Plastic Part Design & Manufacturability — the full webinar

Full transcript

00:01 Hello everyone. Welcome to this webinar on understanding critical aspects of plastic part design and manufacturing. This webinar is jointly hosted by Geometric Limited and ETS Corporation. I am Nikhil Delvi and with me I have our lead speaker Paul A. Tres. Paul A. Tres is a founder, engineering training services, best selling author and an international speaker on plastic product development and design. Having years of experience in design, marketing, selling and manufacture of plastic components and systems. Mr. Tres offers a broad perspective of the plastic industry in general.

00:47 And to the plastic marketing, manufacturing, material selection and part design in special. He will explain the best design practices in detail with examples. Today, plastic parts are essential components of practically every product ranging from cars and high tech electronics to household products like kitchen appliances to toys. Plastic is possibly the most versatile and economical material used by all. It can be used in many applications. If you can see the typical product development process in plastic part design. It involves broad level steps like design, engineering, manufacturing and assembly.

01:34 At different stages, the design changes are requested to the design team to ensure these downstream operations can be executed smoothly. This process of engineering change request affects the time to market, quality and cost. And cost for the organization. As experienced engineers are retiring, knowledge capture is also a concern for many industries. To reduce these design iterations at later stages, many organizations have introduced DFM process. So today we will talk about best design practices for plastic components and also about how to make them easy to manufacture.

02:20 To understand these best design practices for plastic parts. Now I invite Paul to explain the design aspects in detail. Good morning everyone. My name is Paul Tres. I will discuss now the plastic part design and manufacturability. The burger that's shown right now on the screen will be discussed in great detail. We will boss design. What you see on this screen, you see designs going from bad to probably the better. And the difference between the three I will explain now the best I can.

03:03 T stands for wall stock or wall thickness. Everything around the world from any manufacturer and any material supplier is based on only one thickness. That thickness is three millimeters or one eighth of an inch. You could vary from that thickness up and down to as little as half a millimeter to as much as six and a half, even seven millimeters. But those are the thresholds. If you go below, everything that is described in the literature will not be applicable. If you go beyond, you will not be able to get to parts which look the same.

03:45 So the one in the left hand side of the screen right now says bad. The reason is bad. We have three sharp corners. And in addition to it, we have areas which are thicker. Meaning that the resin will still be hot after the part is ejected. The part is ejected from the tool. And those hot spots will create air pockets, voids if you like, and sink marks. So that's the reason the design shown on the left is bad. The one on the middle is a little better. We got the rate of the sharp corner with a nice radius to T.

04:25 But we still have two sharp corners. As you see, I'm pointing with my mouse at those two areas. And those will create stress concentration, sink mark, and voids. So it's still not as good as ideal. Ideal would be to have something as shown on the right hand side. Where we move the boss internally slightly in. And attach it to the walls, the external walls via very thin ribs. Half of the thickness. We break out any sharp corners. And we have a continuous wall. Without the component.

05:08 A good explanation for understanding why corners are detrimental to plastic part design. The number one rule is not to have sharp corners. Goes back to a developed company called de Havilland. Which in 1952, they designed the first jet plane for passengers. They were the first in the world. And when they started flying the plane. The plane was flying from the sky.

05:51 The reason being, as you see by pointing the arrow. Is the window in the jet plane. So when they pick up the pieces from the ground. And they put it together. You notice the pattern. Showing that the initiation was starting at the sharp corner around the window. And the reason that was the case. Is because no plastic was there. It was metal. But pressurization and depressurization of the fuselage. Created the stress. After a couple of minutes the plane was crashing.

06:35 What you see on this slide. You see another component. This is for a Ford Taurus. The first edition. Back in the mid 80s, late 80s, early 90s. And this is the reflect using the headlamp. And what you see on the middle of the screen. Right about here. This is a bus. So. It goes as follows. Design. Molded the part. And then they shipped the part to China, Michigan. Which is a city in Michigan.

07:15 Not China, China. China, Michigan. It's a city. In Michigan. And to provide the plating. To create the reflector. And they did an aluminum deposition based on Ford requirements. However, in order to have that metal adhere to the polymer. Which is a polycarbonate amorphous resin. Which are, you know, very sensitive to chemicals. They had to put a primer. To have the adherence of the metal to the polymer. And that primer, when it was sprayed. Some areas went around this pin. This bus. And when they put the screw in.

