Processing of Plastic

Plastics

There are two types of plastics: Thermosets and Thermoplastics. Thermosets are NOT recyclable because they undergo a permanent

chemical change when heated. This is known as heat hardening. Thermoplastics are recyclable because they only undergo temporary physical

change when heated. When the heat is removed, they return to their original state. This is known as heat softening.

There are a wide variety of manufacturing processes that exist for plastic production. These processes include: extrusion, lamination, thermal form, foaming, molding, expansion, solid-phase forming, casting and spinning. Within the molding processes, there are five different methods: Compression, transfer, blow, rotational and injection.

The two different materials are used for the different processes. For example, Thermoplastics are usually used in Injection Molding, Extrusion, Blow Molding, Calendering whereas, thermosets are usually used in Compression Molding, High Pressure Lamination, and Reaction Injection Molding 33/2

Advantages Light Weight High Strength-to-Weight Ratio Complex Parts - Net Shape Variety of Colors (or Clear) Corrosion Resistant Electrical and thermal Insulation High Damping Coefficient Low pressures and temperature required

Disadvantages Creep Thermally Unstable- can not withstand Extreme Heat U-V Light Sensitive Relatively low stiffness Relatively low strength Difficult to Repair/Rework Difficult to Sort/Recycle

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Processing of Plastics

There are a wide variety of manufacturing processes that exist for plastic production. A wide variety of plastic manufacturing processes exist Extrusion Lamination (Calendaring) Thermoforming Casting Molding

Compression MoldingTransfer MoldingRotational MoldingReaction Injection MoldingBlow Molding Injection Molding

Expansion Foaming Spinning Solid-Phase Forming

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Extrusion

One of the most common process for creating plastic for further processing is extrusion. The Extrusion process combines color pigments and performance additives with resin by pushing it through rotating screws. The heat and pressure produced within the screw barrels disperses and melts the ingredients into homogeneous molten mixture. At the end of the mixture is usually cool die. The mixture is pushed through the die and onto the “finishing” operation such as pelletizing, calendaring, or molding. The pictorial of the process is shown here.

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Lamination (Calendering)

In calendering, sheets of plastic are laminated together by rolling through heated roller. Basically, warm or molten plastic (usually from an extruder) is fed through a series of heated rolls as in this figure. The gaps between the rolls determine the final sheet size. Each additional roll would reduce the sheet thickness further. Then, once the laminated sheet is the correct thickness, the sheet is then stripped off.

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Schematic illustration of calendering. Sheets produced by this process are subsequently used in thermoforming.

Reinforced- plastic components for a Honda motorcycle. The parts shown are front and rear forks, a rear swingarm, a wheel, and brake disks.

Thermoforming

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In thermoforming, a plastic sheet is heated in an oven to the sag point but not to the melting point. The sheet is then removed from the oven and placed over a mold and through the application of a vacuum is pulled against the mold. Typical parts are advertising signs, refrigerator liners, packaging, appliance housings, and panels for shower stalls. The parts cannot have openings or holes or the vacuum cannot be maintained.The sheets used for thermoforming are made using the calendaring process.

Casting

Casting used for both thermosetting and thermoplastic materials. Basically, casting places plastic in a mold then hardens it into a rigid article or form. In potting, the plastic is cast around a part, and the case becomes part of the final component. In encapsulation, the component is covered with plastic and the component and plastic are ejected from the mold. Both processes are used extensively in electronics for insulation and dielectric properties

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Schematic illustration of (a) casting, (b) potting, (c) encapsulation of plastics.

Molding

Molding is the most common plastic forming or finishing method. There are many different methods in plastic molding. Methods for molding include: Compression Molding Transfer Molding Rotational Molding Reaction Injection Molding Blow Molding Injection Molding

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Compression Molding

In compression molding, a pre-shaped part, a pre-measured powder or a viscous mixture of liquid resin is placed directly in a heated mold. Forming is done under pressure with a plug or the upper half of the die. Compression molding is similar to forging, and has the same problem with flash.Typical parts are dishes, handles, container caps, fittings, electrical and electronic components, washing machine agitators and housings. Fiber reinforced plastics may also be formed by this process, though mainly thermoset plastics are used.

