Process description and process selection information for Fiber Reinforced Plastics

Introduction
Fiber-reinforced plastics (FRP Composite) can be fabricated using several processes – hand layup and spray-up lamination, continuous lamination, spin casting, resin transfer molding and its variations, injection molding, etc. However, let’s now focus not on individual processes, but on 3 important categories to which these processes belong
• Open molding
• Low volume closed molding
• Compression molding

Process Descriptions
A. Open Molding – Open molding is the simplest and most widely used process to produce FRP parts. It is done in ambient shop conditions. The mold is generally fabricated from FRP. The cosmetic surface of the part is fabricated next to the mold. The back of the mold is open. While most fabrication processes involve the application of the exterior coating after the main structure of the part has been built, open mold parts are built from the exterior to the interior. The first step in open molding is to apply the gel coat (the exterior coating of the part) to the mold. The remaining layers of the laminate design, which will include some but not all of the following, back the gel coat :
1. Barrier Coat – this is applied behind the gel coat. A barrier coat improves part cosmetics, reduces cracking, and improves osmotic blister resistance in marine parts.
2. Skin Laminate – a relatively thin glass fiber reinforced laminate fabricated behind the gel coat. Skin laminates improve cosmetics and osmotic blister resistance.
3. Print Blocker – a sprayable syntactic foam material used behind a skin coat to improve laminate cosmetics.
4. Coring Materials – lightweight materials used to build part thickness and stiffness without adding weight.
5. Bulk Laminate – the main portion of the laminate that provides most of the structural properties.

Glass fiber reinforcement used in skin and bulk laminates can be applied by hand layup or spray-up techniques. Emissions from open mold processes are significant and are regulated by Federal NESHAP standards and, in some cases, State and Local regulations.

B. Low volume closed Molding –
The category of low-volume closed molding processes includes processes in which liquid resin is transferred into a closed cavity mold containing reinforcing materials. Over time, many variations of low-volume closed molding processes have evolved such as
• Vacuum infusion
• Seamann Composites Resin Infusion Manufacturing Process (SCRIMP®)
• Conventional RTM
• Light RTM (shell laminate RTM)
• Silicone bag RTM
• Closed Cavity Bag Molding (CCBM®)
• Multiple Insert Tooling (MIT®) RTM
• Zero Injection Pressure (ZIP®) RTM.

Parts fabricated using these processes may or may not have a gel coat on the exterior surface. The part size for these processes is limited by mold and part handling considerations. Part-to-part consistency is better than with open molding due to less dependence on operator skill. Also, two-sided cosmetic parts can be produced. Emissions from these processes are still regulated; however, they are much lower than with open molding due to the closed portion of the process. Emissions from the gel coat application, if used, are the same as for open molding.

C. Compression molding
Compression molding is another closed molding process. It uses clamping force during mold closure to flow a pre-manufactured compound through a mold cavity. A hydraulic press generally provides the clamping force. Compression molds are generally made from chrome-plated tool steel. Sheet molding compound, bulk molding compound, and wet molding compound are examples of pre-manufactured compounds.

If an external coating is needed on a compression molded part, it is generally post-applied; however, in-mold coatings are available. Part size is limited by press platen size. Part-to-part consistency is excellent. Emissions from compression molding are still regulated; however, they are much lower than with open molding due to the closed nature of the process.

The following table gives you the gist of the differences between the processes we just discussed

PROCESS COMPARISON
Part Characteristic	Open Molding	Low Volume Closed Molding	Compression Molding
Maximum Part Size	Any Size	Any Size	Up to 100 Square Feet
Factors Limiting Part Size	Mold and Part Handling	Mold and Part Handling	Press Size
Part Surface	One Side	Two-Sided, Smooth or Textured	Two-Sided, Smooth or Textured
Part to Part Consistency	Fair	Good to Excellent	Excellent
Cross Section	Completely Variable	Better if Uniform	Easily Varied
Number of Parts Per Year	<1000	<10,000	>5,000
Parts Per 8-Hour Shift Per Mold	1-2	16-90	100-500
Mold Construction	Composite	Aluminum Nickel Shell, or Composite	Chrome Plated Tool Steel
Mold Lead Time	2-4 Weeks	4-8 Weeks	16 Weeks or More
Tons of Composite Per Tons of Emissions	371	135-16302	135-16302

1. Numbers taken from unified emissions factors; 35 percent styrene content resin; mechanical non-atomized application; and 30 percent fiberglass.

2. Numbers are taken from EPA AP-42 emission factor; 35 percent styrene content resin; compound paste (25 to 100 percent resin); and 30 percent fiberglass.

