A Comprehensive Guide to Low Pressure Injection Molding

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 manufacturing process.

What is Low Pressure Injection Molding?

Low pressure injection molding is a process where molten plastic material is injected into a mold cavity at relatively low pressures, typically ranging from 1.5 to 40 bar (21.8 to 580 psi). The pressure of low pressure injection molding is 0.15~4Mpa. It works by melting a plastic material and then injecting it slowly 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.

Key Aspects of the Low Pressure Injection Molding Process:

• Uses much lower pressures (1.5-40 bar) compared to traditional injection molding (350-1300 bar).
• Lower processing temperatures, typically 180-240°C vs 230-300°C for high pressure molding.
• 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 Molding Process

The process of low pressure injection molding typically involves the following steps:

1.Insert:
The electronic components or parts to be encapsulated are inserted into the mold cavity. This usually takes about 5-10 seconds for loading.

• 2.Overmolding:
  The low pressure molding machine injects the molten material (usually a thermoplastic) into the mold at low pressure (typically 1.5-40 bar).
• 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.

What is the Purpose of the Low Pressure Injection Molding?

Low Pressure Injection Molding (LPIM) is a manufacturing process primarily used to enable the efficient production of complex and delicate parts, especially those that cannot tolerate high injection pressures or temperatures. It is particularly effective for encapsulating and protecting electronic components, delicate parts, and assemblies.

Common Used Low Pressure Injection Molding Materials

Low pressure injection molding is compatible with various materials, each offering specific advantages depending on the application. Here’s an overview of the types of low pressure molding materials and their advantages.

Thermoplastics:

• Polyamides (Nylons)
• Polyolefins (Polyethylene, Polypropylene)
• Polyurethanes (PU)
• Polycarbonates (PC)
• Polyvinyl chloride(PVC)

Thermosets:

• Epoxy Resins
• Silicones
• Phenolics

Those plastic materials have higher mechanical strength, chemical resistance, thermal stability, flexibility impact resistance and electrical insulation. All these low pressure molding materials are suitable for electronics related product applications for UV light resistance, thermal stability, solvent resistance, and chemical resistance.

Examples of Low Pressure Molding

The low pressures (typically 1.5-40 bar) and temperatures used in the low pressure injection molding process make it ideal for encapsulating or overmolding sensitive components without damaging them. It’s particularly useful for creating waterproof seals, providing protection against shock, vibration, moisture, and corrosive agents, and integrating different materials like fabrics or PVC skins directly onto plastic parts.

Electronic Component Encapsulation:

• Circuit board protection
• Connectors containing PCBAs
• Wire harnesses and cables
• Waterproof connectors
• Sensors
• Microswitches
• Inductors
• Antennas

Overmolding of Fragile Components:

• Antennas (enhances radio signal while protecting)
• Circuit boards
• Magnets
• Batteries
• Threaded inserts
• Metal rods, bolts, and clips

Medical Devices:

Encapsulation of sensitive electronic components

Advantages and Disadvantages of Low Pressure Injection Molding

Advantages of Low Pressure Injection Molding

1. High Strength: The skin material bonds seamlessly with the plastic base, ensuring there is no risk of separation.
2. High Molding Efficiency: The process is more environmentally friendly as it eliminates the need for adhesive application required in cladding processes.
3. 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.

Disadvantages of Low Pressure Injection Molding

1. 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.
2. Limited Pattern Retention: Under high stretch, the three-dimensional quality of patterns may be compromised.
3. Higher Costs: The tooling and material costs are higher, leading to increased overall expenses.
4. 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

Ductility and fluidity are critical properties for the resin to flow smoothly into the mold cavity and around inserts/components. Polyamides (PA) and polyolefins are commonly used for their high fluidity, thermal stability (180-240°C range), and adhesion properties. Bio-based polyamides derived from renewable sources like plant oils offer sustainability benefits. Resin viscosity and filler content impact flow behavior and must match part geometry.

Optimize Mold Gating and Venting Design

Proper gating (inlets for injecting resin) and venting (outlets for air escape) are essential for uniform filling without knit lines or voids. Gating should direct resin flow into thin sections first, with vents at last points to fill. Venting allows trapped air to escape as the resin flows through the cavity. Mold materials like aluminum or silicone rubber accommodate lower pressures versus steel tooling.

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

1. This defect occurs when the molten plastic fails to completely fill sharp internal corners or edges within the mold cavity.
   Solutions:
   Increase melt and mold temperatures to improve plastic flow.
2. Increase injection pressure slightly to better fill tight areas.
3. Modify part design to add radii to sharp corners where possible.

Flow Marks or Knit Lines

1. 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.
2. Increase melt and mold temperatures for better flow.
3. Adjust venting to allow trapped air to escape smoothly.

Air Entrapment or Burning

1. 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.
2. Adjust injection speed – too fast causes air traps, too slow allows premature freezing.
3. Ensure proper mold venting maintenance and cleaning.

How Low Pressure Molding Differs from Traditional Injection Molding

Here are the key differences between low pressure injection molding and traditional injection molding:

Injection Pressure

• Low pressure moulding uses injection pressures ranging from 1.5 to 40 bar (21.8 to 580 psi)
• Traditional injection molding uses much higher pressures, typically 350 to 1300 bar (5,000 to 18,800 psi)

Molding Temperatures

• Low pressure molding operates at lower temperatures around 180-240°C
• Traditional high pressure molding uses higher temperatures, typically 230-300°C

Mold Materials

• Low pressure uses lower-cost aluminum or silicone rubber molds
• Traditional high pressure requires more expensive steel mold tooling to withstand the extreme pressures

Resin Properties

• Low pressure resins like polyamides (PA) have high fluidity to flow at low pressures
• Traditional high pressure resins have lower fluidity but can fill complex geometries

Part Adhesion and Sealing

• low pressure moulding is the ability to gently overmold and encapsulate delicate components like PCBs and connectors without damage from extreme pressures and temperatures.
• Traditional high pressure parts often have poorer adhesion and sealing capabilities compared to low pressure molding.

Production Volume

• Low pressure is well-suited for low to medium volume production runs.
• Traditional high pressure injection molding favors medium to high volume manufacturing.

Conclusion

Low pressure injection molding offers a unique and advantageous approach for encapsulating delicate components, achieving excellent sealing properties, and producing complex thin-wall geometries. Operating at much lower pressures and temperatures than traditional injection molding, this process provides significant cost, quality, and sustainability benefits. From automotive interior trim to industrial cable sealing, low pressure molding enables innovative applications across sectors. Its suitability for low to medium production volumes makes it an economical choice for encapsulating specialized components and customized products.

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