Low pressure injection molding (LPM) is a specialized subset of traditional injection molding, distinguished by its use of lower pressure to inject the material into the mold. It is crucial in several industries due to its unique advantages. In this post, we will dive into low pressure injection molding and explore the benefits, applications, and challenges of this on-demand manufacturing process.
What is Low Pressure Injection Molding?
Low pressure injection moulding is a process where molten plastic material is injected into a mold cavity at relatively low pressures, typically ranging from 0.15 to 4 MPa. It works by melting a plastic material and then slowly injecting it into a mold using low pressure. The low pressure allows the molten plastic to flow gently into the mold cavity without excessive force that could damage the mold or the part being produced.
Learn about: Principle of injection molding process
Key Aspects of the Low Pressure Injection Molding Process:
- Lower processing temperatures, typically 180-240°C.
- Can use aluminum molds instead of steel due to lower pressures.
- Often uses hot melt materials like polyamides or polyolefins.
- Cycle times are short, usually 15-60 seconds total.
- Provides good adhesion between the molded material and encapsulated components.
- Allows for delicate overmolding of fragile electronic parts without damage.
- This process is particularly well-suited for encapsulating electronic components, creating waterproof seals, and producing parts with integrated fabrics or skins.
The Key Steps of Low Pressure Injection Moulding Process
The process of low pressure injection molding typically involves the following steps:
1. Low Pressure Insert:
The electronic components or parts to be encapsulated are inserted into the mold cavity. This usually takes about 5-10 seconds to load.
2. Low Pressure Overmoulding:
The low pressure molding machine injects the molten material (usually a thermoplastic) into the mold at low pressure. The material flows around and encapsulates the inserted components. This step takes about 15-60 seconds.
3. Cooling and Ejection:
The molded part is allowed to cool and solidify. Once cooled, the part is ejected from the mold. The finished part can be handled and tested immediately after molding.
Commonly Used Low Pressure Injection Molding Materials
The success of low-pressure injection molding depends on the right material selection. Different materials not only affect molding performance but also determine the reliability and durability of the final part in applications such as electronics, electrical appliances, and automotive.
1. Thermoplastics (Main Category)
- Polyamides (PA, hot-melt adhesives)
- Polyolefins (PE, PP, EVA blends)
- Polyurethanes (PU, specialty grades)
2. Specialty Materials
- Modified Epoxy Hot-Melts
- Silicones (LPM-compatible grades)
Key Advantages of Low Pressure Molding Materials:
- Low viscosity for smooth flow and complete encapsulation at low pressure
- Strong adhesion & sealing to metals, plastics, and electronic components
- Electrical insulation to protect sensitive circuits and connectors
- Chemical and solvent resistance for durability in challenging environments
- Thermal stability & UV resistance for outdoor and automotive applications
Applications of Low Pressure Molding
Low pressure injection molding is widely used across various industries.
- Electronics: Encapsulates circuit boards and connectors, providing insulation and protection.
- Automotive: Seals sensors and electronic components, enhancing thermal resistance and durability.
- Consumer Electronics & Appliances: Protects small electronic parts from water and dust.
- Medical Devices: Encapsulates sensors and precision components, ensuring safety and reliability.
Advantages and Disadvantages of Low Pressure Injection Molding
Advantages of Low Pressure Injection Molding
- High Strength: The skin material bonds seamlessly with the plastic base, ensuring there is no risk of separation.
- High Molding Efficiency: The process is more environmentally friendly as it eliminates the need for adhesive application required in cladding processes.
- Design Flexibility: Offers greater freedom in internal structure and surface modeling compared to cladding processes, resulting in clearer design features, enhanced aesthetics, and improved comfort.
Low Pressure Injection Molding Service
Disadvantages of Low Pressure Injection Molding
- Complex Process: The low-pressure molding process is more intricate, making it susceptible to the influence of mold quality and material ductility, which can lead to a higher scrap rate.
- Limited Pattern Retention: Under high stretch, the three-dimensional quality of patterns may be compromised.
- Higher Costs: The tooling and material costs are higher, leading to increased overall expenses.
Key Considerations for Low Pressure Injection Molding
Successful low pressure injection molding (LPIM) requires careful consideration of several factors, including resin selection, mold design, and process parameters.
Select High Fluidity Resin
High-flow resins ensure smooth filling of the mold and complete encapsulation of inserts. Polyamides (PA) and polyolefins (PE, PP) are widely used for their excellent flowability, thermal stability, and strong adhesion, while bio-based polyamides offer added sustainability. Resin viscosity should match part geometry to ensure proper filling and avoid stress concentrations.
Optimize Mold Gating and Venting Design
The gate should direct resin to fill thin sections first, with vents placed at the last areas to be filled. Venting allows trapped air to escape as the resin flows, preventing air bubbles and incomplete filling, and ensuring part quality.
Control Process Parameters
Melt temperatures typically range from 180-240°C based on the resin. Injection pressures are low, from 1.5 to 40 bar, to gently fill the mold cavity. Injection speed must be optimized – too fast causes air traps, too slow allows premature freezing. Holding pressure and cooling time are adjusted to achieve complete filling and part quality.
Common Defects and Troubleshooting
Here are some common defects in low pressure injection molding and potential solutions:
Breakdown at Sharp Corners
This defect occurs when the molten plastic fails to fill sharp internal corners or edges within the mold cavity.
Solutions:
- Increase melt and mold temperatures to improve plastic flow.
- Increase injection pressure slightly to better fill tight areas.
- Modify part design to add radii to sharp corners where possible.
Flow Marks or Knit Lines
Flow marks are visible lines or knit lines on the part surface caused by the merging of separate melt flow fronts.
Solutions:
- Optimize gate location and number of gates to direct uniform melt flow.
- Increase melt and mold temperatures for better flow.
- Adjust venting to allow trapped air to escape smoothly.
Air Entrapment or Burning
Air can get trapped in the mold cavity, resulting in voids, burns, or degraded material properties.
Solutions:
- Improve venting channels and locations in the mold design.
- Adjust injection speed – too fast causes air traps, too slow allows premature freezing.
- Ensure proper mold venting, maintenance, and cleaning.
Low Pressure Injection Molding vs. High Pressure Injection Molding: Key Differences
When choosing an injection molding pressure, both low-pressure and high-pressure injection molding have their advantages. The comparison table below will help you make the best decision.
| Feature | Low pressure Injection molding | High pressure Injection molding |
|---|---|---|
| Injection Pressure | 0.15–4 MPa | 35–130 MPa |
| Molding Temperature | 180–240 °C | 230–300 °C |
| Mold Materials | Aluminum or silicone rubber molds | High-strength steel molds |
| Resin Properties | Low-viscosity resins (e.g., polyamides, hot-melt adhesives) with high fluidity | Wide range of thermoplastics and thermosets, typically lower fluidity |
| Part Adhesion & Sealing | Excellent for encapsulating delicate components, provides strong adhesion and sealing | Less suitable for encapsulation, adhesion and sealing performance often lower |
| Mold Cost | Lower due to reduced stress on molds | Higher due to need for durable molds |
| Cycle Time | Longer due to lower pressure | Shorter with rapid cooling |
| Part Complexity | Suitable for delicate or electronic encapsulation | Ideal for high-precision, complex geometries |
| Production Volume | Best for low to medium volume runs | Best for medium to high volume mass production |
Conclusion
Zhongde excels in low pressure injection molding, delivering precise encapsulation, reliable sealing, and complex thin-wall components. With our expertise, we ensure high-quality, cost-effective solutions for automotive, industrial, and custom applications, helping you bring specialized components to market efficiently.
