RTM Mold Graphic Production Tutorial

 

Dear composite material friends, have you ever thought about the detailed production process of RTM molds? If you are asked to be a technical leader and direct the workers to make RTM molds step by step, can you do it? After reading this article, you can almost do it with your eyes closed. Please hold back your surprise, this article is destined to be another article full of sparks and sparks.

 

Trains are not pushed, and cowhide is not blown, let’s get back to the point. Now, we are going to make RTM molds for the products shown below.

This is a container lid with flange. The product has uneven thickness, a stepped shape on the front, and text on the reverse side (back). When this product is in working condition, the front side is visible, with high requirements for gloss and flatness, and the reverse side is invisible. We make the front side with the lower mold and the reverse side with the upper mold.

 

To make the upper and lower molds of RTM, we must first make the mold base. The mold base is made of substitute wood (epoxy substitute wood, polyester substitute wood) through CNC machine tools, which has much higher dimensional accuracy and surface flatness than the pure handmade mold base (generally made of wood). The premise of CNC processing substitute wood mold base is to have a three-dimensional model of the mold base. Before giving the model, we first understand a few concepts:

 

1. The front of the product comes from the lower mold of the RTM mold.
2. The lower mold of the RTM mold comes from the wood mold base, named the lower mold base.
3. The upper mold of the RTM mold comes from the wood mold base, named the upper mold base.
4. After the upper and lower molds of the RTM mold are molded, the gap in the middle contains the full size of the product.

 

5. The upper mold of the RTM mold can sometimes be molded on the surface of the RTM lower mold by pasting wax sheets to make the product thickness, and then spraying gel coat on the surface of the wax sheet to make the RTM upper mold. The premise of doing this is that the product thickness is relatively uniform and the surface modeling is simple. Otherwise, this method is not recommended because it is difficult to make precise shapes with wax sheets by hand.

 

6. Strictly speaking, the upper and lower molds of RTM mentioned in this article are both fiberglass molds, which are relatively lightweight compared to metal molds. RTM mechanical injection molding is not used, and no press is used for mold clamping and layering. Therefore, it can be understood as a mold type for lightweight RTM process. Of course, if the upper mold uses a silicone soft mold, it will be lighter.

 

7. The 3D models of the upper and lower mold bases are designed by extracting the surface of the product 3D model (designed in 3D software, such as UG, Catia, ProE, etc.). Seeing this, half of the friends will ask: “What if I can’t draw a 3D model?” It doesn’t matter. Almost every company has a professional model drawing engineer. You just need to guide him to draw. If your company happens not to have one, you can also entrust the supplier of the wooden mold base to help you draw.

 

8. The product model used to design the upper mold base and the lower mold base should be enlarged appropriately before being used for design. The purpose is to reduce the dimensional error caused by the shrinkage of FRP when remaking FRP RTM molds and producing FRP products. Generally, the shrinkage rate of FRP molds is less than 0.03%, while the shrinkage of FRP products is about 0.05%. This requires that the product model be enlarged by “1.0005 to 1.001” times before being used to design the mold base. This is a range, and the specific value depends on the shrinkage of the resin and layer structure commonly used when you make FRP molds and products. Alternatively, keep the product model size unchanged, enlarge it by 1.0003 times after designing the mold base, and then enlarge it by 1.0005 times again on this basis, and then use the enlarged 3D model of the mold base for CNC machining.

 

9. The upper mold base and the lower mold base of the substitute wood mold base cannot be molded together. This one is difficult to understand, but it doesn’t matter, it does not affect the mold remaking. Only when you repeatedly demonstrate it on the 3D software or have a very strong 3D imagination, you can understand why the mold cannot be closed. Simply put, the thickness of the upper and lower mold bases intersect, so they cannot be molded together (the picture below shows the situation of thickness intersection). The reason for giving this point is that many friends often try to mold the upper and lower mold bases together to check the size when inspecting the wood mold bases entering the factory. This is incorrect because they cannot be molded together at all.

