Part-Filling Issue in Injection Molding Pens-Venting Concept Development

PANDIARAJAGURU: So hello, all. So warm welcome to everyone who are all attending this Autodesk University session. So today, I'm going to present about the importance of venting in injection molding. So I'm Pandiar [INAUDIBLE], and my full name is Pandiarajaguru. And I am from India from Chennai location.

So let me tell about you in detail. So I'm basically a beta polymer technologist. So I'm having around 14 years of experience.

So I lived in south part of India, and I'm from Chennai. So I also have completed my associate certification in Autodesk, and I currently work in Newell brands as the senior engineer. So previously, I was employed with the Hero Moto Corp and Delphi Connection Systems, which is now called as Active and in Thomson Corporation as well.

OK, so in my work, I support various divisions like writing, baby, et cetera, in Newell Corporation [INAUDIBLE] in Newell brands. So I carry-- so like preliminary DFM with the help of Moldflow software. so works closely with the design team and [INAUDIBLE] team in validating the design.

And I also support the project engineering team to resolve the issues which you come across in the shop floor during injection molding. So I worked closely with the tooling team as well in mold evaluation while building the tools and addressed a number of issues in product design stage as well as in production stage and was part of complete design and development of the product in our previous organizations as well and also in my current organization. I supported even material engineering team during material selection, which could be used for as a raw material for molding.

OK so the agenda of the session is how venting is renting is crucial in injection molding. And also, let us see how it's important in driving the product quality and as well as the process. So the objectives are what are the causes of short [INAUDIBLE] during injection molding, and let us see the importance of venting in molds.

And also, I'm going to show you how simulation is used to predict the failure and how we are going to compare it with the real time scenario as well and also how to avoid flow marks and weldmarks during molding. OK, so let us get into the objectives one by one. So what are the reasons behind the filling concerns. So what is short fill?

Short fill is nothing but a filling issue or a quality issue which is kind of a non-filling of mold clarity. OK, so short shot or short fill issue are generally due to the below listed causes. So they are material-- they could be due to the material selection.

So if the wrong material is selected for molding, they need-- it will obviously end in a short fill issue, which means-- so you should like, if you are going to kind of mold a thin molded component, and if you are choosing a high viscous material, then obviously, you might end up in filling issues, short filling issues. So then the process-- so the molding parameters are also a key part in designing the filling behavior and filling contents. And then the final thing is the mold and part designs.

So the part designs should be optimized before building the tool, and also, the tool design should be having proper runner and gate dimensions. So otherwise, what will happen is that there will be flow restrictions in the mold design, so which means the restrictive flow parts, like restrictive runner and gate dimensions, might cause again filling concerns. So hesitation-- hesitation may be due to the improper part design.

So if the part design is not optimized before building the tool, then there will be concerns with the hesitation. So hesitation is nothing but, like, variable wall thickness, which is kind of causing filling concerns during molding. Then obviously, we would be having problems in filling if you are having a lack of venting in the mold.

OK, so let us see how the material selection is going to impact the filling. Selecting the right material is always a key to achieve 100% cavity filling. So generally during molding, the molten resin or the plastic flows through the cavity. So it touches the mold walls and starts to freeze out.

And the resin is generally selected based on the flow properties. So mainly all the engineering teams will look for the melt flow index value of the particular resin. And they will choose the material accordingly.

OK, so if an improper material is selected, then it might lead to short filling issue. That's for if you are going to select a high viscous material on a thin-rolled component. Then you may end up with the filling concern.

Say, for example, if a point [INAUDIBLE] at the component, which is kind of a thin walled component. So if the material selected is a highly viscous material-- say, for example, if you are going to select an ABS material or a BC material, which is of highly viscous-- then you might end up in filling issues. In that particular design, if you are going to have an easily fillable resin kind of like a polystyrene or even PP, then there may not be any filling concerns.

But it always does not work in that way. Sometimes due to the application or need, the teams might be selecting a particular material. So in that time, you have to kind of play with the design. So you have to have an optimum thickness, the required areas, and optimum gate locations for avoiding the filling concerns.

OK, so I have listed out some of the commercially available plastic materials which are used in consumer goods industry. So PP material, it's kind of a cheap material which is generally used across the globe in the consumer goods. So it's the same crystalline material, and it's having good flow properties. Even thin walled components can be made out of PP material easily.

And then you are having GPPS material, general purpose polystyrene. So it is another beautiful material. It is a transparent material.

It is also cheap, and one problem with the GPPS is it's kind of a brittle material where you'll be having problems with the brittleness. Hence, it is amorphous. It can break easily.

So then you have HIPS material-- High Impact Polystyrene-- then ABS. So whatever problems you face in GPPS will be solved by ABS. So ABS has the tendency to absorb the loads.

