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Mold Requirements And Manufacturing Process Of Glass Fiber Composite Material Components

Jun 11, 2020

1. Mold

When designing the mold, the following requirements must be comprehensively considered:

①Meet the precision requirements of product design, the mold size is accurate and the surface is smooth;

② Must have sufficient strength and rigidity;

③Convenient demoulding;

④ Sufficient thermal stability;

⑤ Light weight, sufficient material source and low cost.

The mold material should meet the following requirements:

①Can meet the requirements of dimensional accuracy, appearance quality and service life of products;

②The mold material must have sufficient strength and rigidity to ensure that the mold is not easily deformed and damaged during use;

③ It is not eroded by resin and does not affect resin curing;

④Good heat resistance, the mold does not deform when the product is cured and cured by heating;

⑤Easy to manufacture and easy to demold;

⑥ Reduce the weight of the mold during the day and facilitate production;

⑦ The price is cheap and the materials are easily available.

Basic requirements of release agent:

① Does not corrode the mold, does not affect the curing of the resin, and the adhesion to the resin is less than 0.01MPa;

②Short film-forming time, uniform thickness and smooth surface;

③ Safe to use and non-toxic;

④Heat-resistant, it can be heated and cured at a temperature;

⑤ Convenient operation and cheap price.

2. Hand layup

(1) Raw materials

Reinforced materials

Requirements for reinforcement materials for hand lay-up:

①Reinforcement materials are easily penetrated by resin;

②It has enough deformability to meet the molding requirements of complex shapes of products;

③ Bubbles are easy to deduct;

④ Can meet the physical and chemical performance requirements of product use conditions;

⑤ The price is reasonable (as cheap as possible), and the source is abundant.

The reinforcing materials used for contact molding are glass fiber and its fabric, carbon fiber and its fabric, aramid fiber and its fabric, etc. Among them, the commonly used glass fiber reinforced materials are as follows: untwisted roving, untwisted roving cloth, twisted cloth, chopped glass fiber felt, glass fiber fabric.

Matrix material

The requirements for the base material of the hand lay-up process: ① It is easy to soak the fiber reinforced material under the hand lay-up condition, it is easy to exclude bubbles, and it has strong adhesion to the fiber; ② It can gel and solidify at room temperature, and it requires small shrinkage and volatilization There are few materials; ③ The viscosity is suitable: generally 0.2~0.5Pa•s, which can not produce the phenomenon of glue flow; ④ non-toxic or low toxicity; ⑤ the price is reasonable and the source is guaranteed.

In hand lay-up molding technology, the most commonly used is unsaturated polyester resin, followed by epoxy resin, phenolic resin and furan resin, vinyl resin, etc. There are also a few applications.

Auxiliary materials

The auxiliary materials in the contact molding process mainly refer to two types of fillers and colorants, and curing agents, thinners, tougheners, etc., belong to the resin matrix system.

(2) Process flow

Production preparation

Venue: The size of the hand lay-up molding work site should be determined according to the product size and daily output. The site should be clean, dry and well ventilated. The air temperature should be maintained between 15 and 35 ℃. The post-processing and refurbishment section should be equipped with exhaust dust removal And sprinklers.

Mold preparation: preparations include cleaning, assembling and applying release agent.

Resin glue preparation: When preparing, pay attention to two issues:

①Prevent air bubbles from being mixed in the glue;

②The amount of glue should not be too much. Make sure to use it up before the resin gel every time.

Preparation of reinforced materials: The types and specifications of reinforced materials are selected according to design requirements.

Pasting and curing

Laying and pasting: manual laying and pasting are divided into wet and dry methods:

① Dry layup uses prepreg as raw material, cut the pre-learned material (cloth) into bad material according to the model, heat and soften it when laying, and then adhere to the mold layer by layer, and pay attention to exclude the layer Bubbles in between make it dense. This method is mostly used for hot pressing cans and bag pressing.

②Wet lay-up The reinforcement material is dipped directly on the mold and adheres to the mold layer by layer, deducting air bubbles to make it dense. The general hand lay-up process uses this method to lay layers. Wet lay-up is divided into gel coat layer paste and structural layer paste.

Curing: The curing of the product is divided into two stages: hardening and aging: it usually takes 24h from gel to triangulation. At this time, the curing degree is 50% to 70% (Bakur hardness is 15), and it can be demolded. It can be cured for 1 to 2 weeks under environmental conditions to make the product have mechanical strength, which is called aging. Its curing degree is more than 85%. Heating can promote the aging process. For polyester glass fiber reinforced plastics, heat at 80℃ for 3h. For epoxy glass fiber reinforced plastics, the post-curing temperature can be controlled within 150℃. There are many heating and curing methods. Small and medium-sized products can be heated and cured in a curing furnace. Large-scale products can be heated in mold or infrared.

Demolding and trimming

Demoulding: Demoulding should ensure that the product is not damaged. There are several methods of demoulding:

① Ejection and ejection The ejection device is pre-embedded on the mold, and the screw is rotated during ejection to eject the product.