The picture below is the lower mold base:

The working surface of the lower mold base is to extract the front of the product. After a simple chamfer is made on the edge, it extends 150-200mm wide in all directions and adds a positioning pin (convex). When the RTM lower mold is turned, this chamfer will form a mirror chamfer on the RTM lower mold to assist in glue flow. The following picture is a close-up of the chamfer on the lower mold base:

 

 

Let’s take a look at the upper mold base:

 

 

The working surface of the upper mold base is to extract the reverse side (back side) of the product. After a simple chamfer is made on the edge, it is extended to 150-200mm wide (the same width as the extended width of the edge of the lower mold base), and a positioning pin (concave) is added. The inner surface of this concave positioning pin should be about 0.2mm smaller in inner diameter and depth than the convex positioning pin on the lower mold base. In other words, the positioning pin on the RTM upper mold produced by the concave positioning pin on the upper mold base is convex, and this convex positioning pin is smaller than the concave positioning pin on the RTM lower mold produced by the convex positioning pin on the lower mold base, which makes it easier for the RTM upper mold to be molded with the RTM lower mold. The above paragraph is a bit like a tongue twister, and you will understand it slowly.

 

There is a vacuum reserved boss on the RTM upper mold base. The function of this boss is to form a pit on the RTM upper mold for installing the vacuum joint for mold closing. This joint will be introduced in detail later. The upper mold base also has a chamfer, which has the same function as the chamfer on the lower mold base. It will form a mirror chamfer on the RTM upper mold to assist in glue flow. The following picture is a close-up of the chamfer of the upper mold base:

 

Next, let’s look at the reasons why the upper and lower mold bases cannot be molded together as mentioned above:

 

We close the upper and lower mold bases on the 3D model with positioning pins, cut the upper mold base, and display the cross section in red. The lower mold base is transparent. It can be seen that the red surface has penetrated into the lower mold base, resulting in a thickness intersection. If you can’t see it clearly, please see the partial enlarged picture below:

 

 

The thickness cross indicates that the upper and lower mold bases cannot be molded together. The upper and lower mold bases are transition molds used to remake the upper and lower RTM molds, also called transition molds. The RTM upper and lower molds can really be molded together. If you still don’t understand why the upper and lower mold bases cannot be molded together after reading this, then skip it. It will not affect the continued mold remaking. One day you will figure it out.

 

Let’s first look at the lower mold base remaking the RTM lower mold. Spray low-viscosity easy-to-grind gel coat on the surface of the lower mold base, polish the surface after curing, and install the retaining edge after applying the release agent:

 

 

Install the flange around the lower mold base and fix it with screws. The flange is made of a smooth plate, such as FRP plate, aluminum plate, aluminum-plastic plate, etc., with a thickness of 3-10mm. The smooth side of the flange faces inward, and the gap between the flange and the lower mold base is filled with plasticine. The function of the flange is to be used as the flange of the RTM lower mold. Before installing the flange, apply mold release wax on the smooth surface and wipe it with a clean cotton cloth. The next step is the process of conventional FRP mold turning.

 

The following is a reference process: spray mold gel coat (wet film thickness 0.6-1.2mm); paste surface layer (1 layer of surface mat); after curing, remove bubbles, paste transition layer (2 layers of EMC300 chopped mat) + structural reinforcement layer (2 layers of EWR400 + 2 layers of EMC450 alternating); buffer layer (1 layer of EMC300 + 1 layer of 3mm strong core mat + 2 layers of EMC450); anti-deformation bracket structure (plywood plug-in + 1 layer of EMC450 outer coating). Each small section of the above layup separated by “;” is a one-time molding and curing layup.

 

The resins used in each layer are as follows: It is recommended to use vinyl mold resin from the surface layer to the end. Of course, you can also refer to the following layup: vinyl mold resin for the surface layer, zero shrinkage mold resin for the transition layer and structural reinforcement layer, and unsaturated mold resin for the outer covering layer of the buffer layer and anti-deformation support structure. The detailed details of the hand layup operation are limited by space and will not be described here. It should be noted that the structural design of the RTM lower mold takes into account the anti-deformation ability (rigidity) and durability. The following is a schematic diagram of the RTM lower mold after the paste is completed.

 

The RTM lower mold after the paste is completed and has not yet been demolded:

 

 

If the mold is relatively large, you can add mold legs to the support structure and install wheels, which will make it much easier to move the mold. If the mold is relatively small, it can be placed on a platform with wheels after demoulding. In short, the purpose is to facilitate the movement of the mold and avoid manual handling. Of course, if you are a strongman, you like to move the mold, just as a physical exercise, and you can also show your muscles, then I really have no tricks.

 

After the paste is completed, the burrs are cut off after demoulding, the surface is polished, and the release agent is applied for standby use. For details on the specific method of applying the release agent and the operation method of hand lay-up, please refer to another article in this public account “Fiberglass Hand Lay-up Technology” (search on this blog). The following picture shows the finished RTM lower mold:

 

 

Seeing this, you may think that there is nothing profound, which is normal, because the essence of RTM mold lies in its upper mold. Now let’s start making RTM upper mold from the upper mold base.

 

Spray low-viscosity easy-to-grind gel coat on the surface of the upper mold base, grind and polish after curing, and install the retaining edge after applying the release agent. The following figure is a schematic diagram of installing the retaining edge of the upper mold base, the same method as the lower mold base:

 

 

Install the flange around the upper mold base and fix it with screws. The flange is made of a smooth plate, such as FRP plate, aluminum plate, aluminum-plastic plate, etc., with a thickness of 3-10mm. The smooth side of the flange faces inward, and the gap between the flange and the upper mold base is filled with plasticine. The function of the flange is to be used as the flange of the RTM upper mold. Before installing the flange, apply mold release wax on the smooth surface and wipe it with a clean cotton cloth.

 

The above paragraph is the same as the method of installing the flange on the lower mold base, so it looks very familiar. The difference is the next step, the essence is about to shine, please see the following picture:

 

 

In the above figure, explanations from A to E:

1. A is an outer square silicone strip, which is used to form a groove on the outer circle of the RTM upper mold. This groove is used to bond the porous rubber sealing ring, which plays a sealing role when the RTM upper and lower molds are closed.
2. B is a wax sheet (1-2mm thick), which is used to form a gap between the upper and lower molds of the RTM. The width of this gap is equal to the width of the wax sheet. This gap is used to form a vacuum adsorption effect when the mold is closed, so that the upper and lower molds are tightly closed.
3. C is an inner trapezoidal silicone strip, which is used to form a groove on the inner circle of the RTM upper mold. This groove is used to bond the hollow silicone sealing ring, which plays a sealing role when the RTM upper and lower molds are closed.
4. The distance between the wax sheet B between A and C is about 10mm from A and C. This 10mm gap is used to form a positioning circle when closing the mold to avoid excessive fit when the upper and lower molds are closed by vacuuming.
5. D is a semicircular silicone strip, which is used to form a glue flow groove (a circle path for resin flow).
6. E is a wax sheet (about 1mm thick and 10mm wide), which is used to form a circle of gap between the upper and lower RTM molds. This circle of gap is used to guide the resin in the flow groove to the product layer in the mold cavity. This circle of wax sheet is close to the chamfered edge of the upper mold base.

 

The following figure is an enlarged view from A to E for easy observation:

 

 

The total width from A to E is generally 150-200mm, and the specific dimensions of each ring material have specific requirements. For example, the length and width of the cross-section of A are related to the size of the groove of the RTM upper mold it forms. This groove is used to bond the porous (microporous) rubber seal ring. The seal ring is elastic and can be compressed into the groove when the mold is closed and pressed, and it can also play a sealing role. From A to E, including the porous rubber seal ring and hollow silicone seal ring that will appear below, they can be purchased from professional composite material related companies.

