Ethan Clarke
Plastic moulding is a process that involves injecting molten plastic into a mould to create custom plastic parts. The process was invented in 1868 by John Wesley Hyatt, who discovered a way to make plastic billiard balls by injecting celluloid into a mould. Today, plastic moulding is used to create a wide range of products, from car parts to surgical tools. The process involves filling the mould as quickly as possible, ensuring that the plastic pushes out all the air through vents, and then allowing the mould to cool and solidify. The mould's geometry, size, and layout are critical factors that impact the final product's quality and integrity.
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What You'll Learn
Runner geometry, size, layout, and cooling
The runner is a crucial component of the injection molding process, guiding the molten plastic from the injection nozzle to the mold cavities. The geometry, size, layout, and cooling of the runner are all critical factors in ensuring the success of the molding process.
Geometry
The geometry of the runner, including its shape and dimensions, is important in ensuring the smooth flow of plastic melt and the discharge of the item. Trapezoidal or semi-circular cross-sections are commonly used, with the exterior finished to minimise stream resistance and produce a quicker filling speed. The dimensions of the runner vary depending on the type of plastic material, the dimensions of the final product, and the thickness of the item. For most thermoplastics, the runner’s cross-sectional circumference ranges from a minimum of 2-3 millimeters to a maximum of 8-12 millimeters.
Size
The size of the runner is critical in ensuring the uniform filling of cavities and minimising waste. Smaller cross-sectional areas are preferable as they reduce plastic consumption and cooling time. However, larger cross-sectional areas are sometimes necessary to ensure adequate packing pressure. The length of the runner should also be minimised to avoid excessive cooling and pressure losses, with runs ideally kept below 2 feet in length.
Layout
The layout of the runner system should be optimised to ensure the rapid filling of cavities and the efficient cooling and ejection of parts. A balanced runner system, where the molten plastic enters the cavity simultaneously from various gates under identical temperature and pressure conditions, is advantageous as it produces uniform parts and minimises cycle time. The layout of the runner should also take into account the specific requirements of the molding process, including the product design, mold construction, and injection molding process parameters.
Cooling
The cooling of the runner is essential to ensure the plastic solidifies and retains its shape. The cooling method depends on the size and shape of the mold, with temperature control technology used to regulate the heat level. Drilling cooling water lines inside the mold and using a moving cool liquid to remove heat is a popular method. The cooling time can be shortened by creating a smaller cross-sectional area in the runner, reducing the volume of plastic that needs to be cooled.
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Injection pressure and speed
The injection speed controls the melt filling rate, and a higher injection speed is generally preferred as it reduces heat loss and increases melt temperature. The exterior of the runner should be finished to minimise stream resistance and produce a quicker filling speed. The dimensions of the runner vary according to the type of plastic material, the dimensions, and the thickness of the item.
The relationship between injection pressure and speed is interactive and interdependent. At the same injection speed, higher injection pressure improves the plastic's flow capability, enhancing the dimensional precision and surface smoothness of the product. However, excessive injection pressure can cause excessive mould force and destabilise the injection process. Therefore, injection pressure and speed must be carefully adjusted based on specific production requirements and material characteristics to achieve optimal moulding results.
The injection pressure should be high enough to fill the mould cavity completely without causing any defects or voids in the finished product. It is crucial to monitor and control the injection pressure to ensure consistent and high-quality results. The fill pressure, pack pressure, and hold pressure are the three types of pressure parameters encountered in the injection moulding process. The fill pressure fills the mould to a certain level, the pack pressure ensures the plastic reaches all corners and crevices, and the hold pressure is applied during the plastic cooling phase to minimise shrinkage.
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Packing time and pressure
The packing pressure helps to ensure a dense part, moulded with uniform pressure and controlled shrinkage. It compensates for the shrinkage of the plastic as it cools, preventing defects such as "sink" and "void". Packing pressure is typically applied when the mould is about 95% full, with the remaining 5% filled under packing pressure. This pressure is maintained until the gate freezes, at which point the cooling process begins.
