Alexei Volkov
Plastic shrinkage is a critical parameter in the production of plastic products, affecting their dimensional stability and surface quality. It is influenced by factors such as material type, composition, moisture absorption, and mould temperature. To reduce shrinkage, manufacturers can adjust the temperature of the plastic while it resides in the barrel, use fillers, and optimise the mould design and cooling system. Additionally, maintaining consistent moulding temperatures, increasing injection pressure, and prolonging pressure can also decrease shrinkage. Understanding shrinkage allows designers to optimise their designs and select suitable materials, reducing waste, rework, and delays, ultimately enhancing cost efficiency.
| Characteristics | Values |
|---|---|
| Pressure | Prolonged pressure helps maintain shrinkage reduction. |
| Temperature | A consistent molding temperature reduces shrinkage. Higher melt temperatures reduce shrinkage. |
| Injection Speed | High injection speeds slightly increase shrinkage. |
| Injection Pressure | Excessive injection pressure increases shrinkage. Increased injection pressure decreases shrinkage. |
| Wall Thickness | Uneven wall thickness can lead to uneven shrinkage. Thicker walls lead to longer cooling times and greater shrinkage. |
| Part Design | Parts with inserts have lower shrinkage rates. Elongated parts show lower shrinkage along their length. |
| Gate Design | Larger gate size reduces shrinkage. |
| Mold Design | A well-designed cooling system can reduce the overall shrinkage rate. |
| Time | Longer cooling times within the mold decrease shrinkage. |
| Plastic Type | Different plastics exhibit varying shrinkage levels. Crystalline plastics typically exhibit greater shrinkage than amorphous plastics. |
| Fillers | Adding fillers to plastics significantly reduces shrinkage. |
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What You'll Learn
Maintain pressure until plastic solidifies
Pressure is a key variable in the plastic injection moulding process. It directly impacts the plasticization of the plastic and the quality of the final product. The injection pressure is necessary to maintain the desired polymer flow rate during the filling stage, ensuring that the mould cavity is adequately filled with the molten material.
Once the filling stage is complete, the injection moulding machine automatically switches to the packing stage, where the machine controls the injection pressure. This pressure is maintained to complete the product. The pressure is maintained until the gate freezes or the pressure loss is removed by cycle timer control. This is known as the holding pressure. Holding pressure is used to adjust the product weight, size, deformation, and shrinkage. It is the holding pressure that determines shrinkage, not the initial injection pressure.
The required level of holding pressure for semi-crystalline thermoplastics is crucial to ensure the quality of the final product. If pressure is relaxed before the plastic has cooled to its point of solidification, the shrinkage will increase because the molecules have been allowed to move again.
The higher the temperature of the plastic, the greater the amount of shrinkage. This is because of the activity of the individual plastic molecules; as the temperature rises, these molecules expand more and take up more space. Therefore, it is important to control the temperature of the plastic while it resides in the barrel to alter the amount of shrinkage for a specific product or material.
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Adjust temperature in the barrel
The temperature in the barrel plays a significant role in determining the shrinkage of plastic products. This is due to the behaviour of plastic molecules when subjected to changes in temperature. As the temperature rises, plastic molecules expand and take up more space. Conversely, when the temperature decreases, the molecules contract, leading to a reduction in their overall volume.
To minimize shrinkage in plastics, it is essential to adjust the temperature in the barrel. Lowering the temperature will result in reduced expansion of the plastic molecules, subsequently leading to less shrinkage as the plastic cools. As a general guideline, a 10% alteration in barrel temperature will result in a 10% variation in the shrinkage rate. For instance, if a material exhibits a shrinkage rate of 0.005 in/in at a barrel temperature of 500 degrees, decreasing the temperature to 450 degrees will lower the shrinkage rate to 0.0045 in/in.
It is worth noting that the relationship between temperature and shrinkage is not solely dependent on the barrel temperature but also on the mold temperature. A hot mold facilitates the free movement of plastic molecules, resulting in less shrinkage. Conversely, a cold mold causes the outer edges of the plastic to dry prematurely, leading to increased shrinkage. Therefore, when working with thin-walled plastic products, it is advisable to use higher mold temperatures to reduce resistance in the melt flow and, consequently, lower the shrinkage rate.
Additionally, the cooling process of plastic products can also impact shrinkage. Rapid cooling techniques, such as quenching or dropping the molded parts into water, can cause the mobility of polymer chains to "freeze in," leading to potential distortion if subjected to high temperatures later. Hence, a gradual cooling process is recommended to reduce the risk of fractures or cracks in the final product.
Furthermore, the injection pressure applied during the molding process also influences shrinkage rates. Higher injection pressure results in tighter packing of plastic molecules, reducing their mobility and overall shrinkage. Therefore, maintaining consistent pressure until the plastic solidifies is crucial in minimizing shrinkage.
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Optimise design
Understanding the concept of shrinkage is crucial for designers to anticipate size changes during production and optimise designs for precise, consistent results. Firstly, it is important to note that all materials have a specific "shrinkage rate" assigned by the manufacturer, which predicts the difference between the plastic product when it is first moulded and after it has cooled to room temperature. This shrinkage is not a function of time but rather the difference in dimensions due to thermal expansion and contraction.
