Thermoforming and Material Selection: PS, PP, and PET

A small lidded cup used for sauces, samples, condiments, dressings, and measured servings.

Portion cups are small, single‑use containers designed to hold measured amounts of food or nonfood products for retail, foodservice, sampling or portion control. They are most commonly manufactured by thermoforming polymer sheet into shallow or deep cavities, then trimmed and optionally lidded or heat‑sealed. Choice of material and process parameters directly affect clarity, stiffness, barrier performance, temperature tolerance and suitability for microwave or freezer use.

Thermoforming process — a technical breakdown

The thermoforming workflow for portion cups typically consists of these stages:

• Sheet production: Polymer sheet is produced by extrusion (single or coextrusion for multi‑layer constructions) to the required gauge and optionally surface‑treated for printing or sealing.
• Heating: Sheets are conveyed through an infrared oven to bring them to a material‑specific forming temperature. Uniform heating is critical to avoid thin spots, tears, or inconsistent wall thickness.
• Forming: The heated sheet is formed into a mold cavity. Common forming methods for portion cups include vacuum forming, plug‑assist forming and pressure forming. Plug assist is frequently used for deeper draws to control material distribution and wall thickness.
• Cooling and stabilization: The formed part is cooled in the mold to set dimensions and crystallinity (if applicable). Cooling rate influences shrinkage and final stiffness.
• Trimming and finishing: Parts are die‑cut or trimmed (mechanical or CNC) to size. Secondary operations may include printing, lidding, sealing, or assembly into multi‑pack formats.

Material behavior during thermoforming

Different polymers require different oven setpoints, draw techniques and mold design due to their thermal and mechanical properties. Important parameters include forming temperature window, melt/crystallization behavior, melt strength, and tendency to sag or thin during draw.

Comparing HIPS, PP and PET for portion cups

• High Impact Polystyrene (HIPS)
• Properties: HIPS offers excellent stiffness, good surface gloss and attractive clarity for low‑cost applications. It is easy to thermoform and accepts printing well. Impact modifiers improve toughness versus general polystyrene.
• Thermal stability: Tg near 95–105°C; heat tolerance is moderate — not recommended for prolonged exposure to hot fillings or microwaving. Freezer use is possible but it becomes more brittle at very low temperatures.
• Barrier & clarity: Decent optical clarity and gloss, but relatively poor oxygen and moisture barrier compared with PET. Not ideal for long shelf‑life oxygen‑sensitive products.
• Typical uses: Cold condiments, ketchup or single‑serve dressings where cost and clarity are primary considerations.
• Polypropylene (PP)
• Properties: PP is semi‑crystalline, chemically resistant (good with oils and fats) and has higher heat resistance than HIPS. It is lightweight, can be made opaque, translucent or clarified for better clarity, and is widely used for microwaveable containers.
• Thermal stability: Melting ~160–170°C; good short‑time heat tolerance and microwaveability when formulated appropriately. Performs well at low temperatures (common freezer compatibility). PP also shows good dimensional stability under repeated thermal cycles if wall thickness and design are correct.
• Barrier & clarity: Moderate moisture barrier and poor oxygen barrier compared with PET. Clarity is lower than PET unless special clarifying additives or copolymers are used.
• Typical uses: Microwaveable single‑serves, hot sauces, soups or items requiring freezer→microwave transitions; also used where grease resistance is important.
• Polyethylene Terephthalate (PET)
• Properties: PET provides outstanding clarity, gloss and surface definition, plus superior oxygen and aroma barrier among these three. It is dimensionally stable for cold applications and displays good mechanical strength.
• Thermal stability: Amorphous PET has a Tg ~70–80°C; crystalline PET has higher thermal resistance but is more opaque. Standard PET is generally suited to cold or ambient products; reheatable or microwaveable PET grades exist but require careful selection and process control.
• Barrier & clarity: Best oxygen and CO2 barrier of the three, excellent clarity for consumer visibility. PET is also widely recyclable (PET #1).
• Typical uses: Chilled condiments, dressings, single‑serve beverages, and applications where product visibility and shelf life are priorities.

Choosing material based on product viscosity

Viscosity of the contents influences sealing, lid selection and wall thickness design rather than material alone, but some material attributes are relevant:

• Low‑viscosity liquids (e.g., vinegar, thin sauces): Require reliable seal integrity to avoid leakage. PET’s superior dimensional stability and sealing compatibility with appropriate lidding films is an advantage. Ensure lid sealer profiles match cup material (e.g., PET film to PET cup or compatible sealant layers).
• Medium viscosity (e.g., ketchup, creamy dressings): PP is favored where hot‑fill or microwave compatibility is desired; HIPS is commonly used for cold servings due to cost, provided sealing is robust.
• High viscosity (e.g., mayonnaise, thick dips): Chemical resistance to oils/fats favors PP. Wall thickness and venting design should prevent product being forced out during lidding.

Microwave and freezer compatibility guidance

• Microwave use: PP is the default choice for microwaveable portion cups. Use food‑grade PP with confirmed microwave performance; validate for steam venting, lid design and short cycle heating. PET is generally not microwave safe unless manufactured to a high‑temperature grade or specially treated; HIPS should not be used for microwaving.
• Freezer use: PP and PET both perform well at low temperatures. HIPS can become brittle and crack if dropped or flexed after freezing. Consider dimensional changes from crystalline transitions—plan for tightness of lid seals at low temperatures.

Practical selection checklist

• Define product temperature profile: refrigerated, frozen, hot or microwave reheated.
• Assess barrier needs: oxygen or aroma sensitivity pushes toward PET or multi‑layer constructions (e.g., EVOH barrier layers in coextruded sheet).
• Consider chemical compatibility: high‑fat or solvent formulations favor PP.
• Decide clarity requirement: consumer visibility favors PET or HIPS; PP can be clarified but typically less brilliant.
• Choose forming approach: deep draws or high draw ratios may require plug‑assist or specialized grades—PP and HIPS form easily; PET often requires precise preheating and plug assistance.
• Verify sealing/lidding compatibility and perform line trials with actual product to confirm leak, peel and storage performance.

Common pitfalls to avoid

• Assuming PET is microwave safe without verifying grade and cycle parameters.
• Over‑reducing wall thickness to save material, which can cause deformation, leaks or poor stacking.
• Choosing material solely on cost without testing with the actual product viscosity, salt/fat content and temperature cycles.
• Ignoring recyclability/regulatory considerations for intended markets.

Real‑world examples

Cold ketchup or salad dressing served in premium clear cups: often PET for clarity and shelf appearance. Takeout hot soup sides intended to be reheated by the consumer: PP portion cups with vented lids. Low‑cost condiment cups included with fast food: HIPS for economy and acceptable cold performance.

In summary, portion cup material selection balances forming behavior, temperature tolerance, barrier needs and product chemistry. For microwaveability and grease resistance choose PP; for highest clarity and oxygen barrier choose PET (or PET‑based multilayer); for low‑cost, cold applications HIPS remains a common choice. Always validate selected material and forming parameters with production trials using the actual product and expected thermal cycles.