The filling and packing time must be sufficient to allow the plastic to reach the furthest extremities of the cavity and pressurise those areas to ensure minimum shrinkage. The holding time, or the time under packing pressure, must exceed the time required for the gate to freeze to avoid losing cavity pressure. The holding time is dependent on the wall thickness of the part being moulded and should not be prolonged beyond the gate freezing time.
The packing pressure is typically lower than the injection pressure and is adjusted based on the part design and dimensional requirements. If the holding pressure is set too high, the product may be prone to flashing, overfilling, or stress near the gate. Conversely, if the holding pressure is too low, excessive shrinkage and dimensional instability may occur.
Overall, the packing time and pressure play a crucial role in ensuring the quality of the moulded part, preventing defects, and controlling shrinkage.
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Venting and ejecting
Venting
Venting is the process of allowing trapped air and gases to escape from the mold cavity as molten plastic is injected. This prevents defects such as air pockets, burn marks, and incomplete fills. There are several venting methods available:
- Parting Lines: Vents are placed at the parting lines, where the two halves of the mold meet. This is a natural location for venting as gases are released when the mold halves come together.
- Vent Pins: Ejector pins with grooves along their bodies are used for venting during ejection. These are particularly useful for deep or intricate molds.
- Tool Clearances: Using the gaps or clearances between mold components, such as around ejector pins, for venting. This method is suitable for molds with deep cavities, complex shapes, or small precision parts.
- Insert-piece Venting: Venting channels or grooves are integrated into the mold inserts. This method provides flexible vent placement without compromising the mold's structural integrity.
- Porous Materials: Using porous materials like sintered metal inserts in the mold allows air to pass through while blocking the molten plastic.
The placement of vents is critical to effective venting. Vents should be placed directly opposite the gate, at runner ends, and at the end of fill. The depth of the vents also varies depending on the type of plastic used.
Ejecting
The function of an ejection system is to enable the removal of the molded component from the mold once it has solidified. There are several ejection methods available:
- Ejector Pins: These are the most commonly used ejection method. Ejector pins apply force to eject the molded part and can also serve as venting paths. They are cost-effective but may leave marks on the finished part.
- Ejector Sleeves: Small round components can be ejected using ejector sleeves, which distribute ejection forces more evenly than pins. However, they are more costly and tend to wear out faster.
- Valve Ejectors: Valve ejectors have various designs and applications, typically offering large projected areas for load transmission.
To ensure smooth ejection, it is important to polish the mold surfaces that come in contact with the plastic melt. This helps to eliminate scratches and indentations that can cause cracks during ejection.
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Mold cooling methods
The success of plastic injection molding depends on the cooling stage, which ensures uniform solidification and minimises distortion, warping, or internal stress within the various injection-moulded parts. The two main types of cooling in injection moulding are standard/traditional cooling and conformal cooling.
Standard/Traditional Cooling
Standard or traditional cooling involves cooling channels machined into the mould tool through standard methods such as drilling or milling. These channels are typically straight lines, though they may intersect or be enhanced with additional devices such as baffles and bubblers. This method is best suited for parts with relatively simple geometries to ensure effective cooling.
Conformal Cooling
Conformal cooling is used for more complex parts, introducing cooling channels that closely follow the geometry of these parts. This process helps cool these parts consistently, despite having difficult-to-reach areas. Conformal cooling is commonly used for complex components, and the cooling time directly impacts the cycle time—the shorter the cooling passage, the better.
Other Cooling Methods
Water cooling is the most commonly used cooling method for most moulds. However, it requires good pipeline sealing and unobstructed upper and lower water pipelines. Air cooling, on the other hand, is a more ideal method as it does not require tight pipeline sealing, can cool moulds with temperatures higher than 100°C, and the cooling speed can be adjusted by the flow rate of the gas.
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Frequently asked questions
Plastic injection moulding involves injecting molten plastic material at high pressure into a metal mould. The mould is then cooled and opened to reveal a solid plastic part.
The mould should be filled as quickly as possible and then switched to packing mode. This is to ensure the plastic does not cool too quickly and to avoid air bubbles.
The most popular way of cooling a mould is by drilling cooling water lines inside it and working with a moving cool liquid to take out the mould's heat.
