1. Blow Molding

2. 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

3. 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

4. Image of Parison

5. Blowing step in the blow molding process

6. Blow Molding Process The parison can be formed by Extrusion process
Injection molding process

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

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

16. 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

17. 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

18. 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

19. 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

20. Accumulator system in intermittent extrusion blow molding

21. 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

22. 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

23. 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

24. 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 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 using high pressure air into bottles 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 approach 60 psi.

25. 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

26. 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

27. Comparison of extrusion and 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

28. 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)

29. 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