Friction behavior of materials in thermoforming

In the thermoforming process, when there is a slide between the material and the thermoforming mold, it is necessary to consider the friction behavior of the plastic. This may occur during the pre-drawing process of the mold in the negative mold, or in the positive mold when the mold comes into contact with the material during the inward push.
When the friction is relatively large, the material and the mold will bond together as soon as it is in contact. Further drafting is not possible where the adhesive is attached, such as lamination with a binder is a typical example.
If there is no friction present, such as coating the surface of a mold or making a mold with PTFE, the processed material will easily slide on the contact surface, which is unfavorable for thermoforming. When the material is too easy to slide through the mold, it is impossible to press the plastic to the bottom of the mold with such a mold, because the bottom will always be too thin. Therefore, in the process of thermoforming, the friction behavior must be paid enough attention.
The factors that affect the side friction of the die are: the material used in the thermoforming die; The temperature of the contact part between the mold and the material; Surface roughness.
The factors that affect the tribological properties of the forming material are as follows: the type of the material on the contact surface; Surface treatment and conditions (whether anti-adhesive or release agent is added); The temperature at which the molding material is in contact with the surface of the mold.
Practical application points are as follows.
①The surface of the mold is slightly sanded or made slightly rough by artificial methods, compared with a very rough surface or a polished surface, this surface can make the molding material better slide, but it needs to be polished into a mirror at the corner of the positive mold, so that the heated material is relatively easy to slide through.
②The mold temperature plays a very important role in the thermoforming, the material in the thermoforming, it is very easy to stick to the mold, reducing the mold temperature will reduce the friction.
③ For forming materials with obvious bonding tendency, such as sheets with heat sealing layers and composite forming materials, it is easy to stick to the mold, and reducing the mold temperature will reduce the friction.But ABS/PMMA double-layer materials do not have such a problem, because ABS and PMMA have the same thermoforming temperature. The SB/PE double-layer composite sheet, when the PE layer is in contact with the die surface, there will be problems when pre-stretching with the plunger.
This sheet is not suitable for vacuum forming, because the thermoforming temperature of SB is at least 160°C, and at this temperature, the PE and seal layer have become sticky and may bond with the mold. Many materials with sealing layers need to be in contact with the mold at a lower temperature during thermoforming, but this method limits the design of the product.
Practical methods to solve the bonding problem in different cases: processing the thermoformed material at the lowest possible temperature; Apply less heat on the sticky side of the material; If the sticky side of the material is in contact with the mold, the molding temperature should be as low as possible; If the sticky side of the material is in contact with the pre-stretched mold, PTFE mold or PTFE-coated aluminum mold should be used; Special attention should be paid to the addition of anti-adhesive (such as PET) coating thermoplastic materials.
It should be noted that the sliding friction behavior of the sheet is very different from whether there is an anti-adhesive coating.
If the geometry of the die used for a molded sheet has been determined, then the sheet to be used next needs to have the same coating. If the coating is different, the mold parameters can also be changed to adjust. In order to form these sheets with different coatings into products with satisfactory wall thickness divisions, a different set of pre-drawn dies is necessary.