Blow molding process is a manufacturing process through which plastics are formed. The process entails pumping in air to inflate soft plastic in a mold cavity, resulting in the formation of hollow-thin walled plastics. Blow molding is an innovative manufacturing process that has made tremendous contributions to a sustainable economy. The essence of the process is seen in various sectors of the economy such as the packaging of medicine in the health sector, and packaging of drinks in the beverage sector. Since these plastics and containers usage is in large volumes, blow molding companies aim at production in very high quantities. The demand for blow molded plastics has escalated and has proven unbeatable in the sense that nearly all industries benefit from plastics.
Products produced by blow molding include:
Blow mold cases
Hollow industrial parts example drums
Quick links to Blow Molding Information
Blow Molding History
The initial idea of blowing hot materials with hot air was actualized by Syrians in the first century BC. Egyptians later revolutionized the approach to blow molding in 1700-1600 B.C. Among the verified individuals who first launched the process were William Kopitke and Enoch Ferngren. Blow molding process typically originated from glass blowing. This profound idea was set in motion when Hartford Empire Company bought a blow molding machine in 1938, and that marked the onset of commercial blow molding. However during the 1940’s custom blow molding product variety was at its minimal and it was not until later on, that the production rates and wide range of variety increased. As new inventions actualized, they brought technologies and technical mechanisms essential in producing hollow bodied pieces. The technologies facilitated plastics manufacturing which is replacing glass in some situations. Plastic bottles mass production took place in America in 1939. Later the blow molding technology was adopted by Germany which has now surpassed all other manufacturers making it a renowned blow molding machine manufacturer.
Advantages of Extrusion Blow Molding
- Few tools are needed
- Fast production
- The design is easily incorporated thus the ability to mold complex parts
Disadvantages of Extrusion Blow Molding
- Specifically limited to hollow parts
- Improving barrier aspects requires parisons with layers of contrasting materials hence not easy to recycle
How Blow Molding Works
The raw materials in the plastic blow molding process are thermoplastic in the form of pellets. Plastics have a high level of strength that is attributed to the thick webbing structures formed by the polymers. This property makes the plastics resistant to chemicals and abrasion. The thermoplastic materials include:
- High Density Polyethylene
These thermoplastic materials are melted down to form a hollow tube called a parison. When melting is complete, compressed air is pumped inside the parison. The high pressure causes the molten parison to expand and balloon out, conforming to the shape of the mold cavity. After the shape has been acquired by the parison, cooling is introduced by either conduction, evaporation of volatile fluids within the container, or by the cold water that is running through the mold cavity. The final product is then removed from the mold and is ready for leak testing before being packaged.
The ultimate purpose of the custom blow molding process is to create a uniform final product that best serves the market regarding high quality, smoothness, airtight capability and without the need for joining parts together. Blow molding is a fast-paced production method in that it can produce close to 20 thousand containers in an hour's time. This capability is the reason that makes it one of the most reliable manufacturing processes to counter the high demand for plastics around the globe on a daily basis. Blow molding process services use three primary methods to produce blow molded plastics namely:
- Extrusion Blowing
- Injection Blowing
- Stretch Blow Molding
- Extrusion Blowing
Extrusion Blowing Process
Extrusion blow molding process is a flexible method in that it allows for a wide variety of designs in container shapes, neck openings and sizes as well as different handleware. The process has similar aspects to glass blowing and uses two mirror molds in the creation of the containers, causing a thin seam where the two patterns join. In this process (EBM), plastics are melted and later pushed through the parison. Then the parison is enclosed into a metal mold that has been cooled. Next, air under a high pressure of 50 to 150 psi is directed into the parison, and it conforms to the mold shape. Cooling is then introduced into the mold, and when complete, the mold is opened and the part created is extracted.
The extrusion blow molding process has two distinct variations namely:
- Continuous Variation
- Intermittent Variations
In the process of continuous extrusion, the parison is pushed out continuously, and the resulting excess part is scraped using a knife (spin trimming). This is of the essence in ensuring production of even walled products.
