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Peak internal air temperature (PIAT) in rotational moulding is the maximum recorded temperature of the air inside the rotating mould cavity during the heating phase, and it must not be confused with the oven setpoint or the mould wall temperature. The value is normally captured by a type K thermocouple passing through a vent or a hollow mould shaft, with the junction shielded from direct radiant energy but open to the internal air. For linear low-density polyethylene powders with melt indices of 3.5–6.0 g/10 min (ISO 1133-1) and densities of 0.936–0.945 g/cm³ (ISO 1183-1), production ovens are commonly set between 280 °C and 310 °C, while the PIAT typically settles between 190 °C and 240 °C. The internal air peak is therefore not a material property; it is a process-state variable reflecting the integrated heat flux through the mould shell, the powder bed, and the developing melt film. In carousel and shuttle machines producing parts with nominal wall thicknesses from 3 mm to 12 mm, the internal air temperature lags the outside mould surface by 20–70 °C during the final stages of the heating ramp. That lag depends on the thermal diffusivity of the mould material, the bulk density of the powder, and the rate of melt densification. A low PIAT is linked directly to incomplete particle fusion and residual interparticle voids, while an excessive PIAT is linked to oxidation, inner-surface embrittlement, and gas-generation porosity. These two failure modes produce very different microstructures but both reduce the drop impact energy of the part. The process task is to hold the peak internal air temperature within a densification window whose lower boundary is set by the time required for bubble elimination and whose upper boundary is set by the onset of measurable chemical degradation. Thin-walled parts of 3 mm may track the mould wall temperature with a lag of only 10–15 °C, whereas thick-walled marine or industrial sections above 10 mm can contain a temperature gradient of 30–50 °C even after the internal air has reached its plateau.
The connection between PIAT and porosity is rate-controlled rather than thermodynamic. Porosity in rotationally moulded polyolefins originates from interstitial air trapped between powder particles during the early sintering stage, dissolved volatiles released during melting, water released by additives or condensation, and degradation gases generated when the inner surface overheats. Bubble elimination depends on two mechanisms: buoyancy- or rotation-driven bubble rise through a low-viscosity melt and diffusion-driven dissolution of gas into the polymer matrix. The terminal rise velocity of a spherical bubble is proportional to the square of its radius and inversely proportional to melt viscosity. In rotational moulding the acceleration field is low and periodic, so macroscopic melt flow is weak; densification is therefore dominated by sintering and bubble dissolution. A bubble in polyethylene at 200 °C with a radius in the range of 10–100 μm moves slowly, and the diffusion coefficient of nitrogen or oxygen in the melt is orders of magnitude lower than in air. Raising the PIAT from 200 °C to 230 °C reduces the zero-shear viscosity of commercial C4- and C6-LLDPE rotational moulding grades by roughly one-half to one order of magnitude depending on long-chain branching and molecular weight distribution. Typical zero-shear viscosities at 190 °C are 2×10³–8×10³ Pa·s, and at 230 °C the value can fall below 1×10³ Pa·s. The practical effect of an additional 20 °C of PIAT is therefore a substantial acceleration of void closure and gas dissolution. However, this improvement is realised only when the time above the crystalline melting point is adequate. A narrow peak with less than 30 s above 210 °C will not fully remove bubbles even if the recorded PIAT appears acceptable. Production process records therefore include the time above 200 °C or 210 °C, and some in-house specifications require a minimum integrated thermal exposure above the crystalline melting point of 120–180 °C. The residual void content after cooling is commonly below 2 % for flat LLDPE sections with sufficient PIAT and above 5 % for parts pulled before the internal air reaches 180 °C. Because these values vary with part geometry and oven air speed, density-based acceptance must be validated against cross-section micrographs from the same part location.
