ME-542 Manufacturing with Non-metallic Materials

Fiber Reinforced Mold Processes

Vacuum Injection Molding

Vacuum injection molding is a process designed to produce large, complex parts in limited quantities.

In this technique a dry, fiber reinforced preform is placed over a male mold. A matching female mold is sealed and locked in place over it and a vacuum is drawn on the mold cavity. Resin is catalized and drawn through the preform. Excess resin is drawn through the preform to ensure complete saturation of the molded part. The cure time of the part is usually several hours, this is not a rapid process

Here are some of the benfits of vacuum injection molding:

Complex, high precision parts can be molded

Generally inexpensive mold cavities

Low pressures used, generally under 2 atmospheres

Molded parts can have excellent finish characteristics

Tooling costs are low

The primary disadvantage to using vacuum injection molding is the slow, labor intensive nature of the process. Parts produced with this technique should limited to a few hundred. Beyond that, labor costs, and the need to replace the molds may make the proccess economically unattractive.

 

Reinforced Reaction Injection Molding

In the reinforced reaction injection molding process, a fiber-based preform made of inserts or previously molded structures, is placed on top of a female mold. The preform serves as a core for the molding process. After the mold is closed a nd heated, a highly reactive two-part monomer is injected at 50-100 psig. The pr eform is then flooded with a thermosetting polymer. The part is removed when the curing cycle is completed.

Benefits of reinforced reaction injection molding:

Low temperatures and processing pressures

Production of relatively large parts with low clamping forces

Fast cycle times (as fast as 50-70 sec)

Inexpensive tooling

The most significant disadvantage of the reinforced reaction injection molding p rocess is that only a small number of reactive resin systems are available for u se. The process also tends to be labor intensive.

 

Resin Transfer Molding

Resin Tranfer Molding (RTM) is a process that has been looked at by some automotive companies as the long range solution which will enable composites to replace the structural steel components of tomorrow's cars and trucks.

In the RTM process, a preform made of chopped or continous fibers, often lightly bonded with thermoset or thermoplastic resin is placed into a heated male or female mold. Then a matching male or female mold is lowered and the cavity is sealed. In parts with high fiber volume fractions, vacuum may be used to assist in drawing resin through the fiber preform. The mold is then further compressed together, compressing the preform to 100-500 psi. This causes the resin to flow and fully "wet out" the preform. The mold is then allowed to cure at elevated temperature then the part is removed and allowed to cool.

 

Thermal Expansion Resin Transfer Molding

Thermal expansion resin transfer molding is used to form fiber reinforced/resin matrix skins around cored structures. The process consists of five steps:

1. A rigid foam core is formed to the desired shape to support the part to be ma de (usually injection or blow molding techniques).

2. The foam core is wrapped with dry fabric reinforcement.

3. The wrapped core is placed into a cold mold and a controlled amount of low-vi scosity thermosetting resin is pumped into the mold. The resin is not allowed to completely fill the fabric cavities.

4. The mold set is rapidly heated to the resin processing temperature. As it is heated, the foam core expands, displacing the remaining resin into the dry areas of the mold cavity.

5. After cure (10-60 min) the mold is removed.

An advantage of thermal expansion resin transfer molding is that parts may have complex configurations. The major cost driver is the labor required to wrap the preform core.

 

Transfer Molding

Transfer molding is a process in which a thermoset base is compressed into matching metal dies. This process is very similar to compression molding and may have small fibers added to the charge. The matching mold consists of two parts. The first is a transfer cavity, which is heated to approximately 300 to 350 degrees F. The second part of the die forms the part's shape, while approximately 3 tons per square inch are applied by the ram. After the ram pushes the bulk molding compound into the cavity the cycle is completed. This process can be executed in approximately 45 to 90 seconds, depending on the part's complexity. Trends in transfer molding:

Production parts are small, complex and have a fiber content normally ranging from 10 to 35 weight percent.

Parts with a cross sectional area larger than two square feet relative to the ram are not economical to be manufactured in this form.

All transfer molding dies are surface polished and are casted from harden steels.

Production is done at high pressures and the mold is heated making the molds relatively costly.

Transfer molding is normally used for parts which can have a yield larger than 20,000 per year.

Labor and material cost for this process are fairly low.

Because the process is cycled at constant temperature energy costs are also low.