Thermoforming and Related Polymer Processing – Lecture Review

These flashcards cover material selection issues, thermoforming fundamentals, heating technology, process variants, design ratios, advantages/limits, and comparison with other polymer processes.

¿Por qué se rompió el filamento al intentar peletizar material de grado de inyección?

Los polímeros de grado de inyección tienen baja resistencia a la fusión, por lo que la hebra fundida no puede estirarse sin romperse.

What key lesson does the filament-break example illustrate about material selection?

You must know the intended processing method (injection, blow, extrusion, etc.) before choosing a polymer grade.

In economies with small production volumes, why is thermoforming attractive?

It allows economically viable short runs because molds are inexpensive, pressures are low, and cycle times are shorter than many alternatives.

Basic sequence of a simple vacuum-forming thermoforming cycle?

1) Clamp sheet, 2) Heat until soft, 3) Apply vacuum to pull sheet onto mold, 4) Cool, 5) Release and trim.

What material property is crucial for a sheet to stretch without tearing in thermoforming?

High melt strength (also called melt strength or extensional viscosity).

Give two typical consumer products made by simple thermoforming.

Disposable drinking cups and margarine tubs (tarrinas).

Main advantages of thermoforming compared with injection molding (name three).

Lower tooling cost, lower clamp pressure, capability for very large parts, and relatively short cycles for shallow parts.

Main limitations of thermoforming (name three).

Sheet supply requirement, scrap skeleton that must be re-ground, thickness variation, difficulty in sharp corners or deep ribs.

Why is IR heater wavelength around 3.2 µm commonly chosen for heating polyolefin sheets?

At ≈3.2 µm the radiation energy matches the C–H bond absorption of most plastics, maximizing heating efficiency.

List four common electric heater types used in thermoforming ovens.

NiCr wire (open coil), Calrod, ceramic elements, quartz tubes.

Gas-fired ovens: main benefit and main drawback?

Benefit: very low operating cost; Drawback: flame safety issues and less precise uniformity.

Define ‘sag’ in thermoforming pre-heating.

The downward displacement of a softened sheet under its own weight, used as a visual indicator of readiness.

Why must sag be limited (not overheated) before forming?

Excess sag thins the center, causing non-uniform wall and risk of rupture during forming.

Vertical oven vs. horizontal oven: one advantage of the vertical arrangement.

It allows more sheets to be heated simultaneously in the same floor area.

Difference in heating behavior between amorphous (e.g., HIPS) and semi-crystalline (e.g., PP) sheets?

Amorphous sheets soften gradually over a wide window; semi-crystalline sheets drop viscosity abruptly near melting, giving a narrow processing window.

What is the Draw Ratio (DR) in thermoforming?

Depth of the part (H) divided by the smaller base dimension (Wmin); DR = H / Wmin.

What is the Area Ratio (AR) in thermoforming calculations?

Surface area of the formed part divided by the original sheet area captured by the mold.

How is final wall thickness estimated from AR?

tfinal ≈ tinitial / AR, assuming uniform stretching.

Typical DR limit for simple vacuum-forming without assist?

About 1.5:1 to 2:1; deeper parts require assist methods.

Name four enhanced thermoforming methods used for higher DR or better detail.

Plug-assist forming, snap-back forming, billow (bubble) forming, and pressure forming.

Key advantage of plug-assist forming over pure vacuum forming.

Plug mechanically stretches the sheet, giving better thickness distribution and finer detail.

What happens first in snap-back forming?

The heated sheet is pulled into a cavity (or bell) by vacuum/air to pre-stretch, then pushed onto the mold for final forming.

Billow (bubble) forming: basic principle?

Inflate the hot sheet into a bubble above the mold, then apply vacuum to pull it down over cavities, achieving very high draw ratios.

Pressure forming vs. vacuum forming: main difference.

After the sheet contacts the mold, compressed air (up to ~6 bar) is applied on the back side to force the sheet into fine detail.

Give two examples of large, high-value parts commonly thermoformed for transport industries.

Aircraft interior panels and automotive door/roof liners.

Why are corners and deep ribs difficult in thermoforming design?

Material thins excessively as it stretches around sharp features, risking holes or weakness.

Explain residual stress concern in thermoformed parts.

Rapid surface cooling can lock in stresses; if part is later loaded or reheated it may warp or crack.

Compare thermoforming and injection molding in three quick points (speed, quality, cost).

Injection: very high speed, high surface quality, high tooling cost. Thermoforming: variable speed, moderate quality, low tooling cost.

What simple lab method was shown to map thickness distribution after forming?

Print a grid of numbers/letters on the sheet, form the part, then visually measure grid distortion and use a thickness gauge.

For disposable egg trays, why is thermoforming preferred over injection molding?

Low tooling cost and ability to use thin, inexpensive sheet make thermoforming economical for low-strength packaging.