How to use a PVC foam board production line to produce carbon crystal boards and impact-resistant panels？

In the plastic extrusion business, the adaptability of a production line is one important aspect that manufacturers seek, especially if they want to enlarge their range of products without large capital investment. Here at Jiangsu Xinhe Intelligent Equipment Co., Ltd., we helped many customers convert their standard PVC foam board production lines to two kinds of popular value added products - carbon crystal board and impact resistant board. The extrusion principle for these boards is not so different from PVC foam board, but the formula, screw setup, die and cooling setup have to be adjusted properly. The guide below describes a practical approach, based on field experience in many countries including Vietnam, Indonesia and Egypt.

Understanding the Material Differences

PVC foam board: it is made using a foaming agent, resulting in a cellular core. Carbon crystal board is usually solid or slightly foamed. With the addition of calcium carbonate, carbon materials etc., it becomes high rigidity and has good thermal stability, with a very smooth and lustrous marble-like surface finish. Impact resistance board uses additive like acrylic impact modifier, or CPE to enhance its toughness. There may be slight foam in the core,with small air bubbles, but the structure is denser than foam board. All these boards can be extruded using same extruder, but their formulation should be different. For carbon crystal board, the addition of filler can go as high as 50-60 phr. For impact resistant board, addition of impact modifiers should be 30-50% higher than formula for foam board.

Screw and Barrel Configuration Adjustments

The screw is the heart. The compression ratio suitable for general PVC foam board extrusion may not be adequate for carbon crystal board extrusion due to the high filler content which needs a longer mixing section and higher shear force for thorough dispersion. A double stage mixing screw design as that used by Jiangsu Xinhe Intelligent Equipment Co., Ltd. is generally recommended for carbon crystal boards. A screw with moderate shear rate is generally good for impact resistant board, as higher shear can degrade the impact modifiers, and so an impact-resistant formula will utilize a more balanced screw. The temperature profile of the barrel would need to be increased for carbon crystal, for example to 180-200°C, as good dispersion is required for high filler loads. Impact resistant panels are cooler, say 160-180 degrees Celsius, to avoid degradation and ensure good impact properties.

Die Modification for Different Panel Characteristics

The die lip gap, as well as the land length would determine the surface quality, dimensions of the board. For carbon crystal boards which need gloss surface like marble, the flow channels within the die need to be precisely polished, and the lip thickness should be adjustable to achieve a dense, smooth surface. For impact resistant board, the dimension precision is more important, and the surface gloss will be of secondary importance. A common die structure is fine for both applications; however, the die lip gap needs to be slightly wider for impact resistant formulations to compensate for the increased melt viscosity. Increasing die lip gap of impact resistance panel by 10-15% from that of foam board is common practice. There should be adequate pre-land length to allow for back pressure.

Cooling and Shaping Table Reconfiguration

For PVC foam boards, usually mild cooling will be adopted to allow cell formation and enlargement. However, solid or lightly filled boards, such as carbon crystal board, require strong immediate cooling post die in order to get firm dimension, so as to avoid warping. Additional water spraying systems, or chiller roller systems may be used. Impact resistant boards will also benefit from prompt cooling, but to prevent internal stresses due to differential cooling rates, it is important to ensure that cooling is as uniform as possible. A longer cooling table is beneficial, with adjustable cooling intensities for different segments of the table, as offered on our advanced extrusion lines. The first zone for cooling of carbon crystal should be at around 10-15 degrees Celsius, while 20-25 degrees Celsius would suffice for impact resistant panel.

Downstream Equipment Considerations

Other processing parameters that will vary are the cutting method and stacking systems. Carbon crystal boards are harder and more brittle, therefore they require a chipless cutting machine or a high-speed sawing machine with carbide-tipped blades. Standard cutting systems might be more suited to impact resistant panels, as these are tougher and thus cause less blade wear. The stacking systems must be adjustable for various lengths, widths and weights. Here at Jiangsu Xinhe Intelligent Equipment Co., Ltd., our standard PVC foam board extrusion lines have downstream attachments that could easily be adjusted, to produce carbon crystal board or impact resistant board.

Practical Start-Up Tips for Line Conversion

For transition from foam board to carbon crystal board, purge the extruder with cleaning compounds, increase temperatures and run at about 50% of the typical speed for 30 minutes, monitoring melt pressure and motor current before returning to typical settings for the final product. For impact resistant panels, the temperature would be decreased. Run a test product for a minimum of 10 meters before finalizing parameters, which is a universal practice when converting between products.