Thermoforming

Thermoforming Processes

Thermoforming processes are possible because thermoplastic sheets can be softened and reshaped, and the new shape is retained when the material is cooled.  Most thermoplastic materials may be formed by this process; however, acetals, polyamides, and fluorocarbons are not usually thermoformed.  Extruded, calendered, laminated, cast, and blown films or sheet forms may be thermoformed.

Forcing a heated thermoplastic material to take the shape of a mold by mechanical, air, or vacuum pressure is common.  Tooling costs are usually low, and parts with large surface areas may be produced economically.  Prototypes and short runs are also practical.  Although dimensional accuracy is good, thinning is a problem in some part designs.

Tooling can run from low-cost plaster molds to expensive water-cooled steel molds, but the most common tooling material is cast aluminum.  Gypsum, hardboard, pressed wood, cast phenolic resins, filled or unfilled polyester or epoxy resins, sprayed metal, and steel also may be used for molds.

One source dates thermoforming back to the ancient Egyptians.  They found that animal horns and tortoise shells could be heated and formed into a variety of vessels and shapes.  In the United States, John Hyatt thermoformed Celluloid sheets over wooden cores for piano keys.

Today, sheets and films may be thermoformed by the basic techniques of straight vacuum forming, drape forming, matched-mold forming, pressure-bubble plug-assist vacuum forming, plug-assist vacuum forming, plug-assist pressure forming, vacuum snap-back forming, pressure-bubble vacuum snap-back forming, trapped-sheet contact-pressure forming, free forming, and mechanical forming.

Items produced by thermoforming include signs, light fixtures, ice-cube trays, ducts, drawers, instrument panels, tote trays, housewares, toys, refrigerator panels, transparent aircraft enclosures, and boat windshields.  Blister and skin packaging of products are familiar applications of thermoforming. Replacement parts and hardware are examples of items that are sometimes skin packaged.  Skin packaging requires no mold; the plastics film is simply formed over the product.  Cookies, pills and other products are commonly packaged by blister packaging.  Single portions of butter, jellies, and other foods are sometimes packaged in blister packs.

1 STRAIGHT VACUUM FORMING

Vacuum forming is the most versatile and widely used thermoforming process. Vacuum equipment costs less than pressure or mechanical processing equipment.

In straight vacuum forming, a plastics sheet is clamped in a frame and heated. While the hot sheet is rubbery, or in an elastic state, it is placed over a female mold cavity.  The air is removed from this cavity by vacuum (Fig. 1) and atmospheric pressure (10 kPa) forces the hot sheet against the walls and contours of the mold.  When the plastics has cooled, the formed part is removed, and final finishing and decorating may be done, if necessary.  Blowers or fans are used to speed cooling.  One disadvantage of thermoforming is that formed pieces usually must be trimmed, and the scrap must be reprocessed.

(A) A clamped and heated plastic sheet is forced down into the mold by air pressure after a vacuum is drawn in the mold.  (Atlas Vac Machine Co.)

(B) Plastics sheet cools as it contacts the mold. (Atlas Vac Machine Co.)

(C) Areas of the sheet that touched the mold last are the thinnest. (Atlas Vac Machine Co.)

Fig. 1 Straight vacuum forming.

Most vacuum systems have a surge tank to ensure a constant vacuum of 500 to 760 mm of mercury.  Superior parts are formed by quickly applying the vacuum before any portion of the sheet has cooled.  Slots are more desirable and efficient than holes in allowing the air to be drawn from the mold.  Slots or holes should be smaller than 0.65 mm [0.025 in.] in diameter to avoid surface blemishes on the formed part.  A hole or slot should be placed in all low or unconnected portions of the mold.  If this is not done, air may be trapped under the hot sheet with no way to escape.  Unless they are collapsible, molds should include a 2 to 7 degree angle (draft) for easy part removal.

