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RTM process has the advantages of good economy, good designability, low volatilization of styrene, high dimensional accuracy of the product and good surface quality up to grade A surface.
RTM molding process requires more accurate size of the mold. rtm generally use yin and yang to close the mold, so the size error of the mold and the precise control of the cavity thickness after closing the mold is a key issue.

1, Material selection
To control the precision of the mold, the selection of raw materials is an important factor. The production of RTM mold used in the mold gel coat should have a high impact toughness, high heat resistance and low shrinkage, generally can be used vinyl ester type mold gel coat.
RTM mold resin generally also requires good heat resistance and rigidity, a certain degree of impact toughness, shrinkage is small or close to zero shrinkage.RTM molds with fiber reinforcement materials can be used 30g / ㎡ non-alkali surface felt and 300g / ㎡ non-alkali short-cut felt. With 300g / m non-alkali short-cut felt than 450g / m non-alkali short-cut felt made of mold shrinkage is lower, higher dimensional accuracy.

2, Process control
The selection of raw materials is to control the size of the RTM mold and cavity thickness of an important link, and in the mold turning process at any time quality control is a more important process. If this process control is not appropriate, even if the raw material meets the use of requirements, it is difficult to turn the mold with accurate dimensions and qualified cavity thickness.
Mold turning process should first grasp the precision of the transition wood mold. In order to ensure precision, in the beginning of the filter wood mold design can be used according to the mold shrinkage rate to leave a certain amount of shrinkage allowance. In addition, attention should be paid to the transition of the surface of the wood mold repair flat, wood mold surface scarring must be dug out. Scars and wood shrinkage is not consistent will cause the surface of the fiberglass mold is not flat. Dig out the scars and remove the surface burrs, the surface of the wood mold must be scraped putty treatment, generally required to scrape 2~3 times. After the putty is cured, use sandpaper to polish the surface until it fully meets the size and shape accuracy requirements.
Wooden mold production must be willing to spend effort, because in general, the dimensional accuracy of FRP mold ultimately depends on the accuracy of the wooden mold. In order to ensure that the surface of the glass fiber reinforced plastic mold is smooth and clean, turn the first piece of glass fiber reinforced plastic mold, gel coat layer using the spray method is more appropriate.
Spraying gelcoat should pay attention to adjusting the air flow of the gun, so that the gelcoat resin atomization is uniform, does not show particles. Spray gun and gun should be outside the mold, so as not to cause local gel coat hanging, affecting the surface quality. After the gel coat layer is cured, paste the surface felt. Surface felt should be outside the mold, so as not to cause local gelcoat hanging, affecting the surface quality.
After the gel coat layer is cured, paste the surface felt, the surface felt should be covered with flat, folded or lap must be cut and trimmed. Stick a good surface felt, brush can be dipped in a small amount of resin to soak through the surface felt, pay attention to the control of the amount of glue, both to be able to fully infiltrate the fiber, but not too much. High glue content, the bubble is not easy to exclude, and cause curing exothermic large, large shrinkage. Surface felt layer resin curing to pick bubbles, pick bubbles can not cut through the gel coat layer.
After picking bubbles, appropriate sanding, remove fiberglass burrs and remove floating dust, hand-paste 300g / m² non-alkali short-cut felt, each time only 1 ~ 2 layers of paste, to be cured after the exothermic peak before you can continue to paste. Paste to the required thickness, you can lay the copper pipe, and laying insulation core block. Modulation of glass beads resin putty, as the laying of thermal insulation core block adhesive, with which to fill the gap between the thermal insulation core block.
After laying, the glass bead putty should be used to smooth the gap on the surface of the insulation core block. Insulation core block layer curing and then paste 3 ~ 4 layers of short-cut felt, you can paste mold steel skeleton. Paste steel skeleton, steel skeleton first annealed to eliminate welding stress, and steel skeleton and the gap between the mold should be filled to prevent the FRP mold deformation with the steel skeleton.
After the first piece of mold is cured, the mold is removed, the excess flying edge is removed, the mold cavity is cleaned of debris, and the wax sheet is applied. The thickness of the wax sheet used should be uniform and the elongation should be small. The wax sheet should not be wrapped in air bubbles, once there are air bubbles, it should be removed and re-pasted to ensure the size of the mold cavity. Lap joints should be cut, and gaps between wax sheets should be leveled with putty or rubber cement. After the wax sheet is applied, a second mold can be turned in the same manner as the first mold. The second mold is usually made after the gelcoat has been sprayed, and injection holes and venting holes have to be arranged. Turn over the second piece of mold, you must first remove the flying edge, weld the positioning pins and locking bolts, to be fully cured after demolding.

3, Mold inspection and remedial measures
After demolding and cleaning, use rubber cement to measure the thickness of the mold cavity. If the thickness and size can meet the requirements, then after grinding and polishing process is completed, the RTM mold will be turned successfully and can be delivered to production. If the test, due to poor process control and other reasons caused by the mold cavity does not meet the requirements, the scrap, re-open the mold is too much of a pity.
According to experience there can be two remedies:
① scrapped one of the mold, re-open a piece;
② use of the RTM process itself to repair the characteristics of the mold, usually a piece of the mold surface gelcoat layer chiseled off, laying on the glass fiber reinforced material, the other piece of the mold is affixed to the wax sheet, spray gelcoat, and then the mold injection, to be cured after the mold processing, can be delivered to the use.