Chloe Zhang
Plastic shrinkage is a critical aspect of the plastic manufacturing process. It occurs when plastic changes size or shape due to cooling and solidification. This can be referred to as the 'shrink rate' or 'shrinkage rate'. During the thermoforming process, plastic is heated and expands, then contracts and shrinks as it cools. The amount of shrinkage can vary depending on the type of plastic and the conditions in which it is produced. The shrink rate is typically expressed in inches per inch or as a percentage. There are several standardised approaches to determine the shrinkage of plastic parts, including the ASTM D955, ISO 294-4, and ISO 2577 methods.
| Characteristics | Values |
|---|---|
| Definition of Shrink Rate | The amount of shrinkage or contraction various materials experience when heat is applied to them |
| Plastic Shrinkage | Refers to the characteristics of a specific plastic |
| Mold Shrinkage | Refers to the dimensional change of the injection-molded object as it cools |
| Mold Shrinkage Calculation | The percentage can be found using the mold cavity dimensions and final part dimensions |
| Formula | S = Shrinkage Rate, Y = Cavity Dimensions, Z = Part Dimensions |
| Example Calculation | If the Cavity Dimension is 1.7 in. and the Part Dimension is 1.5 in., the part shrinkage can be calculated as S = 0.0013 in. or 0.13% |
| Typical Shrink Rates | Between 0.001/in/in and 0.020/in/in |
| Average Shrink Rate | Around 0.006/in/in |
| Factors Affecting Shrink Rate | Wall thickness, cooling rates, temperature, pressure, molding conditions, shape of the plastic part, mold structure, and more |
| Adjusting Shrinkage | Can be adjusted by altering the temperature of the mold and the pressure applied |
| Rule of Thumb | A 10% change in mold temperature can result in a 5% change in original shrinkage, a 10% change in pressure can cause a 10% change in shrinkage rate |
| Standard Methods for Measurement | ASTM D955, ISO 294-4 (for thermoplastics), ISO 2577 (for thermosets) |
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What You'll Learn
The impact of temperature on thermoforming plastic
Temperature plays a critical role in the thermoforming process, significantly influencing the behaviour and characteristics of plastic materials. The impact of temperature on thermoforming plastic can be summarised in the following key points:
Impact of High Temperatures
When subjected to high temperatures, plastic materials undergo a range of changes:
- Expansion: As temperature increases, plastics expand due to their coefficient of thermal expansion (CTE). This expansion rate varies among different plastics.
- Loss of Stiffness: High heat reduces material stiffness, or flexural modulus.
- Distortion: Exceeding the heat deflection temperature (HDT) of a material can cause it to distort. Prolonged exposure to heat, especially under load, can lead to plastic deformation or "creep".
- Degradation: Prolonged exposure to high temperatures causes plastics to lose strength and toughness, making them more susceptible to cracking, chipping, and breaking. This degradation is proportional to both temperature and time of exposure.
Plastic Thermoforming Temperature Range
The ideal temperature range for thermoforming varies depending on the type of plastic:
- Polystyrene: 90°C to 140°C
- Polyethylene (PE): 160°C to 200°C
- ASA/ABS: 160°C to 190°C
- PMMA (Acrylic Glass): Suitable for thermoforming, but no specified temperature range.
Impact of Low Temperatures
While the effects of temperature on thermoplastics are more pronounced at high heat levels, excessively low temperatures can also impact the material's properties. For example, some plastics, such as TPO, are specifically designed to perform well in both cold and high-heat applications.
Other Factors Influenced by Temperature
Temperature also affects various other attributes of thermoplastics:
- Mechanical properties
- Chemical resistance
- Electrical conductivity
- Material fatigue
Impact on Shrinkage
Temperature is a critical factor in determining the shrinkage rate of plastics during the thermoforming process. As plastics are heated, they expand and become malleable, only to contract and shrink as they cool. This shrinkage can continue for several hours or even days until the temperature and moisture content stabilise. Therefore, understanding the shrinkage behaviour of different plastics is essential for achieving the desired final product dimensions.
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Calculating shrinkage rate using Formula 1
When calculating the shrinkage rate of thermoforming plastic, it's important to consider the various factors that can influence the final outcome. These factors include the type of plastic used, the conditions under which it is produced, the shape of the plastic part, the mould structure, and the moulding conditions. By understanding and incorporating these factors into your calculations, you can ensure more accurate results.
To calculate the shrinkage rate using Formula 1, we need to understand the variables involved. Let's break down the components:
- S: This represents the shrinkage rate, which is the relative difference in size between the mould and the corresponding part after cooling.
- D: This denotes the size of the mould at a specific temperature, typically 23°C with a slight allowance for variation (+/- 2 K).
- M: This refers to the size of the plastic part after it has cooled and stabilised. Specifically, M represents the dimensions of the part 16 to 24 hours after moulding, also measured at 23°C (+/- 2 K) and with a standard air humidity of 50% (+/- 10%).
Now that we understand the variables, we can delve into the calculation:
Formula 1: S = (D - M) / D
In this formula, "S" is the shrinkage rate, "D" is the mould size, and "M" is the size of the plastic part after cooling. By plugging in the values and performing the calculation, you can determine the shrinkage rate expressed as a decimal.
For example, let's consider a scenario where the mould size (D) is 2 inches, and the size of the plastic part (M) after cooling is 1.8 inches:
S = (2 inches - 1.8 inches) / 2 inches
S = 0.2 / 2
S = 0.1
So, in this case, the shrinkage rate (S) would be 0.1, indicating that the plastic part shrank by 10% during the cooling process.
