What is Injection Molding and How Does It Work?

The first step in injection molding is putting metal or plastic pellets into the machine’s hopper. Once the hopper is filled with pellets, the pellets travel into the cylinder.

The next step is heating the cylinder. The cylinder is the injection molding machine’s compartment that heats the pellets for molding preparation. Once the cylinder heats the material to liquidation, the liquid flows out of the nozzle.

Third, the nozzle injects the liquidized material through the attached mold channel, referred to as the sprue, and into the mold cavity like a syringe.

Once the material shoots through the sprue and into the mold cavity, the mold cools and hardens into a formed shape. After it adequately cools, the machine releases the formed molded material.

Lastly, the sprue is removed from the final part. Finishing techniques are also applied to make the final product look nice and function well.

Weld lines are thin streaks on the molded material, identifying where the material merged along the split in the mold. These injection molding defects decrease the molded part’s strength and can produce burns or bubbles.

The defect occurs when the resin solidifies too early during merging. There are three solutions to prevent weld lines:

• 1
  Increase the temperature of resin or mold
• 2
  Increase the injection speed or pressure
• 3
  Adjust the gate position and holding pressure

Cracking and mold sticking is a lump in the mold that makes the entire mold crack. This injection defect occurs when mold release resistance is too high.

There are three solutions for fixing cracking and mold sticking:

• 1
  Decrease injection pressure
• 2
  Decrease resin or mold temperature
• 3
  Decrease ejection speed

Voids, also known as bubbles, appear as little holes inside the molded part. These injection molding defects occur when the pressure is not set correctly for the mold.

Here are some common solutions for fixing voids:

• 1
  Decrease thickness
• 2
  Adjust gate position
• 3
  Increase mold temperature
• 4
  Decrease cylinder temperature
• 5
  Increase injection pressure

Sinkage is depressions on a molded part’s surface. It happens when pressure is incorrectly transmitted to the mold cavity.

Four solutions for preventing or fixing sinkage are:

• 1
  Investigate resin and review if there is enough for the entire mold cavity
• 2
  Increase resin or mold temperature
• 3
  Evaluate leakage, clogging, and possible contamination
• 4
  Asses the air vents for errors

Flash or burrs occur when resin seeps out of the mold seams. Injection molding defects like flashburrs may be a result of too much resin or not enough.

Five possible solutions for fixing flash/ burrs are:

• 1
  Evaluate resin: too much? Too little?
• 2
  Decrease injection pressure and speed
• 3
  Ensure the mold is clamped tight enough
• 4
  Lower resin or mold temperature
• 5
  Find and fix chips, scratches, or fatigue in the mold

Short shot errors mean that the resin didn’t fill the cavity all the way.

There are five possible solutions:

• 1
  Check for sufficient resin
• 2
  Assess injection pressure and speed
• 3
  Review resin and mold temperature: make sure it isn’t too low
• 4
  Review for resin leakage or clogging or foreign objects
• 5
  Review errors in the air vent

Black spots are impurity spots on the molded part that look brown or black. These spots occur from contamination. There are two solutions:

• 1
  Perform frequent purging
• 2
  Ensure cleaning after screw extraction

Gate remnants are extra remnants in the tunnel and pinpoint gates. These remnants occur when the gate remainder is inadequately processed.

This problem has four possible fixes:

• 1
  Change gate shape
• 2
  Increase speed for opening mold
• 3
  Increase cooling time
• 4
  Decrease holding time

Burning is discoloration on a molded part that appears black or brown. Burning injection molding defects are different from black spots because it does not result from contamination and appear in streaks. They are, instead, burned results from the air being compressed at too high temperatures.

Countermeasures for burning:

• 1
  Increase air venting
• 2
  Inspect clogging of air vents
• 3
  Change gate position
• 4
  Lower injection speed

Warpage is a type of molded part deformation that results from residual stress.

Solutions for preventing warpage include:

• 1
  Decrease ejection speed
• 2
  Decrease temp of resin or mold
• 3
  Decrease injection pressure
• 4
  Review thickness

Silver streaks occur from insufficient drying or mold contamination from the air or foreign objects.

Here are a few solutions for preventing or countering injection molding defects like silver streaks:

• 1
  Increase drying time
• 2
  Decrease injection speed
• 3
  Increase injection pressure
• 4
  Increase holding pressure
• 5
  Review for contaminates

Jetting is also one of the injection molding problems resulting in a streaked appearance. However, it looks more squiggly than a silver streak. Jetting also results from resin jumping forcefully during initial molding, which is different from streaking.

Possible fixes for jetting are:

• 1
  Increase temp of resin or mold
• 2
  Decrease injection speed
• 3
  Expand the gate

Flow marks are tree ring shapes streaks on the surface of molded parts around the gate.

Solutions for flow marks are:

• 1
  Increase temp of resin or mold
• 2
  Increase injection speed
• 3
  Increase injection pressure
• 4
  Expand the gate