Sofia Martinez
Selecting the right plastic injection moulding machine is a significant challenge for manufacturers. The traditional approach involves detailed calculations of clamping force, mechanical mould evaluations, and hands-on trials, which can be time-consuming and costly. Important attributes to consider include shot weight, clamping force, and injection volume. The injection unit's screw diameter governs the available maximum injection pressure, so it is critical to choose the correct diameter. Other factors to consider are the mould weight and size, injection rate, and screw surface speed.
| Characteristics | Values |
|---|---|
| Screw diameter | Should be considered with the maximum injection pressure in mind, as large screw diameters can cause quality problems due to material degradation |
| Shot weight | Should be considered in relation to the job's needs, not individually |
| Clamp tonnage | Should be high enough to produce quality parts; low clamp tonnage can cause inconsistent weights, flash, short shots, wall section variation, poor surface finish and size variation |
| Injection volume | Should be considered in relation to the job's needs, not individually |
| Injection rate | Alternative to injection speed; expressed in cm3/s |
| Injection speed | Affects injection time; high injection speed is required for moulding thin wall articles |
| Screw surface speed | Each plastic material has a recommended maximum SSS which must not be exceeded |
| Plasticizing capacity | The amount of PS that a PIMM can uniformly plasticize in one hour at max screw rotary speed with 0 back pressure |
| Clamping force | Should be considered in relation to the job's needs, not individually |
| Machine type | Hydraulic machines are defined by the type of clamp design, type of hydraulic pump design and the presence or absence of an accumulator |
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What You'll Learn
Injection pressure and speed
The injection speed refers to the screw's forward speed, but the crucial factor is the flow speed of the melt within the mould cavity, which depends on the cross-sectional area in the flow direction. The injection speed controls the melt filling rate and is adjusted to achieve the desired production requirements and material characteristics. A higher injection speed can improve the plastic's flow capability and enhance dimensional precision and surface quality.
The relationship between injection pressure and speed is interactive and directly impacts the injection moulding process. At the same injection speed, higher injection pressure can improve the plastic's flow capability and enhance the dimensional precision and surface quality. However, it is important to balance the injection pressure and speed to avoid defects or voids in the finished product.
To achieve optimal moulding results, injection pressure and speed must be adjusted based on specific production requirements and material characteristics. The injection pressure should be high enough to fill the mould cavity completely without causing defects or damaging the mould or plastic. The injection speed should be set to ensure a good appearance and quality of the product, with the filling speeds near the gate, in the main body, and at the flow end carefully controlled.
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Screw diameter
The screw diameter is a critical factor in selecting a plastic injection moulding machine. It determines the maximum injection pressure and shot size available. The larger the diameter, the lower the injection pressure and the larger the shot size.
When selecting the screw diameter, it is essential to consider the application requirements. Choosing the largest diameter possible is not always advantageous. Instead, the diameter should be chosen based on the specific needs of the application to ensure optimal quality and productivity.
For example, when working with plastics like PP, PE, and PS, the general-purpose screw provided by most manufacturers may not be suitable. The 20/80 rule suggests that the shot size should be 20 to 80 percent of the injection unit's shot capacity for these resins. Therefore, a mixing screw or a barrier mixing screw may be more appropriate for certain applications.
Additionally, the screw diameter affects other aspects of the injection moulding process. It impacts the injection stroke, injection weight, injection pressure calculations, and injection speed. Engineers use specific formulas to determine these values based on the screw diameter and other variables, ensuring precise control over the injection moulding process.
Moreover, the screw diameter is just one aspect of screw design. The length-to-diameter ratio (L/D) is another critical factor. Different plastics have different L/D requirements. For instance, PA and PMMA plastics typically require a mutant screw with an L/D ratio of 18-20, while PET plastics generally use an L/D of 20.
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Plastic material selection
There are several important factors to consider when selecting a plastic material for injection moulding. The first property to consider is the desired tensile strength, or resistance to being pulled apart, of the product. Next is the Izod impact or toughness, which is the impact resistance of the material. The flexibility or bending stiffness of the material, also known as the flexural modulus, is another important factor. Electrical insulation properties or dielectric strength should also be considered to determine how well the plastic will prevent the flow of electrical charge.
Other factors to consider include the desired yield and tensile strength, the relationship between stress and temperature, the relationship between temperature and ageing modulus, the maximum short-use temperature, and the melt flow rate. The desired end use of the product is also an important consideration, as some plastics are better suited for specific applications than others. For example, LDPE is commonly used for bottles, plastic bags, and plastic wraps, while HDPE is used for more solid items such as toys and recycling bins.
