Designing Plastic Parts For Injection Molding? Run Mold Flow Analysis Before Cutting The Mold
Mold flow analysis is not required for the injection molding process. But maybe it should be, especially considering it can help to predict manufacturing issues before production starts.

Mold flow analysis software* simulates an injection molding cycle using a specific plastic and part design. It evaluates the design for manufacturability before cutting the mold. This allows designers to identify design flaws that would otherwise result in expensive redesign and time delays.

This post will explore the basics of mold flow software, identify how it helps to optimize the injection molding process, and look at sample data generated by the analysis.

First, let’s briefly review three key components that are critical to the injection molding process: design, mold, and material.

Injection Molding Manufacturing
Injection molding is the most widely used method for mass-producing plastic parts. It’s economical, efficient, and can produce simple to complex parts with low waste. For details about the 6 stages included in the process, visit TISS’s injection molding process page.

Mold 
The injection molding process requires a mold, or tool, to produce plastic parts. Mold design engineers design a custom injection mold and then expert mold makers build the mold for production.

Even if the mold is designed and built to exact specifications of the part, issues could still arise during the injection molding process if the part itself isn’t optimized for injection molding or if gate locations aren’t placed in optimal positions for material flow. For example, the plastic may not completely fill the mold cavities resulting in voids, or part defects. Mold flow analysis helps to determine how a given plastic will perform in the mold.

The “plastic” in plastic injection molding
Not all plastics flow, heat, or cool the same. In fact, there are more than 85K polymers to choose from when designing for plastic injection molding. The vast polymer options can make material selection a challenge.

Mold flow analysis enables designers to evaluate the material for variables such as material shrink rate, cooling properties, ability to fill cavities, and potential for aesthetic flaws.

Mold flow analysis: optimize the injection molding process 
Mitigate risk before production begins
Mold flow analysis helps to mitigate risk and create a successful mold from the start. It helps designers to:

correct potential cosmetic and structural problems
determine the appropriate wall thickness
troubleshoot potential problem areas of the mold
identify optimal gate locations
adjust for ample corner radius
create even and clean edges
identify the best material for the desired outcome
create a successful mold from the start
Design Challenges?
Need help optimizing your design for injection molding? Contact TISS’s consultants. We’re here to help!

Mold flow analysis diagnostic reports
The analysis generates color-coded reports to illustrate how the plastic would perform in the mold. Reports include Fiber Orientation, Average Temperature, Knit Lines, Air Traps, Confidence of Fill, Sink and Warp, and Fill Time Result.

We’ll examine elements of two reports: Fill Time Result and Confidence of Fill.

Fill Time Result
The Fill Time Result report presents the position of the flow front at regular intervals as the mold cavity fills. A balanced flow of plastic pattern indicates the plastic part has a good fill time.

The report provides the result in a color-coded diagram. For example, note the contrast between diagram 1, a good fill time, and diagram 2, poor fill time.

How to read the results
The designer evaluates the image for flow paths that finish and reach the edges at the same time. Evenly spaced contours indicate the speed at which the plastic is flowing. Widely spaced contours indicate rapid flow; narrow contours indicate the part is filling slowly. 

Things to look for
The Fill Time Result report provides insight into the following:

Short shot: A part is short shot when the flow of plastic does not completely fill the mold cavity, thereby resulting in an incomplete part
Hesitation: Hesitation occurs when the flow of plastic stops or slows down resulting in asymmetrical and unpredictable flow patterns
Overpacking: The result of one flow path finishing before others. Overpacking can result in high part weight, warp, and non-uniform density distribution
Weld lines: Also known as knit lines, these are molding defects that occur when two flow fronts meet without the ability to “weld”
Air traps: A bubble of air trapped when plastic flow fronts coincide. Air traps can cause structural and visual defects
Racetrack effect: Occurs when the flow races through the thick areas of a mold cavity before the thin sections have filled
Mold Flow Analysis Guide
Download our guide to help interpret results generated by mold flow analysis.

Confidence of Fill Result
Confidence of Fill Result report addresses the probability of the plastic filling the mold cavity. 

The colors displayed in the Confidence of Fill indicate:

All green: Plastic fills the part easily and the part quality will likely be acceptable.
Some yellow: The part can be difficult to mold, or quality is probably not acceptable. As the percentage of yellow increases, the difficulty in molding the part increases, and the part quality decreases.
Some yellow and red: The part is difficult to fill, or quality is probably unacceptable. As the percentages of yellow and red increase, the difficulty in molding the part increase, and the part quality decreases.
Any translucent: The part cannot be molded because a short shot will occur.
Results help to determine the probability of molding a quality part
One way to use the results to determine whether you can mold a quality part is to consider how much of each color is displayed. The results could indicate a need to:

change the design to better balance flow paths
choose a different injection location to ensure the part is completely filled
re-evaluate the material selection
change the processing conditions
Mold flow analysis: optimize the design before cutting the mold
An accurate mold is critical to producing high-quality, repeatable plastic parts. Mold flow analysis software can help to optimize the process before cutting the mold.

TISS is a trusted partner for injection molding solutions. Contact our team to discuss design challenges and upcoming projects.

*Moldflow, owned by Autodesk, produces simulation software for high-end plastic injection molding. All information and diagrams for Fill Time Results and Confidence of Fill in this blog courtesy of Autodesk.