Thermoplastic Molding Process and Shrinkage: A Detailed Overview

The molding process of thermoplastic plastics is a complex procedure influenced by a multitude of factors that ultimately determine the final dimensions and properties of the molded part. One of the most critical aspects to consider is shrinkage, the tendency of the plastic to reduce in size as it cools and solidifies. Understanding the factors contributing to shrinkage is crucial for achieving accurate and consistent parts.

1. Inherent Characteristics of Thermoplastics:
Thermoplastics, due to their molecular structure and behavior during processing, exhibit several inherent characteristics that significantly influence shrinkage:
Crystallization: Many thermoplastics undergo a degree of crystallization during cooling. This process rearranges the polymer molecules into a more ordered structure, leading to a change in volume and contributing to shrinkage. The extent of crystallization is influenced by factors like cooling rate and mold temperature.
Internal Stresses: The molding process can induce internal stresses within the plastic part due to uneven cooling, molecular orientation, and flow restrictions. These stresses can be “frozen in” as the part solidifies, contributing to dimensional instability and shrinkage variation.
Residual Stresses: These are stresses that remain in the molded part after it has been ejected from the mold. They can be caused by a variety of factors, including uneven cooling, differential shrinkage across the part, and the mold design itself.
Molecular Orientation: During the injection process, the polymer molecules tend to align in the direction of flow. This molecular orientation can lead to anisotropic shrinkage, meaning the part will shrink differently in different directions.

Due to these factors, thermoplastics generally exhibit larger and more variable shrinkage rates compared to thermosetting plastics. The shrinkage often displays a noticeable directionality, meaning it’s not uniform in all directions. Furthermore, post-molding shrinkage (occurring after the part is ejected from the mold), as well as shrinkage after annealing (heat treatment to relieve internal stresses) or conditioning (exposure to specific environmental conditions), is typically greater in thermoplastics.

2. Molding Process Factors:
The specific conditions and parameters of the molding process itself play a significant role in determining the degree and directionality of shrinkage:
Cooling Rate and Density Gradient: When the molten plastic comes into contact with the cooler mold cavity surface, the outer layer迅速 cools, forming a solid shell with a relatively low density. Due to the plastic’s inherent poor thermal conductivity, the inner layer of the part cools much more slowly, resulting in a solid layer with a higher density and greater shrinkage. This creates a density gradient within the part. Consequently, parts with thicker walls, which cool more slowly, and have a thicker high-density layer at the core, tend to experience greater overall shrinkage.
Gate Design (Feed System): The form, size, and distribution of the gate (the channel through which the molten plastic enters the mold cavity) directly impact several key factors:

Melt Flow Direction: The gate design dictates the path of the melt as it fills the cavity.
Density Distribution: It influences how the polymer molecules are distributed and oriented within the part.
Packing Pressure and Shrinkage Compensation: The gate design affects the ability to apply packing pressure (additional pressure applied to the melt during the holding phase) to compensate for shrinkage.

Molding Cycle Time: The gate design can influence the overall time required for the molding cycle.

Specific gate characteristics have the following effects:
Direct Gate or Large Cross-Sectional Area: A direct gate (one that delivers the melt directly to the cavity) or a gate with a large cross-sectional area, especially a thicker cross-section, results in less shrinkage but greater directionality (more pronounced shrinkage in the flow direction).
Wider Gate with Shorter Land Length: A wider gate with a shorter land length (the length of the gate) exhibits less directionality.
Proximity to the Gate and Flow Direction: Areas closer to the gate or parallel to the melt flow direction tend to experience greater shrinkage due to the higher pressure and faster cooling in those regions.

Mold Temperature: A high mold temperature results in slower cooling of the melt, leading to a higher density in the core of the part and consequently, greater shrinkage. This effect is particularly pronounced in crystalline materials, as a higher mold temperature promotes greater crystallinity, resulting in larger volume changes and even more significant shrinkage. Mold temperature distribution also plays a critical role. It affects the cooling and density uniformity within and outside the molded part, directly influencing the magnitude and directionality of shrinkage in different areas of the part.

Holding Pressure and Time: The holding pressure (pressure applied to the melt during the holding phase) and the holding time (duration of the holding pressure) significantly impact shrinkage:
Higher pressure and longer holding time generally result in less shrinkage due to improved packing and compensation for the material contraction.

However, this can also lead to greater directionality as the molecular orientation induced by the higher pressure can become more pronounced.
Injection Pressure: High injection pressure results in a lower viscosity of the melt, which reduces interlayer shear stress (the friction between different layers of the melt). This can lead to greater elastic recovery (the tendency of the part to “spring back” slightly after being ejected from the mold) and can also contribute to a slight reduction in shrinkage. A high melt temperature increases shrinkage but generally reduces directionality due to reduced molecular orientation.

3. Compensation During Mold Design:
To account for the anticipated shrinkage, mold designers must carefully consider several factors during the mold design phase:

Plastic Shrinkage Range: The inherent shrinkage range of the specific plastic material being used.
Part Geometry: The wall thickness and overall shape of the molded part.

Gate Design: The form, size, and distribution of the gates, as discussed earlier.

Based on experience and empirical data, the designer estimates the shrinkage rate for each part of the molded product. These shrinkage rates are then used to calculate the cavity dimensions. The cavity is intentionally made larger than the desired final part dimensions to compensate for the expected shrinkage during cooling.

Controlling shrinkage in thermoplastic molding is a complex challenge that requires a thorough understanding of material properties, process parameters, and mold design principles. By carefully optimizing these factors, manufacturers can produce molded parts with the desired dimensions, properties, and consistency. The information provided above offers a comprehensive overview of the factors influencing shrinkage, providing a solid foundation for further study and practical application in the field of injection molding.