Alexei Volkov
Plastic mould shrinkage is the contraction of a plastic moulding as it cools after injection. This shrinkage is influenced by factors such as the type of plastic, the mould design, cooling rate, and part shape. The calculation of plastic shrinkage is crucial for ensuring that moulded parts meet desired specifications and fit their intended applications. The standard method for measuring plastic shrinkage is ASTM D955, and there are also internationally recognised approaches such as ISO 294-4 and ISO 2577. To calculate plastic shrinkage, one must determine the original length of the plastic and the shrinkage percentage, which can be influenced by various factors, before using a specific formula to calculate the final length.
| Characteristics | Values |
|---|---|
| Plastic shrinkage rate definition | Dimensional reduction percentage of a molded plastic part as it cools from its molten state to room temperature |
| Plastic shrinkage rate formula | S = (D-M)/D or D=M/(1-S) where S = shrinkage rate, D = mold size at 23 °C ± 2 K, and M = corresponding part size 16-24 hours after molding, at 23 °C ± 2 K and 50% ± 10% air humidity |
| Plastic shrinkage rate calculation | Subtract the product of the original length and the shrinkage percentage divided by 100 from the original length |
| Plastic shrinkage rate units | Thousandths of an inch per linear inch (0.00X /in/in) |
| Typical plastic shrinkage rate range | Between 0.001/in/in and .020/in/in |
| Average plastic shrinkage rate | Around 0.006/in/in |
| Factors influencing plastic shrinkage rate | Type of plastic, molding process, cooling rate, wall thickness, shape, type, size and position of the gate, molding factors, cycle times, temperatures, mould design, part complexity, direction in which fibres align, and more |
| Plastic shrinkage rate standards | ASTM D955, ISO 294-4 (for thermoplastics), ISO 2577 (for thermosets), ISO 294-3 (thermoplastic shrinkage), German national standard DIN16901 |
| Plastic shrinkage rate charts | Available for reference, provided by material suppliers |
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What You'll Learn
The impact of multiple factors
Plastic shrinkage is a complex process influenced by various factors, making it challenging to calculate with complete accuracy. Firstly, different types of plastics exhibit varying shrinkage rates due to their unique chemical compositions. For instance, thermoplastic energy plastics with 40% reinforced PPS have a lower shrinkage rate compared to other plastics. Additionally, the crystallinity of the resin plays a crucial role, with smaller crystallinity resulting in a reduced shrinkage rate.
The molecular weight of the plastic also affects shrinkage; larger molecular weights tend to have smaller shrinkage rates, while smaller molecular weights exhibit higher shrinkage rates. This is particularly evident in PP resin, which has a shrinkage rate ranging from 1.8% to 2.5%, making it challenging to create high-precision products. Furthermore, the direction of fibre alignment influences shrinkage rates. Plastics reinforced with long glass fibres tend to shrink more in the cross (transverse) direction than in the longitudinal (flow) direction.
The shape and complexity of the plastic part also come into play. Plastics with complex geometries, such as holes or flow fronts meeting at different angles, can be extremely difficult to model accurately for shrinkage calculations. The wall thickness of the plastic part is another critical factor. Thicker walls generally result in higher shrinkage rates, as evident in the provided example of a 0.100-inch wall thickness yielding a shrinkage rate of 0.005-0.007/in/in.
Additionally, the mould design and structure can impact shrinkage rates. The size, shape, and position of the gate in the mould can all influence the final shrinkage rate. It is worth noting that some plastics exhibit asymmetrical shrinkage characteristics, with different shrinkage rates along the X and Y axes. This poses a challenge for mould designers, especially when creating parts with complex geometries.
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Calculating the final length
To calculate the final length, you will need to first determine the original length of the plastic (OL) in millimetres. This is the intended length of the plastic product before any shrinkage occurs. Once you have the original length, you can determine the shrinkage percentage (SP) that the specific type of plastic will undergo. The shrinkage percentage is typically provided by the material supplier in a data sheet, and it represents the expected reduction in the plastic's dimensions due to cooling and solidification.
With the original length and shrinkage percentage in hand, you can use the following formula to calculate the final length (FL):
Final Length (FL) = Original Length (OL) – (Original Length (OL) * (Shrinkage Percentage (SP) / 100))
By substituting the values into the formula, you can calculate the expected final length of the plastic product after it has undergone shrinkage. This calculation is essential to ensure that the manufactured parts meet the desired specifications and fit properly in their intended applications.
It is important to note that plastic shrinkage rates vary depending on several factors. Different types of plastics have different shrinkage characteristics. For example, semi-crystalline polymers (such as PBT or PP) generally exhibit higher shrinkage rates than amorphous polymers (such as PS or PC). Additionally, the direction of fibres within the plastic can impact shrinkage, with the potential for greater shrinkage in one direction compared to another.
Moreover, the shape and thickness of the plastic part play a role in shrinkage rates. Plastics with complex geometries, such as those with holes or flow fronts meeting at different angles, may be challenging to model accurately. In such cases, test moulds or utilising obsolete moulds of similar size can be employed to observe plastic shrinkage firsthand and make necessary adjustments.
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Asymmetrical shrinkage characteristics
Plastic injection-moulded parts contract during the cooling process, and this phenomenon is called shrinkage. The shrinkage rate of plastic is the dimensional reduction percentage of a moulded plastic part as it cools from its molten state to room temperature. The shrinkage rate (S) is the relative difference between the mould size (D, at 23 °C ± 2 K) and the corresponding part size (M, 16-24 hours after moulding, at 23 °C ± 2 K and 50% ± 10% air humidity).
