Selecting the Right Injection Molding Machines
Choosing appropriate injection molding machines is a critical decision in the manufacturing process, directly impacting production efficiency, product quality, and overall costs. This comprehensive guide outlines the key factors and methodologies for selecting the optimal injection molding machines for your specific application.
Fundamentals of Injection Molding Machines Selection
The determination of specifications and models for injection molding machines is primarily based on the size of the plastic parts, the number of cavities, and the production volume. When selecting injection molding machines, several critical parameters must be considered to ensure optimal performance and product quality.
Key considerations for injection molding machines include plasticizing rate, rated injection volume, rated clamping force, effective mold installation area (distance between tie bars of the injection molding machines), ejection stroke, and other essential factors. These parameters form the foundation for matching the right equipment to your specific manufacturing requirements.
If the specifications and model of the injection molding machines have been determined before mold design, designers must verify parameters such as mold installation dimensions and ejection stroke. If these requirements cannot be met, either the number of mold cavities must be changed, the mold size must be reduced, or discussions with the customer about replacing the injection molding machines must occur.
Key Selection Criteria
- Rated injection volume capacity
- Clamping force requirements
- Mold installation dimensions
- Opening stroke capabilities
- Plasticizing efficiency
1. Selecting Injection Molding Machines Based on Rated Injection Volume
One of the primary considerations when selecting injection molding machines is ensuring the equipment can handle the required material volume for each shot. The rated injection volume of injection molding machines directly determines the maximum amount of material that can be injected in a single cycle.
The calculation for determining the appropriate injection volume capacity in injection molding machines follows this formula:
(Total weight of plastic parts in each cavity + sprue system condensate) < (Rated injection volume of injection molding machines × 80%)
This formula ensures that injection molding machines are not operating at their maximum capacity, which can lead to inconsistent results and potential equipment strain. The 80% factor provides a safety margin for optimal performance of injection molding machines.
Important Note
The calculated value must not be rounded. When selecting injection molding machines, you must always round up to the next whole number to ensure sufficient capacity. This conservative approach prevents under-sizing injection molding machines for your production needs.
Consequences of Incorrect Injection Volume
- Insufficient volume leads to incomplete part formation
- Excessive machine capacity causes material waste
- Inconsistent filling affects product quality
- Material degradation from extended residence time
Injection Volume Calculation Steps
- Calculate total part weight per cavity
- Multiply by number of cavities
- Add estimated weight of sprue and runner system
- Divide by 0.8 to determine minimum required injection volume
- Select injection molding machines with equal or greater capacity
2. Selecting Injection Molding Machines Based on Rated Clamping Force
The clamping force is another critical parameter when selecting injection molding machines. It refers to the force that injection molding machines can exert to keep the mold closed during the injection process. Insufficient clamping force in injection molding machines can result in flash (excess material escaping between mold halves) and compromised part quality.
When evaluating the clamping force requirements for injection molding machines, the following formula should be applied:
Sproj × Pm < Fclamp × 80%
Where:
- Sproj: Sum of the projected areas of plastic parts in each cavity on the parting surface (mm²)
- Pm: Average pressure of the plastic melt on the cavity
- Fclamp: Rated (or nominal) clamping force of the injection molding machines
The 80% factor in the formula provides a safety margin to account for variations in material viscosity, processing conditions, and other factors that might affect the actual clamping force requirements during production with injection molding machines.
Melt Pressure Considerations for Different Materials
The average melt pressure (Pm) varies depending on the type of plastic material being processed, which is a crucial factor when selecting injection molding machines:
| Material Type | Typical Melt Pressure (MPa) | Considerations for Injection Molding Machines |
|---|---|---|
| Low viscosity materials (PE, PP) | 30-50 | Lower clamping force requirements |
| Medium viscosity materials (PS, ABS) | 50-70 | Moderate clamping force requirements |
| High viscosity materials (PC, PMMA) | 70-100 | Higher clamping force requirements |
| Engineering plastics (POM, PA) | 60-90 | Depends on specific grade and filler content |
3. Selecting Injection Molding Machines Based on Installation Dimensions
The physical dimensions of the mold must be compatible with the installation space available in the selected injection molding machines. This is a critical practical consideration that is often overlooked but essential for successful production with injection molding machines.
A key dimension to verify is the distance between the tie bars of the injection molding machines. The tie bars are the large rods that guide the movement of the moving platen and provide the structural support for the clamping force.
For proper installation in injection molding machines, the mold's width must be less than the distance between the tie bars:
A > C
Where A is the distance between tie bars of the injection molding machines, and C is the width of the mold. This ensures that the mold can be properly inserted into the injection molding machines and secured between the platens.
