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 on demand manufacturing process.

With competitive price and timely delivery, GD-HUB sincerely hope to be your supplier and partner.

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- 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 to load.

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.

For more information, please visit Low Pressure Overmolding.

Recommended article:
What is uPVC Door Profiles and Why Do We Use Them?

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 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.

FAQ

Low-Pressure Molding and Overmolding

Low-pressure molding is vital for protecting sensitive electronic components against the environment, like moisture, dust and dirt. As an alternative to high-pressure molding, it allows for overmolding on more sensitive equipment. While one may assume a low-pressure method could take more time than a high-pressure one &#; low-pressure molding machines can seal sensitive electronics and have them ready to handle in seconds. 

What injection molding does:

Injection molding is used to manufacture parts in a large volume and is typically used where the same part is being produced many times in succession. As far as assembly goes, injection molding is cost-effective, especially when used in mass production. This process is commonly used to create a wide variety of products and parts, ranging from containers to automotive body parts. Injection molding is a complex process, reliant on several steps and moving parts but with three main components. According to Milacron, these are the feed hopper, the screw and the heated barrel. Plastic is placed in the hopper, typically in granular or powder form, and then fed into the heated barrel. The screw, along with heater bands, melt the materials using frictional action. The plastic, now melted, is injected through a nozzle into a mold. 

Injection molding is generally used to efficiently create plastic materials based on approved molds. This process allows for making intricate geometrical shapes due to the pressure exerted and can allow for using different types of plastic simultaneously. 

Double shot injection molding:

Double shot molding is used for complex plastic objects, consisting of multiple materials and various colors. This, as well as typical injection molding, is generally cost-effective and viable for mass-production. This manufacturing process creates complicated molded parts with two different materials by molding plastic around a metal or plastic insert. The process, broken down into two steps, is &#; the first material is injected into a mold that will make a piece of the product. This is followed up by a second injection, of a different but compatible material with the initial piece. The plastic resins form a molecular bond, creating a multi-resin product, says Midstate Mold. This method is recognized for lowering unit costs, reducing waste, and improving product quality. 

Running an overmolding project:

Beginning an overmolding project requires knowing what materials you wish to work with. You need a substrate &#; the first piece in what will become a singular product. It can be an electrical connector, a molded plastic part or metal. Then, you need the overmolding material, which is typically plastic and heated to a melting point. 

The process of overmolding, using a MoldMan molding machine, goes as follows:

1. &#;Find your low-pressure molding materials (polyamides, polyamide acrylic hybrids, copolyesters).
2. &#;Determine if you&#;re using a single or multi-cavity mold set. 
3. &#;Monitor temperature and pressure using control screen
4. &#;Place low-pressure molding materials into insulated upper-hopper
5. &#;The material will come into contact with heat, before being injected into the cavity. This can allow for quick material changes.
6. &#;The machine&#;s one-ton clamping force will aid in material distribution, even when using copolyesters

Overmolding is also used for a wide variety of products and purposes, from toothbrushes to electronics. Materials can also vary, but with that, compatibility needs to be taken into account. Typical applications of overmolding include: plastic over plastic, rubber over plastic, plastic over metal, and rubber over metal. It can be used as a way to combine parts without the use of fasteners or adhesives, as well. Overmolding, as a method of low pressure molding, is useful for working on a wide range of products in an efficient manner. Using an overmolding machine can allow you to produce a variety of products but with one piece of equipment. 

Compared to other tools of protecting electronics &#; like potting &#; overmolding is a faster, simpler process with less waste (leftover materials can be recycled into new materials or are biodegradable). Using an overmolding machine can be vital for protecting valuable electronics from heat, water, dust, and the general wear-and-tear of use. 

Contact us about electronic overmolding services and we will coordinate with our partners at Mold-Man Systems to provide the tools you need to ensure your electronic assemblies are protected against the strain of the environment.

 

Want more information on Overmolding Services? Feel free to contact us.