It's worth noting that Formula 1 provides a basic calculation for the shrinkage rate. In practice, the actual shrinkage rate can be influenced by numerous factors, as mentioned earlier. These factors include the type of plastic, wall thickness, cooling rates, and other variables. Therefore, it is often necessary to conduct test shots, review material data sheets, and make adjustments during the mould design process to account for these variables and ensure the final parts meet the desired specifications.
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The role of pressure in controlling shrinkage
Plastic material shrinkage is a critical aspect of the plastic manufacturing process. It occurs when the plastic material changes its 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. This includes factors such as amorphous or crystalline materials, cycle times, temperatures, mould design, part shape, and wall thickness.
It is important to note that the pressure must be maintained until the plastic has cooled to its point of solidification. If the pressure is reduced before that point, the shrinkage will increase as the molecules regain mobility. This relationship between pressure and shrinkage is described by the pressure rule-of-thumb, which states that a 10% change in pressure leads to a corresponding 10% change in the shrinkage rate.
Additionally, the temperature of the mould also influences shrinkage. A hot mould creates less shrinkage than a cold one because the plastic skin solidifies more slowly, allowing for higher injection pressure and more molecular compression before solidification. Conversely, a cold mould solidifies the plastic skin sooner, resulting in greater shrinkage as the molecules are not packed as tightly.
By understanding the interplay between pressure, temperature, and shrinkage, manufacturers can fine-tune their processes to achieve the desired results. This may involve adjusting the density of the material, manipulating how hard the cavity is packed, or extending the cooling period in the mould to optimise shrinkage rates and product quality.
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How to fine-tune shrinkage
Fine-tuning shrinkage during the thermoforming of plastic is a critical step in the plastic manufacturing process. The following factors influence the shrinkage rate and can be adjusted to achieve the desired results:
Material Selection
Different types of plastics exhibit varying shrinkage rates. For instance, amorphous polymers like ABS and polycarbonate typically have lower shrinkage rates compared to semi-crystalline polymers like polyethylene or polypropylene, which are more prone to shrinking. Therefore, selecting the appropriate plastic material for your application is essential.
Wall Thickness
Wall thickness plays a significant role in differential shrinkage. Uneven wall thickness can lead to warping or distortion due to differential shrinkage rates. Thicker areas of a part cool slower, contributing to a higher degree of shrinkage relative to thinner areas. Thus, maintaining a consistent wall thickness throughout the part is crucial.
Mold Temperature
The temperature of the mold significantly affects the cooling rate of the plastic. A cold mold accelerates cooling, enhancing shrinkage, while a preheated or temperature-controlled mold can reduce shrinkage by providing a more gradual cooling phase.
Heating Time
Optimizing the heating time is crucial to prevent excessive stretching and shrinkage. Overheating the plastic can lead to over-extension, resulting in greater shrinkage upon cooling. Therefore, the heating cycle should be carefully controlled to ensure even temperature distribution and that the plastic is pliable without being overextended.
Cooling Rate
A rapid cooling process can increase shrinkage due to the quick solidification of external layers while the internal material remains hot and expanded. This leads to built-up stress and warping. A gradual cooling process allows for uniform contraction and minimizes shrinkage.
Density Adjustment
The molder can fine-tune shrinkage by adjusting the density of the material. This can be achieved by manipulating how tightly the cavity is packed or by extending the cooling period in the mold, allowing for a more controlled and gradual cooling process.
By carefully considering and adjusting these factors, manufacturers can minimize shrinkage and produce parts that meet the desired specifications and tolerances.
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International standards for measuring shrinkage
Plastic material shrinkage is an important consideration in the plastic manufacturing process, as it can affect the final dimensions of the product. The rate of shrinkage varies depending on the type of plastic and the conditions in which it is produced and processed.
There are several well-defined and standardised international approaches to determine the shrinkage of plastic parts. The most internationally recognised approaches are:
- ISO 294-4, ISO 294-3 (thermoplastic shrinkage)
- ASTM D955 (for measuring "molding plastic shrinkage")
- ISO 2577 (for thermosets)
These standards provide a consistent method for measuring and comparing the shrinkage of different plastics. By following these standards, manufacturers can predict how their chosen plastic will behave during the cooling process and make any necessary adjustments to the mould design and production planning stages.
For example, using the ISO 294-4 standard, a tooling engineer can scale the mould tooling by 1.00X to compensate for mould shrinkage. This ensures that the final part dimensions match the desired specifications.
Additionally, understanding the factors that contribute to shrinkage can help manufacturers achieve high precision in their products. For instance, a rapid cooling process can enhance shrinkage due to the quick solidification of the external layers, so a controlled cooling process can help to reduce shrinkage.
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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.
Plastic material expands when heated and contracts when cooled. During the thermoforming process, plastic expands and becomes malleable at high temperatures, then contracts and shrinks as it cools.
The shrink percentage can be calculated using the mold cavity dimensions and final part dimensions. The shrinkage rate (S) is the relative difference between the mold size (D) and the corresponding part size (M). The formula for this is S = (D-M)/D.
The shrink percentage is influenced by the type of plastic, wall thickness, cycle times, temperatures, mould design, part shape, and mould structure. The shrinkage rate can also be adjusted by changing the barrel temperature, injection pressure, and mould temperature.
The shrinkage rate of a specific plastic can be determined by performing a test shot on an obsolete mould of similar size, shape, and wall thickness to your part design. This will allow you to calculate a precise shrinkage rate for the chosen material. Alternatively, you can refer to the material data sheet provided by the supplier, which typically specifies a shrinking range for the material.