Thinning at the upper edges of a part is a disadvantage in using relatively deep female molds.  Thinning is caused by the hot plastics sheet first being drawn to the center of the mold.  Sheeting at the edges of the mold must stretch the most and thus becomes the thinnest portion of the formed item.  If preprinted flat sheets are formed, thinning must be kept in mind when trying to compensate for distortion during forming.  Straight vacuum forming is limited to simple, shallow designs, and thinning will occur often in corners.

The draw or draw ratio of a female mold is the ratio of the maximum cavity depth to the minimum span across the top opening.  For high-density polyethylene, the best results are achieved when this ratio does not exceed 0.7:1. Thermoforming equipment and dies are relatively inexpensive.

 2 DRAPE FORMING

Drape forming (incorrectly called mechanical forming) is similar to straight vacuum forming except that after the plastics is framed and heated, it is mechanically stretched over a male mold.  A vacuum (actually, a pressure differential) is applied that pulls the hot plastics against all portions of the mold (Fig. 2).  The sheet touching the mold remains close to its original thickness.  Side walls are formed by the material draping between the top edges of the mold and the bottom seal area at the base.  When the plastics has cooled, it is removed for trimming or post-processing, if needed.  Mark-off (marks from the mold) is on the inside of the product while such marks appear on the outside of the part in straight vacuum forming.

(A) Clamped heated plastics may be pulled over the mold, or the mold may be forced into the sheet.

(B) Once the sheet has formed a seal around the mold, a vacuum is drawn to pull the plastics sheet tightly against the mold surface.

(C) Final wall thickness distribution in the molded part.

Fig. 2. Principle of drape forming plastics. (Atlas Vac Machine Co.)

It is possible to drape-form items with a depth to diameter ratio of nearly 4:1.  High draw ratios are possible with drape forming, however, this technique is also more complex.  Male molds are easy to make and, as a rule, cost less than female ones, but male molds are more easily damaged.

Drape forming has also been used to form a hot plastics sheet over male or female molds by gravitational forces alone.  Female molds are preferred for multicavity forming because there must be more spacing if male molds are selected.

 3 MATCHED-MOLD FORMING

Matched-mold forming is similar to compression molding.  A heated sheet is trapped and formed between male and female dies that may be made of wood, plaster, epoxy, or other materials (Fig. 3).  Accurate parts with close-tolerances may be quickly produced in costly water-cooled molds.  Very good molded detail and dimensional accuracy can be obtained with water-cooled molds, including lettering and grained surfaces.  There is mark-off on both sides of the finished product; therefore, mold dies must be protected from scratches or damage because such defects would be reproduced by the thermoplastic materials.  A smooth-surfaced mold should not be used with polyolefins because air may be trapped between the hot plastics and a highly polished mold.  Sandblasted mold surfaces are usually used for these materials.

(A) The heated plastics sheet may be clamped over the female die, as shown, or draped over the mold form.

(B) Vents allow trapped air to escape as the mold closes and forms the part.

(C) Distribution of materials in the product depends on the shapes of the two dies.

(D) Male mold forms must be spaced at a distance equal, to or greater than their height or webbing may occur.

Fig. 3. Principle of matched-mold forming. (Atlas Vac Machine Co.)

 4 PRESSURE-BUBBLE PLUG-ASSIST VACUUM FORMING

For deep thermoforming, pressure-bubble plug-assist vacuum forming is an important process.  By this process, it is possible to control the thickness of the formed article.  The item may have uniform thickness or the thickness may be varied.

Once the sheet has been placed in the frame and heated, controlled air pressure creates a bubble (Fig. 4).  This bubble stretches the material to a predetermined height, usually controlled by a photocell.  The male plug assist is then lowered forcing the stretched stock down into the cavity.  The male plug is normally heated to avoid chilling the plastics prematurely.  The plug is made as large as possible so the plastics is stretched close to the final shape of the finished product.  Plug penetration should be from 70 to 80 percent of the mold cavity depth.  Air pressure is then applied from the plug side while at the same time a vacuum is drawn on the cavity to help form the hot sheet.  For many products, vacuum alone is used to complete formation of the sheet.  In Figure 7-4, both vacuum and pressure are applied during the forming process.  The female mold must be vented to allow trapped air to escape from between the plastics and the mold.