②Pressure demolding There is a compressed air or water inlet on the mold. During demolding, pressurized air or water (0.2MPa) is pressed between the mold and the product. At the same time, it is beaten with a wooden hammer and a rubber hammer to separate the product and the mold.

③ Demoulding of large products (such as ships) can use tools such as jacks, cranes and hardwood wedges.

④Complex products can be manually demolded by pasting two or three layers of FRP on the mold, peeling it from the mold after curing, and then placing it on the mold to continue pasting to the designed thickness. After curing, it can be easily removed from the mold Down.

Trimming: There are two types of trimming: one is size trimming, and the other is defect repairing.

①Dimensional trimming After molding, cut the excess parts according to the design size;

②Defect repair Including repair of perforation, repair of air bubbles and cracks, reinforcement of broken holes, etc.

3. Injection molding

(1) Classification

The injection molding machine is classified according to the injection method and can be divided into:

(1) High-pressure type:

① Use the pump to send the resin into the spray gun, and use the pump pressure to spray;

②Pressurize the resin tank and curing agent tank with an air compressor, and under this pressure, press the resin and curing agent into the spray gun to spray.

(2) Pneumatic type: Resin, curing agent or their mixture is atomized and sprayed with air by the force of compressed air spray.

According to the mixed form, it can be divided into:

(1) Internal mixing type: The initiator is mixed inside the spray gun and sprayed.

(2) External mixing type: Resin containing accelerator and initiator respectively are sprayed by spray gun and mixed with each other in mist form. There are two types of spraying initiator separately and spraying resin containing initiator. (3) Mixed type: The resin containing accelerator and initiator is prepared in advance and sprayed from the spray gun.

(2) Process flow

Before the injection molding starts, the gel time of the resin should be checked first. The measuring method is to spray a small amount of resin into a small tank to measure its gel time; the ratio of resin to glass fiber must also be checked. Generally, the resin to glass fiber The ratio is between 2.5:1 and 3.5:1.

After the gel coat resin gels {soften without sticking hands), you can start the injection molding operation. If there is no gel coat resin, spray a layer of resin on the mold first. Then the cutter is started and the mixture of resin and fiber is sprayed. The first layer should be sprayed thinner (about 1mm thick), and carefully rolled, first with short mohair, then with a spiral. To ensure that the resin and curing agent are mixed evenly and the glass fiber is completely infiltrated. Operate carefully to ensure that there are no bubbles in this layer, and this layer must completely wet the gel coat resin, and then spray the next layer after this layer of gel. Each next layer is about 2mm thick. If it is too thick, the bubbles are difficult to remove, and the quality of the product cannot be guaranteed. Each layer should be carefully rolled to remove air bubbles. Repeat this way until the design thickness is reached.

To obtain a higher strength product, it must be used in combination with roving cloth. When using roving cloth, a sufficient amount of resin should be sprayed on the mold before spreading the roving cloth and rolling carefully, which is conducive to removing air bubbles. For most injection equipment, the injection rate is generally 2-10kg/min.

As with hand lay-up molding, the last layer can be made of surface felt and then coated with an outer coating. The processes of curing, trimming, post-curing and demoulding are the same as the hand lay-up molding method.

4.RTM molding process

The resin system for RTM should meet:

1) Low viscosity and good wettability, which is convenient for resin to pass through and impregnate fibers smoothly and evenly in the mold cavity;

2) The peak of curing exotherm is low, preferably 100~180℃;

3) High activity, short gel time and curing time, but a longer pot life during injection;

4) The resin system does not contain solvents, and no low-molecular substances are precipitated during curing, and it is suitable for adding fillers, especially the resin defoaming property is better;

5) The shrinkage rate is low to ensure accurate product size, and the required resin should be pre-promoted.

The high-performance resin matrix used by RTM includes: unsaturated polyester resins, vinyl ester resins, epoxy resins, bismaleimide resins, and thermoplastic resins. Currently it is mainly epoxy resin.

The requirements of RTM molding for reinforcing materials are:

1) The distribution of reinforcement materials should meet the requirements of product structure design, and attention should be paid to the direction;

2) After the reinforcement material is paved, its position and state should be fixed, and it should not move due to mold clamping and resin injection;

3) Wetability of resin is better;

4) It is conducive to the flow of resin and can withstand the impact of resin.

Principle of RTM molding process

Composite Resin Transfer Molding (Resin Transfer Molding, RTM) method is a new composite material molding process evolved from wet layup and injection molding process [6]. The so-called resin transfer molding generally refers to pre-laying a pre-formed reinforcement material (including inserts such as bolts, nuts, polyurethane foam, etc.) designed according to performance and structural requirements in the closed mold cavity. The prepared resin is injected into the mold from the injection hole placed at an appropriate position under a certain temperature and pressure, and cured together with the reinforcing material at room temperature or temperature, and finally the mold is opened and demolded to obtain a molded product.