Chapter 15 Thermoforming ETPL 1100

Introdution Thermoforming is the process where plastic materials are shaped from softened sheet The process involves Heating the plastic sheet to a temperature range where it softens Then stretching the softened plastic against a cold surface mold When the sheet has cooled, it is removed from the mold and excess plastic is trimmed

Introduction Thermoforming is a gemeric term for a group of processes Vacuum forming Drape forming Billow forming Mechanical bending Match-Mold forming Pressure forming

Introduction Thermoforming is one of the oldest methods of forming useful articles

Introduction Markets Thermoformed products are categorized as Permanent( Industrial) Disposable

Introduction Typical Industrial Products Equipment cabinets Tote bins Pallets Trays Head liners Shelves

Introduction Typical disposable products Point of purchase containers Hand and power tool cases Meat and poultry trays Egg cartons

Introduction Major growth area for thermoformed products Multi layer containers Returnable containers Modified atmospheric packages Medical device equipment

Introduction Terminology Thermoforming processes are divided by thickness or gauge of the sheet used Thin gauge for sheet less than .06in (1.5mm) Heavy gauge for sheet greater than 0.12in. (3mm) “gray area” between the thickness, depends on whether the material can be rolled or comes in sheets

Introduction Competition to thermoforming Blow Molding Rotational Molding Injection molding

Introduction Advantages of thermoforming Low temperature, low pressure required Only a single surface mold is required Molds are easy to fabricate and use inexpensive materials No need for the plastic to flow Can make very large surface area to thickness ratios

Introduction Disadvantages to thermoforming Plastic material is more expensive because the pellets have to be made into sheets Generally more waste to reprocess One sided Can get a great deal of wall thickness variation

Plastics and Polymers

Plastics and Polymers Thermoforming uses plastic sheet, which is heated, stretched, cooled and mechanically cut The plastic sheet is manipulated as a rubbery solid or elastic liquid The solid or elastic liquid properties are more important than the viscous properties when thermoforming

Plastics and Polymers You can thermoform both amorphous and crystalline polymers Amorphous No organization, glass transition PS, ABS, PVC, PMMA, PC Crystalline Organized region called crystals, glass transition and melting PE, PP, Nylon, Acetal

Plastics and Polymers Thermoforming Window Temperature range over which the polymer is sufficiently subtle or deformable for stretching and shaping Typically amorphous have broader window than crystalline PS – 260 to 360 F PP – few degrees below melting point of 330F

Plastics and Polymers Important thermal properties Enthalpy or heat capacity Thermal conductivity Temperature dependent density

Plastics and Polymers Heat Capacity or specific heat Measure of the amount of energy required to elevate the polymer temperature One measure of energy uptake is enthalpy It increase with increasing temperature When a material goes through a phase change, such as melting, the enthalpy-temperature curve changes dramatically When a material goes the glass transition, the enthalpy-temperature curve changes very little

Plastics and Polymers Heat Capacity or specific heat It takes twice as much energy to heat heat a crystalline polymer from room temperature to above its melt temperature than to heat an amorphous polymer the same

Plastics and Polymers Heat Capacity or specific heat In general enthalpy changes can be calculated as: M is mass Cp is heat capacity T is temperature

Plastics and Polymers Heat Capacity or specific heat If going through a melting transition ∆H gives the energy required

Plastics and Polymers Thermal conductivity The measure of energy transmission through a material Plastics have substantially lower rates than metals Aluminum has 1000 times greater than PS Th rate of heat transfer to the sheet is very important because it determines the time the process takes

Plastics and Polymers Density Mass per volume It decreases with increasing temperature Small change at glass transition, large change at melting temperature At forming temperature amorphous has 10-15% less density than at room temperature At forming temperature crystalline has 25% less density than at room temperature Shrinkage must be accounted for to meet final specifications

Plastics and Polymers Thermal diffusivity Combination of polymer heat properties Thermal conductivity/density/heat capacity Fundamental polymer property in time dependent heat transfer to materials

Plastics and Polymers Infrared energy absorption Most commercial thermoforming heaters emit energy in far infrared wavelength Thermoforming is concerned most with the wavelength range 2.5 to 15 microns Very important when heating sheet

Plastics and Polymers Thermoformable polymers Polystyrene is by far the most widely used Other PS family HIPS ABS SAN OPS – orientated PS PVC PMMA

