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On a production line consisting of a 75 mm grooved-feed extruder with a 30:1 L/D barrier screw, a 400 mm spiral mandrel die, a dual-lip air ring, and an oscillating haul-off, heavy-duty sack film is produced at a nominal thickness of 180 µm and a fixed blow-up ratio of 2.2. The frost-line height under these conditions is normally maintained between 650 mm and 750 mm above the die face. The frost-line position is not merely a cooling indicator; it defines the axial coordinate where the molten polyethylene sheet undergoes the liquid-to-solid transition, and that axial coordinate determines how much of the total molecular orientation generated by the haul-off stress is retained in the solidified web. When the frost-line height is changed by adjusting output, coolant temperature, or air ring airflow without altering the 2.2 BUR, the bubble shape changes in a way that can be quantified by the curvature of the bubble in the neck region. A low frost line produces a short neck and rapid bubble expansion, while a high frost line produces an extended neck that is more susceptible to ambient air currents and gravitational sag.
The fixed blow-up ratio of 2.2 imposes a constant circumferential strain on the extrudate. With a 400 mm die diameter and a 2.2 BUR, the bubble diameter at the frost line is approximately 880 mm. The nominal area draw ratio is the product of the blow-up ratio and the draw-down ratio. For a 2.0 mm die gap and a final film thickness of 180 µm, the nominal draw-down ratio is 2.0/(0.180×2.2)≈5.05, and the areal draw ratio is approximately 11.1. The frost-line height does not change these nominal ratios, but it changes the rate at which the elongational deformation is imposed and the temperature at which the deformation is frozen. A low frost line quenches the film rapidly and limits the time for polymer chains to orient under the haul-off tension. A high frost line extends the molten-state exposure and can allow more relaxation or more orientation depending on the local stress and the Deborah number of the melt. In heavy-duty sack film, the objective is not maximum orientation but the combination of dart impact, tear resistance, and creep performance required by the application. The cooling rate and the frost-line position therefore become the primary levers for shifting the property balance without changing the blow-up ratio.
At constant BUR 2.2, an increase in frost-line height from 600 mm to 900 mm commonly shifts the tensile strength balance toward the machine direction. That shift occurs because the melt is held under haul-off tension for a longer axial distance before solidification, and the molecular segments oriented in the machine direction have less opportunity to relax. In the transverse direction, the constant 2.2 BUR continues to impose the same nominal lateral draw, but the effective transverse orientation depends on how much of the bubble is still molten at the point where the final diameter is reached. If the frost line is located below the point where the bubble achieves its final diameter, the transverse draw is fixed before solidification, and TD orientation tends to dominate in the final film. If the frost line is moved upward so that solidification occurs before the bubble is fully expanded, the transverse draw is incomplete or non-uniform, and the TD tensile strength can be reduced. The property matrix is therefore a function of the relative position of the crystalline front and the bubble diameter profile. Tensile properties measured according to ASTM D882 are typically evaluated in both MD and TD, and heavy-duty sack specifications often require MD tensile strength above 25 MPa and TD tensile strength above 22 MPa for a 180 µm HDPE-rich film. The actual values depend on the high-load melt index of the resin, which is commonly measured at 190°C with a 21.6 kg load according to ASTM D1238, and on the die gap, output, and cooling rate.
Elmendorf tear resistance, measured according to ASTM D1922 or ISO 6383-2, responds to frost-line height in a more complex way because tear propagation follows the orientation of the crystalline lamellae and the tie-molecule population. In LLDPE-rich systems, a high frost line can reduce MD tear strength because the oriented fibrils provide a low-energy path for crack propagation in the machine direction. In HDPE-rich systems, the effect is less pronounced but still measurable. Dart impact, measured according to ASTM D1709 Method A or ISO 7765-1, is particularly sensitive to frost-line height because impact failure involves biaxial deformation and the absorption of energy across both MD and TD. If the frost line is too high, excessive MD orientation can reduce the capacity of the film to deform in the transverse direction during impact, and the dart impact value may fall below the required threshold. If the frost line is too low, the film may have low overall orientation and low stiffness, which can also reduce dart impact due to inadequate load distribution. Industrial experience on heavy-duty sack lines has shown that the optimum frost-line height at 2.2 BUR is often a compromise that cannot be predicted from a single property curve. Published data for this specific configuration is limited, but comparative trials on pilot lines indicate that changing the frost-line height by 200 mm can alter dart impact by 15–25% and MD tear strength by 20–35% depending on the resin blend and cooling geometry.
At a frost-line height of 650 mm on a 2.2 BUR line with a 400 mm die, the bubble diameter reaches 880 mm approximately 400 mm above the die, and the remaining 250 mm of molten web between that point and the frost line undergoes axial orientation under the haul-off stress. When the frost-line height is reduced to 450 mm, the final diameter is reached very close to the frost line, and the axial orientation generated by the haul-off is minimal. When the frost-line height is increased to 950 mm, the final diameter is reached 500 mm before solidification, and the molten web is subjected to a longer residence time in the extended neck region. This longer residence time does not automatically increase orientation; it increases the probability of stress relaxation if the melt temperature remains high. The effective orientation is therefore dependent on the cooling rate and the strain rate. On a line with internal bubble cooling, the heat transfer coefficient from the inside surface can be 80–120 W/(m²·K) depending on the internal air flow rate and temperature. The external air ring typically operates at a supply air temperature of 5–12°C and a volumetric flow rate of 3,000–5,000 m³/h for a 400 mm die. These values define the cooling capacity that determines how much the frost-line height can be lowered before the bubble becomes unstable due to excessive quenching. A frost line that is too low for the available cooling rate can result in bubble flapping, helical instability lines, and uneven thickness that cannot be corrected by the oscillating haul-off.
Gauge uniformity in blown film is controlled by the die gap distribution, the air ring velocity profile, and the stability of the freeze line. At a fixed 2.2 BUR, the bubble diameter is large relative to the die diameter, and the aerodynamic forces on the bubble surface scale approximately with the square of the local air velocity. A high frost line increases the area of the molten bubble exposed to the ambient air and to the external air ring, and this increases the sensitivity of the bubble to fluctuations in air flow, room temperature, and personnel movement. On production lines without internal bubble cooling, operators sometimes compensate for high ambient air temperature by raising coolant flow or reducing output, but these actions also shift the frost line and can move the bubble into an unstable operating window. Ultrasonic thickness scanners mounted on the oscillating haul-off record gauge variation continuously, and the standard deviation of thickness measurements, assessed according to ISO 4593, often increases from ±3% at a frost-line height of 650 mm to ±6% or higher at 950 mm on a 180 µm film. The exact standard deviation depends on the die temperature profile, the air ring design, and the resin melt index. A dual-lip air ring with an adjustable upper lip can mitigate some of the instability by changing the aerodynamic force balance around the bubble, but the correction is often limited to frost-line adjustments of ±100 mm. Beyond that range, the bubble becomes non-axisymmetric, and the resulting thickness variation produces streaks that are visible after the sack is printed and inflated.
If the frost line is forced above 900 mm on a 350 kg/h line with a 400 mm spiral mandrel die and a 2.2 BUR, the molten web remains in the high-temperature viscoelastic state for a longer axial distance, and the bubble is more prone to gravitational sag. The sag phenomenon is particularly significant in thick films because the mass per unit area of a 180 µm polyethylene film at 2.2 BUR is approximately 340 g/m² at solid density, and the weight of the molten bubble between the die and the frost line contributes to axial stress. The axial stress from gravity is small compared with the haul-off stress but is not negligible when the frost line is very high. The bubble may develop a pear-like shape in which the maximum diameter is reached before the frost line, and the lower portion of the bubble below the diameter maximum has a negative curvature that acts as a mechanical weak point. Under these conditions, the thickness profile of the final film often shows a characteristic bird-wing pattern with thinner regions at the 45° positions relative to the die ports. The internal bubble cooling system can be used to lower the frost line, but if the internal air temperature is already at its lower limit of 5°C, the only remaining control actions are to reduce output, increase the melt temperature, or adjust the external air ring. Reducing output lowers the frost line but may reduce the economic viability of the line. Increasing the melt temperature lowers the frost line only if the additional heat is removed rapidly, but it may also reduce melt strength and re-introduce instability. The practical operating window at 2.2 BUR for heavy-duty sack film is therefore bounded by the cooling capacity of the air ring and the melt strength of the resin.
