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Views: 0 Author: Lee Publish Time: 2026-08-28 Origin: Site
Injection molding wall thickness has a direct impact on cooling, shrinkage, and sink-mark risk. Thick sections retain heat longer and shrink more during cooling, especially around ribs, bosses, and mounting features. To prevent sink marks, maintain consistent wall thickness, avoid sudden transitions, optimize rib and boss geometry, and use uniform cooling. Packing pressure, holding time, mold temperature, and cooling time should then be adjusted through mold trials. The best result comes from solving geometry, tooling, and process issues together rather than relying on molding parameters alone.
A sink mark is a localized depression caused by internal polymer shrinkage during cooling. The basic relationship is:
Thick Section → Slower Cooling → Greater Shrinkage → Surface Depression
This is why a thin cosmetic wall can show a visible sink directly above an internal rib or boss.
For plastic enclosures, engineers should focus on wall-thickness consistency rather than simply making the part thinner. A nominal 1.8 mm wall, for example, may mold well, while a local 3.5–4 mm section created by a solid boss can become a significant sink-mark risk.
Injection molding itself consists of heating, filling, cooling, and ejection, with cooling playing an important role in dimensional stability. This basic process is also described by Wikipedia's Injection moulding reference.
There is no universal wall thickness for every injection molded part. Resin grade, flow length, structural requirements, surface finish, and mold design all affect the final specification.
For consumer electronics, a controlled nominal wall is generally preferable to large thickness variations. When additional stiffness is required, ribs are usually more efficient than simply increasing the entire enclosure wall.
For example, a PC/ABS housing with a 1.8 mm nominal wall may use ribs for reinforcement. If those ribs become too thick or several ribs intersect with a boss, the combined material volume can create a localized thermal mass.
The solution may involve reducing rib thickness, hollowing the boss, adding support ribs, or creating smoother transitions.
DFM plastic design should identify sink-mark risks before mold manufacturing.
During a DFM review, engineers should check:
· Wall-thickness variation
· Rib and boss geometry
· Thick intersections
· Gate location
· Cooling-channel position
· Cosmetic surfaces
· Critical assembly dimensions
A practical design question is not simply, “Is the wall thick enough?” It is:
Where is material accumulating, and how will that region cool?
This approach is particularly important for large consumer electronics housings with thin cosmetic shells, PCB bosses, snap-fits, connector openings, and internal reinforcement.
Wall-thickness optimization must be supported by effective mold cooling. Cooling channels should be positioned according to the actual geometry, especially around thick bosses, ribs, corners, and large flat surfaces.
Autodesk Moldflow highlights the importance of analyzing filling, cooling, shrinkage, and warpage during injection molding simulation. This allows engineers to identify thermal and dimensional risks before production.
Molding parameters also require controlled optimization. Insufficient holding pressure or holding time can increase shrinkage, while excessive pressure may cause flash or residual stress.
Cooling time should be long enough for stable ejection and dimensions, but excessive cooling increases cycle time. A production trial should therefore establish the practical balance between defect prevention, dimensional stability, and productivity.
Consider a PC/ABS electronics enclosure with a 1.8 mm nominal wall. During the first mold trial, filling is complete and screw-hole dimensions are within tolerance, but shallow sink marks appear above two internal bosses.
The first response should not automatically be higher holding pressure.
Engineering review may find that the boss creates a local section almost twice the nominal wall thickness, while the nearest cooling channel is relatively far away. Increasing holding pressure may reduce the defect, but the process window remains narrow.
A more robust solution is to reduce unnecessary boss material, add supporting ribs, improve local cooling, and then fine-tune holding pressure and cooling time.
This illustrates an important manufacturing principle:
A geometry problem should not be solved by process parameters alone.
Observation | Likely Cause | Engineering Action |
Sink above boss | Local thick section | Redesign or core the boss |
Sink along rib | Excessive rib thickness | Optimize rib geometry |
Sink on large surface | Uneven cooling | Review wall and cooling layout |
Sink improves with packing | Insufficient compensation | Optimize holding pressure/time |
Defect varies by cavity | Cooling/filling imbalance | Check mold balance |
What is the recommended wall thickness for injection molding?
There is no universal value. The correct thickness depends on resin, geometry, flow length, structural requirements, and tooling conditions. Consistency is generally more important than one fixed number.
How can you prevent sink marks in injection molding?
Maintain uniform wall thickness, reduce material accumulation around ribs and bosses, improve cooling, and optimize packing and cooling parameters.
Why do bosses cause sink marks?
A thick boss can retain heat longer and shrink more than the surrounding wall, creating a visible depression on the cosmetic surface.
Dawang Precision brings 26 years of manufacturing experience and operates a factory with 400+ advanced machine tools, including Röders and Mazak 5-axis machining centers. Our engineering team evaluates part geometry, mold structure, cooling, machining, and tolerance requirements as one manufacturing system.
