PRODUCTION PROCESSES
Injection molding is one of the most important fabrication processes for the plastics industry. Today, it is almost impossible to do anything without using injection molded parts. You can find them in automotive interior parts, electronic housings, medical equipment, compact discs, and sport products and even in your clothes. Injection molding is used to fabricate pallets, toys, tools, crates, and pails, food containers, drink cups, and milk bottle caps, among others.
Handling polyolefin film extrusion resins it is extremely important to keep polyolefin resins clean. Contaminated resins can produce poor products. Strict quality control throughout resin manufacture and subsequent handling, right through delivery to the processor, ensure the cleanliness of the products. The processor must use clean, efficient procedures for unloading the resin. Maintenance of the in-plant materials handling system also is essential.
Process
Resin is fed to the injection molding machine through the hopper. The resins enter the injection barrel by gravity though the feed throat. Upon entrance into the barrel, the resin is heated to the appropriate melting temperature. Then the material is injected into a specific mold generally by a reciprocating screw that offers the advantage of being able to inject a smaller percentage of the total shot (amount of melted resin in the barrel).
The mold is the part of the machine that receives the plastic and shapes it appropriately. It is cooled constantly to a temperature that allows the resin to solidify. The mold plates are held together by hydraulic or mechanical force. The clamping force is defined as the injection pressure multiplied by the total cavity projected area. Typically molds are over designed depending on the resin to be used. Each resin has a calculated shrinkage value associated with in.
Some advantages of Injection Molding
- High production rates
- Design flexibility
- Repeatability within tolerances
- Can process a wide range of materials
- Relatively low labor
- Little to no finishing of parts
INJECTION BLOW MOLDING PROCESS
Injection blow molding is a process used for the production of hollow objects in large quantities. The main applications are bottles, jars and other containers. The Injection blow molding produces bottles of superior visual and dimensional quality compared to extrusion blow molding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash.
Typical Materials Used
- Polyethylene (Low Density) LDPE, LLDPE
- Polypropylene PP
- Polyethylene - Terephthalate PET
- Polyvinyl chloride PVC
- Polyethylene (High Density) HDPE
The Process
The process has three phases:
Injection of the material
The injection blow molding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mould. The preform mould forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body.
Blowing
The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.
Ejection of molded piece
After a cooling period the blow mold opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mold can have many cavities, typically three to sixteen depending on the article size and the required output.
There are three basic stages in the overall film extrusion process:
- Materials conditioning / handling
- Film extrusion
- Film takeo
Materials conditioning / handling
Resin manufacturing plants must have highly effective and efficient systems, such as filters, cyclones, elutriators, etc, to prevent resin contamination during production, storage, loading and shipment.
Since polyolefin resins are non – hygroscopic (they absorb virtually no water), they do not required drying prior to extrusion. However, precautions should be taken to ensure the cleanliness of the polyolefin pellets are they are handled at the processor´s facilities. Polyethylene’s (HDPE, LDPE and LLDPE) are the most common resins in use. Film extrusion is the best way to manufacture any kind of bags, such as bread bags, grocery bags, or any one of thousands of different.
Two basic methods are used for making polyolefin film: cast film extrusion and blown film extrusion. In both methods, the resin is first melted by subjecting it to heat and pressure inside the barrel of an extruder and finally forcing the melt through a narrow slit in a die (also referred to as the die gap). The slit may be either a straight line or ring-shaped. The resulting thin film has either the form of a sheet (cast film) or a tube, also called a “bubble” (blown film). As the film comes out of the die, it is cooled and then rolled up on a core.
The extruder consists of a resin feeding hopper, a heated barrel, a constant rotating screw, a screen changer, a die adapter and a base. Polyolefin resins are dropped into the extruder feed throat through a round or square funnel, called the hopper. An automatic loader on top of the hopper periodically feeds resin into it.
A motor-driven screw rotates within the hardened line of the barrel. As the screw rotates, the screw flights force the resin in the screw channel forward. As the screw channels become shallower, the resin is heated, melted, thoroughly mixed and compressed. Good mixing of the melted resin is essential for obtaining high clarity film with no defects or blemishes.
An adapter guides the resin melt from the barrel to the die as thoroughly and consistently as possible. The film extrusion die is attached to the adapter. A good die designs ensures smooth and complete melt flow, thus preventing resin degradation from overheating. The die forces the melt into a form approaching its final shape. Also it maintains the melt at a constant temperature and meters the melt at a constant pressure and rate to the die land for uniform film gauge, with allowance for gauge reduction.
Air is introduced through a hole in the centre of the die to blow up the tube like a balloon. Mounted on top of the die, a high-speed air ring blows onto the hot film to cool it. The tube of film then continues upwards, continually cooling, until it passes through nip rolls where the tube is flattened to create what is known as a ' lay-flat' tube of film. This lay-flat or collapsed tube is then taken back down the extrusion ' tower' via more rollers. On higher output lines, the air inside the bubble is also exchanged. This is known as IBS (Internal Bubble Cooling).
The lay-flat film is then either kept as such or the edges of the lay-flat are slit off to produce two flat film sheets and wound up onto reels. If kept as lay-flat, the tube of film is made into bags by sealing across the width of film and cutting or perforating to make each bag. This is done either in line with the blown film process or at a later stage.
Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film which is drawn down along the extrusion direction only .
The advantages of polyolefin blow holding resins are processability, light weight, good toughness, outstanding chemical resistance and relatively low cost compare to other plastics.
Major application areas for polyolefin blow molded products include:
- Packaging for such products as milk and others foods, cleaning fluids, medicines, cosmetics and personal care products.
- Automotive items, such as gas tanks, oil bottles and windshields fluids containers, air ducts and seat backs.
- Consumer products, including toys, house wares and sporting goods.
- Objects for materials handling, including 55 gallon drums and chemical carboys
- Industrial products, such as business machine fluid containers.
- Bellows-shaped shields and double wall instrument carrying cases.
Quimtec offers you polyolefin’s that can be blow molded include:
- Low Density Polyethylene
- Linear Low Density Polyethylene
- High Density Polyethylene
- Ethylene Copolymers, such as Ethylene Vinyl Acetate
There are three basic blow molding techniques –continuous extrusion, intermittent extrusion and injections blow molding- and numerous variations of each. However, all the blow molding processes consist of five successive stages:
- Melting the resin
- Forming the parison or perform: The parison is formed by forcing the plastic melt through a die. In injection blow molding, the melt is injected into a mold cavity to form a “test tube” like perform. Multiple performs are usually molded in a single injection cycle.
- Transferring the parison or perform: in injection blow molding, there are two basic methods for transferring the performs, the first one directly to the blowing station or indirectly the preforms cool, are ejected and then stored in inventory for subsequent forming in a separate blow molding unit.
Inflating the perform: all of the various blow molding techniques use the same basic process for forming the hollow, high pressure air injected into the hot parison or preform forces out against the inside surfaces of the mold cavity.
The advantages of polyolefin blow holding resins are processability, light weight, good toughness, outstanding chemical resistance and relatively low cost compare to other plastics.
Major application areas for polyolefin blow molded products include:
- Packaging for such products as milk and others foods, cleaning fluids, medicines, cosmetics and personal care products.
- Automotive items, such as gas tanks, oil bottles and windshields fluids containers, air ducts and seat backs.
- Consumer products, including toys, housewares and sporting goods.
- Objects for materials handling, including 55 gallon drums and chemical carboys
- Industrial products, such as business machine fluid containers.
- Bellows-shaped shields and double wall instrument carrying cases.
Quimtec offers you polyolefins that can be blow molded include:
- Low density polyethylene
- Linear low density polyethylene
- High density polyethylene
- Ethylene copolymers, such as ethylene vinyl acetate
There are three basic blow molding techniques –continuos extrusion, intermittent extrusion and injections blow molding- and numerous variations of each. However, all the blow molding processes consist of five successive stages:
- Melting the resin
- Forming the parison or perform: The parison is formed by forcing the plastic melt through a die. In injection blow molding, the melt is injected into a mold cavity to form a “test tube” like perform. Multiple performs are usually molded in a single injection cycle.
- Transferring the parison or perform: in injection blow molding, there are two basic methods for transferring the performs, the first one directly to the blowing station or indirectly the preforms cool, are ejected and then stored in inventory for subsequent forming in a separate blow molding unit.
Inflating the perform: all of the various blow molding techniques use the same basic process for forming the hollow, high pressure air injected into the hot parison or preform forces out against the inside surfaces of the mold cavity
