What is low pressure injection molding? Automotive low-pressure injection molding products and mold structure introduction

What is low pressure injection molding? Automotive low-pressure injection molding products and mold structure introduction

- May 12, 2019-

1. What is low pressure injection molding?

The low-pressure injection molding process is a packaging process that uses a very low injection pressure (0.15-4 MPa) to inject hot melt material into the mold and cures quickly. The excellent sealing performance of the hot melt material and excellent physical and chemical properties are used to achieve insulation and resistance. Warm, impact resistant, vibration-damping, moisture-proof, waterproof, dust-proof, chemical-resistant, etc., good protection of electronic components.

2. Forming process and application

The skin is placed in the mold in advance, and then the plasticized molten plastic is injected into the closed cavity by the thrust of the screw, and then the product is obtained after curing and setting. The material used for the low-pressure injection molding can be cloth or PVC. At present, the process has been widely used in the production of automotive door guards, pillar guards and parcel guards.

3. Application examples of low-pressure injection molding technology in automotive products

Back side front upper trim panel PVC fabric covering

Automobile A upper column non-woven cover

Automobile B upper column non-woven cover

Car C upper column non-woven covering

4. Advantages of low pressure injection molding process compared to traditional coating process

1) Low-pressure injection molding combines the skin material with the plastic substrate without the possibility of falling off.

2) The low-pressure injection molding process is more environmentally friendly because there is no coating process necessary for the coating process.

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

4) Low pressure injection molding does not require additional secondary coating to increase production efficiency.

5) Low-pressure injection molding products have a good feel and appearance.

6) Low-pressure injection molding products have low injection pressure and can effectively protect internal parts of the products with low scrap rate.

5. Difference between low pressure injection mold and conventional mold

6. Factors to be considered in the low pressure injection molding process

1) Fabric ductility and tensile properties

Now the fabrics for low-pressure injection molding are the underlying non-woven fabric + foam layer + surface fabric layer. Due to the compression of the mold and the extrusion of the molten plastic, the longitudinal and lateral extension of the fabric is different, and the phenomenon reflected on the product is different. . The most prominent problems are: seepage, breakdown, and damage. That is to say, in the case of a constant mold state and constant process conditions, the characteristics of the fabric have a great influence on the quality of the product. Similarly, the elongation has different adaptability to different molds in the longitudinal direction and the lateral direction. Some molds may have high requirements for longitudinal elongation due to design constraints, and some molds may have high requirements for lateral elongation. Therefore, we need to consider this issue comprehensively when trying to make new products and new molds.

2) Plasticity of liquidity

The indicator for assessing the fluidity of a plastic is: the melt index MI value. Everyone knows that the flow of plastic on the fabric is certainly much slower than the flow on the smooth mold cavity, which is why the low-pressure injection mold of the fabric has more gates than the ordinary mold. The slow flow rate will inevitably affect other appearance problems of the product, such as bonding marks and lack of materials.

7. Common defects and improvement schemes of low pressure injection molding process

(1) breakdown

The corners of the product are prone to breakdown, and the parting surface of the sharp corner is matched with a gap. When the amount or speed increases, the molten plastic penetrates from the gap. A thick wall at a sharp corner is also likely to cause breakdown.

(2) seepage

The material is prone to seepage at the corner of the product, the gate temperature of the infiltration site is too high, the mold temperature is too high, and the amount of gate material in the permeate site is excessively controlled.

Improvement plan:

1 The wall thickness at the corner is too thick to cause breakdown, so the wall thickness at the corner of the mold is relatively thinner to prevent the fabric from seeping or breaking down;

2 product prevention design, try to make the product profile smooth transition, reduce the step drop, avoid excessive product transition;

3Adjust the parameters of the injection molding process, reduce the mold temperature and melt temperature, reduce the amount of plastic injection, and reduce the injection pressure.

(3) shrinking at the glue mouth

Reduce the shrinkage, the gate is below Φ3, and the cold runner length is 15 to increase the cooling at the gate.

(4) lack of material

There is a shortage of material at the sharp corners of the product. The reason is that the mold temperature is too low, the gate temperature of the material shortage is low, the gate material of the material shortage is under controlled, the wall thickness of the mold corner is too thin, and the injection speed and compression are small. The speed is low.

