Thermoforming of Recycled PET

 

Background. Thermoforming is the process of shaping thermoplastic sheet to the contours of a mold through the use of heat combined with pressure or vacuum.  The formed part is then trimmed to eliminate edges, decorated, and/or fabricated into an end-use product.  Each step of the forming process is vital to the quality of the finished part.

Sheet quality along with heating, forming, and cooling conditions must be closely monitored and controlled to obtain quality parts.  Quality parts require uniform wall thickness, desired color and surface appearance, and acceptable physical properties for the end-use.  Thermoforming can accommodate a wide range of sheet thicknesses from thin-gauge sheet (as low as ten thousands of an inch) used to form packaging containers to heavy-gauge sheet (greater than 60 thousands) used to make material handling trays and truck bed liners.

Sheet Specifications

When ordering recycled PET (RPET) sheet for the thermoforming process, it is best to specify minimum and maximum tolerances for sheet thickness. This is to ensure that the sheet will have the least number of variations possible, minimizing operator adjustments during the processing and resulting in uniform properties.

It is recommended that the gauge tolerances be ± 5%.  This gauge range offers the operator the best possible chance of establishing a continuously successful process and profiling of the sheet heating portion of the thermoforming process.

Thermoforming Process for RPET

The plastics manufacturing process known as thermoforming has five basic steps, each of which requires proper control and monitoring.  These five steps apply to all materials, but most specifically to RPET.

1.      Heating
2.      Forming (molding)
3.      Cooling
4.      Trimming
5.      Sealing (optional)

Best Practice for Sheet Heating: The process begins with heating the plastic sheet, which must be brought to the correct temperature before placing into the mold. In the case of RPET, the sheet should be heated to a point just below crystallization, typically 250°F-300°F.  The correct oven temperature is most often determined from process knowledge and operator experience with a given material, as well as various equipment designs.  An experienced operator can determine proper temperature by visually observing the state of the sheet as it passes through the heat tunnel or oven.

During sheet heating, the greater number of control zones available, the better the operator can control the process.  The control zones should be equal on the top and bottom of the heat tunnel as this allows for the heat to penetrate the sheet from both sides and provides for thorough heat penetration into the center of the material.

Best Practice for Forming:  The heated sheet is then shaped into product.  Knowledge of the timing and timing controls is essential to the manufacture of quality parts. The sequence of events in forming is as follows:

1.      Top platen down
2.   Bottom platen up
3.      Vacuum on
4.      Form pressure on
5.      Vacuum form pressure off
6.      Air blow off-on
7.      Bottom platen down
8.      Air blow off-on
9.      Top platen up

EVENT 1: Top Platen Down

This should occur as soon as the heated, indexed sheet comes to a stop.  The top platen should also be set to minimize travel distance and event time from start to stop at sheet line.  A distance of ½ to 1 inch above the formed part is a good rule of thumb for setting the stopping position of the platen.  (See Platen Stroke Adjustment)

EVENT 2: Bottom Platen Up

The bottom platen rises and arrives at the sheet line slightly after the top platen. This is accomplished by use of the flow control valves mounted on the hydraulic and/or pneumatic motion system lines.  Ensure that the platen drops down far enough to allow the indexing plastic to enter the form station without dragging on the clamp frame or molds mounted there.

 

EVENT 3: Vacuum On

In the case of RPET, pre-vacuum should be applied, when possible, just before the platens close.  This optimizes cycle time.  This is accomplished by utilizing the vacuum delay time relay.  By observing the process, adjustments may be made while running to ensure proper timing.

EVENT 4: Form Pressure On

Forming pressure is applied just after the vacuum event starts, and is accomplished by using the form pressure time delay.  Once started, the form pressure and the vacuum stay on until the cycle time is complete, which concludes the forming process.

EVENT 5: Vacuum/Form Pressure Off

This takes place automatically when the main cycle timer times out.

EVENT 6: Air Blow Off-On

The air blow off aids in releasing the formed part from the mold.  The air blow off is connected into the vacuum line so that when the vacuuming is complete, the same hose can be used for air blow off.  This event should be timed to start at the end of the pressure/vacuum cycle, and just before the molds begin to open.  Air blow off should continue until the molds begin to separate.

EVENT 7: Bottom Platen Down

The bottom platen will begin to lower just before the top platen starts to open.  This is necessary so that the clamp frames mounted on the mold will not put pressure on the finished product and cause damage.

EVENT 8: Air Blow Off-On

The air blow off continues only until the bottom mold has dropped just slightly below sheet line.  This assists in removing the parts from the molds.  In the case where the mold is of a female design, the blow off continues until the top platen is slightly above the sheet line.

EVENT 9: Top Platen Up

The top platen returns to the initial starting position and actuates the limit switch in preparation for the next sheet.     

In the form step, all events are related to the main cycle timer, and take place in relationship to the start of the cycle.

 

Best Practice for Platen Stroke Adjustment:  Several different methods are used to adjust platen stroke.

1.      Use a stopping block mounted between the fixed platen and the moving platen.  By lengthening the stop, the travel of the platen is shortened, thereby lengthening the stroke.

2.      Use built-in cylinder stroke adjusters.  In this case, the adjustment to the top platen is made with the platen in the down position and the hydraulic pump off.  Adjustments to the bottom platen should be made in the up position with the hydraulic pump off.

3.      In microprocess-controlled equipment, the strokes can be programmed.

4.      Use micro switches to change stroke lengths.

Best Practice for Cooling: Cooling does not mean that the mold must be cold at all times.  Cooling is a relationship between the sheet temperature and the temperature necessary to set the plastic into its new shape in the most desirable time frame.  At times it is necessary to use higher temperatures than usual.  For example, RPET needs to have a cooling water temperature at approximately 80^oF.

