A Brief Discussion on the Material Selection of Road Roller Covers

I. Introduction

Since the first roller was born in the UK in 1860, the main material of its hood has always been sheet metal. With the rapid development of roller technology, industrial modeling has become the third major focus after the functions and performance of rollers. Aesthetically pleasing large arc-shaped and streamlined machine covers have been widely adopted on rollers, making fiberglass reinforced plastic (FRP) the preferred material for roller machine covers due to its advantages in modeling techniques and showing a strong trend of replacing sheet metal parts. Abroad, major companies such as DYNAPAC and BOMAG began to widely use fiberglass reinforced plastic products on the covers of their rollers in 1998. In 2000, XCMG in China was the first to use fiberglass reinforced plastic (FRP) machine covers on its road rollers, which sparked a trend of improving the design of road rollers in the industry.

It is well known that road rollers are generally stored outdoors and often operate in special working conditions (such as high-altitude areas), where the environment is relatively harsh. This requires that the road roller hood not only have the characteristics of light weight, high strength, anti-deformation, anti-aging, water resistance and anti-corrosion, but also meet the usage requirements in special environments such as low temperature, high temperature and high radiation.

At present, the commonly used materials for the machine covers of road rollers in China are fiberglass reinforced plastic and sheet metal parts.

Ii. Application of Fiberglass Reinforced Plastic in the Cover of Road Rollers

1. Basic knowledge of fiberglass reinforced plastic

FRP, scientifically known as glass fiber reinforced thermosetting plastic, is composed of synthetic resin as the matrix material and its auxiliary materials, as well as glass fiber reinforced materials that have undergone surface treatment.

FRP (Fiber Reinforced Plastic) can be classified into polyester FRP, epoxy FRP, phenolic FRP and furan FRP according to its base material. Due to factors such as mechanical strength, manufacturing processability and cost, polyester FRP is mostly used for FRP machine covers at present.

1) Chemical composition and mechanical properties of polyester fiberglass reinforced plastic

Unsaturated polyester resin was first used in the preparation of fiberglass reinforced plastic (FRP) in 1942 and has since been widely applied in industry. Currently, most FRP products in the industry mainly use the general-purpose domestic unsaturated polyester resin 196 #, whose main components are: propylene glycol, diethylene glycol, maleic anhydride, and phthalic anhydride. Main technical indicators: Acid value 17-25, solid content 64-70%, gel time 8-16 minutes; Typical properties of cured polyester resin (without fillers).

2) The forming methods of fiberglass reinforced plastic

According to the different characteristics of the products, there are mainly five molding methods for fiberglass reinforced plastic: hand lay-up, compression molding, winding, pultrusion molding and resin transfer molding. Hand lay-up method is widely used for integral products and large products with low mechanical strength requirements, such as car bodies, ship shells, and covers of construction machinery, etc. The molding method is mainly used for making small parts. For large machine covers, this method requires a huge initial investment and is difficult to form in one molding process, so it is rarely adopted. The other three methods are usually only used to make small parts.

3) Typical fiberglass reinforced plastic structure

The structure of the fiberglass reinforced plastic layer varies with different molding methods and applications, and is mainly determined by experience and experiments. Figure 1 shows a typical structure of the FRP layer for corrosion-resistant FRP equipment made by hand lay-up method.

The corrosion-resistant layer is composed of a surface layer and an intermediate layer. The surface layer is the innermost layer in contact with the medium and is a resin-rich layer reinforced with glass fiber mat.

The middle layer is reinforced with short glass fiber mat, approximately 2mm thick, which ensures that after the medium penetrates the surface layer, it will no longer penetrate the outer layer.

The outer layer, also known as the reinforcing layer, is a layer that meets the strength requirements and is reinforced with untwisted roving fabric and short fibers.

The outermost layer has the same composition as the surface layer, with the aim of preventing the reinforcing layer from being exposed to corrosive environments.

2 The advantages and disadvantages of FRP machine covers

The structure of the roller hood is generally large in size and in small batches. The cost of using the molding method is too high. Currently, all the roller hoods in China are made by hand lay-up method.

The main advantages of FRP machine covers:

The finished product is relatively light in weight. FRP has a low density and high strength. Its specific gravity is only 1/4 to 1/16 of that of ordinary steel, while its mechanical strength is 3 to 4 times that of steel. Due to the structural reasons of the product, the thickness of the finished FRP machine cover is about three times that of the sheet metal part, but its weight is slightly lighter.

2) Good formability. Due to the excellent manufacturing processability of fiberglass reinforced plastic (FRP), the shape design of the machine cover is almost not restricted by processability. It can be designed with large arcs, streamlined shapes and other curved surfaces, presenting a beautiful appearance. This is also the most prominent advantage of FRP machine covers. Figure 2 shows the typical appearance of a fiberglass reinforced plastic machine cover.

3) Short manufacturing cycle of finished products: FRP machine covers have the characteristic of rapid market response, which is particularly suitable for new product development and can significantly shorten the production cycle. Generally, the process from design to finished product can be completed within one month.

4) The weather resistance and corrosion resistance of FRP machine covers are relatively good, mainly depending on the quality of the gel coat resin. FRP not only does not rust like metal or rot like wood, but is also almost not eroded by media such as seawater, oil and salts. It can replace stainless steel in some situations.

The main deficiencies of FRP machine covers:

Fiberglass reinforced plastic (FRP) has a high tensile strength, ranging from 260 to 400Mpa. However, its shear strength is only 50% of the tensile strength, and the layer shear strength is only 5 to 10% of the tensile strength. That is to say, FRP is hard but brittle.

2) The interface treatment of the paint is difficult. If not handled properly, fine cracks and pinhole-like bubbles are likely to form on the paint surface.

3) Although FRP machine covers have good processability, they are greatly affected by human factors. Therefore, technical workers are required to be proficient in operation and have good skills.

4) When the FRP machine cover is damaged, professional personnel need to use special materials, which increases the difficulty of repairing the FRP machine cover.

5) FRP products do not meet the national requirements for environmental protection and energy conservation. The crosslinking agents used in their manufacturing are highly harmful to human health and cannot be recycled after being discarded.

Iii. Application of Sheet Metal Parts on the Cover of Road Rollers

1. The materials used for sheet metal machine covers

Due to the limitations in the processing and forming of sheet metal machine covers, currently, in the case of small batch sizes, manufacturers in the industry mainly use the plasticity of metals for manual production. Generally, hot-rolled steel plates are selected, with Q235-A being the commonly used material, and the thickness is usually 2mm. Cold-rolled steel plates with good surface quality, smoothness and beauty can also be selected. Such steel plates are expensive, almost twice the price of hot-rolled steel plates. The commonly used material is ST12 (Q195), and the thickness can be 1.2 to 1.5.

When mass-produced, cold-rolled steel plates can be selected for pressure processing, which can achieve excellent flow distribution and high material utilization rate, as well as high productivity. It is easy to realize mechanization and automation, but molds need to be made, and the development cost is high. This forming method is currently less applied in China.

2 The advantages and disadvantages of sheet metal parts preparation machine covers

Before the advent of fiberglass reinforced plastic (FRP) machine covers, sheet metal parts dominated the road roller machine covers. Even now, sheet metal machine covers still demonstrate their unique advantages

The structure and manufacturing of sheet metal machine covers without special styling requirements are relatively simple and do not require special equipment. Figure 3 shows the typical appearance of a sheet metal machine cover.

2) The appearance quality of the sheet metal machine cover is easy to control, and the paint effect has a good texture.

3) It has excellent maintainability. When the hood is damaged during use, it can generally be repaired and used on the spot in a timely manner, which is of more practical significance in facilitating users' operation.

4) Sheet metal products comply with national environmental protection and energy conservation requirements. After being discarded, they can be easily recycled and reused, without causing environmental pollution or waste of raw materials.

The sheet metal machine cover also has its shortcomings:

The streamlined design process of sheet metal parts is complex, especially for machine covers with large arc-shaped surfaces, which are very costly.

2) During the manufacturing process of sheet metal machine covers, if the surface is not thoroughly ground, it is very likely to form rust from the inside out of the paint base. If the paint surface ages or is damaged and not repaired in time, it will also cause rust, resulting in high usage and maintenance costs.

Iv. Prospects for FRP Hoods and Sheet Metal Hoods

In recent years, the covers of road rollers have mostly been made of fiberglass reinforced plastic (FRP), mainly due to its advantages such as simple molding process, light weight and high strength. This is not only a manifestation of technological progress, but also a reluctant move of the road roller industry.

1. The awkwardness behind the technological progress of FRP machine covers

Since the emergence of fiberglass reinforced plastic (FRP) as a new material, it has gradually replaced some wood, ceramic and metal products, and has been successfully used to prepare the covers of road rollers. This in itself is a manifestation of technological progress, and this can also be seen from the development of road roller covers abroad.

In recent years, there has been a "reverse flow" of returning sheet metal parts to the covers of road rollers in China. Besides the fact that the preparation process of fiberglass reinforced plastic (FRP) poses a threat to the health of operators, there are mainly some problems in its application. The reasons can be summarized into three: FRP covers are more expensive than sheet metal covers, have poorer surface quality, and are more inconvenient to maintain.

In China, when FRP machine covers are made by hand lay-up method, it is reasonable that the cost of FRP machine covers is slightly higher than that of sheet metal machine covers. However, at present, there is a phenomenon of "excessive profit" in FRP machine covers. The main reasons are the unreasonable reinforcement inside the machine cover and the "adulteration" in the FRP formula, which increases the weight of the FRP machine cover. Such problems can be solved by refining the technical agreement and conducting business negotiations.

The poor surface quality of FRP machine covers can be completely controlled by improving the manufacturing process, choosing the appropriate paint and strictly controlling the quality.

For ordinary users, there is indeed no good solution to the inconvenience of maintaining FRP machine covers. However, as long as all the performances of the FRP machine cover can meet the standards, the probability of damage to the FRP machine cover is very small

2 Sheet metal machine covers still have great potential

Since the industrial production of the automotive industry began, its huge single-variety output and high added value have made the high mold costs paid for styling insignificant in each vehicle. Although fiberglass reinforced plastic (FRP) panels are occasionally seen mixed in, sheet metal parts remain the top choice for automotive panels at present.

Perhaps the future of the road roller hood can be foreseen from the application of sheet metal parts in the automotive industry. Currently, due to the small batch size of road rollers and the high cost of molds, sheet metal hoods are deterred. This is also the reason why the road roller hood has abandoned sheet metal parts and turned to fiberglass reinforced plastic. With technological advancements, this situation is expected to change. Emerging low-melting-point bismuth-based alloy molds can partially replace steel molds, and their cost is much lower than that of manufacturing steel molds, making it possible to use sheet metal parts for low-cost and large-scale production of road roller covers with modeling requirements.

Closing remarks

To sum up, between the cost issue of sheet metal molds and the environmental protection and manufacturing issues of fiberglass reinforced plastic (FRP), the one that resolves the problems earlier and more thoroughly will gain an advantage in the roller covers with shaping requirements. However, at present, FRP covers and sheet metal covers will still occupy their respective unstable territories.