Twin sheet thermoforming

Twin-sheet thermoforming is a process of vacuum or pressure forming two sheets of plastic essentially simultaneously, with a separate mold on the top and bottom platens. Once the plastic has been molded, it remains in the molds, and while still at its forming temperature the two molds are brought together under high pressures, and the two sheets are welded wherever the molds dictate a weld.

Process

The process creates 3 dimensional parts with formed features on both sides. The parts are typically very strong, stiff, and quite light-weight.

Step 1: Two preheated thermoplastic sheets are simultaneously heated between the two molds till they are are entirely plasticized.

Step 2: On reaching the the specific temperature the two molds move together. The two Sheets are deep-drawn and tightly molded to each other in one step. No adhesives are used. There are neither resulting pressure nor strain.

Step 3: To achieve a high level of detail or precision, the forming can be supported by high pressure or (and) vacuum

Step 4: ... and the hollow body, without solvent, molding additives and adhesives, is finished. Furthermore, there are no inner strains.

Materials

• H.M.W. HD Polyethylene
• ABS
• PC/ABS
• Polycarbonate

Typical applications are

pallets, industrial dunnage, portable toilets, medical housings, surfboards, fuel tanks, air/ventilation ducts, electrical enclosures, recreational boats, cases, toys, marine products, doors, tables, spine boards and numerous transportation-related products.

Main Advantages

1. Increased Structural Integrity and Rigidity
2. Enclosed Cross-Section Capability
3. Low Tooling Cost
4. Internal Reinforcement Options: Structural Member, Rigid Foam, Etc.
5. The process has some distinct advantages over blow-molding and rotomolding.
6. The process has some distinct advantages over blow-molding and rotomolding.
7. Compared to blow-molding, the twin-sheet process:
8. Tooling and machines are more cost-competitive for small to modest run sizes.
9. Each sheet may be a different thickness, material or color.
10. More flexibility with parting line structure is possible.
11. Compared to Rotomolding, the twin-sheet process:
12. Many more resin types are available in twin-sheeting.
13. More structural beams can be created in twin-sheeting
14. Much higher production rates

Inline thermoforming

The in-line thermoforming process is designed to take advantage of the hot sheet coming off the extruder. The sheet is mechanically conveyed directly from the extruder through the oven to maintain the sheet at a forming temperature and then to the forming station.

The forming step must be synchronized with the extruder take-off speed. This type of thermoforming is usually limited to sheet 0 125" or thinner and applications that do not require critical thermoforming. i.e.. optimum material distribution and close tolerances.

This process Is more difficult to control than other thermoforming processes· The major disadvantage is that with the extruder and former being tied directly together an upset In one causes a shutdown in both. The majority of roll-fed machines or in-line machines are commonly used for the production of thin-walled products such as cups, trays, lids. internal packaging, and other finished products with a finished wall of 0.003 to 0.060+ in· in thickness. Because of the speed of these machines, secondary operations are incorporated within the unit. These may consist of printing, filling, sealing, die cutting, scrap cutting, or automated removal and stacking of finished product. The normal roll-fed machines consist of the roll station, upper and lower heating banks. form Station, cooling station, and trim station.

Matched die forming

In this process, both halves of the part are formed by molds with no vacuum or air pressure. The sheet is heated until it is soft, and then both mold halves clamp together to form the part. Used with parts that do not have large draws.

Advantages

• excellent definition and dimensional contol on both sides.
• complexity
• high tolerances

Stretch forming

Stretch Forming A plastic sheet forming technique in which the heated thermoplastic sheet is stretched over a mold and subsequently cooled. It is quick, efficient, and has a high degree of repeatability.

Advanced pre-stretch forming or mechanical assisted forming techniques require thermoforming equipment with both top and bottom platens. Automatic machine sequence control is also usually required.

The only real advantage of this process is that only a male form is needed. The disadvantages are many, and include requiring the male form to beconstructed strong enough to resist thelarge forces exerted by the mechanicalequipment that is needed to stretchmany lineal inches of plastic in onedirection. That force may reach manytons.Additionally, minor dirt particles on,or minor deviations of the molds exteri-or surface, will show up as opticaldefects (mark-off) on the concave innersurface of the part. Such defects arevery difficult to remove by later effortsusing abrasives and polishing products.Other problems are excessive thin-ning of the plastic at the deepest por-tion of the formed part and the intro-duction of severe internal stresses thatusually result in early failure when thepart is exposed to sunlight

Drape forming

Drape forming is similar to straight vacuum forming except that after the sheet is framed and heated, it is mechanically stretched, and a pressure differential is then applied to form the sheet over a male mould. In this case, however, the sheet touching the mould remains close to its original thickness. It is possible to drape-form items with a depth-to-diameter ratio of approximately 4 to 1; however, the technique is more complex than straight vacuum forming. Male moulds are easier to build and generally cost less than female moulds; however, male moulds are more easily damaged. Drape forming can also be used with gravitational force alone. For multi-cavity forming, such as tote trays, female moulds are preferred because they do not require as much spacing as male moulds.