 

We explained from A to E, but careful friends have already discovered that there are some details in the corner. Let’s zoom in and see:

 

 

In the above figure, F is a wax sheet, about 5mm thick. There is a gap of about 10mm between the wax sheet and the positioning pin hole. G is plasticine, which is used to fill the narrow gap between the outer ring silicone seal ring of A and the retaining edge. One end of G is made into a rounded corner or a straight edge as shown in the figure, so that the RTM upper mold that is turned out will form a rounded corner or a straight edge at this place.

 

When laying the wax sheet, heating the wax sheet with a hot air gun can make it soft, easier to fit the mold base surface and do a good job of splicing and alignment. It should be noted here that A, C, and D in A to G should be coated with demoulding wax on the surface before use, and wiped clean with a clean cotton cloth, which is easier to demould later. The silicone strips of each circle of A, C, and D are bonded with universal glue at the splicing joint.

 

After the work from A to G is completed, place the glue suction port embedded part in the middle of the upper mold base:

 

 

The embedded parts in the above picture are used for installing the glue suction port joint later. A small amount of double-sided adhesive is used to bond the embedded parts to the wooden mold base, and the threaded hole in the middle of the embedded parts is sealed with plasticine or tape. The following picture is an enlarged view of the embedded parts:

 

 

The outer surface of the embedded part has anti-loosening patterns and can be embossed to prevent loosening. The installation method of the embedded part and the glue suction port joint will be introduced below.

Now start the layering work of the RTM upper mold:

 

Spray mold gel coat (0.6-1.2mm); paste the surface layer (1 layer of surface mat); after curing, remove bubbles and paste the transition layer (1 layer of EMC300 chopped mat) + structural reinforcement layer (2 layers of EMC450); after curing, use adhesive resin to level the pits and uneven places; continue to lay the layers: sandwich layer (1 layer of EMC300 + 1 layer of 5mm strong core mat + 1 layer of EMC300); structural reinforcement layer (3 layers of EMC450); outer surface layer (1 layer of EMC300). Each small section of the above layers separated by “;” is a one-time molding and curing layer.

 

The resins used in each layer are as follows:

It is recommended to use vinyl mold resin from the surface layer to the end. Of course, you can also refer to the following layup: vinyl mold resin for the surface layer, zero shrinkage mold resin for the transition layer and structural reinforcement layer, vinyl mold resin for the sandwich layer, and unsaturated mold resin for the outer surface layer. The detailed hand layup operation details are limited by space and will not be described here.

 

It should be noted that the structural design of the RTM upper mold takes into account the guarantee of deformation resistance (rigidity) and durability, and the foam sandwich can be increased to increase the thickness of the upper mold. Some people tend to make the upper mold more flexible so that the upper mold can compress the layup more tightly when vacuuming. I prefer the upper mold to be more rigid, because if the flexibility is too large, it is easy to form a pit in the middle when vacuuming. When the upper mold is too rigid, the size of the mold cavity can be guaranteed, because the upper mold is not easy to dent or bulge. The following is a schematic diagram of the RTM upper mold after the paste is completed.

 

The RTM upper mold after the paste is completed and has not yet been demolded:

 

 

In the above figure, the middle embedded part position is covered by the layer. After demoulding, measure the embedded part position, drill gently to expose the top surface of the embedded part threaded hole, remove the plasticine or tape in the embedded part threaded hole, and expose the threaded hole, as shown in the following figure

 

 

You will also need to drill holes in other locations:

 

 

A is a through hole for installing the vacuum joint for mold closing. The pit at A is formed by the boss on the upper mold base. The picture of the upper mold base has been given at the beginning of this article, and the role of the reserved boss for the vacuum port for vacuum mold closing has been explained. The diameter of the through hole at A is matched with the vacuum joint, which will be introduced below.

 

B is a through hole, the hole is located in the middle of the glue flow groove, and the diameter is used to match the tube (transparent or translucent) used for glue suction (or injection resin). C is a through hole, located in the inner ring sealing ring installation groove. The diameter of the through hole at C is very small, about 5mm, and is used to install a plastic tube. This plastic tube is used to release the gas in the hollow silicone sealing ring. This place will be introduced again below.