Process Selection
When the best process to use for the fabrication of a specific part is not obvious, process selection should be accomplished through a process trade study. A process trade study involves comparing the part fabrication costs and part performance factors for a specific part fabricated by various processes. Part fabrication costs include but are not limited to equipment costs, tooling costs, material costs, and labor costs. Part performance factors are dependent on the specific part being studied but can include, weight, strength requirements, and appearance requirements. Emissions of Hazardous Air Pollutants (HAP) or other regulated materials vary by process and may also factor into process selection. An example trade study follows.

The subject part is from the deck of a run-about boat. It is a hinged hatch cover that provides access to an under-deck storage compartment or cooler. The step face features a non-skid profile on the external surface. The step face comprises glass skins over a foam-filled honeycomb core. The part measures 11 inches by 25 inches with a 1.5-inch tall perimeter flange. The design criteria include an impact of 300 pounds from a three-foot elevation.

The part is shown in Figure 1 alongside.
Processes considered in the trade study were open molding and several low-volume closed   

 

molding processes including vacuum infusion, silicone bag RTM, light RTM, and conventional RTM. Equipment costs, tooling costs, material costs, and labor costs were calculated for each process on a per-part basis.

 

 

Costs are based on typical values in the year 2005 and are presented as relative costs with open molding at 100 parts produced as the baseline. The number of parts to be produced varied from 10 to 9,000. Parts were to be produced over a three-year time frame with an equal number of parts per year. Process trade study cost results are shown in Table 1 below. The cost per part decreases as the number of parts produced increases. However, the amount of decrease depends on the process, meaning that Figure 1 Trade Study Hatch Cover different processes are the most cost-effective at different production rates.

Table 1- Process Trade Study Cost Results

Property	Open Molding	Vacuum Infusion	Light RTM	Silicone Bag RTM	Conventional RTM
Part Appearance	The part back side is rough	The part back side is matte	The part back side is smooth	The part back side is matte	The part back side is smooth
Strength	Acceptable	Comparable to open molding	Comparable to open molding	Comparable to open molding	Comparable to open molding
Cost Effective Production Run Size	<100 parts	<200 parts	100 to 9,000 parts	100 to 9,000 parts	>1000 parts
Emissions	0.126 lbs/part	0.064 lbs/part	0.064 lbs/part	0.064 lbs/part	0.064 lbs/part

 

Conventional RTM is not a cost-effective option for hatch cover production at production run sizes of less than 1000 parts. For production run sizes greater than 1,000 parts this process becomes competitive with light RTM, but does not become cheaper than light RTM even at production run sizes of 9,000 parts due to the need for a gel-coated surface.

The cost comparison could be different for large production runs of a non-gel
coated part. While competitive with light RTM, silicone bag RTM is never the cost-effective process for hatch cover production. This is due to the cost of the materials needed to fabricate the silicone bags. However, silicone bag RTM can be an excellent process selection for parts with closed contours that are not easily fabricated by other processes.

Light RTM is the low-cost process for hatch cover production at production run sizes greater than 100 parts.

Vacuum infusion is competitive with open molding as the low-cost process for hatch cover production runs of less than 100 parts. At higher production rates vacuum infusion is not cost-effective due to the cost of the consumable materials (vacuum bag film, sealant tape, etc.) needed for each part. Overall process trade study results including part appearance, strength, cost, and emissions, for the hatch cover, are shown in Table 2.

The use of closed a closed molding process reduces emissions by 50 percent in comparison to open molding with low VOC materials. The emissions differences as well differences in part appearance could influence process selection for hatch cover production.

This trade study is provided as an example of the type of evaluation that can be done to make an informed decision on process selection. The conclusions reached are not valid for all part types, sizes, complexity, and specific combinations of labor, material, and capital costs

Table 2. COMPARISON OF RTM PROCESSES
Property	Open Molding	Vacuum Infusion	Light RTM	Silicone Bag RTM	Conventional RTM
Part Appearance	Part back side is rough	Part back side is matte	Part back side is smooth	Part back side is matte	Part back side is smooth
Strength	Acceptable	Comparable to open molding	Comparable to open molding	Comparable to open molding	Comparable to open molding
Cost Effective Production Run Size	<100 parts	<200 parts	100 to 9,000 parts	100 to 9,000 parts	>1000 parts
Emissions	0.126 lbs/part	0.064 lbs/part	0.064 lbs/part	0.064 lbs/part	0.064 lbs/part

Open molding is the low-cost process for hatch cover production runs of less than 100 parts. But it is not cost-effective for production runs greater than 100 parts.

More information
This information on FRP composites is an extract from a book titled ‘Composites Application Guide’.

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