So it's a good material with better stiffness and also some flexibility as well. But ABS is opaque material. So then you have SAN and PC materials which are kind of, again, transparent materials. So they are kind of beautiful materials. And they aesthetically also look good when it is molded into your component.

Then you have HDPE, again a material which is used-- OK, so again, a cheap material, just like PP. So this can be used in caps, et cetera. Then copolyester, another transparent material, which is kind of used as an alternative for PC material. So generally, improper material selection leads to short filling issue.

OK, so let us see how the process or the molding parameters are going to impact filling or filling behavior. So processing is another important factor which may cause us filling concerns. So you should have the right injection speed, pressure, melt temperature, mole temperature, core cavity temperature, holding pressure time, and fill times.

So how injection pressure is going to affect the filling? So for a particular design or a particular cavity, so if the injection pressure occurred for filling is not sufficient, if you're kind of using a low injection pressure, then the filling will not be 100%. So they're really short filling.

So if you come across those kind of issues, you have to increase the [INAUDIBLE] pressure and get rid of the problem, filling issue. OK, so again, if you're not using the right injection speed or RAM speed, so if the velocity profile of the injection is not kind of adequate, then again, you may end up with the filling issues. So also, the melt and mold temperatures also plays an important role.

Say, for example, for PP generally, the molding window is kind of 200 to 240 degrees Celsius. Suppose if the mold makes-- if the molding team is kind of using a less range, kind of 180, 190 degrees Celsius for molding. And then since the melt temperature is less, so what will happen is like during molding, again, like, as soon as the [INAUDIBLE] hits the mold walls, it tends to freeze, right?

So along with that, we are using a temperature, melt temperature, which is not recommended by the material supplier. So again, in this case, you will end up with the filling concerns. So the complete part may not get filled due to this.

OK, so even the mold temperature is also like that. So we can take an example like PC material, requires a mold temperature of 80 to 100 degrees Celsius. So generally, all the molding-- OK, so all the mold makers generally really use oil cooling for PC material.

So it's of course, if you're not setting the oil temperature to 80 degrees Celsius, then probably you will-- like, say for example, you were using some 30 to 40 degrees Celsius for freezing molding. Then that will also kind of cause some surface defects, and also, filling issues also may occur.

So for avoiding that, generally, it is highly recommended to use the optimum mold and melt temperatures for injection molding, which is recommended by the material supplier. OK, so then comes the packing pressure and packing time. It is also important to use sufficient packing pressure and packing time.

Say, for example, generally during filling phase, so maybe 90% to 98% of your part is getting filled. And after this [INAUDIBLE] point, the holding takes part. So it takes care of the remaining filling.

And the remaining 2% to 10% of the part will be filled during the packing phase, so which means if you're not applying enough packing pressure or packing time, then it will also result in short filling. Finally, fill time is another important factor.

So if you're not going to use optimum fill time, then you might end up in a lot of troubles. Say, for example, if a pot which was required to be filled in one second was filled by using three to four seconds or more than two seconds. Then it's kind of obviously long fill time.

So what will happen with respect to long fill time is so the part will kind of get filled halfway, kind of like-- the other areas of the part will be kind of frozen, so which means that there will be chances of short filling-- so if the fill time is just kind of very long.

If you are having a very short fill time, there are kind of other issues as well. So material shearing may occur with respect to short fillings, short fill times. So that might also cause concerns. But it will be kind of [INAUDIBLE] kind of burn [INAUDIBLE] due to the material sharing.

OK, so let us jump into the mold or part design. OK, so if you take the mold and part design, so if you take the mold design, there'll be flow restrictions in the name of gate and [INAUDIBLE], which means if the gate [INAUDIBLE] is used or not, enough or appropriate, then there will be issues with the flow restrictions.

And also, if the gate location used for filling is not appropriate, then you will also end up with filling issues. So you need to have a gate, right gate at the right location and the other right [INAUDIBLE]. That drives part quality.

So it should not be a longer flow length. The filling balance should be there. There should not be any filling imbalance.

OK, so let us see about flow hesitation. So flow hesitation generally occurs in the part design. So if the part is not optimized for cutting the tool, then there will be concerns. And also, if the gate is not kept at the correct location, then that also can cause part hesitation.

So you can see the right side images. So at the top, you can see a gate at the top on the cavity side. So it was at a symmetric location.

But at the bottom, you are having a [INAUDIBLE] gate at one side. So here, what happens is, so generally, the bottom area is kind of a thick area. The thick areas are getting filled first. And so the nearby, you are having some snaps. And [INAUDIBLE] center, some [INAUDIBLE] is there. So those areas are kind of comparatively thinner than the bottom area.