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Types of compression molding, a process similar to forging: (a) positive, (b) semipositive, and (c) flash. The flash in part (c) has to be trimmed off. (d) Die design for

making a compression- molded part with undercuts.

Transfer Molding

Transfer molding is a further development of compression molding. The thermosetting material is heated and then injected into a heated, closed mold. Pressure is applied somehow to force the material into the mold. The flow process heats the material and homogenizes it. Curing takes place by cross linking. Typical parts include electrical and electronic components and rubber and silicone parts.

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Sequence of operations in transfer molding for thermosetting plastics. This process is particularly suitable for intricate parts with varying wall thickness.

Rotational Molding

Rotational molding is used for large plastic parts. The thin-walled metal mold is a split female mode made of two pieces and is designed to be rotated about two perpendicular axes. A premeasured quantity of finely ground plastic material is placed inside a warm mold. The mold is then heated, usually in a large oven, while it is rotated about the two axes. The action tumbles the powder against the mold where heating fuses the power without melting it. In some cases, a cross linking agent is added to the powder, and cross linking occurs after the part is formed in the mold by continued heating.Typical parts are tanks, trash cans, boat hulls, buckets, housings, toys, carrying cases, and footballs. Various metallic or plastic inserts may also be molded into the parts.

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The rotational molding (rotomolding or rotocasting) process. Trash cans, buckets, and plastic footballs can be made by this process.

Reaction Injection Molding

In reaction-injection molding (RIM), a mixture of two or more reactive fluids is forced under high pressure into the mold cavity. Chemical reactions take place rapidly in the mold and the polymer solidifies, producing a thermoset part. Major applications are automotive bumpers and fenders, thermal insulation for refrigerators and freezers, and stiffeners for structural components. Initial injection pressures typically are much lower than traditional injection molding.

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Schematic illustration of the reaction-injection molding process.

Blow Molding (Bottles)

Blow molding is a modified extrusion and injection molding process, wherein a tube is extruded (usually turned so that it is vertical) and clamped into a mold with a cavity much larger than the tube diameter. Air is blown inside the tube opening and the plastic expands to fill the mold cavity shape. Blow molding is similar to blowing up a balloon inside a bottle. Mostly, blow molding forms hollow plastic parts with relatively thin walls. Examples of blow molding products are bottles, bumpers, bags and ducting.

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Blow Molding (Plastic Bags)

Here is a picture of a the blow molding process for plastic bags. The tube is raised and the plastic bag is blown to size. Then pinch rollers compress the bag for further packaging.

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Injection Molding

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The last and more popular process is injection molding. This process is used to form complex plastic parts. Typical injection molded parts are fittings, containers, bottle tops, housings, and much more. Here is a pictorial of a typical injection molding machine and part. It is the most common of the plastic forming processes today, accounting for approximately 30% of all plastics produced.

Injection Molding

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Basic working of injection molding is as follows- plastic material is heated to the glass transition point (or beyond) and pressurized. The mold is closed. Pressurized plastic melt is forced into the mold. The mold remains closed while part hardens. Upon cooling, the mold opens, the part is removed and the process repeats.The key points to this are getting the plastic to fill the mold and keeping it there, and then ejecting the part after it has solidified. To help ensure good fill, you must employ good fluid flow principles when designing the cavity. This means one should fill the parts from thick to thin. In other words put the plastic in the thickest section and let it flow to the thinner sections. Also, one should try to avoid sharp corners in the runner system. In addition, after the part is filled, the packing pressure is maintained so the part will not shrink away from the wall during solidification. Another key design consideration is the process of ejecting the part. The part should hang on the moving side as it retracts pulling free of the fixed side. Then ejector pins push the part out of the moving side mold. To allow for ejection, one must include some taper or draft in the part. Typically, this is between 0.5o to 3o.

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Injection Mold Layout: The plastic melt flows from the injection nozzles and enters the mold at the sprue. From the sprue the plastic flows into the runners and ultimately through the gates into the part. Gate and runner design is an important part of the mold design. To help ensure that the mold fills completely, one should balance the mold so that all cavities fill at the same time. When the cavities are the same, a symmetric layout is used. If the cavities are all markedly different, often the gates and runners must be sized/shaped differently in order to allow all cavities to fill in the same amount of time.

