three types of blow molding

There are many different ways plastics are shaped to make the items we use everyday, from plastic bottles and forks to trash bags and Lego toys. KW Plastics, a company that buys recycled plastics, has been taking a look at some of the processes of making plastic, from extrusion to injection molding.

In this blog post, we’ll take a look at the three types of blow molding, which is a process to mass produce plastic containers by melting the plastic and blowing it into parisons, or hollow tubes. If you are in need of plastic resin pellets or need to sell your scrap plastic, contact KW Plastics today!


Extrusion Blow Molding

Extrusion blow molding can produce complex plastic parts very quickly. In extrusion blow molding, the plastic is melted and then pushed, or extruded, into the parison, or hollow-shaped tube. The parison is closed and then air is blown into the parison until it reaches the desired shape while it is still warm. The plastic is allowed to cool in the parison and then it is ejected, free to perform the extrusion blow molding process all over again.

Here, the extrusion blow molding process is broken up yet again into two processes: continuous extrusion blow molding or intermittent extrusion blow molding. Like the name implies, the parison is extruded either continuously, using a knife to cut off each piece of plastic when it is finished, or the parison will be extruded intermittently where there is a pause, or a stop, between each mold as it comes out of the processing system.

In general, extrusion blow molds are less expensive than injection blow molds and much faster. However, the plastic is not as thick as in injection blow molding and you will have a greater amount of scrap plastic from the cutting process.

If your company does use extrusion blow molding, give KW Plastics a call. We buy scrap plastics and then reuse it ourselves to make plastic resin pellets. Call us today for pricing!

Injection Blow Molding

The slowest of the three blow molding methods, injection blow molding injects the plastic or polymer material into a core pin, or coe rod. This core pin is then transferred to the blow mold machine where it is rotated until it is made into the desired shape by forced air. The plastic is then cooled and ejected.


Stretch Blow Molding

Stretch blow molding is the most complicated of the three blow molding processes. It takes the plastic resins and stretches them — both vertically and horizontally — to create cross patterns. This yields a much higher quality plastic material, increases its barrier strength, and helps prevent porosity. It also increases its drop impact. Because the plastic is stronger recycled plastic material manufacturers are able to use less material, which is a cost savings of between 10 and 15 percent,. This method is more expensive. However, for lower volume orders, stretch blow molding works great. PET bottles for juices, water, and soda are usually made through the stretch blow molding method.

There are two types of stretch blow molding: single-stage stretch blow molding and two-stage stretch blow molding. Single-stage stretch blow molding has an extruder that injects parison into a rapid cooling preform mold, which is then reheated and put in the container mold. The parison now stretches as air fills the container. The plastic then cools, hardens, and then is ejected as a finished product.

With the two-stage stretch blow molding, two separate machines are involved, one that creates the preforms and the other that heats the containers to fit the mold. This makes the switching out of using preforms super simple and helps to save costs. However, you will need an injection stretch blow molding (ISBM machine), which are relatively expensive. Stretch blow molding is used for peanut butter jars, water bottles, and liquor bottles.


Since 1981, KW Plastics based in Troy, Alabama, has been helping you by buying your unwanted scrap plastic and then turning that into plastic resin pellets, which can then be reused by a recycled plastic manufacturer to make more plastic goods. We have the largest capacity for blow molding, injection molding, extrusion, and blown film. We also can supply you with the best plastic resin pellets made from scrap plastic that will make your plastic products sturdy and durable. We buy bales of high-density polyethylene (HDPE) or polypropylene (PP) scrap material for our use. When you partner with KW Plastics, a plastic recycling company, you will be doing your part to keeping scrap plastic out of landfills and requiring less plastic to be made in the future by recycling your unwanted plastic.

When it comes to plastics, KW Plastics has got your recycled plastic material manufacturing needs covered. Contact us today to find out more about our delivery program!