Processing steps

Step 1. The plastic sheet is clamped in a frame and heated. Heating can be timed or electronic sensors a can be use to measure sheet temperature or sheet sag.

Step 2. Drawn over the mold - either by pulling it over the mold and creating a seal to the frame, or by forcing the mold into the sheet and creating a seal. The platen can be driven pneumatically or with electric drive. In some very small machines the platen can be manually moved up or the clamped sheet can be manually pushed over the mold.

Step 3. Then vacuum is applied through the mold, pulling the plastic tight to the mold surface. A fan can be used to decrease sheet cooling time.

Step 4. After the plastic sheet has cooled, the vacuum is turned off and compressed air is sent to the mold to help free it from the plastic. The platen then moves down pulling the mold from the formed part. The formed sheet is unclamped, removed, and a new cycle is ready to start.

Main techniques

differing by the position of the mold during the first stage.

1st Method: The sheet (without masking) is placed on top of the mold in its basic, flat state. Both sheet and mold are then slid into a hot-air circulating oven and heated to about 150-155?°C (300-312?°F). When the sheet (and mold) reaches the required temperature it sags and drapes over the heated mold. Both are then pulled out of the oven and quickly helped, by gloved hands, to conform more precisely to the mold. It is then allowed to cool down.

2nd Method: The sheet is placed into a hot-air circulating oven (without masking), and heated to about 150-155?°C (300-312?°F). When the sheet reaches the required temperature it is quickly pulled out of the oven and placed on top of the mold. there the sheet sags, aided quickly by the gloved helping hands, and takes the accurate shape of the mold. For better results we recommend pre-heating the mold to about 80-100?°C (175-210?°F) before putting the heated sheet on top. Then it is, likewise, allowed to cool down.

Advantages

• better part dimensional control on inside of part
• lower mold costs
• ability to grain surface (tubs, showers, counter tops, etc.)
• faster cycle times.
• Disadvantage is more scrap due to larger clamps and trim area

Applications

Drape forming is widely used for large panels that require retaining a simple non-flat shape as in a curved display wall. Another useful application of this process is for the construction of wide sections of odd-shaped walls that will still retain overall even material thickness.

Pressure forming

Pressure forming is a variation of vacuum forming that utilizes both vacuum and compressed air to force the plastic sheet against the mold. As the platens are closed, the vacuum pulls on one side of the sheet and compressed air pushes on the other. Specially shaped tooling is used to match the top and bottom halves of the mold creating a seal to maintain pressures of up to 500 psi, therefore, the platens must be locked together. This compressed air pressure reduces the cycle time and makes it possible to run at lower temperatures, it also improves the distribution of the material creating a more even wall thickness and enhances the detail of the part to a nearly-injection-molded quality. After the part has been formed, the platens unlock and one of the platens moves out of the way to speed up the cooling process.

The increased air pressure will require a stronger mold and a locking device for the platens so consequently a higher tooling expense will be incurred.

Materials

Theoretically, any thermoplastic material can be pressure formed. However, some materials are more difficult to work with than others. Polyethylene, for instance, flows easily and causes few problems for pressure formers. With vacuum alone, polyethylene can be intricately formed. On the other hand, polycarbonate, which chills quickly, can cause manufacturers to be concerned about tool design and plug assists.

Medical device manufacturers usually specify that their products should be formed of a material that passes the Underwriters Laboratories (UL) 94 V0 or 94 5V tests for flammability. The resins most commonly used in pressure-formed medical products are flame- retardant grades of acrylonitrile butadiene styrene (ABS).

In many cases, assists are used to help distribute material evenly and to coin it into sharp or narrow corners. Depending on its complexity, the design of a product's tooling may require the former to use matched heated molds and assists; otherwise, assists can be made of low-heat-transferring materials such as wood.