The actual shrinkage rate can only be approximated due to the influence of multiple factors. The shrinkage rate of plastics is not a fixed value but a range. The shrinkage rate of the same material produced by different factories may be different, and multiple batches of materials produced by a factory may also have different shrinkage rates. Therefore, factories generally provide users with a shrinkage range.
Another layer of complexity is added for materials with asymmetrical shrinkage characteristics. These are plastics that have different shrinkage rates depending on the part orientation. Polymers filled with long glass fibres will shrink more in the cross (transverse) direction than the longitudinal (flow) direction. This poses a challenge for the mould designer as the material supplier documentation will state a different shrink rate along the X-axis than along the Y-axis. This is not much of a problem for long, straight parts like candy sticks or rulers, but it can be challenging for parts with complex geometries.
If the part is complex and includes features like holes or flow fronts that meet at different angles, it can be impossible to calculate and model the plastic shrinkage with complete accuracy. The time and cost to model the result would be expensive and unnecessary, even if it could be performed to a desired level of confidence and reliability. It is best to avoid resins with asymmetrical shrinkage if the design calls for tight tolerances.
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Standard methods to measure shrinkage
The standard methods to measure the shrinkage of plastic involve various procedures and calculations. The plastic shrinkage rate is the dimensional reduction percentage of a moulded plastic part as it cools from its molten state to room temperature. This rate is influenced by multiple factors, including the type of plastic, wall thickness, cooling rates, and part complexity.
Firstly, it is important to note that the shrinkage rate of plastics is typically provided as a range rather than a fixed value. This is because the same type of plastic produced by different factories may have varying shrinkage rates, and even different batches from the same factory can exhibit different shrinkage behaviours. Therefore, manufacturers usually provide a shrinkage range for their materials.
Secondly, the calculation method for plastic shrinkage rates is often based on established standards. One such standard is the German national standard DIN16901, where the shrinkage rate (S) is calculated using the formula: S = (D - M) / D, where D is the mould size at a specific temperature and M is the corresponding part size after a set period following moulding. Another standard method is ASTM D955, which measures the contraction of the plastic compared to the injection moulding tool.
Additionally, there are several well-defined and standardised approaches to determine plastic shrinkage, including ISO 294-4 and ISO 294-3 for thermoplastic shrinkage, and ISO 2577 for thermosets. These standards provide specific guidelines and formulas to calculate and address plastic shrinkage.
Furthermore, when dealing with complex part geometries, it is essential to consider the orientation of the part. Some plastics exhibit asymmetrical shrinkage characteristics, shrinking differently along the X and Y axes. This can be influenced by the direction in which fibres are aligned within the plastic.
Finally, to directly observe plastic shrinkage, a test shot can be performed using an obsolete mould of similar size, shape, and wall thickness to the desired part. By injecting the chosen resin into this mould, the resulting parts can be used to calculate precise plastic shrinkage for the selected material. This step can help prevent potential issues and save time and costs associated with reworking or scrapping moulds.
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The impact of different plastics
Polyvinyl Chloride (PVC), commonly used for pipes and cable insulation, is known for its high shrinkage rate upon cooling. On the other hand, Polypropylene (PP), often used in packaging and automotive parts, exhibits notable shrinkage after molding, especially when not filled with other materials. High-Density Polyethylene (HDPE), a polymer used in containers and pipes, also has a significant shrinkage rate, particularly when molded in thicker sections.
The direction in which fibres align within the plastic can also impact shrinkage rates. Polymers filled with long glass fibres, for example, tend to shrink more in the cross (transverse) direction than in the longitudinal (flow) direction. This asymmetrical shrinkage characteristic poses a challenge for mold designers, especially when dealing with parts that have complex geometries.
Additionally, the type of plastic used can influence the accuracy of shrinkage calculations. The shrinkage rate of the same type of plastic produced by different factories may vary, and even multiple batches of materials from the same factory can exhibit different shrinkage rates. This variability makes it difficult to calculate and predict shrinkage with complete accuracy, and it is often necessary to rely on the shrinkage range provided by the manufacturer.
By understanding the unique shrinkage characteristics of different plastics, manufacturers can make informed decisions when selecting the appropriate plastic for specific applications. Properly accounting for shrinkage ensures that the final product meets the desired specifications and that all parts fit and function as intended.
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Frequently asked questions
Plastic shrinkage is the contraction of a plastic moulding as it cools after injection.
The plastic shrinkage rate is the dimensional reduction percentage of a moulded plastic part as it cools from its molten state to room temperature.
The shrinkage rate (S) is the relative difference between the mould size (D) and the corresponding part size (M). It is calculated according to Formula 1: S = (D-M)/D.
First, determine the original length of the plastic (OL) in millimetres. Next, determine the shrinkage percentage (SP) that the plastic will undergo. Then, calculate the final length (FL) of the plastic in millimetres using the formula: FL = OL – (OL * (SP / 100)).
The shrinkage rate of plastic is influenced by various factors, including the type of plastic, the mould design, the wall thickness, the cooling rate, and the processing conditions. Additionally, the direction of fibre alignment can impact the shrinkage rate, with differential shrinkage occurring in different directions.