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Gate

Runner

Sprue

MoldCavity

Plunger Type Injection Molding Press: In this molding press, the plastic is fed into the mold when a cylinder plunger extends and forces the plastic into the mold. After the plunger retracts more material can be fed from the hopper to the shooting pot. (Thus the stroke of the plunger determines the additional material fed in each time.) Of course the shooting pot is long enough to hold several shots, so the plastics stays in the pot for a while, giving the band heaters time to heat and melt the plastic. Notice the torpedo, which is basically an obstruction to the plastic flow in the shooting pot. As the plastic moves around the torpedo, it is better mixed.

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Plunger

HopperShooting

Pot

Torpedo

NozzleBand Heaters

Screw Type Injection Presses: The original plunger type has had one important modification. A reciprocating screw now forces material into the mold. This screw action ensures that the same amount of material is always metered in, and it is equally dense along the length of the screw. Additionally the material will be much better mixed by the screw action which helps to maintain better consistency from shot to shot. Since the screw action generally helps to pack the material in better, a given plunger travel will push more material into the cavity. Finally the action of the screw, as it rotates and mixes, adds energy to the melt. However, band heaters are still needed to fully heat the melt. All of this results in a much better and more consistent part. This is why the screw press is essentially the only press found in industry. Small plunger presses are still made for prototype/lab purposes.

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Screw meters plastic, plunger provides pressure

Band Heaters

Shooting Pot

Nozzle

Hopper

Reciprocating Screw

Injection Molding Screws: The injection molding screw plunges forward to provide holding and packing pressure. The screw rotates as it retracts to meter and plasticize the melt. The screw is broken up into 3 regions. The Feed Section draws material from the hopper & starts movement into the shooting pot. In this section, channels between the flights are deep and the depth is constant. The next section, called the Transition Section, compresses and melts the plastic pellets. Most plasticization occurs in this section. The root diameter tapers, causing the channel depth to decrease. In the last section, the Metering Section, the correct fill is precisely measured out. This section has a constant channel depth.

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FeedSection

TransitionSection

MeteringSection

Screw Configuration Design: There are a number of design variations that can be made to the injection molding screw configuration. The extruder screws are composed of various screw segments connected together to create a complete configuration design. Dulmage Mixing Section: Some the segments used in plastic extrusions

are the dulmage mixing segment and mixing pins segment. The Dulmage Mixing design is used at the end of the screw to enhance mixing. Usually, several sections are put together.

Mixing Pins: Another configuration option are the mixing pins. These are usually inserted between the final flights to enhance the mixing.

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Dulmage Mixing Section Mixing Pins

Vented Barrels: Vented barrels with two stage screws are another options in some design. The first stage meters and compresses the material. Then, the material is vented, compressed and metered forward in the second stage.

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Vent

Material from

Hopper

Vented Barrels

Barrier Flight Screws: In Barrier flight screws, the channel is split into two sections: one for solids and one for the melted plastic. The barrier flight is not quite as large as the primary flight. The barrier flight has small passage for the melt to flow from the solid channels into the melt channels. The solid channel becomes smaller and the melt larger along the length of the screw. The functioning of the barrier flight is shown here.

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Solids ChannelMelt Channel

PrimaryFlight

BarrierFlight

Press Parameters: Generally, there are three common parameters used to describe the injection molding press capacity: clamping force, shot size, and injection pressure. Clamping force is usually the most common method to refer to the injection

molding press capacity. Thus, presses are talked about as being 20 ton, 50 ton, etc. The clamping force is the force available to hold the platens together. The platens contains the mold cavities. Clamping can be achieved using “in-line” hydraulic cylinders, mechanical toggle clamps, or a combination of the two (called hydro-mechanical).

Shot size is the amount of material that can be transferred into the mold in one shot. Shot sizes are usually specified in cubic centimeters or ounces.

Injection pressure is the pressure at the sprue that forces or injects the plastic melt into the mold. Specification by this parameter refers to the maximum injection pressure.