Plastics and Polymers Thermoformable polymers Cellulosics PC PET PE PP If a polymer can be made into a sheet it can be thermoformed

General Forming Concepts

General Forming Concepts The simplest thermoforming process consists of simply heating the sheet and forcing it against a solid shaped mold There are many variations of this simple method

General Forming Concepts Simple heating and stretching The basic thermoforming process is of differential stretching Only the sheet that is not touching the mold stretches As stretching continues, the sheet becomes thinner and thinner Area last formed are the thinnest, most orientated and weakest Final part has very non-uniform wall thickness

General Forming Concepts Drape forming Earliest method The sheet is heated and then manually shaped over the mold Uses a male or positive mold Yields a part that is thinner along its side walls, rim and corners than at the bottom Used to make heavy gauge products such as signs and refrigerator liners

General Forming Concepts Vacuum forming The sheet is heated and stretched into a female mold Requires a vacuum system The part is thinner in the bottom and corners than at the top Used to make heavy gauge products such as signs and thin gauge product such as picnic plates

General Forming Concepts Free forming Billow or free bubble forming Does not use a mold The sheet is heated to its forming temperature, then air pressure is applied against the sheet, and the sheet expands As the bubble expands it touches a shut off device for the air, controlling the final size of the bubble

General Forming Concepts Free forming Because the bubble never touches a solid surface, it remains mar free The bubble is uniform in thickness Transparent polymers are most used Heavy gauge, free fromed shapes are used as skylights and aircraft windows Thin gauge, free fromed shapes are used in blister packaging

General Forming Concepts Assisted forming Used to improve the wall thickness uniformity for deep draw parts Three types Non uniform heating Pneumatic preforming Plug assist

General Forming Concepts Non uniform heating For heavy gauge froming its produces a sheet that is hotter in certain ares than others Hotter sheet stretches more that cooler sheet Regions of the sheet that would normally be over thinned are not heated as much as regions that would normally be over thick

General Forming Concepts Pneumatic preforming The heated sheet is first inflated with air A mold is plunged into the inflated sheet Vacuum is applied to the mold to ensure the sheet replicates the mold surface

General Forming Concepts Plug assist Plugs are mechanically driven, shaped solid structures that are pressed into the softened sheet prior to forming Used to locally stretch a sheet Used for heavy gauge products such as tote bins and equipment cabinets Used for thin gauge products such as drink cups

General Forming Concepts Pressure Forming When the pressure difference across the sheet exceeds 1 atmosphere, 14.7 psi or 0.1MPa Pressure forming uses air pressure up to 10 atmospheres on the free side of the sheet and vacuum on the sheet surface closest to the mold Pressure formed parts have surface texture and radii that rival injection molded parts

General Forming Concepts Pressure Forming Used in thin gauged forming to improve cooling cycle times by rapidly stripping the sheet from the plug and driving it against the cold mold Heavy gauge pressure forming is used with stiffer filled polymers

General Forming Concepts Twin sheet forming Competes with blow molding Two halves of the finished product are made then glued together

General Forming Concepts Contact forming The thin sheet is brought into contact with a heated, non stick metal surface, then quickly transferred to a mold for forming Its not economical to heat very thin plastics by non contact means

General Forming Concepts Diaphragm forming The diaphragm is usually a high temperature rubber such as neoprene The sheet is clamped against the diaphragm and heated The diaphragm is for support with plastics of low forming strength The diaphragm and hot sheet are deformed against the mold surface Part wall thickness is very unifrom

General Forming Concepts Mechanical forming For simple bends, strip heaters are placed against the sheet held in a bending structure When the local area of the sheet is hot, the bending fixture is activated The sheet is allowed to cool in its final bent form Care must be taken to ensure that the plastic is sufficiently heated, otherwise some spring back occurs.