Advantages

• Sharp, crisp lines and details
• Low tooling costs
• Short lead time
• Textured surfaces and molded in colors
• Formed in undercuts
• Ideal for short runs
• Zero degree draft on sidewalls
• Embossed and Debossed areas
• Highly detailed openings
• Superior, uniform tolerance control

Applications

Use pressure forming when the part will be the "face" of the product expecting a long life. You can pressure form a company logo or model designation with styling lines, surface texture or other features in a light weight and durable part. Use pressure forming when you have undercuts or rims and a greater depth of draw.

Processing Steps

Material is heated to proper temp then moves over the mold.

Platens close and lock.

Vacuum and air pressure are applied.

Free Forming

Free Forming This method of thermoforming does not use a mold. Instead, an acrylic sheet is clamped in a frame and either a vacuum or compressed air draws the material to a desired depth. An electric eye determines when the proper depth has been reached and cuts off the pressure. Since only air touches the sheet of material, there is no markoff. Free forming is used to create windshields for planes, skylights, or anything where optical quality is necessary.

Advantage is achieving high clarity.

Vacuum snapback Thermoforming

Vacuum-snap back is an excellent and often used process for forming deep draw products with uniform wall thickness. Vacuum is used to pre-stretch the hot plastic before the mold makes contact with the sheet. Vacuum snap-back, while more complex than plug assist, can produce deeper drawn products with better wall uniformity and less mark-off. A vacuum pre-stretch box is required. The pre-stretch box is sealed against the hot sheet and vacuum is applied. The plastic is drawn into the box as a hemisphere with the height of the hemisphere usually controlled by a photocell. Other methods can be used to control the hemisphere height, but a photocell works well.

The steps of vacuum snap-back

After the plastic sheet is heated and the sheet cart returns to the forming station, the bottom platen moves up sealing the vacuum pre-stretch box against the hot sheet. Vacuum is then applied. When the stretching plastic crosses the photocell beam, vacuum is turn off.

The mold is moved into the formed hemisphere. When the mold is sealed against the hot plastic, vacuum is applied to the mold and vacuum is released from the pre-stretch box causing the plastic to snap to the contours of the mold.

The pre-stretch box is then lowered and cooling air is blown against the hot plastic. After the plastic cools, the mold vacuum is released, air eject is applied through the mold and then the mold is removed from the formed plastic part.

Placing the mold on the bottom platen and the pre-stretch box on the top platen will also work for vacuum snap-back.

Advantages

well controlled part thickness

though longer cycle times

Plug assist forming

Plug assist forming is a widely used forming technique and requires the use of a female (cavity) mold. The limited depth of draw of female molds is improved by the use of plug assist. With plug assist the plastic sheet is mechanically pre-stretched by a plug that is pushed into the hot plastic before the application of vacuum to the mold. The plug has a geometry that is usually 10 - 30 percent smaller than the interior of the female mold cavity. The plug is constructed of materials with low thermal conductivity or is heated. Low thermal conductivity plugs or heated plugs must be used to keep the plastic sheet from cooling when the sheet comes in contact with it. Materials such as wood, syntactic foam, and cast thermoset plastics can be used to make a low thermally conductive plug. This insulator type plug can be covered with felt to reduce mark-off.

Aluminum with temperature controlled electric heaters can also be used. Aluminum plugs produce excellent results but are usually more costly than insulator type plugs. Different wall and bottom thickness can be produced by controlling how deep the plug goes into the mold and by controlling and varying plug temperatures.

A thermoforming method that combines either pressure or vacuum force with a mechanical device to force plastic material onto a mold.

Learn more about plug-assisted forming in the class "Principles of Thermoforming 265" below.

Processing steps

The steps in plug assist forming are:

After the sheet is heated and the sheet cart moves back to the forming area, the bottom platen moves up to the plastic sheet and seals.

The top platen with the plug moves down pushing the plug into the hot plastic.

After the plug reaches the required depth, vacuum is applied to the female mold forming the plastic to the contours of the mold.

The top platen moves back up and cooling fans cool the plastic covering the inside of the female mold.

Advantages

better wall thickness uniformity especially for cup or box shapes

reduces stretching or thinning of material during forming.

Types of thermoforming

Vacuum Forming

Vacuum forming is a plastic thermoforming process that involves forming thermoplastic sheets into three-dimensional shapes through the application of heat and pressure. In general terms, vacuum forming refers to all sheet forming methods, including drape forming, which is one of the most popular. Basically during vacuum forming processes, plastic material is heated until it becomes pliable, and then it is placed over a mold and drawn in by a vacuum until it takes on the desired shape.

