Why PET Sheet Crease or Break Easily during forming?

Why does PET sheet crease or break easily during forming? This question can be answered from the perspective of PET production technique. Generally speaking, PET sheet is produced in the following steps:

1. Material preparing and mixing

The main raw materials of PET sheet are matrix chip and lustrous chip. The former refers to the PET chip that contains additives including silicon dioxide, calcium carbonate, barium sulfate, kaolin. The proper matrix chip can be chosen depending on particular usage of PET sheet.

PET is usually made from matrix chip containing certain amount of silicon, together with some lustrous chip. Silicon dioxide particles distribute in PET sheet and increase its roughness so as to prevent sheet from adhesion. The proportion between lustrous chip and matrix chip is certain. After metering and preparing , they are mixed and fed into the next station.

2. Crystallizing and drying

Polymers such as PET, PA or PC are likely to absorb moisture. Therefore, before extension in length and breadth, the polymers should be crystallized and dried. The goal of this process is to heighten their softening point, avoid adhesion or agglomeration during drying and melting extrusion, remove moisture and prevent ester polymers from hydrolysis or generating bubbles during melting extrusion.

Normally, filling plant with crystal bed is adopted for crystallizing and drying. Besides, there are some dry air preparing devices, including air compression, molecular sieve dehumidifier and heater... The temperature of this process should be controlled within 150-170°C and time within 3.5-4 hours. The moisture content of PET chip after drying should be about 30-50PPm.

3. Melting extrusion

After crystallizing and drying, the PET chip is heated, melts and plasticizes in the single-auger extruding machine. The auger structure plays an important role in good plasticization of PET chip and stable extrusion pressure.

Barrier shape of the auger is specially required, except for length-diameter ratio, function and so on. Barrier structure enables material to be well plasticized, keeps material temperature uniform, makes material to be discharged stably and increases extrusion efficiency.

Besides, if the extrusion amount is not large, air-exhausting double-auger extrusion machine is recommended. Such an extrusion machine is equipped with two air exhausting ports connecting with two vacuum systems so that it can exhaust air and dehumidify effectively. As a result, a set of complex crystallizing and drying system is not needed. The investment and production cost can be both reduced. A point that should be noticed is that the machine temperature should be set at 210-280°C.

4. Casting

Die head, the key for casting, directly determines the casting shape and thickness. Coat-hanger die head is commonly used for PET production. Its openness is adjustable by several pull-push differential bolts with heating coils. Then the online thickness gauge can measure thickness and transmit feedback to the heating bolts to fine-tune the openness. Besides, the temperature of die head should be controlled within 275-280°C.

Instant cooling roller or casting roller rotates at constant speed and cools PE melt to below glass transition temperature (Tg) quickly so that glassy casting with uniform thickness is formed. The goal of instant cooling is to reduce crystallization of the casting and avoid negative effect on the following extension. Therefore, the instant cooling roller is required to have uniform surface temperature, good cooling effect and constant and stable rotating speed. In addition, 30°C cool water is fed into the roller to cool casting to below 50°C.

Another device is electrostatic adsorption device. It can make the casting contact with the punch roller firmly to avoid air entering while instant cooling roller is rotating and to guarantee heating and cooling effect. This device is composed of wire electrode, high-voltage generator and electrode torque motor and so on. Its working principle is as follows: the high-voltage generator generates thousands of volt of direct voltage, which turns electrode wire and cooling roller into positive and negative respectively. The instant cooling roller should also be grounded. Casting can be charged by electrostatic induction to gain the opposite polarity to the cooling roller in the high-voltage electrostatic filed. Under the action of heteropolar absorption, the casting and the instant cooling roller are closely adsorbed together to achieve the purpose of removing air and good heat transfer.

5. Vertical stretching

The casting is heated and vertically stretched by the vertical stretching machine. The machine consists of preheating roller, stretching roller, cooling roller, tension roller, rubber pressing roller, infrared heater, heating units and driving devices. Casting is normally stretched based on a single point, but sometimes also on two or three points. The vertical stretching ratio is the speed ratio of slow stretching roller and quick stretching roller, normally reaching 3.3-4.

6. Horizontal stretching

The horizontal stretching machine is made up of oven, chain clip, guide rail, static pressure box, guide rail width adjusting device, clips, hot wind circulating system, lubricating system and EPC, etc.. Its function is to preheat, stent, heat set, cool and horizontally stretch the PET sheet. The horizontal stretching rate is 3.5-4.

7. Traction rolling and cutting

The equipment for this process is composed of several traction and guiding roller, cooling roller, flattening roller, tension roller, tracking roller, cutting device, thickness gauge and corona processor. The vertically and horizontally stretched sheet can be rolled and slit after edge cutting, thickness measuring and corona processing. The product which is tested to be qualified is the end product.

However, the sheet which is processed by the above technique is of different physical mechanical properties in length and breadth. The large internal stress of the sheet makes it to crease or break easily during vacuum molding and the product pass rate is pretty lower.

Application of PET

On our mono dose plastic ampoule and bottle forming filling and sealing machine,  mono dose blister capsule packing machine, PET is widely used to pack food, cosmetic and medicines. Because of the special features of PET, we need to improve our machine to adopt perfectly with PET, including the forming mould , sealing and cutting mould.