07:56 To mount it on a GOR grill opening retainer on the vehicle. Were cracking right there. So it was a quarrel between the two companies. For about 2.7 million dollars. And I will explain how this was fixed in a few slides. Also, what you see in here. You see a very lousy design. A yellow with the bottom of the screen. Shows you a good design practice. Where you have no sharp corners. You core out all the heavy wall stock.

08:37 And you use gasses and so forth. To link them to the main walls. Another Ford product is shown in this one. This is called an HVAC system. Stands for heat ventilation air conditioning system. It's a large component. This is for a Ford Focus 2008. And it's hidden by the IP. The instrument panel in the vehicle. Okay. And what you see on this slide. You see a very good depiction of ribs. Properly designed. And removed all the sharp corners. As you see as I'm pointing with my mouse here and here.

09:17 Everywhere. You don't have any sharp corners. So this is a very good practice. If you look from a different point of view. You do now a vertical cross section to a bus. The one on the left is not good. Because the pin forming the inside of the bus. Is not going deep enough. A line allows a very wall. Heavy wall stock to stay. Which will be shrinking outside the tool. Creating the void. And the sink mark. And that's a very bad approach. The one which is better is shown in the middle. However I want to caution you here. If you design a product with two resins.

10:00 Very famous resins. One is called polypropylene. For short PP. The other one is known as acetal. Okay. They shrink a lot. So even if you use this good approach shown in the middle. Where you go with the pin down up to about half of the thickness. You will still see the sink marks. Okay. There is no way you could get rid of them. Using polypropylene and acetal. They don't have any sink marks whatsoever. And if you have a clad surface. The only way you could get rid of them. Is to have a hole. Now going back to that Ford headlamp.

10:45 What Ford did during that quarrel. I was telling you legal quarrel. You know the two point some million dollars in. They start shipping the parts shown in this picture. So let me try to show you another view. Which is shown in here. You see the Ford logo. And the boss was placed with a pin. And they didn't use any more thread forming screw. They used a primer being sprayed on this pin.

11:28 Will not create that pin to fit. So that's how they fixed it. It was error incorporated in the design by Ford Motor Company. Because they told this company Masco. What kind of. Because they should use. And what kind of aluminum. And the process. The standard. What you see on this slide. You see how you could. Both in a plastic. The melt flow. Direction of the resin flowing through the tool. And you see the four pins.

12:09 Forming those holes and behind them usually four well lines one two three and four which are forming opposite to the direction of the flow and then the way you design something like this you should use the minimum two or minimum 2D whichever is the least to lay out the holes into the plastic part ribbing if you never designed ribs before which you see on this slide probably very good what i'm saying here i'm saying that if you use ribs which are less than one and a half.

12:50 The wall stock the kind of money you are spending to put those ribs into the tool and the kind of outcome you get in meaning the stiffness are not worth it because it's a very small increment in stiffness and you spend an arm and a leg in that's the reason I said it would be minimum one and a half thickness and on the other side five times the wall stock should be the maximum the reason i'm saying that is if you increase the rib height beyond five times the wall stock you create vacuum into the tool.

13:30 Preventing the part from ejecting the material from the wall stock so that the material from the wall stock would be the maximum for me injecting from the tool okay so those are the number one reasons when you have to keep the ratio for the rib height within those ranges okay very nice design this comes from France is from French company and what you see on here it's about.

14:11 2007 at the rate of the handbrake mostly used in Europe they got the rate of the ribs so the ribs are designed such that when it's engaged the weight of the car is handled by those ribs and the ribs are in compressive loading only that's ideal.

14:51 For here we have a phone telephone housing obviously as many lines and has many holes as you see even if you use an amorphous resin which is.

15:31 A predictable and larger the better if you have texturing on the surface of the rib then you should increase the draft angle the one to two even three degrees per percent to eighty percent of the wall.

16:11 Stock and it's a range every time you open a handbook from any vendor that they give you a specific range for the part what kind of polymer you are using if you are using a highly shrinkable material.