(A) The plastics sheet is heated and sealed across the mold cavity.

(B) Air is introduced, blowing the sheet upward into an evenly stretched bubble.

(C) A plug shaped roughly to the cavity contour presses downward into the bubble, forcing it into the mold.

(D) When the plug reaches its lowest point, vacuum is drawn to pull the plastics against the mold walls.  Air may be introduced from above to aid forming.

Fig. 4. Pressure-bubble plug-assist vacuum forming. (Atlas Vac Machine Co.)

 5 PLUG-ASSIST VACUUM FORMING

To help prevent corner or periphery thinning of cup- or box-shaped articles, a plug assist is used to mechanically stretch and pull additional plastics stock into the female cavity (Fig. 5).  The plug is normally heated to just below the forming temperature of the sheet stock.  The plug should be from 10 to 20 percent smaller in length and width than the female mold.  Once the plug has forced the hot sheet into the cavity, air is drawn from the mold, completing the formation of the part.  The plug design or shape determines the wall thickness, as shown in cross-section in Figure 5D.

(A) Heated, clamped plastics sheet is positioned over mold cavity.

(B) A plug, shaped roughly like the mold cavity, plunges into the plastics sheet to pre-stretch it.

(C) When the plug reaches the limit of its travel, a vacuum is drawn in the mold cavity.

(D) Areas of the plug touching the sheet first form thickened areas due to chilling effect.

Fig. 5. Plug-assist vacuum forming. (Atlas Vac Machine Co.)

Plug-assist vacuum and pressure forming allows deep drawing, and permits shorter cooling cycles and better control of wall thickness.  Close temperature control is needed, however, and the equipment is more complex than straight vacuum forming (Fig. 6).

(A) Clamp layout

(B) RAM part clamps.

Fig. 6. Restricted-area molding (RAM), with individual part clamps build into the mold.  This helps to control material draw and reduces draw ratio. (Brown Machine Co.)

 6 PLUG-ASSIST PRESSURE FORMING

Plug-assist pressure forming is similar to plug-assist vacuum forming in that the plug forces the hot plastics into the female cavity.  Air pressure applied from the plug forces the plastics sheet against the walls of the mold (Fig. 7).

(A) Heated, clamped sheet is positioned over the mold cavity.

(B) As the plug touches the sheet, air is allowed to vent from beneath the sheet.

(C) As the plug completes its stroke and seals the mold, air pressure is applied from the plug side, forcing the plastics against the mold.

(D) Plug-assist pressure forming is capable of producing products with uniform wall thickness.

Fig. 7. Plug-assist pressure forming . (Atlas Vac Machine Co.)

 7 VACUUM SNAP-BACK FORMING

In vacuum snap-back forming, the hot plastics sheet is placed over a box and a vacuum is drawn that causes a bubble to be forced into the box (Fig. 8).  A male mold is lowered and the vacuum in the box is released, causing the plastics to snap-back around the male mold.  A vacuum may also be drawn in the male mold to help pull the plastics into place.

(A) Plastics sheet is heated and sealed over the top of the vacuum box. (Atlas Vac Machine Co.)

(B) Vacuum is drawn beneath the sheet, pulling it into a concave shape. (Atlas Vac Machine Co.)

(C) The male plug is lowered and a vacuum drawn through it.  At the same time, vacuum beneath the sheet is vented. (Atlas Vac Machine Co.)

(D) External deep draws can be obtained with this process to form luggage, auto parts, and other items. (Atlas Vac Machine Co.)

Fig. 8. Vacuum snap-back forming. (Atlas Vac Machine Co.)

Vacuum snap-back forming allows complex parts with recesses to be formed.

