Blow Molding Machines Guide
Blow molding machines are used within the plastics manufacturing industry to make bottles of all types. Like other molding processes, blow molding machines generate their finished product through use of heated plastic and a mold cavity in which the liquid is shaped and cooled into its final form under pressure.
Extrusion Blow Molding Machines
Extrusion blow molding machines use a plastic manufacturing process to produce bottles of various sizes and shapes. Plastic resin in the form of pellets are fed into an extruder which uses a rotating flighted screw to melt the resin and push it through the head of the extrusion blow molding machine. The tube-like plastic that exits the head is called a parison. clamp with molds in the shape of the final product close around the parison. A blow pin inserts into the middle of the parison and blows the plastic into the shape of the surrounding mold, which then cools to form the bottle. Many heads are programmable to form parisons with varying wall thickness along the length of the parison. This method is used to counter the effects of the temperature gradient along the length of the parison that would otherwise lead to bottles with non-uniform wall thicknesses.
Examples parts made by the EBM process: most polyethylene hollow products, milk bottles, shampoo bottles, automotive ducting, watering cans, and hollow industrial parts such as drums.
- Low tool and die cost
- Fast production rates
- Ability to mold complex parts
- Handles can be incorporated in the design
- Limited to hollow parts
- Low strength
- Parisons are often made of mixed (multilayer) materials, to increase their barrier properties, and are thus not recyclable
- To make wide neck jars spin trimming is necessary.
Injection Blow Molding Machines
Injection blow molding machines are used for high volume production of smaller bottles such as medical vials and other pharmaceutical bottles, single serve bottles, and jars. Injection molding machines employ the use of three stations arranged in a rotary fashion. The first station is the injection station where the injection blow molding machine injects melted plastic into a heated mold that contains a core rod. By the time the process at this station is complete the pre-form has been made which contains the finished neck of the bottle or jar being made, attached to a polymer tube which will form the body. The mold opens and the core rod is rotated to the next station where a hollow, chilled blow mold clamps around it before compressed air flows through an opening in the core rod to blow the body of the part into the finished shape. Once sufficiently cooled, the injection blow molding machine rotates the finished part to the final station for ejection. Typical material run on these machines includes LDPE, HDPE, PP, and PVC.
The advantages and disadvantages:
- It produces an injection molded neck for accuracy.
- Only suits small capacity bottles as it is difficult to control the base center during blowing.
- No increase in barrier strength as the material is not biaxially stretched.
- Handles can’t be incorporated.
Accumulator Blow Molding Machines
Accumulator blow molding machines employ a type of blow molding process called intermittent extrusion blow molding. These machines are similar to continuous extrusion blow molding machines except that the plastic that exits the extruder first builds up in an accumulator head before a hydraulic system pushes the accumulated plastic out through the die to form the parison. The two halves of the mold are clamped around the parison which is blown with air into the form of the mold. This system is ideal for larger and heavier products in which the weight of the parison tends to cause a large amount of pull upon exiting the die head, thus creating forces that lead to non-uniform wall thickness. Allowing the plastic to accumulate first and then get rapidly pushed through the die via a hydraulic system such as a press ram helps to mitigate the wall thickness dilemma. Many well-built machines include parison programming which allows the machine to alter the thickness of the parison as it is being formed. The combination of the above processes allows accumulator blow molding machines to efficiently produce heavy, hollow plastic products with uniform wall thickness. Examples of the types of products made on accumulator blow molding machines are large drums, garbage cans, and gas tanks using resins such as HDPE, PP, and PP.
Injection Stretch Blow Molding
The 2 main methods of Injection Stretch Blow Molding as single stage and two stage.
Injection Stretch Blow Molding has two main different methods, namely Single-stage and Double-stage process. The Single-stage process is then again broken down into 3-station and 4-station machines.
In the single-stage process both preform manufacture and bottle blowing are performed in the same machine. The older 4-station method of injection, reheat, stretch blow and ejection is more costly than the 3-station machine which eliminates the reheat stage and uses latent heat in the preform, thus saving costs of energy to reheat and 25% reduction in tooling. The process explained: Imagine the molecules are small round balls, when together they have large air gaps and small surface contact, by first stretching the molecules vertically then blowing to stretch horizontally the biaxial stretching makes the molecules a cross shape. These “crosses” fit together leaving little space as more surface area is contacted thus making the material less porous and increasing barrier strength against permeation. This process also increases the strength to be ideal for filling with carbonated drinks.
Highly suitable for low volumes and short runs. As the preform is not released during the entire process the preform wall thickness can be shaped to allow even wall thickness when blowing rectangular and non-round shapes.
Restrictions on bottle design – only a champagne base can be made for carbonated bottles.
In the two-stage injection stretch blow molding process, the plastic is first molded into a “preform” using the injection molding process. These preforms are produced with the necks of the bottles, including threads (the “finish”) on one end. These preforms are packaged, and fed later (after cooling) into a reheat stretch blow molding machine. In the ISBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high-pressure air into bottles using metal blow molds. The preform is always stretched with a core rod as part of the process.
Very high volumes are produced. Little restriction on bottle design. Preforms can be sold as a completed item for a third party to blow. Is suitable for cylindrical, rectangular or oval bottles.
High capital cost. Floor space required is high, although compact systems have become available.