The History of Pastic Injection Moulding

In 1868, Hyatt developed and improved a plastic material and makes it in the shape of an article.  Then in 1872, he and his brother Isaiah registered the first patent plunger-type injection molding machine. Basically it is running like a giant hypodermic needle. This huge needles inject plastic into the mold through a heated cylindrical.

In the 1940s, during the Second World War, ​ huge demand of cheap mass-produced for the product made it popular.

In 1946 American inventor James Watson Hendry build the first injection molding machine, which makes the injection more precisely controlling the speed and quality of an article. In 1951 the United States developed the first screw injection machine.

In the 1970s, Hendry then developed the first gas-assisted plastic injection moulding process and allows the production of complex, hollow products, of rapid cooling. This greatly increases the flexibility and strength of the design and the end parts manufactured while reducing production time, cost, weight and waste.

How to do Plastic Injection Mould in the Factory?

The mould tool is the key component in the plastic injection moulding

  1. It provides a passageway for molten plastic to travel from the injection cylinder (barrel) to the mould cavity.
  2. It allows the air which would be trapped inside when the mould closes to escape.  If the air could not escape (be vented) then the moulded component would contain voids (air bubbles) and have a poor surface finish.
  3. It cools the moulding until it sets.  The temperature of the mould is controlled because it is important that the moulding cools at the correct rate to avoid distortion and stress.  In most systems water circulates through channels drilled through the mould similar to a car engine cooling system.
  4. By means of ejectors the finished moulding is pushed from the mould.

 

THE PLASTIC MOULD TOOL

 mould

  1. Guide Pins – fixed to one half of the mould and align the two halves by entering the holes in the other half.
  2. Runner – passageways in the mould connecting the cavities to the sprue bush.
  3. Gate – Frequently the runner narrows as it enters the mould cavity.  This is called a gate and produces a weak point enabling the moulding to be easily broken or cut from the runner.
  4. Sprue Bush – Tapered hole in the centre of the mould into which the molten plastic is first injected.
  5. Locating Ring – Positions the mould on the fixed platen so that the injection nozzle lines up with the sprue bush.
  6. Mould Cavity – The space in the mould shaped to produce the finished component(s).
  7. Ejector Pins – These pins push the moulding and sprue/runner out of the mould.
  8. The Shot – Total amount of plastic injected into mould.
  9. Sprue – Material which sets in the sprue bush.

What is the Process of Plastic Injection Moulding?

The plastic injection moulding process produces large numbers of parts of high quality with great accuracy, very quickly.

Plastic material in the form of granules is melted until soft enough to be injected under pressure to fill a mould.  The result is that the shape is exactly copied.  Once the plastic moulding has cooled sufficiently to harden the mould opens releasing the part.  The whole injection moulding process then repeats.

 

 

04

Close Mould

CLAMPING – the moving and fixed platens of the injection moulding machine (see next section) holds the mould tool together under pressure.

                        01

Inject Molten Plastic

INJECTION the molten plastic that has been melted from pellet form in the barrel of the moulding machine (see next section) is injected under pressure into the mould.

 

02

Hold Mould Closed

DWELLING after the molten plastic has been injected into the mould pressure is applied to ensure all cavities are filled.

COOLING the plastic parts are then allowed to solidify in the mould.

                                      03

 

Open Mould and Eject Part

OPENING the moving platen moves away from the fixed platen (see next section) separating the mould tool.

EJECTION – rods, a plate or air blast then aids ejection of the completed plastic moulding from the injection mould tool.

The length of time from closing the mould to ejecting the finished plastic moulding is the cycle.

Plastic Forming——Injection Mould

Plastic forming

There are two types of plastic. One is called thermosettting resin which does not soften agina once it is formed and hardened, and other is called thermoplastic resin which becomes soft or hard when its temperature rises or falls. Although thermosetting resin has an older history, the majority of the presently used plastic are made of thermoplastic resins.

There is plastic forming method :

Plastic injection moulding process

(1) In general ,material arrangement and coloring are carried out in separate process.

(2) The drying process may be required for some plastics but not required by others.

(3) Materials are supplied to a molding machine from a Hopper .

(4) Materials are heated to be plasticized with a heating cylinder. There are various methods for this process. The most popular method is as follows: the screw in-line feeds materials forwards while the screw itself goes rearwards, when a specified volume of plasticmaterial for plasticity is completed, the screw is moved forwards and a plastic resin is injection and charged into a closed die to mould a shaped.

(5) The resin that has filled up the inside of the cavity of die is cooled in the die and becomes solid.

(6) The die is opend to take out molded products. Parts other than products(supuremrunner, fin, defective products) which are produced by this process are regarded as non-conforming products and they can be used agina as moulding materials.

 

Mould Injection System – Hot Runner System

In mould manufacturing, how to decide the mould injection system and how to choose hot runner system, it is a very important issue. In order to get a top quality mould, we have to know the details of mould injection system (hot runner system or cold runner system).

Mould injection system can be devided into two system:

Hot runner plastic injection moulding system can be devided into below tow big types:

  • Open hot runner nozzle
  • Close hot runner nozzle

Open hot runner nozzle has below different types:
-pin point gate nozzle
-direct gate hot nozzle.
please see below picture to see the two types hot nozzle’s differences.

Close type of injection nozzle, we call it valve gate injection nozzle.
-Pneumatic valve gate nozzle ( use air cylinder to drive the vlave pin moving)
-Hydraulic vlave gate nozzle  ( use hydraulic cylinder to drive the valve pin moving)
-Mechanical vlave gate nozzle ( use mechanical moving to drive the valve pin)

If you use soft and high MFI plastic material, then you can choose pin point gate, of course the plastic parts should be so big, for example the part’s weight should not bigger then 200G, otherwise, you should use 2 or more then 2 tips of pin point gate.  The suitable material is such like, PP,PE..

But if the plastic part is big then you have to use direct gate or valve gate. But the direct gate shortcoming is that there will be a gate mark and we have to cut the gate tail manually. If you use valve gate, you do not need to do any after moulding treatment and the gate mark is very nice.

Below advantage and shortcoming of each hot nozzle can help you to make better selection.

  • Pin point nozzle, suitable for small volume injection, with the very small gate mark do not need to do any treatment to the moulded plastic parts.   Compare to valve gate it is much more enconomical cost, but need high pressure injection moulding.
  • Direct gate nozzle, suitable for big volume injection moulding, but we need to cut the gate tails after, the cost is similar the pin point gate nozzle, the advantage is easy filling.
  • valve gate advantage, easy filling, nice gate marks, no after moulded treatment, the only shortcoming is that the cost will be higher then the other two types of nozzle. In the other hand, the mould cost also will be bigger since we need thicker mould plate to housing the valve gate nozzle.

Please check above advantages and shortcomings of the different type of hot nozzle to decide how to choose hot runner system for your mould injection system.

Anole hot runner system is one of the leading hot runner technologies innovators in China, it can help you to solve where to make the injection gate, how to choose the suitable hot nozzle size and hot nozzle type. How to design injection pressure balance,how to make the colors change during your injection moulding, how to make the fast filling, how to design the hot nozzle tips and nozzles’ quantity.