Transfer presses

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Attention, this article mixes several manufacturing technologies and is not clearly differentiated!

The resin transfer molding ^BE or Resin Transfer Molding ^AE (RTM) is a process for the production of moldings from thermosetting plastics and elastomers . In comparison to pressing, the molding compound is injected into the mold cavity by means of a piston from a mostly heated pre-chamber via distribution channels, in which it hardens under heat and pressure .

Formaldehyde resins ( PF , MF , etc.) and reactive resins ( UP , EP ) with small filler particles and elastomers can be used as molding compounds .

Classification

Depending on the arrangement of the injection pistons, there are three basic methods:

• the lower piston process (two-piston process), in which the injection cylinder sits below the mold
• the upper piston process (single piston process), in which the injection cylinder sits above the mold
• horizontally arranged automatic spray guns, which mostly have a screw preplasticization.

Differential Pressure Resin Transfer Molding (DP-RTM) is a process further developed by the German Aerospace Center (DLR).

Standard flow

At the beginning of a cycle there is a pre-plasticized and metered molding compound in the antechamber. First the mold is closed. The molding compound is then injected into the mold and left in the mold for a certain time. During this so-called dwell time, the molding compound reacts or vulcanizes. It depends on various factors (resin type, filler, processing pressure and temperature). When the dwell time has ended, the tool can be opened. The previously filled molding compound is now solid (hardened) and is now referred to as a molded part. This can now be removed from the mold. The tool is then cleaned and a new cycle can begin. The amount of molding compound required for pressing and re-pressing should always be greater than the volume of the final molded part so that the tool is completely filled. This guarantees that the molded part is completely formed and that no air is pressed in. The excess molding compound left in the antechamber, also known as residual cake, must be removed before the start of the new cycle and replaced with new molding compound.

Processing of preforms

In order to process long fibers or pre-fixed semi-finished fiber products (preforms) , these are first placed in the tool and infiltrated with the resin. In order to avoid air inclusions, the cavity (mold cavity) is usually also evacuated .

Types of sprue for preform processing

The injection strategies can be classified according to the number and design of the resin sprues. In the following, the introduction of the resin into the semi-finished fiber product is referred to as injection , regardless of how the pressure gradient is generated.

• Point injection : The resin is only injected into the semi-finished product at one point. With a point gating, the flow front can trap air, which leads to imperfections.
• Multipoint injection : multiple injection sites allow the mold to be filled with resin faster. The inclusion of air can be prevented by clever positioning.
• Line injection : In the case of line injection, the injection is not carried out at a punctiform point, but as a line at the edge of the mold. This can be an advantage for components with a large aspect ratio, since only the shorter edge length has to be traversed.
• Flow channel injection : The resin is injected through a wide channel that lies above or below the semi-finished fiber product.
• Cascade injection : To keep the pressure gradient low, several injection points are placed in the direction of the flow front. To do this, however, it is necessary to open and close the injection lines following the flow front.

Shape types

• Solid forms
• Soft shapes
• Mixed forms (e.g. blow molding RTM)
• Dual forms

Reactive resins for preform processing

Resins which have a low viscosity are used as the injection resins . As a result, the flow resistance when flowing through the mold remains low and smaller pressure differences are necessary for filling. Reactive resins for RTM processes are offered as special injection resins which consist of a resin and hardener component. Low reactive resin systems can be mixed before the infusion. If you want to use highly reactive resin systems, the resin and hardener can only be mixed directly in the infusion line or the mold. In this way, shorter cycle times are possible. Processes in which the injection resin components are only mixed immediately before injection are known as RIM processes ( Reaction Injection Molding ).

Conclusion

This process is well suited for small to medium series (1000–10,000 pieces / year) and can achieve high fiber volume contents of up to 65%. Due to the closed process, there are only minimal emissions at the workplace compared to open processes and reproducible quality is ensured. In addition, the surface quality is good on both sides. However, there is only limited freedom of design and high cycle times of around 20 minutes.

Individual evidence

1. ↑ Christian Bonten: Plastics Technology Introduction and Basics , Hanser Verlag, 2014.