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Clamping Mechanisms: There are a few different ways to provide the clamping force for the mold. The “in-line” hydraulic cylinder provides good force control, but requires large hydraulics that tend to be slow. The toggle clamps move quickly but provide poor force control. The hydro-mechanical clamping system is a combination of the two clamps. It uses a toggle mechanism for most of the travel and then uses the hydraulic cylinder for the locking force.

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HydraulicCylinder

HydraulicCylinder

ToggleClamp

“In line” hydraulic cylinder

Toggle Clamp

Injection Molding Defects

Injection molding can create defects on the finished product. Some of the more common Injection molding defects are

Short Shot Flashing Weld Lines Jetting Ejector Pin Marks Sink Marks Warpage/Residual Stresses

When designing for a part and the associated mold, one should keep these defects in mind as well as the fundamental objectives of filling, holding, packing and removing the part.

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Short Shot: Short shot occurs when there is insufficient material to fill the mold cavity and/or the material solidifies too soon. It has several causes, including insufficient injection pressure, or insufficient time allowed during the injection process. Sometimes the material will freeze in a given section before it can reach the edges of the mold.

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UnfilledSectionsof Mold

UnfilledSectionof Mold

Flashing: Flashing occurs when there is too much material and it pushes its way out of the die; basically, the material overflows the cavity. This can be caused by too much injection pressure, too much injection time, or insufficient clamping force. It also can be caused by a poorly machined die that does not properly seal off the cavity.

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Plastic Pushesinto Area BetweenMating Surfaces

CavityWall

FlashFlash

Part Moderate-Heavy Flash

Weld Lines: Weld lines occur when flow fronts meet in the mold. In addition to being aesthetically unappealing, weld lines decrease the strength of the part. The cooler the fronts are when they meet, the less the plastic will be able to “meld” together; thus, the weld lines are more pronounced and the part is much weaker at this point. The amount of plastic that cools is directly related to how far it must travel. For this reason, multiple gates are sometimes used on parts with solid cores. There will be more weld lines, but each one will be stronger. Additionally, weld lines are much more pronounced if flow fronts are moving in completely opposite directions, as opposed to when the flow fronts share some components of velocity. When the flow fronts are at least partially moving together, mixing is enhanced. Generally, weld lines should be avoided if possible. For example, the solid square pictured here, should be gated in one place to reduce the number of weld lines. If you have a part with a solid core, as with the second example, weld lines are unavoidable. However, things can be done to maximize the knitting and thus minimize the weld line defect. For instance, one can keep the sections hotter when they meet. 33/31

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Unavoidable w/ Solid CoresCan result from poor gate placement

Weld lines are more pronounced if melt is cooler when fronts meet.Also if flow fronts are moving into one another (butt weld, or weld

as opposed to streaming weld, or meld).

Gate Gate

Gate

Weld Line

WeldLine

Jetting: Jetting is generally caused when one gates a part in such a way that the material flow enters an open section with much space between the gate and the opposite wall. When the flow area is squeezed through the gate, the velocity increases, and the plastic melt shoots into the empty cavity mold. If there is nothing to break its path, it will shoot all the way through to the opposite wall, where it will quickly solidify. Successive incoming material streams then fold over the previously frozen stream, and the stream lines become locked into place. Often air pockets can be trapped in between the successive folds and further folds do not fully join creating a weaker part. To reduce the risk of jetting, one should always gate the part so that incoming material flow is directed into a nearby wall. After the stream has impinged on the wall, the plastic melt will spread in the appropriate fashion. Melt moves rapidly, cools unevenly and traps flow lines.

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Jetting Alternate gating

eliminates jetting

Ejector Pin Marks: Once the part is sufficiently cooled, the cavity opens and ejector pins push the part out. The pins usually leave marks in the area where the ejector pins pushed the part. There are four different possible causes of the pin marks: 1) the pin is above the flush line, 2) the pin is below the flush line, 3) there is clearance around the pin and 4) material is too soft at ejection and one pushes through the part.

Ejector pin marks are virtually unavoidable. It is almost impossibly to have the pin surface be perfectly even with that of the cavity wall. However, one can minimize the severity by trying to get the pins as flush with the cavity wall as is possible. Also, there should be very little clearance between the ejector pins and the holes in the die cavity, otherwise the plastic will “flash” around the ejector pins. Since some level of defect is unavoidable, one should take care to hide the pin marks as best as possible in inconspicuous places of the mold.