(5) Fabric crushing

Reason: Generally, the mold temperature used in low-pressure injection molds is low (10-15 degrees). If the anti-rust measures are not done well, rust spots will be generated on the pressed fabric frame and the slider parting surface, and the fabric will be pressed during production. Excessive, the fabric has limited ductility, which crushes the fabric.

(6) The R corner is bright

Check if the R angle is at a sharp corner, if the product has a large warpage angle but the R angle is not large enough, and the ductility of the skin meets the requirements.

(7) Epidermal strain

Check if the draft angle is sufficient; if the mold surface finish is satisfactory.

8. Several ways to hang the low pressure injection mold

The first type: fixed hanging needle

The hanging needle is not adjustable. It is suitable for relatively flat products. It utilizes the ductility of the fabric itself to save the mold space, but it is easy to tear the fabric. When the 2-hole product is used, it needs to be hanged separately.

Second: adjustable hanging needle

It is suitable for products with large drop and complicated shape. The hanging needle itself can be adjusted to effectively protect the fabric.

The third type: using the hook form

The fabric is clamped in the form of a hook to vertically suspend the fabric. This form is easy to operate, and the sharp part of the hanging needle is prevented from being tied to the person. The fabric can be automatically adjusted during the mold clamping process, and is not easy to wrinkle.

9. Several ways to press the low pressure injection mold

The first type: press cloth form

The thimble is modified, the processing is simple and convenient, but the pressing needle is easy to crush the cloth.

10. Summary of low pressure injection molding dies

1) The release angle of the product is required to be more than 5 degrees. If it is less than 5 degrees, the skin will be strained. The angle of the corner R is more than 2 times the thickness of the rubber.

2) The product can not have sharp corners, sharp angles and other defects, which will cause the product skin to be easily broken (the skin is easy to break and brighten the non-woven fabric; the composite skin with leather, sponge and non-woven fabric is easy to collapse, thus affecting the product) Beautiful), the undulation of the product is not easy to be too large.

3) The surface of the mold surface needs to be R-transition, and it is not possible to have favorable angles to prevent scraping and crushing the fabric.

4) When designing the mold, it should be noted that the gate cannot be fed by conventional feeding methods such as side feeding and latent, and should be designed with flip-chip design and large nozzle design.

5) The hot runner should be controlled by sequential valve to better adjust the injection molding process; the hot runner is different from the conventional hot runner, the nozzle temperature is not too high (about 200 degrees) to prevent burnt skin; the nozzle lining is not too long, not More than 15mm to prevent surface shrinkage.

6) The parting surface is inconsistent with the common injection mold. The main parting surface needs to be avoided, and the reserved space is 1-2mm thicker than the thickness of the skin, so that the skin has an extension space. The side parting surface needs to reserve compressed skin space, the non-woven fabric and the compression amount is about 0.6mm, and the skin, sponge and non-woven fabric have three layers of composite skin, and the pre-compression should not be less than about 1mm.

7) The actual stretch ratio of the fabric at the corner is smaller than the theoretical stretch ratio (assuming the thickness of the skin is the theoretical data position X, then the actual thickness of the skin after stretching is X/1.16, the actual stretch ratio = the length of the data line 16.67 / process The length of the wire is 14.4=1.16. The longitudinal and transverse stretch ratio of the fabric is 25%, so it can meet the requirements)

11. Automotive A column product introduction

1) In addition to the decorative effect, the A-pillar upper trim panel assembly must ensure the clearance from the instrument panel, the front windshield, the canopy and the peripheral parts after installation;

2) The clearance between the A-pillar upper trim panel assembly and the A-pillar inner panel sheet metal shall ensure the electrical harness space;

3) The surface fillet is too soft, and the fillet size of the specific area meets the requirements of the internal projections;

4) Meet the A-bar barrier angle regulations, consider the comfort of the human body, take into account the aesthetics of the shape, and meet the instrument panel and peripheral components.

A column conventional injection mold physical

A-column low-pressure injection mold

12. Automotive A column mold structure introduction

The surface coating of the product cannot be glued. The low-pressure injection mold generally adopts the flip-chip mode, and the needle valve hot runner directly feeds into the product B surface.

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