Common Defects and Solutions for RTM Products

RTM process can produce high-performance, large-size, high-integration, medium-volume products. The number of products is between 1,500 and 40,000 pieces, which is suitable for RTM process. Therefore, RTM is a very promising molding process. At present, RTM process has been widely used in construction, transportation, telecommunications, health, aerospace and other fields. Because the process is relatively young and technically difficult, some defects often occur in the products. This article analyzes the common defects of RTM products for reference.

 

1. The surface of the product is partially rough and dull

The main reason for this phenomenon of RTM products is that the product is slightly sticky to the mold. Touch the mold with your hand, and when you touch these parts, the feel is extremely rough. Usually, this problem will occur after the product has been produced for a period of time, and the mold needs to be cleaned in time. First, use water sandpaper to polish the rough parts of the mold, then wipe the entire mold with cotton thread dipped in acetone, and finally apply a release agent to the mold.

 

2. Wrinkling of gel coat

The main reason for wrinkling of gel coat is that the gel coat resin is not completely cured before pouring the resin, and the styrene monomer in the pouring resin swells the gel coat resin, resulting in wrinkles. Therefore, pay attention to the following aspects when spraying gel coat:
(1) The thickness of the gel coat layer is usually 0.3-0.5mm or 400-500g/㎡, and its thickness can be increased when necessary;
(2) Check the amount and mixing ratio of the accelerator and curing agent. The amount of accelerator and curing agent must be sufficient;
(3) Wait until the gel coat is completely cured before pouring the resin.

 

3. Leakage of glue

The main reason for leakage of glue is that the mold is not tight after closing or the sealing gasket is not tight. Before closing the mold, check whether the sealing ring is intact and whether there are cracks, etc., and replace it in time if any problems are found. When closing the mold, the mold should press the sealing gasket tightly. If leakage is found during pouring, the bolts around the leakage part should be tightened immediately.

 

4. Bubbles

Theoretically, there are four reasons for the generation of bubbles:

(1) The heat release of the resin curing reaction in the mold cavity is too high, and the curing time is too short, so the gas in the mold cavity is not completely discharged;

(2) When the resin is injected into the mold cavity, too much air is brought in, and it cannot be completely discharged during the injection time, and the bubbles are even always at the top of the mold cavity, and the resin from top to bottom cannot carry them out;

(3) The viscosity of the resin is too high, and the bubbles cannot completely overflow from the product during the injection time. Moreover, due to the high viscosity of the resin and the slow flow, the bubbles may even be carried to the middle area of the side of the product (when the product is higher), so that the product after demolding contains more bubbles (or pit-shaped glue shortage);

(4) The pressure of the resin injected into the mold cavity is too high, causing the bubbles to be contained in the resin and difficult to be discharged.

 

The Corresponding Solutions Are:

(1) Appropriately reduce the amount of resin curing agent used for injection;

(2) Design an exhaust port on the mold;

(3) Test the viscosity of the resin at 25°C. Usually, the viscosity of the resin for RTM is 0.5-1.5PaS. If the resin viscosity does not exceed the standard, you should consider whether the ambient temperature is too low. If the temperature is too low, you can preheat the resin to a constant temperature before pouring it, but the temperature selection must be appropriate. Too low resin viscosity will affect the mechanical properties of the product;

(4) Reduce the resin injection pressure and increase the resin injection volume, thereby reducing the flow rate of the resin in the mold cavity and increasing the seepage volume.

 

5. Dry spots on products

Dry spots on the inside of RTM products are mainly caused by insufficient impregnation of glass fiber. If dry spots appear on a certain part of a product in the same period, it may also be caused by contamination of the glass fiber cloth. Usually, dry spots on the inside of the product are also related to the viscosity of the resin, so the viscosity should be analyzed and adjusted first; check whether the mold flow channel is too long or too narrow, and improve the mold and feed pipe in time to increase the feed point.

 

6. Wrinkles

Wrinkles are also a common problem for many RTM products. Wrinkles are different from wrinkles. Wrinkles are caused by wrinkling of the cloth layer. After demolding, the appearance of the product can clearly see the cloth folds, but the cloth folds are filled with resin. The main reasons for wrinkles are: (1) When closing the mold, wrinkles are generated due to the extrusion of the mold on the cloth layer; (2) When the resin flows in the mold, the cloth is squeezed and deformed, resulting in wrinkles. Therefore, when producing RTM products, attention should be paid to the closing operation method and the thickness of the cloth layer. Improper mold closing or too thick a cloth layer can easily cause wrinkles; use glass fiber cloth, preformed blanks or knitted composite mat with good erosion resistance to reduce injection pressure, thereby reducing the impact of resin on the cloth layer.

 

7. Cracks
There is still a certain gap between domestic RTM technology and foreign technology. The fiber content of domestic RTM products is low (generally about 50%), and due to the immature technology of the process, the fiber content of some products is unevenly distributed, resulting in cracks in the part with too low fiber content.

 

From a theoretical analysis, there are two reasons for the generation of cracks:

(1) The product is not completely cured in the mold cavity. Even after post-curing treatment, the interior of the product is still slowly curing, and the curing shrinkage rate of the resin is large. In this way, the strength of the load-bearing carrier is insufficient in the parts with low fiber content in the product. Due to the effect of curing internal stress, cracks are formed on the surface of the product;

(2) The product itself has been completely cured, but due to the large temperature difference during transportation, thermal expansion and contraction, large internal stress is generated, and cracks are generated in the weak parts of the product with the lowest fiber content.

Therefore, the process parameters should be adjusted according to the actual process conditions to improve the fiber content and fiber distribution uniformity. At the same time, attention should be paid to the curing shrinkage rate of the resin used in the RTM process. A large curing shrinkage rate of the resin will not only affect the surface effect of the product, but also increase the difficulty of demolding due to the internal stress generated by curing shrinkage. At this time, low shrinkage additives should be used, or a mixed resin system should be used to reduce the curing shrinkage rate of the resin.

 

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With the improvement of composite material manufacturing level, the continuous expansion of production scale and the great progress of automation, composite materials are more widely used in aerospace, automobile, rail transportation and other fields, and their application level has become an important indicator of the advanced nature of the new generation of equipment. my country’s composite materials account for about 30% of the global market share.

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HP-RTM process is a new RTM process technology introduced in recent years to cope with the mass production of high-performance thermosetting composite parts. It adopts matching preforming technology and completes the impregnation and curing of resin on fibers through high-pressure mixed injection in a closed vacuum mold cavity, which can achieve low-cost, short-cycle (large-scale) and high-quality production.