Production process of EPP light-weight seat

Abstract

The invention relates to the technical field of foamed polypropylene forming and manufacturing, and discloses a production process of an EPP light-weight seat, which comprises the following steps: establishing a bead unsaturated filling state, controlling steam to climb pressure at a quasi-static rate, inducing to generate a condensed water film in a thin-wall area of a special-shaped part by utilizing thermal response difference, and keeping a thick-wall area dry; high-low pressure pulse circulation is executed, steam is driven to penetrate through the thick-wall area through the impact rate, and flash evaporation heat absorption of the condensate water film is induced through sudden pressure drop; and finally closing the mold for pressure maintaining and shaping. According to the method, gas-liquid phase state distribution is regulated and controlled through the pressure change rate, the thermodynamic contradiction between thin-wall overheating collapse and poor thick-wall welding in the large-wall-thickness-ratio special-shaped part forming process is solved, and the homogenization welding quality and size precision of the total cross section of a product are improved.

Description

Technical Field

[0001] This invention relates to a manufacturing process for lightweight EPP seats, belonging to the field of foamed polypropylene molding and manufacturing technology. Background Technology

[0002] Currently, foamed polypropylene (EPP) materials are widely used in the manufacture of structural components for automotive seat systems due to their excellent specific strength, energy absorption characteristics, and thermal stability. In the mainstream EPP steam molding process, the basic process includes filling pre-foamed beads into the mold cavity, introducing high-temperature steam to penetrate the micro gaps between the beads, using the enthalpy carried by the steam to bring the surface of the beads to the melting temperature and bond them together, and then cooling and demolding them. The core of the process is to establish a stable penetrating heat flow field to ensure that the beads in the mold cavity receive enough heat to complete sintering.

[0003] As the automotive industry increasingly demands lightweighting and space utilization, modern EPP seat structures exhibit geometrically irregular features, often featuring designs with extremely high wall-to-thickness ratios. For example, a seat cushion support area exceeding 100 millimeters in thickness may coexist with a thin-walled area only a few millimeters thick. This extreme geometric difference leads to a non-uniform distribution of thermodynamic response characteristics within the mold cavity. The thin-walled area, due to its small heat capacity and typically located in a low-resistance steam flow channel, experiences extremely rapid heating. Conversely, the thick-walled core, with its large heat capacity and location at the end of the flow resistance, suffers from delayed heat transfer. Under traditional constant-pressure or simple pulse steam heating modes, process parameters are often forced to compromise to meet the molding requirements of the thick-walled area, i.e., extending the ventilation time or increasing the steam pressure to ensure the core... To address the need for lightweighting, existing technologies often focus on adjusting the physical structure of the seat. For example, Chinese invention patent CN109822810B discloses a manufacturing process for lightweight EPP seats, which uses steel pipes or strip steel plates bent to form the outer contour skeleton of the seat. This skeleton is then positioned in a mold and foamed integrally with the EPP material, followed by PUR foam composite. Although this solution introduces a high-strength skeleton to reduce dependence on EPP density, it still uses conventional temperature and pressure control logic for the EPP foaming and molding process. It emphasizes the stacking of hard structures while neglecting the control of the thermal field in soft processes. For complex-shaped EPP parts, the traditional steam heating mode cannot solve the thermodynamic contradictions caused by the large wall thickness ratio.

[0004] Therefore, the technical problem to be solved by this invention is how to solve the problems of thermal response mismatch and flow channel self-locking under high pressure penetration in the molding process of large wall thickness ratio irregular parts by reconstructing the physical field of the molding process without increasing the complexity of the mold structure and the hardware cost, so as to achieve homogeneous welding of the whole cross section. Summary of the Invention

[0005] To address the problems mentioned in the background art, the technical solution of the present invention is as follows: A manufacturing process for an EPP lightweight seat, comprising the following steps:

[0006] Step S1: Fill the mold cavity with EPP beads and establish an unsaturated filling state;

[0007] Step S2: Perform the slow-pressure condensation pre-setting operation; control the steam flow rate into the mold cavity so that the pressure inside the mold cavity increases at a slow rate R. slow Rise to the preset condensation pressure; utilizing the physical property that the heat absorption response rate of the thin-walled region of the EPP irregular part is faster than that of the thick-walled region, at a slow rising rate R slow During the continuous pressurization process, the surface temperature of the thin-walled area preferentially reaches the dew point and generates a condensate film, while the thick-walled area is kept in a dry steam environment, thus forming a gas-liquid two-phase distribution in the mold cavity where the thin-walled area is wet and the thick-walled area is dry.

[0008] Step S3: Perform at least one high-low pressure pulse cycle based on the gas-liquid two-phase distribution; the high-low pressure pulse cycle includes alternating pressurization and rapid discharge phases; during the pressurization phase, control the steam to rise at a rapid rate R. fast Entering the mold cavity, the rapid rise rate R fast Set to be greater than the ramp rate R slow More than three times that of the previous method, the steam penetration force generated by rapid pressure increase is used to overcome flow resistance and enter the gap between beads in the thick-walled area; in the rapid discharge stage, the steam input is cut off and the exhaust is opened, so that the pressure in the mold cavity is rapidly reduced to the low pressure value that induces flash evaporation of the condensate film, and the heat of the thin-walled area is carried away by the vaporization of water.