Plug assist forming

Plug assist forming is a widely used forming technique and requires the use of a female (cavity) mold. The limited depth of draw of female molds is improved by the use of plug assist.

Vacuum snapback

Vacuum-snap back is an excellent and often used process for forming deep draw products with uniform wall thickness. Vacuum is used to pre-stretch the hot plastic before the mold makes contact with the sheet. Vacuum snap-back, while more complex than plug assist, can produce deeper drawn products with better wall uniformity and less mark-off. A vacuum pre-stretch box is required.

Billow Forming

A method of thermoforming sheet plastic in which the heated sheet is clamped over a billow chamber. Air pressure in the chamber is increased causing the sheet to billow upward against a descending male mold.

Free Forming

This method of thermoforming does not use a mold. Instead, an acrylic sheet is clamped in a frame and either a vacuum or compressed air draws the material to a desired depth. An electric eye determines when the proper depth has been reached and cuts off the pressure. Since only air touches the sheet of material, there is no markoff.

Pressure Forming

This process is similar to vacuum forming, except with the addition of pressure, which pushes the sheet into the shape of the mold. This process is mainly used for parts that require styling and aesthetic qualities because pressure forming creates greater detail, allowing for textured surfaces, undercuts and sharp corners, which are not as easily created with vacuum forming.

Drape forming

In drape forming, a sheet of plastic is heated and stretched down, generally over a male mold. Next, depending on the shape of the mold, gravity alone will pull the material to the mold or commonly, a vacuum is applied to draw the sheet to the mold which will more detail to the inside of the part.

Stretch Forming

A plastic sheet forming technique in which the heated thermoplastic sheet is stretched over a mold and subsequently cooled. It is quick, efficient, and has a high degree of repeatability.

Matched Die Forming

In this process, both halves of the part are formed by molds with no vacuum or air pressure. The sheet is heated until it is soft, and then both mold halves clamp together to form the part. Used with parts that do not have large draws.

Inline thermoforming

In this process the plastic film moves from a roll onto the inline equipment and through the heating section. The heated material advances into the forming section where pressure and/or vacuum force the plastic onto a mold. It then proceeds to another station where formed parts are die-cut.

Twin sheet forming

Twin-sheet thermoforming is a process of vacuum or pressure forming two sheets of plastic essentially simultaneously, with a separate mold on the top and bottom platens. Once the plastic has been molded, it remains in the molds, and while still at its forming temperature the two molds are brought together under high pressures, and the two sheets are welded wherever the molds dictate a weld.

Thin and thick gauge thermoforming

There are two general thermoforming process categories. Sheet thickness less than 1.5 mm (0.060 inches) is usually delivered to the thermoforming press in rolls. Thin-gauge, roll-fed thermoforming applications are dominated by rigid or semi-rigid disposable packaging.

Vacuum Forming

Vacuum forming, commonly known as vacuforming, is a simplified version of thermoforming, whereby a sheet of plastic is heated to a forming temperature, stretched onto or into a single-surface mold, and held against the mold by applying vacuum between the mold surface and the sheet. The vacuum forming process can be used to make most product packaging, speaker casings and even car dashboards.

Normally, draft angles must be present in the design on the mold (a recommended minimum of 3°), otherwise release of the formed plastic and the mold is very difficult.

Vacuum forming is usually – but not always – restricted to forming plastic parts that are rather shallow in depth. A thin sheet is formed into rigid cavities for unit doses of pharmaceuticals and for loose objects that are carded or presented as point-of-purchase items. Thick sheet is formed into permanent objects such as turnpike signs and protective covers.

Relatively deep parts can be formed if the form-able sheet is mechanically or pneumatically stretched prior to bringing it in contact with the mold surface and before vacuum is applied.

Suitable materials for use in vacuum forming are conventionally thermoplastics, the most common and easiest being High Impact Polystyrene Sheeting (HIPS). This is molded around a wood, structural foam or cast/machined aluminum mold and can form to almost any shape. Vacuum forming is also appropriate for transparent materials such as acrylic which are widely used in applications for aerospace such as passenger cabin window canopies for military fixed wing aircraft and "bubbles" for rotary wing aircraft.

problems with vacuum forming

Moisture absorption: absorbed moisture can expand forming bubbles within the plastic's inner layers. This may be solved by drying the plastic for an extended period at high but sub-melting temperature.

