1. Blow Molding My presentation is on the process of blow molding.
December 09, 2006
Mitchel Plamondon

2. Introduction Blow Molding Characteristics
Modified extrusion and injection-molding process
Characteristics
Hollow thin-walled parts of various sizes
High production rates
Low cost for making beverage and food containers
Blow Molding is a manufacturing process by which hollow plastic parts are formed.
Some of the characteristics of this process are:
Hollow thin-walled parts of various sizes
High production rates
Low cost for making beverage and food containers
December 09, 2006
Mitchel Plamondon

3. General Process The process can be simplified into 3 steps
1) Melting the resin
2) Forming a preform tube
3) Blowing the preform tube into the shape desired
Steps 1 and 2 can be done either in an extruder or in an injection molding machine.
The process at its very simplest is 3 steps:
Melt the resin
Forming a preform, which is an injection molded part that is to be later heated and then formed into a shape for blow molding
Blowing the preform tube into the shape desired
December 09, 2006
Mitchel Plamondon

4. Methods There are two types of blow molding
Extrusion Blow Molding
Injection Blow Molding
Injection Molding gives rise to a third process called stretch blown molding
There are fundamental differences in the these two types of blow molding.
I will talk mostly on the two main types of blow molding:
Extrusion blow molding and,
Injection blow molding
And I will briefly go over a third method that came about as a result of Injection Blow Molding called Stretch blown molding
There are fundamental differences between the two main types of blow molding
December 09, 2006
Mitchel Plamondon

5. Extrusion Blow Molding
(1) and (2) parison is pinches at the top and sealed at the bottom around a metal blow pin as the two halves of the mold come together
(3) the tube is inflated so that it takes the shape of the mold cavity
(4) mold is opened to remove the solidified part.
The first method is extrusion molding, this figure shows how the extrusion blow molding works.
In step one the parison, which is the cylindrical tube that is trapped within the mold and then blown to fill the mold, is extruded through a die into the open mold cavity.
It is then pinched off at the top and sealed at the bottom around a metal blow pin
The tube is then inflated so that it takes the shape of the mold cavity
Once the mold is sufficiently cooled, it is opened and the solidified part is removed.
December 09, 2006
Mitchel Plamondon

6. Problems The continuous nature of the extrusion process is a problem
with extrusion blow molding.
Six ways to solve the problem of interference:
Intermittent Extrusion Blow Molding
Continuous Extrusion Blow Molding
Accumulator Head (Ram Extension)
Rising Mold System
Parison Transfer System
Multiple-Mold System
There is a problem with extrusion blow molding in that the continuous nature of the process causes interference.
There are six ways to solve this interference problem.
(read slide)
Figure: Parison transfer system for continuous extrusion blow molding.
December 09, 2006
Mitchel Plamondon

7. Solving Interference Problems
Intermittent Extrusion Blow Molding
Stop the extrusion of the parison while the part is cooling in the mold.
Continuous Extrusion Blow Molding
An accumulator receives the material from the extruder and the creates the parison as needed.
Accumulator Head or Ram Extrusion
The extruder output flows into a reservoir that is external to the extruder itself. A ram pushes the resin out of the reservoir and through the die that is mounted at the end of the accumulator to create the parison.
(read slide)
The figure is a Rotary wheel blow molding systems which are used for the high-output production of a wide variety of plastic extrusion blow molded articles. Containers may be produced from small, single serve bottles to large containers up to liters in volume - but wheel machines are often sized for the volume and dimensional demands of a specific container, and are typically dedicated to a narrow range of bottle sizes once built. Multiple parison machines, with high numbers of molds are capable of producing over one million bottles per day in some configurations.
December 09, 2006
Mitchel Plamondon

8. Solving Interference Problems
Rising Mold System
When the mold closes on the parison and air injection has occurred, the mold is moved off to the side. The pinching off the mold creates a break point in the parison so that when the mold moves aside, the parison is not disturbed.
Parison Transfer System
The parison is cut from the die by a knife and transferred by a mechanical arm to a mold where it is blown, cooled, and ejected.
Multiple-Mold System
Several Molds are mounted to a rotating wheel. While one mold is closing the capture the parison, the mold ahead is in a position for the blowing of the part, and other parts are closed for cooling, and further up a mold opens to eject the part
(Read Slide)
for multiple-mold system refer to figure
Plastics Materials and Processing
December 09, 2006
Mitchel Plamondon

9. Injection Blow Molding
Injection blow molding:
Parison is injection molded around a blowing rod
Injection mold is opened and parison is transferred to a blow mold
Soft polymer is inflated to conform to a blow mold
Blow mold is opened and blown product is removed.
The second method is injection blow molding, this figure shows how the extrusion blow molding works.
In step one the parison is injection molded around a blowing rod.
The injection mold is opened and parison is transferred to a blow mold
The soft polymer is inflated to conform to the blow mold cavity
Once the mold is sufficiently cooled, it is opened and the solidified part is removed.
December 09, 2006
Mitchel Plamondon

10. Extrusion vs. Injection Blow Molding
Bottles over ½ pound
Tooling costs are 50% to 75 % less
Generates 20 to 30% scrap
Additional machinery needed to grind scrap
Total cycle time is shorter
Wider choice of resins
Greater flexibility in part design
Injection
Long runs and smaller bottles
No trim scrap
Higher accuracy in the final part
Uniform wall thickness
No seam lines or pinch marks
Better transparencies
Improved mechanical properties
This slide shows a comparison of the two types of blow molding. As you see extrusion blow molding allows you to produce bottles over ½ pound while having a wider choice of resins. while injection blow molding is capable of long runs and smaller bottles and high accuracy of the final part.
December 09, 2006
Mitchel Plamondon

