6. Blow molding 299 blowing operation that provides radial stretch and orientation (Figure 6.11). Blowing pressures range up to about 40 bar. The blow mold temperature is relatively high at 35 to 65C in order to minimize strain in the bottle. For a given bottle size, the degree of orientation is determined principally by the parison length and diameter. Stretch ratios are relatively high. In the wall thickness of the bottle body, the amount may be as high as 15:1. Axial stretch is about 4:1; diametrical stretch ranges about 3.5:1 Figure Gotl Example of stretched injection blow molding using a rod (left) and example of stretched injection blow molding by gripping and stretching the preform With the two-stage process, processing paramctcrs for both preform manufacturing and bottle blowing can be optimized. A processor does not have to make compromises for preform design and weight, production rates, and bottle quality as done on single-stage equipment. One can either make or buy preforms. And if one chooses to make them, one can do so in one or more locations suitable to the market. Both high-output machines and low out-put machines are available. The two-stage process, which permits injection molding of the preform and then shipping to BM locations, has allowed companies to become preform producers and to sell to BM producers. Thus companies that wish to enter the market with oriented containers can minimize their capital requirements. Extrusion stretch blow molding (ESBM) is a one-stage or two-stage process using two mold/mandrel sets where one is for preblow and the other for final blow. An extruded parison is first pinched off and blown 300 Plastic Product Material and Process Selection Handbook conventionally in a relatively small preblow mold to produce a closed- end preform. The preform is then transferred to the final blow mold where usually an extending stretch rod within the blowing mandrel bears on the closed preform end to stretch it axially. The stretched preform is then blown to impart circumferential stretch. Standard BM machines can be converted for extrusion stretch BM. The process is most often used for PVC bottles. Oriented PVC containers most commonly are made on single-stage, extrusion-type machines. The parison is extruded on either single- or double-head units. Temperature conditioning, stretching, and thread forming are done in a variety of ways depending on the design of the machine. Many of the processes in use are proprietary. Dip Blow Molding The dip BM process bears some resemblance to IBM in that it is a single-stage process performed with a preform on a core/blow pin.The difference is in the way the preform is made. The process uses an accumulator cylinder that is fed by an extruder. The cylinder has an injection ram at one end while the other is a free fit over the blow pin. The blow pin is dipped into the melt so that a neck mold on the pin seals the end of the accumulator cylinder. The injection ram is advanced to fill the neck mold; then the blow pin is withdrawn at a controlled rate so that it is coated with a melt layer extruded through the annular gap between the pin and the accumulator cylinder. The thiclcness of the coating can be varied or profiled to an extent by varying the speed of the blow pin and the pressure on the injection ram. After trimming, the preform is BM in the same manner used for IBM. The process results in a seam and flash free container with a high quality molded neck. The preform is produced at a lower pressure than that used for injection molding, so the machine can be lighter and of lower cost constructed. The preform is formed under relatively low stress. Process is best suited to the production of smaller containers. Multiblow Blow Molding The process is used for high volume BM of very small containers such as pharmaceutical vials and whiskey bottles. A multi-cavity mold is used with an extruded parison whose circumference approaches twice the total width of the closely spaced cavities. Before the mold closes, the parison is stretched and semi-flattened laterally so that it extends across the full width of the cavities. The process is usually combined with blow/fill/seal techniques. 6-Blow molding 301 Sequential Extrusion Sequential EBM is a special multi-material technique used for the pro- duction of special designed products. The different plastics are chosen typically to contribute complementary mechanical properties and are present in distinct sequential zones in the finished part. Normally two materials are used but three or more are also used. Separate external ram accumulators for each material serve the die head. These are operated sequentially, typically in A-B-A sequence, to produce a parison with three distinct material zones in axial succession. The parison is subsequently BM by normal techniques. An example for sequentially BM polypropylcnc is an automotive air duct in which a central flexible zone (Figure 6.12) joins rigid end sections. The flexible zone allows for installation mismatches, accom- modates thermal expansion, and damps vibration noise. The rigid portions allow for direct connection to other mechanical elements in the assembly. Figure 6~ 12 Examples of different shaped sequential extrusion blow molding products 302 Plastic Product Material and Process Selection Handbook Blow/Fill/Seal The blow/fill/seal process is a complete packaging technique that integrates the extrusion or IBM and container filling steps. This can provide for aseptic filling of the hot as-blown container and is used