Injection Molding

There are many plastic products you may see in day-to-day life. Have you wondered how these plastic products are manufactured? Plastic parts like bottles, and toys are manufactured by the plastic injection molding process.

What is injection molding

The following GIF explains about plastic injection molding process in a few seconds.

Stage 1: Plastic granules/resin feed into the hopper.
Stage 2: Plastic granules are pushed into the barrel through the plunger and handle.
Stage 3: Plastic granules are melted with the help of a heater that heats plastic resin above the melting temperature.
Stage 4: Molten plastic is injected into the mold to form the desired part shape.
Stage 5: In this stage molten plastic is allowed to cool and solidify.
Stage 6: Final Molded plastic product injected by opening mold.

Plastic injection molding technology/process manufactures varieties of parts/components of plastic such as plastic bottles, toys, electronics components like TV remotes, LCD TV cabinets, cellphone bodies, automobile trims, car dashboards, and varieties of parts.

Injection Molding Process

Above GIF is old technology of plastic injection molding which is heavily dependent on operator skill and all manual work. High volume production is not feasible with old technology.

The following graphics help you to understand the concept of a plastic injection molding machine. The injection molding machine is divided mainly into two sections, one is the injection unit and another one is the clamping unit.

The plastic injection molding process is being conducted in the following steps,

Blending and Mixing:

In this step, plastic raw materials mainly known as plastic pellets or resin are heated for a few minutes to remove moisture. This blending process is also helpful to mix plastic pellets with masterbatch in a defined ratio.

Hopper Feeding:

Post mixing of plastic granules/resin, it is fed into the hopper. In many plastic injection molding shops, a centralized hopper feeding system is used to better space control and flexibility in day-to-day operations.

Barrel Feeding:

Plastic granules/resin guided into barrel where screw conveyor starts pushing plastic granules into the heating zone by applying pressure. The screw conveyor is driven with the help of a motor and gear mechanism.

Heater:

The function of the heater is to increase the temperature inside the barrel to melt plastic granules. Multiple heaters are used in the plastic injection molding machine depending on material type, melting temperature range, and other process parameters.

Molten Plastic:

Molten plastic is fed into the mold via sprue. To avoid back-flow of molten plastic from mold to barrel a non-return valve is used. This non-return valve will only allow molten plastic to flow from barrel to mold.

Sprue:

Sprue is a primary feed of molten plastic into the mold from a barrel. Sprue is connected to multiple runners. Sprue is the first point where the injection unit ends and clamping units start.

Runner:

A runner connected to sprue, which helps to allow the flow of molten plastic into the gate of each cavity. In the mold, multiple runners are used for each cavity at equal length. Equal length is important to achieve a uniform flow of molten plastic in each cavity.

Gate:

Gate is a small passage that connects the runner and cavity. The molten plastic flows from runner to gate and then it flows into the cavity.

Cavity & Solidification:

Molten plastic is injected into the cavity. Cavity generates the final product after the solidification of plastic.

Ejection:

Once the solidification process completes, the moveable platen is pulled back with the help of a hydraulic cylinder and toggle clamp which separates both molds and actuates the ejector mechanism to eject plastic parts from the mold.

Plastic injection molding raw material

Mechanical Properties	Yield Stress Yield Strain  Strain at Break  Tensile Modulus  Charpy Impact (notched) Temperature of Deflection under Load Gripping Distance	ISO 527-1
Physical Property	Density	ISO 1183
Molding Parameters	Melt Temperature Vicat Softening Temperature	ISO 306
Biological Properties	Biological evaluation	ISO 10993
Food contact material requirements	Food contact material	67.250

Molten plastic flow direction

The molten plastic flows in the following direction

a. Barrel
b. Sprue
c. Runner
d. Gate
e. Cavity

Process Control in Plastic Injection Molding Process

Barrel temperature and Fluidity:

A lot of barrel heater temperature directly affects the fluidity of plastic resin material. It’s very important to set heater temperature as per plastic resin types i.e. barrel temperature should be higher than the melting point of plastic resin.

Moisture and hydrolysis:

Excess moisture leads to hydrolysis effects in the plastic injection molding process. The hydrolysis effect is nothing but a reaction of polymers, alternation of the molecular structure due to the presence of moisture/water.

           The hydrolysis effect in the plastic injection molding process further leads to,

                  a. Loss of gloss

                  b. Reduced transparency (clouding)

                  c. Reduced mechanical strength of plastic

                  d. Embrittlement

Plastic Raw material / Resin selection:

1. Yield Stress
2. Yield Strain 
3. Strain at Break 
4. Tensile Modulus 
5. Charpy Impact (notched) 
6. Density 
7. Mass Flow Rate 
8. The temperature of Deflection under Load 
9. Vicat Softening Temperature 
10. Melt Temperature

Injection molding defects:

Following defects may appear in the plastic injection molding process if process parameters are not followed properly. To discover the root cause of defects in the plastic injection molding, use the following details along with fishbone diagram or 5-Why problem-solving tools.

1. Short shot
2. Flow marks
3. Bubbles
4. Sink marks
5. Gloss Defects
6. Burns
7. Galling

Burn:

This defect can be visibly noticed on plastic parts. The black burn mark appears on the plastic due to overheating of trapped air in the mold or burning of resins itself. A thin section of the plastic part is more prone to burn defects.

How to avoid burn defects in plastic injection molding?

Machine: 

1. By providing appropriate venting line/provision air trapping in the mold can be avoided.
2. Better provision of trapped air escape from parting line.
3. Regular mold maintenance and inspection

Method:

1. By maintaining proper injection speed (neither too low nor too high)
2. By maintaining proper charge quality (neither too low nor too high)

Mold Deposit:

Deposition of excess plastic material in the corner of the core or the gap between core and insert known as mold deposit.

Key Causes of Mold deposit:

1. Improper exhaust of trapped air and gas
2. Thermal decomposition of the resin
3. Presence of Oil/grease in the core, sprue, gate, runner.

How to avoid mold deposit defects in plastic injection molding?

1. Proper design of a mold for smooth flow of trapped air and gases.
2. Selection of process parameters as per plastic resin properties.
3. Usage of organic solvents to remove oil/grease from the core, sprue, gate, and runner area.