[0009] Step S4: Stop steam input and use mold closure and pressure holding for shaping.

[0010] Preferably, in step S2, the gradual increase rate R slow The pressure is controlled to be between 0.02 MPa / s and 0.05 MPa / s; the condensing pressure is controlled to be between 0.06 MPa and 0.08 MPa; in step S3, the rate of increase R... fast The pressure should be controlled to be greater than 0.2 MPa / s.

[0011] Preferably, in the high and low pressure pulse cycle of step S3, after the rapid discharge stage and before the next pressurization stage, a gap recovery stage is also included; in the gap recovery stage, steam is controlled to be introduced into the mold cavity, so that the pressure in the mold cavity rises back to the recovery pressure and is maintained for the recovery time; wherein, the recovery pressure is greater than the low pressure value and less than the peak pressure of the pressurization stage; the compression effect generated by the recovery pressure overcomes the rebound expansion of the EPP beads at the instantaneous temperature in the mold cavity, and reopens the steam flow channel between the EPP beads.

[0012] Preferably, the recovery pressure is set to 30% to 50% of the peak pressure during the pressurization stage; the recovery time is set to 0.5 seconds to 1.0 seconds; the recovery time allows steam to fill the steam flow channel without causing substantial adhesion on the surface of the EPP beads.

[0013] Preferably, the process further includes an adaptive adjustment step based on the characteristics of the exhaust pressure drop: in the rapid exhaust stage of step S3, the pressure drop rate at the exhaust port of the mold cavity is monitored using a pressure sensor; the pressure drop rate is compared with a preset reference rate range; if the pressure drop rate is higher than the upper limit of the reference rate range, the pressure rise time in step S2 in the next molding cycle is extended or the number of high and low pressure pulse cycles in step S3 is increased; if the pressure drop rate is lower than the lower limit of the reference rate range, the number of high and low pressure pulse cycles in step S3 in the next molding cycle is reduced.

[0014] Preferably, after step S2 and before step S3, a local pre-cooling resistance-increasing step is performed: cooling water is introduced into the cooling circuit in the mold corresponding to the thin-walled area of ​​the EPP shaped part for a preset time to harden the EPP beads on the surface of the thin-walled area to increase the vapor permeation resistance; taking advantage of the difference in vapor permeation resistance, during the pressurization stage of step S3, the vapor is guided to flow preferentially to the thick-walled area of ​​the EPP shaped part with relatively low permeation resistance.

[0015] Preferably, in step S2, the surface temperature of the inner wall of the mold cavity is monitored; when the difference between the surface temperature and the saturation temperature of the introduced steam is less than a preset temperature difference value, the gradual increase rate R is terminated. slow The pressurization process is then transferred to step S3.

[0016] Preferably, the set value of the recovery pressure follows a dynamic setting rule based on the material's springback characteristics: P restore =α⋅(P peak -P valley )+P valley , where P restore To restore pressure, P peak P is the peak pressure during the pressurization phase. valley α represents the low-pressure value during the rapid displacement stage, and α is a preset elastic recovery coefficient related to the density of EPP material, ranging from 0.3 to 0.6.

[0017] Preferably, the low pressure value is controlled as a negative pressure between 0MPa and -0.05MPa; the negative pressure environment is used to lower the boiling point of the condensate in the mold cavity and enhance the flash cooling effect.

[0018] Preferably, in step S1, the back pressure of the mold cavity is set to 0.05MPa to 0.1MPa, and the gaps between the EPP beads are kept in a connected state after filling, serving as the initial channel for steam flow in step S2.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] 1. In the production of EPP lightweight seats, a quasi-static pressure ramp-up gradient is established during the initial penetration stage. Utilizing the physical difference in thermal response time constants between thin-walled and thick-walled regions, steam preferentially completes heat exchange and transforms into a liquid water film on the surface of the thin-walled region with lower heat capacity during the slow pressure rise. In the thick-walled core with higher heat capacity, gaseous penetration is maintained, creating a gas-liquid two-phase spatial distribution within a single mold cavity. The pre-placed liquid water film undergoes flash evaporation phase change during the pressure drop stage, absorbing the heat accumulated in the thin-walled region using the latent heat of vaporization. The thick-walled region, lacking a liquid medium, only undergoes gaseous expansion and diffusion. By utilizing the fluid phase change dynamics, the heating and cooling functions of different geometric feature regions within the mold are automatically decoupled and precisely aligned in the time dimension. This resolves the contradiction between thin-walled overheating and collapse and poor welding of thick-walled regions during the molding of irregularly shaped parts. The process is based on a spontaneous response of physical laws, eliminating the need for complex partitioned temperature control hardware or cooling medium channels.