16:51 Such as polypropylene or acetal you should go to the least amount amorphous resin usually fall into the middle somewhere around zero point six or three percent of the wall so the reason I.

17:31 Give you this range is to thinking about problem what you see in here you see that we use a radius this radius actually makes the problem it's on the right hand side of the.

18:12 Screen right will have larger voids and largest thing marks and warping okay so there's a reason that's not a good design what you see on the left hand side have to use some nice.

18:52 Radio a t shape rib design shown on this slide the one on the left in red that really bad and you start with that area the one at the top which shows in.

19:32 The middle of their is achieved via what is known as gas assist injection molding process or water assist we have different designs using very thin rebound at the bar get there.

20:13 very nice design is on this slide this is another for product shifter and actually is will give you an example on how actually to design with okay so for example let's say you do a calculation and you come up that you need a plate which is 10 inches long and 340,000 stick okay if you are in metric that's about 250 millimeters

20:54 about 10 12 millimeters okay which is very very thick for plastics and two components molded in the same tool we're using the same resin using the same process will be like a potato chip so they will not be the same so you cannot have a duplication so in order to make this what I did below I used ideal wall stock polymers which is three millimeters and then I added a bunch of ribs as you see so then I'm trying to now calculate the stiffness of one

21:34 versus the other okay the stiffness is the same I'm trying to calculate the savings okay how much less material I use in the design B versus design A so I start with a rib which is tapered two degrees per side as you see the rib height is three times the wall stock and at the base the rib is 80,000 which is about sixty some percent of the typical wall stock which is three millimeters and I have a small number called B which is the small base of the trapezoid so what I'm trying to do

22:14 I'm trying to find out the centroid the centroid because it is a trapezoidal is on the horizontal axis is in the middle but on the vertical axis is not in the middle is shifted and is shifting towards the large base okay so y1 through 9 that's the value okay now I do the same thing the plate which in both cases is in the center because it's a rectangular cross section and then I combine the two moments of inertia actually the nine for the ribs one with a plate and then I get the total and then when I compare areas

22:55 versus the area original versus redesign I obtain a saving of about 60% so what that means means that I use 60% less resin that I have a duplicate component I could make over and over accurately I reduce the cycles time and I have the same stiffness so this works very well which is you on this slide you see a door handle for an automotive it's an old part made about 20 years ago in in Morteau which is just outside Lake Geneva and

23:36 the only metal parts in this door handle are the two inserts on the left and the right here and here and the metal spring the rest is all plastic and the ribs are designed exactly as I showed you on the previous slides wall stock as I said ideal wall thickness is 3 millimeters you could go up and down from like half a millimeter to as much as six and a half if you go below or above you will not be able to use you have to foam it gas assisted water assisted to go beyond or to go below you have to go into very precision injection molding process and what you

24:20 see on the top you see poor design why is poor because you have an abrupt change from thin to thick the thinnest area of the part if you place a gate in the thinnest area of the part let's say here you will be unable to actually fill the part okay plus the sharp corners and the thinnest area of the part will be created a stress concentration forcing this part to fail miserably okay so a better choice would be to taper or the transition between thick to thin as shown in here to 3t but in this case again you have to gate in the thickest area you have to get here in the middle or here or below you cannot gate in the thin area

25:04 area of the part the best design however is to core out the heavy wall stock and to have the wall stock throughout the part as shown at the bottom. When you reach the thin wall as I explained to you half a millimeter everything that is in the literature will not be applicable and what you see in here you see the tool and you see the red is the resin flowing through this opening in the tool and the yellow is the frozen layer which provides the resin and it's like cholesterol in the arteries you see like a blood flow going through an old

25:44 guy which has cholesterol so if he still eats lots of fat the yellow could top and bottom and prevent the blood to flow and he will have a heart attack the same here if this channel to which the resin should flow is very long and very narrow frozen layers touching each other and then if you increase the pressure in the tool in the injection molding machine you have flash and you will not be able to actually mold apart you will split up okay and in order to make sure that that's not the case this is the maximum injection pressure formula based on the