So those areas are kind of getting filtered last. So it is mainly because of the wall thickness variation in the part design. OK, so the melt stops and flows down in the cavity due to the thickness variations, and this is called the hesitation effect. So the thicker areas are getting filled at first, and the thinner areas are getting filled last. It You can relate this with the [INAUDIBLE] tracking effect.

OK, so let us come to the last reason for the filling concern, which is kind of lack of venting. So doing mold filling, the melt flowing through the runner's cavities pushes the air inside the cavity. So the air will try to escape out of the cavity through the venting in the mole cavities. The air or gas trap will occur due to the lack of venting.

If you're not going to have adequate venting in the mold, then you're obviously going to end up in trouble. So you'll not get any good quality products. So I have found some of the images here. So you can see the venting channels.

So this is kind of your cavities. So this is the venting provided in the cavities. OK, so then let us see why the venting is more important.

So let us see what is the air trap. So the air trap is a molding effect which commonly occurs in the last filling area of the cavity. Air trap is nothing but the air trapped into cavity walls during molding due to the converging flow fronts. So if these air [INAUDIBLE] can't escape out of the cavity, they will cause short shot or burn marks. So that is why we need to relieve these air traps for getting higher quality products.

OK, so I'm going to kind of let you know about one of the issues which we have faced in one of our projects. OK, so let us understand through this example why venting is more important in the mold. So let me share about the project information.

So it was a multi-cavity mold for a plunger component of a pen. So plunger is a component which is a part of retractable pin. So the mold was a 64 cavity mold. The label used for the toolmaker is a balanced layout. OK, so you can see all the 64 cavities with the uniform runner sizes, and the gate dimensions are also the same for all the 64 cavities.

OK, so let me tell about the problem statement. So we face difficulties in molding the components due to the gas trap. OK, the short filling is noticed in all the cavities.

So you can see a hole at the top. So the hole was formed due to the air flap. And it was kind of an aesthetic defect and a fault issue. It completely makes this component unusable.

So on looking the samples closer, we were able to also see some flow marks. OK, so the short fill is noticed in all the cavities at the same place. OK, so a few more observations for better understanding.

What we have did is we have gathered all the required information from the toolmaker. So we have checked the tool drawing, product drawing, and the injection parameter suits, everything from the toolmaker. So from the tool drawings, we kind of came to know they used a sub gate. The gate diameter was 0.6mm.

The gate location is noticed near the bottom of the plunger, which means near the cores die, they have used a gate. So the runner used was a modified trapezoidal runner. The material used was an ABS material, but it was a high flow grade. So it was having [INAUDIBLE] for more than 25.

OK, so then, based on the observations, we kind of found the reasons for the filling issues. It's listed up here. So let us see the causes for the filling issues in this project.

So our main concern is the air trap. We need to get rid of that, and that is causing the short fill. And one more thing is since we are having a gate location at kind of a symmetric location-- and also, it's at the core site.

So it seems it is at the core site. The cavity site is generally expected to fill up last, which means the air trap is expected at the last filling area, which is your cavity site. So the venting is not provided in the cavities, which means in the cavity side, there was no venting. Though the toolmaker has provided some venting in the core site, that was not sufficient to get rid of the air which was trapped inside the cavity.

So they have not provided venting in the mold initially in the last filling areas, which is the cavity area. There were also traces of flow marks in the samples, which indicates that it's kind of pressure and temperature drop in the cavities. The runner and cavity dimensions kind of controls the temperature drop as well, so kind of like these flow molds as well.

OK, so let me share you how we used the simulations to correlate these failures. So we kind of used the Autodesk mold for simulation, for predicting and correlating this failure with the actual scenario. The molding defect observed in the molding is replicated in the simulation.

OK, so we have got the complete mold drawing from the toolmaker. So we have used a mold for software to replicate the molding scenario. The full mold setup was modeled in Moldflow simulation. The 64 cavities were modeled along with the gate runner setup. We have also modeled the cooling channels as well and carried [INAUDIBLE] cool from simulation. The multi-cavity flow simulation was carried out.

OK, you can see the runner layout which was modeled using Moldflow. So all the 64 cavities has been modeled. And the gate, you can see the gate location. So gate is kept at the side of the plunger.

OK, so you can see like after modeling, we have run the cool [INAUDIBLE] simulations. And you can see the multi-cavity animation here. So from the animation, like from the filling animation, you can see where the last filling area is at the top, which is at the cavity side. OK, so you can see the filling results as well as the air trap results shown here.

So from the filling results, we can see the last filling area, which is kind of similar to the area where hole was formed. OK, so you can see the air trap as well. The last filling area, there was an air trap which needs to be [INAUDIBLE] out of the cavity. But since venting was not provided, we were not able to remove that air trap.

