In chemical engineering, it also study about plastic and rubber.
Among these, there are various kinds of polymer processing techniques.
(a) Extrusion-Blow molding
(b) Injection Blow Molding
(c) Stretch-Blow Molding
6.Compression and Transfer Molding
The first three polymer processing techniques are more popular than others.
· Parts of infinite length can be produced
· Some parts, like blown film, can only be made with extrusion
· Extrusion is a continuous process capable of very large production volumes
· Most thermoplastics, and some thermosets can be extruded
· Since the dies contain no moving parts, only continuous cross sections can be manufactured
· Complex geometry cannot be produced
· Complicated feature can be very hard to produce because of difficulties in producing a balanced die
apply in food processing
Injection Molding Machine
Injection molding machine also known as presses, holds the mold in and components are shaped inside it. Presses are rated by tonnage, which expresses the clamping force that the machine can generated. This pressure keeps the mold closed during the process. Tonnage can vary from less than 5 tons to 6000 tons.
The injection molding equipment or machine has three basic components (functional units) (1) injection units (plasticator), (2) mold, and (3) clamping unit.
The function of plasticator is to prepare the proper plastic melt and through the injection unit transfers the melt into the next component that is mold. The clamping system closes and opens the mold. The machine perform certain essential functions –
1. Plasticizing: heating and melting of the plastic in the plasticator.
2. Injection: injecting from the plasticator under pressure a controlled volume shot of melt into a closed mold, with solidification of plastics beginning on the mold’s cavity wall.
3. After filling: maintaining the injecited material under pressure for a specified time to prevent back flow of melt and to compensate for the decrease in volume of melt during solidification.
4. Cooling: cooling the thermoplastic (TP) molded part in the mold until it is sufficiently rigid to be ejected until it is sufficiently rigid to be ejected or heating: heating the thermoset (TS) molded part in the mold until it is sufficiently rigid to be ejected.
5. Molded part release: opening the mold, ejecting the part, and closing the mold so it is ready to start the next cycle with the shot of melt.
Injection molding processes
1. The mold is first closed as its two halves come together.
2. The nozzle moves toward the mold, until the parts adjoin.
3. The material is injected into the mold via a hole in the mold, called the sprue.
4. The mold is cooled before the product is ready to be ejected.
5. The material at the end of the screw is decompressed and the shot size of the material is metered.
6. The mold opens.
7. The part is ejected.
• High production rates
• Design flexibility
• Repeatability within tolerances
• Can process a wide range of materials
• Relatively low labor
• Little to no finishing of parts
• Minimum scrap losses
• High initial equipment investment
• High startup and running costs possible
• Part must be designed for effective molding
• Accurate cost prediction for molding job is difficult
It can apply from micro parts to large components such as bumpers and wheelie bins.
Containers, household goods, auto components, electronic parts, flower pots.
mold က ေစ်းၾကီးေသာ္လည္း injection molding ကုိအသံုးမ်ားပါတယ္.. အမ်ဳိးမ်ိဳးထုတ္လုိ ့ရလုိ ့ပါ.. ေက်ာင္းမွာ တစ္ခါႏွစ္ခါေလာက္ေတာ့ practical လုပ္ဖူးပါတယ္.. machine က control လုုုပ္ရတာသိပ္မခက္ပါဘူး..
short-run or prototype products[/li]
- [li]Poor material distribution: the wall thickness varies and is difficult to control
Consider finishing is required and considerable waste
the polymer has to heated twice, once to form the sheet (extrusion) and again during vacuum forming
the range of shapes available is limited[/li]
The largest application is for food packaging such as toiletries, pharmaceuticals, electronics.
Food related applications are meat trays, microwave and deep freeze containers, ice-cream and margarine tubs, snack tubs, sandwich packs and vending drink cups.
Other non-food applications include manufacturing collation trays and blister packaging.
- [li]economical for small to medium production runs
low tooling costs
high strength to weight ratio
no need for painting; the color and texture are formed in[/li]
1. The heated plastic sheet is clamped and sealed across the pressure box.
2. Control air pressure blows the bubble, giving the pre-stretch of 35-40%.
3. The plug is moved down into the bubble; air-pressure keeps the bubble and the plug contact.
4. The plug is lowered to its final position, a vacuum is pulled between plug and sheet, and the pressure in the pressure box causes the sheet to take up the shape of the plug.
Baths and shower trays, ski-boxes, refrigerator liners, parts of vehicle cabs, sandwich boxes, exterior shop signs
1. Extrusion-Blow Molding
low initial mold tooling costs
flexibility of tooling
flexibility in production: Neck inner diameters (I.D) can easily controlled to varying requirements. Bottle weights are adjustable.
container sizes can range from less than 1 oz. to 55 gallons and up
container shape is not restricted by blow-up ratios. Bottles can be long and flat or have handles.
wide selection of machine sizes: Molds can geared to volume requirements
bottles and containers
automotive fuel tanks
Pharmaceutical products, consumable bottles
Carbonated and soft drink bottles
Cooking oil containers
Bathroom and toiletry products
Compression and Transfer Molding
The polymer flows over shorter distance, thus reducing frozen in stress
Polymer is not forced through small gates which can lead to points of weakness in a molding.
Low mold maintenance costs.
Mold design is not complicated by spruce and runner layout
Its ability to mold large and fairly intricate parts
Offers least product consistency
Difficult to control flash
Not suited for some types of parts
economically produced large products
minimum design constraints
no polymer weld lines
comparatively low mold costs
diesel fuel tanks