The reason why the shrinkage problem of hard plastic parts is more difficult to solve than that of soft plastic parts
Shrinkage in injection molded parts is caused by uneven volumetric shrinkage during melt cooling, manifesting as surface depressions or internal shrinkage cavities, impacting both the product’s appearance and mechanical properties. Practice has shown that shrinkage in rigid plastics (such as PC, PS, and ABS) is more challenging to address than in flexible plastics (such as PE, PP, and TPU), resulting in scrap rates 20%-30% higher. This difference stems from the differences in the physical properties, shrinkage mechanisms, and molding processes between the two materials. The high rigidity, low elasticity, and complex shrinkage behavior of rigid plastics make shrinkage more challenging to control. For example, shrinkage in rigid PC lenses can lead to a loss of surface flatness and affect light transmittance. In contrast, minor shrinkage in flexible PE tubing generally does not affect performance and can be mitigated through post-processing correction. In-depth analysis of the causes of shrinkage in rigid plastics is crucial for developing targeted solutions.
The fundamental reason for the varying degrees of difficulty associated with shrinkage issues lies in the differing shrinkage characteristics of rigid and flexible plastics. While rigid plastics typically have a lower shrinkage rate (0.5%-1.5%), they exhibit significant shrinkage nonuniformity, particularly at locations with varying wall thicknesses, which can easily lead to localized shrinkage. For example, the average shrinkage rate of ABS plastic parts is 0.8%, but where the wall thickness increases dramatically from 2mm to 5mm, the shrinkage rate can rise to 1.2%, resulting in noticeable shrinkage. Flexible plastics, on the other hand, have a higher shrinkage rate (1.5%-4.0%), but shrinkage is more uniform. Furthermore, due to their excellent elasticity, even minor shrinkage differences can be compensated for by their inherent elasticity, minimizing visible shrinkage. For example, PP has a shrinkage rate of 2.0%-3.0%, but due to its softness, differential shrinkage at varying wall thicknesses can be absorbed by elastic deformation, resulting in less noticeable surface shrinkage. Furthermore, the high glass transition temperature of rigid plastics restricts molecular chain movement during cooling, making any uneven shrinkage difficult to correct through subsequent shrinkage. However, flexible plastics maintain a certain degree of flexibility at room temperature, allowing their molecular chains to adjust slowly, minimizing shrinkage.
The high rigidity of rigid plastic parts makes them highly resistant to shrinkage stress, making shrinkage difficult to correct with external forces. After molding, rigid plastics exhibit high rigidity and elastic modulus (e.g., 2.4 GPa for PC and 2.2 GPa for ABS). Once shrinkage causes surface depression, it’s difficult to restore it to a flat surface through external pressure (e.g., holding pressure). For example, if the shrinkage depression in a PC part exceeds 0.1mm, even extended holding times won’t eliminate it because the material’s rigidity hinders the flow of the melt. Soft plastics, on the other hand, have a low elastic modulus (0.8 GPa for PE and 0.1-0.5 GPa for TPU). During the holding phase, the material retains a certain degree of fluidity and flexibility. External pressure can encourage the melt to flow into the shrinkage area, reducing shrinkage. For example, in a PE part, under holding pressure, the molecular chains rearrange, filling the shrinkage space and reducing the shrinkage depth from 0.2mm to below 0.05mm. In addition, hard plastic parts shrink less after demolding. Once shrinkage occurs during molding, it cannot be improved by natural shrinkage later. However, soft plastic parts still have 5%-10% post-shrinkage after demolding, which can partially offset the shrinkage during molding.
The molding process window for rigid plastic parts is narrow, and parameter fluctuations can easily lead to shrinkage. Rigid plastics have a narrow molding temperature range (for example, PC’s molding temperature is 260-300°C, with a temperature difference of only 40°C). Temperature fluctuations exceeding ±5°C can affect melt fluidity, leading to insufficient filling or uneven shrinkage. For example, when PC material temperature falls below 260°C, melt viscosity increases sharply, slowing filling speed and easily causing shrinkage in thick walls. Temperatures above 300°C can cause material degradation, generate bubbles, and exacerbate shrinkage. Soft plastics, on the other hand, have a wide molding temperature range (PE’s is 160-220°C, with a temperature difference of 60°C), and parameter fluctuations have a lesser impact on shrinkage. For example, a ±10°C fluctuation in PE material temperature only changes shrinkage by 0.2%-0.3%. Injection pressure has a more significant impact on the shrinkage of hard plastic parts. Due to the high viscosity of hard plastic melt, higher pressure (80-120MPa) is required to ensure filling and holding effects. Pressure fluctuations exceeding ±5MPa may cause shrinkage. Soft plastics, on the other hand, require lower pressure (40-80MPa), so the impact of pressure fluctuations is relatively small.
Rigid plastic parts are highly sensitive to wall thickness, and variations in wall thickness can easily cause shrinkage. Rigid plastics respond more strongly to variations in wall thickness. When a part has uneven wall thickness (a difference exceeding 2x), the thicker parts cool more slowly, shrink more, and are more susceptible to shrinkage. Thinner parts have already cooled and solidified, preventing the melt from replenishing. For example, when the wall thickness of a PC part increases from 2mm to 6mm (a 3x difference), the thicker parts can experience shrinkage of up to 1.5%, resulting in noticeable concavity. Due to the softness of soft plastics, any shrinkage caused by thickness variations can be mitigated by elastic deformation, resulting in less noticeable surface shrinkage. For example, when the wall thickness of a PP part increases from 2mm to 6mm, the shrinkage in the thicker parts can be absorbed by slight deformation, resulting in a surface concavity of only 0.05-0.1mm, which is generally within the acceptable range. Furthermore, rigid plastic parts with ribs, bosses, and other structures are more susceptible to shrinkage due to the significant difference in thickness between these areas and the main body. Ribs in soft plastic parts, on the other hand, can reduce shrinkage through elastic deformation.
The rapid cooling rate of rigid plastic parts and the short melt replenishment time exacerbate shrinkage issues. Rigid plastics have low thermal conductivity ( PC : 0.2W/(m・K) and ABS : 0.18W/(m・K) ), but their high glass transition temperature leads to rapid solidification during cooling. This shortens the melt’s flow time within the mold cavity, limiting the time available to replenish shrinkage during the holding phase. For example, the effective holding time for PC parts is only 3-5 seconds. Failure to adequately replenish the melt within this time period can lead to shrinkage. Soft plastics have higher thermal conductivity ( PE : 0.4W/(m・K) and PP : 0.22W/(m・K) ), but their melting point is lower, leading to slower cooling. This allows for a longer holding time of 8-15 seconds, allowing more time for melt replenishment and minimizing shrinkage. For example, the holding time for PE parts can be set to 10 seconds, which is sufficient to address most shrinkage requirements. In addition, the mold temperature of hard plastics is usually higher (80-100℃ for PC), but the cooling water channel design is difficult, which can easily lead to local uneven cooling. However, the mold temperature of soft plastics is low (20-40℃ for PE), cooling is easier to control, and shrinkage is more uniform.
