Omar Khalid
Plastic can become brittle due to a variety of factors, including weak molecular bonds, low temperatures, and the use of certain plasticizers. The ductility of plastic, or the ability of its long, chain-like molecules to stretch and slip past each other, is essential to its resilience. When molecular motion is restricted by factors such as low temperatures or improper mixing during manufacturing, the plastic becomes more susceptible to breakage. Additionally, the use of volatile plasticizers like dinonylphthalate, which was commonly used in the past, can lead to brittleness over time as the plasticizer evaporates, requiring the remelting and re-addition of the plasticizer.
| Characteristics | Values |
|---|---|
| Plasticizers evaporating | Dinonylphthalate was a common plasticizer used in the past, but its volatility meant it would evaporate over time, causing the plastic to become brittle. |
| Weak molecular bonds | Insufficient mixing and melting of the plastic resin can cause weak bonding between molecules, resulting in brittleness. |
| Thermal degradation | Localized thermal degradation can cause a separation of molecules and hinder proper molecular bonding, leading to brittleness. |
| Low injection pressure | Insufficient injection pressure can cause weld line areas, resulting in weak spots that appear brittle and prone to cracking. |
| Gate and runner restrictions | Small gates and runners can restrict the flow of molten plastic, causing thermal degradation and weak molecular bonding, which makes the plastic brittle. |
| Condensation | Condensation on cold molds in humid conditions can introduce moisture, disrupting molecular bonding and causing brittleness. |
| Low ductility | Plastics with low ductility cannot absorb and dissipate stress effectively, leading to cracks and fractures. |
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What You'll Learn
Low compression ratios
The use of an injection screw with too low a compression ratio is a common cause of weak molecular bonds in plastic, resulting in brittleness. This occurs because the material is not properly mixed and melted, leading to weak bonding between individual plastic resin molecules.
The solution to this issue is to utilise an injection screw with the appropriate compression ratio. While the general-purpose screw that typically comes with the machine is often adequate, specific screw designs are available for almost any particular material. It is recommended to consult the material supplier for guidance on selecting the most suitable screw for the specific application.
Additionally, it is important to ensure that the overall machine cycle time is sufficient to allow the material to attain the proper consistency through adequate heating and melting. Increasing the cycle time can enhance the melting process, although this may increase the cost of moulding. As an alternative, increasing the barrel temperature, screw rpm, and back pressure can achieve similar results without extending the cycle time. However, caution must be exercised to avoid excessive increases that may lead to material degradation due to increased shear heat.
Moreover, low injection pressure can also contribute to brittleness in moulded plastic parts. Insufficient injection pressure can lead to the formation of weld line areas caused by a non-filling condition. This occurs when the plastic solidifies prematurely, resulting in weak areas that exhibit a tendency to crack or appear brittle. Increasing the injection pressure is recommended to address this issue, as it helps to force the weld line areas together, minimising the propensity for cracking or brittleness.
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Weak molecular bonds
Plastic is known for its ductility, or the ability of its long, chain-like molecules to stretch and slip past, around, or through one another. This ductility allows plastic to absorb energy and collectively dissipate stress from impact, preventing breakage. However, when molecular bonds in plastic are weakened, this ductility is compromised, leading to brittleness.
Another factor contributing to weak molecular bonds is thermal degradation. This can be induced by excessive back pressure, screw rpm, or injection speed during processing. Additionally, restrictions in the flow of molten plastic due to small gates and runners can cause shearing friction, leading to thermal degradation and weak molecular bonding. Similarly, condensation on the surface of a cold mold in humid conditions can introduce moisture, creating a barrier to localized molecular bonding and resulting in brittleness.
To address weak molecular bonds and the resulting brittleness, several solutions can be implemented. Increasing the cycle time, barrel temperature, screw rpm, or back pressure can enhance melting and reduce the likelihood of weak bonds. However, caution is advised to prevent excessive values that may lead to thermal degradation. Optimizing the size and shape of the runner and gate through computer simulation can also alleviate restrictions in the flow of molten plastic, minimizing thermal degradation and promoting stronger molecular bonds.