Some common plastic materials used in injection moulding include ABS, a tough and impact-resistant plastic widely used across industries, and polypropylene (PP), which is cheap but lacks rigidity and cosmetic appeal. Nylon is another versatile injection moulding material used in various applications such as apparel, car tires, and electrical insulation. Acrylic is a strong, clear thermoplastic that provides a lightweight and shatter-resistant alternative to glass, although it is limited in heat resistance and can crack under pressure. Polycarbonate is a strong and lightweight thermoplastic with good transparency, although it contains BPA, so it is not suitable for food preparation or storage.
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Shot weight
The shot weight is influenced by the screw diameter, which determines the maximum injection pressure. A larger screw diameter can cause quality problems due to material degradation, especially with small shot sizes. This is because a smaller shot size will spend more time being heated by the barrel heaters, leading to inconsistent shot weights and material degradation. Therefore, it is important to ensure that the shot size is appropriate for the screw diameter to avoid these issues.
The shot weight also affects the injection capacity of the machine. The injection capacity is typically specified in terms of General Purpose Polystyrene (GPPS) by manufacturers. To use a different material, the melt density of that material must be considered to calculate the injection capacity accurately. The injection capacity should be within the recommended range of 25%-65% to ensure good quality parts.
When selecting a machine, it is recommended to choose one capable of producing 30-40% greater shot sizes than required. This allows for flexibility in the production process and ensures that the machine can handle higher capacities if needed. Additionally, factors such as tie bar spacing, ejector stroke, platen size, and tonnage should also be considered when selecting a plastic injection moulding machine.
In summary, the shot weight plays a crucial role in determining the screw and barrel assembly, injection capacity, and overall machine selection for plastic injection moulding. By considering the shot weight and other relevant factors, one can choose a suitable machine that ensures efficient production and high-quality parts.
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Clamp design
Clamp Geometry
The geometry of the clamp refers to its design and configuration, including the shape, size, and arrangement of clamping components. Proper clamp geometry ensures uniform force distribution across the mould surface, preventing uneven pressure points that could lead to mould deformation or premature wear. When moving a mould between machines with different clamp designs, such as toggle and hydraulic clamps, the geometry can influence part geometry and surface finish.
Clamp Force
The clamp force is the pressure applied to keep the mould halves securely closed during injection. It must be greater than the injection pressure to prevent the mould from opening prematurely. Insufficient clamp force can lead to part distortion, variation, and excess plastic escaping from the mould (known as "flash"). The required clamp force depends on factors such as the projected area of the part, the number of cavities, the material used, wall thickness, injection pressure, and mould temperature.
Type of Clamp
The type of clamp used can vary, with common options including T-slot clamps and forged, closed-toe clamps. The choice depends on the specific application, including the size and weight of the moulds, the machine size, and the required clamping force.
Safety Considerations
Safety is a crucial aspect of clamp design. The clamp must securely hold the mould to protect workers from potential hazards. This includes ensuring that the clamp bolts are of the correct length and thread type and are properly lubricated to reduce friction and increase preload. The safety strap and eyebolts should also be in the correct position and in good condition to prevent the mould from tilting or causing damage to the platen.
Machine Compatibility
The clamp design must be compatible with the plastic injection moulding machine. For hydraulic machines, the clamp design is a defining factor, along with the hydraulic pump design and the presence or absence of an accumulator. The machine manuals will often specify the minimum mould size and other important considerations for clamp selection.
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Frequently asked questions
Important attributes to consider when selecting a plastic injection moulding machine (PIMM) include quantifiable attributes like shot weight, clamping force, and injection volume. These attributes should be considered together based on the job's needs. Other factors to consider are the injection unit's screw diameter, injection rate, and injection speed.
Shot weight is the measured weight of the plastic injected when the nozzle is freestanding. It is calculated as shot weight in resin multiplied by specific gravity (SG) of resin and divided by 1.05.
The injection unit's screw diameter determines the maximum injection pressure. Therefore, choosing the correct diameter is critical when selecting a PIMM.
Machine learning can enhance PIMM selection efficiency and aid decision-making by considering criteria such as product type, material properties, and mould specifications. It can accurately predict machine selection, as demonstrated in a study using data from the automotive plastics industry. However, the traditional approach of detailed calculations, evaluations, and trials may still be preferred for its ability to provide hands-on experience and expertise.
