No matter what product you make or what process you choose, as long as it is a FRP product, you will find an unavoidable problem, that is, the shrinkage and deformation of the product.
When designing, the dimensional data is very accurate, and the process seems to be no problem, but the products that come out are shrinking and deforming. Which link has the problem?
The root cause of the shrinkage of the product is the shrinkage of the resin, and there are three main reasons why the resin shrinks.
①When the resin is cured, the chemical bond is broken, and the cross-linking reaction is reversed. The original distance between molecules becomes the bond length distance, and the occupied volume becomes smaller.
②In the viscous flow state, the molecules of the resin are distributed in disorder, and during the curing process, it gradually changes into an ordered distribution. Molecules are tightly packed, resulting in a smaller occupied volume.
③ During the curing process, the temperature of the system rises due to the exothermic reaction. After reaching equilibrium, the temperature begins to drop, the thermal motion of the molecular chain gradually weakens, and the free volume decreases at this time.
To better understand these three points, take an example. In a room, originally packed with 30 people, at first everyone stood, sat or lay down at will, but the room was full. Later, I asked everyone to sort them, and 30 people stood in line one after another. The crowded room suddenly became more spacious. During the sorting process, the confusion caused by walking is annoying, but as everyone quiets down, it begins to cool down slowly, and the space is more open at this time.
The shrinkage of the resin causes internal stress in the product, which is the source of the deformation of the product. The root causes of product shrinkage and deformation were found at once, which was caused by resin shrinkage. The next step is to find a way to solve the problem from the root.
1. Resin
Since resin is the root cause, can we improve the shrinkage rate of the resin itself? The answer is yes. Resins of different systems have different shrinkage rates due to different reaction mechanisms. For example, the volume shrinkage rate of common unsaturated polyester resins is generally 7-10%, while the volume shrinkage rate of epoxy resins is generally 2%. about. It can be seen that there is a very large gap in the shrinkage rate when different resins are selected.
The same unsaturated polyester resin can also fundamentally control the shrinkage rate by selecting different raw materials in the synthesis stage. The volume shrinkage of some new UP resins can be controlled at 2-3% or even lower.
However, in many cases, due to various reasons such as process and cost, we do not have much space when selecting resins. When the shrinkage rate of the resin is too large, we need to look at the following suggestions.
2. Filler
The problem with resin shrinkage is that after curing, its space volume becomes larger, so the shrinkage rate can be reduced by adding some fillers.
Commonly used inorganic fillers are calcium carbonate, talc, aluminum hydroxide, etc. These fillers generally do not participate in chemical reactions and are inert materials. The shrinkage rate is reduced by reducing the resin content per unit volume and the way of occupying the pit.
In addition, shrinkage can be controlled by adding a low shrinkage agent (LPA), which is mainly composed of some thermoplastic resins, elastomers or combined polymers. In addition to occupying the pit, the thermoplastic resin will also expand to a certain extent during the heat release of the resin, which can offset the volume shrinkage caused by the curing process of the thermosetting resin, thereby reducing the shrinkage rate more effectively.
Polystyrene (PS) commonly used in the production of SMC sheets is a thermoplastic resin.
Although the addition of filler or LPA can effectively reduce the shrinkage rate of the resin, it will also reduce the mechanical properties of the product to a certain extent, which is not a foolproof solution.
3. Process
When the raw materials cannot be changed, we can also control the shrinkage rate from the process.
Controlling the cure speed is critical. Too fast curing speed and violent reaction often result in too high exothermic peak. If the temperature of the system changes too much, it is easy to cause thermal shrinkage. Therefore, under the premise of ensuring production efficiency, the curing speed must be well controlled, and it should not be too fast.
Control the resin content. The source of shrinkage is mainly the resin, so controlling the resin content can also control the shrinkage rate. Especially in the hand lay-up process, in many cases, workers increase the amount of resin in order to rush work, resulting in excessive resin content in the product, which is more prone to shrinkage and deformation.
Control the number of single layers. This is mainly reflected in the hand lay-up process. There is a very obvious difference in the shrinkage rate of one-layer pasting and five-layer pasting at a time. The more layers, the more concentrated the reaction heat, which is more likely to lead to thermal shrinkage. .
Post-curing treatment. Through heating and post-curing, the curing degree of the product is improved, and the monomer continues to react after demolding, resulting in shrinkage and deformation. This link is neglected in many normal temperature processes (hand lay-up, vacuum introduction, RTM), and in order to improve production efficiency, the mold release is often too fast, so it is easy to cause the product to be placed for a period of time, and the deformation will appear before it leaves the warehouse.
4. Layer design
This is mainly for the lay-up design of reinforcing materials and core materials.
The selection of different reinforcing materials has a certain influence on the content and distribution of the resin, so this issue needs to be considered when designing the layup.
Whether the product chooses the core material will also affect the resin content. For example, for 20mm products, one solution is pure glass fiber reinforcement, and the other design solution is to use 10mm sandwich foam + 10mm skin, and the shrinkage rates of the two are also different.
Of course, all choices must be made based on the requirements of the product, and it is necessary to take into account factors such as performance and cost in order to make a better solution.
5. Local strengthening
Shrinkage will lead to internal stress, and the existence of internal stress will lead to deformation of the product, thus affecting the use. In order to suppress this deformation, some local reinforcements can be made on the product to offset the internal stress and prevent the deformation of the product.
In FRP molds, steel structures are basically used for post-reinforcement, which can prevent deformation when the mold is subjected to external force or internal stress. In the product, the reinforcement ribs are used to ensure the rigidity of the product.
With the current technical means, it is unrealistic to completely eliminate the shrinkage of the resin itself, and the shrinkage is only a matter of large and small. There are so-called "zero shrinkage" resins on the market, which are not really zero shrinkage, but only control the shrinkage rate to a few thousandths.
Where there are people, there are rivers and lakes. Where there is resin, there is shrinkage.
Shrinkage and deformation are big problems, which will seriously affect the subsequent installation and use of the product. However, many companies do not pay attention to these problems in the production process, and use violence if they cannot be assembled. I have seen many scenes of beating.
It is possible to mix up products with low requirements. What if you encounter customers with high standards?
Of course, as long as we follow the standard and keep the shrinkage within a reasonable range, the problem will be solved.