Choosing an injection molding screw is not only about selecting a machine with enough clamping force. You also need a screw and barrel that match your shot size, material and production requirements.
Many injection molding machines offer A, B and C screw options. These options usually have different screw diameters. A smaller screw can provide higher injection pressure, while a larger screw can provide a larger shot.
So, which one should you choose? The answer depends on what you are molding.
What Is the Difference Between A, B and C Screws?
For many injection molding machines, the A, B and C options represent different screw diameters for the same machine model.
- A screw: Usually the smallest diameter.
- B screw: Usually the middle diameter.
- C screw: Usually the largest diameter.
A smaller screw generally provides a smaller shot capacity and higher available injection pressure. A larger screw generally provides a larger shot capacity and lower available injection pressure.
The exact specifications depend on the machine and injection unit. Always check the manufacturer’s data sheet before making a decision.
For example, one machine manufacturer lists a model with 55 mm, 60 mm and 65 mm A/B/C screws. As the diameter increases, its listed shot volume increases and its listed injection pressure decreases. This is an example of the comparison, not a specification for every machine. Source: Union Power machine specifications
A, B and C Screw Comparison
| Feature | A screw | B screw | C screw |
|---|---|---|---|
| Typical diameter | Smaller | Medium | Larger |
| Shot capacity | Smaller | Medium | Larger |
| Available injection pressure | Higher | Medium | Lower |
| Typical starting point | Smaller shots or higher pressure needs | General-purpose production | Larger shots |
The comparison applies to typical options for the same machine or injection unit. Actual values vary by manufacturer and model.
Why Does Screw Diameter Affect Pressure and Shot Size?
During injection, the screw moves forward and pushes molten plastic into the mold.
If the injection unit applies the same forward force, a smaller screw area can produce higher pressure. A larger screw area spreads that force over a larger area, so the available pressure is lower.
Shot volume also depends on screw diameter and injection stroke. When the stroke is the same, a larger diameter provides more space for the melt pushed forward.
However, machines can have different strokes, drive systems and pressure limits. That is why you should compare the actual specifications instead of relying on diameter alone.
When Should You Consider an A Screw?
An A screw is often a useful starting point when your application needs a smaller shot or more injection pressure.
It may be worth considering for:
- Small parts or molds with a low total shot weight.
- Thin-wall parts that need higher filling pressure.
- Materials or mold designs that create high resistance to flow.
- Applications where a large barrel may keep the material inside for too long.
For example, if a mold uses a small amount of plastic each cycle, an oversized barrel may increase material residence time. A smaller injection unit can help bring the shot size closer to a suitable operating range.
But an A screw is not automatically the best choice for every engineering plastic or precision part. You still need to check the required shot capacity, plasticizing rate and material processing conditions.
When Should You Consider a B Screw?
A B screw can be a practical starting point when both shot capacity and pressure fit the job.
It may suit a factory that runs different molds and common materials on the same machine. Its middle size can offer a balance between the smaller A option and the larger C option.
But “middle size” does not mean it is always the safest choice. If your shot is too small, the material may spend too long in the barrel. If your shot is too large, the machine may struggle to prepare enough melt within the required cycle time.
Choose B because its specifications fit your production, not simply because it is in the middle.
When Should You Consider a C Screw?
A C screw is often considered when the mold needs a larger shot.
It may be suitable for:
- Larger parts.
- Multi-cavity molds with a higher total shot weight.
- Applications where the B option does not provide enough capacity.
- Products that can be filled with the pressure available from the larger injection unit.
For example, a larger screw may help when the machine has enough clamping force but the smaller injection unit cannot provide the required shot.
However, a larger shot capacity comes with a trade-off: the available injection pressure may be lower. Before choosing C, check whether the machine can still fill and pack the mold under your actual processing conditions.
How to Choose the Right Screw Size
1. Calculate the Total Shot Size
Start with the amount of material needed for one complete molding cycle.
For a cold-runner mold, include:
- The weight of all molded parts.
- The weight of the runner and sprue.
- Any other material injected as part of the shot.
For a hot-runner mold, calculate the shot based on the material actually injected each cycle. Do not add the entire hot-runner inventory as if it were discharged every cycle.
