Discussion on the Problem of Flow Induction in Closed Mold Process and Glass Fiber Pattern Printing of Products

 

1. Introduction to Closed Mold Molding Process RTM/Silicone Mold/Vacuum Bag Pressing

1. RTM process (RTM/LRTM/VARTM, etc.)
   a.   RTM advantages: good product stability, suitable for complex components, relatively high glass fiber content, only requires an injection molding machine… Disadvantages: high mold cost, high equipment cost, difficult mold repair, low efficiency

b. LRTM Advantages: less investment in equipment and molds, simple operation, high production efficiency… Disadvantages: relatively poor product stability

c. Advantages of VARTM: uniform resin flow, fewer product defects, good product stability…

Disadvantages: relatively large equipment investment.

2. Advantages of silicone bag molding process: reusable, high production efficiency, reduced garbage, product surface is better than vacuum bag pressing… Disadvantages: large one-time investment in equipment and molds, product structure cannot be too complicated, only applicable to unsaturated polyester resin…

Discussion on the solution to the glass fiber texture of closed mold products

1. Use low shrinkage resin
2. Advantages: simple operation…

3. Disadvantages: obvious cost increase

 

2.Use fiberglass surface felt

Advantages: good flame retardancy, good corrosion resistance, good moldability…

Disadvantages: poor versatility and poor operability…

3. Use polyester surface felt

Advantages: relatively simple operation, good UV resistance, no chemical adhesives…

Disadvantages: poor versatility, poor flame retardancy

4. Use Dymisolation Mat

Advantages: good mold filling, good flow conductivity, simple operation, easy laying, no chemical adhesive (suitable for closed mold process)…

Disadvantages: poor flame retardancy

 

5. Use composite felt, multi-axial or strong core mat

 

1. Closed mold process printing solution
2. RTM is a process method in which the designed reinforcement material is laid in the mold cavity, and the resin is injected into the mold cavity under the action of pressure or vacuum or both. The resin completes the infiltration of the reinforcement material and solidifies and forms during the process of flowing and filling the mold, thereby obtaining a molded product.

 

Process characteristics:

The thickness of the mold cavity determines the thickness of the product, so its reinforcing material needs to have certain flow conductivity and mold filling properties…

PAN70 Surface Veil: can be flow-conducted and mold-filled;

Polyester Surface Veil: soft, easy to wrinkle when injected with resin;

Glass Fiber Surface Veil: relatively brittle and easy to crack;

Composite felt and strong core mat: used only under special conditions;

Therefore: The characteristics of the RTM process determine that PAN70 Surface Veil is more in line with the process operation requirements.

 

PAN70 Surface Veil + RTM Close Mold Mat (Flow Mat)

 

1、RTM

2. Silicone bag mold

Process characteristics: The silicone bag mold process takes into account the advantages and disadvantages of RTM and vacuum bag pressing processes. The rigid upper mold of RTM is changed into a silicone soft mold, which is equivalent to a layer of silicone bag mold. The fiber is laid between the soft mold and the mold. The vacuum negative pressure is used to introduce the resin from the other end of the mold into the mold through the pipeline, and the fiber is impregnated, solidified and demolded. Therefore, the generation of its fiber texture is basically caused by pressure.

To solve the grain printing problem, we should start from the following aspects

1. Low shrinkage resin (select appropriate resin)
2. Form a fat-rich layer on the surface of the product (polyester/glass fiber surface felt)
3. The surface fiber has a certain flow conduction and mold filling property (PAN Surface Veil/ DymIsolation)

 

PAN Surface Veil + RTM Resin Flow Mat + Combination Mat

 

3. Vacuum Bag Pressing

Use a single-sided mold, vacuum bag film, and sealing strips to establish a closed system. Use a vacuum pump to extract the air in the closed system to form a negative pressure. The resin uses the pressure difference to enter the fiber layer through the pre-laid pipeline and fill the entire cavity. After the resin is cured, remove the vacuum bag material and demold to obtain the product.

 

The thickness of the product is formed by fiber accumulation. This process is very easy to produce fiber texture. Generally, this process is basically used for large products

Glass Fiber/Polyester Surface Veil: Single-layer laying cannot solve the problem of grain printing;

DymIsolation Mat PAN70: The fluffy characteristics make this process difficult to operate on the facade hanging cloth;

DymIsolation Mat V30/300/V30: It is composed of polyester fiber + glass fiber non-woven, which can form a fat-rich layer and increase the surface strength of the product.

Composite Mat/Strong Core Mat: Strong core felt has a more obvious suppression of grain printing (high cost, low strength) V30/300/V30+Composite Sandwich Mat + Composite Mat+Axial Cloth

3. Discussion on the solution of resin diversion in closed mold process
4. Introduction of diversion medium
5. Diversion net:

Advantages: fast diversion speed, uniform resin flow, no mold filling elasticity (for single layer)

Disadvantages: waste of auxiliary materials and resin.

 

2. PP flow-conducting layer:

Advantages: It has the mold filling and flow-conducting properties required by the RTM process, and has good compatibility with resin.

Disadvantages: It has low strength and is not flame retardant.

3. Continuous Filament Mat:

Advantages: It has a certain degree of fluffiness, good flame retardancy and good strength.

Disadvantages: It has poor adhesion and high cost.

 

4. Glass fiber self-winding flow-guiding medium: Advantages: mold filling, uniform flow conductivity, suitable for high viscosity resin, good flame retardancy. Disadvantages: high cost.

B. Discussion on the closed mold process resin diversion scheme

 

1. RTM The characteristics of the RTM process determine that its reinforced fabric needs to have a certain elasticity to adapt to the change of its mold thickness;

It has a certain diversion property, so the above-mentioned diversion medium PP and glass fiber self-winding type are more suitable.

From the perspective of flow diversion, RTM special closed mold felt or Weitong double-layer flow diversion net composite felt is generally used.

Locally high rigidity (surface) As the core material of a sandwich structure, RTM closed mold felt utilizes the crossbar effect.

Schematic diagram of resin shrinkage:

2. Silicone bag mold

 

The process of silicone bag mold is to use vacuum to form negative pressure to make the resin enter the “mold cavity”, so there is a certain pressure and void in the “mold cavity”, and the pressure and void ratio are between RTM and vacuum bag pressure.

Therefore, it is determined that its flow-guiding medium must have a certain mold filling property. It is recommended to choose a composite felt with PP as the flow-guiding medium.

From the perspective of flow diversion, composite felt such as chopped yarn + PP (flow diversion net) + warp knitted fabric is generally used.

3. Vacuum Bag Pressing

The characteristics of vacuum bag pressing process are that the product thickness and resin flow are mainly formed by vacuum pressure. The guiding principle is that the resin flows along the guiding medium in the direction of pressure. Therefore, the guiding medium needs to have a certain pressure resistance and be able to maintain a certain porosity.

From the perspective of diversion, a suitable diversion net or a composite felt with built-in diversion net is generally selected.

• Typical applications of Closed Mold Process