Created on 01.28
Avoid Warpage and Sink Marks: Preheat Profiles That Work
Warpage and sink marks are often treated as “different defects,” but they share a core driver: non-uniform shrinkage created by non-uniform temperature and solidification. If you use IR preheating (sheet heating, part/insert preheat, localized heating near forming), your goal is not “make it hotter.” Your goal is shape the temperature field so shrinkage is predictable.
This article shows three repeatable preheat profiles, how they map to warpage/sink mechanisms, and how to validate the profile without turning commissioning into trial-and-error.Cycle-time-safe preheating approach (for injection molding)
Why warpage and sink marks show up together
Warpage: differential shrinkage and thermal gradients
Warpage is strongly tied to differential shrinkage—regions cool and shrink at different rates and magnitudes, creating residual stress that releases after ejection.
Temperature deviation between mold surfaces (core vs cavity side) is a known contributor to shrinkage imbalance and warpage.
Sink marks: thick areas cool slower, shrink more
Sink marks commonly occur in thicker regions where cooling is slower; the surface can pull away from the cavity wall relative to adjacent areas.
When packing is insufficient (or gate freezes too early), the interior shrinkage can’t be compensated, and the surface collapses.
Preheating matters because it changes the starting temperature field (thermoforming) or the thermal balance around a critical feature (local heating / insert preheat), which changes how and where shrinkage concentrates.
What “preheat profile” means in plastic molding
A preheat profile is the intentional temperature distribution you create before forming/molding outcomes are locked in.
- In thermoforming and related processes, the final thickness distribution is strongly dependent on the preform/sheet temperature distribution prior to forming, so the heating stage must be controllable.
- On sheet heating systems, independently controlled zones (including perimeter zones) are used to improve uniform heating; perimeter zones can be set higher than the center to compensate edge losses.
The three preheat profiles that actually work
Profile A: Ramp + equalize (the default “safe” profile)
Use this when you see inconsistent geometry or mixed thickness behavior.
- Ramp quickly to get into the workable temperature band.
- Equalize briefly to reduce gradients before the part “locks in.”
Why it helps: gradients are what convert shrinkage into distortion; reducing gradients reduces warpage risk.
Profile B: Edge-compensated zoning (sheet width control)
Use this when you see edge-underheat, edge tearing, or cross-width thickness drift in thermoforming.
ACRYLITE’s thermoforming guidance explicitly recommends perimeter zones set to higher output than center zones to ensure uniform heating across the sheet.
Practical rule: if your defects are “repeatable by lane” or “always worse at the edge,” start with zoning, not overall power.
Profile C: Surface-limited, core-reached (thick sections without blister/overheat)
Use this when thick regions show surface damage or you cannot reach stable core condition.
Thermoforming guidance notes a key limitation: for thicker sheets, surface heating can outpace heat transfer to the core; extending radiant heating may overheat the surface (blister risk), which is why air convection is preferred for thicker sheet heating in that guidance.
Interpretation for your retrofit: if your product is thickness-dominated, you may need lower surface intensity + longer dwell, or a hybrid stage that supports core equalization.Distance–time–power tuning order (for defect fixes)
Defect-to-cause matrix (preheating-specific)
Defect symptom | What your preheat profile is probably doing | Corrective profile change |
Warpage increases after speed-up | Equalization time collapses → gradients persist | Add a brief equalize stage or reduce peak ramp intensity |
Warpage flips direction (curl to one side) | Core vs cavity thermal balance changed | Check side-to-side thermal symmetry; reduce temperature deviation across faces |
Sink marks worsen at bosses/ribs | Thick zones cool slower; packing window too short | Reduce localized overheat, improve thermal uniformity at thick features; ensure adequate packing/hold window |
Sheet forms thin at one region every run | Heating field is wrong for the draw | Use patterned heating (screening/shading) or zoning to reshape the temperature field |
Surface looks “done” but core behavior is unstable | Surface temperature is misleading | Treat pyrometer readings as surface-only and validate core/equalization behavior |
How to measure and tune without chasing noise
Pyrometers are convenient, but thermoforming guidance warns they indicate surface temperature, not core temperature, and rely on emissivity (wavelength dependent).
Your tuning workflow should therefore aim for repeatability rather than “one perfect number.”
Step 1: Build a repeatable “uniformity map”
Pick a fixed measurement moment (same dwell time, same distance, same part presentation), then record cross-width or feature-to-feature temperature.
Thermoforming guidance suggests using pyrometers and even temperature tapes to identify different locations and adjust zone outputs to obtain more uniform heating.
Step 2: Validate by outcome, not just temperature
For injection molding, process-parameter reviews emphasize that holding time and cooling time coordination strongly affects deformation after ejection; too-short cooling can lead to large deformation after ejection.
Your acceptance check should therefore include the “after-ejection” outcome window, not only the in-station reading.
Case example (illustrative): reducing warp and sinks without slowing production
A line sees edge-related forming drift (thermoforming) and occasional sink at thicker areas (downstream molded features). The team:
- implements edge-compensated zoning to stabilize cross-width heating
- reduces peak intensity and adds a short equalize window to lower gradients
- documents a uniformity map and repeats it at two points in the shift to confirm stability
Outcome pattern: fewer “lane” rejects and fewer thick-feature cosmetic defects because the shrinkage field becomes more predictable.
FAQ
Does a hotter preheat always reduce sink marks?
Not necessarily. Sink marks are linked to how thick regions cool and shrink and whether shrinkage is compensated; thicker sections cool more slowly and are higher risk.
Why do we still see warpage when dimensions look stable in the heater?
Because warpage often manifests after ejection when residual stresses release; it’s tied to differential shrinkage and thermal gradients.
How do I know if I’m only heating the surface?
If your surface reading rises quickly but forming/molding behavior is unstable, you likely need more equalization or a lower-intensity/longer-dwell approach. Thermoforming guidance explicitly distinguishes surface vs core limitations.
Call to action
[Defect goal] Warpage direction + sink-mark locations (photo helps)
[Profile target] Ramp / equalize / edge zoning concept
[Validation] Repeatable map + pass/fail acceptance at production speed
Share polymer, thickness range, part/sheet size, target cycle time/line speed, and available heater length. YFR can propose a preheat profile and zoning approach designed to reduce gradients and stabilize shrinkage.
Data sources
Last modified: 2026-01-27
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