Introduction to low-pressure injection molding process

Characteristics and classification of low-pressure injection molding process

Characteristics of the low-pressure injection molding process compared to the traditional coating process：

1.  Low-pressure injection molding integrates the epidermis material with the plastic substrate, and there is no possibility of falling off.

2.  Since there is no gluing process necessary for the coating process, the low-pressure injection molding process is more environmentally friendly and conducive to the air quality in the car.

3.  The internal structure of low-pressure double-layer injection molded parts can be designed arbitrarily, the freedom of surface modeling is greater than that of the coating process, and the modeling characteristics are clearer and tougher.

4.  Low-pressure injection molding does not require additional secondary coating to improve production efficiency

5.  Low-pressure injection molded products have a good feel and appearance, and excellent surface texture

6.  Low-pressure injection molding products have low injection pressure, which can effectively protect the internal parts of the product, and the scrap rate is low.

7.  Can cover appearance defects such as fused wiring

Classification of low-pressure injection molding process：

1. Injection compression molding

The punched fabric is covered in a mold, the resin is injected into the back of the fabric and then combined and compressed into a mold. The extrusion injection molding process can only be realized in a special (special) injection molding machine.

2. Injection molding

The punched fabric is covered in the mold, and then the mold is closed for direct injection molding. Injection molding can be achieved on a general injection molding machine.

Injection compression molding process characteristics：

    The working principle of special (special) low-pressure injection molding equipment is to hang a coating on the opposite side of the injection filling nozzle, and reserve a certain distance (gap) when the front and rear molds are closed. The injection filling nozzle emits a predetermined weight of injection material, and then the mold is closed, pressurized and locked tightly, and the injection glue is continued to hold the pressure until it is formed.

    Injection compression molding can increase the ratio of flow injection length/wall thickness of injection molded parts; use smaller clamping force and injection pressure; reduce the internal stress of the material; and improve processing productivity.

The distinctive feature of injection compression molding is that the mold cavity space can be automatically adjusted according to different requirements. According to different modes of operation, the size of the cavity space can be controlled accordingly during or after the injection of the material is completed, so that it is compatible with the injection process, so that the polymer maintains an appropriate state of compression, and achieves the effect of compensating for the shrinkage of the material.

Key points of injection compression molding technology：

1.  This process can be used in ordinary injection molding machines, but it is important to use a valve-type/sequentially fed hot runner system to prevent the return of sol.

2.  The structural design of the PL surface of the mold is very critical, and the activity of the coating must be well controlled. (Good re-skin activities can minimize the damage to the skin texture on the surface of the re-skin).

3.  In order to minimize the damage to the covered leather, the leather must be evenly and flatly covered on the mold.

4.  For the filling of more tendon positions, before the mold is completely closed to provide pressure, the mold cavity must have a sufficient and accurate capacity of sol.

5.  We set up a side touch guide surface at the sealing material of the mold. The coating of the mold is not completely compressed before it is completely closed, which can effectively control the overflow of sol.

6.  Before the mold is designed, it is necessary to conduct a detailed mold flow analysis of the product and accurately locate the gate.

Key points of product design and points of attention for common defects

1. The positioning structure is consistent with other ordinary injection molded parts

2. The release angle of the product is≥8° (combined with the elongation of the fabric, it is judged according to the technical ability of the professional manufacturer)

3. The material thickness of the product should neither be too thick nor too thin, and the optimal solution should be determined if the strength requirements of the product are met.

4. The length of the product flanging is preferably ≤20mm;

reason:

①the fabric is wrinkled, crushed or broken when the flanging is too long during injection molding

②the elongation of the fabric cannot meet the requirements

The fabric elongation of the fabric at 50N is generally: 11.28% in the horizontal direction and 20.28% in the vertical direction.  37%

5. The edge contour design of the product is as smooth as possible

reason:

After the low-pressure injection molding of the product is completed, there will be a certain width of elastic fabric on the edge. According to the appearance requirements, ultrasonic cutting or welding methods are generally used. If the edge structure of the product is complex, it will have a greater impact on cutting and welding, resulting in defects such as uneven edge contours of the product, and increasing the scrap rate.

6. The parting line on the back of the product is designed with a fixed area of 10-15mm fabric inward, and the rest of the structure is not designed.

7. It is not easy to have too many corners of the product. Try to split large parts into small parts to achieve the purpose of low-pressure injection molding.

Common defects

1. The fabric is subjected to the impact of fluid, and the high temperature of the melt has defects such as cracking, curling, deformation, and burning.

2. The repeatability and stability of the manipulator affect whether the fabric is flat when it is hung in the mold, otherwise the material will wrinkle after injection molding.

3. The pressure and speed parameters are not set appropriately, and the product is prone to under-material.

4. The product is prone to warping and deformation

5. Burns occur at the gate of the product

6. Epidermal breakdown, broken skin, and epidermal strain

7. The R corner is shiny

The core technology of low-pressure injection molding

1. Equipment accuracy requirements:

The most important thing for low-pressure injection molding equipment is the mold closing system. It is generally a two-plate template. It needs to have the injection compression function, that is, the template is opened during the mold closing and injection molding process, and the distance from the template opening ranges from 0mm to 5mm (depending on the product). High-precision hydraulic mold closing system is the most critical technology of low-pressure injection molding equipment.

2. Low-pressure injection mold

The main characteristics of low-pressure injection molds are as follows:

(1) Flip mold, that is, there is no ejection part on the fixed side (moving mold) of the decorative fabric, and the ejection system is on the injection (fixed mold) side.

(2) Use a hot runner system

(3) The precision of the mold is very high, and the cooperation is precise, otherwise it will easily cause damage to the film or decorative materials.

(4) The design requirements of the mold waterway are high, and the mold needs to be cooled evenly.

3. Surface decoration materials:

The requirements for fabrics for low-pressure injection molding mainly include:

(1) Insensitive to pressure and temperature:

(2) The surface structure is highly resilient;

(3) The ductility of decorative materials is suitable for the geometry of the product;

(4) Good adhesion to basic materials;

(5) Insensitive to fusion wiring performance:

(6) Suitable for soft tactile requirements:

4. Control of low-pressure injection molding process

Product debugging needs to coordinate the manipulator, fabric thickness and position, mold movement and equipment matching, and injection process parameters. At the same time, the entire production process has a long time and the appearance quality of the product is demanding. A slight error in any link will cause the quality of the product to fail to meet the requirements, so it requires a comprehensive process planning and strict process control in the early stage, as well as a relatively long debugging process.

5. Pre-deformation technology:

During low-pressure injection molding, due to the influence of injection speed, pressure, material temperature, fabric thickness, product structure and other factors, the product is prone to warping deformation and is not easy to change: preliminary analysis can be carried out to confirm the amount of deformation before the mold is processed to avoid subsequent complicated work.

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