Ethan Clarke
Plastic shrinkage is a crucial property of plastics that directly impacts product molding. It refers to the percentage decrease in dimensions from the initial, uncooled size to the cooled, room-temperature size. This phenomenon is not solely due to thermal expansion and contraction but also related to various molding factors, leading to the term mold shrinkage. The dimensional change occurs as the injected material cools, affecting the final shape of the molded part. Optimizing designs, material selection, and cost efficiency are all influenced by understanding shrinkage rates, which vary depending on the plastic type, composition, moisture absorption, mold temperature, and wall thickness.
| Characteristics | Values |
|---|---|
| Definition | Plastic shrinkage refers to the percentage decrease in dimensions from the initial, uncooled size to the cooled, room temperature size. |
| Factors Affecting Shrinkage | Material type, composition, moisture absorption, mold temperature, thickness, molding processes, and environmental conditions. |
| Impact | Shrinkage affects part dimensional accuracy, assembly precision, fitting performance, surface irregularities, product aesthetics, texture, and production costs. |
| Standard Measurement Methods | ASTM D955, ISO 294-4 (for thermoplastics), ISO 2577 (for thermosets), and ISO 294-3 (thermoplastic shrinkage). |
| Units | Thousandths of an inch per linear inch (0.00X /in/in). |
| Typical Shrink Rates | Vary between 0.001/in/in and 0.020/in/in, with an average of around 0.006/in/in. |
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What You'll Learn
- Plastic shrinkage is the percentage decrease in dimensions from the initial, uncooled size to the cooled, room-temperature size
- The shrinkage of plastics depends on factors such as material type, composition, moisture absorption, and mould temperature
- The direction fibres align in can impact shrinkage, with less shrinkage in one direction than the other
- Plastic injection-moulded parts contract during the cooling process, this is called shrinkage
- Plastic shrinkage can be calculated using internationally recognised approaches, including ISO 294-4 and ASTM D955
Plastic shrinkage is the percentage decrease in dimensions from the initial, uncooled size to the cooled, room-temperature size
Plastic shrinkage is a crucial property of plastics, referring to the percentage decrease in dimensions from the initial, uncooled size to the cooled, room-temperature size. It is an important consideration in injection moulding, where designers must account for the shrinkage and contraction rate of the injected material and how it will affect the final product's shape.
Plastic shrinkage is influenced by various factors, including material type, composition, moisture absorption, mould temperature, and wall thickness. For example, crystalline plastics typically exhibit greater shrinkage than amorphous plastics. The direction in which fibres align within the plastic can also impact the shrinkage rate, with the potential for varying degrees of shrinkage in different directions.
Standard methods for measuring plastic shrinkage include ASTM D955 and international standards such as ISO 294-4 for thermoplastics and ISO 2577 for thermosets. The shrinkage rate is typically expressed as thousandths of an inch per linear inch (0.00X /in/in), with average rates around 0.006/in/in. It can also be stated as a percentage.
Understanding shrinkage is essential for optimising designs, selecting suitable materials, and ensuring cost efficiency in production. By anticipating size changes due to shrinkage, designers can create precise and consistent products. Additionally, addressing shrinkage issues early can reduce waste, rework, and delays, leading to shorter development cycles and faster product launches.
The moulder can fine-tune the shrinkage rate by adjusting the material's density or extending the cooling period within the mould. Test shots are recommended before injecting parts with large dimensions, tight tolerances, or new materials to ensure that the final product meets the desired specifications.
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The shrinkage of plastics depends on factors such as material type, composition, moisture absorption, and mould temperature
The shrinkage factor of a plastic material is influenced by several factors, primarily the type of plastic, its composition, moisture absorption, and mould temperature.
Material Type
The type of plastic is a significant factor in determining the shrinkage rate. Semi-crystalline polymers generally exhibit higher shrinkage compared to amorphous polymers. The inherent characteristics of the polymer, such as molecular weight and the presence of fillers or fibres, also influence the degree of shrinkage. For instance, resins with higher molecular weight tend to have higher viscosity and pressure drop during filling, resulting in increased shrinkage if not compensated by higher packaging pressure.
Composition
The composition of the plastic material plays a crucial role in shrinkage. The addition of pigments, fillers, and fibres can significantly impact the final shrinkage. Pigments act as nucleating agents, promoting crystallization and increasing shrinkage, particularly in semi-crystalline polymers. Fillers and fibres, commonly used to modify plastic properties, tend to shrink significantly during cooling due to their low coefficient of thermal expansion.
Moisture Absorption
Moisture absorption can affect the shrinkage of plastics. While this is more commonly associated with concrete, where moisture loss leads to contraction and shrinkage, it is also a factor in plastics. For example, recycled sand mortar, which has a higher moisture absorption capacity, exhibits increased plastic shrinkage values.
Mould Temperature
The mould temperature and processing conditions, including cooling rate, are critical factors in plastic shrinkage. Most of the shrinkage occurs while the plastic is cooling in the mould, and it continues to a lesser extent after ejection. The rate of dimensional change in plastics is influenced by the stress level and the temperature at which the plastic is held under stress. Therefore, accurate prediction and control of mould temperature are essential to achieving the desired dimensions of the final product.
