Introduction
Injection moulding is one of the most widely used manufacturing processes for producing plastic components with high accuracy, repeatability, and surface finish. However, the quality and feasibility of an injection-moulded product do not depend only on mould design or raw material selection. An equally critical factor is the proper selection of the injection moulding machine.
Choosing an unsuitable machine can lead to defects such as short shots, flash, sink marks, excessive cycle time, mould damage, or even machine failure. Therefore, before finalizing a moulding machine for a particular product, several machine specifications must be carefully checked and matched with product and mould requirements.
This article explains the key machine specifications that determine the suitability of an injection moulding machine for a given injection-moulded product, along with examples and reference tables to aid practical understanding.
Why Machine Selection Matters in Injection Moulding
An injection moulding machine performs three major functions:
- Plasticizing and injecting molten material into the mould
- Holding the mould closed against injection pressure
- Opening the mould and ejecting the product
If any of these functions are inadequate, the moulding process becomes unstable. Correct machine selection ensures:
- Complete cavity filling
- Dimensional accuracy
- Absence of defects like flash and short shots
- Longer mould life
- Optimized cycle time and production cost
Key Machine Specifications to Be Checked
To determine whether a moulding machine is suitable for a specific injection-moulded product, the following specifications must be evaluated.
1. Maximum Shot Weight Capacity
Definition
The shot weight capacity of an injection moulding machine is the maximum weight of molten plastic that can be injected in one cycle.
Requirement
The maximum shot weight capacity of the machine must be greater than:
- Weight of the moulded article
- Weight of all articles (in case of multicavity moulds)
- Weight of the runner system (sprue, runners, and gates)
Why It Is Important
If the shot capacity is insufficient:
- The machine cannot deliver enough molten material
- Short shots occur
- Inconsistent filling leads to poor part quality
Recommended Practice
Generally, the actual shot weight should be 30–80% of the machine’s maximum shot capacity. Operating below 30% leads to poor melt control, while operating above 80% strains the injection unit.
Example
| Parameter | Value |
|---|---|
| Weight of one article | 40 g |
| Number of cavities | 4 |
| Total article weight | 160 g |
| Runner system weight | 40 g |
| Total shot weight required | 200 g |
| Machine shot capacity | ≥ 250 g (recommended) |
2. Injection Pressure Requirement
Definition
Injection pressure is the pressure applied by the machine to force molten plastic into the mould cavities.
Requirement
The machine’s maximum injection pressure must be sufficient to completely fill all cavities without short shots.
Factors Affecting Injection Pressure
- Flow length and wall thickness of the part
- Type of plastic material
- Melt temperature
- Gate size and runner design
- Number of cavities
Thin-walled parts and high-viscosity materials require higher injection pressure.
Consequences of Insufficient Pressure
- Short shots
- Incomplete filling
- Poor surface finish
- Weak weld lines
Typical Injection Pressure Ranges
| Material | Typical Injection Pressure (bar) |
|---|---|
| PP | 800 – 1500 |
| HDPE | 700 – 1400 |
| ABS | 1000 – 1800 |
| Nylon (PA) | 1200 – 2000 |
| Polycarbonate | 1400 – 2200 |
3. Clamping Tonnage Requirement
Definition
Clamping tonnage is the force applied by the machine to keep the mould closed during injection.
Requirement
The clamping force must be adequate to resist the cavity pressure, otherwise the mould halves may separate, causing flash.
Calculation of Clamping Force
Clamping force depends on:
- Projected area of the moulded part
- Number of cavities
- Injection pressure
Basic Formula
Clamping Force (tons) = Projected Area (cm²) × Cavity Pressure (kg/cm²) ÷ 1000
Consequences of Insufficient Clamping Force
- Flash formation
- Poor dimensional control
- Damage to mould parting surfaces
Example
| Parameter | Value |
|---|---|
| Projected area per cavity | 50 cm² |
| Number of cavities | 4 |
| Total projected area | 200 cm² |
| Estimated cavity pressure | 400 kg/cm² |
| Required clamping force | 80 tons |
| Recommended machine | 100-ton machine |
4. Daylight Opening of the Machine
Definition
Daylight opening is the maximum distance between the stationary and moving platens when the mould is fully open.