On production carousel machines, type K thermocouples with ungrounded junctions and glass-fibre insulated leads are installed through the mould vent or through a hollow mould shaft, and the signal is transmitted by a battery-powered data logger rotating with the arm. The sampling interval is normally 2–5 s, but a 2 s interval is required to resolve the PIAT plateau in parts with wall thickness below 5 mm. Fixed oven air thermocouples are not equivalent to internal air temperature; the difference at the heating endpoint can reach 80–100 °C in large moulds because the mould shell and powder absorb significant heat. Batch-to-batch PIAT variation of ±5 °C is common when the total shot mass changes by 10–15 % on the same arm even if the oven setpoint is unchanged. That variation is sufficient to move a 6 mm C6-LLDPE wall from ductile puncture to partially brittle cracking when the lower limit of the PIAT plateau is crossed. On multi-station carousel lines with index times of 8–12 min, the operator cannot rely on oven temperature alone because the internal air temperature is a response to mould mass, wall thickness, part shape, and powder packing. The recorded trace should include the full internal air curve, the peak value, the time from 150 °C to peak, the time above 210 °C, and the cooling rate between the melt temperature and 80 °C. In a production setting, the PIAT trace operates as a release criterion: parts are not opened until the internal air trace has demonstrated the required plateau, because premature cooling of a low-PIAT part locks in voids and reduces impact resistance in a way that cannot be corrected after demoulding. This is particularly critical for double-wall parts and fuel tanks, where internal porosity cannot be repaired once the outer shell has been released.
Drop impact performance of rotationally moulded polyolefins is assessed on flat plaques cut from the moulding or on complete articles using drop-weight or striker-mass methods. The most widely cited instrumented plaque method is ISO 6603-2, which specifies a through-penetration test with a striker diameter of 20 mm, a support ring diameter of 40 mm, and square specimens of 60 mm × 60 mm or 100 mm × 100 mm, conditioned according to ISO 291. The standard records force-deformation, energy at peak, total energy, and failure mode. A related North American method, ASTM D3763, provides high-speed puncture properties over a range of striker velocities. For low-temperature assessment, product specifications often require impact at −40 °C because the ductile-to-brittle transition of linear polyethylene can occur near that temperature for high-molecular-weight grades. The total puncture energy of a defect-free 3 mm C6-LLDPE plaque at 23 °C is commonly in the range of 15–40 J, but 2–5 % volumetric porosity can reduce total energy by 30–60 % and shift the fracture from hinge-dominated ductile perforation to radial cracking with low post-peak deformation. The failure mode classification in ISO 6603-2 distinguishes ductile failure with a crown of drawn material from brittle failure with crack propagation before full penetration. Resin supplier technical literature and production trials often identify an optimum PIAT range for C4- and C6-LLDPE rotational moulding grades between 200 °C and 230 °C, with lower values associated with incomplete densification and upper values associated with surface oxidation, colour shift, and loss of low-temperature ductility. In a poorly densified part, pores visible at 10× to 50× optical magnification concentrate stress at the crack tip, and the instrumented puncture curve shows an early peak load followed by unstable crack growth and a sharp energy drop. In an oxidised part, the inner surface may display microcracks and the impact energy at −40 °C falls because the molecular weight distribution has narrowed through chain scission or extended through oxidative crosslinking. The transition between these mechanisms is not always visible as a change in total energy at room temperature; therefore drop impact testing must include a sub-ambient condition and, preferably, a ductility index calculated from the energy after peak divided by energy to peak. Full-scale product drop tests are geometry-dependent and cannot be transferred between parts of different wall thickness and angle.
