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The seven fundamentals of thermoforming.



The long-term consistency necessary for product quality and profitability can be achieved by following the seven operating fundamentals.

Long-term consistency throughout the thermoforming process is essential for profitability, competitiveness, quality, and ease of production. The achievement of consistency requires the contributions of the designer, the resin supplier, the sheet/film extruder, and the thermoformer. Everyone must help determine part specifications at the beginning, consulting all people necessary for a practical assessment of requirements.

It has been the author's experience-in forty years in the industry-that 72% of the complexities and variables in the thermoforming process lie in the manufacture of resin and sheet material, and the remaining 28% in the thermoforming process. This article details the seven fundamentals of successful thermoforming, beginning, appropriately, with determining resin and sheet specifications. Basic heat and pressure requirements are also considered.

1. Resin

The type of resin and its essential specifications must be determined very early in the production process. Tolerances on such properties as melt flow, odor, and gels must also be established. Wherever possible a single source of supply should be used in order to guarantee responsibility, continuity, consistency, and the provision of technical help. Starting with a consistent, homogeneous material that meets specifications helps ensure that the sheet manufacturer can produce the necessary product. Close communication with resin and sheet suppliers is necessary.

2. Sheet and Film

Consistency in meeting specifications, along with a uniform, homogeneous mixture throughout production runs, is essential for trouble-free, quality thermoforming. The most common serious problem encountered is sheet or film from extrusion runs in which resin, additives, and regrind are not consistently and thoroughly mixed, so that materials from the beginning, the middle, and the end of the runs are not identical. The user/designer must be sure that the thermoformer can procure the proper materials. Practical sheet specifications must be set with the sheet supplier at an early stage in planning. See the Box on p. 32 for a list of sheet and film purchasing specifications. (The Society of the Plastics Industry, Inc., defines sheet as being >0.01 0-in thick and film as being <0.010-in thick.)

3. Heating the Sheet

Assuming the thermoformer has good sheet to work with, the next most important factor is its proper heating. The core of the sheet must be heated uniformly to the processing temperature. In order to avoid degradation, care must be taken not to overheat the surfaces. When the core is below the processing temperature, excessive stresses may be created in the formed part, producing negative results such as warpage and lower impact strength. Small panel ceramic or quartz sandwich radiant heaters are the most efficient and preferred type of heating.

4. Vacuum and Compressed Air

To take full advantage of the thermoforming process, most parts are formed on a " one-sided mold. " A large percentage of thermoforming is done by straight vacuum pressure. The highest pressure obtainable from a vacuum at sea level is 14.7 psi. To attain this level, all vacuum systems should have a rated pressure of 29 inches of mercury (Hg), preferably more. To obtain stress free, top quality parts, very rapid vacuum is normally used. There should be no 90' elbows anywhere in the vacuum system, from the surge tank to the mold, but only straight-in connections and flexible vacuum hose. In the few cases where a slow rate of vacuum is desired, flow control valves can be used, with full, fast vacuum applied as soon thereafter as possible.

The addition of 50 psi of compressed air pressure greatly helps the thermoforming rate and improves the properties and detail of the finished part. The use of compressed air along with the vacuum is called pressure forming. Once the forming rate and amount of pressure are determined, they should be maintained from then on.

5. Mold Temperature

To consistently achieve quality parts at an economical cost, temperature controlled aluminum molds must be used.

There can be no compromise on this requirement. Molds with cast-in cooling, machined cooling channels, and cooling pins or cooling plates must be used. Epoxy, wood, polyester, flocked suede rubber-covered molds, and molds made from other heat sink type materials should be used only for very special projects and very short runs. Otherwise, cost and quality suffer greatly.