Additional Installation Dimensions to Verify
Fig. 2-23: Mold width must be less than tie bar spacing in injection molding machines
- Minimum and maximum mold height: Ensures the mold can be properly clamped between the platens of injection molding machines
- Platen size: Must accommodate the mold's overall dimensions without interference
- Clamping bolt pattern: Must match or be adaptable to the mold's mounting holes
- Ejection system location: Must align with the mold's ejection requirements
Installation Checklist for Injection Molding Machines
- Tie bar spacing > mold width
- Platen size accommodates mold dimensions
- Mold height within machine specifications
- Mounting hole pattern compatibility
- Ejection stroke sufficient for part removal
- Access for mold cooling connections
- Space for part removal mechanism
- Compatibility with hot runner systems (if used)
4. Selecting Injection Molding Machines Based on Opening Stroke
The opening stroke of injection molding machines refers to the maximum distance the moving platen can travel away from the fixed platen. This dimension is critical because it determines whether the mold can open sufficiently to allow for part removal. Different models of injection molding machines have varying ejection devices and maximum ejection distances, so it's essential to ensure compatibility between the injection molding machines' capabilities and the mold's requirements.
Two-Plate Mold Opening Stroke
Fig. 2-24: Two-plate mold opening stroke requirements for injection molding machines
1- Panel; 2- Fixed mold A plate; 3- Moving mold B plate; 4- Support plate; 5- Spacer block; 6- Base plate; 7- Locating ring
For two-plate molds, the calculation for the minimum opening stroke required is:
Minimum opening stroke for two-plate molds = H1 + H2 + (5~10mm)
Where:
- H1: Minimum distance required to eject the plastic part
- H2: Total height of the plastic part and sprue system condensate
- 5~10mm: Safety margin to ensure proper part removal
This calculation ensures that injection molding machines can open sufficiently to allow for easy removal of the molded parts while maintaining proper mold alignment and function.
Three-Plate Mold Opening Stroke
Three-plate molds have more complex opening requirements due to the additional plate that separates the sprue from the part cavities. For these molds, the calculation for the minimum opening stroke required when selecting injection molding machines is:
Minimum opening stroke for three-plate molds = H1 + H2 + A + C + (5~10mm)
Where:
- H1: Minimum distance required to eject the plastic part
- H2: Total height of the plastic part
- A: Sprue gate; for three-plate molds, this is the sprue system condensate height B + 30mm, and the distance A must be greater than 100mm to facilitate removal
- C: 6~10mm, representing the distance between mold plates when opened
- 5~10mm: Safety margin to ensure proper operation
Fig. 2-25: Three-plate mold opening stroke requirements for injection molding machines
1- Sprue bushing; 2- Sprue puller; 3- Guide pin; 4- Panel support plate; 5- Runner stripper plate;
6- Fixed mold A plate; 7- Moving mold B plate; 8- Spacer block; 9- Base plate
Selection Principles for Opening Stroke in Injection Molding Machines
When evaluating injection molding machines, two critical stroke-related parameters must be verified:
-
Maximum stroke capability
The maximum stroke of the moving platen (Smax) of the injection molding machines must be greater than the minimum opening stroke required by the mold. This ensures that the mold can open sufficiently to allow for part removal.
-
Minimum platen spacing
The minimum distance between the moving and fixed platens (Hmin) of the injection molding machines must be less than the minimum thickness of the mold. This ensures that the mold can be properly clamped when closed.
These principles ensure that the selected injection molding machines can properly accommodate the mold throughout the entire molding cycle, from the closed position during injection to the fully open position for part removal.
Fig. 2-26: Opening stroke range of injection molding machines
Key Stroke Relationships
- Mold thickness: Hmold
- Minimum platen spacing: Hmin < Hmold
- Maximum opening stroke: Smax > Required stroke
- Maximum platen spacing: Hmax = Hmin + Smax
Comprehensive Selection Process for Injection Molding Machines
Selecting the right injection molding machines requires a systematic approach that considers all the factors discussed. This comprehensive evaluation ensures that the chosen injection molding machines will meet production requirements, maintain product quality, and operate efficiently.
Step-by-Step Selection Process
- Calculate total shot weight requirements
- Determine required clamping force based on part geometry and material
- Measure mold dimensions and verify compatibility with injection molding machines
- Calculate necessary opening stroke for part removal
- Evaluate plasticizing capacity for production volume requirements
- Consider additional features based on specific application needs
- Verify all parameters against manufacturer specifications for injection molding machines
Balancing Multiple Factors
Selecting injection molding machines often involves balancing competing requirements:
- Machine size vs. energy consumption
- Production speed vs. part quality
- Initial investment vs. long-term productivity
- Specialized features vs. versatility
- Current requirements vs. future expandability
By carefully evaluating all these factors and following a systematic selection process, manufacturers can choose injection molding machines that will provide optimal performance, reliability, and value. Properly selected injection molding machines form the foundation of efficient, high-quality plastic manufacturing operations, ensuring that production targets are met while maintaining consistent part quality.
The proper selection of injection molding machines is a critical decision that impacts every aspect of the manufacturing process. By following the guidelines outlined here and carefully evaluating all relevant parameters, you can ensure that your chosen injection molding machines will meet your production needs, maintain product quality, and operate efficiently for years to come.
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