To make injection molding products have a good feel and appearance, using hard plastic as the base material skeleton, fabric, and soft plastic for the surface layer is now popular.
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Throughout this process, using a hard plastic skeleton and then adding a layer of soft plastic (insert, secondary) shows its practicality because it eliminates the need for follow-up processing. In contrast, fabric and skin low-pressure injection molding require additional steps.
What is Low-Pressure Injection Molding?
Low-Pressure Injection Molding involves injecting an encapsulated material into a mold at a very low injection pressure and curing it quickly.
This safe and meticulous process, situated between high-pressure injection molding and potting, is ideal for circuit board protection due to its short cycle times and low pressures.
In the automotive industry, it is used for interior trim molding and electronic component packaging.
Low-Pressure Injection Molding Application Examples
Interior parts: Using very low injection pressure (300-600 Bar), PVC skins or knitted fabrics are integrated into the molding process. Common materials include leather, PVC skin, TPO skin with PP-Foam, PUR skin, knitted fabric with non-woven fabric, fiber felt, and composite injection between surface material and skeleton material. Many mid- to high-end car interior parts, such as Mercedes-Benz and BMW door panel inserts, employ this technique.
Electronic components encapsulation: This process uses minimal injection pressure (1.5-40 bar) to inject and quickly cure encapsulation material.
The pressure and temperature for low-pressure injection molding are significantly lower than those for normal injection molding. The process is simple and requires basic equipment and molds.
Low-Pressure Injection Molding Process
The process is an in-mold assembly technique, similar to IMD, where the fabric/skin is molded directly onto the product. If the injection pressure is too high, the fabric may be damaged.
There are two types of low-pressure injection molding: open mold and closed mold. The difference lies in whether there is a gap between the moving and fixed molds when the mold is closed.
Throughout the filling process, a precise amount of melt is injected smoothly. The flow of plastic melt on the fabric is much slower than on a smooth mold cavity, so fabric low-pressure injection molds have more gates than ordinary molds.
Low-Pressure Injection Molding Features
1. Advantages
a. High strength: The skin material integrates with the plastic base material, ensuring it does not detach.
b. High efficiency and environmentally friendly: No glue application is needed, improving air quality in vehicles.
c. Design flexibility: Complex internal structures and surface modeling features are possible, making the products both beautiful and comfortable.
2. Disadvantages
a. Complexity: The process is susceptible to mold influence and fabric/PVC skin ductility, leading to a relatively high scrap rate.
b. Pattern retention: High stretch reduces the three-dimensional sense of patterns.
c. Higher costs: Higher tooling costs and more expensive fabrics.
Considering the above advantages and disadvantages, low-pressure injection molding has distinct characteristics. These include parting surface with a closing gap, sequence valve-controlled gates, and fabric clamping and product pick-up functions.
Composite layers may include PVC/PU, fabric, etc. Skeleton materials often consist of modified PP and PC/ABS with better flowability.
Low-Pressure Injection Molding - Important Influencing Factors
Fabric composition: May include non-woven layer, sponge layer, foam layer, surface knitted fabric layer, or soft plastic layer (PVC).
Fabric performance: Attributes like longitudinal static elongation, residual elongation, transverse static elongation, and residual elongation. More elongation makes it harder to deform and more prone to wrinkles.
Mold structure: Usually features a point gate sequential valve with more gates than ordinary mold skeleton material.
Design of cavity exhaust: Similar mold structures for fabric and PVC skin injection, focusing on cavity and core space exhaust.
Other technical requirements: R angle at product corners no less than 3.0; the gluing thickness should prevent product shrinking. Related articles and more information on rapid tooling can be found on our website.
Low-Pressure Injection Molding Plastic Flowability
The melt index (MI) value measures plastic flowability. Plastic flow on fabric is slower than on smooth mold cavities, necessitating more gates in fabric low-pressure injection molds.
The slow flow rate affects product appearance, leading to issues like bonding marks and missing material. Generally, PP material's MI value ranges from 20 to 55. Imported fabrics and plastic particles are not necessarily required; qualified low-pressure molding products can be made with domestic materials.
Structure of Low-Pressure Injection Molding Moulds
Typically, the ejector is in the fixed mold, using a valve gate to control the material quantity at each gate. The fixed mold has a pressed fabric frame and mechanisms to fix the fabric, such as pins or air suction cups.
Low-Pressure Injection Molding - Common Defects and Handling Measures
(1) Lack of material: Sharp corners are prone to lack of material.
(2) Infiltration: Corners are prone to infiltration due to high gate temperature, high mold temperature, or excessive gate material control.
(3) Penetration: Sharp corners are prone to penetration due to parting surface gaps. Increased material or injection speed can exacerbate this issue.
(4) Fabric pressure breakage: Rust spots on molds can lead to excessive pressure and limited fabric ductility, causing breakage.
Features of PVC Skin Low-Pressure Injection Molding
1. The Nature of PVC Skin
Exhaust differs because fabric is porous and allows gas to escape easily, while PVC skin traps gas. The fabric substrate is non-woven and offers high resistance to molten plastic flow, while PVC skin, having a smooth plastic foam layer, offers low resistance.
The structure gap also varies: post-extension, fabric gaps are larger, allowing easier plastic penetration, while PVC skin maintains strong resistance against penetration.
2. Structure of the Mold
Compared to fabric injection molding, PVC skin injection molds focus more on cavity and core space exhaust to expel air compressed during molten plastic injection and avoid defects like bonding marks, shrinkage, and scorching.
3. Common Defects and Handling Measures
(1) Product Surface Shrinkage
Caused by large gaps in mold parting surfaces, allowing material escape and poor compaction. Also caused by thick product walls and poor cooling at reinforcement bars.
(2) Melt Marks
Caused by poor exhaust in mold cavities and core, leading to compressed air at the gate resulting in visible fusion marks. Low mold temperature and low plastic melt temperature can also cause these defects.
(3) Skin Crushing
Like fabric injection molding, rust spots on molds can lead to excessive pressure and limited ductility, resulting in skin breakage.
(4) Melt Through Sprue
Occurs if plasticized molten plastic is injected too quickly or with excessive shear, causing it to melt through. Reducing gate, temperature, or mold temperature can mitigate this but requires careful balancing of resin liquidity to avoid material shortage.
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