Class A surfaces

Low-profile additives, which control shrinkage, have emerged as a distinct science and class of additive. Unsaturated polyester resins, as do all thermosetting polymers, shrink when cured. Low-profile additives are a major class of additives used to control shrinkage, which vastly improves surface quality. This science is credited with the opening of automotive markets where surface quality is of prime importance. In exterior automotive body panels, Class A surfaces are required for market acceptance. 

Sheet molding compounds (SMCs) and bulk molding compounds (BMCs) are the dominant materials used in automotive applications. These composites of unsaturated polyester resin, fillers and fiberglass have advantages of high stiffness, heat resistance and low coefficient of expansion. Coupled with low creep resistance, which is a distinct advantage over thermoplastic competition, and low-profile additives, which can yield Class A surfaces, these materials are well suited for applications from exterior body panels to under the hood components. 

This moldable material primarily consists of TS polyester resin, glass fiber reinforcement, and filler. Additional ingredients, such as low-profile additives, cure initiators, thickeners, and mold-release agents are used to enhance the performance or processing of the material. As with any material, such as metallics and plastics, SMC can be formulated in-house or by compounders to meet performance requirements of a particular application such as tensile properties or Class A surface finish. Varying the type and percentage of the composition will result in variations in mechanical properties and processability. 

SMC typically requires post mold rework to provide a Class A surface. Consequently, additional manpower 1s allocated for Inspection and repair for SMC to fill and sand the parts In preparation for painting. Some manpower 1s allocated for minor repair of PU-RRIM parts. The Inspect/reject function 1n an Injection molding operation 1s normally handled by the clamp operator. 

Surface Quality. Class "A surface (after painting) is considered by the automotive industry as their first requirement for an exterior part. Comparing the surface quality of several reinforcements in RRIM urethane panels, it was found by visual examination that the best surface quality was the glass flakes and the worst was the integral chopped strands. Milled fiber is slightly worse than glass flakes, but it is still considered a "Class A" surface. 

Atryl. [Owens oming Fiberglas] Low profile resin for achieving class A surface in comixession-molded automotive parts. 

The largest potential applications are in the areas of vertical body panels, which require Class A surface aesthetics. Due to the extreme requirements... 

Blends of PPE and PA-66 are suited for exterior automotive applications, where they produce class A surface on body panels, because they combine the processing and solvent resistance of polyamides with the moisture resistance and dimensional stability of PPE. Therefore, they have good heat resistance, impact resistance, moldabUity, paint adhesion and chemical resistance. 

SRIM has been restricted to non-appearance parts because of problems with glass read-out and surface porosity. Recently, SRIM is used more and more for exterior automotive body panels due to its improved surface qualities. Studies have shown that a Class-A surface comparable to SMC can be obtained by selecting the proper processing parameters, reinforcements (e.g. continuous-strand glass reinforcement mat) and resins. 

Mainly used in TPs, mica reinforcement improves the tensile and flexural strength and flexural modulus. Heat distortion temperature is increased and the coefficient of linear thermal expansion is reduced. Shrinkage and creep are significantly reduced, and warpage is virtually eliminated. Chemical resistance is high and permeability is reduced. Mica can also help to produce a Class A surface finish (Table 2.11). 

Over the last few years, the injection-molded bulk molding compound (BMC/Chapter 4) hatchback program of Citroen has developed to a two-part design using an outer skin for class A surface and an inner panel providing mechanical properties, injected from a single port on the parting line (which produces fewer weld lines, no need for a cold... 

The nearly shrinkage-free SMC qualities are used to produce class A surfaces in automobiles. On the other hand, fumre use of such SMC applications is being called into question at present due to extremely stringent surface quality specifications and... 

Textiles and artificial leather for vehicle interiors, carbon look and imitation wood foils for the dashboard, paint foils in the car color for radiator masks, decor strips, and door sUls. The remaining difficulties include the class A surface quality demanded by carmakers as well as so-called color-matching (same optical impression of sheen and color adjacent to conventionally painted parts). In these fields, the back injection process competes more directly with painting processes than with laminating processes. Here as well, this technology offers excellent opportunities for greater cost efficiency in future production processes (Figs. 28 and 29). 

Customers will no longer be willing to pay for the exaggerated demands made of class A surfaces. The integration of the painting process in the molding system is a plus for cost-conscious consumers. 

Ribs (B) and metal inserts (E, H) are not recommended for preforms, but they can be used with the other versions. The minimum inside radius (IR) is 0.125 in for preform-molded parts and 0.062 in for SMC-and BMC-molded parts. Long-fiber compression-molded parts can be trimmed in the mold. For Class A finishes in SMC parts, the inside radius at the base of a rib should be 0.020 in otherwise it should not be less than 0.060 in. The thickness at the base of the rib (B) can be equal to the nominal wall thickness (W), however, it must not exceed 0.75W under Class A surfaces. The height of the rib (C) should be 2 to 3 times the nominal wall thickness (W) and the draft should not be less than 1° per side except for ribs imder Class A surfaces (which can handle the cost premium) where it can go down to 0.5° per side. These same rib design rules apply to boss wall thicknesses for these parts. In-mold coating can be used for high-class finishes. 

The conventional process has still priority in the industry. Therefore, virtual surfaces (CAD surfaces) which are used for conceptual studies are still derived from the clay model and processed by designers to obtain class A surfaces. Further analyses regarding ergonomics, aerodynamics, handling and production are based on these virtual surfaces to validate the car concept. The concept phase provides a... 

Class A surfaces difficult to obtain. Considered an intermediate volume process. 

Noryl GTX 910 380 = F (193°C) HDT at 66 psi (0.45 MPa) dimensionally stable grade useful in automotive online painted components gives Class A surface appearance... 

---