat drives sink, voids, and longer cooling.
Cooling time and sink risk often rise faster than the stiffness you wanted. Coring out keeps a nominal wall. Ribs carry stiffness after the island is gone. NetProto flags thick sections that will pull sink onto the opposite cosmetic face. DFM review uses your section views before steel is quoted.
The wall thickness page picks the nominal skin. Uniform wall covers transitions. Bosses own fastener posts. Sink marks and voids own those defects. The design guide hub lists every topic.
Roles
Which page answers which thick-section question
Stay here for when a mass is the problem. Open a sibling for how to hollow it or the defect it causes.
| Page | Use it for |
|---|---|
| Thick section design (this page) | When a local mass sinks, voids, or holds heat |
| Coring out thick sections | How to hollow the mass |
| Wall thickness | Pick the nominal wall first |
| Uniform wall thickness | Ramp out of a thick island |
| Boss design | Screw posts that start as thick pillars |
| DFM review | Section-view check before steel |
Checks
What DFM review looks for on thick sections
Qualitative checks. NetProto does not publish a maximum wall thickness.
Voids in the core
The last plastic to freeze can leave a void inside the mass. Section the suspected island on the CAD.
Opposite cosmetic face
A class-A skin over a solid boss is a common fail. Call that face out on the RFQ.
Core or rib
Remove mass that is not needed. Add ribs if stiffness must stay. Do not leave a solid block for CAD convenience.
Decision
A solid thick block versus a cored or ribbed section
Wrong: leave a solid pillar. Right: keep a nominal wall, core the island, and add ribs if stiffness is still needed.
What goes wrong
- Sink on the wall opposite a solid boss or island
- Long cooling while the core is still hot
- A void in the center of a thick section
- A drawing that never sections the suspected mass
What to try first
- Keep a nominal wall and core the island
- Tie a boss to the wall and core the center
- Add a rib grid for stiffness after the mass is gone
- Show section views through every suspected thick zone
Drivers
Where thick sections usually hide
Qualitative drivers. This page does not publish a thickness table.
| Where it hides | Why it matters |
|---|---|
| Solid bosses | A fastener post is a thick pillar until it is cored. |
| Heavy corners and leftover islands | A wall change that was never ramped leaves a hidden mass. |
| Glass-filled or high-shrink grades | Filled and high-shrink resins can make read-through worse. Put the grade on the RFQ. |
| Cooling that cannot reach the core | Steel cooling still has to pull heat out of leftover mass. |
RFQ
Send these notes with CAD for a thick-section review
The quote can start from CAD alone. These notes make the DFM pass specific.
Cosmetic faces opposite thick bosses or islands
Section views through suspected thick zones
Coring already modeled, if you have a preference
Resin grade if glass-filled or high shrink
Which masses must stay solid for function
Rib plan if stiffness must remain after coring
Related
Related design guidelines
This page owns thick islands. Open a sibling for how to hollow, the feature, or the defect.
Sink marks in thick walls
The cosmetic defect a thick island often pulls onto the opposite face.
Sink marks in thick wallsDFM review
How NetProto checks section views before steel is cut.
DFM reviewFAQ
Injection molding thick section design FAQ
What is a thick section in injection molding?
A thick section is a local mass that stays hot after the nearby skin has frozen. Solid bosses, heavy corners, and leftover islands after a wall change are common. That extra heat drives sink, voids, and longer cooling.
Why do thick sections sink or void?
The core of a thick mass cools last. As it shrinks, it can pull a sink on the opposite face or leave a void inside. A show face behind a solid boss is a common case. The sink-marks article and voids page own those defects.
Will sink always show on a thick boss?
Not always, but an uncored boss on a thin cosmetic face is a top sink driver. Resin, pack, and whether the opposite face is class-A change the risk. Flag those bosses on the RFQ instead of hoping the face stays flat.
Should I core out a thick section or add ribs?
Core the mass when the extra plastic is not needed for function. Add ribs when you still need stiffness after the island is gone. The coring-out page owns how to hollow. The rib page owns the stiffener. This page decides that the island is the problem.
Should I avoid thick sections entirely?
No. Some features must stay locally thick. Control them with coring, ribs, and gradual transitions, and mark the opposite cosmetic face. Do not leave a solid block because the CAD was easier that way.
How is this page different from coring-out or wall thickness?
Wall thickness picks the nominal skin. Uniform wall covers transitions. Coring-out owns how to hollow a mass. Bosses own fastener posts. This spoke owns when a section is too thick and what to do first. DFM review checks your section views.
Next step
Upload CAD for DFM review of your thick sections
Share geometry, resin, cosmetic faces, and section views so review can flag islands that will sink, void, or hold heat before steel is cut.