for pharmaceutical, food, and cosmetic products. The process employs a two-part mold in which the container body mold cavity blocks are separate from the neck-forming members. The body mold closes on the parison that is blown normally by a neck calibrating blow pin. Immediately, with the mold still closed, the liquid contents are injected through the pin. The pin is then withdrawn and the neck is formed and sealed under vacuum by the neck-forming members. Both mold parts then open to eject a filled and sealed container. Small containers may be formed entirely by vacuum rather than blowing. Blow Molding 3-D Because EBM is performed on a cylindrical parison, the conventional process is not well suited to the production of products with complex forms that deviate substantially from the parison axis. Such forms can be produced by conventional BM equipment, but only by using a parison that in its form blankets the complex mold cavity. This 3-D process in the past usually developed an excessive amount of pinch-off scrap. During the past few decades developments in parison handling robot equipment and in blow mold design make it possible to manipulate a relatively small parison into the complex mold cavity. The result is a BM largely free of flash and scrap and offering considerable process savings. There arc many such techniques, some of them proprietary property, and they are collectively lcnown as 3-D blow molding. Examples are shown in Figures 6.13 and 6.14. Blow Molding with Rotation The injection molding with rotation (MWR) is an example of processing at lower temperatures, pressure, etc. It is also called injection spin molding or injection stretched molding. This BM process com- bines injection molding and IBM, as performed in IBM reviewed, except it has the additional step of with melt orientation (Dow patent). The equipment used is what is commercially available for IM except the mold is modified so that either the core pin or outside cavity rotates. The rotated melt on its preform pin is transferred to a blow mold. The end product can come directly from the IMM mold or bc a result of two-stage fabrication: malting a parison and BM the parison. 164 This technology is most effective when employed with articles having a polar axis of symmetry; having reasonably uniform wall thickness; and 6. Blow molding 303 Figure 6o t :3 Example of a suction extrusion blow molding process fabricating 3-D products (courtesy of SIG Plastics International) whose dimensional specifications and part-to-part trueness are important to market acceptance. The MWR process requires no sacrifice of either cycle time or surface finish. Both laboratory and early (past) commercial runs identify good potentials for reducing cycle time; for either reducing the amount of plastic required or improving properties with the same amount of plastic, or both; and for sub- stituting less expensive plastics while achieving adequate properties in the fabricated product. During fabrication using the MWR process, two forces act on the plastic: injection (longitudinal) and rotation (hoop). The targeted balanced orientation is a result of those forces. As the product wall cools, additional high-magnitude, cross-laminated orientation is developed frozen in and throughout the wall thiclmess. Orientation on molecular 304 Plastic Product Material and Process Selection Handbook Figure 6,14 Examples of 3-D extrusion blow molded products in their mold cavities (courtesy of SIG Plastics International) planes occurs as each layer cools after injection. This orientation can change direction and magnitude as a function of wall thiclcness. The result is analogous to plywood or reinforced plastics (Chapter 15) and the strength improvements are as dramatic. In the MWR process, there is an infinite number of microscopic layers each of which has its own controlled direction of orientation. By appropriate processing conditions, both the magnitude and direction of the orientation and strength properties can be varied and controlled throughout the wall thickness. MOLD Blow mold usually consists of two halves, each containing cavities which, when the mold is closed, define the exterior shape of the BM (Chapter 17). Multiple cavity molds are used. Because the process produces a hollow article, there are no cores to define the inner shape. Mold details and actions will vary considerably according to the geometry of the product and the BM process in use. Even though the following review concentrates on EBM, the information can also be applied to IBM. The two halves that meet on a plane are known as the parting line. The plane is chosen so that neither cavity half presents an 6-Blow molding 305 undercut in the direction of mold opening. For most bottle designs, this requirement presents little or no difficulty. For products of asymmetrical cross-section, the parting line is placed in the direction of the greater dimension (Figure 6.1 5). Guide pillars/pins and bushings to ensure that there is no mismatch between the cavities align the two mold halves. With EBM the parison passes across the mold in the axis of the cavity and is pinched and compressed between the faces of the closing mold at the neck and base regions of the cavity. These are known as the pinch-off zones. Separate inserted mold blocks typically form the base and neck regions of the mold. The mold includes channels for the circulation of cooling water. F{gure 6,! 