Maya Ellison
Plastic mould shrinkage is a critical aspect of the plastic manufacturing process. It occurs when the plastic material changes size or shape due to cooling and solidification processes. The rate of shrinkage varies depending on the type of plastic and the conditions in which it is produced. To calculate the shrinkage allowance, one must take into account various factors such as the shape of the plastic part, mould structure, and moulding conditions. The calculation method for plastic shrinkage rate is generally based on the German national standard DIN16901. The shrinkage rate is the relative difference between the mould size and the corresponding part size. While there are several well-defined and standardised approaches to determine the shrinkage of plastic parts, the actual shrinkage rate can only be approximated due to the influence of multiple factors.
| Characteristics | Values |
|---|---|
| Definition of plastic shrinkage rate | Dimensional reduction percentage of a molded plastic part as it cools from its molten state to room temperature |
| Factors influencing shrinkage rate | Shape of the plastic part, mould structure, molding conditions, wall thickness, type of plastic, temperature, cycle times, mould design, part shape, and mould thickness |
| Calculation method | Generally based on the German national standard DIN16901. 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) |
| Formula for mould size | D=M/(1-S) |
| Formula for calculating shrinkage rate | Determine the difference between the mould and part dimensions, then divide that number by the mould dimensions |
| Standard approaches | ISO 294-4, ISO 294-3 (thermoplastic shrinkage) |
| Fine-tuning | Adjusting the density of the material by manipulating how hard the cavity is packed or extending the cooling period in the mould |
| Typical shrinkage rates | Vary between 0.001/in/in and 0.020/in/in, with an average of around 0.006/in/in |
| Plastic materials with high shrinkage rates | Delrin or Polyoxymethylene |
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What You'll Learn
The impact of mould design and material type
Additionally, the material type used for the mould can impact the shrinkage allowance. Different materials have varying shrinkage rates, and this variability must be considered when designing the mould. Some materials, such as thermoplastics, exhibit expansion upon heating and shrinkage upon cooling, which must be factored into the mould design to ensure the final plastic part meets the required specifications.
The type of plastic material being moulded also influences the shrinkage allowance. Different plastics have distinct shrinkage characteristics. For instance, amorphous and crystalline materials have different shrink rates, and even the same type of plastic produced by different factories may exhibit varying shrinkage rates due to differences in manufacturing processes and batch variations. Therefore, manufacturers must carefully consider the specific plastic material being used and consult the factory's provided shrinkage range to accurately calculate the necessary shrinkage allowance.
Furthermore, the design of the mould should take into account additional factors that can influence the shrinkage rate, such as cycle times, temperatures, part shape, wall thickness, and gate size and position. By considering these factors in the mould design, manufacturers can fine-tune their processes to ensure the final plastic parts meet the desired dimensions and precision requirements.
Lastly, it is important to note that the calculation of shrinkage allowance is a complex process, and the actual shrinkage rate may only be approximated due to the influence of multiple variables. Mould designers and manufacturers must carefully consider the specific plastic material, mould design, and various process parameters to make informed decisions and adjustments, ensuring the final product meets the required standards and specifications.
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Calculating shrinkage rate
Plastic shrinkage rate is the dimensional reduction percentage of a molded plastic part as it cools from its molten state to room temperature. If this shrinkage is not calculated in advance and compensated for in the mold design, the molded plastic parts cannot meet the project requirements.
The calculation method for plastic shrinkage rate is based on the German national standard DIN16901. The shrinkage rate (S) is the relative difference between the mold size (D, at 23 °C ± 2 K) and the corresponding part size (M, 16-24 hours after molding, at 23 °C ± 2 K and 50 % ± 10 % air humidity). It is calculated according to Formula 1. If the size of the plastic part and the shrinkage rate of the material are known, then the mold size can be calculated as D=M/(1-S). To simplify the calculation, Formula 2 is generally used to calculate the mold size.
However, when calculating the shrinkage rate, the actual shrinkage rate can only be approximated due to the influence of multiple factors. Firstly, the shrinkage rate of various 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. Secondly, the actual shrinkage rate during the forming process is influenced by factors such as the shape of the plastic part, mold structure, and molding conditions.
To fine-tune the shrinkage of parts, the molder can adjust the density of the material by manipulating how hard the cavity is packed or by extending the cooling period in the mold. It is recommended to perform test shots when injecting parts that are large, have tight tolerances, or use a new or unusual material. The molder will shoot the desired resin into an obsolete mold of a similar size to your project for a small fee. These parts can be used to calculate a precise plastic shrinkage for the chosen material.
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Approximating shrinkage rates
Plastic shrinkage rate is the dimensional reduction percentage of a moulded plastic part as it cools from its molten state to room temperature. The rate of shrinkage varies depending on the type of plastic and the conditions in which it is produced. The actual shrinkage rate is influenced by several factors, including the shape of the plastic part, mould structure, and moulding conditions. Therefore, the shrinkage rate can only be approximated.
The shrinkage rate formula is calculated based on the German national standard DIN16901. 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). To simplify the calculation, Formula 2 is generally used to calculate the mould size: D=M/(1-S).
