Blow Molding

Introduction

• Plastic formation process-manufacture of bottles and hollow-shaped parts • Competitive with other processes, injection molding • Advantages; cycle is very short (low cost), lower mold cost than injection molding, high production rates in producing hollow parts with small or large opening

Blow molding process; general steps

• Melting the resin- done in extruder • Form the molten resin into a cylinder or tube (this tube is called parison) • The parison is placed inside a mold, and inflated so that the plastic is pushed outward against the cavity wall • The part is allowed to cool in the mold and is then ejected • The part is trimmed

Image of Parison

Blowing step in the blow molding process

Blow Molding Process

• The parison can be formed by – Extrusion process – Injection molding process

Extrusion blow molding

– Parison is formed from by forcing the plastic through an extrusion die. – Material enters the die, flow around the mandrel so that extrudate would be cylindrical – The die would have a hole at the center so that air could be blown into the cylinder – In some blow molding operations, the air is introduced from the bottom through an inlet

Extrusion blow molding

• This process can be; – continuous extrusion blow molding • During the process, the extrusion runs continuously, thus making a continuous parison.

• using multiple mold to match the mold cycle to the extrusion speed – Intermittent extrusion blow molding • During the process, the extruder is stopped during the time that the molding occur • use either reciprocating screw or an accumulator system

Continuous extrusion blow molding

• In this system, the output of the extruder is matched by having multiple molds which seal and blow the parison and then move away from extruder to cool and eject • In practical case, the mold cycle is longer than time required to extrude a new parison • If the mold cycle is twice than time needed for creating a parison, a two mold system can be used • The method is sometimes called

rising mold system

- system of which two or more molds are used to mold parts from one extruder during continuous process

Two-mold system in continuous extrusion blow molding

• In this system, one mold captures the parison (Mold A) • While a part is cooling in Mold B

Two-mold system in continuous extrusion blow molding

• Mold A then withdraws the parison from the extruder • While the part is ejected from in Mold B

Two-mold system in continuous extrusion blow molding

• The part is cooling in Mold A (moved out from extruder) • Mold B is moving into position around the parison

Two-mold system in continuous extrusion blow molding

• The cycle is completed by cooling of the part in Mold A • Capture a new parison in Mold B, blow the part into the mold cavity

Parison transfer system for continuous extrusion blow molding • System in which the parison is removed from the die and transferred to a mold, thus give space for another parison to be formed while the first part is cooling

Multiple mold system for continuous extrusion blow molding • Use multiple molds to match the output of the extruder when forming the parison

• Disadvantages of rotating mold system-the rotating wheel is much more complicated mechanically, cost of additional molds is quite high • Advantage of rotating mold system – mold cycle that are much longer than extrusion rate can be accommodate • Continuous extrusion blow molding methods are suitable for production of small to medium size parts

• If the parts are large, the time to form the parison by continuous extrusion is relatively long • Long extrusion time results in excessive cooling of the parison- cannot be effectively blown • Therefore method to forming parison that is faster than extrusion speed must be used • The rapid forming has been accomplished by two methods; the reciprocating screw system and accumulator system • In both systems, the parison is formed intermittently rather than continuously, and used only one mold

Reciprocating Screw

• Specific amount of resin is ejected to form a parison • A parison can be formed in one or two seconds • During the subsequent blowing and cooling, the extrusion screw is retracting and accumulate another charge • No parison is formed during the part of the cycle so the parison formation is intermittent • If the blowing and cooling time are long, the screw may stop when sufficient material has accumulate for the next parison shot • The stoppage may cause some disruptions in the extruder, so another method which used accumulator and allow continuous running of the extruder is developed

Accumulator system

• The accumulator system also produce parison intermittenly • In this system, the extrudate flow from the extruder into an external chamber or accumulator • At the appropriate moment in the cycle, a ram in the chamber injects the resin through a die to form parison • The accumulator is heated to maintain the proper temperature of the resin

Accumulator system in intermittent extrusion blow molding

Injection Blow Molding

• The parison is formed by the injection of molten resin into a mold cavity and around a core pin • The parison is not a finished product, but it is subjected to subsequent step to form the final shape • Second step, blowing of the intermediate part in a second mold • Because of distinct separation of the two steps, the parison made by injection molding is called a preform

Injection blow molding process

• The mold is closed • Resin is then injected to form a cylindrical part • The mold is opened, and preform is ejected

Injection blow molding process

• The preform is transferred to a blowing station while it is still hot or it can be reheated • After the second mold is closed, air is injected into the heated preform (through a hole) • Blow the preform, against the inside walls, mold is open and the part is ejected

Stretch blow molding

• the plastic is first molded into a "preform" using the Injection Molded Process. These preforms are produced with the necks of the using high pressure air into bottles bottles , including threads (the "finish") on one end. These preforms are packaged, and fed later (after cooling) into an EBM blow molding machine. In the SBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown approach 60 psi. using metal blow molds. Usually the preform is stretched with a core rod as part of the process. The stretching of some polymers, such as PET (PolyEthylene Terepthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages, which typically

Injection blow molding process

• The preform can be stored until the finished blow molded is needed • The flexibility of separating the two cycles has proven useful in manufacture of soda pop bottle

Comparison of extrusion and injection blow molding

• Extrusion blow molding – It is best suited for bottle over 200g in weight, shorter runs and quick tool changeover – Machine costs are comparable to injection blow molding – Tooling costs are 50% to 75% less than injection machine – It requires sprue and head trimming – Total cycle is shorter than injection (since the parison and blowing can be done using the same machine) – Wider choice of resin – Final part design flexibility

Comparison of extrusion and injection blow molding

– Injection blow molding • Best suited for long runs and smaller bottles • No trim scrap • Higher accuracy in final part • Uniform wall thickness • Better transparencies with injection blow molding, because crystallization can be better controlled • Can lead to improve mechanical properties from improved parison design

Common plastics for blow molding

• HDPE (stiff bottle, toys, cases, drum) • LDPE (flexible bottle) • PP (higher temperature bottle) • PVC (clear bottle, oil resistant containers) • PET (soda pop bottle) • Nylon (automotive coolant bottle, power steering reservoir)

• Thickness of the part- wall thickness is varied (bottom part is thicker, and thin toward the top) • Stiffness of the final part is dependent on the wall thickness (good stiffness is required to stand straight & to withstand pressure of filling and labeling) • Important parameter - Blow ratio= Mold diameter/Parison Diameter • Blow ratio = 1.5 to 3 are common