Machinery for the Forming Process

Machinery for the Forming Process Currently the machinery market is $500 million Most machines are homemade or substantially modified from original design Low temperature, pressure process Important things in commercially manufactured machines is to produce quality parts with high productivity, high reliability and low maintenance costs

Machinery for the Forming Process We will look at 5 major systems Thin gauge, roll fed machines Heavy gauge sheet forming Thin gauge form, fill and seal Extrusion/forming line Matched mold forming machines

Machinery for the Forming Process Several defining characteristics shoul be considered when choosing a roll fed machine Platen dimensions Maximum depth of draw Nature of the forming process Type of power drive for the platen and sheet indexing Type of heater and controls

Machinery for the Forming Process Sheet take off The incoming sheet is delivered as a roll The roll stand should be capable of handling 2000 lbs and 6 foot diameter rolls The roll stand must have passive braking Rapid roll changer is an optional feature

Machinery for the Forming Process Pin chain and pin chain rail Typically the thin gauge sheet is advanced through the machine on pins spaced along the length of continuous bike-like chain Self lubricating chains have advantages, but chains without lubricants are required for medical and food applications Servo driven chain advancement is usually standard and offers smooth and constant sheet acceleration and deceleration rates and constant sheet speed during advancement

Machinery for the Forming Process Oven It is necessary to provide as uniform energy input to the sheet as possible Many machine have modular ovens to allow more than one “shot” to reside in the oven The oven can have more than one section for hard to heat polymers and foam APET,PVC and PP have 3 oven sections Low density foams have 5 sections Most ovens have opening for infrared temperature measurement

Machinery for the Forming Process Press Once the sheet is hot, it must be quickly pressed against the mold surface The press has many functions It must close smoothly and rapidly against the sheet, without banging the mold It must retain alignment for the platen, mold and any ancillary devices

Machinery for the Forming Process Forming assist devices Plug assist and pressure forming are common for deep draw parts such as drink cups Plug assist features should include the capabilities for Rapid replacement of individual plugs Internal heating and cooling aluminum plugs Relatively easy adjustment of the plug assist platen Easy adjustment of plug travel and rate of travel

Machinery for the Forming Process Trim means With inline trimming, the sheet with the formed parts still attached to the web is conveyed away from the thermoforming machine to a separate inline trimming press With in machine trimming, the sheet with the formed part is conveyed from the forming press to a separate in machine trimming station With in place trimming, the trim die is an integral part of the forming die

Machinery for the Forming Process In machine stacking Many machines include means to stack the product The parts are punch separated from the web using either mechanical piston linkage or rack and pinion servo devices

Machinery for the Forming Process Trim or web take up station The excess material needs to be ground up into pellets, eventually There are two general way of treating the web One is to cut the web directly from the machine and feed it directly to the grinder The other is to continuously wrap the web into a roll and send it off to be ground

Machinery for the Forming Process Condition monitor Sheet temperature should be measured using infrared thermocouples Mold and coolant temperature should be monitored Heater temperature should be closely monitored Air pressure and vacuum pressure should be monitored Photoelectric eye and alarms are recommended in the oven for excessive sheet sag and “out of sheet” indications

Machinery for the Forming Process Process control Most machines are equipped with PLC controllers and color monitors Most controls are based on time, how long the sheet is in the oven Most important are the automatic protocols for emergencies Fire Power outage Safety cage breech

Machinery for the Forming Process Heavy gauge sheet forming Two general types Shuttle presses Rotary presses Machine Criteria Platen dimensions Depth of draw General nature of the forming process Types of motive power for moving things Types of heater and max energy output

Machinery for the Forming Process Shuttle presses %70 of machines Very versatile and capable of forming parts of nearly unlimited dimensions Economically inefficient

Machinery for the Forming Process Shuttle presses Simplest thermoforming press The sheet is clamped in a four sided clamp frame The sheet and frame are moved into the oven When the sheet reaches its forming temperature, the sheet and clamp are moved to the forming station

Machinery for the Forming Process Shuttle presses The formed part is help against the mold until it cools The part with excess material is removed from the mold The formed part is then trimmed in a secondary trimming operation

Machinery for the Forming Process Shuttle presses In this simple process, no other sheet is formed while a sheet is heated. Only one thing happens at a time Two ways to overcome this inefficiency Double oven press – one sheet is formed while the other is heated Use a rail system similar to that used in a roll-fed system

Machinery for the Forming Process Rotary Press Very efficient Requires more care in set up Are limited in the size of the part that can be formed

Machinery for the Forming Process Rotary Press The heart of the rotary press is the carrousel that carries the sheet from station to station The stations are A load/unload station An oven station A forming press station A second oven station is added in a four station machine

Machinery for the Forming Process Rotary Press A four station machine is used when heating the sheet limits the process or when sequential twin sheet thermoforming is being done The sheet is clamped on all sides at the load/unload station It is then indexed into the oven The heated sheet is then indexed into the forming press The formed part is then rotated to the load/unload station, where it is removed and a new sheet is clamped in the frame