Proper mold temperature should be determined and maintained throughout the run. The hotter the mold, the more the final shrinkage of the part. Also, the hotter the mold (along with a very fast vacuum), the fewer the internal stresses in the formed part. 6. Cooling the Part Ideally, heat should be removed from both sides of the part at the same rate and in the same manner. Because a one-sided mold is used in most thermoforming, the cooling rate and temperature can only be accurately controlled from the mold side. To get consistent parts, it is important to use the same method from part to part for cooling, and the same length of time on the mold, whether the method be ambient air, forced air, vapor spray mist, or carbon dioxide. Polytetrafluoroethylenecoated molds slow part cooling because of PTFE's insulating properties. To obtain final shrinkage of parts made of high mold shrinkage materials such as polyolefins, while maintaining optimum forming cycles, postcooling may be necessary.

7. Trimming

Trimming can be the thermoformer's Waterloo." Sheet (or film) must in some way be held in position while heated and formed. Thus, in most applications, the excess or selvage must be trimmed from the finished unit. Where possible, the trim should be designed in "one plane" for more economical removal.

Trimming parts at the same temperature each time helps tremendously in maintaining size tolerance. On continuous in-line thermoformers this is usually done automatically. However, with cut-sheet material, trimming is normally done at a later time. When close tolerances are required in the finished part, it is essential that it be trimmed at the same time and temperature after removal from the mold, or after all the post-shrinkage has occurred.

Proper collection and handling of the trim is of great economic importance and should be treated as such. Clean, uncontaminated trim that has not been degraded or allowed to gather dust makes far better sheet when recycled, resulting in fewer problems for the extruder and ultimately the thermoformer.

Trim is never to be called "scrap" unless it is to be thrown away. This will help everyone realize the value of handling it properly. Some of the proper terms are trim, selvage, and drop-off.

Heat Requirements

Temperature and vacuum and/or compressed air are critical factors in the forming process. Any variation in temperature of the hot sheet dramatically affects the hot strength or elasticity of the plastic. Under normal conditions, it is essential that the sheet material be heated uniformly throughout. Then the faster the vacuum, the better the material distribution, since it does not have time to cool as it is formed. This produces a minimum of internal stresses and parts with the best possible physical properties. Likewise, when pressure forming is used and the material is moved even faster than by vacuum alone, material distribution will be better and the parts even more stressfree. There are exceptions to fast vacuum, one being material such as crosslinked cast acrylic in very deep draws. This material has great hot strength, which allows use of slower vacuum. However, a very hot mold must be used in these cases.

Processing temperature ranges for some of the popular thermoforming thermoplastics are shown in Table 1. A discussion of the parameters follows.

Mold and set temperatures. The set temperature is the temperature at which the thermoplastic sheet hardens and can be safely demolded. This is generally the heat distortion temperature at 66 psi. The closer the mold temperature is to the set temperature without exceeding it, the fewer internal stresses will be molded into the part. If post shrinkage is encountered, for a more rapid cycle time, postcooling fixtures can be used so that parts may be pulled early.

Lower processing limit is the lowest possible temperature for the sheet before it is completely formed. Parts formed at or below this limit will have severely increased internal stress-another reason for rapid vacuum or forming pressure. The least amount of internal stress is obtained by using a hot mold, hot sheet, and very rapid vacuum and/or compressed air.

Orienting temperatures. Biaxially orienting the molecular structure of thermoplastic sheet by 275% to 300% at these temperatures and then cooling greatly enhances properties such as impact and tensile strengths. Careful matching of heating, stretch rate, mechanical stresses, and so on, is required to achieve maximum results. During thermoforming, good clamping of the oriented sheet is important. The sheet is heated, as usual, to its proper forming temperature. Hot forming temperatures do not realign the molecular structure; therefore, the improved properties of the oriented material are carried into the finished part.

Normal forming temperature is the temperature the core of the sheet must reach for proper forming under normal circumstances. This value is determined by heating the sheet to the highest temperature below the degrading temperature at which the sheet still has enough hot strength to be handled.

Upper limit is the temperature at which the sheet begins to degrade or decompose. It is crucial that the sheet stay below this temperature. If radiant heat is used, the surface temperature should be carefully monitored while the temperature of the core climbs to forming temperature. These limits can be exceeded for only a short time with minimum impairment to sheet properties.

