RTM process has the advantages of good economy, good designability, low styrene volatilization, high product dimensional accuracy, good surface quality, and A-level surface. It accounts for an increasing proportion in the FRP molding process.
The RTM molding process requires a mold with a more precise size. RTM generally uses male and female molds, so the dimensional accuracy of the mold is a key issue for the control of the cavity thickness after the mold is closed.
1. Raw material selection
To control mold accuracy, the selection of raw materials is an important factor. The mold gel coat used to make RTM molds should have higher impact toughness, heat resistance, rigidity and lower shrinkage. Generally, vinyl ester mold gel coats can be used.
The fiber reinforced material for RTM mold can be 30g/㎡ alkali-free surface mat and 300g/㎡ alkali-free chopped strand mat. The mold made of 300g/㎡ alkali-free chopped strand mat has lower shrinkage rate and higher dimensional accuracy than 450g/㎡ alkali-free chopped strand mat.
2. Process control
The selection of raw materials is an important part of controlling the size of the RTM mold and the thickness of the cavity. The quality control in the mold remanufacturing process is a more important process. If the process is not properly controlled, even if the raw materials meet the requirements for use, it is difficult to remake Make molds with accurate dimensions and qualified cavity thickness.
In the process of mold remaking, the accuracy of the transitional wooden mold must be grasped first. In order to ensure accuracy, a certain shrinkage margin can be reserved according to the shrinkage rate of the mold used when the design of the filter wood mold is started. In addition, pay attention to repairing and leveling the surface of the transitional wooden mold, and the scars on the surface of the wooden mold must be dug out. The inconsistency between scarring and wood shrinkage will cause uneven surface of FRP mold. After digging out the scars and removing the surface burrs, the surface of the wooden mold must be treated with putty scraping, generally requiring scraping 2-3 times. After the putty is cured, smooth it with sandpaper until it fully meets the requirements of size and shape accuracy.
The wooden mold must be made fine, because the dimensional accuracy of the FRP mold ultimately depends on the accuracy of the wooden mold. In order to ensure that the surface of the FRP mold is smooth and clean, it is more appropriate to use the spray method for the gel coat layer when turning the first FRP mold.
When spraying gel coat, attention should be paid to adjusting the air flow of the spray gun, so that the gel coat resin is atomized uniformly and does not appear granular. The starting and closing of the spray gun should be outside the mold to avoid local gel coat sagging and affect the surface quality. After the gel coat layer is cured, the surface felt is attached. The surface felt should be outside the mold to avoid local gel coat sagging and affect the surface quality.
After the gel coat layer is cured, the surface felt should be applied. The surface felt should be spread evenly, and the fold or overlap must be cut and trimmed. The surface felt is pasted, and a small amount of resin can be impregnated with a brush to soak the surface felt. Pay attention to controlling the amount of glue. The glue content is high, the bubbles are not easy to be eliminated, and the curing exotherm is large, and the shrinkage rate is large. After the resin of the surface felt layer is cured, pick up air bubbles, and pick up the air bubbles to not scratch the gel coat layer.
After picking the air bubbles, sanding properly, removing glass fiber burrs and removing floating dust, hand lays 300g/㎡ alkali-free chopped strand mat, and only 1~2 layers can be pasted at a time. Pasting can be continued after the curing exothermic peak. After the paste is made to the required thickness, copper pipes can be laid and thermal insulation pellets can be laid. Prepare glass microbead resin putty as an adhesive for laying insulation core blocks, and use it to fill the gaps between insulation core blocks.
After laying, the gaps on the surface of the thermal insulation pellets must be smoothed with glass bead putty. After the insulation core block layer is solidified, paste 3-4 layers of chopped strand mat to paste the mold steel skeleton. When the steel frame is pasted, the steel frame should be annealed first to eliminate welding stress, and the gap between the steel frame and the mold should be filled first to prevent the FRP mold from deforming with the steel frame.
After the first mold is cured, it is demolded, the excess flash is removed, the mold cavity debris is cleaned, and the wax sheet is attached. The wax flakes used should have uniform thickness and low elongation. Air bubbles should not be wrapped in the wax sheet. Once there are bubbles, remove and re-attach to ensure the cavity size. Cut the lap joints, and use putty or plasticine to fill the gaps between the wax slices. After the wax sheet is pasted, the second mold can be turned over as the first mold. The second mold usually needs to be equipped with injection holes and vent holes after the gel coat layer is sprayed. After turning over the second mold, you must first cut off the flash, weld the positioning pins and the locking bolts, and demold after the curing is complete.
3. Mold inspection and remedial measures
After demoulding and cleaning, use plasticine to measure the thickness of the cavity. If the thickness and size can meet the requirements, the RTM mold will be remanufactured after the grinding and polishing process is completed, and it can be delivered to production.
If the mold cavity does not meet the requirements due to poor process control and other reasons, there are two remedies: ① scrap one of the molds and reopen one; ② use the characteristics of the RTM process to repair, usually glue one of the mold surfaces The clothing layer is chiseled off, and glass fiber reinforced material is laid. The other mold is pasted with wax, sprayed with gel coat, and then closed and injected. After curing, it can be delivered for use.