Most people that have a familiarity with the plastic manufacturing industry have heard of plastic blow molding and plastic injection molding. However, there remains some confusion by engineers and product designers who are new to the industry just what these differences are.
The plastic molding process has long been a standard for certain applications. Having a clear understanding between these two manufacturing processes can save on product design and production time. This will also translate into money savings and faster delivery of your plastic parts.
Determining if blow molding or injection molding is right for you starts with understanding the basics.
Plastic blow molding and plastic injection molding have several significant differences:
- Design considerations
- Manufacturing process
- Margins of error and tolerances
- Process role in intended result
Below, we will discuss these differences in the manufacturing process to help you determine which molding process is right for you.
Product Variations
- Plastic injection molding is used for the production of solid parts.
- Plastic blow molding produces parts that are hollow.
The most noticeable differences between plastic injection and plastic blow molding parts, is the products that they produce. If your product requires a rigid wall, injection molding is your best bet. Injection molding creates parts such as coffee cup lids, hair combs, bottle camps, television and computer housing, and much more.
If you require a structure that is rigid, flexible, or has the ability to hold fluid, blow molding is the right choice for you. The most popular example of plastic blow molding products is plastic bottles. Every year, billions of bottles are manufactured around the world at low prices using plastic blow molding techniques. Blow molding can also produce a variety of more industrial parts and shapes such as stadium seats, fuel tanks, and coolers.
The process of blow molding can produce a wide variety of plastic parts with shapes that are complex. However, it is very possible for the wall thickness to vary from area to area dependent on how the material is stretched and blown in order to achieve the desired shape. Injection molding, on the other hand, is determined by the core to mold relationship.
Understanding your product needs is the first step to choosing a molding process that is best for product development stage.
Different Processes
Injection Molding
Injection molding starts with the injecting of a melted resin into a hollow mold. This mold is held together with intense pressure. This pressure is sufficient enough for the entire interior to fill with the molten high-pressure plastic resin. The production cost and machine size will vary on the “tonnage” pressure that is required in order to hold the mold together.
Blow Molding
Blow molding techniques begin with the heating a plastic tube. This plastic tub is then filled with air until it becomes a balloon of hot plastic, known as a parison. Then, a mold is clamped around this balloon shape. This traps the plastic while air continues to stretch the plastic into the shape of the part. The associated costs and machinery size required to produce blow-molded products are based on the overall weight of the plastic “shot” needed in the mold.
Mold Variations
Injection Molding
Plastic injection molds require a high precision match between each mold half so that the flow of material is flawlessly controllable. The actual mold creation in this method is 90 percent of the battle. When you have a mold that is error-free and functional, the remaining production process is a fairly simple routine to maintain. The precision needed in this mold creation makes injection molding more expensive than the blow molding technique.
Blow Molding
Plastic blow molding has a bit more freedom in design between each mold half shape. This is because each mold half will expand to forms its own wall. Because of this, the mold itself is only 50 percent of the battle. Other variations and concerns in this method include air leaks, wall thinning, streaks, and flash. Periodically monitoring the manufacturing line is important for quality control and product consistency. If the plastic is stretched too thin cracks in seals and small pinholes can result. Reviewing uniformity of wall thickness to ensure the correct tolerances of the product is also vital to product satisfaction. Implementing a solid set of control systems is necessary to product high quality, consistent, and functional parts.
Plastic blow molding production concerns:
- Consistent product dimensions
- Poor function
- Wall thickness
- Process temperature
- Speed of mold closing
- Pressure or velocity of blown air
- Strength or flexibility
- Leaking seals
- Pinhole leaks
Questions to Ask Yourself
- What length, width, and depth does your product require?
- Is your product solid and rigid or flexible?
- Does it need to be food safe?
- Will it hold liquid?
- Does it need to adjust to PSI or specific pressure levels?
- Does the part need to withstand extreme levels of heat or cold?
- Is the item’s weight vital to its function?
- Does the part need to condense, expand, move, or pivot?
- Will the part work in tandem or connect with other working parts in order to achieve a specific result?
- Does the item stand-alone?
- How does the plastic part need to function overall?
Reviewing these questions with an expert plastic molding engineer will help ensure that the design and production of your item is a good idea and that the best plastic molding technique is used. It’s important to keep in mind that there are a large variety of plastics to choose from. The relationship between product accuracy and the type of manufacturing technique is best when determined early on. If many cases, even a single millimeter in production differences can render a product unusable in its intended field. In these situations, creative solutions and accuracy are essential.
Additional Considerations
Because the manufacturing and production of plastic parts contain so many variables, there is no one size fits all solution. Your plastic engineering expert will help you review all of the variables involved based on the desired end use of your product, to develop a custom plastic molding solution that works for you.
Consider these variables during your molded part production:
- Product dimensions
- Wall thickness
- Temperature withstanding levels
- PSI pressure rate required
- Strength or flexibility
- Type of plastic used
- Recyclable considerations
Once all of the variables have been considered and challenges and opportunities have been identified, the entire production process will go very smoothly. The designing and fine-tuning of molds to utmost perfection can take time to develop, but as soon as it is production can begin.