An operator opens a mold and finds a field of small round voids just below the surface of a polyurethane part. The first response is often to add more defoamer or raise the vacuum. Both help when the source is dissolved gas or entrained air, but neither fixes moisture, trapped mold air, mixing turbulence, or a cure-related shrink cavity. The most dependable start is to classify the void before changing any setting.
The same sequence works for cast elastomers, rigid foam, microcellular soles, and RIM components. The weighting changes with the process, but the physical sources do not. A bubble is a sign that gas was trapped, formed by reaction, or left behind by solidification, and each source leaves a different fingerprint.
First, Classify the Bubble Type
Cut a representative part and inspect the defect in section. If the part was designed as a solid polyurethane, a density measurement can separate local voids from a uniform microcellular texture. ASTM D3574 [1] is the practical framework for flexible cellular PU, ISO 845 [2] defines apparent density for cellular plastics, and ISO 1183-1 [3] applies to non-cellular material.
Shape and position are usually enough for a first classification. A trapped-air void is smooth and round. A moisture-driven pore is small and dispersed, often with a denser ring because the water–isocyanate reaction creates both carbon dioxide and polyurea [4]. A shrink void is irregular and follows the thickest wall or the last region to freeze.
| Defect pattern | What it looks like in the part | Most likely source | First check |
|---|---|---|---|
| Open-surface pinholes in the same cavity area | Small voids at the skin, repeated at the same location | Trapped air from poor venting or flow front collapse | Vent position, fill pattern, short shot |
| Fine, dispersed internal pores | Tiny round voids spread through a solid section | Moisture or dissolved gas in the component stream | Material moisture, degassing, feed line sealing |
| Large round voids under the top skin | Smooth, spherical cavities below the surface | Air pulled into the mix or gas released during pressure drop | Mixer vortex, cavitation, ratio stability |
| Irregular center cavity in a thick section | Rough-walled hollow in the last region to solidify | Shrinkage after gel | Demold time, mold temperature, feed path |
Shrink voids share the same solidification logic. Causes Of Shrinkage In Polyurethane Foam How To Control It covers how cooling and conversion interact to leave sink or internal voids.
Check the Material Path for Moisture and Dissolved Gas
Water is the most overlooked bubble source in solid polyurethane. It competes with polyol for isocyanate and forms carbon dioxide; the same reaction produces polyurea [4]. That chemistry is useful in water-blown foam, but it is a defect in a solid elastomer, seal, or structural part.
The first check is not the machine; it is the material conditioning. Open containers, cold drums moved into warm production areas, and day tanks with head-space condensation can all add moisture before the metering pump sees the component.
Dry or degas only to the suitability limits in the material TDS. A generic moisture target can be wrong because polyester polyols, polyether polyols, plasticizers, and additives differ in how much water they can tolerate before bubble formation becomes visible. Vacuum degassing removes dissolved gas and helps reduce residual moisture, but it does not correct a poorly sealed supply line.
Inspect Molding and Venting Flow
A repeated bubble in the same corner is rarely a chemistry problem. It is usually trapped air that the part geometry cannot push out. The fill should start low and advance as a single rising front so air is pushed continuously from the mix path to the vent. If the stream drops through a sprue and splashes against a wall, it can wrap air into the material before the cavity fills.
Vents should be the last point to fill and should not close before the section is packed. Small vent grooves, contamination, or heavy release residue can hold a bubble at the same location. A short shot is the fastest diagnostic: it reveals the true flow front and shows whether air is being trapped or released.
Isolate Metering and Mixing Variables
After material and tooling are cleared, shift the machine variables one at a time. On mechanical mixers, a visible vortex above the blade pulls air into the liquid. On high-pressure impingement heads, low mixing pressure relative to viscosity can leave unmixed or partially mixed streams that later create gas inclusions. The standard PU processing reference describes how mixing energy, viscosity, and component ratio interact during reaction injection [4].
Metering problems usually make fine, evenly distributed voids. A starved pump inlet, a blocked filter, a leaking seal on the suction side, or a low conditioning temperature can cause cavitation before the mix head. The bubbles are not injected later; they are created in the component stream.
Ratio and shot weight should be checked together. A small shot-weight shift may mean the A/B ratio is moving, which changes gel time and exotherm. An early gel can freeze bubbles that would otherwise have escaped, while a slower reaction can give gas more time to separate [4].
A drifting shot weight usually means the metering system or the material path has changed. How To Calculate Shot Weight In Polyurethane covers the calculation and how to use shot weight to catch ratio or feed problems before they become part defects.
Bubble release depends on how long the reaction mixture remains fluid. The Role Of Rise Time In Polyurethane Foaming covers rise profile and how faster gel can trap voids before they reach the vent.
Bubble defects rarely have a single cause. When the void pattern changes between cavities or batches, the interaction is usually material preparation, tooling, and machine variables working together. The fastest way to narrow it down is to gather the data at the same time: part photos, hardness, target density, A/B ratio, vacuum level, mold temperature, and shot weight. Email those details to Info@chinahaifeng.com for a process-focused review.
Run the Troubleshooting Checklist
The following order keeps the search economical. Change one variable at a time; if vacuum and mold temperature are changed together, the root cause will stay hidden.
- Classify the void from a cut section.
- Verify material moisture and degassing against the TDS target.
- Inspect suction lines, filters, and seals for cavitation.
- Confirm shot weight and A/B ratio are within control limits.
- Observe a short shot to confirm the flow front and vent function.
- Test mold temperature and demold time only after the first five are stable.
- Document a before/after image from the same cavity.
Get a Process Review for Bubble Defects
If the defect remains after the checklist, the issue may be in the equipment interaction rather than a single setting. Send the following information for a process review:
- cut-section and surface photos
- material system, target density, and hardness
- actual and set shot weight, A/B ratio, vacuum level, and mold temperature
- fill pattern or short-shot observation
Contact Info@chinahaifeng.com or WhatsApp: 86 13566296633.
Frequently Asked Questions
What is the difference between trapped-air bubbles and shrink voids?
Trapped air produces smooth, spherical voids that often sit near the flow front or vent. Shrink voids are irregular and follow thick sections or the last material to solidify. Moisture-driven reaction pores are smaller, more dispersed, and usually associated with a solid formula that is turning microcellular.
Will vacuum degassing remove all bubble defects?
No. Degassing mainly removes dissolved gas and helps reduce some residual moisture. It will not stop trapped air from poor mold venting, cavitation in a metering pump, or water-driven reaction gas. It is one step in the sequence, not a total fix.
Why do bubbles always appear in the same mold area?
A fixed location usually points to a local flow or venting condition: gate splash, an air pocket at the last-to-fill point, or a cold or contaminated spot on the mold surface. Before changing the formula, run a short shot and compare the flow front.
References
[1] ASTM D3574-17, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, ASTM International, 2017.
[2] ISO 845:2006, Cellular Plastics and Rubbers—Determination of Apparent Density, International Organization for Standardization, 2006.
[3] ISO 1183-1:2019, Plastics—Methods for Determining the Density of Non-Cellular Plastics—Part 1: Immersion Method, Liquid Pycnometer Method and Titration Method, ISO, 2019.
[4] G. Oertel, Polyurethane Handbook, 2nd ed., Hanser Publishers, 1994.
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What Is Semi Rigid Polyurethane Foam
The Role Of Rise Time In Polyurethane Foaming
What Is Spray Polyurethane Foam Composition And Application