Requirement
The daylight opening must be greater than:
- Total mould height (core + cavity)
- Height of the moulded article
- Additional space required for part removal or automation
Why It Is Important
Insufficient daylight opening can:
- Prevent mould opening
- Obstruct ejection of parts
- Restrict use of robotic pickers or conveyors
Example
| Parameter | Height (mm) |
|---|---|
| Mould height (closed) | 400 |
| Article height | 80 |
| Clearance for removal | 70 |
| Required daylight opening | 550 mm |
| Machine daylight opening | ≥ 600 mm |
5. Minimum Mould Height of the Machine
Definition
Minimum mould height is the smallest mould thickness that can be safely mounted and clamped on the machine.
Requirement
The minimum mould height specified for the machine must be less than the actual mould height.
Why It Is Important
If the mould is thinner than the minimum mould height:
- Proper clamping cannot be achieved
- Toggle mechanisms may not lock correctly
- Risk of platen damage increases
Practical Note
If mould height is less than the machine’s minimum, spacer plates may be used, but this should be avoided where possible.
6. Distance Between Tie Rods
Definition
Tie rods support the clamping unit and limit the maximum mould size that can be mounted.
Requirement
The distance between tie rods must be greater than the overall mould dimensions (length and width).
Consequences of Inadequate Tie Rod Spacing
- Mould cannot be mounted
- Limited access for mould handling
- Risk of damaging tie rods or mould
Example
| Parameter | Value |
|---|---|
| Mould size | 520 × 480 mm |
| Tie rod spacing | 600 × 600 mm |
| Suitability | Acceptable |
7. Screw and Barrel Specifications
Definition
The screw and barrel are responsible for melting, mixing, and conveying plastic material.
Key Parameters to Check
- L/D ratio (Length to Diameter ratio)
- Screw tip design
- Compression ratio
L/D Ratio
| Material | Recommended L/D Ratio |
|---|---|
| PP, PE | 18:1 – 22:1 |
| ABS | 20:1 – 24:1 |
| Nylon | 20:1 – 24:1 |
| PVC | 16:1 – 18:1 |
Higher L/D ratios provide better melting and mixing but increase residence time.
Screw Tip Design
| Material Type | Recommended Screw Tip |
|---|---|
| General thermoplastics | Standard non-return valve |
| Glass-filled materials | Hardened screw with wear-resistant tip |
| Heat-sensitive materials | Low shear screw design |
Importance of Proper Screw Selection
Incorrect screw design can cause:
- Poor melting
- Material degradation
- Inconsistent shot weight
- Excessive wear
Summary Table: Machine Specification Checklist
| Specification | Condition to Be Satisfied |
|---|---|
| Shot weight capacity | Greater than total shot weight |
| Injection pressure | Sufficient to fill cavities |
| Clamping tonnage | Higher than required holding force |
| Daylight opening | More than mould + part height |
| Minimum mould height | Less than actual mould height |
| Tie rod spacing | Larger than mould dimensions |
| Screw & barrel | Suitable for material type |
Conclusion
Selecting the right injection moulding machine is a critical engineering decision that directly impacts product quality, production efficiency, and tooling life. By carefully evaluating machine specifications such as shot weight capacity, injection pressure, clamping tonnage, daylight opening, mould height limits, tie rod spacing, and screw-barrel design, manufacturers can avoid costly production issues and ensure stable moulding operations.
A systematic approach to machine selection not only reduces defects but also improves consistency, lowers maintenance costs, and enhances overall productivity. For every new mould or product, these specifications should be verified before machine allocation, making machine suitability analysis an essential step in injection moulding process planning.