This is a procedure within blow molding that is used to clear excess molding material from the final products. During the molding process, some containers such as jars could have excess material which needs to be shredded. This is done through a knife that removes the excess materials. These extra materials are later recycled to create new products, significantly complementing the efficiency of the process. Even walls are best formed through rotational molding. Spin trimmers come in hand on materials like PE +, LDPE, PVC as well as HDPE. It's crucial to note that materials have different characteristics that affect trimming. Amorphous components prove daunting in cutting, compared to crystalline made ones. In trimming, titanium blades are preferred unlike steel since they increase life factor.
The process of Intermittent variation consists of two known methods namely:
Straight Intermittent Method
The straight intermittent method resembles injection molding, in that the screw moves and it stops to push out molten materials. Injection molding is of high importance in ensuring quality production.
In the accumulator method, the accumulator gathers all the molten plastics as it waits for cooling of the previously pushed mold to take effect. Then a rod moves the molten material to form the parison. During situations like this, to perform the task, the screw could move intermittently or continuously.
It is worth noting that continuous extrusion has a hindrance where the weight of the parison is the force that drags the parison, making calibration of the wall thickness a problematic process. The reciprocating screws or the accumulator head makes use of a hydraulic system, pushing the parison speedily out and minimizing weight effect. More so, enabling efficient and accurate control of wall thickness.
Continuous and Intermittent Extrusion Blow Molding
Therefore, extrusion blow molding could be continuous or intermittent in nature. This specific possibility enables custom blow molding equipment to be classified into two categories:
Intermittent Extrusion Machinery
- Reciprocating Screw Machinery
- Accumulator Head Machinery
Continuous Extrusion Machinery
- Shuttle Machinery
- Rotary Wheel Blow Molding Systems
Injection Blow Molding
The process of injection blow molding comes in handy when producing hollow plastics as well as glass. Here, the plastic materials are wholly melted and later moved along to a station where the molding procedures take effect. However, the above process has not been adopted in full production and is mostly used to manufacture custom plastic bottles and medical glass equipment. It involves three steps:
The machine used in the injection blow molding process works continuously to melt the material components. Then the fluid substance is heated and later injected into a mold cavity. At this stage, the external design shape that takes effect resembles the cavity.
The next step is where the preform is rotated and clamped onto the blow mold. Then through an opening in the core rod, pressured air is pumped to inflate the preform which assumes the surrounding design shape. When cooling has taken effect, an opening in the blow mold facilitates rotation of the core rod to the ejection phase.
The final product is scrapped off the core rod. At this point, it is advisable to perform leak test before packing. The blow mold and preform could have several cavities ranging from 3 to 16 depending on required output. Also, there is an arrangement of core rods which facilitate the processes to take effect one after the other without missing any crucial procedures.
Stretch Blow Molding Process
Stretch blow molding process has two categories: Two-stage & Single Stage Injection
The Single stage further includes: 3 Station & Four Station Machine
At this phase, the components are heated to a molten state, preform, which has bottlenecks containing threads. It’s advisable to perform cooling measures to ensure smooth shape recovery as well as maintaining the strength of the product. Immediately after cooling is complete, preforms are packed into the rsb machine. ISB method entails heating of preform to high temperatures then blowing it with a lot of pressure. The process involves stretching the preform with a core rod as a standard procedure. Stretch blow molding machine facilitates effective production at all phases.
Advantages of Two-stage injection
- Production of large volumes
- Limited restriction on bottle design
- Works efficiently for rectangular, oval or cylindrical bottles
Disadvantages of Two-Stage Injection
- Ample floor space needed to perform production
Single Stage Injection
During this method, preform manufacture, as well as the blowing procedures, are so designed to be performed in similar machines. The previous four station procedure is more expensive than the three stations since it does away with the reheat stage and replaces it with latent heat which saves up on energy cost and tools.