When designing plastic products, maintaining reasonable dimensions is essential to control the shrinkage rate and avoid excessive size differences. Thicker walls, for instance, lead to longer cooling times and greater shrinkage. Additionally, uneven wall thickness can result in uneven shrinkage, causing sink marks and warping. Therefore, designers should aim for consistent wall thickness to minimise these issues.
The type of plastic used also plays a significant role in shrinkage rates. Different plastics exhibit varying shrinkage levels during moulding, with crystalline plastics generally exhibiting greater shrinkage than amorphous plastics. For instance, ABS shrinkage typically falls between 0.4% and 0.7%, while PP shrinkage ranges from 1.0% to 2.5%. Thus, designers should carefully select materials based on design requirements and their understanding of shrinkage characteristics.
Furthermore, the design of the mould's cooling system directly impacts the cooling efficiency of the plastic and the overall shrinkage rate. A well-designed cooling system can expedite the cooling and solidification process, reducing shrinkage. However, excessively high injection temperatures and pressures can increase the shrinkage rate, so these parameters must be carefully controlled.
Finally, designers should consider the shape and complexity of the moulded part. Complex shapes tend to have smaller shrinkage rates, and parts with inserts exhibit lower shrinkage rates. Taking these factors into account during the design phase can help optimise the final product and minimise shrinkage-related problems, enhancing cost efficiency and product quality.
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Add fillers
Additives such as fillers, colorants, and lubricants can affect the mould shrinkage of plastic parts. Fillers can significantly reduce shrinkage in plastics. Fillers that have lower thermal expansion than polymers will lower shrinkage. For example, resins filled with glass fibre, wood, or mica will generally shrink less than unfilled resins.
Fillers that orient may lower shrinkage values but increase the ratio of flow to cross-flow variation. For example, polymers filled with long glass fibres will shrink more in the cross-flow direction than the longitudinal direction. Very small fillers may increase nucleation sites to create higher crystallinity and thereby increase overall shrinkage.
The type of filler and the amount used can have very different effects on shrinkage. For instance, unfilled nylon has a typical shrinkage ratio of 1.5%, but 30% glass-filled nylon has a shrinkage of 0.3% in the flow direction and 1.0% shrinkage in the cross-flow direction. The material with the lowest shrinkage is carbon-filled materials, which may be near zero depending on the carbon loading and the substrate resin.
It is important to note that the addition of fillers can also increase the stiffness of the plastic, which results in higher shrinkage during cooling.
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$11.87
Choose the right plastic
When choosing the right plastic, it's important to consider the unique requirements of your product design and the specific application of the plastic component. Different plastics have distinct molecular structures and exhibit varying shrinkage rates, so selecting the most suitable plastic will ensure safer and more reliable products.
Crystalline plastics, for example, typically exhibit greater shrinkage than amorphous plastics. Amorphous materials, such as resins, tend to have reduced orientation effects with increasing wall thickness, which helps to maintain consistent shrinkage. Thicker walls, however, generally lead to longer cooling times and greater shrinkage, so this trade-off must be considered.
The type of plastic you choose will depend on the desired characteristics of your final product. For instance, ABS and Polycarbonate shrink in the range of 0.004”–0.006” per inch, while nylon and acetal offer much higher shrink rates of 0.022”–0.030” per inch. If you require a smaller part, switching to a material with a higher shrinkage rate, like acetal, may be beneficial.
Additionally, certain plastics have specific advantages. For instance, resins filled with glass fibre, wood, or mica can exhibit lower shrinkage rates than unfilled resins. Plastics with good flowability also tend to have smaller moulding shrinkage, and adding fillers to plastics can significantly reduce shrinkage.
It's worth noting that colour consistency can be challenging when using different moulded plastic materials. Even with the same colour, variations in material properties can result in slight differences in the final product's appearance. Therefore, it's crucial to understand how different injection-moulded materials will behave and design your products accordingly.
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Frequently asked questions
Plastic shrinkage is the phenomenon where the volume or dimensions of plastic decrease during the injection moulding process as it cools and solidifies from a liquid state.
The shrinkage rate is influenced by the type of plastic, crystallinity, molecular weight, modifications, and processing conditions. The wall thickness of the mould, gate design, and melt and mould temperature also play a role in determining the shrinkage rate.
Higher mould temperatures generally increase shrinkage. However, excessively high injection temperatures can reduce the viscosity of the plastic melt, accelerating the cooling and solidification process, which then increases the overall shrinkage rate.
Increased injection pressure can decrease shrinkage. Maintaining pressure for a prolonged period during the cooling process also helps maintain shrinkage reduction.
Product designers should understand the shrinkage characteristics of different plastics and select suitable materials based on design requirements. Optimising designs by anticipating size changes can lead to precise and consistent results. Additionally, a well-designed cooling system can speed up the cooling process and reduce the overall shrinkage rate.
