Sink Marks

Sink marks are a prevalent issue in plastic injection molded parts, and our aim is to assist product developers in minimizing their impact through effective design practices. While it might be challenging to completely eliminate sink marks in certain instances, we believe our expertise in mold manufacturing can provide valuable suggestions. The root cause of sink marks lies in the uneven cooling and shrinking rates of hot liquid resin in different areas. When there is a substantial disparity in mass between different sections of your product design, the area with more mass tends to shrink more upon contact with the tool wall. Here is pertinent information to inform your design decisions and address the occurrence of sink marks.

Packing Pressure

Increasing packing pressure is a strategy to enhance mold filling before the gate solidifies, but it comes with potential drawbacks if implemented in isolation. To achieve a balanced approach and avoid introducing other issues, consider the following guidelines:

1. Balance Packing Pressure with Lower Injection Speed:

Adjust the packing pressure while concurrently reducing the injection speed to maintain equilibrium. This prevents the creation of molded-in stress. However, be mindful that this approach may elongate the cycle time, which may be acceptable for small production runs.

2. Modify Mold Coolant Temperature:

Altering the coolant temperature in the mold can shift the location of sink marks. However, it's essential to recognize that this adjustment may relocate the sink mark rather than eliminate it entirely.

3. Reposition the Gate Location:

Strategically changing the gate location can ensure that the sink mark appears only in a non-cosmetic area of the part. Keep in mind that this adjustment may necessitate compromises in other aspects of the design.

4. Utilize a Blowing Agent:

Incorporating a small amount, typically no more than 0.5%, of a blowing agent increases the internal pressure of the material. This enhancement aids in achieving more comprehensive packing.

Sink marks resulting from the bosses

Ribs And Bosses

The proximity of sink marks is most pronounced in the regions adjacent to ribs and bosses in a part design. Bosses, being crucial support structures for inserts or self-tapping screws that secure assemblies, play a vital role. Employ the following design tips to enhance the optimization of bosses.

Balancing Rib and Wall Thickness

The flow of liquid resin in plastic injection molding tends to favor the path of least resistance, prioritizing thicker wall sections before filling thinner rib sections. As a consequence, the thicker wall sections cool more rapidly upon contact with the tool wall, leading to the formation of sink marks. Designers should be mindful of this phenomenon and aim to minimize the difference between the wall thickness and the thickness of the rib at its base. Additionally, it is advisable to restrict the rib height to no more than three times the wall thickness to mitigate potential issues.

The Seven Degree Rule

Employing a gentle 7° slope at the base of any rib in your design is a prudent practice. This approach offers the benefit of steering clear of surface blemishes while promoting a more uniform packing of the mold.

It's crucial to keep in mind that the gate should be positioned in close proximity to the base to facilitate easy and effective packing.

Design of Bosses

A boss serves as a reinforced post designed to secure a threaded insert or a self-tapping screw. Proper design of bosses is essential to avoid potential sink marks resulting from additional mass. Consider the following key points for optimal results:

1.Equalize Boss Wall Thickness and Inner Diameter:

Ideally, maintain the wall thickness of the boss equal to the inner diameter of the hole.

2. Limit Outer Wall Thickness for Stronger Bosses:

For enhanced strength in bosses, ensure that the maximum outer wall thickness does not exceed 2.5 times the inner diameter.

3. Incorporate a 7-Degree Radius at the Base:

Integrate a gradual 7-degree radius at the base of the boss. Proximity to the gate is crucial to enable complete filling of this area.

4. Avoid Immediate Placement Against Outer Wall:

To prevent excessive mass, refrain from placing the boss directly against an outer wall section. Instead, attach the boss to the wall using a rib or core out the wall section immediately adjacent to the boss.

A Design Feature to Avoid

An approach that some product designers may consider involves thinning the base area around a boss to counterbalance its mass, potentially preventing sink marks. While this tactic may address one issue, it introduces additional problems, prompting our recommendation against its use. Firstly, a thinner base compromises strength, increasing the risk of part failure.

Moreover, a slender section disrupts resin flow during injection, resulting in stress marks. Therefore, we advise our partners to adhere to the design suggestions provided earlier for optimal outcomes.

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The preceding considerations underscore the crucial need for meticulous design of mold tools and part geometries to accommodate the nuances of thermal stresses in plastic injected parts. Our engineers conduct a thorough design-for-manufacturing review for all plastic injection molding projects, aiming to eliminate the risk of sink marks. If you wish to discuss your upcoming project with one of our experts, feel free to submit your CAD file for a complimentary quote and design review.

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Texture

Textured or coarse surfaces extensively applied to the interior walls of mold cavities may result in a firm grip on the cooled plastic part, leading to scratching or binding during ejection. To address this issue, product developers should incorporate larger draft angles into the design.

In cases where this isn't feasible or desirable, an alternative approach involves molding these surfaces by introducing additional sliding cores. However, it's essential to note that this solution contributes to increased costs and complexity in the development of the mold tool.

Wall Thickness

Achieving uniform wall thickness in your design is essential for effectively controlling heat distribution within the part. To optimize results, consider the following design tips for wall thickness:

1. Aim for uniform thickness in adjacent walls, with variations not exceeding ~50% where possible.

2. Instead of increasing thickness, provide support for long wall sections through the use of ribs or gussets.

3. Incorporate a draft angle of 1 degree for every 2 cm in wall height to facilitate easier release during the molding process.

4. Introduce a slight radius at the base of walls to alleviate stress, minimizing the risk of cracking or deformation.

You want to create equal wall thickness in order to control the rate of shrinkage.

Weld Lines

A weld line is a distinct region where two separate streams of flowing liquid plastic converge and solidify within a mold cavity. As these opposing fronts meet, they undergo partial cooling, potentially trapping gas and leaving a characteristic mark on the surface of the finished part. While weld lines may be challenging to completely avoid, there are strategies to minimize their impact. These include the use of conformal cooling channels where feasible, enhancing venting at the point of contact, and adjusting melt temperatures. Additionally, rough surface textures or painting can be employed to effectively disguise weld lines. Our collaborative approach involves working closely with you to explore alternative methods for mitigating weld lines, some of which may necessitate compromises in other aspects of the part or tool design.

Weld lines are created where two fronts of thermal resin come together and cool at different rates.

Tolerance and Shrinkage

Tolerance and shrinkage are crucial considerations in the design of plastic parts and mold tools, as different plastic resins exhibit varying rates of contraction. It's essential to account for this shrinkage in various features of the design, such as threads, holes, ribs, pockets, flat planes, and more. Our collaborative process involves working closely with you to elucidate the impact of shrinkage on these specific areas. For a comprehensive overview, you can refer to our injection molding tolerance guide, which outlines the common plastic types and provides insights into the expected degree of shrinkage in typical applications.