Webs may form around the mold, which is due to overheating the plastic and so must be carefully monitored. Webbing can also occur when a mold is too large or parts of the mold are too close together.

Thin and thick gauge thermoforming

There are two general thermoforming process categories. Sheet thickness less than 1.5 mm (0.060 inches) is usually delivered to the thermoforming machine from rolls or from a sheet extruder. Thin-gauge roll-fed or inline extruded thermoforming applications are dominated by rigid or semi-rigid disposable packaging. Sheet thicknesses greater than 3 mm (0.120 inches) is usually delivered to the forming machine by hand or an auto-feed method already cut to final dimensions. Heavy, or thick-gauge, cut sheet thermoforming applications are primarily used as permanent structural components. There is a small but growing medium gauge market that forms sheet 1.5 mm to 3 mm in thickness.

Heavy-gauge forming utilizes the same basic process as continuous thin-gauge sheet forming, typically draping the heated plastic sheet over a mold. Many heavy-gauge forming applications use vacuum only in the form process, although some use two halves of mating form tooling and include air pressure to help form. Aircraft windscreens and machine gun turret windows spurred the advance of heavy-gauge forming technology during WWII. Heavy gauge parts are used as cosmetic surfaces on permanent structures such as automobiles, refrigerators, spas, and shower enclosures, and electrical and electronic equipment. Unlike most thin-gauge thermoformed parts, heavy-gauge parts are often hand-worked after forming for trimming to final shape or for additional drilling, cutting, or finishing, depending on the product. Heavy-gauge products typically are of a "permanent" end use nature, while thin-gauge parts are more often designed to be disposable or recyclable and are primarily used to package or contain a food item or product.

Thermoforming has benefited from applications of engineering technology, although the basic forming process is very similar to what was invented many years ago. Microprocessor and computer controls on more modern machinery allow for greatly increased process control and repeatability of same-job setups from one production run to the next, usually with the ability to save oven heater and process timing settings between jobs. The ability to place formed sheet into an inline trim station for more precise trim registration has been hugely improved due to the common use of electric servo motors for chain indexing versus air cylinders, gear racks, and clutches on older machines. Electric servo motors are also used on some modern and more sophisticated forming machines for actuation of the machine platens where form and trim tooling are mounted, rather than air cylinders which have traditionally been the industry standard, giving more precise control over closing and opening speeds and timing of the tooling. Quartz and radiant-panel oven heaters generally provide more precise and thorough sheet heating over older cal-rod type heaters, and better allow for zoning of ovens into areas of adjustable heat.

An integral part of the thermoforming process is the tooling which is specific to each part that is to be produced. Thin gage thermoforming as described above is almost always performed on in-line machines and typically requires molds, plug assists, pressure boxes and all mounting plates as well as the trim tooling and stacker parts that pertain to the job.

Thick or heavy gage thermoforming also requires tooling specific to each part, but because the part size can be very large, the molds can be cast aluminum or some other composite material as well as machined aluminum as in thin gage. Typically thick gage parts must be trimmed on CNC routers or hand trimmed using saws or hand routers. Even the most sophisticated thermoforming machine is limited to the quality of the tooling. Some large thermoforming manufacturers choose to have design and tool making facilities in house while others will rely on outside tool-making shops to build the tooling.

Thermoforming

Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and certain material types, is heated in an oven to a high-enough temperature that it can be stretched into or onto a mold and cooled to a finished shape.

In its simplest form, a small tabletop or lab size machine can be used to heat small cut sections of plastic sheet and stretch it over a mold using vacuum. This method is often used for sample and protoype parts. In complex and high-volume applications, very large production machines are utilized to heat and form the plastic sheet and trim the formed parts from the sheet in a continuous high-speed process, and can produce many thousands of finished parts per hour depending on the machine and mold size and the size of the parts being formed.

Thermoforming differs from injection molding, blow molding, rotational molding, and other forms of processing plastics. Thin-gauge thermoforming is primarily the manufacture of disposable cups, containers, lids, trays, blisters, clamshells, and other products for the food, medical, and general retail industries. Thick-gauge thermoforming includes parts as diverse as vehicle door and dash panels, refrigerator liners, utility vehicle beds, and plastic pallets.

