To make injection molding products have a good feel and appearance, and take into account the advantages of other low-pressure molding materials, it is now popular to use hard plastic as the base material skeleton, fabric, and soft plastic for the surface layer of the injection molding process.
Throughout this type of process, from the development point of view, injection molding skeleton and then injection molding a layer of the good feel of the soft plastic process (insert, secondary) will show its practicality because it does not need to follow the processing, while the fabric, skin low-pressure injection molding has a lot of follow-up processing.
What is Low-Pressure Injection Molding？
Low-Pressure Injection Molding is the process of injecting an encapsulated material into a mold at a very low injection pressure and curing it quickly.
Low pressure injection molding is a safe and delicate process – positioned in between high pressure injection molding and potting. With its short-cycle times and low pressures, it is the ideal solution for circuit board protection.
In the automotive industry, this process is used for some automotive 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 added to the molding process.
Commonly used material types include: leather, PVC skin, TPO skin with PP-Foam, PUR skin, knitted fabric with non-woven fabric, and fiber felt, as well as the composite injection between the surface material and the skeleton material; many mid- to high-end car interior parts use low-pressure injection molding process, such as Mercedes-Benz, BMW door panel inserts, etc.
Electronic components encapsulation: The encapsulation process uses a very small injection pressure (1.5~40bar) to inject the encapsulation material consumption into the mold and cures it quickly (5~50 seconds).
The pressure and temperature used for low-pressure injection molding are much lower than the normal injection molding process, and the process is relatively simple and does not require high equipment and molds.
Low-Pressure Injection Molding Process
The process is as follows:
Low-Pressure Injection Molding 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 will be damaged).
From the characteristics of the molding process, there are two types of low-pressure injection molding: open mold and closed mold.
The difference lies in whether there is a certain 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 the fabric low-pressure injection molds have more gates than ordinary molds.
Low-Pressure Injection Molding Features
1. Advantages
a. High strength, the skin material is integrated with the plastic base material, there is no possibility of falling off
b. High molding efficiency, more environmentally friendly, no glue application process necessary for the cladding process, improving the air quality in the car
c. The internal structure can be designed arbitrarily, and the freedom of surface modeling is greater than the cladding process, the modeling features are clearer, the product is beautiful, and the comfort is high.
2. Disadvantages
a. The low pressure molding process is more complex, vulnerable to the influence of the mold and the ductility of the fabric/PVC skin, and the scrap rate is relatively high
b. Pattern retention, in the high stretch, pattern three-dimensional sense is poor
c. Higher tooling cost and more expensive fabrics
Considering the above advantages and disadvantages, low-pressure injection molding has the following characteristics.
Mold: parting surface with a closing gap; the gate is controlled by sequence valve (controlled by valve type hot runner)
Manipulator: both fabric clamping function and product pick-up function
Composite layer: PVC/PU; fabric, etc.
Skeleton material: modified PP and PC/ABS with better flowability, etc.
Low-Pressure Injection Molding important influencing factors
Fabric; composition of fabric: may include non-woven layer, sponge layer, foam layer, surface knitted fabric layer, or soft plastic layer (PVC).
Fabric performance: longitudinal static elongation, longitudinal residual elongation, transverse static elongation, transverse residual elongation; fabric elongation is larger, not easy to deformation, wrinkles will be more obvious fabric elongation is smaller, easy to deformation, not easy to wrinkle.
Mold structure: usually selected in the form of point gate sequential valve, and the number of gates is more than ordinary injection molding skeleton material, the total length of the product L = 1000mm, wall thickness T = 2.5mm, the recommended value of the number of gates = 1000/2.5/FPP material F value less than 100, ABS or PC material less than 80.
Design of cavity exhaust: The structure of PVC skin injection mold is the same as that of fabric injection mold, except that it focuses on the exhaust of the cavity and core space.
Other technical requirements： the R angle at the corner of the product is not less than 3.0; the design of the thickness of the product glue bit should not be too thick to prevent the product from shrinking,
The thickness of the tendon bit should not exceed 3/5 of the thickness of the glue bit; deep hole products need to be split; the edge of the product appearance needs to reverse wrap the fabric workplace, the back of the product should not have a tendon bit, etc.
Low-Pressure Injection Molding Plastic Flowability
The index of plastic flowability is the melt index MI value. As we all know, the flow of plastic on the fabric is much slower than on the smooth mold cavity, which is why the fabric low-pressure injection molds have more gates than ordinary molds.
