In precision mold manufacturing, molding shrinkage is a core factor affecting product dimensional accuracy. Its fluctuations directly lead to dimensional deviations between the plastic part and the mold cavity, thus impacting assembly accuracy and functional achievement. Mold structure optimization, as a key means of reducing shrinkage, requires a comprehensive approach encompassing multiple dimensions, including the runner system, gate design, cooling system, cavity layout, mold materials, venting structure, and insert design.
Runner system optimization is a fundamental step in reducing shrinkage. Traditional cold runner systems, due to the melt cooling and solidifying within the runner, experience pressure attenuation at the filling end, easily leading to uneven shrinkage. Hot runner systems, on the other hand, maintain a constant melt temperature through heating devices, ensuring the melt fills the cavity in a stable flow state and reducing shrinkage differences caused by temperature gradients. Furthermore, shortening the runner length and optimizing the runner cross-sectional shape (such as using trapezoidal or semi-circular runners) can reduce melt flow resistance, resulting in more uniform pressure transmission and thus suppressing shrinkage fluctuations.
Gate design directly affects the melt filling pattern and pressure holding effect. Large plastic parts are best suited for multi-point gates or fan-shaped gates. This disperses melt inlet pressure, preventing uneven shrinkage caused by localized stress concentration. Smaller, precision parts are better suited for point gates or submarine gates, which shorten holding time and reduce residual stress at the gate. The gate size must match the part's wall thickness; too small a gate will result in insufficient filling, while too large a gate may cause jetting marks and orientation shrinkage. Optimizing the gate location and number using simulation software can achieve balanced melt filling and significantly reduce anisotropic shrinkage.
The uniformity of the cooling system is crucial for shrinkage control. Uneven mold temperature distribution leads to different cooling rates in different areas of the part, resulting in uneven shrinkage and warpage. Optimizing the cooling system should follow the principle of "strong cooling near the gate, slow cooling further away from the gate," minimizing the temperature gradient on the cavity surface by adding cooling water channels, using conformal cooling channels, or irregularly shaped cooling pipes. Furthermore, using mold materials with high thermal conductivity (such as beryllium copper) or embedded heat pipe technology can accelerate heat conduction, shorten the molding cycle, and improve dimensional stability.
Cavity layout and parting surface design must consider both the plastic part's structure and shrinkage characteristics. For plastic parts with complex shapes and uneven wall thickness, insert structures can decompose the cavity into multiple independent units, improving overall precision by controlling the shrinkage rate of each unit. The parting surface should be chosen to avoid areas of weak strength in the plastic part, and the draft angle and shrinkage compensation direction must be considered to ensure uniform release of shrinkage stress during demolding. For long, strip-shaped plastic parts, rotary demolding or side-pulling mechanisms can reduce deformation caused by shrinkage.
The coefficient of thermal expansion and rigidity of the mold material directly affect the shrinkage compensation effect. Using materials with a low coefficient of thermal expansion (such as H13 steel) can reduce dimensional changes in the mold due to temperature fluctuations, thereby reducing the impact on the shrinkage rate of the plastic part. Enhancing mold rigidity (such as increasing wall thickness and optimizing the layout of reinforcing ribs) can suppress elastic deformation during molding, ensuring effective pressure transmission to all parts of the plastic part during the holding pressure stage and reducing shrinkage differences. For precision molds, high-rigidity materials such as ceramics or hard alloys can be used to further improve dimensional stability.
Optimized venting structures can prevent localized high temperatures and abnormal shrinkage caused by gas compression. If gas within the mold cavity cannot be expelled in time, it will be compressed during melt filling, forming high-temperature zones that accelerate cooling of the plastic part in those areas and cause uneven shrinkage. Venting grooves (typically 0.02-0.05mm deep) at the parting line, core edges, or deep ribs can effectively expel gas, ensuring smooth melt filling. For transparent or high-gloss plastic parts, a vacuum venting system can further eliminate air bubbles, improving surface quality and dimensional accuracy.
The design of inserts and metal parts must consider shrinkage rate matching. When a metal part is embedded within a plastic part, if the difference in shrinkage rates is too large, uneven shrinkage can easily lead to stress cracking or delamination. Optimization measures include: preheating the metal part to near the mold temperature to reduce shrinkage differences caused by temperature variations; designing anti-slip textures or undercut structures on the insert surface to enhance mechanical interlocking force; and using an elastomer buffer layer to absorb shrinkage stress. For high-precision assemblies, simulation analysis is needed to predict the shrinkage synergy between the insert and the plastic part to ensure that the final dimensions meet assembly requirements.