In the most common method of high-volume, continuous thermoforming of thin-gauge products, plastic sheet is fed from a roll or from an extruder into a set of indexing chains that incorporate pins, or spikes, that pierce the sheet and transport it through an oven for heating to forming temperature. The heated sheet then indexes into a form station where a mating mold and pressure-box close on the sheet, with vacuum then applied to remove trapped air and to pull the material into or onto the mold along with pressurized air to form the plastic to the detailed shape of the mold. (Plug-assists are typically used in addition to vacuum in the case of taller, deeper-draw formed parts in order to provide the needed material distribution and thicknesses in the finished parts.) After a short form cycle, a burst of reverse air pressure is actuated from the vacuum side of the mold as the form tooling opens, commonly referred to as air-eject, to break the vacuum and assist the formed parts off of, or out of, the mold. A stripper plate may also be utilized on the mold as it opens for ejection of more detailed parts or those with negative-draft, undercut areas.

The sheet containing the formed parts then indexes into a trim station on the same machine, where a die cuts the parts from the remaining sheet web, or indexes into a separate trim press where the formed parts are trimmed. The sheet web remaining after the formed parts are trimmed is typically wound onto a take-up reel or fed into an inline granulator for recycling.

INJECTION MOLDING TERMS

Air Hole : A hole in a molding by air or gas trapped in the melt during solidification.

Air Traps : Converging flow fronts surround and trap a bubble of air or gas which will cause a surface blemish in the part .

Bottom Plate: The plate fixed to the moving half of die and facilitates attachment to the injection molding machine .

Cavity : The mold or die impression that gives a molding its external shape.

Core: Male potion of the mould, which forms internal shape of the component.

Cooling: Channels located within the body of a mold through which a cooling Channels medium is circulated to control the mold surface temperature.

Cycle Time : The time required by an injection molding system to mold a part and return to its original position/state.

Draft : An angle or taper on the surfaces of a pattern or insert that facilitates removal of parts from a mold or die.

Edge gate:Entrance to the part from the runner located on the parting line.

Ejector: A mechanism that pushes the solidified molding out of the die.

Feeding: In a molding, providing plastic melt to a region undergoing solidification, usually at a rate sufficient to fill the mold cavity ahead of the solidification front and to compensate for any shrinkage accompanying solidification.

Flash: A thin section of plastic formed at the parting surface.

Temperature :The material temperature at each point as that point was filled. The result shows the changes in the temperature of the flow front during filling.

Gate: Is a channel or orifice connecting the runner with the impression.

Guide Pillars : Cylindrical members meant to align the mold halves and are & Bushes made of hardened steel. Bushes are meant for wear resistance between housing and pillars.

Hesitation : Hesitation is a surface defect that results from the stagnation of polymer melt flow over a thin-sectioned area, or an area of abrupt thickness variation.

Housing : A metal block which houses inserts (core and cavity), pillars, return pins etc.

Injection: The process of forcing molten melt under pressure into molds.

Injection Location :The injection location is the place where the molten plastic is forced into the mold cavity.

Insert : A part formed from a second material, usually a metal that is placed in the molds and appears as an integral structural part of the final molding.

Locating ring: Is a circular member fitted to top-plate of the mould. Its purpose is to register the mould in its correct position on the injection machine.

Molding: Plastic part molded to the required shape by pouring or injecting melt into a mold, as distinct from one shaped by cutting or a mechanical working process.

Mold Temperature: The temperature at which the mold is maintained. Often the most important benefit of raising mold temperature is that it allows a slower injection rate without the plastic getting too cold.

Nozzle: Hollow metal hose screwed into the extrusion end of the heating cylinder of an injection machine designed to form a seal under pressure between the cylinder and the mold.

Parting Plane : In molding, the dividing plane between mold halves.

Runner: A Channel through which melt flows from one receptacle to another.

Sprue Bush: A bush with a tapered hole, which connects nozzle with runner.

Tie Bar: A bar-shaped connection added to a casting to prevent distortion caused by uneven contraction between two separate members of a casting.

Vent : A small opening or passage that facilitates the escape of gases when the melt is fills the die cavity.

EFFECT OF ALLOYING ELEMENTS

a) Effect of Carbon:

Carbon makes the steel hard and more wear resistance. Steels having less than 0.3% carbon cannot be hardened. Beyond 3% carbon steels can be through hardened. Hardness of steel increases upon 0.8% carbon and beyond that, hardness does not increase but wear resistance increase.

b) Effect of Chromium:

Chromium increases hardenability, toughness and wear resistance. Chromium causes greater hardness penetration.

c) Effect of Nickel:

It improves toughness and wear resistance. When used with increasing hardness element like chromium it lowers hardening temperature and tends the steel towards oil hardening. Higher the percentage of nickel along with the chromium results in rust free steel – stainless steel.