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1) Pin above flush

2) Pin below flush

3) Clearance around pin

4) One tries to eject the part before it solidifies and pins push through the part.

Place the ejector pins on hidden areas of the part.

Sink Marks: Sink marks are also common injection molding flaws. They are caused by excessively thick sections or abrupt changes in thickness. The pressure level in the cavity is fairly high during the injection. After the gate freezes, the pressure drops sharply. The walls in the thick and thin sections cool. However, the material in the center of the thicker sections cools slower than that in the thinner sections. Thus plastic in the thick sections is still molten after everything else, including the gate, is frozen. When the plastic melt in the thick section solidifies in the absence of any packing pressure, it will shrink away from the wall, causing sink marks. To reduce and in some cases eliminate sink marks, one should try to minimize or remove material from excessively thick sections. In general, it is good design practice to core out thick sections and use ribs for added support. In the example to the left, notice all the cross-hatched areas. This is the material that is removed in the improved design. Note that these principles also help to eliminate residual stresses as well. Of course in some cases sink marks are unavoidable. For example when using ribs for support, there is a thick section where the rib meets the wall. In these cases, sink marks are often masked by surface textures or by intentionally designing grooves in the surface.

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BadDesign

ImprovedDesign

When unavoidable, sink marks can be masked by surface texture

Thick sections cause sink marks

Warpage/Residual Stresses: Warpage is the “out of plane” distortion of an injection molded part, generated by constraining the part while cooling. Warpage is typically caused by anisotropic shrinkage. Several causes for anisotropic shrinkage are: Variations in thickness, Differing shrink rates due to melt orientation, Uneven cooling, Differences in the mold cavity pressure. If the part is massive enough to resist warpage, residual stresses will result. Since gates are usually highly oriented and have extremely fast cooling rates, residual stresses are always present near the gates.

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Suggested design changes to minimize

distortion

Less Common Methods

Expansion expandable polystyrene bead fill mold and bond (steam)

Foaming liquid chemicals combine & cure (isocynate polyal)

Spinning produces plastic fibers similar to extrusion

Solid Phase Form forming plastic below glass temperature similar to stamping or forging

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Common Polymers

ABS (Acrylanitrile Butadiene Styrene): amorphous, good Impact Strength, excellent appearance, easy to process computer housings, small appliances, automotive interior, & medical componentsAcrylic: amorphous polymers, excellent clarity, excellent weatherability optical & outdoor applicationsCellulosics: among the first thermoplastics developed: smell funny, very flammable Nylon 6: semi-crystalline polymer, good cost to performance ratio, lower numbered nylons, 6 ,6-6, 4-6, absorb moisture and change their properties as a resultPolycarbonate: amorphous material, excellent Impact Strength, clarity, & optical properties currently long lead times for this material.Polyethylene High Density: widely used, inexpensive, thermoplastic, easy to process, good to excellent chemical resistance, soft & not for use above 150 FPolypropylene: semi-crystalline material, low temperature material, excellent chemical resistance difficult to mold to extremely close tolerances Polystyrene High Impact (HIPS): few cents more than crystal styrene, to pay for the rubber modifier, opaque & very widely used, lower modulus, better elongation, & less brittle than crystal styrenePVC Polyvinyl Chloride Rigid: properties similar to ABS (except appearance) at a slightly reduced cost primarily for water pipe and pipe fittings, occasionally for electrical enclosures *in plastic phase PVC is corrosive to molds & machines (non corrosive as a solid)

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Shaping Processes for Thermoplastics

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Shaping Processes for Thermosets

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Sheet Molding

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The manufacturing process for producing reinforced-plastic sheets. The sheet is still viscous at this stage; it can later be shaped into various products.

Prepregs

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(a) Manufacturing process for polymer-matrix composite (b) Boron-epoxy prepreg tape.

(b)(a)

Examples of Molding Processes

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(a) Vacuum-bag forming. (b) Pressure-bag forming.

Manual methods of processing reinforced plastics: (a) hand lay-up and (b) spray-up. These methods are also called open-mold processing.

Pultrusion

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Schematic illustration of the pultrusion process.

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