Vacuum Needs

Under the majority of conditions, the faster the vacuum and/or the compressed air, the better the part. Cycle time is also improved through more intimate contact with the mold, resulting in more efficient cooling, with better details and tolerances.

The vacuum gage should never drop below 25 inches of Hg during processing. If the pressure drops as low as 20 inches, the machine should be shut down and the problem corrected. At elevations above 2000 ft, two large surge tanks with a pressure valve in between should be used. When the first tank drops below the set pressure, it is shut off and the valve kicks in the second tank.

The basic vacuum pressure measurements are shown in Table 2 and pressures at various altitudes in Table 3. Pressure definitions are as follows.

Psig: Gage pressure in psi is the amount by which pressure exceeds the atmospheric pressure, negative in case of a vacuum.

Psia: Absolute pressure in psi is measured with respect to zero pressure (absolute vacuum = 29.92 inches of Hg). In a vacuum system it is equal to the negative gage pressure subtracted from the atmospheric pressure. Hence:

psig + atmospheric pressure = psia; 1 inch of Hg = 0.4912 psi of atmosphere on the part; and 1 psi = 2.036 inches of Hg. Reduced vacuum and free air delivery is to be expected at altitudes above sea level, because as altitude is increased, vacuum pressure is reduced proportionately to the absolute pressure. Although other variables are involved, Table 3 can be safely used to approximate performance. Speed of air evacuation also slows because of reduced pressures. Most manufacturing plants are at 500to 1000-ft elevations; consequently, a drop in vacuum to 20 inches will be trouble. The 9-psi pressure (at best) is enough to move the hot plastic sheet, but not fast enough to obtain the best available physical properties and material distribution. This will cause deficiencies such as warpage, lower physical properties, and lack of detail.

For optimum vacuum system performance, all 90' elbows between the surge tank and the mold must be eliminated, flexible vacuum hose used where necessary, and straight-in connections used where possible. Each 90' angle slows the vacuum by 30%. When such a connection is unavoidable, a 45 degrees or a large radius "bend" 90' elbow should be used. When splitting vacuum, use a Y" connection. When a slower vacuum is required, a full opening type ball or globe valve is used.

A vacuum pump with a minimum rated capacity of 29 inches of Hg and a vacuum storage or surge tank are essential for achieving the rapid vacuum and necessary pressure for proper thermoforming. The surge tank or tanks should have a total volume of at least six times the cubic displacement that has to be evacuated (for an accurate computation method, see the Box on p. 33). A surge tank permits use of a smaller vacuum pump with longer forming cycles, and "levels out" short cycle pulsations. The tanks should be located as physically close to the forming stations as possible. Hose clamps and mold plate sealing edges must be vacuum tight.

In central plant systems where one source supplies all machines, each thermoformer should have its own vacuum surge tank, and, if it is also a pressure former, its own air accumulator. Vacuum line and air valves should be the full-opening type and are sized according to the machine forming area as follows: up to 30 x 36 in, 1-in diameter; 36 x 48 in up to 84 x 108 in, 1.5-in diameter; and 96 x 120 in and above, 2-in diameter.

Compressed Air Needs

A compressed air supply has requirements similar to that of a vacuum system. An adequate accumulator tank as close to the machine as practical, properly sized lines and valves, and a compressor must be provided. Air should be very dry -40 degrees F dew point) and absolutely oil free.

Care must be used when exhausting air from the pressure box prior to opening. A two-tank system can be used. When pressure forming, all safety precautions applicable to pressure vessels must be obeyed. They must have appropriate safety ratings and over-pressure relief diaphragms. Pressure forming stations need to have pressure interlocks that prevent opening when internal air pressure is above a fixed (relatively low) level.
COPYRIGHT 1990 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Author:McConnell, William K., Jr.
Publication:Plastics Engineering
Date:Dec 1, 1990
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