Injection rate is the amount of material that has been injected into a mold cavity for a specific period. This can be measured in pounds per hour, cubic centimeters per minute, or kilograms per hour. It is important to know how to control injection rates to maintain an even flow and prevent defects such as sink marks and gate marks from forming.
This article will explain the process related to injection rate in the plastic injection molding process.
The Process
The molding material enters the injection unit through the hopper. The material is then heated to its molten state, except for thermoplastic elastomers (TPE), which are typically not heated because they do not require much force to fill a mold. The molten material is pushed under high pressure into the barrel via the feeder mechanism. This causes the material to flow through the screw channel.
The barrel is one of the primary components in the plastic injection molding process. The barrel is composed of a metal tube with a screw attached to it. As this screw rotates, it pushes forward and forces plastic against the walls of the barrel until eventually, it pushes out through an opening in the back of the barrel, known as the screw tip or nozzle.
The opening of the barrel is called the sprue plate. The sprue plate allows molten plastic to be delivered into a mold cavity before it hardens. It also prevents air from entering an injection molding system and acts as a funnel for all plastics to go through.
The injection unit is a mechanism that pushes molten plastic through a metering device, which meters the flow of molten material into a mold. The amount of plastic flowing from the metering device determines the injection rate. Injection units have been used in many different ways over past years until finally being used today with single, multi-cavity, and multi-station injection molds.
Calculation
To calculate injection rate, you must know these three variables:
- Cycle Time
- Scrap Percentage
- Shots-per Hour or Minute
Example of Injection Rate Calculation for a Injection Machine Using the Formula: CR = C/(D*L). CR = Cycle Time (minute), C = Number of Cavities in mold, D = Fast / Slow shot ratio, L = Number of shots per hour
What is the injection flow rate?
The flow rate should not be confused with the injection rate, which deals with how much material is injected into a mold cavity in a certain period of time. Injection flow rate deals with the amount of material that flows through all components of the injection molding system for a specific time period.
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