Not only does your product have to be functional, but to a certain extent, it must look good, too. Injection molding is an excellent way to create plastic parts in bulk, but there are many problems that can cause cosmetic defects ranging from minor to major. Here are some of the most common cosmetic defects and how to avoid or fix them.
There are 5 different grade levels of cosmetic standards for plastic parts:
Each grade has its own standards and allowances for defects, with Grade 1 having the highest standards and lowest allowance for defects.
Now, let’s talk about the most common defects and how you can fix or avoid them by keeping things in mind as you design your mold and run your first few batches of parts.
Flash happens when some molten plastic escapes from the mold cavity, protrudes over a part’s edge, and creates a small flap of thin extra plastic on the edge of the part.
Flash is most commonly caused by:
Depending on what is causing the flash, possible fixes include:
A sink mark is a small depression or crater that may appear in thicker areas of your part when shrinkage occurs in the inner parts of the finished product.
Common causes of sink marks include:
To avoid or fix sink marks:
Also called flow marks, flow lines are discolored lines, streaks, or waves near the gate part of the mold, where the molten plastic is injected. While they don’t affect functionality, flow lines may impact a part’s aesthetic appeal.
Flow lines can be caused by:
To help prevent flow lines, keep these things in mind while designing your mold:
If you already have a mold and are experiencing flow lines, try these fixes:
Also known as weld lines, knit lines resemble cracks or hairline fractures on the surface of a plastic part, usually at the edge of a hole or indent or where two melt fronts converge. While usually a cosmetic issue, knit lines can create failing points on areas that receive stress.
Knit lines are caused by inadequate bonding between two or more flow fronts as a result of partial solidification of the molten plastic.
When designing your mold, keep these things in mind to help prevent flow lines:
If you’re currently having problems with weld lines, you can try:
You can learn more about avoiding knit lines here.
Voids are air bubbles trapped in or near the surface of an injection-molded part. While voids are primarily a cosmetic problem, multiple bubbles can weaken the part.
The most common cause of voids is insufficient molding pressure or parts with very thick wall section. They may also be caused by a mold with two halves that don’t align properly.
There are several ways to avoid or fix vacuum voids, including:
Discoloration is when a product is a different color than it should be and is typically only apparent in one part of the product or as a few streaks on the surface of the part. Discoloration is a cosmetic issue that does not affect a part’s functionality.
Typical causes of discoloration include:
To help avoid discoloration:
Warping is a deformation caused by uneven cooling and shrinking in different areas of the mold, resulting in a part that is uneven, twisted, or bent.
Warping is typically caused by different cooling rates in various parts of the mold, causing internal stresses. Glass filled materials have a higher propensity for warp as well.
The best ways to avoid warping are to:
Short shots are where the molten plastic doesn’t fill the entire mold cavity, resulting in an area with no plastic. Since they are incomplete, short shots are typically unusable since the product’s functionality is impaired.
Many different things can cause short shots, including:
There are many ways to prevent or fix short shots, including:
Jetting is a type of deformation where the plastic solidifies in a way that shows the wavy folds of the jet stream on the surface of the part because the molten plastic failed to stick to the mold surface.
Jetting is caused by a jet of molten plastic entering the mold and cooling quicker than the rest of the material, usually as a result of high injection pressure or low melt temperature.
To avoid or prevent jetting:
Burn marks are discolorations (usually rust-colored or black) on the surface of injection molded parts. They’re usually harmless as long as the burn hasn’t degraded the plastic.
Burn marks are typically caused by overheated resin or trapped air bubbles due to excessive heating or injection speeds that are too fast.
The best ways to fix burn marks are to:
Surface delamination is when thin surface layers start to peel away from the surface of the part. It may cause a part to weaken and lose functionality.
Surface delamination is usually caused by contamination of the raw materials with particles of a foreign substance. The contaminant can’t properly bond with the plastic, which causes the flaking. Delamination can be dangerous if the product is a crucial safety component. Another cause of delamination is that the product hasn’t dried properly and has moisture on its surface.
To prevent delamination:
Ready to get a quote? Want to learn more about how Universal Plastic Mold can help you avoid cosmetic problems with your parts? Click here or call 1-888-893-1587 today.