Resin selection interacts with frost-line height through the high-load melt index and the molecular weight distribution. A bimodal HDPE with a high-load melt index of 8 g/10 min at 190°C and 21.6 kg can be processed at lower melt temperatures and shorter frost-line heights because its melt elasticity stabilizes the bubble. A unimodal HDPE with a narrow molecular weight distribution may require a lower frost-line height to avoid melt fracture and bubble instability, but the lower frost line then reduces the time available for the development of machine-direction orientation. The addition of 10–20 wt% LLDPE to the HDPE matrix, a common formulation for heavy-duty sacks, increases the bubble stability at a given frost-line height because the long-chain branching in LLDPE contributes to strain hardening. However, the LLDPE component also reduces the density and the stiffness of the final film, and the crystalline front moves to a slightly lower temperature because of the lower melting point of the LLDPE crystallites. The position of the frost line must therefore be adjusted when the LLDPE content is changed, even if the blow-up ratio remains at 2.2. A change from 0% LLDPE to 20% LLDPE at constant output and cooling has been observed in industrial trials to lower the frost line by approximately 50–80 mm, but the exact shift depends on the specific LLDPE grade and the cooling air temperature.
The frost-line height on a heavy-duty sack line is best measured with a non-contact infrared pyrometer operating in the 8–14 µm wavelength band and calibrated against a contact thermocouple reading of 120–125°C for the solidification front of a 0.948 g/cm³ HDPE resin. The emissivity setting for polyethylene is typically 0.94–0.97, but the exact value must be validated against the contact surface temperature on each line because the bubble curvature and the air flow around the bubble can affect the infrared reading. A hand-held surface thermocouple with a response time of less than 1 s can be used to verify the position at which the surface temperature reaches the crystalline plateau. The measurement uncertainty of the frost-line position is often ±20 mm on a stable bubble, and this uncertainty must be included in the control limits. The control band for the frost-line height on a 2.2 BUR line producing 180 µm heavy-duty sack film is typically ±40 mm around the target. Exceeding this band can produce measurable changes in the key mechanical properties. The rheological requirements for the resin are determined by capillary rheometry at 190°C according to ISO 11443 or ASTM D5422. For a heavy-duty sack line with a 400 mm die and a 2.2 BUR, the apparent shear viscosity at 100 s⁻¹ is often in the range of 600–900 Pa·s, and the melt strength at 190°C, measured as the maximum force during a melt tension test, is typically 0.15–0.35 N depending on the HDPE grade. A resin with a melt strength below 0.10 N is unlikely to sustain a stable bubble at a frost-line height above 800 mm at 2.2 BUR, while a resin with a melt strength above 0.40 N may require a higher melt temperature to avoid excessive die pressure. These boundaries are not fixed; they depend on the die gap, the output rate, and the cooling configuration.
| Frost-line height above die face | MD tensile strength (ASTM D882) | TD tensile strength (ASTM D882) | MD Elmendorf tear (ASTM D1922) | TD Elmendorf tear (ASTM D1922) | Dart impact (ASTM D1709 Method A) | Gauge variation (ISO 4593) |
|---|---|---|---|---|---|---|
| 400–500 mm | 26–30 MPa | 24–28 MPa | 3.8–5.0 N | 6.0–7.5 N | 500–600 g | ±5–8 % |
| 650–750 mm | 30–34 MPa | 26–30 MPa | 3.0–4.0 N | 7.0–9.0 N | 550–650 g | ±3–5 % |
| 900–1000 mm | 34–38 MPa | 27–32 MPa | 2.2–3.0 N | 8.0–10.0 N | 450–550 g | ±6–10 % |
The values in Table 1 are directionally consistent with the known effect of frost-line height on semicrystalline blown polyethylene: MD tensile strength increases as the frost line is raised, while MD tear strength decreases and TD tear strength increases. The dart impact curve is non-linear and peaks in the intermediate frost-line range. The exact position of the peak depends on the density, the high-load melt index, and the concentration of LLDPE. Below 400 mm, the bubble becomes so fluid that gauge variation increases and the film cannot meet the thickness tolerance required for high-speed sack converting. Above 1000 mm, the bubble is so extended that the web is easily deformed by ambient air currents, and the resulting gauge variation reduces the reliability of the final sack. The specific values in Table 1 should not be used as a purchasing specification; they are representative of a 180–200 µm heavy-duty sack film and may differ for a specific resin grade.
Production-scale observations from a 90 mm extruder with a 450 mm die at a 2.2 BUR indicate that the frost-line height cannot be treated as an isolated setpoint. When the ambient air temperature in the extrusion hall increases from 25°C to 35°C, the frost-line height on an open bubble rises by 100–150 mm if the air ring temperature is not adjusted. The resulting change in film properties can be detected in the final sack as a drop in dart impact of 10–15% and an increase in the standard deviation of the thickness profile. To maintain the frost-line height within the control band, the plant must either lower the external air temperature, increase the internal bubble cooling, or reduce the output by 5–10%. These adjustments are not always possible in high-demand periods. Some plants install frost-line control systems that use a laser or infrared camera to track the position of the crystalline front and automatically adjust the air ring flow or the internal bubble cooling valve. The control loop time constant is typically 10–30 s, and the dead time between the air ring adjustment and the observed frost-line movement is 20–60 s depending on the output. This lag makes manual control difficult and can lead to oscillation of the bubble if the operator responds too aggressively to a frost-line deviation.
Internal bubble cooling removes heat from the inside surface of the bubble. Without IBC, the external air ring is the only cooling mechanism, and the frost-line height is strongly coupled to output: an increase in output raises the frost line because the additional polymer carries more heat into the bubble. With IBC, a portion of the heat is removed from the inside, and the frost-line height can be held constant while output is increased by 20–30% on a typical heavy-duty line. The internal air flow rate and the internal air temperature become additional control variables. The IBC system on a 400 mm die may circulate internal air at 3,000–6,000 m³/h and maintain an internal bubble pressure needed to sustain the 2.2 BUR. The internal air temperature is controlled by a heat exchanger and can be maintained between 5°C and 20°C depending on the cooling water temperature. The effect of IBC is not uniform: it cools the bubble surface from the inside, which can produce a different temperature profile through the film thickness than external cooling alone. The crystalline front on the outside surface may appear at a lower position than the crystalline front on the inside surface, and the measured frost-line height may differ depending on which surface is observed. This asymmetry is usually less than 30 mm but can be larger in thick films. The practical benefit of IBC is that it permits a lower frost-line height at the same output, which improves bubble stability and gauge uniformity while maintaining enough thermal stress to develop the required mechanical properties. However, IBC cannot fully decouple frost-line height from output because the external air ring still removes a substantial fraction of the heat, and the internal air flow is limited by the bubble pressure and the size of the heat exchanger.
Overstretching occurs when the haul-off speed imposes a strain rate in the molten web that exceeds the ability of the polymer chains to relax before the temperature drops below the crystallisation onset. At 2.2 BUR, the transverse strain is fixed by the bubble diameter, but the machine-direction strain is determined by the haul-off speed and the die gap. If the frost-line height is very short, the molten web is stretched rapidly and then quenched, which can produce a high level of residual stress and a narrow processing window for the subsequent converting operations. If the frost-line height is very long, the molten web is stretched over a long axial distance and may undergo substantial stress relaxation, which reduces the orientation but also can improve the tear propagation resistance. The optimum frost-line height for heavy-duty sack film is therefore not the minimum that gives the highest tensile strength, nor the maximum that gives the best tear resistance. It is the height that produces the most uniform distribution of tie molecules between the crystalline lamellae. Small-angle X-ray scattering data from oriented polyethylene films show that the long period and the lamellar thickness are influenced by the stress at the crystalline front. When the frost-line height is increased, the lamellae tend to align more in the machine direction, and the tie-molecule concentration in the amorphous phase increases. When the frost-line height is decreased, the lamellae are more randomly oriented, and the tie-molecule concentration is lower. The practical consequence is that a film made at a frost-line height of 850 mm may have a higher MD tensile strength but a lower dart impact than a film made at 650 mm, and the difference can be detected by the end user in terms of sack drop performance. Drop tests on heavy-duty sacks are often evaluated according to ISO 7965-1 or ASTM D5276, and the failure mode changes from a ductile burst to a brittle crack when the MD orientation is too high. Published data correlating frost-line height to sack drop performance at 2.2 BUR is limited, but industrial reports suggest that the drop height to failure can decrease by 10–20% when the frost line is moved from 700 mm to 1000 mm without adjusting the resin blend.
Post-consumer recyclate reduces the average molecular weight and broadens the melt elasticity spectrum compared with the virgin HDPE/LLDPE blend. A heavy-duty sack line running 20–30 wt% PCR at a 2.2 BUR often requires a frost-line height 50–100 mm lower than the equivalent virgin film to maintain bubble stability. The lower frost-line position compensates for the reduced melt strength by solidifying the web earlier, but it also reduces the time for molecular orientation and may lower the MD tensile strength below the virgin specification. To counteract this, some processors increase the high-load melt index of the virgin component or add a processing aid. The processing aid is typically a fluoroelastomer added at 200–500 ppm, and it reduces the die pressure and improves the surface quality of the bubble. The addition of processing aid does not directly change the frost-line height, but it allows the processor to use a lower melt temperature, which can lower the frost line without sacrificing bubble stability. The use of PCR in heavy-duty sacks is often governed by the requirements of ISO 14021 for recycled content claims and by the relevant national packaging regulations. The processing window for frost-line height is narrower for PCR-containing film because the gel particles and contaminants in the PCR act as stress concentrators at the crystalline front. A frost line above 850 mm increases the residence time of the molten web in the presence of these stress concentrators, which can lead to bubble tears. A frost line below 500 mm may not allow the gel particles to pass through the die lips without creating flow marks. Melt filtration with a 120 µm screen pack is recommended for PCR-containing heavy-duty sack film to remove the largest contaminants. Published data for this specific configuration is limited, but industrial experience indicates that the maximum PCR content for a stable 2.2 BUR heavy-duty sack bubble is typically 30–40 wt% depending on the PCR source and the filtration level.
The external air ring imparts a jet of air that follows the surface of the bubble due to the Coandă effect. The air velocity and the angle of the upper lip determine the heat transfer coefficient and the position of the frost line. A higher air velocity increases the heat transfer coefficient and lowers the frost line, but it also increases the aerodynamic drag on the bubble, which can deform the bubble and cause instability. At a 2.2 BUR, the bubble diameter is relatively large, and the surface area available for cooling is greater than at a lower BUR. The heat transfer coefficient from the external air ring is typically 40–80 W/(m²·K) for a single-lip ring and 80–150 W/(m²·K) for a dual-lip ring with a high-velocity lip. The internal bubble cooling system adds an additional 20–60 W/(m²·K) from the inside surface. The total heat flux from the bubble is the sum of the internal and external contributions, and the frost-line height is the position where the cumulative heat removal equals the heat of fusion and the sensible heat of the melt. The film thickness affects the frost-line height because a thicker film contains more sensible heat per unit area, and the heat must be conducted through the film to the surfaces. For a 180 µm film, the thermal conductivity of polyethylene is approximately 0.33 W/(m·K), and the thermal diffusivity is approximately 1.1 × 10⁻⁷ m²/s. The characteristic conduction time through the thickness is on the order of 0.3 s. The residence time from the die to the frost line is typically 2–5 s depending on the output and the bubble length. This means that the film thickness does not reach thermal equilibrium before solidification, and the temperature gradient through the thickness can be significant. The frost-line position observed on the outer surface may therefore differ from that on the inner surface, and the average crystalline front is located somewhere between the two. This difference is important for the interpretation of frost-line height measurements.
On a 75 mm extruder with a 400 mm die at a 2.2 BUR, the frost-line height is also influenced by the screw design and the melt temperature profile. A barrier screw with a Maddock mixing section produces a more uniform melt temperature and reduces the temperature variation across the die gap. The die gap itself has a direct effect on the draw-down ratio and therefore on the frost-line height. A wider die gap of 2.5 mm at the same output and final thickness increases the draw-down ratio and the axial stress on the molten web, which tends to raise the frost line. A narrower die gap of 1.5 mm reduces the draw-down ratio and can lower the frost line, but it may also increase the die head pressure and the risk of melt fracture. The choice of die gap is therefore coupled to the frost-line control strategy. A 2.0 mm die gap is common for heavy-duty sack film because it provides a balance between die head pressure and draw-down ratio at 2.2 BUR. The die head pressure on a 400 mm spiral mandrel die with a 2.0 mm die gap and a 0.948 g/cm³ HDPE at 210°C is typically 250–350 bar. The precise pressure depends on the resin viscosity, the output rate, and the die temperature profile. If the die head pressure exceeds 400 bar, the screen changer pressure and the extruder motor load may approach their limits, and the frost-line height could be affected by the resulting melt temperature increase. The pressure at the screen changer is typically 150–250 bar, and the melt pressure at the end of the extruder is 300–400 bar. These pressures are within the operating limits of most heavy-duty lines, but they reduce the available margin for increasing output.