 

Let’s take a look at the reverse side. The positions of A, B, and C in the figure below correspond to those of A, B, and C in the figure above:

 

 

In the above picture, it is necessary to drill a hole at B, and the direction is from the back (gray) of the mold to the front (red). Do not drill through, and leave 2-3mm at the bottom. The hole size is used to install the embedded part of the injection port joint. See the enlarged picture after the embedded part is installed at B:

 

 

The right side of the above picture shows the embedded parts. The embedded parts and the RTM upper mold are bonded with structural adhesive, and a reinforcing gasket is added on the top. After polishing the mold surface, it is time to install the sealing ring:

 

 

In the above picture, the outer ring sealing strip and the inner ring sealing strip are installed in the corresponding grooves and bonded with universal glue. The grooves are formed by the silicone strip on the upper mold base. You can review it by looking at the description and pictures of the upper mold base above. The outer ring sealing strip is made of porous (microporous) soft rubber material, and the inner ring sealing strip is made of hollow silicone material. The following is a schematic diagram of their cross-section:

 

 

As mentioned above, the through hole at C on the back of the RTM upper mold is used to install a plastic tube and release the gas in the hollow silicone seal ring. Because the hollow seal ring is compressed when the mold is closed, the gas must be released. When bonding the seal ring, poke a hole on the bottom side of the hollow seal ring (the side that fits the mold surface) at the position corresponding to the through hole at C to install the plastic tube. The plastic tube is bonded with universal glue, and the gap between the plastic tube and the mold is sealed with sealant. As shown in the figure below:

 

 

The right side of the above picture shows the plastic tube cut open for easy observation. My friends, I have painstakingly drawn many pictures to make it clear for you. If you don’t pay attention, it would be a shame for you to have spent so much time looking at it. Now let’s install the vacuum joint, as shown in the following picture:

 

 

The picture above is not very clear, let’s zoom in and take a look:

 

 

The left side of the above picture shows the RTM upper mold in a transparent manner. You can see that the glue suction (glue injection) joint is screwed into the embedded part through the thread, and the blue part of the joint and the RTM upper mold are sealed with sealant. This joint is more durable than ordinary pneumatic joints. It is a stainless steel part. The glue suction (glue injection) tube passes through the joint, so that the resin does not contact the joint, making the joint more durable. Let’s take a closer look at this joint, as shown in the following figure:

 

 

The connector shown in the picture above is used in the electrical industry and can be used in RTM molds. The price is not expensive, and it is estimated that each one is only about 10 yuan. It is also very convenient to use. Loosen the top cover of the connector, pass the glue suction (glue injection) tube through the entire connector to the appropriate position, and then tighten the top cover of the connector. The top cover will press the flower plastic parts to squeeze the rubber locking tube, and the rubber locking tube will lock the glue injection (glue suction) tube.

 

Let’s take a look at the vacuum mold joint:

 

 

The vacuum mold joint is a copper plumbing pipe joint that can be purchased at the hardware market. When the upper and lower molds of RTM are closed, this vacuum joint will connect to another quick joint on the vacuum pipeline, which is convenient to connect and saves time and effort. Its inner diameter is about 20mm. When the mold is closed and vacuumed, the air flow is large and the mold is closed quickly and powerfully. The vacuum mold joint and the mold are bonded with structural adhesive and sealed with sealant. Let’s take a closer look at this joint:

 

 

At this point, the basic work is done, and the mold surface is coated with a release agent for standby use. After the product layer is prepared on the lower mold of the RTM mold, the RTM upper mold is molded onto the lower mold, and then it is time to connect the vacuum pipeline and use the RTM process to produce the product. It should be noted that the product layer generally uses RTM closed mold mat. To increase the strength, biaxial cloth or multi-axial cloth and continuous mat are often used for interleaving. The product layer here is not our focus. Let’s take a look at the next step, how to connect the vacuum pipeline:

 

 

Is the picture above dazzling? It doesn’t matter, I will enlarge the details for you to see. The following explains the working principle of this connection pipeline.