 8 PRESSURE-BUBBLE VACUUM SNAP-BACK FORMING

As the name implies, the sheet is heated and then stretched into a bubble shape by air pressure (Fig. 9).  The sheet pre-stretches about 35 to 40 percent. The male mold is then lowered.  A vacuum is applied to the male mold while air pressure is forced into the female cavity.  This causes the hot sheet to snap-back around the male mold.  Mark-off is on the male mold side.

(A) Heated plastics sheet is clamped and sealed across a pressure box.

(B) Air pressure is introduced beneath the sheet, causing a large bubble to form.

(C) A plug is forced into the bubble, while air pressure is maintained at a constant level.

(D) Air pressure beneath the bubble and a vacuum at the plug side create a uniform draw.

Fig. 9. Pressure-bubble vacuum snap-back forming. (Atlas Vac Machine Co.)

Pressure-bubble vacuum snap-back forming allows deep drawing and the formation of complex parts, but the equipment is complex and costly.

 9 TRAPPED-SHEET CONTACT HEAT PRESSURE FORMING

This process is like straight vacuum forming except that air pressure and a vacuum assist may be used to force the hot plastics into a female mold.  Figure 7-10 shows the steps of this process.

(A) A flat, porous plate allows air to be blown through its face.

(B) Air pressure from below and a vacuum above force the sheet tightly against the heated plate.

(C) Air is blown through the plate to force the plastics into the mold cavity.

(D) After forming, additional pressure may be exerted.

Fig. 10. Trapped-sheet contact-heat pressure forming.  (Atlas Vac Machine Co.)

 10 FREE FORMING

In free forming, air pressures of over 2.7 MPa may be used to blow a hot plastics sheet through the silhouette of a female mold (Fig. 11).  The air pressure causes the sheet to form a smooth bubble shaped article. A stop may be used to form special contours in the bubble.  Skylight panels and aircraft canopies are well-known examples of this technique.

(A) Basic setup.

(B) Air injection

(C) Examples of free-form shapes that can be obtained with various openings. (Rohm & Haas Co.)

Fig. 11. Free forming of plastics bubbles.

 11 MECHANICAL FORMING

In mechanical forming, no vacuum or air pressure is used to form the part.  It is similar to matched molding; however, close-fitting matched male and female molds are not used.

This process is sometimes classified as a fabrication or post-forming operation. The forming process may make use of simple wooden forming jigs to give the desired shape using ovens, a strip heater, or heat guns for the heat source. Flat stock may be heated and wrapped around cylindrical shapes or stock may be heated in a narrow strip and bent at right angles.  Tubes, rods, and other profile shapes may be mechanically formed.

 12 PLASTIC PROCESSING

 12.1  Injection molding

 12.1.1  Process:  Similar to die casting of metals, a thermoplastic molding compound  is heated to plasticity in a cylinder at a controlled temperature and then forced under pressure through sprues runners, and gates into a cool mold; the resin solidifies rapidly, the mold is opened, and the parts ejected; with certain modifications, thermosetting materials can be used for small parts.

 12.1.2  Advantages:  Extremely rapid production rate and hence low cost per part, little finishing required; excellent surface finish; good dimensional accuracy; ability to produce a variety of relatively complex and intricate shapes.

 12.1.3  Limitations:  High tool and die costs; high scrap loss; limited to relatively small parts; not practical for small runs.

 12.2  Cut extrusions

 12.2.1  Process:  Thermoplastic molding powder is fed through a hopper to a chamber where it is heated to plasticity and then driven, usually by a rotating screw, through a die having the desired cross section; extruded lengths are either used as is or cut into sections; with modifications, thermosetting materials can be used.

 12.2.2  Advantages:  Very low tool cost; material can be placed where needed; great variety of complex shapes possible; rapid production rate. 

 12.2.3  Limitations:  Close tolerances difficult to achieve; openings must be in direction of extrusion; limited to shapes of uniform cross section (along length).

