Thin Sheet Thermoforming

Lesson 1 - Thin Sheet Thermoforming Study Guide

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Thin Sheet ThermoformingLesson 1

Introduction, Machines and Materials

I. IntroductionParts made by thermoforming:

• Drinking cups

• Food containers

• Blister packs

• Meat trays

A. Process is called: Thin-walled or Roll-fed thermoforming

1. Roll fed thermoforming for food products and packaging is often less than 10 thousandths of an inch, .25 mm.

2. We will use an upper limit of sheet that can be stored on a roll as 80 thou-sandths of an inch or 2 mm thick.

3. Cut-sheet or Heavy-gauge sheet

a. sheet that is too stiff to be rolled

b. cut into specified lengths

II. Thermoforming Processing StepsThe plastic is:

1. Heated

2. Formed

3. Cooled

4. Trimmed


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III. Types of Thermoforming MachinesA. Single Station Shuttle Machine

Single Station Shuttle Machine

This machine has one station where the sheet is clamped into a frame. The sheet then moves to the oven where it’s heated. Then the sheet shuttles back over the mold where a vacuum or pressure pulls the sheet down onto the surface of the mold to form the part. The part is cooled by the mold and then removed and the machine is ready for the next cycle.

B. In–line Roll Fed Thermoforming Machine

In-line Roll Fed Thermoforming Machine

A roll-fed sheet first goes to a heating station, also called the heating tunnel, where multiple banks of heaters, heat the plastic in stages. The sheet moves or indexes to the forming station where the parts are formed and cooled. After forming, the parts are trimmed, usually at a separate trim station where they are cut out of the plastic sheet. The part of the plastic sheet that was not used, called the web or the skeletal web, is recycled. The movement of the sheet through this machine is intermittent - the sheet must move or index, then stop so the heated sheet can be formed in the mold. The length of each index is determined by the length of mold.


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C. Thin Gage Rotary Thermoforming Machine

Thin Gage Rotary Thermoforming Machine

Hot sheet directly from an extruder die feeds molds on a rotating wheel. The sheet is formed by a vacuum and cooled. The parts are then released from the molds and are carried to a matched metal trim press where they are trimmed and stacked. The web is usually fed directly to a grinder for reprocessing.

IV. Sheet Extrusion

Parts of the Extruder

The plastic pellets are first fed into the hopper. A screw, much like an auger, ro-tates and conveys the granules through a heated barrel where the plastic is melt-ed and its pressure is increased. The plastic then flows into the sheet die where the plastic spreads and flows out the die lips. The plastic then goes to the roll stack that consists of a set of steel cooling rolls. The first two rolls set the sheet thickness and begin to cool the plastic. The sheet then winds around any remain-ing chill rolls for additional cooling.

From the roll stack the sheet travels over support rollers to give the plastic additional time to cool. Then the sheet is pulled by take-away rolls, past a trim station then on to the wind up station. The roll of sheet is now the raw material for thermoforming.


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A. Sheet Quality

1. Start with quality plastic sheet

a. The sheet must have the correct thickness or gauge

b. The gauge must not vary

c. The internal stress within the sheet must be within acceptable limits and consistent

V. What Are Plastics? The three most widely used plastics for roll fed thermoforming:

• Polyethylene

• Polypropylene

• Polyethylene terephthalate, usually called PET.

