The growing appeal of thermoformed polyethylene terephthalate (PET) packaging stems from the fact that it’s an infinitely recyclable material and that post-consumer recycled PET (rPET) can be used in food-contact applications without the risk of contaminating the products. rPET is commonly used to manufacture bottles and plastic sheeting for punnets and trays.
The strength of formed PET and rPET packaging is evident from its popularity in hinged fruit and vegetable clamshell containers, compartmentalised and ovenable food trays, and convenience packs for hardware items like nails, nuts, and bolts. PET’s toughness is key to replacing other plastics such as PVC, HIPS and OPS when a package has a hinge or snap-fit feature or must meet extended durability and shelf life requirements.
Prospects for PET are further boosted by the lower temperatures required to form PET sheeting (compared to PVC sheeting), as well as shorter forming cycles that translate into reduced energy use. Additionally, PET sheeting can run on thermoforming equipment used for many other plastics, with only minor modifications to hardware and processing parameters.
But for all its positives attributes, PET converting isn’t without challenges. Its toughness, which makes it difficult to trim, poses the biggest challenge to PET’s success in replacing other plastics.
Compared to softer plastics, PET requires greater force during machining, increasing the strain imposed on machine punches and dies. This typically results in greater die damage, higher wear rates and longer trim cycles than for other materials.
Improper trimming of thermoformed PET containers generates particulates in the form of fuzz, angel hair and dust, which can contaminate trim scrap and degrade the final product quality. Imperfect trimming also often creates edge defects or cracks, meaning that rims, perimeters and holes of parts are impaired, leading to a higher reject rate.
To obtain a clean PET cut and avoid damage to the product, it’s vital to use only top-quality cutting tools made from carefully-selected steels. To maintain optimal wear resistance and dimensional stability, Renlaw cools the punches and dies it manufactures in liquid nitrogen immediately after hardening.
Ensuring that the surface finish of all tooling is in good condition is another critical step in minimising wear rates and maintaining the minimal clearance between the punch and die is making certain that the surface finish of all tooling is in good condition. Machine operators also need to double-check the accuracy of alignment when installing punches and dies, as clearances between the parts are in the thousandths of a millimetre range.
Another simple but easily overlooked point to keep in mind is the importance of keeping cutting tools sharp. A professionally resharpened punch and die set has a sharp-life of up to 90% of a new set, which makes financial sense during these challenging times. PVD coating the tools will also help to decrease cutting force and increase the lifespan.
Systematically addressing PET trimming issues is critical because this promising material is set to grow in popularity.