Weld lines and sink marks – just a question of visual appearance?

At the beginning, when LEWITT entered the microphone market, most of the produced parts were die-casting components. Sometimes small plastic parts were used in the assemblies. They were low in complexity, significantly small in size, but most of all hidden from the customer's view and not really challenging from an engineering and quality point of view. Whilst expanding into other product segments of the professional audio industry, the need to use larger and more complex plastic injection molded parts became a reality. As a result, the engineering team had to face new challenges, namely mastering the design and the production of such parts according to the market requirements. Despite truly designing with manufacturing in mind and applying their excellent plastic injection practices, the team was soon confronted with challenging facts: unexpected results and defects in the injected parts, often resulting in lengthy discussions and improvement loops with the suppliers. It quickly became clear that the engineering team needed a powerful tool providing helpful and constructive assistance during the design process. A tool which is able to assist the team in the early stage of product design to anticipate, correct and avoid errors, all of which potentially could cause injection problems later during the production process. Additionally, the aim was to reduce DFM times, to gain the upper hand in discussions regarding part defects with suppliers, and to reduce the time from T0 to T1 parts.

After a fairly extensive search of suitable tools on the market, LEWITT decided for Moldex3D, one of the leading 3D CAE technology for detailed analysis, verification and optimization of injection molded parts, tools and processes. The engineering team was intensively trained and found in SimpaTec, one of the leading engineering and software companies for the plastics processing industry and exclusive reseller of Moldex3D in German-speaking countries, a strong sparring partner to get the best out of simulation. Today, after just two years of intensive use, the software became an integral and fundamental part of the design process.

This article explores LEWITT's commitment to innovation and excellence, focusing on plastic injection simulation using Moldex3D. Through a detailed case study of a plastic part designed for an audio interface, LEWITT's engineering team optimization of design parameters to ensure the optimal quality of the parts that compose their products is highlighted.
 

LEWITT's CONNECT 2 is designed to be an easy-to-use audio interface that delivers superior sound for vocals and instruments. Audio interfaces convert analog microphone and instrument signals into a digital format that computers and software can recognize (analog-to-digital, also known as AD conversion). They also convert digital signals to analog signals so that audio from a computer can be played through headphones or studio monitors (digital/analog, also known as DA conversion). The interface is designed to be placed on a desktop, probably at the same level as the keyboard, allowing quick access to all controls with small hand movements. Designed entirely in-house, CONNECT 2 has a touch-based user interface and provides intuitive controls with clear visual feedback via LEDs shining through the plastic cover analyzed in this article.

The plastic component in question is quite large, at least by LEWITT's standards, and has a lot of features embedded, making it a perfect case study. It is also the part where the user's hand will rest and interact with the product - basically in contact all the time. If all parts are important to be flawless, then this one especially is.

During the development phase, it became clear that cosmetic surface is an issue due to the mentioned features as well as the multiple openings in the back of the part. Especially, the situation that on the visible surface probably sink marks could appear and weld lines are to be expected because of the multiple openings.
 

To be able to fix it, first of all the problem must be analyzed and understood. What are sink marks and weld lines? How do they occur and how can we control them?
 

In order to process the plastic, it must be melted first. In this particular case at 210°C. Since the density of plastics not only depends on temperature but also on pressure, the pvT diagram is used to describe it: p is the pressure, T is the temperature and v is the specific volume, which is nothing more than the reciprocal of the density.

If following the injection process in the pvT diagram, one can clearly see what is happening inside the part [figure 2]. At step one, the pressure increases. With good process settings the melt temperature should remain almost constant. The specific volume decreases as the pressure increases. The density increases. At the second step, the cavity is filled. The pressure should be kept constant to compensate for volumetric shrinkage. The temperature decreases. The third step is to seal the gate. The holding pressure can no longer act. Pressure drops to ambient. The temperature continues to drop.
At the fourth and final step, the part continues to cool at ambient pressure.

Using Moldex3D, this behavior can be evaluated at any point in the part and one will find that these optimal process conditions cannot be achieved everywhere in the part. Due to the geometry, there are some areas that show higher shrinkage because these areas cannot be optimally supplied with packing pressure.

Two possibilities now can result in these areas. First possibility, the frozen surface layer is still very thin and unstable. In this case, a sink mark will occur as the shrinking melt can pull the flexible outer skin inward. Secondly, the frozen surface layer is more stable than the melt stiffness. In such a case, there is no visible defect in the outer skin. On the inside, however, the melt is torn apart and a void is formed. This defect is also visible on transparent parts and looks like an air trap.

This behavior is very clearly visible in CONNECT 2. SN1 (orange curve) is in an area close to the gate with average wall thickness, while SN2 (blue curve) is in a thick-walled area that cannot be supplied with sufficient holding pressure. The orange curve shows an almost perfect behavior in the pvT diagram, as expected. The blue curve is missing the entire second stage. This means that the volumetric shrinkage cannot be properly compensated.