 12.3  Sheet moldings (thermoforming) VACUUM FORMING

 12.3.1  Process:  Heat-softened sheet is placed over a male or female mold; air is evacuated from between sheet and mold, causing sheet to conform to contour of mold.  There are many

modifications, including vacuum snapback forming, plug-assist, drape forming etc..

 12.3.2  Advantages:  Simple procedure; inexpensive; good dimensional accuracy; ability to produce large parts with thin sections.

 12.3.3  Limitations:  Limited to parts of low profile.

 12.4  Sheet molding (thermoforming) BLOW OR PRESSURE FORMING

 12.4.1  Process:  The reverse of vacuum forming in that positive air pressure rather than vacuum is applied to form sheet to mold contour.

 12.4.2  Advantages:  Ability to produce deep drawn parts; ability to use sheet too thick for vacuum forming; good dimensional accuracy; rapid production rate.

 12.4.3  Limitations:  Relatively expensive; molds must be highly polished.

 12.5  Sheet molding (thermoforming) MECHANICAL FORMING

 12.5.1  Process:  Sheet metal equipment (presses benders, rollers creasers. etc.) forms heated sheet by mechanical means.  Localized heating is used to bend angles; where several bends are required, heating elements are arranged in series.

 12.5.2  Advantages:  Ability to form heavy and/or tough materials; simple; inexpensive; rapid  production rate.

 12.5.3  Limitations:  Limited to relatively simple shapes.

 12.6  Blow moldings

 12.6.1  Process:  An extruded tube (parison) of heated plastics within the two halves of a female mold is expanded against the sides of the mold by air pressure; the most common method uses injection molding equipment with a special mold.

 12.6.2  Advantages:  Low tool and die cost; rapid production rate; ability to produce relatively complex hollow shapes in one piece.

 12.6.3  Limitations:  Limited to hollow or tubular parts; wall thickness difficult to control.

 12.7 Slush rotational dip castings

 12.7.1  Process:  Powder (polyethylene) or liquid material (usually vinyl plastisol or organosol) is poured into a closed mold, the mold is heated to fuse a specified thickness of material adjacent to mold surface, excess material is poured out, and the semifused part is placed in an oven for final curing.  A variation, rotational molding, provides completely enclosed hollow parts.

 12.7.2  Advantages:  Low cost molds, relatively high degree of complexity; little shrinkage.

 12.7.3  Limitations:  Relatively slow production rate; choice of materials limited.

 12.8  Compression moldings

 12.8.1  Process:  A partially polymerized thermosetting resin, usually pre-formed, is placed in a heated mold cavity; mold is closed, heat and pressure applied, and the material flows and fills mold cavity; heat completes polymerization and mold is opened to remove hardened part.  Method is sometimes used for thermoplastics, e.g., vinyl phonograph records; in this operation, the mold is cooled before it is opened.

 12.8.2  Advantages:  Little waste of material and reduced finishing costs due to absence of sprues, runners, gates, etc.; large, bulk parts possible.

 12.8.3  Limitations:  Extremely intricate parts involving undercuts, side draws, small holes, delicate inserts etc., not practical, extremely close tolerances difficult to achieve.

 12.9  Transfer moldings

 12.9.1  Process:  Used primarily for thermosetting materials, this method differs from compression molding in that the plastic is 1) first heated to plasticity in a transfer chamber, and 2) fed, by means of a plunger, through sprues, runners, and gates into a closed mold.

 12.9.2  Advantages:  Thin sections and delicate inserts are easily used; flow of material is more easily controlled than in compression molding; good dimensional accuracy; rapid production rate.

 12.9.3  Limitations:  Molds are more elaborate than compression molds and hence more expensive; loss of material in cull and sprue; size of parts somewhat limited.

 12.10  Reinforced plastics moldings CONTACT

 12.10.1  Process:  The lay-up, which consists of a mixture of reinforcement (usually glass cloth or fibers) and resin (usually thermosetting), is placed in mold by hand and allowed to harden without heat or pressure.

