Causes And Avoidance of Sink Marks in Plastic Injection Molding Products

Introduction

plastic injection molding products often have shrink marks. We want to assist product designers in mitigating the damage caused by sink marks through design best practices. While they might not be an option in every case, we believe that we can provide helpful suggestions based our knowledge in mold manufacturing.

What are the causes of injection molding sink marks? Sink marks can be caused by areas of hot resin that cool down and shrink at different speeds. If your product design contains significantly more mass than the other, it will shrink more once it touches the tool wall. Below are some tips to help you make right design decisions.

Packaging Pressure

To ensure that the mould for plastics is completely filled before freezing, you can increase the packing pressure. You should be aware that increasing pressure alone can cause other problems. We recommend that you follow these guidelines to maintain a balanced approach.

Lowering the injection speed will balance the packing pressure. This is done to prevent molded-in stress. This approach may slow down the cycle time but isn't a problem for small production runs.The mold's coolant temperature can be adjusted. This can cause the sink mark to move from one location to the next. 

You can move the gate. This method will allow you to make sure that the sink mark does not appear on any non-cosmetic areas of the part. It is possible to move the gate without making compromises in other parts of the design.

Use a blowing agent. Use a small amount of blowing agent, not more than 0.5% to increase the internal pressure of your material, which will help it pack more fully.

Ribs and Bosses

Sink marks are most commonly found in areas that are near ribs or bosses. Bosses are an important feature because they serve as support structures for inserts and self-tapping screws which hold the assemblies together. These design tips will help you optimize the use of bosses.

Wall Thickness and Balancing Rib

Liquid resin fills thicker sections of wall first, following the path that is least resistant. The liquid resin fills the thinner sections of the rib sections if the resistance in "T", becomes too great. The wall sections will become more brittle when they come in contact with the tool wall. This causes a sink mark. Designers should limit the difference between the wall thicknesses and the thickness of ribs at the base. In addition, avoid the height of the ribs to be more than 3 times the thickness of the wall.

The Seven Degree Rule

A gradual 7 degree slope is a good idea for the bottom of any rib used in your design. This helps avoid surface imperfections and allows the mold to pack more evenly. To facilitate packing, the gate must be placed close to the base.

Design of Bosses

A boss is a reinforced support that can hold a threaded insert, or self-tapping screws. A boss can cause sink marks if it isn't designed properly. Here are some important things to consider if you want the best results :

• If possible, make the boss's wall thickness equal to the inside diameter of the hole.

• To make bosses stronger, limit the outer wall thickness to no more than 2.5x the inner diameter.

• A gradual 7-degree radius should be used at the base to the boss. The boss must be as close as possible to the gate in order to fully fill this area.

• The boss should not be placed directly against the outer wall section. This will cause too much mass. Instead, attach the boss by using a core or rib to attach it to the wall.

Avoid Design Features

Sometimes product designers will try to reduce the area surrounding a boss to lessen its mass which can avoid a sink mark easily.

But this design has other issues, so we do not recommend it. One, the base is weak and can cause part failure. A thin section can also interrupt the resin flow, which can lead to stress marks. We recommend that our partners stick to the design suggestions above.

Build in Last

For all plastic injection molding products project, our engineers conduct a design-for-manufacturing review to eliminate the risk of sink markings. If you have any requirement for next project, send us your CAD file and ask for a free quote now!