Injection Molding 101

Introduction

The Injection molding process converts materials such as plastics, resins, powders, pellets and other forms into usable products. One thing they have in common is that they all require some level of force in order to accomplish this transformation. Some will require less force, like resins and others will require large amounts of force, like plastic pellets. One method that is commonly used today In manufacturing is Injection Molding. This section will give you an overview of Injection Molding and some of the processes involved.

Injection Molding

Injection molding is a process that converts plastics into product. We see these products all around us every day. Take some time to look at them closely some time. Look at the way they are made. Study the parts and try to imagine the flow of plastic entering into the mold and traveling to all part until the cavity is full. Many times you will be able to find where the plastic enters, where the part is pushed out from and many other fascinating characteristics hidden from the average eye.

Injection molding is appropriate for all thermo plastics. A thermo plastic is any plastic that can be heated and then cooled without degrading the mechanical value of the plastic itself. This is not saying that any plastic that can be melted and hardened again is capable of being used for Injection Molding. This is just a general guideline to let you know what happens with Injection Molding.

The injection process involves heating the plastic to a specific melt temperature, adding pressure behind the melted plastic to push that plastic into a closed mold, allowing the melted plastic to solidify by cooling then opening the mold and letting the ejection system push the part out. 

Elements of molding cycles

1. The mold is closed

2. As the screw starts to move forward, the non-return valve at the front of the screw is closed off thus preventing any material to move backwards along the screw. As the screw moves forward the plastic is caused to flow into the mold. 

3. The screw will maintain pressure until the plastic has cooled or set. This is the point at which that gate will freeze. This freezing of the gate effectively separates the part from the melt. Any further pressure form this point forward is wasted energy.

1. Fill time is the time required to fully displace the air within the cavity.

2. Pack time is the time required to completely fill the part and achieve gate freeze.

3. Cooling time is the time required for the melt to cool or set enough for the part to be ejected.

4. Dead time is the time required to open the mold, eject the part and return the mold to ready state.

1. Injection pressure is relieved as the screw starts its return cycle to draw in more material from the hopper. The amount of material brought into the screw is regulated based on the size of the shot. Once the specified amount has been reached the screw retracts a small amount more to relieve pressure at the tip.

2. The machine then moves into ejection cycle where the clamping force is released and the mold is opened. Once the mold is opened the ejector pins push the part out of the mold and the part is conveyed out from the machine.