The moulder can fine-tune the shrinkage of the parts by adjusting the density of the material. This can be done by manipulating how hard the cavity is packed or by extending the cooling period in the mould. It is recommended to perform test shots when dealing with large parts, tight tolerances, or new materials. This can be done by asking the company to shoot resin into a similar mould in size, shape, and wall thickness to your part design. These parts can be used to calculate a precise plastic shrinkage for the chosen material.
When considering mould shrinkage calculations, the tooling engineer simply scales the mould tooling by 1.00X. The material supplier will usually provide a material data sheet that specifies a shrinking range for the material. For example, a material data sheet may specify a shrinking range of 0.005-0.007/in/in for a 0.100-inch wall thickness. If the wall is 0.100 inches during validation and inspection, the shrink average along those walls is expected to be 0.006 inches.
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Injection moulding and shrinkage
Injection moulding is a process where molten plastic material is injected into a mould cavity under high pressure. As the plastic material cools, it shrinks and solidifies. This shrinkage is typically stated as a percentage and is influenced by various factors such as the type of plastic, wall thickness, cooling rates, and mould design.
Plastic shrinkage, also known as mould shrinkage, is an important consideration in injection moulding. It refers to the dimensional change or contraction of the injected material as it cools, which can affect the final shape of the moulded part. Most shrinkage occurs while the plastic is still within the mould, but a small amount of shrinkage can also occur after ejection as the part continues to cool. This additional shrinkage can last for several hours or even days until the temperature and moisture content stabilize.
To calculate the shrinkage allowance for plastics in injection moulding, several standardized approaches can be used, such as ISO 294-4 and ISO 294-3 for thermoplastic shrinkage. The German national standard DIN16901 provides a general calculation method for the shrinkage rate (S), which is the relative difference between the mould size (D) and the corresponding part size (M) after a specific cooling period. By knowing the shrinkage rate and the size of the plastic part, the mould size can be determined using Formula 2: D=M/(1-S).
However, calculating the exact shrinkage rate is challenging due to multiple influencing factors. These factors include the shape of the plastic part, mould structure, moulding conditions, plastic composition, additives, and stress levels during the injection moulding process. To compensate for shrinkage, manufacturers often oversize the mould cavity or add additional material, which is then trimmed after moulding. Test shots with similar moulds can also be performed to observe plastic shrinkage firsthand and calculate precise shrinkage rates for specific materials.
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Adjusting for precision
When calculating the shrinkage allowance for plastics, precision is paramount. The process of plastic moulding involves many variables, and a slight deviation in any of these factors can significantly impact the final product. To ensure accuracy, several considerations must be made regarding the plastic material, the mould design, and the manufacturing process.
Firstly, it is essential to recognise that the shrinkage rate of plastics is not a fixed value. It varies depending on the type of plastic, the manufacturing process, and even the specific batch of material. Therefore, it is crucial to refer to the shrinkage range provided by the factory or utilise standardised references, such as ISO 294-4 and ISO 294-3 for thermoplastics. Additionally, the material supplier typically provides a data sheet specifying the expected shrinkage range for a given material, aiding in the selection of appropriate values for calculations.
The shape and dimensions of the plastic part play a crucial role in determining shrinkage. For instance, the wall thickness of the plastic part affects the shrinkage rate, with thicker walls requiring the upper limit of the shrinkage range and thinner walls necessitating the lower limit. Moreover, the overall shape of the part must be considered, as shrinkage can vary between different sections of the product. For example, vertical shrinkage may differ from horizontal shrinkage, and this disparity should be accounted for in the calculations.
The mould design itself is a critical factor in achieving precision. The mould structure, including its shape, size, and wall thickness, influences the shrinkage rate. By selecting plastics with a narrow shrinkage range for high-precision parts, manufacturers can better control the final dimensions. Additionally, leaving room for correction and modification in the mould design allows for fine-tuning during the trial moulding stage. This iterative process involves gradually adjusting the mould until the desired dimensions and precision are attained.
To further enhance precision, the moulder can manipulate the density of the material. This can be achieved by adjusting how tightly the mould cavity is packed or by extending the cooling period. Test shots or trial moulding with obsolete moulds of similar size, shape, and wall thickness can provide valuable insights into the plastic shrinkage behaviour of the chosen material. These preliminary tests may incur additional time and costs but can prevent the need for costly reworks or scrapped moulds later in the manufacturing process.
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Frequently asked questions
Plastic shrinkage is the dimensional reduction of a molded plastic part as it cools from its molten state to room temperature.
The shrinkage rate varies depending on the type of plastic and the conditions in which it is produced. Factors such as the shape of the plastic part, mould structure, wall thickness, and moulding conditions can all influence the shrinkage rate.
The shrinkage rate is calculated by determining the difference between the mould and part dimensions and then dividing that number by the mould dimensions. The formula for the shrinkage rate (S) is: S = (D - M) / D, where D is the mould size and M is the corresponding part size.
Plastic shrinkage is important to consider when designing parts for injection moulding. If the shrinkage is not calculated in advance, the moulded plastic parts may not meet the project requirements. Calculating plastic shrinkage allows for precise manufacturing and ensures that the final product meets the desired specifications.
Yes, there are well-defined and standardised approaches to determine plastic shrinkage, such as ISO 294-4 and ISO 294-3 for thermoplastic shrinkage. Additionally, the German national standard DIN16901 provides a calculation method for the plastic shrinkage rate. It is recommended to refer to these standards and consult material suppliers for accurate data and calculations.
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