At high output rates above 400 kg/h on a 400 mm die, the frost-line height becomes very difficult to control without internal bubble cooling. The external air ring may not be able to remove heat fast enough to keep the frost line below 800 mm, and the bubble becomes highly unstable if the frost line exceeds 1000 mm. Some processors install a second external air ring or a water-cooled calibration basket to increase cooling, but these devices add complexity and may introduce their own stability issues. A water-cooled calibration basket can reduce the frost line dramatically, but it may also produce water marks or thickness distortions if the water temperature is not controlled within ±1°C. The use of a calibration basket is rare in heavy-duty sack film because the film is too thick and the bubble too large. Most heavy-duty sack lines rely on a dual-lip air ring and internal bubble cooling, with the frost-line height controlled by a closed-loop system. The closed-loop system samples the frost-line position every 2–5 s and adjusts the internal cooling valve or the external air ring by a small increment. The gain of the controller must be tuned to the specific bubble dynamics, and a poorly tuned controller can make the frost-line height oscillate with a period of 30–60 s. Such oscillation produces periodic gauge variation and can cause roll blocking during winding.
After the film leaves the nip, the frost-line history is retained in the final roll as variations in the coefficient of friction, the blocking tendency, and the surface treatment retention. A film produced at a very high frost line may have a higher surface crystallinity and a lower surface energy, which affects the adhesion of printing inks and the retention of the corona treatment. The corona treatment is usually applied in-line after the haul-off, and the surface energy is measured according to ASTM D2578. A film produced at a low frost line may have a lower surface crystallinity and a higher surface energy, but the higher amorphous content can lead to blocking in the roll if the film is wound warm. The winding tension is typically 15–25 N/m for heavy-duty sack film, and the roll hardness is controlled to avoid telescoping. The frost-line height influences the film modulus and therefore the winder tension control. A film with a higher MD modulus requires a higher winding tension to maintain the same roll hardness, and the winder must be adjusted when the frost-line height is changed. The converting operations, such as printing, folding, and sealing, are all affected by the MD/TD property balance that originates at the frost line. The sealing strength is measured according to ASTM F88, and the seal initiation temperature is influenced by the film crystallinity. A film with a high frost line may have a higher seal initiation temperature because the crystalline structure is more developed. This can be disadvantageous on high-speed sack-sealing machines. The overall process window for frost-line height is therefore not solely determined by the blown film line; it must be validated through the entire converting and filling operation.
| Test property | Standard method | Typical heavy-duty sack specification range for 180–200 µm film at 2.2 BUR |
|---|---|---|
| Tensile strength, MD | ISO 527-3 / ASTM D882 | 25–35 MPa |
| Tensile strength, TD | ISO 527-3 / ASTM D882 | 22–32 MPa |
| Elongation at break, MD/TD | ISO 527-3 / ASTM D882 | 500–700 % |
| Elmendorf tear, MD | ASTM D1922 / ISO 6383-2 | 2.5–5.0 N |
| Elmendorf tear, TD | ASTM D1922 / ISO 6383-2 | 6.0–10.0 N |
| Dart impact | ASTM D1709 / ISO 7765-1 | 500–700 g |
| Puncture resistance | ASTM D5748 | 120–180 N |
| Thickness tolerance | ISO 4593 | ±5 % |
| Density | ASTM D1505 / ISO 1183 | 0.940–0.950 g/cm³ |
| High-load melt index | ASTM D1238 / ISO 1133-1 | 8–12 g/10 min at 190°C, 21.6 kg |
The compliance matrix is not intended to represent a particular standard; it is a composite of specification ranges commonly cited in supplier data sheets and heavy-duty sack purchase orders. The frost-line height must be controlled within a band that keeps the measured properties within these ranges. A line producing film at the lower end of the tensile strength range may still pass the customer's specification if the dart impact is high, but a frost-line height that produces a high tensile strength at the expense of dart impact may fail the field performance test. The relationship between process conditions and final properties is non-linear, and the direction of the shift is more important than the absolute value.