5 Example of a 3-part mold to fabricate a complex threaded lid With injection BM the preform only has a pinch-off at the neck. In EBM the pinch-off zone performs two functions. It must weld the parison to make a closed vessel that will contain blowing air, and it must leave pinched-off waste material in a condition to be removed easily from the blown product. 164 Flash caused by the pinch-off is an unavoidable evil in EBM. Ability to control the adverse effects of the flash is critical to success of the process. 228 Pinch-off generates excess material in the form of flash that is usually twice the thickness of the parts wall. This thicker plastic cools slower than the blown product. It is subject to fold-over and can adhere to the blown product. Flash imposes costly limits on BM efficiency. It has potential for significantly extending the molding cycle, primarily by increasing the time needed to cool the thick flash. This cycle increase could approach twice what would normally be required. Removal calls for a post-molding trim step that requires secondary equipment and poses a risk of damaging good parts. To reduce the time cycle a fabricator has some damaging options such as ejecting the part before the flash is sufficiently cooled. Because it is 306 Plastic Product Material and Process Selection Handbook still soft and pliable when ejected, it can create other problems such as a fold over on itself and adhering to adjoining surfaces of the part after ejection of the molding. Flash is also considerably more difficult to handle and trim while hot. In either case, the resultant penalty may be a significant increase in the part reject rate. By locating cooling lines as close as possible to the flash heat transfer to the cooling water will reduce cycle time. So it is critical to appreciate maximizing the heat transfer as much as possible to the flash area. By keeping the water turbulent takes advantage of operating the water in the proper Reynold's number (Chapter 17). When a parison is blown, a large volume of air must bc displaced from the mold cavity in a short time. Because blowing is carried out at relatively low pressure, it is essential to provide venting to allow this air to escape without resistance. Unless a gloss finish is required on the molding, it is common practice to sandblast the cavity to a fine matt finish. This helps air to escape as the expanding parison touches the cavity face but it is not sufficient in itself. Vent slots may bc cut at appropriate points into the mold parting face to a depth of 0.05 to 0.15 mm. The appropriate point is where there is a possibility for air to collect as the hot plastic expands in the cavity. Venting can also bc provided within the mold cavity by means of inserts equiped with vent slots, porous sintercd plugs, or by holes with a diameter not greater than 0.2 ram. Such holes are machined only to a shallow depth and arc relieved by a much larger bore machined from the back of the mold. Efficient mold cooling is essential for economical BM. As in injection molding typically, up to 80% of a BM cycle is devoted to cooling. Molds arc constructed as far as possible from high thermal conductivity aluminum alloys, and water cooling channels arc placed as close as possible to the surface of cavities and pinch-off zones. Because BM is a relatively low pressure process, the channels can be quite close to the surface and quite closely spaced before mold strength is compromised. The actual dimensions will depend on the heat transfer rate and cooling temperature requirements for the material of construction and plastic being processed. As a guide, channels may approach within 10 mm of the cavity and center spacing should not bc less than twice the channel diameter. If the mold body is cast, the cooling channels can be fabricated in copper pipe to closely follow the cavity contours before being cast in place. If the mold is machined, drilling and milling will produce channels, and it is not usually possible to follow the cavity contours so closely (Chapter 17). 6. Blow molding 307 An alternative in cast molds is a large flood chamber (Figure 6.16). However, efficient water cooling requires turbulent flow and this may not be attained in a flood chamber or in large coolant channels (Chapter 17). Many small channels are better than a few large ones. The cooling circuits will normally be zoned so that different areas of the mold can be independently controlled. The coolant flow rate should be sufficient to ensure turbulent flow and to keep the temperature differential between inlet and outlet to about 3C. Figure 6~ 6 Examples of water flood cooling blow molding molds THERMOFORMING Introduction Thermoforming is a process for converting thermoplastics into shell forms, using plastic sheet or film as a preform. Processes permit forming many small to large durable varied shapes. The various forming techniques permit manufacture of products individually or on mass production-continuous belt type production that are used in many different markets. Products include machinery and tool housings, industrial pallets, boat hulls, computer housings, transportation [auto, bus, aircraft, etc.] components, refrigerator door liners, etc. Typical products are high production items such as plates, cups, lids, trays, containers, etc. Many different methods of thermoforming are used. Figure 7.1 provides an introduction to the thermoforming methods. With the exception of a few such as matched mold, hybrid billet [combines thermoforming and blow molding, 24s and twin sheet thermoforming, the forming process uses an open mold that defines only one surface of the thermoformed part. The second