Sink Marks in Injection Molding: Design and Process Solutions
Published
A sink mark forms where a thick section keeps shrinking after the surface over it has set, and the shrinking core pulls the surface inward. Hold pressure can reduce it only while the gate is still open; once the gate freezes, more hold time or pressure does nothing for that part. That is why the reliable cure is in design (thinner ribs and bosses, cored-out sections, gates near thick areas) and why process changes should be checked against a gate-freeze study rather than turned up blindly.
How a Sink Forms
- The melt touches the mold and a skin freezes against the steel.
- The interior stays molten longer, especially where the section is thick: at rib and boss bases, wall junctions, and thick walls.
- As the interior cools it shrinks. While the gate is open, hold pressure pushes in more melt to make up the volume.
- When the gate freezes, no more material can enter. Remaining shrinkage in the thick area pulls the skin in (a sink) or, if the skin is too rigid, tears a void inside the section.
Sinks and voids are the same shrinkage expressed differently, so changes that move a sink can create a void, and a part that “passes cosmetics” can still hide voids in a section that carries load.
Design: Removing the Thick Section
| Feature | Design rule of thumb | Why |
|---|---|---|
| Ribs | thinner than the wall they stand on | the junction stays close to nominal thickness |
| Bosses | cored so the boss wall stays thinner than the nominal wall, connected by ribs | no solid cylinder of plastic |
| Thick walls | shelled out to nominal with internal ribs | removes the slow-cooling mass |
| Wall changes | gradual tapers, thick areas near the gate | packing can reach them |
Fictiv puts bosses, ribs, and gussets “generally” at 40 to 60% of nominal wall thickness to prevent them “from becoming heat-retaining hot spots” (Fictiv on cycle time and part design). On glossy, high-visibility surfaces the thinner end of that range lowers sink risk; on textured or hidden surfaces a thicker rib may be acceptable. The wall side of the design is covered in wall thickness guidelines.
Placement and finish help as well: locate ribs and bosses away from the most visible surfaces, and consider texture, which hides minor sinks that a polished surface shows.
Process: What Can and Cannot Be Changed
The process levers act during the packing window, between skin formation and gate freeze:
| Lever | Effect on sink | Limit |
|---|---|---|
| Hold pressure | more material into the shrinking area | flash, overpacking near the gate, stress |
| Hold time | keeps pushing until the gate freezes | no effect after gate freeze |
| Gate size or location | longer packing window, closer to thick areas | mold change |
| Mold temperature near the sink | changes when the skin sets | can lengthen the cycle or shift sink to a void |
| Melt temperature | changes viscosity and shrinkage | can increase total shrinkage |
Using a Gate-Freeze Study
For example, assume a gate-freeze study gives these part weights (illustrative values):
| Hold time | Part weight |
|---|---|
| 2 s | 41.2 g |
| 4 s | 41.9 g |
| 6 s | 42.3 g |
| 8 s | 42.3 g |
| 10 s | 42.3 g |
Weight stops increasing at about 6 seconds, so the gate is frozen there. If sink remains at 8 seconds of hold, raising hold time to 10 or 12 seconds will not change it: the gate is closed and no more material can reach the part. The remaining options are hold pressure within the 6-second window, a larger gate or one closer to the thick area, local mold temperature, or the design. The study method is described in injection pressure versus hold pressure.
Defining an Acceptable Sink
Sink is a cosmetic defect on visible surfaces and a functional one on sealing or mating surfaces. Acceptance should say:
- Where it matters. Surface classes on the drawing (for example, A-surfaces visible to the user, B-surfaces hidden).
- How it is judged. A visual standard under defined lighting and viewing distance, or a measured depth with a stated method (profilometer, dial gauge on a fixture, or optical scan).
- Limit samples. Boundary samples agreed with the customer, because “no visible sink” depends on light, color, gloss, and the observer.
Painting and plating can make sinks more visible, so parts that will be finished are judged after finishing or against a standard that accounts for it.
Working Out Why a Sink Persists
- Locate the sink and identify the thick feature behind it.
- Check gate freeze: is hold ending before or after the gate freezes?
- Check hold pressure against flash and overpacking near the gate.
- Check local mold temperature at the sink area.
- If the sink remains, section a part to see whether shrinkage has turned into a void.
- Plan the design or mold change: thinner rib, cored boss, gate relocation, or texture.
Sinks are one of several defects that share the same packing physics; the broader map is in common injection molding defects. A sink that survives a correct gate-freeze study and sensible hold pressure is a design message, not a process problem.
Leave a Reply