Generally speaking, customers with many years of experience in the injection molding industry are often capable of judging and selecting appropriate injection molding machines for their production needs. However, in certain situations, customers may require assistance from manufacturers to determine which specifications of injection molding machine to adopt.
Even customers who only have product samples or concepts may consult manufacturers about whether their machines can produce the desired products or which type of machine is more suitable. Additionally, some special products may require special equipment such as accumulators, closed-loop systems, or injection compression to produce more efficiently.
This highlights the extreme importance of selecting the right injection molding machine for production. Whether considering new equipment or evaluating used injection molding equipment, the same careful selection process applies. The key factors influencing injection machine selection typically include the mold, product, plastic material, and molding requirements. Therefore, before making a selection, the following information must be collected or available.
I. Key Factors in Injection Molding Machine Selection
Mold Specifications
- Mold dimensions (width, height, thickness) and weight
- Special designs or features that may affect machine compatibility
- Number of cavities and their arrangement within the mold
- Any special requirements for ejection or cooling systems
Plastic Materials
- Type(s) of plastic to be used (single material or multiple plastics)
- Material properties including melt flow index, viscosity, and shrinkage rate
- Required processing temperature range and heat sensitivity
- Whether the material requires special handling or processing conditions
Product Specifications
- External dimensions (length, width, height, thickness)
- Weight of the finished product
- Wall thickness distribution and structural complexity
- Surface finish requirements and cosmetic considerations
Molding Requirements
- Quality requirements and allowable tolerances
- Production speed and cycle time expectations
- Any special processes or techniques required
- Production volume and long-term manufacturing goals
Expert Insight
When evaluating used injection molding equipment, these factors become even more critical. While used injection molding equipment can offer significant cost savings, ensuring it meets all your specific requirements requires careful inspection and testing. Always verify that used injection molding equipment can handle your exact mold specifications, material requirements, and production demands before making a purchase decision.
II. Injection Molding Machine Selection Process
After obtaining information about the mold, product, plastic material, and molding requirements, you can follow these steps to select the appropriate injection molding machine, whether you're considering new equipment or quality used injection molding equipment.
Machine Type Selection
Determine the machine type and series based on the product and plastic material.
With the wide variety of injection molding machines available, the first step is to correctly determine which type or series of machine is suitable for producing the specific product. This decision matrix applies equally whether evaluating new equipment or used injection molding equipment.
Key Considerations:
- Material type: Whether it's general thermoplastic, thermosetting plastic, PET, or other specialty materials
- Color requirements: Monochromatic, two-color, multi-color, sandwich, or mixed color capabilities
- Special production requirements: High stability (closed-loop), high precision, ultra-high injection speed, high injection pressure, or rapid production (multi-loop) capabilities
When evaluating used injection molding equipment, it's crucial to verify that the machine's specifications match your exact material and production requirements. Many used injection molding equipment options can deliver excellent performance if properly selected and maintained, offering significant cost advantages over new machinery.
Determine Mold Installation Dimensions
Evaluate whether the machine's tie bar spacing, mold thickness capacity, minimum mold dimensions, and platen size are appropriate.
Proper mold fit is essential for efficient production, regardless of whether you're using new or used injection molding equipment. The mold must fit securely and operate correctly within the machine's parameters to ensure safe operation and quality output.
Tie Bar Spacing Check
The mold's length and width must be less than or at least one side must be less than the distance between the tie bars (格林柱内距).
Platen Size Consideration
The mold's length and width should ideally fit within the platen dimensions to ensure proper support and alignment.
Mold Thickness Range
The mold's thickness must fall within the machine's specified mold thickness range to ensure proper clamping force distribution.
Minimum Mold Dimensions
The mold's length and width must meet the minimum mold size recommended for the injection molding machine; too small a mold can cause issues.
When inspecting used injection molding equipment, pay special attention to the platen surfaces and tie bars for signs of excessive wear or damage that could affect mold alignment. Many reputable dealers of used injection molding equipment can provide detailed measurements and inspection reports to verify these critical dimensions.
Determine Product Ejection Requirements
Evaluate whether the mold opening stroke and ejection stroke are sufficient for product removal.