 12.10.2  Advantages:  Low cost; no limitations on size or shape of part.

 12.10.3  Limitations:  Parts are sometimes erratic in performance and appearance; limited to polyesters epoxies and some phenolics.

 12.11  Reinforced plastic moldings VACUUM BAG

 12.11.1  Process:  Similar to contact except a flexible polyvinyl alcohol film is placed over lay-up and a vacuum drawn between film and mold (about 82 kPa).

 12.11.2  Advantages:  Greater densification allows higher glass contents, resulting in higher strengths.

 12.11.3  Limitations:  Limited to polyesters epoxies and some phenolics.

 12.12  Reinforced plastic moldings PRESSURE BAG

 12.12.1  Process:  A variation of vacuum bag in which a rubber blanket (or bag) is placed against film and inflated to apply about 350 kPa.

 12.12.2  Advantages:  Allows greater glass contents.

 12.12.3  Limitations:  Limited to polyesters epoxies and some phenolics.

 12.13  Reinforced plastic moldings AUTOCLAVE

 12.13.1  Process:  The vacuum-bag setup is simply placed in an autoclave with hot air at pressures up to 1.38 MPa.

 12.13.2  Advantages:  Better quality moldings.

 12.13.3  Limitations:  Slow rate of production.

 12.14  Reinforced plastic moldings MATCHED DIE

 12.14.1  Process:  A variation of conventional compression molding, this process uses two metal molds which have a close-fitting telescoping area to seal in the resin and trim the reinforcement; the  reinforcement, usually mat or pre-form is positioned in the mold, a pre-measured quantity of resin is poured in, and the mold is closed and heated; pressures generally vary between 1.04 and 2.75 MPa.

 12.14.2  Advantages:  Rapid production rates; good quality and excellent reproducibility; excellent surface finish on both sides; elimination of trimming operations; high strength due to very high glass content.

 12.14.3  Limitations:  High mold and equipment costs; complexity of part is restricted; size of part limited.

 12.15  Reinforced plastic moldings FILAMENT WOUND

 12.15.1  Process:  Glass filaments, usually in the form of rovings, are saturated with resin and machine wound onto mandrels having the shape of desired finished part; finished part is cured at either room temperature or in an oven, depending on resin used and size of part.

 12.15.2  Advantages:  Provides precisely oriented reinforcing filaments; excellent strength-to-mass ratio; good uniformity.

 12.15.3  Limitations:  Limited to shapes of positive curvature; drilling or cutting reduces strength.

 12.16  Reinforced plastic moldings SPRAY MOLDING

 12.16.1  Process:  Resin systems and chopped fibers are sprayed simultaneously from two guns against a mold; after spraying, layer is rolled flat with a hand roller. Either room temperature or oven cure.

 12.16.2  Advantages:  Low cost; relatively high production rate; high degree of complexity possible.

 12.16.3  Limitations:  Requires skilled workers; lack of reproducibility.

 12.17  Castings

 12.17.1  Process:  Plastics material (usually thermosetting except for the acrylics) is heated to a fluid mass, poured into mold (without pressure), cured, and removed from mold.

 12.17.2  Advantages:  Low mold cost; ability to produce large parts with thick sections; little finishing required; good surface finish.

 12.17.3  Limitations:  Limited to relatively simple shapes.

 12.18  Cold moldings

 12.18.1  Process:  Method is similar to compression molding in that material is charged into a split, or open, mold; it differs in that it uses no heat - only pressure.  After the part is removed from mold, it is placed in an oven to cure to final state.

 12.18.2  Advantages:  Because of special materials used, parts have excellent electrical insulating properties and resistance to moisture and heat; low cost; rapid production rate.

 12.18.3  Limitations:  Poor surface finish; poor dimensional accuracy; molds wear rapidly; relatively expensive finishing; materials must be mixed and used immediately.