Injection molding

What is cold material?

Cold material refers to a small section of plastic that is cooled and solidified in advance in the nozzle or runner of the injection machine. You can think of it as an "ice pick" mixed into the hot paste - during the next injection, this cold material will be pushed by the hot melt material into the mold cavity, leaving scars or marks on the product surface.

What are the consequences of cold material?

• Cold scars, flow marks, and whitish patches appear on the product surface

• Stress concentration points are generated internally, reducing the product strength

• It may clog the gate, resulting in insufficient filling

• In severe cases, it may scratch the mold

The three main causes of cold material production

1. Temperature control issues

• The nozzle temperature is too low: The nozzle directly contacts the cold mold, losing heat quickly, and the front plastic is prone to solidification

• The runner temperature is too low: The molten material gradually cools during flow

• The mold temperature is too low: The cold mold walls quickly absorb the heat of the molten material

2. Mold design flaws

• Lack of cold material wells: There is no "small pit" specifically for collecting cold material, and the cold material has nowhere to go

• Insufficient capacity of cold material wells: They are too small to hold it, and still enter the product

• Small gate size: Cold material is prone to getting stuck at the gate

3. Improper molding process

• The melt temperature is too low: The plastic has poor liquidity (flowability)， and the front material quickly solidifies

• The screw retraction is too large： The retraction distance is too large， possibly sucking the cold material back into the barrel

• Inappropriate injection speed： Too slow will cause premature cooling， and too fast may result in uneven heat distribution

How to solve it?

The core idea for addressing the issue of cold material is "dilution and interception".

Dilution: Increase the temperature of the nozzle/flow channel/mold, optimize the injection speed, and make the plastic flow more smoothly.

Interception: Design a cold material pit at the end of the mold flow channel - a small depression specifically used to store the leading cold material. This is the most effective and economical solution, and it is used in almost all precision injection molds.