Views: 0 Author: Site Editor Publish Time: 2025-06-17 Origin: Site
Selecting the correct sprue bushing is critical for achieving high-quality injection molded parts and ensuring process efficiency. A well-chosen sprue bushing improves flow, prevents defects, and helps control cooling time. This guide will walk you through how to choose the right sprue bushing step by step, covering types, sizing, material compatibility, and common pitfalls to avoid.
A sprue bushing is a mold component that guides molten plastic from the injection molding machine’s nozzle into the mold. It connects the machine to the runner system, ensuring smooth material flow. Selecting the right sprue bushing type, size, and material directly impacts molding quality and cycle efficiency.
Pick the right sprue bushing type. Use cold for simple, low-number molds. Use hot for complex, high-number molds. This helps save money or cut down on waste.
Match the sprue bushing material and head type to your mold and machine. This stops leaks and damage. It also helps seal better.
Choose the right orifice size for the part weight and plastic type. This makes the flow smooth. It helps stop defects and keeps cooling time balanced.
Check the fit by measuring diameter, length, taper, and spherical radius. This helps stop leaks and keeps things lined up right.
Use cooling features like channels or copper alloy bushings. Try special options like string eliminators. These help make parts better and speed up the cycle time.
There are two main types of sprue bushings: cold and hot. Each type suits different production needs.
Cold Sprue Bushing: Guides molten plastic into the mold and allows it to solidify inside the sprue. The sprue must be manually removed after each cycle. Suitable for simple molds and low-volume production. Lower cost but produces more waste.
Hot Sprue Bushing: Maintains the plastic’s temperature using built-in heaters, reducing material waste and improving cycle times. Suitable for high-volume and complex molds. Higher initial cost but minimizes waste.
| Type | Best For | Key Benefit | Drawback |
|---|---|---|---|
| Cold Sprue | Simple, low-volume | Lower cost | More waste |
| Hot Sprue | Complex, high-volume | Less waste, faster | Higher cost |
The head type determines how the bushing connects to the mold and machine nozzle.
Flat Head: Sits flush with the mold plate and is common for standard molds.
Radius Head (B Series): Matches the machine nozzle’s curve for better sealing.
Tapered Head: Provides tight sealing and prevents leakage.
Some sprue bushings include advanced features:
String Eliminators: Prevent plastic stringing during mold opening.
Cooling Channels: Control temperature around the sprue for better cooling and faster cycle times.
Choose a sprue bushing material that can handle your process temperature and injection pressure. Stainless steel is a common choice due to its durability and rust resistance. For abrasive or corrosive plastics, select tougher alloys or coated bushings.
When using hot runner systems or molds with special requirements, ensure the bushing is designed for high heat or additional cooling.
The sprue bushing must precisely fit the mold and injection machine to prevent leaks and misalignment. Key measurements include:
Outer Diameter: Must match the mold base hole.
Head Thickness and Shape: Must sit flush with the mold plate.
Length: Must match the mold stack height.
Head Radius: Must align with the machine nozzle tip.
Check these dimensions carefully to ensure a secure fit.
The standard taper is 1/2 inch per foot to ensure smooth plastic flow and prevent leakage. The spherical radius at the bushing head must match the nozzle tip to ensure proper sealing.
Always double-check the taper and spherical radius based on your injection machine’s specifications.
The orifice size controls the flow of molten plastic into the mold. Choosing the wrong size can cause slow filling, high injection pressure, or molding defects.
For most applications, an orifice diameter between 4 mm and 8 mm is suitable.
Recommended orifice sizes:
| Part Size | Typical Orifice (mm) |
|---|---|
| Small (<10g) | 2 – 4 |
| Medium (10-100g) | 4 – 6 |
| Large (>100g) | 6 – 10 |
A simple formula to estimate orifice size:
Orifice Diameter (mm) = √(Part Weight (g) / 1.5)
Adjust the size based on plastic flow behavior and part complexity.
Use smaller orifices for thin-walled parts.
Use larger orifices for thick-walled or large parts.
Increase orifice size if injection pressure is too high.
Decrease orifice size if stringing or drooling occurs.
A smaller orifice increases shear rate and can overheat the plastic. Excessive shear may degrade heat-sensitive materials.
Cooling time depends on orifice size:
Larger orifices take longer to cool, which may slow the cycle.
Smaller orifices cool faster but may cause stringing if not managed well.
You can improve cooling by:
Adding cooling channels around the bushing.
Using shorter sprues.
Selecting copper alloy bushings for faster heat transfer.
Use shorter bushings with the correct taper to improve flow and ease sprue removal.
Choose copper alloy bushings to improve cooling efficiency.
Add cooling channels to control sprue temperature.
Match the head type to your mold and machine nozzle.
Use string eliminators to prevent unwanted plastic strings.
| Mistake | Result |
|---|---|
| Poor fit | Leaks, flash |
| Wrong head type | Nozzle damage |
| No relief (3-plate mold) | Mold damage |
| Wrong orifice size | Defects, slow cycles |
| Missing cooling channels | Overheating, longer cycles |
Always double-check measurements and material compatibility before ordering parts to avoid costly production issues.
Follow this step-by-step checklist to ensure you select the right sprue bushing:
Identify Mold Type
Cold or hot sprue bushing?
Two-plate or three-plate mold?
Check Material Compatibility
Select material based on molding plastic.
For abrasive plastics, choose tougher alloys or coatings.
Verify Machine and Mold Fit
Measure outer diameter, length, and head shape.
Confirm bushing head matches machine nozzle.
Set Taper and Spherical Radius
Use standard taper unless specified otherwise.
Match spherical radius to nozzle tip.
Determine Orifice Size
Start with 4-8 mm or use the orifice size formula.
Adjust based on part size and flow behavior.
Plan for Cooling
Add cooling channels if cycle time is long.
Consider copper alloy bushings for better heat dissipation.
Review Special Features
Decide if string eliminators or other features are needed.
Careful planning ensures a perfect fit and smoother molding operations.
What is the purpose of a sprue bushing?
A sprue bushing directs molten plastic from the injection machine nozzle into the mold. It prevents leakage and ensures smooth material flow.
How do you check if the sprue bushing fits the machine?
Measure the outer diameter, head shape, length, and spherical radius. Ensure the bushing aligns with the machine nozzle without gaps or leaks.
Can you reuse a sprue bushing in a new mold?
Yes, if the size, head type, and material are compatible with the new mold. Always inspect the bushing for wear before reuse.
What happens if the orifice size is wrong?
An incorrect orifice size can cause high pressure, slow filling, stringing, drooling, or long cooling times.
