Comparison of various molding processes
Hand laying and jet forming allow for fast product development cycles because the tool manufacturing process is simple and relatively low cost.
Hand paste molding
The hand-lay molding process, also known as contact molding, is the earliest molding method used and widely used in the production of resin matrix composites. The hand-laying process is mainly manual operation, less use of mechanical equipment, it is an unsaturated polyester resin or epoxy resin as a matrix material to bind the reinforcing material together a molding method.
Although the hand-laying operation is simple, it requires high operator skills. It requires the operator to have a serious working attitude, skilled operation skills and rich practical experience. The product structure, material properties, mold surface treatment, gel coat quality, glue content control, cutting and laying of reinforcing materials, uniformity of product thickness and various factors affecting product quality must have a more comprehensive understanding, especially the judgment and treatment of common problems in practice, not only need to have rich practical experience, but also have certain chemical knowledge and have a certain ability to read maps.
Jet molding
Similar to hand-lay molding, jet forming offers greater shape complexity and faster production. Jet molding uses low-cost open molds, room-temperature cured resins, making it ideal for producing low to medium volumes of large parts. Short-cut fiber reinforcement materials and catalytic resins are deposited in the mold from the shredder/spray gun.
As with the layup, manual rolling removes entrained air and wets the fiber-reinforcing material. Non-woven roving is usually added to specific areas to obtain thickness or greater strength. Coloring gelcoats can be used to produce smooth, colored surfaces.
merit:
Wide range of part size potential
The part has a finished surface that needs to be trimmed twice
Ideal for small batches, large and/or complex components
Ideal for < 1,500 parts per year
Lowest cost tool options
Accommodates single or multi-piece molds
The preferred method of prototyping – design changes are easy
Molded SMC/BMC
Compression molding is the most common choice for high-volume composite parts and is often associated with SMC (Sheet Form Composite Materials) and BMC (Block Form Composites) materials.
The process produces high-strength, complex parts of various sizes. The matching metal mold is installed in the hydroforming machine. The material charge is placed into the mold manually or robotically, the heated half mold closes, and a pressure of up to 2,000 psi is applied. The cycle time is 1-5 minutes, depending on the part size and thickness. Features such as ribs, bosses, and inserts can be molded.
Molded parts are characterized by net size and shape, two excellent finished surfaces and excellent part-to-part repeatability. Trimming and finishing costs are extremely low.
In-mold coatings can be used to improve surface applicability without the need for a primer.
Color compounds can be customized.
Molds are usually made of mold steel. When the pressure < 400psi, other tool materials can be used.
Molds are typically capable of running at temperatures up to 150°C and pressures of approximately 1000psi, depending on the profile.
Each mold can typically make 60,000 parts.
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LCM (Liquid Composite Molding) process
PRIME (Prepositioned-Reinforcement-Ensuring-Manufacturing-Excellence) is a proprietary LCM process for prepositioned reinforcement. It offers performance characteristics similar to smCs, as well as similar cost and volume advantages. The key differentiator is the excellent structural efficiency that improves the mechanical and cosmetic properties of the molded parts. This advantage makes it popular in high-volume automotive and truck parts that require higher structural integrity.
PRIME uses pre-placed reinforcement materials to precisely align the fibers with the load settings of the product. This results in increased fiber efficiency and lower cost per weight. The use of advanced robotics minimizes changes in fibers and labor, further improving performance. These factors expand the permissible scope of the design and reduce energy costs by 50%.
How to use PRIME stacked up against SMC?
Figure 1 plots the impact energy of PRIME and SMC. At various stages of the impact, PRIME can absorb more energy than the SMC
Figure 2
When comparing the tensile strength of PRIME to SMC (Figure 2), the average of the two materials is very close — but the range of PRIME parts is much smaller. This is due to the more uniform distribution and orientation of the fibers. Lower variations allow product designers to use materials that are very close to their design limits – resulting in stronger, lighter and more efficient parts.
Figure 3
Automotive OEMs have been using PRIME for sports cars since 2007 (Figure 3). The PRIME part reliably meets the necessary fiber content and mechanical specifications, but contains less total fiber. This makes the vehicle lighter and faster.
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Resin transfer molding (RTM) process
RTM is a vacuum-assisted resin transfer process. This process improves the compression rate of the laminate, the high fiber resin ratio and the excellent strength and weight characteristics. RTM parts have two finished surfaces.
RTM is suitable for medium volume production of larger parts, and resin transfer molding is often considered an intermediate process between a relatively slow spray mold cost and a rapid compression molding method with a higher mold cost.
Recommended for products with high strength to weight ratio requirements
Ideal for medium volume production, producing 200 to 10,000 parts per year
Gelcoats can be used to provide high-quality, durable finishes
Molds can be made from many different materials – polyester, nickel shell, aluminum or even mild steel.
Vacuum Perfusion Machining (VIP), RTM Light (LRTM, Resin Infusion, VARTM, SCRIMP)
Vacuum Infusion Molding (VIP)
VIP and RTM Light are vacuum-assisted closed-mold resin processes. For compression of the B-side surface, the VIP uses flexible sheets or vacuum bag membranes.
Through these processes, dry reinforcement materials are placed in molds, often in combination with cores or other inserts. A vacuum is then applied to compact and remove air. The resin is introduced, sucked in by a vacuum and evenly distributed within the reinforcing material.
Vacuum Infusion Processing (VIP)
VIP can use very low viscosity resins and proper filling times to manufacture very large parts using vent films and other exhaust devices. Due to the relatively high fiber content, this resin infusion process enables very low porosity and excellent mechanical properties. Fiber content is determined by fiber structure and pressure. With RTM Light, resin is introduced through a pumping mechanism of less than 2psi. More flexibility in resin and filler types is possible, as well as an intermediate range of fiber content. The benefits are better surface treatment and variations in part thickness, as well as faster cycle times to reduce costs. Tools can be aluminum, steel, or composites.
Lightweight RTM
Light RTM is recommended for products with high strength-to-weight ratio requirements, or for products with slight design regression, edge overhang, or high release angles that result in clamping of rigid B-side mold surfaces.
Ideal for medium volume production of 200 to 5,000 parts per year
Environmentally friendly – This closed mold process does not involve resin exposure
Preset reinforcement can be used to achieve the best strength-to-weight ratio
In-mold gelcoat finishes can be used for the desired cosmetic finishes
Note: Number of products suitable for production - The recommended amount is an average and is provided as a general guide only. Actual volumetric efficiency is a more complex issue that requires detailed statistics about the part to be manufactured.
Source of this article: moldedfiberglass, translated by Carbontech
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