Weld lines usually arise when two melt fronts meet. Depending on the material, the angle of impact and the temperature at the flow front, weld lines are different. Therefore, a weld line is not only an optical but also a mechanical weak point. What makes the weld line visible is a small notch on the surface. The notch forms because the temperature on the mold wall drops rapidly and the air in the area usually cannot escape. The greater the angle of impact and the warmer the melt temperature, the better the weld usually is. However, there are other effects that can improve or degrade the quality of the weld line. Weld lines are often located in areas that are not visible on the final product. In most cases, attention is only paid to areas where the weld line is identifiable. But often not, how and if the weld line moves through the part. This is usually done by underflow, which can have a positive effect on weld line strength. Later in this article more details are pointed out.

During the DFM phase, the supplier shared a few details about the injection process and conditions with the engineering team. Gate location and size were known and replicated in the simulation but details such as pressure/temperature or cooling system description remained unknown. For this reason, typical settings were used that provided some assurance that they were not too far off from reality. Unfortunately, this often is the situation the engineering team is faced with, allowing only limited information about the understanding and prediction of the simulation models behind it.

The material selected for the simulation is a fairly common ABS, the same as used in the manufacture of the part (ABS PA757-GJ08 CHIMEI). This material was available in the Moldex3D Material Wizard.
 

The goal of the simulation for LEWITT is to be able to estimate whether the part can be manufactured with this geometry and where potential problems might occur so that they can react accordingly in advance.

The simulation shows no critical areas during filling. The part can be produced with the selected process settings. The melt temperature increases slightly in the area of the weld lines but does not exceed the maximum acceptable melt temperature. This allowed the critical injection speed to be determined, which can also be beneficial for weld line formation.

Simulation shows no critical areas during filling.

Moldex3D offers a wide range of results to assess the risk of sink marks. This is necessary because the formation of voids and sink marks is very complex in detail and depends on many factors. The most important points are certainly whether a sink mark will occur, how noticeable the sink mark will be and whether one can influence the sink mark.

With the results "Sink Mark Displacement" and "Sink Mark Indicator" it is very easy to estimate how high the risk of a sink mark will be and how noticeable it is going to be. The "Molten Core" result can be used to determine the ideal packing time and when a certain area can no longer be supplied with packing pressure. This is very important information to prevent or at least reduce sink marks.
 

Influence of holding pressure time on the sink marks (showing with Moldex3D and rendered by B-Velopment GmbH)
 

The frozen surface layer can also be determined this way and, with the appropriate experience with the material to be processed, it can be estimated whether a void or a sink mark will occur. From the results and the size of the sink marks shown, it can be assumed that no sink marks will be visible on the cosmetically important surfaces. Although the real process parameters and the mold information are not yet known at this stage, it is an essential part of the design phase and allows a preview of any small cosmetic defects. The comparison with the real injected part shows that it could be produced without visible sink marks on the cosmetically important surfaces, as shown in the simulation.

The position of the weld lines in the simulation shows some unavoidable possible critical areas, clearly around one of the rear openings required for the product's functionality. However, no criticality was detected in the meeting angle or temperature of the weld lines. The Particle Tracer can also be used to estimate where the weld line will move and whether or not it will be underflowed.

The comparison with the real part shows the same problem as in the simulations, the meeting angle could not be clearly verified, but the defect does not seem to be a structural problem, only a cosmetic one, which can be easily covered by painting (an already planned product requirement).
 

Openings in the back of the real injected part (left) and microscope pictures around opening 1(right)

The results of the simulation were positive, so mold production was started. Additionally, the simulation results were considered in advance by planning how the mold could be easily modified to quickly respond to a potential failure during the initial trials; in particular, there was some room to increase the wall thickness in the area of the rear openings if necessary. After trial production, there was no structural failure, validating the design and allowing pilot production to proceed with no problems associated with the manufacture of this part.

In a constant quest for optimization, LEWITT moved to integrate plastic injection simulation into the design process. With Moldex3D, the engineering team gained the ability to much better predict and correct problems in advance that otherwise would only surface once molds are produced and parts are coming off the injection machines, impacting product time-to-market and adding cost to projects.

For the particular case analyzed in this article, the part - CONNECT 2’s top housing - is a perfect example to demonstrate how the integration of plastic injection simulation is fundamental to move quickly in the production of the product, without delaying its launch date or stopping any of the production runs. Especially, since the design of a part with sizes and features that were relatively new for LEWITT and thus a challenge for the team, Moldex3D is a very valuable tool to anticipate any problems and to be ready and reactive in case of problems.
 

Thanks to the author's:
Alessandro Berioli, Ricardo Almeida Roque, LEWITT GmbH
Florian Aichberger, SimpaTec GmbH

Copyright:
_ Moldex3D is a registered trademark of CoreTech System Co., Ltd., Taiwan.
_ Pictures (Header, 1 & 6) with kind permission of LEWITT GmbH
_ Picture 4 with kind permission of B-Development GmbH