d) Effect of manganese:

It helps to make the steel sound while casting as an ingot. It lowers the critical point and hence the hardenability by 1.5%. Addition of more manganese increases hardenability. The depth of penetration of hardness increases.

e) Effect of Tungsten:

It increases wear resistance when added in fairly large quantity (1.5%). 12 – 20% tungsten with chromium gives a new property names as “Red Hardness”. It unites with the carbon to form tungsten – carbide, which attains very high hardness and wear resistance.

f) Effect vanadium:

It helps in obtaining fine-grained steel. It increases red hardness property. Small percentage increases tensile strength, yield strength, hardness and wear resistance.

g) Effect of Molybdenum:

It has both properties of chromium and tungsten. Like chromium it increases hardness penetration and inclines the steel towards oil or oil hardening. Like tungsten it increase wear resistance and red hardness. 5 – 12% with chromium, tungsten and vanadium forms a molybdenum based HSS material.

SELECTION OF TOOL STEEL

The selection of tool steel is very important for any tool upon which the development and economical operation of the process depends. Care should be taken especially while injection moulding plastics, which may affect the factors like mould life, quality, cost of mould and machine etc.

The steel used in the manufacture of mould varies depending on the applications. Proper material selection and proper combination of alloys in varying percentages are required for finished moulds.

Following properties are desired from the steels used in mold making

• Characteristics permitting economical machining
• Capacity for heat treatment without problems
• Ease of polishing
• High Wear resistance
• High Thermal conductivity
• Good Corrosion resistance
• Good Toughness
  

GENERAL CONSIDERATIONS OF MOULD DESIGN

In the design of injection mold following machine specifications are to be considered. As frequently an existing machine or a certain machine size poses an important limitation, to which the design engineer has to submit.

1. Injection pressure:

This is the pressure by which thermo-plastic material is injected into the cavity.

2. Shot capacity:

The amount of melt in grams, which can be conveyed into the mold with one stroke of the screw.

3. Plasticizing rate:

This is the maximum amount of material that may be plasticized to an acceptable melt quality per unit time. It is expressed in terms of weight of polystyrene plasticized per hour.

4. Clamping force:

Clamping force is the force required to keep the mold halves together when injection takes place. Clamping force machine should be more than clamping tonnage required of the mold to prevent escape of the melt at parting surface. If the clamping force is insufficient, “flash” occurs at the mold joint or interface of the mold.

5. Distance between tie the bars:

The tool length and width should be within the distance between the tie bars to mount the tool easily without constraints. This decides whether the tool can be accommodated in the machine or not.

6. Max & Min daylight:

The minimum ‘day light’ is the distance between the mold platens when tool is in closed condition. The maximum daylight is the distance between the die platens when the tool is in open condition. The mold height is required to be larger than the minimum daylight and less than the maximum day light.

7. Ejection stroke:

The depth of the component decides the ejection stroke. Ejection stroke is indicative of the maximum ejection movement available.

BASIC TYPES OF INJECTION MOULDS

There are two main types of injection molds:

1. Cold runner.
2. Hot runner.
A runner is the channel in the mold that conveys the plastic from the barrel of the injection molding machine to the part.

1. Cold runner

In a cold runner mold, the runner is cooled and ejected with the part. Every cycle, a part and a runner are produced. The obvious disadvantage of this system is the waste plastic generated. The runners are either disposed of, or reground and reprocessed with the original material. This adds a step in the manufacturing process. Also, regrind will increase variation in the injection molding process, and could decrease the plastic's mechanical properties.
Despite these disadvantages, there are many significant advantages to using a cold runner mold. The mold design is very simple, and much cheaper than a hot runner system. The mold requires less maintenance and less skill to set up and operate. Color changes are also very easy, since all of the plastic in the mold is ejected with each cycle.

2. Hot runner

In a hot runner mold , the runner is situated internally in the mold and kept a temperature above the melting point of the plastic. Runner scrap is reduced or eliminated. The major disadvantages of a hot runner are that it is much more expensive than a cold runner, it requires costly maintenance, and requires more skill to operate. Color changes with hot runner molds can be difficult, since it is virtually impossible to remove all of the plastic from an internal runner system.
Hot runners have many advantages. They can completely eliminate runner scrap, so there are no runners to sort from the parts, and no runners to throw away or regrind and remix into the original material. Hot runners are popular in high production parts, especially with a lot of cavities.