OK, so as the gate location is near the core site, the last filling area is expected to be in clarity site. So the melt started to flow from the gate orifice near the petal area of the plunger. Slowly fill the shroud area, and the last filling area was expected to be the top of the plunger, which is opposite to the gating side.

The air present in the cavity was pushed by the converging polymer flow front. Hence, there was no venting provided by the toolmaker in the [INAUDIBLE] to relieve the gas from the cavity. The air or gas could not escape or got trapped to form a hole or short shot in the part. So we have observed a short shot because of that.

OK, so from the mold results, again, we can kind of confirm that the critical air trap is noticed in the last filling area on the cavity side. There was also flow hesitation noticed, and there was some temperature and pressure drop. The air trap formed in the core site can always be easily removed by core [? pins. ?]

But here in our case, it's on the charity side, and it's generally tough to eliminate without venting. OK, so let me share how we sorted out these problems and what accommodations you have made to the toolmaker. So you can see the images shown here.

So the first image at the left shows the existing cavity, which is one piece cavity. And the second one is modified cavity, two piece cavity. OK, so what is two piece cavity?

So what we did is so we have kind of cut the cavity or drilled the cavity down. And at the top, we have provided a two piece insert. So the top insert was kind of having a venting.

OK, so to relieve the air trap or gas trap, it was recommended to provide venting at the top of the plunger where hole was formed. So the cavity was splintered into two pieces.

So these were two piece cavity in concept. And the gas vent clearance of 0.02mm was used, and 3mm landing was planned. The flash is not expected even in mass production or in long term because the cavity insert will not get separated out during part ejection.

OK, so then it was suggested to increase the gate dimension to 0.8mm from 0.6mm to allow more material into the cavity. It was also suggested to increase the runner dimensions by 0.5mm to aid the flow. OK, so initially, what we did is we have carried out the modifications in two cavities out of the 64 cavities to check the venting concept.

OK, so then we have did the tool trails after affecting the tooling modification. The toolmaker has made the changes in the tool. A few key process parameters were suggested to the tool maker based on the Moldflow simulation results as well.

So initially, higher injection time was used by the tool maker. So later, it was reduced to one second for better filling behavior. After tooling modifications, the filling results were great, and we were able to kind of get 100% filling. The hole formed earlier does not exist anymore. Since the trial with two cavities were successful, it was suggested to replicate the modification in the remaining 62 cavities as well.

OK, so I have shown some images of the modified cavities here. OK, so these are cavity inserts. OK, so you can see the venting shown here.

Like, this is your two piece insert. So it's a cavity insert. And this is a two piece insert that sits on top of the cavity insert. So the other side, so you can see the venting. So on the two sides of the plunger, venting was provided. OK, so this is your bottom piece, and this is your top piece.

OK, so then one other trail was successful. Like, we have affected the modification in all the 64 cavities. So after the modifications, we got all the cavities filled completely.

OK, but one problem we have observed-- so few of the cavities showed some visible weld marks and flow marks, which is a surface defect. The well marks were visible in deep. These well marks affect the product quality aesthetically. And also, there were some flow marks as well in a few of the cavities.

OK, so I have shown how the cavities are filled completely after affecting the modification. So you can see the image. So the part is completely filled here.

OK, so the other issue which I was telling is deep visible weld marks as well as flow marks were also observed in a few of the cavities. OK, so for avoiding these well marks, what we did is we have kind of increased the venting land over these well marks. So as per the suggestion, the toolmaker has widened the venting land area.

So initially, the venting land area provided was around 2mm. So it was increased to 2.8mm now. So additionally, the venting was provided on all the four sides.

So initially, only two sides were having venting. So we have made it to four sites now. So trial has been conducted with the modified venting. So after affecting these modifications, we were able to get all the components without any well marks.

So you can see the modification which we have did for optimizing the venting concept. OK, so initially, we had only two slots, two venting slots here. But later, it was changed to four, and the venting land was also increased. By doing this, we were able to achieve well marks and flow marks-free components.

OK, so you can see the components before providing venting. So you can see holes everywhere, short filling due to venting. So after affecting the modifications after providing venting in the mold, we were able to get high quality products which are kind of defect-free now.

So this shows the importance of venting in the injection molds. It is highly important to have sufficient and adequate filling in all the injection molded components for producing defect-free components. So venting not only provides you good quality products-- it also kind of reduces the injection pressure which is required for filling as well.

So if you're not having venting, then you have to put more pressure to push the material inside the cavity. So if you're having proper venting at the proper places, then there will not be concerns with the injection pressure [INAUDIBLE], which means your tool life will also be better. OK, so thank you, everyone, for providing this opportunity.

And also, thank you, everyone, for hearing this session. And I would like to thank Autodesk University for providing this opportunity to share my thoughts and knowledge to all of you. Thank you, everyone.