In summary, weak molecular bonds in plastic are a critical factor contributing to brittleness. By understanding the causes, such as inadequate mixing, melting, and thermal degradation, we can implement effective solutions to enhance molecular bonding and maintain the desired ductility of plastic materials.
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Poor mixing and melting
Other factors that contribute to inadequate mixing and melting include low injection pressure, which can cause weld line areas in the moulded part, resulting in weak spots that tend to crack or appear brittle. Gates and runners that are too small can also restrict the flow of molten plastic, causing thermal degradation and weak molecular bonding. Condensation on the surface of a cold mould in humid conditions can introduce moisture, which acts as a barrier to localised molecular bonding, resulting in brittleness.
To address these issues, adjustments can be made to various parameters such as increasing cycle time, barrel temperature, screw rpm, back pressure, injection pressure, and nozzle temperature. However, caution must be exercised to avoid excessive increases that may lead to thermal degradation of the plastic.
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Thermal degradation
The process of plastic becoming brittle is known as "brittle fracture". This occurs when the long, chain-like molecules within the plastic are unable to stretch and slip past, around, or through one another. Instead, they become restricted and rigid, leading to a loss of ductility.
One critical factor is the size of the gates and runners in the mould. If they are too small, the flow of molten plastic becomes restricted, leading to increased shearing friction and heat build-up. This excessive heat causes the resin to overheat and form weak molecular bonds, resulting in a brittle final product. Therefore, it is essential to ensure that the gates and runners are appropriately sized to prevent thermal degradation.
Additionally, condensation on the surface of a cold mould in humid conditions can introduce moisture into the molten plastic. As the plastic cools and solidifies, this moisture can act as a barrier to proper molecular bonding, leading to brittleness. To mitigate this issue, it is crucial to minimise condensation and control the humidity during the moulding process.
The injection pressure also plays a role in preventing thermal degradation. Insufficient injection pressure can lead to weld line areas in the moulded part, known as a "non-filling condition". This occurs when the plastic solidifies before fully packing the cavity, resulting in weak areas that exhibit brittleness and cracking. Increasing the injection pressure can help minimise these tendencies and create a more robust product.
Furthermore, the overall machine cycle time should be carefully considered. If the cycle time is too short, the plastic may not have sufficient residence time in the injection barrel to melt properly, leading to inadequate mixing and melting. This, in turn, can result in weak molecular bonds and brittleness. Extending the cycle time allows for better heating and melting, reducing the likelihood of thermal degradation and producing a higher-quality product.
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Low injection pressure
To address this issue, it is crucial to ensure that the injection pressure is sufficient for the specific material being processed. Increasing the injection pressure can help to force these weld lines together, reducing the likelihood of cracking or brittleness. It is recommended to consult with material suppliers to determine the optimal processing parameters for a given plastic resin.
Additionally, other factors such as screw design and compression ratio can play a role in ensuring proper mixing and melting of the plastic. A screw with an inadequate compression ratio may not generate enough heat and pressure to properly mix and melt the material, contributing to weak bonding and brittleness. Therefore, it is important to select the appropriate screw design and ensure proper machine setup to avoid issues with brittleness.
Furthermore, low injection pressure can be a result of various issues within the injection moulding machine or process. For example, a short machine cycle time may not allow enough residence time for the plastic to melt properly. Understanding the root cause of low injection pressure is crucial to implementing effective solutions and ensuring the production of high-quality plastic parts.
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Frequently asked questions
Brittleness in plastics is the tendency of a moulded plastic part to break, crack or shatter under conditions in which it would not normally do so.
Plastic becomes brittle due to weak molecular bonds. This can be caused by:
- Low injection pressure
- Localised thermal degradation
- Condensation on the surface of a cold mould in humid conditions
- An injection screw with too low a compression ratio
- Excessive back pressure, screw RPM, or injection speed
To prevent plastic from becoming brittle, the following measures can be taken:
- Increasing the injection pressure
- Reducing nozzle temperature
- Adjusting the size and shape of the runner and gate
- Increasing the barrel temperature, screw RPM and back pressure
- Increasing the cycle time
Yes, plastic that has become brittle can be softened by remelting the plastic and adding new plasticizers.