surface is only in- directly defined by the mold. This second surface will lack precision definition of features to an extent dependent partly on the sheet thickness and thickness tolerance as well as the uniformity in heat subjected to the sheet prior to forming. ~, 28, 194, 229, 230, 231,232-237, 476 There is no direct control over wall thickness of the formed part; this MI1 vary from feature to feature according to the degree of stretch and thinning experienced at that point. Normally, it will be a target in thermoforming to obtain as even a wall thiclmess as possible in the finished part. Because the basic process uses a sheet preform and a single-surface mold, it is not possible to create independent features on the second surface. These processing consider- ations confine most thermoformed parts to relatively simple shapes however there are different complex 3-D parts formed. [...]... twin-sheet and pressure-formed plastic products Fueled mostly by advances in mold technology, material developments, and thermoforming machinery capabilities, technology improvements in the form of machine controls have led to machine designs that are faster and more consistent than was previously possible As an example advanced materials and 31 1 312 Plastic Product Material and Process Selection Handbook. .. the forming The process is particularly suitable for molds that have both male and female features Blister Package Forming Thin plastic film is thermoformed into simple to very complcx shapes for packaging different products such candy, fruit, toys, auto parts, etc Certain packaging products can bc used as molds Machines arc used 326 Plastic Product Material and Process Selection Handbook chiefly for... extruded profile is pulled Process can include heating that is usually well below the plastic' s melt or thermoforming temperature Thermosct (TS) plastics such as Bstage can be used (Chapter 15) The main difference between metal and plastic forming is the time dependency or spring-back, or recovery 330 Plastic Product Material and ProcessSelection Handbook in thermoplastics All materials exhibit some strain... is related to the thicl~ess of the plastic processed There are thin-gauge and thick-gauge or heavy-gauge thermoforming processes Thin-gauge identifies sheet thickness that is less than 0.06 in (0.15 cm) Film forming is a form of thin-gauge forming where the plastic thickness is less than about 0.01 in (0.025 cm) 309 310 Plastic Product Material and Process Selection Handbook Heavy-gauge means that the... atmospheric pressure, pressure forming processes greatly expand the design envelope and market applications for thermoforming Much thicker and stronger sheets can be formed, the replication of mold surface detail is greatly improved, and it becomes possible to form relatively sharp corners and undercut features 322 Plastic Product Material and ProcessSelection Handbook Vacuum-Air Pressure Forming There... Postforming identifies plastic sheet, rods, tubes, and other profile shapes that are formed into different shapes (coils, corrugated tubings, tubular nets, etc Its major use is inline with an extruder processing 332 Plastic Product Material and Process Selection Handbook thermoplastics As the plastic exits the die it is formed into different shapes Details on postforming thermoplastics are in Chapter... mold In parts where fine detail or textured patterns must be accurately reproduced, vent holes less than 0.5 in (0.013 cm) apart is usually necessary 319 3 2 0 Plastic Product Material and Process Selection Handbook Undercuts used in a mold require them to provide a means to easily remove the formed product Use of split section molds can be designed to be disassembled permitting removal of the product. .. molding is used 328 Plastic Product Material and ProcessSelection Handbook Matched Mold Forming Method requires sufficient driving force of the equipment press platens, proper cavity air evacuation, and a realization of the depth-of-draw limitations This process can be considered a take-off of compression forming, compression molding, or reinforced plastic (RP) stamping (Chapters 14 and 15) Matched mold... perform such as single-stage, double-stage, three-stage (Figures 7.4), five-stage (Figures 7.5), and rotary There are special designed thcrmoforming machines that starts with plastic extruded tube, flattened by rolls, and formed in molds on a rotary wheel 31 6 Plastic Product Material and Process Selection Handbook Figure 7,3 Schematicof roll-fed thermoforming line Figure 7~ Schematicexample of a rotating... female or negative, and mixed having both positive and negative characteristics Parts drawn over male molds tend to have greater draft angles, heavier bottoms and corners, and thinner rims with the inside of the product replicating the mold surface Parts drawn into female molds tend to have smaller draft angles, thinner bottoms and corners, and heavicr rims, with the outside of the product replicating . molding products 302 Plastic Product Material and Process Selection Handbook Blow/Fill/Seal The blow/fill/seal process is a complete packaging technique that integrates the extrusion or IBM and. machine designs that are faster and more consistent than was previously possible. As an example advanced materials and 312 Plastic Product Material and Process Selection Handbook machinery enable. frozen in and throughout the wall thiclmess. Orientation on molecular 304 Plastic Product Material and Process Selection Handbook Figure 6,14 Examples of 3-D extrusion blow molded products