What is Flow mark in plastic?
Flow mark refers to the wavy lines or patterns appearing on the surface of the molded product. This usually happens when the injection speed is slow, or when the surface layer solidifies faster than resin filling.
What are bosses in plastic?
In plastic parts, bosses are typically used to assist in assembly, as a receptacle for a screw or threaded insert or as the locator for a mating pin on another part. Because of its function, a boss must have sufficient strength to do its job. This dictates a minimum size for the feature.
What is flow lines in injection Moulding?
Flow marks, also known as flow lines, are molding defects that can occur in the manufacturing process of injection molding. They are best described as “off tone” wavy lines/streaks or patterns in the molded part around the injection ports.
What are the four plastic variables?
The four plastics variables are: temperature, flow, pressure, and cooling. These are in the order that the plastic itself undergoes. It is essential that we check the variables in this order so that we do not affect earlier parts of the process as we go.
What causes in flow mark?
Flow lines are often the result of variations in the cooling speed of the plastic as it flows in different directions through the mold. They also can occur when you’re creating pieces of differing thicknesses, as the thinner areas cool before thicker areas are fully filled, causing flow lines.
What is rib and boss?
RIBS: A reinforcing member of a fabricated or molded part. It is included to support and strengthen the part. · BOSS: Projection on a plastic part. It is included to: 1) add strength, 2) to facilitate alignment during assembly and 3) to provide for fastening.
What is a boss component?
A boss is a cylindrical protrusion placed on a part’s wall. A boss can have various functions, such as the following: Positioning aid – To help align parts during assembly; for example, a pin on one part will fit a hole in the other part.
How do you set cooling time in injection molding?
First, divide the square of the part thickness by the product of the following: the square of the thermal diffusivity, multiplied by 6.28 (or two times “pi”). This number is then multiplied by a logarithmic equation. If you’re not an engineer, now might be the time to give one of your engineers a call.
Which variables determine the injection time to?
As pressure and speed are interrelated, here are the factors that affect injection time: Speed setting. Pressure setting. Material viscosity.
How do you get rid of flow marks?
How to Prevent Flow Lines
- Increase the injection speed, pressure, or material temperature: Slow-moving molten plastic is more likely to cool quickly and cause flow lines.
- Round the corners of the mold where wall thickness increases: This helps keep flow rate consistent to the thicker sections and prevents flow lines.
What causes splay in plastics?
Splay is caused by tiny gas bubbles that are dragged across the surface of the part when the mold cavity is filled. Splay is usually caused by water that is absorbed within the plastic granules.
What is difference between rib and gusset?
Ribs are thin, wall-like features typically designed into the geometry of a part to add internal support to walls or other features like bosses. In a similar fashion, gussets are support features that reinforce areas such as walls or bosses to the floor.
What is screw boss?
Boss features are commonly found in injection molding designs. They are used to aid in the assembly of molded parts by providing a channel for a screw. Designing plastic screw bosses is fairly straightforward but there are a few considerations to ensure a strong connection between parts and mitigate cosmetic defects.
What is boss casting?
Bosses are the integral features added to a casting for a number of potential functions, including mounting. Critical to die cast parts that need to be mounted, bosses are added to serve as stand-offs and mounting points. Bosses must have universal wall thickness for maximum integrity and strength for the component.
What is boss hole?
A bosshole is a combination of a boss and, well, an anal sphincter. Bossholes are workplace bullies who yell at the drop of a hat and generally make their employees feel tired, lousy, and unappreciated.
Why use flow leaders and restrictors in molding?
Used correctly, flow leaders and restrictors can improve the quality of molded parts when conventional means cannot. As a moldmaker, you need to create a mold that is cut to very tight tolerances and that produces good parts with a robust processing window.
What is the best way to design a flow leader?
Three approaches come to mind (in order of least-preferred to most-preferred): • Design at the press. Steel is hogged out of the mold’s B side as it is hanging in the machine during the mold trial. • Design by the moldmaker. The mold is returned to the shop to be properly machined to specifications for the flow leader.
What is the flow path of plastic in injection molding?
In general, when fluids have a choice between two different flow paths, they take the one with the least resistance. In an injection mold, the plastic fluid flows and spreads out in many directions as it comes out of the gate.
Should you add flow leaders or restrictors for Knit-line control?
Adding flow leaders or restrictors for knit-line control or air-trap avoidance becomes an exercise in trial and error, even in the simulation world, but it is not as expensive as mold iterations. There are three main design rules for incorporating flow leaders: