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Efficient and Economical Cutting in Thermoforming Production

Thermoforming is to heat the plastic sheet to a certain temperature, when it reaches the glass-like or soft state, then use negative pressure, positive pressure or both of them to stretch it to the outline of mold for cooling &shaping.It needs to be cut into individual products after forming.PVC,PS,PP,PET,APET&PETG are widely using in production. Although most materials need only partial cutting to break, there are also hard materials (such as PET) that have to be cut completely to break, which requests higher requirements of equipment. Therefore,"cutting technology"has become the biggest challenge on thermoforming machines.

At present,there are 2 kinds of cutting methods widely used in thermoforming machine--laser cutting mold and hardware cutting mold.

Laser Cutting Mold -- Using the strong energy of laser to ablate blade template, which forms a gap same as the shape of bending blade.The blade is fixed by the gap of boards to maintain upright state.Laser blade template and back plate achieve cutting function, its process is simple and delivery time is even within 1 day, so the cost is low. Its disadvantage is that it is difficult to exhaust, and can not achieve side cutting.

Hardware cutting mold is one kind of cutting form which is similar to principle of scissors according to the characteristics of product and requirements of cutting shape. It adopts way of module combination, install the cutting mold into the base of hardware punching mold for cutting&exhausting.It is with complex structure,high requirements of matching precision, raw material& heat treatment,so it takes longer production cycle and costs more.Its disadvantages are complex structure, large volume and high waste rate.

Therefore, in thermoforming production, we consider both technical and economic aspects. Generally speaking,we will give priority to the use of laser mold cutting, unless it can not achieve the technical objectives. Our company engaged in thermoforming machine research and production for more than 20 year, who have accumulated a lot of experience of cutting process, following will make a summary.

1,The thinner the sheet and the larger the cutting ranges, the higher the accumulated parallelism of the mold table and the cutting mold, and the higher the repeatability precision of mold closing.According to one test result from our company,”Thermoplastic sheet, such as OPS, PVC, is easy to cut, it can be broken when it is cut about 75% of thickness, and there is almost no defect on the edge after cutting, but PET is difficult to cut, it must be almost completely cut to break cleanly and separate.”

Take 0.10mm thickness PET sheet for example.To cut the whole layout reliably, the cutting edge of the laser cutting mold needs to be cut into the surface of the sheet sufficiently deep.Cause the sheet is too thin, if the parallelism of the mold table and the cutting mold is too low, or the repeatability precision of the mold closing is too low, there will be parts that can not be separated due to the cutting is not complete. The following drawing shows the cutting layout of our customer in actual production. The PET sheet thickness is 0.10mm, total length of cutting knife line of layout is 11.8m,actual production cutting speed is in 30cycle/min,waste rate of production is only 23.2%.

2. The longer the cutting length, the greater the cutting force required.on the basis of ensuring the accumulated parallelism and mold closing accuracy of the above mold table and cutting mold, the calculation is based on the shear strength of common PET sheet δ=45Mpa,to break 1 meter long PET sheet takes 4.5 ton pressure. In above example, it takes about 53 tons of pressure to completely cut the whole layout. Considering that there is a certain movement resistance on the mold table unit, and it total needs about 60 tons cutting force.

The "dual-power composite link rod mechanism" developed by our company with patent ownership(Patent No.: CN 203779847U).The upper mold mechanism as shown in below drawing,which adopts dual-power driven force-increasing connecting rod mechanism,the upper is mold closing motor,and the lower is cutting motor, both of them combine.The Max.cutting force reaches 45ton, when the distance between cutting edge and sheet surface is 2.5mm.The distance decreasing,and the cutting force increasing rapidly.When the cutting edge of laser cutting mold is close to cut the sheet completely,the Max. cutting force is over 100ton.The cam is used as the dynamic input in this mechanism,when closing molds, the force is increased steadily, and the damage degree of cutting edge is obviously decreased in the process of cutting.Our customers feedback that it can use about 1 year and accumulated cutting more than 8 million modules.

3. To improve cutting reliability, we use stainless steel plate as the cutting back plate of the laser cutting mold, and add a certain elastic pad under the stainless steel plate(Using pp board and board in practice.),which will help prolong the service life of laser cutting mold.Meanwhile,we add auxiliary function -- electric heating on the upper cutting mold to increase the cutting edge temperature of the laser cutting mold, which is beneficial to improve the cutting reliability and reduce wire drawing and powder from sheet fracture, and improve the sanitary cleanliness of the products. The following drawing is our cutting upper and lower mold.

Postscript: This article is based on some experience we have accumulated during development and using of thermoforming machine, and put forward how to cut efficiently and economically.We hope that more people in the same industry can give more suggestions in depth on development of thermoforming machine.