26:29 diameters okay and then what's happening when you reach that the magic number 0.5 wall stock everything goes asymptotic so everything that is discussed in the literature will not be applicable once you've reached the level of thickness or you go beyond it knockouts knockouts are increased areas in the tool to allow you to eject the part from the tool without deforming the part so stress is defined as force divided by the area if you increase the area you have the same force this will be lower to be able to sustain it draft

27:12 angles what I'm saying here I'm saying that when you open a tool and you you like the the part to stay on the core so the core draft angle should be about quarter degree less than the cavity the zero draft angle is not a smart idea because will cost you an arm and a leg in maintenance you have to lubricate the tool you have to service it lots of labor so in order to prevent that from happening I would highly recommend you not to use zero draft angles okay comparison between two typical type of presence the first one is called ABS it's an amorphous resin shrinks anywhere

27:53 from five to seven thousand inch per inch the second one is a polypropylene which is highly a shrinkable material shrinks anywhere from twenty to thirty thirty two thousand inch per inch so what says here that the more the resin shrinks the last ejection force you need the note I have to tell you about that as well SPI a three on p20 what that means SPI is a society of plastic industry a trade organization in Washington DC which besides dealing with the government and promoting plastics they also have some standards one of them is the surface finish goes from a1 to a6 a3

28:38 is right in the middle so it's not very rough it's not very smooth it's somewhere in between p20 is one of the most commonly used steel in the plastic industry probably the cheapest okay if you have better quality you could use in your product production you could use an h13 which cost wise it's about 50% more or you could go to stainless steel you could double and then furthermore if you do lenses for optics okay you need mirror surface you have to use a stainless steel texturing means that you

29:22 have to add a draft angle as I said earlier today so if you're not quite married to your son you might want to find a new one of these these are industrial ones but you can also use the Sabra X b into the god you haveieron because the guys have control software that the we are paid for'm I mean you know this one here I can disturb the ignition you have the rotation of the part of the paint that willriend the of course thank you yeah thank you for for all the best practices that you mentioned now i will just

30:08 like to take it further so as paul just described designers need to understand all these factors to ensure that the parts can be produced without any manufacturing problem there are guidelines and checklists prepared by the organizations to take care of these issues at the design stage but they are not frequently referred to many of these design considerations can well be defined as the rules of design to help the effective implementation of design for manufacturing where data is already available in digital format automating some of these tasks can

30:49 definitely provide value to the organization geometric brings DFMPro an automated dfm review solution as you see in this workflow DFMPro helps you to understand the design process and help design engineers to quickly validate the design while they are creating or updating it the best practices of organizations related to manufacturing assembly engineering they can be integrated into DFMPro as the rules these tests can be executed at the early stage in the product development cycle and detecting all the possible manufacturing issues and reducing

31:30 the lead time required for design change we will now see a live demonstration of DFMPro as you see i have opened one plastic component and you see DFMPro as a menu which is available within the cad right now we are showing it in pro engineer or creo elements pro i start DFMPro and it asks two inputs one is the rule file and second is the process with which this part is going to be manufactured in this case since it's an injection molded part i have selected injection molding as the process

32:14 in the meantime it also asks for the pulling direction it has already automatically identified that and user has a choice to select that as well at this time DFMPro automatically identifies all the manufacturing features which are like ribs bosses undercut and this is identified using feature recognition which is geometric propriety tool which is our patented technology so here we are not referring to the cad feature tree but we are identifying all the manufacturing

32:55 features from this geometry and we are checking the parameters of this feature using the rules that are defined in DFMPro as you see once DFMPro runs all the violations in this window result window in this window you can also see you click on this rule and it gives the significance of that particular rule for the plastic design designer you can drill down and click on the specific instance to know what is the problem in this particular feature in a complex shape it is difficult to identify this feature and

33:44 it also gives the facility to zoom to selection so that you can exactly reach the area that you are interested in as you see in this particular case the boss doesn't have the fillet and it is indicated that it is recommended to have the fillet of radius 0.65 so designer can take the decision immediately and he can remove this particular violation by changing the fillet of radius to 0.65 and this will give you the standard result which is the