 

Compressed air is converted into vacuum pressure through a vacuum converter, and the vacuum pressure gauge adjusts the pressure to about -0.04MPa. The vacuum pressure in the resin collection is consistent with the vacuum pressure gauge. Of course, a vacuum pressure gauge can also be added to the resin collector to indicate the real-time vacuum pressure reading. The vacuum pipeline is connected to the vacuum mold joint on the RTM upper mold through a quick connector. The details of how to connect here will be enlarged in the following text. The suction hose can be connected to the RTM injection machine, or a stainless steel tube about 20cm long can be put on the end and directly placed in the resin barrel to naturally absorb the resin by the vacuum suction in the mold cavity. The function of this stainless steel tube is to give the suction hose a gravity so that it can hang firmly in the resin barrel, eliminating the need to hold it by hand or fix it with a tooling clamp. It can also be used to stir the resin, killing two birds with one stone. Let’s show the details of each place below:

 

 

The above picture is an enlarged view of the resin collector. The transparent cover is a transparent PC board about 20mm thick, and the main body of the resin collector is a stainless steel bowl, which can be bought in supermarkets. Wax the bowl before use to facilitate the cleaning of the resin later. There is a circle of rubber sealing ring between the transparent cover and the stainless steel bowl. The material and cross-sectional shape of this sealing ring are the same as the outer rubber sealing ring of the RTM upper mold. The glue suction (glue injection) joint is screwed into the pre-punched hole (through hole) on the PC cover through the thread (apply adhesive on the thread first) and sealed with sealant. Such a resin collector is relatively low in cost, very convenient to move, and easy to clean the resin. Let’s take a look at the quick connector on the mold vacuum pipeline:

 

 

On the left side of the above picture, the quick connector on the vacuum pipeline and the vacuum mold joint on the RTM upper mold are connected by a snap, and on the right side, this quick connector is shown separately. Like the vacuum mold joint, the quick connector is also a copper plumbing pipe fitting, a ball valve, which can be bought in the hardware market. Such a snap-on rotary connection is very fast and convenient. When the mold is vacuumed, the air flow is large and the mold is closed quickly and powerfully. Let’s cut open this quick connector and see what it looks like inside:

 

After cutting it open, you can see that the internal structure is a ball valve structure, and the specific model and vacuum mold joint can be matched with each other. Let’s take a look at more interesting combinations:

 

 

The above picture shows a simple combination that replaces the expensive “vacuum conversion box” used in the RTM process. The vacuum conversion box costs thousands of yuan and is difficult to repair if it breaks down. The combination in the above picture is simple, practical, and cheap, costing about 400 yuan. Ten such combinations only cost 4,000 yuan, which is very cost-effective.

 

Its working principle has been explained above, and I will repeat it here: compressed air is converted into vacuum pressure through a vacuum converter, and the vacuum pressure gauge adjusts the vacuum pressure to about -0.04MPa. The reason for adjusting to -0.04MPa is that the RTM process requires the vacuum pressure in the mold cavity to be -0.04 to -0.06MPa. When the resin viscosity is high (such as flame retardant resin), the vacuum pressure approaches -0.06MPa, and when the resin viscosity is not high, the vacuum pressure approaches -0.04MPa. The minimum value of the vacuum pressure is -0.1MPa (the reason why it is called the minimum is because it is a negative value). If -0.1MPa is used for RTM process to produce products, the resin will flow too fast due to excessive vacuum suction, resulting in dry areas on the product layer (the resin does not penetrate the layer). Let’s zoom in and take a look at this combination:

 

 

Vacuum pressure gauges and vacuum converters are commonly used pneumatic components, which can be sold in hardware and pneumatic tool stores or e-commerce platforms such as Alibaba and Taobao. It makes operation more convenient, reduces costs, and makes work more efficient. Isn’t this a very interesting combination? I hope that my friends can invent and create some interesting things at work like me and share them with everyone.

At the end of the article, let’s take a look at the space in the mold cavity after the RTM upper and lower molds are closed:

 

 

The above picture shows the full longitudinal section after mold closing. It is not clear, so let’s zoom in on the left side of the injection port to see: