What is Injection Molding Overmolding? DIY Guide

OVERMOLDING UNDER LOW PRESSURE

Firstly, low-pressure overmolding has won a well-deserved place in the foundry industry. The essence of the method is that the melt under low pressure of air or gas fills the mold.

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Controlling the pressure in the metal flow is crucial to overcome various resistances in its path. The over-molding mold according to this method is filled from the bottom up directly.

Advantages of the low-pressure overmolding method:

• Possibility to automate and mechanize the mold filling process.
• Overmolding quality improves due to excess pressure in the liquid plastic, offering the capability to create over moldings with very thin walls (3-4 mm) and a dense structure.
• Minimal heat loss during the melt's journey from furnace to mold.
• The process of automatic filling of the mold cavity with plastic typically takes 1.5 to 6-8 seconds, depending on the weight of the overmolding.
• The result is overmoldings with high mechanical properties, smooth surfaces, and precise geometric shapes, eliminating the need for additional machining.

Design and features of overmolding

The overmolding includes a retarding filter that ensures smooth braking of the rising plastic flow, increasing the static pressure, and avoiding the deformation of the sand core. The filter allows the displaced air to exit freely while blocking the metal. This retarding filter is always made of all metal.

The braking element consists of a slot labyrinth formed by the outer surface of the central part and the inner surface of the detachable parts of the housing.

What is the speed of filling in overmolding?

The process begins slowly, with the melt filling the mold up to the collector level, which closes the electrical contact, activates additional pneumatic pressure, and rapidly fills the mold cavity. Upon the mold cavity’s completion, the melt solidifies thoroughly, maintaining the pressure via automated systems.

How many types of molding machines?

When the overmolding completely solidifies, technological time switch releases pressure in the crucible, opening the mold and ejecting the molded part. This process yields thin-walled parts in different forms. Currently, more than 20 varieties of thermoplastic for injection molding are known. Each type adapts to specific products and can be used in universal or specially equipped injection molding machines.

Traditional injection molding

The most common type uses universal Injection Molding Machines (IMCs) for parts with wall thicknesses of 1.5 to 5 mm. Traditional overmolding accounts for over 90% of molded thermoplastic parts.

Injection molding, such as compression molding and press molding, involves injecting a melt into a mold partially closed. Final molding occurs with increasing pressure during full mold closure, producing thick, simple-shaped parts.

Is the overmolding process high-speed?

High-speed overmolding produces thin-walled products (0.6 to 1.2 mm thick) using specialized molding units and ventilation systems. It requires fundamental adjustments in overmolding modes compared to traditional methods.

Overmolding thick-walled parts (10–30 mm) employs a similar technique to traditional methods, but the holding and cooling times are extended, and sprue diameters increased.

How many producing stages of overmolding?

The overmolding process occurs in two stages. Initially, the mold is partially filled with melt during screw rotation. The final fill and pressure hold occur as the screw-piston moves forward without rotation. This method is used when the molded part’s volume exceeds the injection volume capacity of the machine.

How is Sandwich overmolding characterized?

Sandwich overmolding involves sequentially injecting two polymers through one sprue. The first polymer creates a surface layer, while the second inflates the surface from inside. Machines must be equipped with two injection units for this process.

Two-component or two-color injection molding uses two polymers injected in succession using molds of different shapes and volumes, requiring machines with double injection units.

The molding machine uses high injection rates

Foam molding expandable thermoplastics uses high rates on hydraulic accumulator machines. The original thermoplastic contains physical or chemical blowing agents to achieve specific properties.

How does the molding machine work?

Overmolding with gas begins with incomplete melt injection, followed by introducing gas (typically nitrogen) into the melt, inflating the polymer shell and pressing it against the mold cavity. The structure has a pronounced gas cavity surrounded by monolithic polymer, often used in thick-walled products.

Low-pressure injection molding and traditional ... - GREFEE mold

Low pressure injection is a method using low injection pressure (0.15-4MPa) to inject the material into the mold, forming quickly and solidifying in a short period (5-50 sec).

Low-pressure injection often includes other parts as inserts, wrapped by plastic to form integrated assembly parts.

The main difference between low-pressure and traditional injection molding is the low injection pressure to avoid damage to wrapped or bonded parts. Major applications include:

1. The electronic and electrical field focuses on connectors containing PCBA, conductors, packaging of sensitive components like circuit boards, automotive products, mobile batteries, sensors, and cables.

2. Automobile industry applications involve parts like door guards, column guards, wrapped frame guards with materials like cloth or PVC fabric.

Electronic and electrical field

Electronic and electrical field parts are often delicate, vulnerable to external factors. Packaging these requires protection from stress, humidity, temperature, dust, and chemical solvents, with internal properties for high/low temperature resistance, impact resistance, and insulation.

1. Comparison between low-pressure injection molding process and traditional high-pressure injection molding process:

Low pressure injection molding, similar to traditional molding, has advantages in packaging electronics by using special hot-melt adhesive. This material’s low viscosity and good fluidity allow for low-pressure mold filling, offering great protective capabilities.

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2. Comparison with traditional potting process:

Low-pressure molding is more environmentally friendly and cost-efficient than traditional methods. It allows faster production cycles and reduces material waste.

Low pressure injection molding process

1. Add materials and insert components into the mold.

2. Using a hot melt adhesive machine, inject adhesive into the mold.

3. After about a minute, open the mold and remove the parts.

Advantages over traditional potting process:

1. Traditional potting's chemical reaction versus the physical reaction in low pressure injection molding allows for recyclable materials.

2. Traditional potting takes about 24 hours to cure, whereas low-pressure molding finished in seconds, speeding production.

3. Requires fewer components and has a simpler flow, resulting in lower costs.

4. Low-pressure molding does not require waiting times to cure, occupying less space.

5. Less material is used due to efficient cavity design.

Typical applications of low pressure injection molding in the electronic and electrical field

1. Packaging of sensitive electronic components, protecting them from environmental damage while serving as shells.

2. Waterproof seals for connectors using low-pressure hot melt techniques to ensure watertight integrity.

3. Field injection molding of grommets, enhancing protection and providing quicker production times.

4. Serving structural functions post-molding for diverse assembly requirements.

Automotive industrial field

Low-pressure injection molding's application in the automotive field is not just for electronic devices but also for interior trims like door guards and column guards.

The demand for aesthetic and comfortable car interiors has led to increased use of fabric covers combined with low-pressure injection molding, offering higher efficiency and environmental benefits compared to manual processes.

Low pressure injection molding of automotive interior parts

1. Advantages over traditional coating process:

(1). Integration of material with strong bonding and reduced risk of detachment.

(2). Environmentally friendly, reducing VOC and improving air quality inside cars.

(3). Design freedom for internal structures without the limitations of coating processes.

(4). Higher production and pass rates due to the simplified process.

Challenges of low pressure injection molding:

(1). Complex processes that can be influenced by mold and fabric properties, leading to higher scrapping rates.

(2). Poor stereoscopy retention in pattern design due to high stretch points.

(3). High cost of mold tooling and special materials.

(4). Difficulty in handling complicated structures due to low injection pressure.

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