Blow molding vs injection molding, what are the differences and their uses?

Two types of molding are commonly used in manufacturing. There is blow molding and injection molding. Let’s look at the differences between these two methods.

What is blow molding?

Blow Molding

Blow molding uses high-pressure air to create molded plastic parts. The parts are blown into shape using an air compressor, and then they are cooled and hardened using heat or cold. This process is more labor-intensive than injection molding but less expensive than other manufacturing techniques.

There are three types of blow molding which include extrusion, injection, and stretch. Extrusion blow molding is used to manufacture parts such as bottles and jars. Injection blow molding is typically used for smaller parts such as medical syringes. Stretch blow molding is used in creating parts that need to be very strong, such as car bumpers.

The advantage of blow molding over other manufacturing techniques is that it can create shapes that are more complex quickly and cheaply. This is especially beneficial in the automotive industry, where intricate designs and complicated parts are common.

What is injection molding?

Plastic Injection Molding

The process known as injection molding uses heat and pressure to create objects from plastic. It’s a method widely used in the industry today, and for a good reason. One of the main benefits of injection molding is that it’s fast. It can produce products in as little as 60 seconds. That means you can quickly get your product out to market without waiting for a traditional production process.

This method creates plastic parts where liquid, super-heated plastic is injected under high pressure and temperature into the mold’s cavity, which cools and solidifies to create the part. Injection molding has several advantages over other manufacturing processes, including speed, accuracy, and flexibility.

Another great thing about injection molding is that it’s versatile. You can use it to create any object you can imagine. This makes it an ideal choice when you need to create something quickly and with high precision. On the downside, injection molding is not without its risks. It can be difficult to troubleshoot problems and may not be suitable for all products.

Differences between blow molding and injection molding

blow molding VS Injection molding

Injection molding and blow molding are two very popular methods of manufacturing plastic products. Though both processes are similar in that they involve melting and shaping plastic, there are several key differences between the two methods.

This method injects hot plastic into the cavity of the mold. The plastic then hardens and forms the shape of the cavity. In contrast, blow molding involves blowing hot air into a tube of molten plastic. The plastic expands and forms around the mold. One major difference in these two methods is the amount of pressure required.

Injection molding requires much higher pressures than blow molding. This is because injection molded plastics must be forced into the mold cavity under high pressure.

Blow molded plastics, on the other hand, are not under nearly as much pressure since they are simply blown into the mold. Another difference between these two methods is the type of products that can be made. Injection molded products tend to be more complex in shape than blow molded products. This is because injection molding allows for precise control in shaping the product.

Blow molding is better suited for making products that have a more simple shape, such as bottles or cups. Finally, injection molding is typically faster than blow molding. This is because injection molded plastics harden quickly once they are injected into the mold cavity. Blow molded plastics, on the other hand, take longer to cool and harden.

Blow Molding vs. Injection Molding #1: Processing Steps

Blow Molding Process Steps

The blow-molding process begins with melting plastic pellets into a liquid form. The molten plastic is then fed into a mold, where it is blown into mold cavity for shaping. Once the plastic has cooled, it hardens, and it is then removed from the mold and trimmed to create the finished part.

  • 1. Plastic Resin Feeding or Charging

Plastic resins are fed into the blow molding machine either manually or via an automated process. The plastic resin is then melted and injected into the mold cavity. It then solidifies as it cools and creates the desired shape. The blow molding machine then opens the mold, and the now-solidified product is ejected.

  • 2. Plasticizing or Melting

In the second step of the blow molding process, plastic material is fed into a heated barrel. As the material is heated, it begins to melt and flow. At this stage, the molten material is plasticized, meaning it is softened and made pliable enough to be molded.

  • 3. Parison Extrusion or Preform Injection

Preform injection injects hot liquid material into the mold to create a preform. Preform injection is typically used for materials that are not able to be blow molded, such as glass or metal.

Parison extrusion is the process of extruding molten material through a die to create a parison. A parison is a tubular shape that can be inflated to form the desired shape. Parison extrusion is typically used for materials that are not able to be blow molded, such as glass or metal.

  • 4. Inflation or Blow molding

Inflation or blow molding is the process of inflating hot liquid plastic into a mold to create a hollow product. This process creates products with a range of sizes and shapes.
The first step in inflation or blow molding heating plastic until it is in a molten state. Next, is to inject the material into the mold. Once the mold is full, the plastic is allowed to cool and harden. The last step is to remove the product from the mold.

  • 5. Trimming

After the parison is blown and cooled, it is cut to the desired length using a trimming knife. The cutting leaves a smooth, finished edge on the part.

  • 6. Leak Test

Leak testing is important to ensure that the preform is free of defects that could cause problems during the blowing process. To do this, air is injected into the preform, and then it is placed in a water bath. If there are any leaks, bubbles will be seen coming from the preform.