In most cases, it is recommended that the RPET sheet be cooled as quickly as possible to achieve the best results.  If high levels of crystallinity are desired in the finished product (e.g., thermally stable parts), hot mold temperatures (> 200^oF) are used.

Best Practice for Trimming: Trimming takes place during forming, but is independently controlled. The use of cold rolled steel for the striker plate is recommended.  The trim timer and the cut timer are the controls that regulate this step.  The total trim time should not exceed the total form time.  When setting the cut timer, the shortest time that achieves a cut with the least amount of pressure should be used.

In order to achieve the lowest pressure and the shortest cutting time, it is necessary for the setup operator to do an accurate “make-ready” cutting die per the following steps:

1.      Mount new cutting die and striker plate in the trim station, being careful not to damage the trim dies.

2.      Place a sheet of Kraft paper (or substitute), on the striker plate.  This paper should be cut to the same size as the striker plate and temporarily held in place by tape.

3.      Lower the cut pressure to the minimum the equipment allows, also set the cut timer at 0.5 seconds.

4.      In manual mode, close the trim press and activate the cut for the shortest time possible.  Open the press and check the paper for cut.  If there is no cut, increase the pressure of the press, gradually, until there appears to be approximately a 40% cut in the paper.

5.      Remove the paper from the trim press, and using either scotch tape or accu-shim, place tape over the portions of the paper that did not cut, putting more tape where the least amount of pressure is indicated.  This process raises the plate in the areas needed to achieve cut.

6.      Place the trimmed paper under the striker plate, being sure to locate it properly, and secure the plate.  Take another sheet of paper and place it on the striker plate, close the press, and again activate the cut pressure.

7.      Remove the second sheet of paper and check the cut.  At this point, the trim is almost complete.  Those areas not yet cut will be minimal, and should require little additional make-ready.

8.      Start forming and trimming sheets.  If the parts do not trim, increase the pressure until cutting is achieved.  When the cutting operation begins, continue to reduce the cut time until the shortest cycle time for adequate cutting is achieved.

At first, the make-ready cutting process may seem cumbersome, but with practice, this procedure should only take 20 to 30 minutes.

Best Practice for Cutting: The goal of cutting is the separation of material by the use of a sharp-edged device. Maintenance of the sharp edge and protection from damage is very important for efficient cutting.

The cutting process begins as the sharp edge first enters the material to be separated and starts to exert horizontal force against the material.  (See figure below). 

	Sheet
Sheet

Striker
Plate

 

As the edge further enters the material, greater horizontal force is built until the force (shown by arrows) generated parts the material. This should happen at the moment just as the sharp edge kisses the surface below the material being separated.  (See below).

Striker Plate	Sheet

 

When the sharp edge repeatedly strikes the surface with too much force the sharp edge is flattened.  Once this happens, you are no longer using a cutting process, but using a crushing process.  (See figure below).  When separating PET, constantly monitor the condition of the cutting edge and prevent flattening or dulling.  Unlike many materials, PET must be separated by a sharp edge.  When sufficient pressure is applied to other materials they will part, but this is crushing, not cutting.  

Striker Plate

Sheet

Best Practice for Sealing RPET: There are four methods utilized to seal RPET – heat, sonic, radio frequency and UV.  All have different degrees of success, but the most successful of these is sonic sealing. 

Each method has several variables that are operator controlled and require certain experience-based knowledge.  The following recommendations give an operator a baseline from which to establish the operating parameters for each process.

SONIC SEALING.  The sonic sealing process involves the use of high frequency sound waves to generate heat in the RPET material by creating molecular motion.  The heat then causes the two plastic surfaces to bond.  This process has been very successful when used in clamshell sealing.

There are only two variables that affect this process – time and pressure.  The gauge of the sheet determines time.  The thinner the material, the shorter the time required to create a seal.  Generally 0.5 to 2.0 seconds should accomplish any sealing. 

Pressure is most affected by the size of the sealing area under the sealing head.  The larger the area, the more pneumatic pressure required to create the sealing condition.  A pressure between 20 and 40 psi should be sufficient for sealing.  As in all manufacturing processes, the operator must be able to adjust to existing conditions.

HEAT SEALING.  Heat sealing involves the adhering of a plastic blister to a coated card.  This is one of the most common methods for packaging products.  When using this method with RPET, there are some important conditions to be aware of:

1.      It is important that the blister be as stress free as possible.  Any stress in the blister will result in molecular motion that will prevent the adhesive on the card from attaching to the RPET blister.  Stress in the blister also can result in a deformation in the plastic.

2.      There are three physical/mechanical events intrinsic to heat sealing.  They are heat, pressure, and time.  These variables are subject to change based on prevailing conditions, such as card thickness, plastic thickness, adhesive, and size of area to be sealed.  Common starting conditions can be as follows: heat: 350^oF-400^oF; pressure: 40-60 psi; time: 1.5-3.0 seconds.  These conditions are only starting points and are not to be taken as absolutes.

RADIO FREQUENCY SEALING.  Radio frequency (RF) sealing commonly is used to weld the surfaces of clear clamshell packaging.  RPET is difficult to RF seal because of its tendency to crystallize and must be done with precise control of operating conditions.  When using RPET packaging material, the following guidelines are recommended. These are only starting points and will vary with different equipment.

PREHEAT:  The temperature of the platen range is from 225^oF-250^oF for 0.5-1.0 seconds

SEALING:  Sealing time is 2.0 to 2.5 seconds, divided between the two stages.  Low power sealing should last 0.5 to 1.0 seconds, and high power sealing should last 1.0 to 2.0 seconds.