Thick-walled rotational mouldings above 8 mm present a process conflict because the internal air temperature can reach 220 °C while the polymer near the mid-wall is still close to the crystalline melting point. The outer skin is fully fused, the inner skin is partially compacted, and the core contains residual particles and interconnected voids. Under these conditions, the PIAT signal may satisfy the production target, but the part remains under-densified at the centreline because heat transfer through the polymer is poor and the internal air temperature is not in equilibrium with the melted wall. The thermal conductivity of semicrystalline polyethylene is approximately 0.4 W/m·K in the solid state and 0.2 W/m·K in the melt, so the gradient across a 10 mm wall can remain 20–40 °C even after the internal air reaches a plateau. A long oven residence time pushes the PIAT beyond 240 °C in an effort to remove core porosity, but this exposes the inner surface to oxidative degradation and may generate degradation-gas microbubbles that reduce density no matter how long the part is held. The processing window for thick parts is therefore narrower than for thin parts, and the optimum PIAT may shift upward by 5–10 °C while the cooling rate is reduced to avoid differential crystallisation and warpage. Field data from production-scale carousel machines indicates that a 10 mm wall LLDPE tank section may require a PIAT of 230–240 °C and an internal air time above 210 °C of at least 5–8 min to reach a void content below 3 %, but published experimental studies that isolate PIAT from oven setpoint and residence time for this specific geometry are limited. Instrumented drop impact on thick plaques cut from the flat portion of such a part often shows lower energy per unit thickness than injection-moulded plaques of the same nominal polymer, because the core retains a population of small voids that are not visible on the surface. The failure path in a thick, partially densified wall bends toward the core where voids coalesce, and the fracture surface appears rough with partially melted particles. This is distinct from oxidative embrittlement, where the fracture surface remains relatively smooth and shows discolouration near the inner surface. For thick parts, post-moulding inspection includes through-section microtomy or X-ray computed tomography to detect voids that density measurements may average out across the entire wall. If the PIAT has been raised to remove core porosity at the expense of inner-surface degradation, the part may pass a visual surface check and still fail the −40 °C ISO 6603-2 test because the brittle inner layer initiates cracks under the striker.
After the PIAT plateau is reached, the cooling rate modifies the relationship between peak temperature and impact behaviour. Fast cooling from an excessively high PIAT suppresses crystal growth and produces a lower-density, more amorphous morphology with lower tensile modulus but possibly improved thin-section impact ductility; slow cooling from an adequate PIAT increases crystallinity and stiffness but may decrease the zero-shear melt time required for bubble removal. A part with a PIAT of 230 °C and forced-air cooling can therefore exhibit very different porosity and impact energy from a part with the same PIAT and slow ambient cooling. In production, the cooling stage is controlled by water mist, forced air, or ambient rotation with defined rates, and the internal air temperature trace during cooling is recorded because a pause in the cooling rate between 120 °C and 80 °C can induce post-crystallisation shrinkage voids in thick sections. Mould release agents can also influence the apparent porosity near the inner surface: excessive release agent or silicone spray volatilises during late heating and creates additional gas pressure at the melt-release interface, producing local bubble clusters even when the PIAT would otherwise be sufficient for bubble dissolution. These bubbles are not eliminated by further heating because they are trapped against the mould wall and are fed by decomposition products. Their effect on drop impact is detected as a reduction in total energy and a change in the first portion of the force-displacement curve, before visible puncture occurs. Oxidative degradation of the inner surface at elevated PIAT is detected by yellowing or brown discolouration, a change in melt flow rate, and an increase in the carbonyl index measured following ASTM D5576. In polyethylene, the inner surface exposed to internal air can undergo chain scission and carbonyl formation after prolonged exposure above 230 °C, and this chemical change correlates with a fall in drop impact energy at −40 °C. The practical control is to set a moderate oven temperature and allow enough residence time for the PIAT to rise gradually, rather than to use a very high-temperature rapid heating cycle that overshoots the surface while the core remains cold.