Example:
If a mold produces four parts weighing 20 g each, and the runner and sprue weigh 10 g, the total shot weight is:
4 × 20 g + 10 g = 90 g
Use this figure as a starting point when comparing injection unit options.
2. Compare Shot Size with Machine Capacity
A shot that is too small for the barrel can lead to long residence time. A shot that is too close to the machine’s maximum capacity can leave too little room for stable operation and the required cushion.
A practical review range often discussed is 30% to 70% of rated shot capacity, but it is only a starting point. The suitable range depends on the material, machine and cycle.
For example, if the total shot is 90 g and the machine’s comparable rated capacity is 200 g:
90 ÷ 200 × 100% = 45%
That gives you a starting point for evaluating the injection unit.
Make sure the two values are comparable. Some machines list shot volume in cm³ or shot weight based on a reference material such as polystyrene. Your actual resin may have a different density. Ask the machine supplier to confirm the capacity for your material.
For more guidance, see YIZUMI’s guide to screw diameter and shot size.
3. Check Injection Pressure
Enough shot capacity does not guarantee that the mold will fill.
A thin-wall part, a small gate or a long flow path may require more pressure. A larger C screw may have enough capacity but may not provide enough available pressure for the application.
Compare the mold’s requirements with the machine’s pressure specification. If possible, use mold-flow analysis or production data to support the decision.
4. Check Plasticizing Capacity and Cycle Time
The screw must prepare enough melt before the next cycle begins.
A smaller screw may provide enough injection pressure, but it also needs enough time and plasticizing capacity to prepare the required shot. If screw recovery takes too long, it can limit the production cycle.
Check:
- Required shot weight.
- Target cycle time.
- Material melting behavior.
- Machine plasticizing capacity.
5. Consider the Material and Screw Design
Screw diameter is only one part of the decision.
The screw’s length-to-diameter ratio (L/D), compression ratio, channel geometry and surface treatment can also affect processing. These features are not necessarily the same across A, B and C options.
A longer L/D ratio does not automatically mean better mixing or better melt quality. Screw geometry, material and operating conditions also matter.
For glass-fiber-filled or abrasive materials, review wear resistance as well. Depending on the application, a wear-resistant screw and barrel may be appropriate. Learn more about YINGDA’s bimetallic barrel wear protection and how to discuss the right solution for your material and machine. Confirm the material, filler content and processing conditions before selecting a surface treatment or alloy.
Do Transparent Parts Always Need an A Screw?
No. An A screw may be suitable for some transparent parts, especially when the shot size is small or higher pressure is needed. But transparency alone does not determine the correct screw size.
Clear parts also depend on:
- Material drying.
- Melt temperature and residence time.
- Screw and barrel condition.
- Mold design and venting.
- Injection speed and pressure.
Choose the injection unit based on the full process, not the appearance of the finished product alone.
Can You Change from a B Screw to a C Screw?
It may be possible on some machines, but you should check with the machine manufacturer first.
A different screw diameter usually requires a matching barrel and may also require changes to other components or settings. The machine must be designed to support the alternative injection unit.
Before ordering a replacement, confirm:
- Machine manufacturer and model.
- Existing screw diameter.
- Required screw and barrel dimensions.
- Compatibility with the injection unit.
- Required shot capacity and pressure.
- Material and operating conditions.
Do not order a replacement based only on the letters A, B or C. Those labels do not define a universal screw size.
Frequently Asked Questions
Is an A screw always better for engineering plastics?
No. An A screw may offer higher available pressure, but the right option depends on the material, mold, shot size and machine. Always check both pressure and capacity.
Is a C screw always better for large parts?
Not always. A C screw may provide a larger shot, but the machine also needs enough pressure and plasticizing capacity to produce the part consistently.
Does a longer screw always provide better mixing?
No. L/D is one design factor. Mixing also depends on the screw profile, mixing sections, material and processing conditions.
What information do I need for a replacement screw and barrel?
Start with the machine model, screw diameter, drawings or measured dimensions, material, filler content and production requirements. Photos of the existing assembly can also help with the initial discussion.
Need Help Choosing an Injection Molding Screw and Barrel?
The right screw and barrel should match both the machine and the material being processed. Before choosing an A, B or C configuration, compare the available shot capacity, injection pressure and plasticizing requirements.
If you need a replacement or a custom screw and barrel, send YINGDA your machine model, drawings, material details and production requirements for review.