Common quality defects and solutions of rigid polyurethane sheets

According to years of experience in polyurethane sheet production, several common problems are listed for your reference only.

1 Defects

The foam structure is irregular and accompanied by layering effects.

Possible causes

The foam gel is too slow, so the foam at the pressing material is easy to deform when the foam is extruded or rolled.

Corrective methods

Increase the amount of catalyst used to increase the gel rate.

Increase the shelving time and extend the time before pressing.

Reduce the speed of the transmitter and extend the time before pressing.

Increase the pressure template temperature and increase the gel speed.

2 Defects

Near the edge, the thickness of the plate is generally reduced.

Possible causes

The foaming mixture is poorly distributed and cannot fill the edges.

Corrective methods

Reduce the lateral round-trip motion rate. to increase foaming mixture deposition near the edge.

3 Defects

・About 10 cm near the edge, the foam is thicker when entering the press roller.

Possible causes

Poor foam distribution, excessive filling of edges.

Corrective methods

Increase the lateral round-trip movement rate to reduce the deposition of foaming mixture near the edges.

Note: The secondary effect of overfilled edges depends on the gel state of the foam when entering the press roller and also on the type of processing system (PU or PIR).

4 Defects

The foam structure near the edge of the board is stained, layered, and irregular.

Possible causes

Excessive rim filling.

Corrective methods

Increase the lateral round-trip movement rate to reduce the deposition of foaming mixture near the edges.

5 Defects

Intermittent longitudinal voids near the underlying surface layer.

Possible causes

Excessive rim filling.

Corrective methods

Increase the lateral round-trip movement rate to reduce the deposition of foaming mixture near the edges.

6 Defects

Inclusions or voids passing through the laminate under the top surface.

Possible causes

Filling too much edge support floating platen, thus preventing the upper soft surface layer from contacting the foam.

Corrective methods

Increase the lateral round-trip movement rate to reduce the deposition of foaming mixture near the edges.

7 Defects

There are transverse ripples on the top surface.

Possible causes

・The top layer of paper is stretched too long, forming wrinkles, which is the result of the top belt moving faster than the bottom belt through the conveyor part (uneven belt pull).

Corrective methods

Reduce the pressure of the top belt press roller, or tighten the slack part of the bottom belt with the adjustment roller.

8 Defects

The whole plate is irregular and uneven.

Possible causes

Due to uneven tension or insufficient vacuum of the shelving table, the bottom surface layer is not placed flat, and sometimes the paper is stored and dry

Caused by deformation during dryness.

Corrective methods

Check the passage and tension of the bottom fabric, such as paper deformation, should be scrapped and replaced.

9 Defects

・There is a loose area between the top surface layer and the foam, and the foam hair in the loose area is dark in color.

Possible causes

Due to the shear force from the incorrect arrangement of the press rollers, the top paper surface layer is disengaged after contact with the foam.

Corrective methods

Check if the press roller is 0.4mm below the first plate position. In production, whether the press roller rotates continuously and is exactly parallel to the pressure platen.

10 Defects

The foam adjacent to the top surface layer has a higher density and dark color.

Possible causes

・The roll pressing of the soft foam at the pressing material causes the liquid foam mixture to overload the foam layer.

Corrective methods

Increase the shelving time; Increase the amount of catalyst; Increase the pressure plate temperature to improve foam ripening at the roll gap; Lower transfers

Machine speed and/or pressure at the press.
11 Defects

• Long-lasting longitudinal depressions appear in the lower surface layer.

Possible causes

Foam or other materials stuck to the surface of the bottom platen or heated pressure plate are severely deformed.

Corrective methods

Clean the shelving table and heated pressure platen.

Replace the deformed plate.

12 Defects

• Wavy edges (paper panel)

Possible causes

The edges of the paper are loose.

Due to the fluctuating lateral round and round movement, the foaming mixture is not evenly deposited at the edge.

Corrective methods

Before use, the paper is stored at the specified humidity to avoid edge swelling, causing sagging and, if necessary, clean or modify

Unqualified lateral round-trip motion machinery.

13 Defects

• The edge foam is soft and underripe.

Possible causes

The edge temperature of the laminate is too low, which inhibits the curing speed.

Corrective methods

By machine category, increase thermal insulation to reduce heat loss at the edge of the pressure platen, or increase the heat transfer of the edge heater.

14 Defects

The upper surface is uneven due to the looseness of the upper surface.

Possible causes

The surface viscosity of the foam at the pressing place is small.

The front floating pressure plate does not float normally.

Due to uneven foam shelving, gas is trapped between the top surface layer and the foam surface layer.

Corrective methods

Reduce shelving time, or increase conveyor speed.

Reduce the amount of catalyst and reduce the temperature of the pressure plate.

Inspection and overhaul of pressure plate connections and operating machinery.

Improve foam shelving with the following points: clean the nozzle and adjust the flow rate of mixed/nucleated air gases;

Check and adjust the uniformity of round-trip operations;

Check if the nozzle diameter is suitable for foam output.

15 Defects

Longitudinal wrinkles appear in the laminate.

Possible causes

・There are stretch wrinkles in the lower paper layer.

Corrective methods

Reduce the tension of the lower surface layer and/or feed roller to keep the lower pressure plate continuously flat.

16 Defects

・The foam near the lower surface layer has low strength and is wool-like

Possible causes

When heated, the excess water vapor emitted by the lower surface layer reacts with the foaming mixture to form a porous soft foam layer.

Corrective methods

Preheat the lower surface layer to remove water vapor, preferably using a surface layer with poor vapor permeability, such as paper coated with polyethylene.
17 Defects

When using polyethylene coated paper, there are trachoma or small bubbles on the surface of the foam.

Possible causes

Moisture in the paper is discharged through the pinholes of the polyethylene coating.

Corrective methods

Use high-quality, thick-textured polyethylene coated paper, or the pressure plate temperature should be as low as possible during operation.

18 Defects

The paper panel is bent due to the transfer press

Possible causes

Due to the excessive pre-drying of the paper, one side shrinks greatly, and there is a difference in the top and bottom belt speeds.

Corrective methods

Excessive preheating of the surface layer so that it becomes concave in the middle, or less heated on the reverse side of the surface layer.

Adjust or repair the pressure controller of the press roll, and adjust the tension of the upper and lower drive belts to the same.

19 Defects

The plate bends due to the machine or when stacked.

Possible causes

The density gradient of the entire foam thickness is too large, for example, the density of the bottom surface is too low, while the density of the top surface is too high, and the thickness is uneven (thick edge, thin middle).

Corrective methods

Reduce the pressure plate temperature and/or increase the amount of catalyst.

Properly install or repair incorrectly adjusted or faulty round-trip devices.

Thermoplastic Polyurethane (TPU) is a type of polymer resin that is known for its flexibility, durability, and versatility. It is a thermoplastic material, which means it can be melted and reprocessed multiple times without losing its original properties.

TPU is commonly used in various industries, including automotive, electronics, footwear, and sportswear. It is often used to make products such as phone cases, cables, hoses, seals, gaskets, and inflatable structures.