| PIAT region and thermal signature | Densification and porosity outcome | Drop impact response | Test basis |
|---|---|---|---|
| Below 185 °C, time above 150 °C less than 2 min | Incomplete particle fusion; interconnected voids; apparent density low; void content often above 5 % | Brittle perforation; radial cracks; low total energy at 23 °C and −40 °C | ISO 6603-2, ISO 1183-1, cross-section microscopy |
| Plateau 200–230 °C, time above 210 °C for 1–3 min at 3–6 mm wall | Sintered, isolated spherical voids; void content 1–3 %; density near void-free plaque | Ductile puncture; high energy to peak and post-peak energy; stable hinge | ISO 6603-2, ASTM D3763 |
| Peak above 245 °C, brown-grey inner skin, oxidative odour | Inner-surface degradation gases, microcracks, colour shift; apparent density may remain high but embrittled | Reduced low-temperature energy; brittle inner-skin initiation; variability between plaques | ISO 6603-2 at −40 °C, carbonyl index by ASTM D5576 |
Porosity in rotationally moulded polyethylene is measured by density difference, polished cross-section microscopy, or computed tomography. The density method uses ISO 1183-1 or ASTM D792 for the apparent density and compares it with a compression-moulded void-free reference plaque of the same formulation. The void content is calculated from the ratio of apparent density to reference density and is expressed as a volume percentage. In production quality control, the specimen is cut from the flat section where impact plaques are taken, because porosity varies with wall thickness and radiused corners behave differently. A void content below 2 % is a common target for parts requiring low-temperature drop impact, but the acceptance value depends on the part wall thickness and purchaser specification. Some automotive and industrial container specifications combine density-based porosity with a minimum total energy at −40 °C measured according to ISO 6603-2. Optical image analysis of polished cross-sections at 20× to 50× magnification resolves voids larger than 5 μm, while micro-CT can resolve voids below 10 μm and provide spatial distribution through the wall. Image analysis commonly reports number density and area fraction rather than volume fraction, and these values cannot be interchanged without knowing the void shape factor. For process development, computed tomography is preferred because it detects interconnected porosity in the core of thick sections that an outer-skin density measurement may miss. The relationship between PIAT and porosity is best established on a production-scale machine by holding oven setpoint and part mass constant and varying only the heating time. When the PIAT is increased stepwise from 190 °C to 230 °C, the apparent density rises and cross-section micrographs show a transition from irregular, elongated voids between powder particles to isolated spherical bubbles. Above 240 °C, the density may plateau or decrease slightly as degradation gases form new bubbles. Peak internal air temperature therefore cannot be used as a linear predictor of porosity; it must be paired with a measurement of time above melt temperature and inspection of the inner surface for oxidation.
The use of peak internal air temperature as a release criterion fails when the part wall is thick, when the mould shell has variable heat transfer, or when the thermocouple is not truly measuring the internal air. A thermocouple placed too close to the mould wall reads the metal temperature, while one placed directly in the oven air reads the oven temperature; both produce a false PIAT. For a valid reading, the junction must be isolated from the wall and shielded from direct radiant heat, but still open to the internal air. The response time of the assembly must be 2 s or less, otherwise the recorded peak is attenuated and the part may be underprocessed. Even with a valid reading, PIAT is an integrated thermal state and not a direct measure of pore elimination. A part might reach 230 °C for 20 s and retain voids because the time above 210 °C was insufficient. Another part might reach 220 °C for 6 min and be fully densified. The most robust process control therefore uses PIAT as one boundary variable and integrates the internal air temperature trace above the melt temperature. A minimum time above 210 °C, rather than a single peak value, correlates more directly with drop impact energy in semi-crystalline polyethylene. In moulding shops where the process is controlled by oven time alone, PIAT can be recorded offline with a modified mould lid and then transferred to production, but the safety margin must account for differences in ambient temperature, mould mass, and powder bulk density. The measured PIAT can vary between winter and summer production by 5–10 °C if oven temperature recovery is sluggish, and this variation can be enough to change the porosity profile of a 6 mm part from acceptable to marginal.
Peak internal air temperature effects on porosity and impact are material-specific. Hexene-based C6-LLDPE offers a wider PIAT process window than C4-LLDPE because of its higher toughness and chain architecture, but the density of the powder and the stabiliser package determine the oxidative upper limit. Some rotational moulding grades tolerate a PIAT of 240 °C without colour shift, while others discolour at 220 °C. Polypropylene requires a higher PIAT to reduce viscosity and achieve densification, often 210–240 °C, but it is more prone to oxidative degradation at these temperatures and usually requires a higher loading of antioxidant and a lower oven residence time. The effect of porosity on drop impact is more pronounced in polypropylene because the matrix is already less ductile than LLDPE at −20 °C. Polyamide 6 rotational moulding grades require polymerisation-activated or low-viscosity resins and a peak temperature above 240 °C to complete densification and crystallisation; the viscosity is highly temperature-sensitive and residual moisture must be controlled to avoid steam porosity. For these materials, the relationship between PIAT, porosity, and impact is steeper, and the processing window can be as narrow as ±5 °C when a fast-sintering grade is used. In such cases, the internal air temperature trace must be measured on every shot rather than inferred from oven conditions. Published industrial data for a specific formulation and geometry is limited because compounders and equipment suppliers often keep PIAT maps confidential, but the general response is consistent with melt viscosity and gas dissolution kinetics.