Machinery for the Forming Process Rotary Press – Sheet handling When sheet of standard dimensions is used or when production runs are long, the sheet is picked and placed mechanically Sheets are either lifted with vacuum suction cups or elevated with pneumatic lifting tables

Machinery for the Forming Process Rotary Press – Sheet Clamp Clamp pressure should be at least 50 psi The clamping area should be at least 0.5 in on .1 in thick sheet and at least 2 in on .4 in thick sheet The clamp frame and associated equipment should be able to withstand 800 F for 20 minutes The clamp frame should be easily adjustable for varying sheet dimensions

Machinery for the Forming Process Rotary Press – Oven The oven temperature should be step controlled Side baffles or oven side walls should close off the sheet and clamp during heating to minimize drafts Oven should be provided with ports for in oven infrared temperature measurement Provisions should be made for emergency shut down The spacing between the sheet and the top and bottom halves should be easily adjustable

Machinery for the Forming Process Rotary Press – Press Adequate press capacity is the most critical part of heavy gauge machine design The press should have sites for platen leveling, rapid platen alignment and capabilities for rapid mold change over

Machinery for the Forming Process Rotary Press – Pre-stretching means Pre inflation or pre-stretching of the heated sheet is common in heavy gauge thermoforming Two methods are used Bubble stretching Vacuum or draw box

Machinery for the Forming Process Bubble stretching The hot sheet is temporarily clamped over a lower mold element that acts as a blow box The sheet is then inflated with extent of inflation controlled by an electric eye Vacuum or draw box The sheet is drawn down by vacuum, with the extent controlled by a photo electric eye

Machinery for the Forming Process Vacuum and pressure Vacuum pumps should be capable of maintaining 28.5 in (735 mmHg or 35 torr) at their inlet Surge tanks should be capable of maintaining 25 in (635mmHg or 125 torr) at their inlet The surge tank volume should be 6 to 20 times the combined volumes of the deepest cavity

Machinery for the Forming Process Vacuum and pressure Pressure forming need air pressure of at least 100 psi Forming of multilayer, filled or reinforced sheet requires air pressure of 200 psi or more

Machinery for the Forming Process Thin gauge form and seal Many disposable, rigid, medical and food packages and certain point of purchase containers are produced with FFS equipment Thin gauge sheet, typically 10 to 20 mils, is continuously fed from a roll to the heating station For wide or heavy sheet, a pin and rail system is used

Machinery for the Forming Process Thin gauge form and seal Non contact infrared heater are used for heavier sheet, while direct contact heating is used for thin sheet The sheet is heated to temperature lower than traditional thermoforming temperatures Mechanical pressing is used rather than vacuum or pressure forming The formed containers, still connected to the web are fed beneath a filling station

Machinery for the Forming Process Thin gauge form and seal The containers are then filled with product Manually in simple cases Automatically in most cases After filling, the containers proceed to a sealing station May involve snapping a lid closed or heat sealing The containers are then trimmed from the web Then the containers are stacked and placed into cartons

Machinery for the Forming Process Extrusion/Forming line For dedicated thermoformed products, the extrusion process is often coupled with the thermoforming process Refrigerator door liners are often produced this way More complex process because it involves both extrusion and thermoforming

Machinery for the Forming Process Extrusion/Forming line Most line are found in the thin gauge forming operations for the production of drink cups and deli containers Care must be taken to match the output of the extruder to the through put of the thermoformer The thermoforming cycle dictates

Machinery for the Forming Process Match Mold forming machines Two types Low density foams because they are hard to heat and stretch Highly filled or reinforced polymers because they require high clamping and forming forces

Methods of Sheet Heating

Methods of Sheet Heating The first step in forming a plastic part involves heating the sheet too the proper forming temperature Heat transfer and the method of heating dominate thermoforming technology, because it’s the only way to produce a good part and is a major component of the cost.

Methods of Sheet Heating Concepts in Heat Transfer There are three basic methods of heat transfer Conduction Convection Radiation

Methods of Sheet Heating First we must understand how much energy is required to heat a unit mass of sheet from room temperature to the forming temperature Where Q is the total amount of heat needed

Methods of Sheet Heating Q, the heat needed to raise the temperature from room temperature to the forming temperature is attained through one of the methods

Methods of Sheet Heating

Methods of Sheet Heating

Methods of Sheet Heating

Methods of Sheet Heating

Methods of Sheet Heating