The growing appeal of thermoformed PET packages springs from their positive environmental image, outstanding rigidity and clarity, and extraordinary toughness. Ironically, PET’s toughness also poses the biggest single challenge to its success in replacing other clear, amorphous polymers, namely efficient and trouble-free parts trimming.
The muscularity of formed PET packaging is evident from its inroads in hinged egg cartons, fruit and vegetable clamshells, compartmentalized and ovenable food trays, and convenience packs for hardware items like nails, nuts, and bolts. PET’s toughness is a key to replacing PVC, HIPS, and OPS, since it makes PET the best option when a package has a hinge or snap-fit features or must meet extended durability and shelf-life requirements.
Prospects for PET vs. PVC are further boosted by the lower oven temperatures required to form PET sheet (450 F vs. 600 F), as well as shorter forming cycles that translate into reduced energy use. PET’s processing advantages arise from its lower glass-transition temperature than PVC (158 F vs. 170 F) and higher thermal conductivity. Also, recycled PET lends itself well to reuse in clear packaging. Finally, PET sheet runs on thermoforming equipment used for PVC, HIPS, and OPS with only minor modifications of hardware and processing parameters.
Yet its toughness also makes PET a bear to trim. When PVC, HIPS, and OPS are being die-cut, penetration of the die through 50% of the thickness is generally enough to get a clean, particulate-free break. In contrast, dies have to slice through virtually 100% of PET thickness for a clean fracture to occur (Fig. 1).
PET’s trimming behavior has adverse consequences, since greater force and strain are imposed on die-cutting equipment. That means die damage and wear rates are typically higher than for other materials. And trim cycles are usually longer for PET.
PET trimming should be assigned to well-trained workers. Since dies must evenly contact the cutting or striker plate to ensure a clean PET cut and avoid damage, accurate “make-ready” of the die to cutting plate is critical. Disciplined make-ready procedures are critical to efficient PET trimming.
Improper trimming generates particulates in the form of fuzz, angel hair, and dust, which can contaminate trim scrap and degrade final part quality. Imperfect trimming also often creates edge defects, so rims or perimeters of parts are impaired, leading to a higher reject rate.
Systematically addressing PET trimming issues is thus critical. This promising material will maintain momentum in thermoformed packaging markets only if users select appropriate die-cutting methods and implement proper trimming procedures.
Selecting a trim method
The three most commonly used methods for trimming PET involve using steel-rule dies, forged dies, and matched-metal dies. The first two methods involve “kiss-cut” or “clamp-cut” action—i.e., forcing the die through the sheet to the precise point where the die tip touches the cutting plate. This plate is usually heated for PET, whereas PVC seldom requires a heated plate. Kiss-cutting generally is best suited for low- to intermediate-volume programs.
Steel-rule trimming uses a sharp knife that is usually mounted on a laminated wood die board. Cutting is typically done in place (in the mold) or in line. This method generally requires modest skill, is low cost, and best suits lower-volume applications. A central- or double-bevel steel die with 50 to 55 C Rockwell hardness is recommended. With thin sheet (under 0.01 in.) or complex parts, a die with only 45-50 C hardness will reduce the chances of die breakage.
Another op tion in kiss-cutting is the forged-steel die. This generally works as a cut-in-place method using a heated base plate, and functions best for intermediate-volume uses. Forged metal dies are typically used in the “heat-assisted” die-cutting mode—i.e., the die initially contacts the PET sheet at low pressure and waits while heat is conducted from base plate to the cutting point. Heat softens the sheet and makes cutting easier. After a short delay, a high-pressure cut completes the trim. It is critical to keep the base plate temperature just below the roughly 160 F at which PET sheet will adhere to the plate.
The preferred material for cutting plates is 58 C Rockwell (or higher) tool steel (aluminum has a shorter life span and tends to splinter). The hardness of the cutting plate should always exceed that of the die, since this minimizes plate damage and the dulling of die bevels. Insulation of the base plate is advised to improve control over heat levels (i.e., avoid PET adhesion to the plate) and optimize energy use.
Ideally, the die force for kiss-cutting would push the die consistently and fully through the part, but not beyond it. Excess force bends or rapidly dulls the cutting edge of the die. A common rule of thumb is to use 400 lb of force per lineal inch of die length.
It is important to compensate for part shrinkage that occurs between forming and trimming, even though exact measurement of shrinkage is difficult. A common practice is to assume 0.3% to 0.6% shrinkage (or 0.005 in./in.) in PET sheet.
The third alternative in die-cutting formed PET parts is the matched-metal die (MMD) method, also called punch-and-die cutting. With its scissors-type cutting action, MMD is often the surest and fastest way to trim PET parts. But it requires a separate cutting press, through which the continuously formed sheet passes. This is far more capital-intensive than the other trimming methods, making it best suited for higher-volume applications.
The favored approach in MMD is to combine a harder punch (i.e., 55 Rockwell C) with a softer (43 C) die. The critical need is to maintain a minimum die clearance at all times. That involves a process called peening, which allows recovery of minimal clearance by spreading the edge of the die back to its original size using an air-operated hammer. MMD cutting is sometimes done using a punch and die of equal (62 C) hardness, with the minimal clearance maintained via continual resurfacing of the die and punch.
Coining is a technique used to reduce the required cutting force and extend matched-metal die life. It occurs during forming and involves thinning the areas to be cut by up to 50% through designing ridges into the pressure box. Coining of parts takes place as the pressure box is clamped to the mold.
Other simple but easily overlooked precepts in PET trimming are to always keep dies sharp and to use properly guarded cutting devices to maximize die life. Effective trimming of PET parts is inevitably a trial-and-error process.
Larry Koester is v.p. of marketing and sales for the Lavergne Group in Ville d’Anjou, Que. The company compounds thermoplastics and extrudes virgin and recycle-content PET sheet. The author may be contacted at email@example.com.