11. Stretch Blow Molding
Stretch blow molding produces a part with biaxial molecular alignment. In the process a preform, or parison, elongated mechanically in the mold and than expanded radially in a blowing process. A desirable resulting molecular orientation yields a material with increased strength. This means that products that are strength-based designs can be produced using less material than if they were to be produced using simpler blow molding techniques.
A: Stretch-Blown Pin; B: Air Entrance; C: Mold Vents; D: Preform;
E: Stretch Rod Extended; F: Cooling Channels
Greater rigidity so that the pouring would be easier
Reduced permeation over nonorientated bottles because the orientation moved the molecules closer together, thus making gas through the molecular layers more difficult
Greater precision in dimensional control
Elimination of bottom flash, which is present when the bottom is pinched off rather than molded
Ability to match the wall dimensions of the preform to the expansion of the bottle so that the wall thicknesses were more uniform
Increased burst pressure
December 09, 2006
Mitchel Plamondon

12. Materials Considerations
One critical requirement is that the
polymer must have good melt
strength.
The common blow molded plastics are:
Polyolefins
HDPE
LDPE
Polypropylene (PP)
PVC
PET
When blow molding a part it is critical that the polymer has a good melt strength, that being said, these are some of the more common blow molded plastics:
High density polyethylene
Low density polyethylene
Polypropylene
Polyvinyl Chloride
Polethylene tretraphthalate
PET bottles are excellent barrier materials and are widely used for soft drinks
December 09, 2006
Mitchel Plamondon

13. Polyolefins
ADVANTAGES
The polyolefins are easy to process and have the advantage of good electrical properties, moisture resistance, and low cost.
DISADVANTAGES
Sensitive to oils and can have stress crack problems
PROCESSING PARAMETERS
These resins are usually blow molded in the range of 320o – 410oF. Mold temperatures should range from 50o – 100oF.
High and low density polyethylene and poly propylene are all categorized as polyolefins, meaning that they are plastics that contain only carbons and hydrogens, and they share the same characteristics.
HDPE used in products and packaging such as milk jugs, detergent bottles, margarine tubs, and garbage containers.
LDPE is used in cable covering, toys, lids, buckets and containers, pipe.
PP is used in a wide variety of applications, including food packaging, textiles, plastic parts and reusable containers of various types
(read slide)
December 09, 2006
Mitchel Plamondon

14. Polyvinyl Chloride (PVC)
ADVANTAGES
Very desirable properties of clarity, smooth surface low glass and aroma permeation, high stiffness with low wall thickness, high chemical resistance, and easy labeling and printing
DISADVANTAGES
Very temperature sensitive, therefore not normally injection blown because the process requires two heat cycles
PVC is a widely used thermoplastic polymer. As a building material, PVC is cheap and easy to assemble. In recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas.
(refer to slide)
PROCESSING PARAMETERS
Typical melt temperature range for PVC is 375o – 420oF.
December 09, 2006
Mitchel Plamondon

15. Polyethylene terephthalate (PET)
ADVANTAGES
Easily recycled, very lightweight, it makes a good gas and fair moisture barrier. It is strong and impact-resistant. It is naturally colorless and transparent.
DISADVANTAGES
Have to be careful to avoid a temperature that is two high or acetaldehyde forms.
PROCESSING PARAMETERS
Temperature range for PET to blow mold is 200o – 212oF. Mold temperatures should be in the range of 35o – 40oF
Polyethylene terephthalate is a thermoplastic polymer resin of the polyester family that is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber.
PETG, a copolymer of PET in which the glycol component of the copolymer resin is slightly different from that used in PET, provides improved resistance to dilute mineral acids, bases, salts, soaps, aliphatic hydrocarbons, alcohols, and some oils.
December 09, 2006
Mitchel Plamondon

16. Conclusion – Wrap Up ADVANTAGES RAW MATERIALS APPLICATIONS
This process lends itself to any designs involving hollow shapes. Equipment availability is good in most geographical locations. Can save tooling dollars over injection molding.
RAW MATERIALS
Most commodity grade and engineering grade resins may be blow molded. The viscosity of the plastic must be high to keep the parison from stretching too much prior to mold closure.
APPLICATIONS
All types of bottles, toys, air ducts for automobiles, chemical & gasoline tanks, household goods. Can be obtained from these blow molding processes.
DISADVANTAGES
Cycle times are slower than injection molding. Piece prices are higher than injection molding.
Advantages:
Designs involving hollow shapes
Equipment is available
Saves money over injection molding
Disadvantages:
Cycle times are slower than injection molding
Prices are higher than injection molding
Raw Materials:
Most commodity grade and engineering grade resins
Viscosity has to be high to prevent the parison from stretching to much
Applications:
All types of bottles, toys, and household goods can be obtained from blow molding processes
December 09, 2006
Mitchel Plamondon

17. Works Cited
Strong, A Brent. Plastics: Materials and Processing. New Jersey: Pearson, 2006.
Kalpakijan, Serope and Steven R. Schmid. Manufacturing Processes for Engineering Materials. New Jersey: Pearson, 2003.
Rodriguez, Ferdinand, Claude Cohen, Christopher K. Ober, and Lynden A. Archer. Principles of Polymer Systems. New York: Taylor & Francis,2003.
PlasticsDome. 10 Dec Blow Molding. 10 Dec. 2006     <
Polymer Processing. 10 Dec Processing. 10 Dec. 2006     <
Wikipedia. 12 Nov Blow Molding. 10 Dec. 2006     <
Morrison, David.  Polymer (Plastic) Processing. <
December 09, 2006
Mitchel Plamondon