Other plastics used in thermoforming:

• ABS

• PVC

• Polystyrene

• Acrylic

• Polycarbonate

A. The Structure of Plastics

1. All plastics are made from short molecules called monomers

2. Polymer molecules are long molecules made from monomers

3. Polymerization - the process of making long molecules (polymers) out of short molecules (monomers)


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4. Size of polymer molecules

a. A typical polymer molecule length might only be 1/100,000 of an inch long. (about 1/100th the thickness of a human hair)

5. Thermoplastic vs. Thermoset

a. Thermoplastic - Molecules remain un-attached to each other when they are heated

• they can be heated and re-melted

b. Thermoset - Molecules form cross-links or chemical bonds between each other when they are heated

• they cannot be re-melted

B. Additives

1. Additives are chemicals put into the plastic to improve specific properties or to protect the plastic from damage due to the environment where they will be used

2. Common additives:

a. Heat Stabilizers - reduce damage to the polymer caused by excessive heat during processing

b. Anti-oxidants - reduce oxygen attack on the plastic which would de-grade the properties of the plastic

c. UV stabilizers - increase the resistance to ultraviolet light

d. Colorants - change the color of the plastic, and there are many more additives used for specific purposes

Polymerization

Monomers Polymers


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VI. Plastic Properties A. Thermal Conductivity - the ability of a material to transfer heat

1. Plastics have a low thermal conductivity

• 1/100th that of steel

• takes more time to both heat and cool the plastic

• large temperature differences within the plastic as you try to heat or cool it

B. Thermal Degradation - the loss of desirable properties caused by overheating

1. Occurs if some of the molecules become broken or if some undesirable chemical change occurs to the molecules.

C. Molecular Orientation - the lining up of polymer molecules in the same direction

1. The plastic becomes stronger in the aligned direction but weaker perpendicu-lar to the alignment.

NOTE: Anisotropy – The properties of a material vary when measured in dif-ferent directions.

D. Polymer Flow

When polymer molecules flow through a sheet die, the molecules nearest the die walls flow slower than those further in, as shown by the arrows on this graph. Any differences in the flow speed of molecules that are next to each other force them to become aligned in the flow direction. This alignment of molecules caused by flow is the cause of molecular orientation. In the sheet die the highest molec-ular orientation develops near the die walls where the flow speed differences are the greatest.

Polymer Flow


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NOTE: In thin sheet, molecular orientation may extend completely through the cross-section.

E. Viscosity

1. Viscosity - a fluid’s resistance to flow

a. High viscosity - the fluid resists being made to flow

• example - a heavy motor oil

b. Low viscosity - the fluid flows easily

• example - water

NOTE: In the extrusion process, the viscosity of the raw material is an import-ant property of the plastic. If the viscosity changes, it affects the prop-erties of the extruded sheet.

F. Melt Flow Index

1. MFI test - used primarily to determine if the viscosity of the plastic raw material has changed

a. In this test, a small amount of the plastic is heated in the test instrument. Then a weighted plunger forces a specific volume of melted plastic out a small orifice. The length of time it takes is measured. Then a calcu-lation determines how many grams of plastic would have flowed in 10 minutes. The result is called its melt flow index or MFI number.

• higher MFI number (the plastic flows more easily) = lower viscos-ity

• lower MFI number (the plastic does not flow easily) = higher vis-cosity

b. Change in the MFI number

• indicates a change in the viscosity of the plastic

• possible change in the extruded sheet properties

NOTE: When the MFI test is used in either an extrusion plant or a thermoform-ing plant, upper and lower acceptance limits of MFI for each plastic must be specified


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G. Stress Relaxation Test

Internal stress and molecular orientation are inherent properties of extruded sheet. Thermoforming companies should have control limits on the amount they will accept.

1. Stress relaxation test steps

a. cut squares of plastic from the incoming sheet and mark the extrusion direction

b. put a square in an oven at a temperature that will cause the oriented molecules to relax

• the temperature and time in the oven depends on the type and thickness of the plastic

c. remove the square and allow to cool

d. measure the dimensions

Notice the shrinkage of the heated square on the left. Stress relaxation caused 15% shrinkage in the extrusion direction and almost no shrinkage in the trans-verse direction. This tells you that the plastic sheet has too much stress caused by molecular orientation. If the shrinkage is more than 5% in the extrusion direc-tion, you may have difficulty maintaining part dimensions.

Stress Relaxation Test

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Thin Sheet Thermoforming