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The direction fibres align in can impact shrinkage, with less shrinkage in one direction than the other
Plastic shrinkage is the reduction in a plastic part's dimensions as it cools from its molten processing temperature to room temperature. This is due to the polymer chains packing more closely together upon cooling and solidification. The amount of shrinkage varies depending on the type of plastic, with semi-crystalline polymers shrinking more than amorphous ones. Factors like mould design, processing conditions (temperature, pressure, cooling rate), and part geometry also influence the final shrinkage.
The direction in which fibres align can also become a factor, as shrinking may occur less in one direction than in the other. Filler and fibre reinforcements in composites result in shrinkage that varies from virgin polymers. Most fillers and fibres have a relatively low coefficient of thermal expansion, so they tend to shrink significantly when a part cools down during processing. The reduction in shrinkage is approximately proportional to their concentration.
Polymers filled with long glass fibres shrink less along the direction in which the fibres align. This shrinkage occurs in the flow direction compared to the transverse direction. Recycled fibre-reinforced polymers exhibit different mould-shrinkage characteristics than those of virgin resin.
The more the polymer molecules are oriented, the more shrinkage will occur in that direction. This can be observed in the difference between transverse and machine direction shrinkage, indicating how much unbalanced orientation there is in the extruded part. This unbalanced stress can cause parts to crack or become misshapen. Orientation is seldom uniform across the part cross-section due to varying drawdown, shear stress, and cooling rates.
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Plastic injection-moulded parts contract during the cooling process, this is called shrinkage
Plastic injection-moulded parts contract during the cooling process, and this phenomenon is called shrinkage. It is a crucial property of plastics that directly impacts product moulding. Shrinkage is the percentage decrease in dimensions from the initial, uncooled size to the final, cooled size. It is not only due to thermal expansion and contraction but also related to various moulding factors, hence the term "moulding shrinkage".
The extent of plastic shrinkage depends on factors such as material type, composition, moisture absorption, and mould temperature. For example, crystalline plastics typically exhibit greater shrinkage than amorphous plastics. Shrinkage also varies with wall thickness. The material supplier will usually provide a data sheet specifying a shrinking range for different wall thicknesses. This information is critical for designers to consider during the tooling design stage to ensure the final parts meet the model specifications.
Shrinkage rates can be fine-tuned by adjusting the density of the material. This can be achieved by manipulating how hard the cavity is packed or by extending the cooling period in the mould. Understanding shrinkage is essential for product designers to anticipate size changes during production and optimise designs for precise, consistent results. It also aids in selecting suitable materials based on design requirements and expediting product launches.
Additionally, addressing shrinkage issues early can enhance cost efficiency by reducing waste, rework, and delays in production processes. Shrinkage can also impact product performance and appearance. For instance, in the automotive industry, shrinkage can affect the smooth operation of components like doors and windows, influencing overall vehicle performance and safety.
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Plastic shrinkage can be calculated using internationally recognised approaches, including ISO 294-4 and ASTM D955
Plastic shrinkage refers to the contraction of moulded plastic compared to the mould cavity. This is influenced by factors such as mould and melt temperature, fill times, and packing conditions, and the alignment of fibres in the material. Several internationally recognised methods are used to calculate plastic shrinkage, including ISO 294-4 and ASTM D955.
ISO 294-4 is one of the most internationally recognised approaches for determining the shrinkage of plastic parts, particularly thermoplastic shrinkage. It is often used in conjunction with other standards, such as ISO 294-3 and the appropriate material specifications, to improve the reproducibility of the test.
ASTM D955 is a standard test method for measuring shrinkage in thermoplastics. It quantifies the contraction in moulded articles compared to the mould cavity used in compression or injection moulding processes under specified conditions. ASTM D955 is used to adjust injection moulds to achieve the desired final dimensions of the plastic part. This standard is part of a broader set of ASTM standards that guide testing and analysis for polymers, plastics, ceramics, metals, alloys, and paints and coatings.
The calculation of plastic shrinkage using ASTM D955 is presented as inches per inch, percent, or mm per mm for the stated dimensions and conditions. This standard focuses on measuring the difference between the mould cavity dimensions and the moulded specimen dimensions, which can vary based on the mould design and moulding process, such as injection or compression moulding.
By employing these internationally recognised approaches, manufacturers and researchers can accurately determine the shrinkage characteristics of plastic materials. This knowledge is essential for predicting and controlling the final dimensions of plastic parts, ensuring they meet the required specifications and function as intended.
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Frequently asked questions
The shrinkage factor in plastic materials refers to the percentage decrease in dimensions from the initial, uncooled size to the cooled, room temperature size.
Plastic shrinkage occurs due to the polymer chains packing more closely together upon cooling and solidification. The amount of shrinkage depends on factors such as material type, composition, moisture absorption, and mould temperature.
Understanding shrinkage is vital for product designers as it allows them to anticipate size changes during production and optimise designs for precise, consistent results. It also aids in selecting suitable materials based on design requirements.
Plastic shrinkage is calculated by measuring the dimensional change of the injection-moulded object as it cools. Standard methods for measuring shrinkage include ASTM D955, ISO 294-4, and ISO 2577.
Plastic injection moulding shrinkage varies with wall thickness. The material supplier will provide a data sheet specifying a shrinking range for different wall thicknesses. The moulder can fine-tune shrinkage by adjusting the density of the material or extending the cooling period.
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