34:31 following result in the next four or five minutes so this is the first report let us see the evaluation here at the adjoining point you can see that we did a lot of research basing it on x 0.3 and then in the actual case it is 0.8 that's why it is shown as a violation similarly in this case the rib height to wall thickness ratio is 8 and the recommended is less than or equal to 3 again these values can be configured using the rule manager that we have in DFM Pro if you look at the draft angle rule in this

35:21 case there is no draft given and as I've all mentioned earlier if you don't apply a drop it can create problem in taking the component out so it is again indicating what is the recommended value for the drop and the actual value is also highlighted it also indicates that there are some undercuts in this part which are difficult to manufacture because it needs a separate operation for each of the components and it also indicates that there are some undercuts in this part which are difficult to manufacture because it needs a separate operation for these undercuts uniform wall thickness is also identified it indicates what is

36:08 the thickness at this place and what is the recommended one the way you identify the thickness of the component the tool also gives that the mold wall thickness here is less so it may create a thin steel condition for the mold and the value is shown here so these are some examples in which in which the designers can actually take the decision and make the design better. We also have a facility to create the report. There are three types of reports which are created.

36:50 One which can be opened in the browser. It shows which are the rules that failed with the actual values. DFMPro also creates the report in the 3D report which is eDrawings report where you can see all the features that failed just like you see it in the CAD platform. So you can click on the specific instance. It will zoom to that area and it also gives the indication what exactly has gone wrong there. On this report you can also put the comment like I am saying that we need to add a radius here

37:39 and this can be shared and the design collaboration can happen. Within this single file where all the comments are stored. It creates Excel reports as well where you can see all the failures and when you click on the specific rule it gives the details about it. This report also helps in tracking because it has different parameters like what is the action to correct it and status, check, remark, different columns and this can be customized as well.

38:21 Right now what we just saw is all the rules related to the injection molded process. Similarly we also have rules related to milling, sheet metal, assembly where it identifies all the violations related to those areas and gives the indication to the designer and gives the guidance to the designer to help him making the design better. As we just saw this is an automated DFM review process. So such process definitely helps organizations in many aspects. One is the productivity improvement.

39:01 Obviously because of the reduction in the design iteration. Reduced number of defects result in improvement in the quality. Standardized reports automatically generated by the tool improves the communication. Identification of areas of the design which can be difficult and expensive to manufacture benefits the product cost. And the DFM review solution also facilitates capture of different best practices and it provides a framework to retain and disseminate the manufacturing knowledge in the organization.

39:48 The statistics of this recurring defect can also help in identifying the training. So this DFM review solution helps actually bridging the gap between desired and achieved measurement. These tools definitely help guide the design engineers to take right decisions. At the same time it is also important that training design engineers is very very critical because all the best design practices can be actually explained for the design engineers and they provide better understanding for the engineers on the manufacturability aspect

40:32 and also make the effective use of this tool. Here we come to the end of our presentation. We hope that the presentation would have given you idea about designing plastic components with best design practices and also making them easy to manufacture. Now we will move on to question and answer session. So I will hand over to Prashant and if you have any questions you can raise your hand or type in your questions in the questions window. Thank you Nikhil which most of the people have started typing now. So here is the first question. Paul this is the question for you. It says that somewhere on your slide which shows draft angle on the core.

41:20 So it says that isn't it that the PP shrinks more onto core than ABS? Right because polypropylene shrinks more than ABS. On the core when you eject you will have easier ejection force than you will have with ABS material. This resin which shrinks less so you know you need a higher ejection force.

42:02 The next question is to Nikhil. The question is what is the difference between typically a simulation tool or CI tool and DFMPro? DFMPro is not really an analysis tool. If you look at the overall product development process. Analysis is a process which starts after the design is actually released. But DFMPro is something which the designer will make use of. So it is part of the design process itself. So while the designer is creating the design or modifying the design. He will be able to run DFMPro, get the results and he will be able to correct it immediately.

42:44 So it is something which is a preventive measure and not really effective. And not really a corrective step. This is also again next question to you Nikhil. It says how DFMPro is different on PRO-E compared to model check? Okay so model check. If you look at the model check rules that are there in PRO-E. So those are more towards the design standard. It may mention like your design whatever you are making. Or the drawings that you are making. The size of the font or it should be something.