The slow flow rate will inevitably affect other appearance problems of the product, such as bonding marks, missing material, etc.
Generally, the MI value of PP material is between 20 and 55, which requires us not to blindly use imported fabrics and imported plastic particles in product development.
Because these are needed after the application of the combination, as long as and mold with good, domestic material, as usual, can make qualified low pressure molding products.
Structure of Low-Pressure Injection Molding Moulds
Usually, the ejector of this kind of mold is in the fixed mold, using the valve gate to control the material quantity of each gate, the fixed mold has a pressed fabric frame, and the fixed mold has a fabric pin or air suction cup to fix the fabric.
Low-Pressure Injection Molding common defects and handling measures
(1) Lack of material: The sharp corners of the product are prone to lack of material.
(2) Infiltration: The corner of the product is prone to infiltration, the reason: the infiltration part gate temperature is too high; the mold temperature is too high; the infiltration part gate material amount control is too much.
(3) Penetration: The sharp corner of the product is easy to appear penetration, the reason: the sharp corner of the parting surface with a gap, when the amount of low pressure molding material or speed increases molten plastic from the gap inside penetration, first drive the non-woven fabric and then penetrate the surface fabric.
The wall thickness at the sharp corners is too thick, so the wall thickness at the sharp corners of the mold is relatively thin to prevent the fabric from seeping through the material or breakdown.
(4) Fabric pressure breakage Cause: Usually the mold temperature used in low-pressure injection mold is low (10-15 degrees), if the anti-rust measures are not done well, it will produce rust spots on the pressed fabric frame and slider parting surface, which will produce excessive pressure on the fabric and limited ductility of the fabric, thus breaking the fabric.
Features of PVC skin Low-Pressure Injection Molding
1. The nature of PVC skin
Compared with the fabric, there are many differences between the two. First of all, the exhaust: the fabric is pore-like and loses structure, so the gas is easy to run out from inside; while PVC skin is a layer of PVC plastic on the surface, so it is difficult for the gas to run out from inside.
Secondly, the substrate: the fabric substrate is non-woven fabric, so the flow resistance to molten plastic is quite large; while the PVC skin substrate is a plastic foam layer, the surface is smooth, and the flow resistance to molten plastic is small.
Then is the ductility: fabric extension after the structure gap is large, molten plastic more easily penetrate; PVC skin extension still has strong resistance, and molten plastic is not easy to penetrate.
2. Structure of the mold
Compared with fabric injection molding, the biggest difference lies in the design of cavity exhaust, the structure of PVC skin injection mold is the same as fabric injection mold, except that it focuses on the cavity and core space exhaust.
As the air exists inside the core and cavity after closing when the molten plastic is injected in, the air inside is slowly compressed, if the gas is not discharged in time, it will inevitably cause great trouble to the filling and many defects. Such as bonding marks, shrinkage, and scorching.
3. Common defects and handling measures
(1) Product surface shrinkage
Cause: The gap between mold parting surfaces is too large, resulting in material escape, resulting in the need for material not compacted and thus shrinkage;
As with ordinary injection molding, the thick wall of the product, where there are reinforcement bars, due to poor cooling, resulting in volume shrinkage caused by shrinkage.
(2) Melt marks
Reason: If the mold cavity and core are not exhausted well, the air at the front of the gate where the fronts meet is compressed and cannot be fused naturally, and the combination of traces can be reflected on the skin, resulting in obvious fusion marks and affecting the appearance.
Of course, like ordinary plastic injection molding, the low mold temperature and low temperature of plastic melt will also cause obvious defects of fusion marks.
(3) The skin is crushed
Reason: Like fabric injection molding, the mold temperature of low-pressure injection mold is usually low (10-15 degrees), if the rust prevention measures are not done well, rust spots will be produced on the pressed fabric frame and slider parting surface, and the pressure on the epidermis will be excessive during production, and the ductility of the epidermis will be limited, thus breaking the epidermis.
(4) Melt through the sprue
Reason: Plasticized molten plastic is injected from the gate, subject to shear, heating circle, its central position temperature is high if open mold injection and the distance increases can effectively protect reduce the melt through phenomenon; can reduce the gate, temperature, mold temperature, but at the same time need to take into account the resin liquidity problems, otherwise there may be a lack of material.
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