Advantages of Single Stage Injection
- Best for short runs and low volumes
- Allows shaping of preform wall thickness during blowing objects into different designs
Disadvantages of Single Stage Injection
- There is a high restriction on the design
What to Consider When Choosing the Right Manufacturer
When deciding to settle for a company that can best deliver your order, you need to consider several factors that include:
Standard and Custom Molds
A company that can efficiently deliver your product has standard shapes which considerably save up on time and production cost. Custom molds can be uniquely designed to be a conventional design in the future. Standard molds are mostly used for jars and familiar container shapes. For a unique mold, it will have to be craftily custom designed to fit your needs. The company should be in a position to offer these both design aspects.
A manufacturer that offers product prototypes is efficient before moving onto the more significant cost of production. Prototyping is essential is revolutionizing design and improving manufacturer lead time. It has been a recurring mistake where manufacturers mass produce a client’s order without consulting, only for the client to end up in disappointment. Therefore, it is of utmost importance for prototypes to be designed and presented to the client for approval before production commences.
Plastic Blow Molding Types
A manufacturer offering more than one option of the three types of blow molding process is ideal since you can discuss the most suitable method by looking at the pros and cons of each.
Materials and Construction
Different plastic resins are available to choose from during production. Therefore, you need a blow molder and supplier who can work to achieve quality for the functionality required. The right manufacturer should have an eye for keenness to identify the most appropriate blend of plastic resins that will solve the needs presented, be it heat tolerance for food cans or withstanding abrasion for tools kit.
Quality and Timely Delivery
Last but not least, comes the question of quality. There are well-established companies out there that stand tall in the quality aspect. Their products have overwhelming reviews. Hence those are the partners you should be linking with. Timely delivery is of great essence as well. The right manufacturer ought to be in a position to deliver high quality products promptly to meet order deadlines since lateness could adversely affect schedules of many great businesses that depend on the end products.
As the blow molding industries continue to revolutionize plastic products production, demand is escalating, and they are working efficiently to meet those needs. There is much detailed production revolving around the blow molding process as evident from the above outline. Hoping now you are in the know on the subtler aspects that are part of the process.
Blow Molded Plastics Terms
- The lack of haze or cloudiness in a plastic material.
- A material that at room temperature has the ability to return to its original shape after being stretched up to twice its length.
- Ethylene Vinyl Alcohol Copolymer (EVOH)
- A high-barrier plastic resin packaging material.
- Also known as the “E” dimension, it is the material enclosing the neck opening of containers that accommodate particular closures.
- Extra plastic attached to a molding along the parting line. Flash must be removed (i.e. deflashed) before the part can be considered finished.
- Flame Treating
- A technique of making inert thermoplastic items responsive to adhesives, inks and paints. The submersion of the item in an open flame induces surface oxidation.
- A procedure that diminishes permeability and increases the chemical resistance of different plastics and elastomers. The exposure of polymers to fluorine gas causes molecular structural surface changes.
- Minimum Wall
- The thinnest part of the wall of a bottle.
- Mold Cavity
- The hollow part of a group of molds that forms a container on bottle machines.
- Mold Seam
- A vertical line formed at the point where the mold halves meet, the prominence of which depends on the accuracy with which the halves are matched.
- Multi-layer Bottles
- Bottles consist of layers of special plastic materials that are co-extruded to preserve and use the unique characteristics of each material to improve the barrier qualities of the container and increase the product shelf life.
- Natural Color
- Refers to the natural translucent appearance of the plastic material of containers and closures.
- Neck Ring
- The component of the molding apparatus that produces the bottle finish.
- The collapse of a portion of a container structure, due to a loss of interior pressure, which occurs during aging and storage.
- The empty cylinder of plastic melt that emerges from the extruder die head and expands in the mold space from air pressure, producing the molded part.
- Parting Line
- The mark on a plastic bottle where the two halves of the mold meet at the point of closure.
- A heat-softened polymer shaped like a thick test tube with neck threads that is inflated while inside a blow mold.
- A procedure for mechanically altering the weight, size or wall thickness of the parison as it forms.
- A decrease in dimension of an article after the molding process, which is caused by the contraction of the plastic material upon cooling.
- Side Seam
- The seam that joins both edges of a blank forming a body.
- Surface Treating
- Any treatment technique, including chemical, electronic and flame treatments, that makes plastic receptive to inks, lacquers and adhesives.