43:24 So these are more of the rules related to the design standard. Whereas in DFMPro what we are addressing is the manufacturability issue. So this is definitely different than model check. And if it is combined it can be a complementary tool. Where all the checks can be performed. So I could add one more comment. I think this tool that Nikhil showed us is an excellent tool. The only thing I want to caution the audience. If you have very little experience you really need to get up to speed.

44:07 And understanding the capabilities and the rules which will be applied. Because you could have major errors if you do not understand what you are doing. Understanding and applying. The next question is from Mr. Rafi. It says that this is to you Paul. It says can you please provide guidelines for both designs suitable for brass inserts. What will happen in case if it is M2.5 up to a die of let's say 4 mm.

44:48 Oh the brass inserts. Okay. Inserts into the plastic part. Okay. So let's say. What will happen in case the die is M2.5 and insert die of 4 mm. Okay. There are two ways actually many ways of inserting metal components into a plastic part. Okay. One of the ways used in production is to insert inside the tool and then mold around them. Okay. If you plan to use this technique. The inserts have to be in the tool.

45:39 And the temperature of the inserts should be very close to the TG. Glass transition temperature of the polymer. This is if you use inserts inside the tool. When heated in the tool you create stress concentration around the metal component. Could be brass. Could be aluminum. Could be anything. And that will cause stress concentration will create an unpredictable failure mode. Okay. Another way of inserting is to use a ultrasonic approach. Ultrasonic approach is an approach to which you use friction generated by a horn to melt

46:23 the polymer superficially in order to engage the metal insert. However there is a brand new technology which came from Germany. Was launched about two years ago. In Dusseldorf. The largest plastic show in the world. Called K-Show or Kunsthof in German stands for. A company called the first ever machine developed. Electromagnetic heating within a second. So you blink your eye.

47:03 Imagine that you have an insert. Heat it to the TG or just above the TG of a given polymer. And then insert it into a plastic part. All within a second. Which is incredible. So those are the three techniques used for inserts commonly. Did I answer the question? Yes. The next question is for which CAD software does the DFM Pro is available? So DFM Pro is integrated with Pro Engineer SolidWorks. And very soon we will be coming out with NX.

47:46 And we also have a standalone version available. Which can open all the neutral file formats. Yes. There are also questions related to plastics again. This question is about like how to take I think Paul you have to handle this. Does this guidelines are also useful when you use composites? Or when you fuse different plastics and metal materials together? Yes. In general yes. You have to design the metal parts such that you increase the surface area of the contact.

48:29 So a typical application are to use mechanical interlocking designs. To have the plastic fully engaged. If you do not use mechanical interlocks and you use just making the parts. You could delaminate the part in time. Because plastic has certain metal. And you have that delamination taking place. Components using metal and plastic you have to design mechanical interlocking there.

49:10 The next question is to Nikhil. It says that what about if I would like to add my own rules in DFM Pro? Yes. So DFM Pro provides the framework where you can add your own rules. So there is a customization framework which is available with the software. With little bit of programming in visual basic or C++. You can add your own rules and keep your proprietary information with yourself. I think we are getting lot of questions. But it would be very difficult to try to answer all of those here. We will try to answer those questions offline because it would be impossible.

49:53 So thank you everybody. Thanks to Paul and Nikhil for joining. So Paul do you have any suggestions, any views? No. I'm all set. I appreciate that you guys came on and listened to our presentations. And hopefully you learned a few tricks. I wish you the best of luck with your plastic part design. It's a wonderful field. And it's growing by large percentages in the last few years. Thank you Paul. So this is just to reiterate that if you have further questions. We have mentioned the contact number and email ID of ETS Corporation.

50:33 If you would like to pose any questions to Paul. You can send it to info at ets-corp.com. If you would like to know more about DFMPro or download or try to evaluate the tool. You can visit www.dfmpro.com or write to us. In case if you have joined late for this webinar. We have recorded this session. And will be soon made available for the participants. In case if you have joined late. Again thank you very much for your time. And keep visiting DFMPro.com for such interesting webinars. And if you would like to stay updated. You follow DFMPro on Twitter or on Facebook.

51:13 And we will keep you updated with this webinar. Again thank you very much. And have a great day ahead.