2.8.1 INJECTION MOULDING MACHINE PARAMETERS

Injection Moulding Machines have become highly sophisticated, complex, computerized machines with many features that need to be considered when deciding which machine will be able to make a particular product.

In the design of injection mould, the following machine specifications are to be considered.

1. Injection pressure

This is the pressure in the barrel at the point of injection, expressed in kg/cm2 or kbar. A higher L/D ratio produces greater Injection Pressure. The greater the Injection Pressure the better the quality of product produced.

2. Shot weight

The shot weight is the weight of the component including the feed system. Calculated shot weight should be less than the machine shot weight.

The shot weight of a machine does not tell us the maximum volume of part that that machine can produce, because the pressures for moulding are higher than for extruding. Typically, the maximum volume of a low quality part will be 85% of the Shot Weight, or around 75% for a high quality part.

Parts need to weigh between 35% and 85% of the Shot Weight of a machine, any lower and the machine may be damaged during production, any higher and the mould cavity or cavities will not be filled.

3. Clamp force

The melt is injected into the mould under very great pressure, which the mould halves, and any other components or cores, must be able to resist in order to maintain their shape and to avoid any seepage. The larger a component is, the greater the pressure and so, the greater the clamp force.

The clamping force required to keep the mould closed during injection must exceed the force given by the product of the opening pressure in the cavity and the total projected area of all impressions and runners.

4. Distance between Tie bars

It is the minimum distance between the edges of the tie bars that guides the moving platens. This measurement limits the size of moulds that can be placed between the tie-bars and into the moulding machine. Tie bars are used in moulding machine to hold the platens. It also guides the moving platen during closing and opening of the mould.

5. Minimum and Maximum Mould Height

It is the minimum and maximum distance that can be adjusted between the stationary and moving platens. The mould shut height should always lie in between the maximum and minimum mould height.

6. Mould opening stroke

This is the distance that the moving mould half moves from mould closed to mould open. Because the injection moulded part has to clear the mould and have room to be removed from the machine, the Opening Stroke must be greater than:[(2*Mould Height)+Length of Sprue]

7. Ejection force

The force required to eject the component after cooling of the plastic material in the impression. It is about 3-5% of the locking force.

8. Maximum Ejection Stroke

This is the maximum distance through which the component can be ejected during the open condition. The ejection stroke to be adjusted on the machine depends on the component depth.

CLASSIFICATION OF INJECTION MOULDING MACHINE

Injection Moulding machines are broadly classified into two types,
1) Plunger type Injection Moulding Machine.
2) Screw type Injection Moulding Machine


1) Plunger type Injection Moulding Machine

The earliest moulding machines were of plunger type as illustrated in fig and still many of these types of machines are used. In this type of injection moulding machine the resin is fed from hopper into the barrel and heated through the input of thermal energy from heaters around the barrel. The molten resin collects in a pool in the barrel called the injection chamber.
The molten resin is then pushed forward by action of a plunger driven by a hydraulic system at the head of the machine. To facilitate the melting of any residual solid material and to give better mixing of the melt, the molten resin is pushed past a torpedo or spreader that, along with a back pressure plate, imparts shear to the melt. Then the resin flows through a nozzle into the mould.

2) Screw type Injection Moulding Machine

Figure shows a schematic drawing of the injection end of a reciprocating screw machine. The extruder screw, which is contained in the barrel, is turned most often by a hydraulic motor (as contrasted to an electric motor attached to a gear system) point, at which time the screw stops. The rotary shutoff valve is rotated so that when the injection plunger advances, the material is injected into the mold. The main advantages of a two-stage screw are that the material passes over the whole As the screw turns, it picks up material from the hopper. As it progresses down the screw, the resin is compacted, degassed, melted, and pumped past the nonreturn flow valve assembly at the injection side of the screw. This, in essence, is a check valve, which allows material to flow only from the back of the screw forward.

As the material is pumped in front of the screw, it forces back the screw, hydraulic motor, and screw drive system. In so doing, it also moves the piston and rod of the hydraulic cylinder(s) used for injection. Oil from behind the piston(s) goes into a tank through a variable resistance valve, called the back pressure valve. Increasing this resistance requires higher pressures from the pumping section of the screw, and results in better mixing, a slower cycle, and greater energy consumption.

The screw will continue to turn, forcing the carriage back until a predetermined location is reached. Then the rotation is stopped, and an exact amount of melted material is in front of the screw and will be injected into the mold at the appropriate time in the cycle. This is accomplished by using the hydraulic injection cylinder(s).