Ejection System Requirements:
- The mold opening stroke must be at least greater than twice the height of the product in the mold opening/closing direction, including the length of the runner system.
- The ejection stroke must be sufficient to completely eject the finished product from the mold cavity.
- When using a robotic arm for automation, additional space must be considered for the mechanical arm and gripper width during the ejection process.
For used injection molding equipment, thoroughly testing the ejection system is crucial. Verify that all ejection mechanisms function smoothly and can be adjusted to meet your specific requirements. Many suppliers of used injection molding equipment can perform these tests and provide demonstrations before purchase, ensuring the ejection system meets your production needs.
Determine Clamping Force
Calculate the required clamping force based on the product and plastic material characteristics.
When molten material is injected into the mold cavity at high pressure, it creates a mold expansion force. Therefore, the clamping unit of the injection molding machine must provide sufficient clamping force to prevent the mold from opening during injection. This is a critical consideration for both new and used injection molding equipment, as worn components can affect clamping force performance.
Clamping Force Calculation Method
Clamping Force = Projected Area × Number of Cavities × Mold Cavity Pressure
Projected area in cm², Mold cavity pressure in kgf/cm²
Mold cavity pressure varies with different materials, typically ranging from 350-400 kgf/cm² for general materials
The machine's clamping force must be greater than the calculated mold expansion force. For safety, the machine's clamping force should generally be more than 1.17 times greater than the mold expansion force.
When evaluating used injection molding equipment, it's essential to verify that the clamping force remains within the manufacturer's specifications. Many factors can affect the actual clamping force of used injection molding equipment, including wear on tie bars, bushings, and other critical components. Reputable sellers of used injection molding equipment can provide clamping force verification tests to ensure the machine meets the required specifications for your application.
Determine Injection Capacity
Calculate the required injection volume based on product weight and number of cavities.
The injection capacity must be determined based on the product weight and the number of cavities, then select the appropriate screw diameter. This calculation is equally important when evaluating used injection molding equipment, as worn screws and barrels can affect actual injection volume and consistency.
Injection Capacity Guidelines:
- Calculate the total shot weight considering the number of cavities (how many parts per mold)
- For safety, the injection capacity should be at least 1.35 times the total product weight
- Conversely, the product weight should be within 75% of the machine's maximum injection capacity
- Consider material density variations and runner system volume in calculations
When assessing used injection molding equipment, inquire about the condition of the screw and barrel assembly, as these components directly affect injection capacity and material processing. Many dealers of used injection molding equipment can provide detailed inspection reports on these critical components, helping you make an informed decision about whether the machine can meet your production requirements.
Determine Screw Configuration
Select appropriate screw compression ratio and injection pressure based on plastic material.
Certain engineering plastics require higher injection pressure and appropriate screw compression ratio design to achieve better molding results. Therefore, to ensure optimal product quality, the screw must be selected based on injection pressure requirements and compression ratio, whether you're purchasing new equipment or evaluating used injection molding equipment.
Screw Selection Considerations:
- Material viscosity and flow characteristics determine optimal compression ratio
- Heat sensitivity of the material influences screw design and L/D ratio
- Generally, smaller diameter screws can provide higher injection pressure
- Screw geometry affects melting efficiency and mixing capabilities
When considering used injection molding equipment, the condition and configuration of the screw are paramount. Many suppliers of used injection molding equipment offer screw and barrel reconditioning services or can provide machines with screws specifically configured for your material requirements. Investing in properly configured used injection molding equipment can yield production results comparable to new machines at a fraction of the cost.
Conclusion
By following these comprehensive steps, you can select the optimal injection molding machine for your specific production requirements. Whether you're investing in new equipment or evaluating quality used injection molding equipment, this systematic approach ensures that all critical factors are considered.
Used injection molding equipment can be an excellent investment when properly evaluated using these selection criteria. Many used injection molding equipment options offer years of reliable service at significantly lower upfront costs, making them particularly attractive for small to medium production runs or specialized applications.
Remember that the selection process requires careful consideration of your mold specifications, material characteristics, product requirements, and production goals. Taking the time to properly evaluate each factor will result in a more efficient production process and higher quality end products.
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