Continuous extrusion machine (generally bottle machines).
Continuous extrusion means that the extruder is constantly producing a parison (tube) out of its head. The mold halves then grab the parison and then transfer it to a blow station where the air molds the part to the configuration of the mold.
Accumulator head blow molding machine.
These are generally used for industrial parts in which the melted material is accumulated in the head and then pushed out from the head by cylinders.
Reciprocating screw blow molding machine
These machines are a semi injection machine and a semi accumulator head machine. They first melt the plastic and collecting the shot in front of the screw and then push it out over the mandrel creating the parison and then form the part in the same way as type 1 and type 2.
Injection blow machine or IBM’s.
These are machines that are a cross between an injection molding machine and a blow molding machine. They first squirt liquefied plastic material into a closed mold (steel) forming a pre-form. The machine then opens its clamp and indexes the pre-form on a mandrel to a blowing station where the air is applied to form the part (usually a bottle) into the shape desired. The machine then indexes a second time to the ejection station.
Two-step re-heat and blow machine or RHB
In this process an injection molded pre-form is unscrambled and placed into a serpentine belt system and re-heated. (The re-heating is programmed by quartz heaters to allow the exact form to more easily be blown when it reaches the mold cavities.) When it reaches the mold cavities a rod pushes the parison thereby lengthening it and simultaneously blow air is supplied through the mouth of the container thereby stretching the pre-form in two directions at the same time. This produces a bi-axially oriented product which is capable of providing a co2 barrier thus making a typical pop bottle.
One-step blow molding machine
The process is similar to the above IBM process ( #4) in that a pre-form is molded in the first stage mold halves, indexed to the second stage and stretched and blown at the same time as in step #5.
The particular materials that are generally blow molded are:
PET (polyethylene terephthalate)
HDPE (high density polyethylene)
HWPE (high molecular weight polyethylene)
ABS (acrylonitrile butadiene styrene)
PVC (polyvinyl chloride)
Engineered materials such as zenoi, ppo and other common injection molded materials
Silicone gum rubber
As you can see from the above list most commonly injection molded and extruded materials can be blow molded. The particular materials are chosen for their physical properties, cost and environmental utilization properties.
The sizing of the particular machine is based on the weight of the part plus the flash of the part (if flashed) and the particular molecular weight of the resin. For example if one were going to run a 5 gallon gas can (the red ones in your garage). One would have to first know that it weighs approximately 1500 grams, cycles in about 48 seconds, is molded out of HDPE and generally is molded on a single cavity or double cavity machine. The key point in the above example is that the 1500 gram finished weight of the product has a flashed weight of about 2000 to 2100 grams. Thus a five pound machine (accumulator head) is necessary to mold the product. It can also be molded in a reciprocating machine as well as in a continuous extrusion machine. In the case of the continuous extrusion machine it becomes a bit more difficult to determine what size is necessary. In all cases the platens must be large enough to accommodate the mold. With continuous extrusion one must understand that the out-put is continuous and therefore one must know the cycle time of the product in order to determine the pounds per hour that the extruder must produce. In the case of the red gas can the machine must be capable of 350 to 400 pounds per hour to keep up with the cycle time.
All of the above is a very cursory explanation of the sizing of the machines versus the nature of the product. An additional factor to come into play is the capability of the head to accommodate the tooling which makes the parison the correct diameter. Each machine is capable of a tooling size range. In the case of the red gas can we would need a tooling size of approximately 7 inches in diameter. The way that this is calculated is to first use the circumference of the circle.
The circumference is then divided by two thereby giving us lay flat.
Lay Flat = C=pd
The lay flat is really the parison squeezed flat which is what happens when the mold halves close on the tube.
Parison Diameter x Pie (-3.14) ÷ 2 = Lay flat
In other words the diameter of the tooling determines the diameter of the tube and thus the size of the product that can be made by that head tooling. Each machine has a particular maximum and minimum tooling diameter thereby allowing the machine to be size to make the particular product.
As one might well understand a few paragraphs do an injustice to the technology. This information is provided to help the user in sizing and selecting the machinery necessary for their job. More information and help in sizing any machinery can be provided by speaking with Bob Jackson at Jackson Machinery.