[0021] 2. Utilizing the volume response characteristics of foamed polymer beads to changes in environmental pressure, a specific compressive stress gap recovery stage is created after the low-pressure exhaust action and before the high-pressure inflation action in the breathing cycle. The elastic rebound force of the beads is forcibly overcome by a medium pressure field, causing the gaps between the beads that were closed due to low-pressure expansion to reopen and form a connected steam flow path. The pressure waveform is reconstructed in a stepwise manner, eliminating the fluid channel self-locking effect caused by the elastic expansion of beads in traditional pulse processes. This ensures that subsequent high-pressure steam pulses penetrate to the deep core of the part through convection heat transfer along the low-flow-resistance open channel, avoiding decay into inefficient surface heat conduction. This ensures that the core welding quality of thick-walled parts is not limited by the simple penetration depth of steam, but depends on the duration of fluid channel maintenance throughout the molding cycle, achieving homogeneous molding of the entire cross-section of large-thickness irregular parts.

[0022] 3. By utilizing local pre-cooling hardening to induce changes in vapor permeation resistance, an asymmetric flow resistance field distribution is constructed during the molding process. This causes the surface beads in thin-walled or easily overheated areas to harden and form a high flow resistance barrier. This drives subsequent high-energy heat flow to be deflected and enriched in relatively low-flow-resistance, thick-walled, or difficult-to-form island areas. The differences in the internal geometry of the mold are transformed into a logic switch to guide the flow of vapor. This allows thermal energy to be automatically and preferentially compensated to areas with high thermal inertia and slow heating. There is no need to introduce external flow regulating valve groups. Through time-series control, the interaction between materials and fluids is directly mobilized to establish a heat supply and demand balance in the complex mold cavity. This avoids short-circuiting and loss of vapor in low-flow-resistance areas, which would lead to energy waste and the risk of local overheating. This ensures the dimensional accuracy stability of complex geometric topology parts under continuous production conditions. Attached Figure Description

[0023] Figure 1 This is a flow chart of the gas-liquid two-phase forming process for pressure-rate regulation according to the present invention.

[0024] Figure 2 This is a comparison curve of the thermal response of thin-walled cold protection and thick-walled penetration according to the present invention. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0026] In a manufacturing process for a lightweight EPP seat, the distribution of steam condensation within the mold cavity is controlled based on the pressure rise rate. The EPP beads are heated and molded using the mold. This process establishes an unsaturated filling state for the beads. By controlling the steam pressure rise rate, a two-phase gas-liquid distribution with wet thin-walled areas and dry thick-walled areas is constructed within the mold cavity. The pressure changes during high and low pressure pulse cycles are used to achieve flash cooling of the thin-walled areas and deep fusion of the thick-walled areas.

[0027] In step S1, EPP beads are filled into the mold cavity to establish an unsaturated filling state. The back pressure of the mold cavity is controlled to be 0.05MPa to 0.1MPa. Compressed air is used to inject EPP beads into the cavity, and the volume compression ratio after filling is controlled to be 5% to 10%. This compression ratio is set to maintain the interconnected micro gaps between the EPP beads, avoiding the elastic deformation of the beads due to excessive compression and the closure of the steam flow channel. This ensures that the packing density after filling is in an unsaturated state, reserving physical space for subsequent steam penetration and condensate film formation. After filling, the gaps between the beads are kept in a connected state, serving as the initial channel for subsequent steam flow.

[0028] In step S2, a slow-pressure condensation pre-setting operation is performed, controlling the steam flow rate into the mold cavity so that the pressure inside the mold cavity increases at a slow rate R. slow The pressure rises to a preset condensation pressure at a gradual rate R. slow The pressure is controlled at 0.02 MPa / s to 0.05 MPa / s, and the condensing pressure is controlled at 0.06 MPa to 0.08 MPa, with a slow rate of increase R. slow During the continuous pressurization process, taking advantage of the physical property that the thin-walled region of the EPP shaped part has a faster heat absorption response than the thick-walled region, the surface temperature of the thin-walled region with a thickness of less than 10 mm preferentially reaches the dew point and forms a condensate film, while the thick-walled region with a thickness of more than 50 mm is kept in a dry steam environment. This step forms a gas-liquid two-phase distribution in the mold cavity, with the thin-walled region being moist and the thick-walled region being dry. During this process, the surface temperature of the inner wall of the mold cavity is monitored. When the difference between this surface temperature and the saturation temperature of the introduced steam is less than a preset temperature difference value, the gradual increase rate R is terminated. slow The pressurization process is then transferred to step S3, with the preset temperature difference value set to 5℃.

[0029] After step S2 and before step S3, a local pre-cooling resistance-increasing step is performed. Cooling water is introduced into the cooling circuit in the mold corresponding to the thin-walled area of ​​the EPP shaped part for a preset time, which is 1 to 3 seconds. This operation hardens the EPP beads on the surface of the thin-walled area to increase the vapor permeation resistance. Utilizing the difference in vapor permeation resistance, the vapor is guided to flow preferentially to the thick-walled area of ​​the EPP shaped part with relatively low permeation resistance during the pressurization stage of step S3.