If no leaks are found, the preform is then ready to be blown into its final shape.

Injection Molding Process Steps

injection molding machines
  • 1. Clamping

The first step is clamping. This is done by closing the mold halves together using either a manual or hydraulic press. The pressure from the press keeps the mold halves together during the injection process.

  • 2. Injection

The plastic injection molding process begins with a raw material, such as plastic pellets, that is fed into a hopper. The pellets are then melted and injected into a mold cavity of injection molding machine.

  • 3. Dwelling

During this stage, the melted materials begin to solidify in the mold cavity. This step gives the heated materials time to completely fill the mold cavity. Continual pressure is applied
during this process to achieve full packing of the injection mold.

  • 4. Cooling

Cooling can be achieved either by air or water cooling, depending on the material being used. Air cooling is typically used for engineering plastics, while water cooling is used for metals.

The cooling process begins when the molten material is injected into the mold cavity. As the plastic or hot liquid material cools and solidifies, it takes on the cavity’s shape. Once the desired shape is achieved, the mold is opened, and the part is ejected.

The cooling time will vary depending on the thickness of the part. Smaller parts will cool faster than larger parts. Thicker parts will also take longer to cool than thinner parts.

  • 5. Mold opening and Removal

The opening of the mold is a critical step in the process. The mold must be opened carefully to avoid damage to the mold or the product. The mold is opened by a hydraulic ram or a pneumatic cylinder. The mold is then separated into two halves, and the product is ejected from the mold.

Blow Molding vs. Injection Molding #2: Molding Materials

Molding materials are substances used to produce the desired shape from a softer material.

Blow molding materials can include:

blow molding materials
  • Polyurethane

  • Santoprene

  • Kostrate

  • K-Resin Styrene-Butadiene Copolymer

  • Acrylonitrile Butadiene Styrene

  • High-Density Polyethylene

  • Low-Density Polyethylene

  • Polypropylene

  • Polystyrene

Materials used in injection molding can include:

molding materials


  • Polycarbonate

  • Polyester

  • Ultem

  • Urethane

  • Valox Resin / Polybutylene Terephthalate

  • Acrylic

  • Acetal

  • Delrin (Polyoxymethylene)

  • Fortron Polyphenylene Sulfide

  • Nylon 6/6

  • Polyetheretherketone

  • Poly Vinyl Chloride

Blow Molding vs. Injection Molding #3: Injection Molds

injection molding mould

A production mold is a tool used in manufacturing plastic and metal products. It is a type of tool specifically designed to create one or more copies of a specific object, usually with heat and pressure. A production mold can be made from various materials, including plastic and metal.

The use of production molds can be essential for the manufacturing process of many types of products. For example, a production mold may create copies of products such as cars, phones, and toys. Production molds are also often used for machine parts and other products.

Production molds are often very specific in their design. This is because they are often used to create objects with specific functions or uses. For example, a production mold may create copies of phone cases with specific features or designs.

Blow Molding vs. Injection Molding #4: Applications

Blow molding products:

  • Plastic bottles and containers for household products, food, and beverage packaging

  • Large tanks and vessels for storing chemicals or other liquids

  • Automotive parts such as fuel tanks, dashboards, and light covers

  • Furniture and home goods such as laundry baskets, waste bins, and patio chairs

  • Construction barrels

  • Medical devices and supplies

  • Packaging

Some products made from injection molding products:

  • Housings for electronics.

  • Some toys are made using injection molding, such as dolls and action figures

  • Molded houseware, components, drinkware, containers

  • Many automotive parts are made using blow molding, such as dashboards, door panels, and fenders

  • Single-use, bulk manufactured healthcare products

  • Flooring panels, insulators, roofing vents, deck fasteners

  • Sporting goods

Blow Molding vs. Injection Molding #5: Advantages & Disadvantages

Blow molding is a process where a material is formed by blowing it into a thin, rigid piece. The advantage of the process is that it is relatively easy to produce small pieces, and it can create products with intricate details. Blow molding expansion causes the part to become stiffer and more durable than if it were made using injection molding.

Disadvantages of this process include the potential for inconsistency in the finished product and the need for high-quality equipment and materials.

Injection molding, by contrast, is a more complex and expensive manufacturing process, but it can make larger products. It can produce products with consistent dimensions and shapes, and the ability to create products with greater detail than blow molding is an advantage of the injection method. The injection molding process allows for extremely detailed, complex parts that are difficult to make using blow molding.

The disadvantage of injection is the higher cost of equipment and materials and the potential for greater variation in the finished product.

Which process should you choose for your projects?

To answer this question, you’ll need to consider the specific requirements of your project. Blow molding offers quicker production speeds and lower costs, while injection molding is more precise and versatile. Ultimately, it’s up to you to decide which process is the best fit for your business. With a little bit of research and knowledge, you can make an informed decision on which type of plastic molding


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.