Resin Transfer Molding (RTM) ------
RTM is a technology of injecting liquid resin into a closed mold. There are already stacks of dry fiber layers or preform placed in the closed mold cavity before the injection. During the injection process, resin flows and impregnating and wetting the entire surface of the perform material, when the curing cycle completes, resin become rigid plastic. After demolding, the transfer process ends, composite part is being molded.
So the first step of this process would be getting ready the preform or pre-cut fiberglass layer. The reinforcement comprises a variety of fiber types and forms, such as fiber tows, mats, or woven structures. In this process we need our first set of mold, which is quite simple, because it is function only to create the shape of the preform or reinforcement so we could lay them inside the second set of mold cavity.
The process of Preform
The preform play an important role in RTM whole process. The better preform you get, the better the structural part you have. The preforms are often produced in advance by putting the textile material through a dry preforming process, which require a simple set of tool. This process eliminates the influence of viscous resin displacement. In additional, fiber direction can be optimized and different textile material can be combined. There are contact heating, IR heating, compression molding for this preforming process, here we only talk about the compressing way like above image.
Once the preforms are ready, next step would be insert them into the RTM mold and close and clamp. The resin is pumped under pressure through the injection channels into the mold, and part is cured in the mold. Vacuum assist can be used to enhance resin flow in the mold cavity. This process is referred to as vacuum assisted resin transfer molding (VA-RTM). The liquid resin either is polyester, vinyl ester or epoxy premixed with curing agent and other ingredients. One of the advantage of this process is there are metal inserts like bolts and nuts can be molded into the body during the molding process.
Resin transfer process
Automotive industry has been a prominent application area for RTM molding in general for body panels, hood, fenders, and deck-lid, battery covers, A pillars, front-crash structure。
A complete RTM process require quite amount of equipment. It includes injection unit, heating power unit, hydraulic press, possible automatic tool loading and unloading system. Tooling is what Toolex is specialized.
RTM toolings can range from soft low cost mold to temperature controlled hard metal tooling, like aluminum or steel. It is mainly depend on the expectation duration of the mold run. Soft tooling would be either polyester or epoxy mold. Harding tooling would be cast machined aluminum or steel. Heated molds are required to achieve fast cycle times and product consistency, so when large numbers of parts are needed, steel molds is the better choice, because soft tools can be produced a maximum of 3000parts.
The uniformity of the temperature in the RTM mold not only determines and affects the flow and filling performance of the resin in the mold cavity, but also has a significant impact on the fiber infiltration performance, as well as the overall performance of composite materials and the internal stress of the product. Therefore, it is necessary to use medium heating combined with efficient and reasonable circulating oil circuit design.
The sealing performance of the molding mold directly determines the resin flow and filling characteristics, as well as the emptying ability of the molding process, which is a key factor affecting the performance of the product. It is necessary to design the layout position, method, and quantity of sealing rings based on the product, and solve the sealing problems of mold fit gaps, ejection systems, vacuum systems, and other positions to ensure that there is no air leakage during the resin filling process and ensure the performance of the product.