[0030] In step S3, at least one high-low pressure pulse cycle is performed based on the gas-liquid two-phase distribution. This high-low pressure pulse cycle includes alternating pressurization, rapid discharge, and gap restoration phases. During the pressurization phase, the steam is controlled to rise at a rapid rate R. fast The material is introduced into the mold cavity, and the rapid rise rate R is increased. fast Set to be greater than the ramp rate R slow The pressure is more than three times that of the target material, and the specific value is greater than 0.2 MPa / s. The steam penetration force generated by the rapid pressure increase overcomes the flow resistance and enters the gap between the beads in the thick-walled area. The thin-walled area is covered by the condensate film generated in step S2. The thermal inertia and latent heat of phase change of the water film buffer the thermal shock of the high-temperature steam. In the rapid discharge stage, the steam input is cut off and the exhaust is opened, so that the pressure in the mold cavity is rapidly reduced to the low pressure value that induces the flash evaporation of the condensate film, and controlled between 0 MPa and -0.05 MPa. The negative pressure environment is used to lower the boiling point of the condensate in the mold cavity, enhance the flash cooling effect, and the water vaporization carries away the heat of the thin-walled area, suppressing the overheating of the area. The thick-walled area, due to the lack of liquid water film, only gas expansion and diffusion occur. The pressure drop rate of the mold cavity exhaust port is monitored by a pressure sensor. The preset reference rate range is determined by the following calibration procedure: Before the mold is put into production, at least one... Standard venting tests under empty mold conditions were conducted to obtain the baseline pressure decay curve of the mold cavity in the bead-free state. Based on the packing porosity of the current batch of EPP beads, the theoretical maximum decay rate when the venting channel is fully unobstructed was set as the upper limit of the baseline rate. The rate at which the venting time is extended by 20% due to excessive condensate accumulation was set as the lower limit of the baseline rate. This pressure drop rate was compared with the preset baseline rate range. The dynamic definition procedure for the baseline rate of venting conductivity was as follows: The standard venting curve Pempty(t) under empty mold conditions was collected and the decay constant τ0 was extracted. Combined with the current bead filling porosity ϕ (calculated from the filling mass m and the mold cavity volume V according to ϕ=1-m / (V⋅ρsolid)), the lower limit of the baseline rate λmin=Ppeak / τ0⋅(1-ϕ)2 was set, and the upper limit λmax was set to 1.5 times λmin. If the measured rate v... measure <λ min If the exhaust channel is found to be clogged with condensate, the PID controller is triggered to output an incremental signal ΔN to increase the number of pulse cleaning cycles; if vmeasure >λ max If the liquid film coverage is deemed insufficient, the steam injection time is extended by Δt until v is reached. measure Convergence into [λ] min ,λ max If the pressure drop rate is higher than the upper limit of the reference rate range, it is determined that the condensate content in the mold cavity is insufficient. The pressure rise time in step S2 of the next molding cycle is extended or the number of high and low pressure pulse cycles in step S3 is increased. If the pressure drop rate is lower than the lower limit of the reference rate range, it is determined that too much condensate leads to increased exhaust resistance. The number of high and low pressure pulse cycles in step S3 of the next molding cycle is reduced.

[0031] After the rapid discharge stage and before the next pressurization stage, a gap recovery stage is performed. Steam is controlled to be introduced into the mold cavity, causing the pressure inside the mold cavity to rise to the recovery pressure and maintain the recovery time. The recovery pressure is greater than the low pressure value but less than the peak pressure of the pressurization stage. The compression effect generated by the recovery pressure overcomes the rebound expansion of the EPP beads at the instantaneous temperature inside the mold cavity, reopening the steam flow channels between the EPP beads. The set value of the recovery pressure follows a dynamic setting rule based on the material's springback characteristics: P restore =P valley +α(P peak -P valley ), where P restore To restore pressure, P peak P is the peak pressure during the pressurization phase. valley The low-pressure value during the rapid filling stage is α, which is a preset elastic recovery coefficient related to the density of EPP material, ranging from 0.3 to 0.6. The in-situ calibration protocol for the elastic recovery coefficient α is as follows: a static compression test is performed during the filling stage in step S1, the mold is controlled to open slightly by d=1mm and then close, and the clamping force F is recorded. close Calculate the transient bulk modulus E of the material based on the displacement Δx response curve. bulk =dF / dA⋅L0 / dx; Based on the ASTM D3575 standard, a density-modulus mapping database is established. The rebound potential energy factor of the current batch of EPP material is retrieved and substituted into the correction formula α=0.3+0.3⋅E. bulk -E min / E max -E min Calculate, where E min E max These are the standard modulus boundary values ​​for materials with density grades of 20 g / L and 60 g / L, respectively, from which the target value of the recovery pressure P is quantitatively set. restoreTo overcome the thermal expansion stress of the beads without producing plastic deformation, the recovery pressure is set to 30% to 50% of the peak pressure during the pressurization stage, and the recovery time is set to 0.5 to 1.0 seconds. This recovery time allows steam to fill the steam flow channel without causing substantial adhesion on the surface of the EPP beads.

[0032] In step S4, the steam input is stopped, the mold is controlled to fully close from the slightly open state when it is not saturated filled to the designed position, and the mold water cooling system and vacuum dehydration system are turned on to cool the product to the demolding temperature.

[0033] Example 1: In the specific application of this invention to the production of EPP automotive rear seat cushions with varying wall thicknesses, the maximum wall thickness of the side support areas of the cushion reaches 120mm, while the minimum wall thickness of the central connecting rib area is only 8mm. This extreme difference in geometry leads to a misalignment of the thermodynamic time constant. Specifically, the thin-walled area can reach a welding temperature of 145°C within 2 seconds of steam introduction, while the thick-walled core, due to its large heat capacity and flow resistance, requires 8 to 10 seconds to establish the same energy level. To resolve this thermodynamic contradiction without introducing external physical insulation inserts, the process establishes a back pressure of 0.08MPa within the mold cavity and injects EPP beads with a magnification of 30x, controlling the volume... A compression ratio of 8% is used to construct interconnected microscopic airflow channels. The control system drives the steam proportional valve to perform quasi-static pressurization at a slow rate of 0.03 MPa / s (Rslow) until the cavity pressure reaches 0.07 MPa. During this process, the low thermal inertia of the thin-walled region is utilized to allow the surface to preferentially reach the dew point and precipitate a micron-sized liquid condensate film. Meanwhile, the thermally inert thick-walled region located at the end of the flow resistance remains dry due to the lag in temperature rise. This creates a differentiated phase boundary between thin-walled wetting protection and thick-walled drying and heat-receiving regions within a single cavity space. Specifically, the formation of the two-phase partition follows the following physical logic: due to the extremely small characteristic length of the thin-walled region (8 mm), its thermal response time constant is... Much smaller than the thick-walled area (120mm), in During the quasi-static pressurization period, steam preferentially fills the low-flow-resistance, thin-walled voids via diffusion, providing sufficient latent heat to rapidly condense on the thin-walled surface with its relatively small heat capacity. However, in the thick-walled core, due to its long path (high flow resistance) and enormous heat capacity, the steam permeation rate is slower than the condensation rate on the thin-walled surface, thus... Before the impact, a phase boundary was successfully constructed with thin-walled wet protection and thick-walled dry heat-prepared phases.

[0034] Based on this phase distribution field, the system fully opens the valves, causing steam to rise at a rate of 0.25 MPa / s (Rfast) to form a high-momentum fluid impact on the mold cavity. This pressure wave overcomes the deep flow resistance in the thick-walled region, forcing dry steam into the gaps between the core beads to achieve convective heat transfer. At the same time, the liquid water film covering the thin-walled region absorbs the excessive thermal shock of the high-pressure steam using the latent heat of phase change, thus shielding the surface beads from overheating damage. Then, the system switches to vacuum exhaust mode, causing the mold cavity pressure to drop sharply to a low pressure of -0.03 MPa (P) within 0.5 seconds. valley The abrupt change in thermodynamic state induces violent flash evaporation of the water film in the thin-walled region, which carries away sensible heat to lock in the size, while the thick-walled region mainly undergoes gas expansion and diffusion to homogenize the internal temperature field. The final cross-sectional inspection results show that the 120mm thick-walled core bead interface welding rate of the seat cushion exceeds 95%, and the surface of the 8mm thin-walled area is smooth without orange peel wrinkling. Its dimensional deviation is controlled within ±0.5mm.

[0035] Example 2: This example constructs an experimental system designed to objectively verify the effectiveness and stability of the above-mentioned production process in a real engineering environment. The experiment selects an EPP simulation sample with typical complex geometric features as the verification object. The physical structure of the sample is designed with 120mm thick-walled support areas on both sides, while the thickness of the middle connecting area is reduced to 8mm. This is to simulate the extreme thermodynamic mismatch challenge faced by large wall thickness ratio irregular parts during the molding process. The experimental platform uses an industrial-grade EPP molding machine equipped with a high-frequency response steam proportional valve (response time less than 20ms) and a multi-channel mold cavity pressure-temperature synchronous monitoring system. Miniature thermocouples are pre-embedded in the mold cavity, located on the surface of the thin-walled area and the geometric center of the thick-walled area, respectively, to acquire transient thermal field data during the molding process in real time.

[0036] To objectively evaluate the actual technical effect of the process of this invention (hereinafter referred to as the experimental group) compared with the prior art, a control group was designed. The control group adopted the currently common constant pressure penetration molding process in the industry. Both groups of experiments used the same raw materials (30x EPP beads) and initial mold temperature (60°C). In the experimental group, the dynamic pressure control strategy of this invention was implemented: the steam was controlled to rise at a slow rate of 0.03 MPa / s. slow The steam was injected into the mold cavity, and the pressure was increased to 0.075 MPa within 2.5 seconds and maintained. A pulse cycle was triggered, and the steam pressure was impacted to 0.35 MPa at a rate of 0.25 MPa / s. Immediately afterwards, the vacuum was opened to exhaust the steam, causing the pressure to drop sharply to -0.03 MPa within 0.5 seconds. In the control group, the traditional full-open valve strategy was adopted, that is, a constant steam pressure of 0.35 MPa was directly established after the mold was closed, and the steam was continuously ventilated until the temperature of the thick-walled core reached the welding threshold. The key process data collected during the experiment and the performance test data of the final product are shown in Table 1.

[0037] Table 1: Comparison of Molding Process Parameters and Product Quality under Different Process Paths

[0038] Evaluation indicators Control group (constant pressure penetration) Experimental group (process of this invention) Time taken for the thick-walled core to reach the welding temperature (s) 12.0 6.5 Peak surface temperature of thin-walled structures (°C) 162.5 148.3 Thick-walled core interface welding rate (%) 92 96 Dimensional shrinkage rate (%) of thin-walled region 2.8 0.4 Surface appearance quality assessment Obvious orange peel texture, sunken smooth and flat Steam consumption per cycle (kg) 1.45 1.12

[0039] As shown in Table 1, the control group extended the high-temperature steam introduction time to ensure the thick-walled core reached the welding temperature, causing the surface temperature of the thin-walled material, which has a very low heat capacity, to soar to 162.5℃, far exceeding the overheating collapse threshold of EPP material. This led to deterioration of appearance quality and dimensional shrinkage. In contrast, the experimental group's data showed a decoupling effect: although the thick-walled core achieved a higher welding rate (96%) in only 6.5 seconds, the peak temperature of the thin-walled surface was controlled within the safe range of 148.3℃, and the gradual rise rate R... slow A condensate film is pre-placed in the thin-walled region, which acts as a thermal buffer during high-pressure impacts and lowers the temperature of the thin-walled region through flash evaporation during the pressure drop phase, thereby achieving a differentiated thermodynamic path of rapid heating of thick walls and cold protection of thin walls.

[0040] To further verify the gradual increase rate R in this invention slow To verify the rationality of this key process parameter range setting, the experiment also designed a set of gradient verifications for this parameter. Under the condition that other conditions remain unchanged, the molding effect was tested under three working conditions: a rise rate of 0.005MPa / s (below the lower limit), 0.03MPa / s (within the range), and 0.15MPa / s (above the upper limit).

[0041] Table 2: Verification data table on the influence of the gradient of the rise rate parameter on the molding effect

[0042] Slow ramp rate R_{slow} (MPa / s) Overheat suppression effect in thin-walled region Molding cycle increment (s) Process status evaluation 0.005 efficient +18.5 Production efficiency is significantly reduced, making it uneconomical. 0.03 efficient +2.5 The optimal balance between quality and efficiency 0.15 Failed (slight orange peel effect) +0.8 The pressure rises too quickly, and the water film does not form sufficiently.

[0043] The data in Table 2 clearly reveal the nonlinear influence of the gradual increase rate on the process effect. When the rate is too low (0.005 MPa / s), although a sufficient protective water film can be formed, the entire molding cycle is significantly prolonged, resulting in the loss of efficiency advantages in industrial production. When the rate is too high (0.15 MPa / s), the steam pressure rises too quickly, causing the thermal front to penetrate the thin-walled area before a sufficiently thick condensate film can be established, thus losing the ability to suppress overheating. Only within the specific range of 0.02 MPa / s to 0.05 MPa / s can the system maintain a high production cycle while utilizing the limited thermal response time window of the thin-walled area to establish an effective phase protection barrier.

[0044] Example 3: This example combines Figures 1 to 2 A description of the manufacturing process for a lightweight EPP seat, such as... Figure 1As shown, EPP beads are input, and step S1 is entered to establish an unsaturated filling state of the beads, control the filling back pressure, and reserve micro-gap as the initial steam channel. Step S2 is entered to perform a slow-pressure condensation pre-setting operation, controlling the steam to rise at a quasi-static rate, using the thermal response difference to induce the formation of a condensate film in the thin-walled area and maintain the dry gaseous state in the thick-walled area. Then, step S3 is entered to perform a high and low pressure pulse cycle, including alternating pressurization and rapid discharge stages. In the pressurization stage, the rapid rise rate impact drives the steam to penetrate the thick-walled area. In the rapid discharge stage, the pressure drop induces the thin-walled water film to flash and absorb heat to prevent overheating. Finally, step S4 is entered to close the mold, hold pressure and shape, stop the steam input, and perform final shaping and cooling, outputting the EPP lightweight seat product.

[0045] like Figure 2 As shown in the figure, the horizontal axis represents time in seconds, and the vertical axis represents temperature in °C, displaying the temperature change curves of different regions during the molding process of the experimental group and the control group. The control group's thin-wall temperature rises rapidly and maintains a high temperature plateau above 160 °C after about 3 seconds, showing an overheating trend. In contrast, the experimental group's thin-wall temperature, after an initial rise, experiences a sharp temperature drop in the range of 4 to 6 seconds due to the protection of the condensate film and flash cooling, and remains at a lower level. Meanwhile, the experimental group's thick-wall temperature rises steadily and reaches its peak in about 9 seconds.

[0046] Example 4: This example calibrates the impact of the pressure drop rate during the rapid discharge stage on the flash cooling effect of the condensate film and the final molding quality. The experiment was conducted on an EPP molding machine equipped with a high-response vacuum-assisted exhaust system. The test object was an irregularly shaped sample with an 8mm thin-walled area and a 120mm thick-walled area. Under the premise of keeping the back pressure 0.08MPa, the gradual rise rate 0.03MPa / s, and the condensation pre-pressure 0.075MPa constant, three pressure drop rate gradients were set. Gradient 1 was natural depressurization, in which the pressure dropped from 0.35MPa to 0MPa in about 2.0 seconds, with an average drop rate of about 0.175MPa / s. Gradient 2 was the controlled rapid drop preferred in this invention, in which the pressure dropped from 0.35MPa to -0.03MPa in 0.5 seconds, with an average drop rate of about 0.76MPa / s. Gradient 3 was bursting, in which the pressure dropped to -0.08MPa in 0.1 seconds using a vacuum tank, with an average drop rate of about 4.3MPa / s.

[0047] The experimental results show that under gradient one condition, the surface temperature drop in the thin-walled region is limited, and the final product surface retains traces of thermal shrinkage and exhibits significant dimensional rebound, indicating that the flash evaporation intensity is insufficient to rapidly remove sensible heat. Under gradient two condition, the surface temperature of the thin-walled region rapidly drops below 100°C during the pressure drop, resulting in high dimensional accuracy and a smooth surface in the thin-walled region of the final product, while no structural damage is observed in the thick-walled core. This confirms that this rate range triggers high-intensity flash cooling and protects structural integrity. Under gradient three condition, although cooling is rapid, pits and cracks appear on the surface of the thick-walled region, and tearing interfaces exist between the core beads, indicating that the excessively high pressure drop rate exceeds the structural strength limit of the material at high temperatures. Based on the above gradient experimental data, the key parameter window for the pressure drop operation in step S3 is determined to be: the pressure drop time is controlled between 0.3 seconds and 0.8 seconds, and the target low pressure value P. valley By controlling the pressure within the range of 0 MPa to -0.05 MPa, the system can maximize the phase change heat absorption power of the thin-walled liquid film by utilizing pressure surges, while ensuring the structural integrity of the thick-walled region.

[0048] Example 5: To ensure the stability and consistency of the process of the present invention when facing different batches of EPP raw materials or new molds for the first production run, this example discloses a standardized initial state calibration procedure. Under normal temperature conditions without steam introduction, the actual mold cavity clearance under the rated clamping force is measured using a standard feeler gauge, and the back pressure setting is finely adjusted according to the clearance value until the actual volume compression ratio after bead filling falls within the target range of 5% to 10%. Then, a stepped pressure scanning program is started, and the gradual increase rate R is set to... slow Starting from 0.01 MPa / s, the rate was gradually increased in increments of 0.005 MPa / s to 0.06 MPa / s. At each rate gradient, the heat flux density change curve was monitored in real time using a high-sensitivity heat flux sensor installed on the outer surface of the thin-walled region.

[0049] When the monitored heat flux density curve shows a characteristic latent heat release plateau and the duration of the plateau is greater than 0.5 seconds, the rate of gradual rise at that moment is recorded as the reference rate R. base If, at this reference rate, the thermocouple reading of the thick-walled core is still below 80°C at the end of the preset phase, then the R is determined to be... base This is the acceptable process setting value; if it exceeds this threshold, it indicates premature thermal penetration, requiring a reduction in the reference rate and re-execution of the scan. The determined reference rate R... base The curing process parameters for this specific mold and raw material combination will be input into the control system for subsequent automated mass production, thereby eliminating the uncertainty in molding quality caused by differences in equipment response or environmental fluctuations.

[0050] Example 6: This example describes a standardized engineering calibration procedure used to determine the key control parameter P for the rapid arrangement stage before production, based on different mold geometry characteristics and raw material batch characteristics. valley (Target low pressure value) and t drop (Pressure drop time) A representative irregular part mold was selected, which includes at least one thick-walled reference area with a wall thickness greater than 100 mm and a thin-walled reference area with a wall thickness less than 10 mm. High-frequency response temperature sensors were installed on the inner wall surfaces of these two reference areas of the mold, and a miniature pressure sensor was pre-embedded in the geometric center of the thick-walled reference area. The calibration process was carried out within a standard molding cycle, keeping other process parameters constant. By adjusting the opening of the exhaust valve of the vacuum system and the auxiliary suction power, a series of pressure drop rate gradients were constructed. For each set rate gradient, the surface temperature drop curve of the thin-walled reference area and the pressure decay curve of the thick-walled core were recorded and analyzed in real time. Two critical points were determined through data analysis: the first critical point... The first critical point is the pressure drop rate at which the temperature drop rate in the thin-walled region reaches its maximum value, corresponding to the peak value of the heat absorption power of the condensate film flash evaporation. The second critical point is the pressure drop rate at which the pressure sensor in the thick-walled core detects a pressure fluctuation exceeding a preset safety threshold, such as 0.02 MPa. This point indicates that the internal microporous structure begins to show a risk of mechanical damage due to rapid gas expansion. Finally, the pressure drop rate range between the above two critical points is defined as the safe and efficient window for the combination of mold and raw materials. An intermediate value is selected as the standard production parameter, and the corresponding exhaust valve opening curve and vacuum pump start-up sequence are set to ensure that the thin-walled region obtains maximum cooling effect while preventing structural damage to the thick-walled core.

[0051] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A manufacturing process for an EPP lightweight seat, characterized in that, Includes the following steps: Step S1: Fill the mold cavity with EPP beads and establish an unsaturated filling state; Step S2: Perform the slow-pressure condensation pre-setting operation; control the steam flow rate into the mold cavity so that the pressure inside the mold cavity increases at a slow rate R. slow Rise to the preset condensation pressure; utilizing the physical property that the heat absorption response rate of the thin-walled region of the EPP irregular part is faster than that of the thick-walled region, at a slow rising rate R slow During the continuous pressurization process, the surface temperature of the thin-walled region is preferentially increased to the saturated dew point corresponding to the quasi-static pressurization process and condensation of water film is induced. At the same time, the thick-walled region is kept in a dry steam environment, thereby forming a gas-liquid two-phase distribution in the mold cavity where the thin-walled region is wet and the thick-walled region is dry. Step S3: Perform at least one high-low pressure pulse cycle based on the gas-liquid two-phase distribution; the high-low pressure pulse cycle includes alternating pressurization and rapid discharge phases; during the pressurization phase, control the steam to rise at a rapid rate R. fast Entering the mold cavity, the rapid rise rate R fast Set to be greater than the ramp rate R slow More than three times that of the previous method, the steam penetration force generated by rapid pressure increase is used to overcome flow resistance and enter the gap between beads in the thick-walled area; in the rapid discharge stage, the steam input is cut off and the exhaust is opened, so that the pressure in the mold cavity is rapidly reduced to the low pressure value that induces flash evaporation of the condensate film, and the heat of the thin-walled area is carried away by the vaporization of water. Step S4: Stop steam input and use mold closure and pressure holding for shaping.

2. The manufacturing process of an EPP lightweight seat according to claim 1, characterized in that, In step S2, the gradual increase rate R slow The pressure is controlled to be between 0.02 MPa / s and 0.05 MPa / s; the condensing pressure is controlled to be between 0.06 MPa and 0.08 MPa; in step S3, the rate of increase R... fast The pressure should be controlled to be greater than 0.2 MPa / s.

3. The manufacturing process for an EPP lightweight seat according to claim 1, characterized in that, In the high and low pressure pulse cycle of step S3, after the rapid discharge stage and before the next pressurization stage, there is also a gap recovery stage; in the gap recovery stage, steam is controlled to be introduced into the mold cavity, so that the pressure in the mold cavity rises back to the recovery pressure and is maintained for the recovery time; wherein, the recovery pressure is greater than the low pressure value and less than the peak pressure of the pressurization stage; the compression effect generated by the recovery pressure overcomes the rebound expansion of the EPP beads at the instantaneous temperature in the mold cavity, and reopens the steam flow channel between the EPP beads.

4. The manufacturing process for an EPP lightweight seat according to claim 3, characterized in that, The recovery pressure is set to 30% to 50% of the peak pressure during the pressurization phase; the recovery time is set to 0.5 to 1.0 seconds; the recovery time allows steam to fill the steam flow channel without causing substantial adhesion to the EPP bead surface.

5. The manufacturing process of an EPP lightweight seat according to claim 1, characterized in that, The process also includes an adaptive adjustment step based on the characteristics of exhaust pressure drop: in the rapid exhaust stage of step S3, the pressure drop rate at the exhaust port of the mold cavity is monitored using a pressure sensor; the pressure drop rate is compared with a preset reference rate range. If the pressure drop rate is higher than the upper limit of the reference rate range, extend the pressure ramp-up time in step S2 of the next molding cycle or increase the number of high and low pressure pulse cycles in step S3. If the pressure drop rate is below the lower limit of the reference rate range, reduce the number of high and low pressure pulse cycles in step S3 of the next molding cycle.

6. The manufacturing process of an EPP lightweight seat according to claim 1, characterized in that, After step S2 and before step S3, a local pre-cooling resistance-increasing step is performed: cooling water is introduced into the cooling circuit in the mold corresponding to the thin-walled area of ​​the EPP irregular part for a preset time to harden the EPP beads on the surface of the thin-walled area to increase the steam permeation resistance; taking advantage of the difference in steam permeation resistance, during the pressurization stage of step S3, the steam is guided to flow preferentially to the thick-walled area of ​​the EPP irregular part with relatively low permeation resistance.

7. The manufacturing process for an EPP lightweight seat according to claim 1, characterized in that, In step S2, the surface temperature of the inner wall of the mold cavity is monitored; when the difference between the surface temperature and the saturation temperature of the introduced steam is less than a preset temperature difference value, the gradual increase rate R is terminated. slow The pressurization process is then transferred to step S3.

8. The manufacturing process for an EPP lightweight seat according to claim 3, characterized in that, The setpoint for the recovery pressure follows a dynamic setting rule based on the material's springback characteristics: P restore =α⋅(P peak -P valley )+P valley , where P restore To restore pressure, P peak P is the peak pressure during the pressurization phase. valley This is the low pressure value during the rapid discharge phase. The preset elastic recovery coefficient is related to the density of EPP material, and its value ranges from 0.3 to 0.

6.

9. The manufacturing process of an EPP lightweight seat according to claim 1, characterized in that, The low pressure value is controlled as a negative pressure between 0MPa and -0.05MPa.

10. The manufacturing process of an EPP lightweight seat according to claim 1, characterized in that, In step S1, the back pressure of the mold cavity is set to 0.05MPa to 0.1MPa, and the gaps between the EPP beads are kept in a connected state after filling, serving as the initial channel for steam flow in step S2.

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