10 Case Studies In more than thirty-five years of experience in this business, I have seen all sorts of parts, customers, and problems Anyone in this business who has not been visited by an inventor with a “wonderful” idea that he wishes to implement in plastics must be in a position where he does not deal with the public Here comes a bright-eyed inventor Perhaps the (abbreviated) conversation goes like this I NVENTOR: “I wanna make this here whatsit outa plastic.” MOLDER: “What kind of plastic?” INVENTOR: “Uhhh……Hard plastic?” MOLDER: “Do you know that this invention will require molds that cost thousands of dollars?” INVENTOR (IRATE): “You tryin’ to rip me off? I know plastics are cheap!” and he stalks out in an angry huff Then there is the guy responsible for the case described in Sec 10.5, who cannot be taught or warned about our less scrupulous brethren This chapter deals with a variety of molding experiences, problems, and solutions that stand out in my memory Even though this book’s emphasis is controlling shrinkage and warpage, these “personal experiences” deal with other things as well I hope you find them interesting and informative 10.1 Unexpected Housing Shrink Figure 10.1 shows a housing that contains a rotor with very little clearance between the rotor and the inner bore This part was in a family mold with other simpler parts To avoid three-plate or hot-runner expense, this part was gated at each of the ears marked “G.” The center core pin at “V” was inside a sleeve ejector, and the clearances between the pin, sleeve, and cavity were generous to provide venting The material was 30% glass-filled nylon The mold builder realized that the material flow and fiber orientation would be predominantly radial and assumed a shrink factor higher than published to allow for cross-flow shrink The inner diameter shrank about twice as much as expected, but the outer dimensions were right on spec What was the cause? Notice the many sharp corners and changes in direction between the gates and the vent When the part was placed in an oven and the plastic burned away, there was a significantly lower concentration of glass fiber in the inner cylinder than in the outer cylinder Two things contributed to the shrinkage First, of course, was the reduction in fiber concentration It seems that each corner effectively combed out a small percent of the glass fibers Second, each corner caused energy loss and a reduction in effective holding pressure in the plastic in the inner diameter Adding radii to the various corners would have helped the situation, but ultimately it was necessary to open the bore and increase the core size to provide the clearance for the rotor Sometimes strange things happen during molding operations This case is a good example Figure 10.1 Rotor housing © Plastics Design Library Ch 10: Case Studies 140 10.2 Changing Materials Triggers Warpage Figure 10.2 shows a part that was originally designed in ABS Part of the runner and gate are shown to indicate where the material was introduced into the mold A need for better chemical resistance dictated a change to nylon after the mold was built In order to maintain size, a glass-filled nylon was chosen that had the same published shrink rate as ABS To the molder, it seemed like the solution was easy Using a material with the same shrink rate should yield an identical part Unfortunately, that was far from the reality When the first samples were shot, they looked something like Fig 10.3 Since flatness was a primary concern, parts warped like this were not satisfactory The molder had to find a solution—fast! The nylon supplier was consulted and he explained the phenomena of differential shrinkage based on fiber orientation He drew a picture like Fig 10.4, showing the approximate flow paths in the part with the edges folded up to form a flat pattern It can be seen that the flow path on the gate side is essentially parallel to the long edge and symmetrical on that edge above and below the gate, so the fiber orientation is predominantly along the long axis of that side On the opposite side, the flow is predominantly vertical across that edge Since shrinkage in fiber-filled materials is significantly greater across flow than it is along the flow, the long side opposite the gate was shrinking significantly more than the gate side, causing the warpage shown in Fig 10.3 Fortunately, the solution to the problem was fairly simple By moving the gate to the top center of the narrow end of the part, the warpage was reduced to a satisfactory level This problem occurred long ago when glass-fiber reinforcement was relatively new and before talc- or flake-filled materials were available It is possible that a simple substitution of a talc- or flake-filled nylon for the glass-filled nylon would also have solved the problem Figure 10.2 Views of a part designed for ABS plastic Figure 10.3 The same part molded in glass-filled nylon Ch 10: Case Studies Figure 10.4 Flat pattern of the part showing flow directions © Plastics Design Library 141 10.3 Creep in a Water Heater Stand When building codes required that water heaters be raised eighteen or more inches from the floor to inhibit ignition of heavy flammable vapors, one enterprising entrepreneur proposed to make a plastic stand to lift the tanks Figure 10.5 shows the design of the stand The molder and the mold builder cautioned the customer that there needed to be reinforcement or metal pads under the water heater feet Initial tests indicated that the stand would support many times the weight of an 80-gal tank The customer was sure that his product was fine After all, look at all the reinforcing ribs and the enormous test loads that had been sustained After about four years, the water heater feet started sagging or breaking through the top of the stand Creep had struck again A plastic part can withstand very high stresses for a short time, but sustained, high stresses cause major deformation or failure This failure could have been delayed or prevented if the point loads at the feet of the water heater had been spread over several square inches of area A 4–6-inch diameter steel or aluminum disk under each foot would have been adequate 10.4 Oversize Part Injection-Molding Alkyd Thermoset While thermosets are not addressed in this book, there is one experience that I would like to share with you When injection molding of thermosets was new, my company was asked to bid on making a thermoset box as shown in Fig 10.6 The material was to be alkyd, and the material supplier assured us that it was a simple matter to mold their brand new injection-molding grade We completed the mold and began mold trials The part is gated between the two mounting feet at the top of the picture The walls are about one-tenth inch thick We could not get the part to fill The material would set up before it filled the cavity, leaving a void in the wall at the bottom of the part in the figure We tried everything We changed mold temperature, material temperature, injection pressure, and injection rate without success We opened the nozzle orifice, the gate size, and the runner size The material supplier came to the plant and basically shrugged his shoulders, “We thought it would fill.” Figure 10.5 A plastic hot water heater stand © Plastics Design Library Ch 10: Case Studies 142 Figure 10.6 A thermoset box We were ready to try anything That was when our molding shop foreman asked, “Why don’t you lubricate it?” He suggested adding just a touch of zinc stearate Well, it worked In fact it worked too well The first parts molded after we added the lubricant filled and packed the mold so well that the part did not shrink It actually came out of the mold and cooled larger than the cavity That little bit of lubricant allowed the part to fill so easily that we compressed the plastic more than the shrink rate, causing “negative” shrink By reducing the lubricant loading and the injection pressure, and modifying the injection rates and mold temperature, we were able to produce thousands of good parts We never told the customer or the supplier how we made such good parts 10.5 Inadequate Baby Dish Mold A young man walked into my office, many years ago, with an idea for a baby dish that would not spill It would be clamped onto a highchair tray so the baby could not push it off or turn it over The dish was to look something like Fig 10.7 The gentleman looked and acted in every way like a frugal person But unfortunately he was not very wise I priced the tool He advised me emphatically that he could get the mold built for half that amount I delicately inquired who the builder might be and cringed at his reply The proposed supplier was infamous in our area for building the cheapest molds possible and for making a profit on the initial 50% down payment Figure 10.7 Proposed baby dish design Ch 10: Case Studies © Plastics Design Library 143 Because I was concerned about possible liability, I could not tell this young man that this particular mold builder produced junk Rather I tried to educate him about mold construction, showing him some molds that we had built I advised him to get references and to look at some of the other molds this guy had built and compare them to our molds My education efforts were to no avail Some months later, very late in the day, the young man came into my office struggling to carry what appeared to be a stack of rusty, flame cut square plates I realized that this stack of junk represented what the other mold builder had produced Over my objections, he set the plates on my desk and told me his sad tale The other mold builder had not been able to produce even one part from the mold There was a show in less than a month for which that the young man absolutely had to have parts Figure 10.8 approximates the construction of this mold, but it does not show that the plates were not ground flat nor the edges finished after being flame cut Major deficiencies are visible in this sketch: • The core pieces were surface-mounted on an unground plate and positioned with two dowel pins in each core piece and were retained against the plate with four bolts in each core piece Plastic was injected into the mold at the “ear” on the right side of the cavity (shown at the right in Fig 10.8) The viscosity of the plastic was so high that the forces pushing the core pieces sideways were sufficient to cause the dowel pins to distort the holes in the core pieces and in the mold-support plate, allowing the cores to move out of position • There was inadequate plate thickness on the ejector (left) side of the mold under the cores The plate under the core was flexing so that injected plastic was flowing under the cores, effectively trapping the molded part on the mold so that it could not be ejected In a good quality mold, the plate containing the cores will have at least an inch of thickness under the cores and the cores will be integral with or pocketed into that plate In addition, there is normally an additional plate under the core plate that adds additional support and stiffness to the assembly • Part of the problem with flex in the support plate was the total lack of support pillars Even with the much thicker plates normally found in a quality mold, support posts or pillars are necessary in molds having a span between the side rails Figure 10.8 Sketch of a poorly constructed mold © Plastics Design Library Ch 10: Case Studies 144 of greater than to inches Injection pressures are typically in excess of 5,000 psi and may be as high as 20,000 psi or more This dish had an area of about 40 in.2 With high injection-pressure, the bending force on the core plate could be as high as 800,000 lb Certainly this is sufficient to cause significant deflection or bending of the core plate • There were no leader pins or bushings; instead, dowels were placed in drilled holes through both plates The dowels were allowed to “select” the side on which to stay, depending on the fit and friction at that moment The dowels were about the same size as the dowels that were supposed to position the cores, so there was less area resisting the side force caused by injection pressure at the nominal leader pin location than there was at the core Therefore, the “leader pin” holes were stretched out-of-round, which aggravated the moving cores • The unground plate surfaces resulted in irregular gaps between the two plates The gaps were large enough for plastic to flow into them, causing flash I doubted if that poor excuse for a mold could be made to work properly and I was right After grinding the plates, adding pillars, leader pins and bushings, and adding large keys to position the cores, the best part that could be produced is shown in Fig 10.9 There was some flash all around the part, especially at “A” and “B,” but the largest was at “C.” The young man was satisfied because he was able to trim the flash away and sand the parts smooth enough for demonstration at his show 10.6 Gas Entrapment in Baby Dish Mold There is a sequel to the story More months passed and the young man showed up again This time he had another mold This one was far better but it had a little problem The plastic flowed around the outside of the part faster than it flowed across the part, and trapped some air approximately at point “A” in Fig 10.10 As the air was compressed by the plastic, it was heated to the point that the plastic around the hole was charred black This time the solution was more direct and the mold was salvageable By thickening the bottom of the part by removing some material from the tops of the cores at “A” and “B,” and by making the rib between the dish pockets between “A” and “B” thinner, the plastic flowed across the mold more easily than it did around the edges So we could mold a good dish, as shown in Fig 10.11 Figure 10.9 A part from a reworked junk mold Note the “flash” at locations A, B, and C Ch 10: Case Studies © Plastics Design Library 145 Figure 10.10 A new baby-dish mold with air entrapment Figure 10.11 The final, good, baby dish 10.7 Warpage in a Molded Spool most obvious was the gate design with respect to the part-wall thickness Note that the wall is almost twenty times greater than the gate thickness (0.37 in vs 0.02 in.) The gate length (0.05 in.) is more than twice the gate thickness (0.02 in.) These two errors resulted in an almost immediate freeze-off at the gate as soon as the mold filled This left the spool with only a thin wall, perhaps less than 0.06 in., which solidified while the remaining mass was molten Furthermore, the thin gate resulted in a significant amount of shear heating of the material at the gate, which further raised the temperature of the molten mass in the cavity The higher temperature resulted in greater thermal contraction than would have been experienced with a cooler melt The part represented in Fig 10.12 contains many of the classic mistakes that are made by part and mold designers Only pertinent dimensions are shown The molder had reported “A little trouble with warpage and shrinkage.” In fact, the shrinkage was about twice the published shrink rate and the internal bore had ballooned out a considerable amount Almost certainly, the customer and part designer had indicated a desire for clean ends, without gate marks The molder wanted an easy-to-remove gate and an inexpensive mold These restrictions led an inexperienced mold maker to make several gross errors in the mold design The Figure 10.12 Heavy-wall, glass-filled nylon spool Dimensions are in inches © Plastics Design Library Ch 10: Case Studies 146 Although it is difficult to analyze, it is possible that the jetting effect of the thin gate encouraged the material to flow to the far side of the cavity and then flow down the length of the part This would result in cooler material on the side of the cavity away from the gate and warmer material near the gate In addition, there is a greater heat load from the shear heating at the gate and on the gate side of the center-core pin This means the mold is warmer on the gate side than opposite the gate The turbulence from the gate jetting discourages glass-fiber orientation, so there is not likely to be much, if any, fiber orientation to affect shrink rates Therefore, the likely primary causes of excessive shrinkage, other than the small gate, are the mold and melt temperature differentials Because nylon is a semicrystalline material, it has a higher rate of shrinkage than amorphous materials, and the heavier the wall section and the higher the melt temperature, the greater the percentage of crystalline material in the molded part Higher crystallinity translates into higher shrink rates Higher temperatures on the gate side of the mold cause the part to shrink more on the gate side and assume a shape somewhat like that represented by the dash-dot-dot lines These lines indicate some concavity in the outside walls in addition to the bending effect The inside walls, as molded, are represented by the dotted lines The central core was not adequately cooled and as a result encouraged the ballooning of the inside bore to compensate for the shrinkage in the solid mass around the core There was little foaming or voids in the mass, as would be expected in an unreinforced material The glass fibers helped prevent voids forming in the mass The hot core encouraged the inner wall to sink away from the core to make up the shortage of material that resulted from the high shrink-rate In such a case, the first action to minimize shrinkage is to increase the minimum dimension of the gate to at least 50% of the wall thickness It may be necessary to increase the gate to as much as 70% to 80% of the wall thickness This will allow material to flow from the runner into the cavity, as the material in the cavity cools and shrinks, for a longer time It has been established time and again that longer, effective holding/ packing time reduces shrinkage Notice that the runner and sprue are smaller (0.23 in to 0.25 in.) in cross section than the part (0.37 in.) The material in the runner is cooling faster than the material in the cavity The runner is surrounded by cool mold-plates, while the part is cooled effectively from Ch 10: Case Studies the outside but not from the inside This means that the runner will solidify a significant amount of time before the part does Therefore the runner also needs to be increased in size to approximate or even exceed the cross section of the part Note the 0.12-in diameter cylinder of material at the end of the sprue Here we have a molding-machine nozzle of 1/8-in diameter feeding a part that is 3/8 in thick The material in the nozzle can freeze and stop the flow of makeup material before the material in the cavity solidifies Furthermore, with a larger gate and runner cross section, shear heating will occur at the nozzle instead of at the gate as earlier discussed Therefore the largest nozzle that can be used without drooling (assuming good material drying) should be chosen More effective cooling of the core and a resulting decrease in cycle time and ballooning around the core can be achieved by use of a bubbler or cascade, or a heat pipe, within the core The small core requires a very small feed tube and clearance around it for a bubbler Any water contamination or corrosion would likely block effective flow, resulting in hot spots, or would revert to inadequate cooling Probably the best solution is to have the core built with an integral heat pipe The back, or base, of the core would require a heatpipe extension into a water channel that is about the same length as the heat-pipe exposure to molten plastic, that is, about 3.5 in Fins can be added to the rear extension-tube to reduce this length requirement The region of the part near the gate is always affected by the variation in flow and packing at the gate Depending on conditions, the gate effect could cause either higher or lower shrinkage than the more remote areas Therefore, it is unwise to place a gate in an area where warpage is a concern For this reason, the gate should be moved to the end of the part, perhaps gating axially parallel to the center bore and on both sides of the core to balance pressure from one side of the part to the other Finally, a hot runner or hot sprue should be considered The massive gate and runner could be entirely eliminated with significant savings in waste or reground material Even with a single-cavity mold, a hot-sprue design that centers the core while providing material on two or three sides of the center-core pin should be possible The hot sprue would also reduce the size requirement for the gate in that the heated sprue would help keep the material molten at the gate to provide longer, effective packing time © Plastics Design Library 147 10.8 Daisy-Wheel Breakage When the daisy-wheel printer was new, the wheel was made with a plastic hub, and molded letters were connected together by a stamped-steel spring The spring had one arm for each letter or character Analysis indicated that a wheel that was all plastic, eliminating the steel spring, was feasible A one-cavity mold was produced but when the new, all-plastic daisy wheels, as shown in Fig 10.13, were tested, the spring arms would fail when struck repeatedly (for example, when the underline character was used to create a line across the paper) Samples of the molded part were analyzed and it was discovered that the physical properties of the glassfilled nylon were significantly below what they should have been The question then was, “What is causing the material degradation?” Right away it was discovered that the plastic was not being adequately dried A simple, glass-slide test indicated moisture in the material in the feed throat of the molding machine The glass-slide test requires two glass slides and a hot plate The hot glass slides are placed on a hot plate and heated to just above the melt point of the plastic being tested When the slides are hot, two to four dried plastic pellets are placed on one slide, spaced about one-half inch apart along the centerline of the slide Tweezers are used to position the pellets, and to place the second slide on top of the pellets The slides are pressed together with the edge of a tongue depressor or popsicle stick When the pellets are thinned so that they are translucent, each pellet is about one-half inch in diameter or a bit more, and will be translucent, even if highly pigmented If there is any moisture present, it will appear as bubbles in the flattened pellets Adequately dried plastic was molded with some improvement of properties, but still significantly below what they should have been The part was center-gated with a 0.040-in diameter gate The gate had a cross-section area that was less than twice the area of even one of the many spring arms The mold was difficult to fill even when the mold and the material temperatures were at the upper recommended limit Often the plastic would freeze before the spring arms were fully filled It was taking several seconds to fill the part even when it would fill at all We theorized that the gate was so small that the part could not be filled fast enough to finish filling before the material would freeze in the spring arms Attempts to fill faster with increased injection pressure caused the material to get even hotter from shear heating in the gate This additional heat was causing heat degradation of the material We doubled the diameter of the gate and were able to lower the mold and material temperatures and still fill the part in under two seconds The physical properties improved to the levels expected and the parts no longer failed under test 10.9 PVC Part-Flashing Problems A large part with a projected area of 240 in.2 (10 in × 24 in.) was being molded on a 730 ton clamp molding machine The material was rigid PVC The molding machine shot capacity was about four times the required shot size The average wall thickness was 0.200 in The molder had been plagued with PVC degradation in the barrel of the molding machine When PVC degrades, it can break down into something almost like Figure 10.13 An early prototype of a daisy-wheel print disk © Plastics Design Library Ch 10: Case Studies 148 a thermoset, that is, a powdery solid Because of this history, the molder was wont to keep the barrel temperatures as low as possible and use a slow injectionrate to avoid overheating the PVC The barrel temperature settings were essentially flat at the minimum temperature to melt the PVC The injection rate was set to take a minimum of eight seconds to fill the mold Toward the end of the fill cycle, the injection rate slowed due to viscous back-pressure from the mold, increasing the total time to over ten seconds The problem was that under these conditions, it took over ten seconds with injection pressures equating to over 13,000 psi to fill the mold, even with the sprue centrally located as shown in Fig 10.14 Over half the time the part would flash dangerously near the sprue, even when the part did not quite fill If even half of that injection pressure translated into separation forces, the pressure trying to force the mold open would be 780 tons The clamp pressure setting was less than 700 tons No wonder the mold was flashing Examination of the mold found that the support pillars were essentially the same height as the side rails, or perhaps 0.001 in less The mold builder increased the height of the pillars so that they were preloaded about 0.003 in The molder was persuaded to increase the frontzone barrel temperature to the maximum recommended by the material manufacturer, with each previous zone lower so that the feed zone was at the minimum-recommended temperature The maximum fill-rate was more than doubled, reducing the fill time to less than Figure 10.14 A large part that was having flash problems Ch 10: Case Studies four seconds (The sprue was quite large.) These changes allowed the molder to cut the injection pressure nearly in half The discoloration seen in the far end of the part in Fig 10.14 is residual discoloration from earlier shots when the barrel temperatures and fill rates were lower (The mold builder rarely gets the best parts.) The front barrel temperature could probably be even higher, because the plastic melt rarely raises to the barrel temperature The pillar height adjustment was probably not necessary The higher melt temperature reduced the viscosity The steep temperature gradient in the barrel compensated in part for the relatively small shot-size compared to the maximum shot-size The large sprue allowed a rapid fill-rate without significant shear heating 10.10 Polycarbonate Switch Failure When polycarbonate first came out, some folks thought it was the answer to every plastic problem One company decided to use polycarbonate in a switch inside an explosion-proof housing in an oilfield application The switch required two of the (A) parts and two of the (B) parts in Fig 10.15 Only one switch rotor (C) was required By rotating the contact leafsprings, the five contacts could be either normally open or normally closed Figure 10.15 shows the two polycarbonate parts positioned properly Two more parts assembled in the same way and inverted completed the polycarbonate Figure 10.15 Partially assembled polycarbonate rotary switch © Plastics Design Library 149 parts The gray part was a thermoset part to better resist any electrical arcing as the switch opened and closed It was placed between the electrical contact bars in the center hole of the end pieces The parts worked very well in testing Unfortunately, the initial testing was relatively short and did not include an adequate exposure to the wide variety of aromatic hydrocarbons that are present in a petroleum-producing environment After about six months, some of the switches began to literally fall apart from cracking and crazing No one had studied the chemical resistance of polycarbonate to aromatic hydrocarbons The solution was to change the material to oldfashioned, glass-filled nylon, which has a very high resistance to aromatic hydrocarbons 10.11 Square Poker Chip Tray, Inadequate Shot Size The very first problem I encountered when entering the injection molding industry was one of maximum shot-capacity of available machinery I was working with a start-up company that had only two presses One was a 3-oz shot-size, Van Dorn plunger-type press The other was a 450-ton clamp, 24-oz., Reed Prentice screw-injection machine My predecessor had quoted and accepted a contract to mold square poker chips and a poker chip tray somewhat like the one shown in Fig 10.16, which was to be available in a variety of colors The difference was that the tray was to have 1/4-in.-thick walls The one shown has a wall thickness of about 1/8 in Figure 10.16 A square-poker-chip tray with some chips © Plastics Design Library Apparently, my predecessor had a habit of ignoring or guessing at part weight When the mold was complete the customer was present for the initial test The molding machine setup man set the machine for its maximum shot size, expecting to reduce the shot size later and hoping for a full part on the first shot The injection unit bottomed out, pushing a full 24 oz of plastic into the mold That surprised the setup man When the mold opened and the part was removed, you can imagine the expressions of shock, anger, and dismay when the part was little more than half formed Later calculations indicated that a full part would weigh about 36 oz At that time in that company, no one was aware of how to use a foaming agent when injection-molding plastic That would have probably formed a satisfactory part Other molders with larger-capacity machines would not mold the heavy part with the longer-thanexpected cycle times for the quoted part price Did I mention that the customer gave the impression that his business was on the shady side? The molder was left with only two options: cut down the mold to reduce the wall size, or else As you can see, they chose the former 10.12 Problem Ejecting Square Poker Chips Part of the poker chip deal was that there were to be no visible ejector-pin marks on the 1/8-in.-thick poker chips They were molded of crystal polystyrene with flecks of aluminum They were molded with radii all around and were formed on both sides of the mold with the parting line in the center of the part thickness They really looked nice The same individual who failed to consider shot size was sure that the poker chips would fall right out of the mold with the runner In fact they did, but the formidable customer was dissatisfied with the pinpoint parting-line gate Besides, there was a fair amount of labor removing the chips from the runner The parting-line gates were plugged and tiny tunnel gates were cut into the 1/16-in.-deep ejector side of all forty-eight cavities, as shown in Fig 10.17 That solved the problem of gate blemishes, but then fewer than half of the poker chips dropped from the mold They usually stuck to the ejector side of the mold This time the solution to the ejection problem was to modify the radius of each chip on the injection side of the mold opposite the gate, to create a small undercut, as shown in the upper right part of Fig 10.18 This caused the poker chip to pivot around the gate Ch 10: Case Studies 150 and out of the ejection side of the mold as the mold opened The ejection system in the runners severed the gates, leaving the poker chips virtually hanging in thin air The sound of those chips tinkling into the collection chute was the sound of money Figure 10.17 A square poker-chip cross section showing the tunnel gate and modified radius on the opposite edge that the material could be placed directly into the cavity without compacting it Most compression-molded parts enter the compression mold as preformed blocks The preforms are usually formed slightly smaller in diameter than the cavity into which they are placed There are machines that take standard pellets and compress them into preforms The machines can reduce the volume of the raw material to about 25% to 50% of its original density This material had to be compressed to less than 10% of its original volume to make the preforms It was reduced to about 3% of its original volume in the final product That is a lot (this is a bit of a “stretch”) of shrinkage We had to design and build a special preforming machine to complete this contract The second reason this stands out in my memory is that the same guy who forgot to calculate the shot size of the poker-chip tray did not realize that this special machine was necessary And he forgot to include the cost of the raw material in the quoted price Did you ever try to get out of a government contract? Have you heard the concept, “If you are loosing money on every part you mold, you have to make it up in volume?” It does not work The profit on this job was shrinking too Too much! Figure 10.18 Edge detail of the square poker-chip 10.13 Military Cup Material “Shrinkage” I don’t know what the military uses for dinnerware now but, at one time, they had coffee mugs, serving trays, and cups made of a linen-reinforced melamine One of the cups is shown in Fig 10.19 This little project stands out in my memory for two reasons The first is that this material, as received, was less dense than the cotton stuffing often found in an aspirin bottle when first opened It was light and fluffy With a little bit of effort, one could pack enough of it into a large coffee can to make a single molded cup Most plastics come in a granulated form and are reasonably dense Once again I have strayed into the thermoset field (sorry) The cup was molded in a four-cavity compression mold All the material for a part had to be placed in the cavity before the mold closed There was no way Ch 10: Case Studies Figure 10.19 A drinking cup once used by the U.S Army © Plastics Design Library 151 10.14 Core-Deflection Problems Steel is rigid, right? Consider the part shown in Fig 10.20 It appears to be a straightforward molding problem The part is not very big and the core is almost 5/8-in thick There are no side cores How simple a mold can you imagine? If you imagined that you are wrong As expected, the mold was built with a freestanding core from the parting line at the open end of the box The part was sprue-gated in the center of the closed end Ejection was by stripper bars across the long sides of the core The base of the core was about 2-in wide, 6-in long, and almost 2-in thick The first test-shots resulted in large voids near the gate/sprue on one of the large flat sides The plastic was flowing down the opposite side and the ends to the base of the core, then around the base, trapping air near the closed end on the opposite side Drat, and other expletives Some misbegotten toolmaker had obviously made a mistake grinding the core or cavity off-center Inspection revealed that in fact this was so, but only by a very few thousandths of an inch Not nearly the amount that would be required for the wall thickness variation at the closed end The closed side was over 0.06 in thicker than it should have been opposite the air-entrapment void We puzzled over that awhile and decided that the core to retaining-plate fit was too loose, allowing the core to pivot in the retaining plate The core was precisely centered, and the retaining plate-core fit was adjusted to a tight shrink-fit The core might as well have been machined from a solid block of steel The next molding trial was even more frustrating The void moved from side to side Some shots would have the void on one side of the core and others would have the void on the other side We could scarcely be- Figure 10.20 A proposed electronics case © Plastics Design Library lieve what was happening The core could not be moving that much without breaking, but it was The core was flexing almost 1/16 in from side to side, each side of center We finally figured out that, on a random basis, the plastic flow would start down one side or the other of the core As soon as the flow started down one side, the injection pressure would flex the core slightly, encouraging flow down the thicker side and inhibiting flow down the thinner side It was a vicious circle: • Thicker wall, easier flow • Easier flow, more pressure • More pressure, more flexure • More flexure, thicker wall Okay, we can solve this problem We sought and obtained permission to put a couple of alignment dowels between the core and cavity The electronics were to be potted into the case and the potting material would seal the holes We placed a 0.250-in.-diameter dowel near each end of the core on each side of the gate The dowels would keep the core centered The dowels lasted an average of less than a dozen shots before breaking The pressures in an injection mold are sometimes almost beyond belief Consider that the injection pressure was in the vicinity of 10,000 psi If one side were to be mostly filled with molten plastic before the other side started to fill, the pressure on the filled side would be × × 10,000 lb That’s 160,000 lb! That’s a bending moment of about 320,000 lb-in or 16 ton-inches No wonder the core was flexing The dowels were only about 0.05 in.2 each, and if each carried one-quarter of the side load, that translates to a shearing stress of 40,000/0.05 = 800,000 psi No wonder the dowels were breaking Figure 10.21 Modifications to keep the core in the center of the box Ch 10: Case Studies 152 After much anxiety, tears, and prayer, someone suggested changing the core design as shown in Fig 10.21 This had the effect of a diaphragm gate that would not be removed It would tend to force relatively uniform flow to both sides of the part In addition, if the core tried to flex, it had the effect of closing the gate slightly on the thick side and opening it on the thin side It was what is known as a “negative feedback” system It worked We molded thousands with only minor wall-thickness variations A sample part is shown in Fig 10.22 Notice the classic hourglass shape of the open end, caused by higher shrinkage at the hot corners of the core as compared to the cooler cavitytemperatures in the corner As usual, I only get to keep rejects See the missing letter “D” in the phrase “FRAGILE _O NOT DROP.” elevator, one on each side at the top and bottom of the elevator car I understand that the nylon gibs were direct replacements for cast bronze gibs; perhaps that explains in part the massive cross section The gibs were held in cast and machined iron brackets that restrained the gibs on all surfaces except those in contact with the guide rail The molding problem related to this part that I want to mention is the warpage that is shown in the drawing and shaded images The sidewalls are supposed to be parallel, but because of slower cooling and higher shrinkage at the end and corners of the mold core, the sidewalls were drawn toward one another as shown Our best efforts to cool the core more rapidly were of little help The only way to control the warpage was with a shrink fixture Two pieces of steel about ¾-in thick, 2-in wide, and 4-ft long were machined on a taper, reducing the ¾-in thickness, with the narrow end of the taper the same width as the bottom of the groove in the nylon gib The sides of the steel bar were machined with a taper so that the gib was held with the groove slightly wider at the open side than the closed Figure 10.23 Elevator guide gib, approximately 1.5 in across, in tall, and in long Figure 10.22 Photograph of the molded box with a “flexible” core 10.15 Elevator Gib Warpage Most personnel elevators are guided as they move up and down the elevator path (it can hardly be an elevator shaft when it is outside a building), with rollers or wheels that roll against two guide rails, one on each side of the elevator At one time, and perhaps in some cases today, the U-shaped nylon gib shown in Fig 10.23 and Fig 10.24 replaced the rollers There were four of these on each Ch 10: Case Studies Figure 10.24 Shaded image of the elevator gib, as molded © Plastics Design Library 153 side when the gib was placed on the bar The two bars provided cooling space for eight or ten gibs After each one-cavity mold cycle, the coolest gib was removed from the bars and replaced with one fresh from the molding machine This allowed about fifteen minutes of cooling time out of the press and on the shrink fixture If the nylon gibs were slightly wider at the open side than the closed side, it was felt that the iron holding-devices on the elevator would hold the sides parallel when they were installed The holes in the nylon gibs were simply to remove mass When the gibs wore to the point that the holes were exposed to the guide rail, it was time to replace the gibs There were some cored areas across the closed outside edge of the gib as well These are not shown 10.16 Sucker-Rod Guide Brittleness Oil-well pumps are usually several thousand feet below ground Steel rods called sucker rods, that extend to the pump jack on the surface, drive them The drilled oil-well hole and the pipe that lines the hole are not completely straight However, the sucker rod, being under tension, tries to assume a straight line This causes the sucker rod to rub against the pipe that lines the hole Over time, the sucker rod can wear out or wear a hole in the pipe Then, instead of pumping oil to the surface, oil is pumped into the strata where the hole is worn This is not good from both an economic and an environmental standpoint To avoid wearing through the pipe, nylon guides, like the one shown in Fig 10.25, are placed on the sucker rods every so often The nylon wears faster than the pipe by far, and also distributes the wear over a larger area Figure 10.25 One of several designs for a sucker-rod guide © Plastics Design Library These rod guides are supposed to be installed by placing the groove around the sucker rod and driving the rod guide against the rod so that the rod snaps into the circular center section of the rod guide The grip against the rod by the rod guide holds the rod so that the rod guide moves with the rod as it moves up and down Installation requires a BIG hammer We made several test installations when we first tried the new mold, entirely successfully The rod guides were still warm The molding problem we encountered was that when the customer came, he performed the same test, with his big hammer and tested parts that had cooled overnight The nylon rod guides may as well have been glass They shattered into a dozen pieces The customer explained that the rod guides are often installed in the arctic They have to be tough enough to be taken from a deep freezer and installed while cold The cure for the problem was to boil the rod guides for several hours This forced them to rapidly absorb water Once they were thus properly moisture-treated, they could be installed while cold, using a 10-pound sledge, without breaking 10.17 Bottle-Cap Thread Distortion Sometimes, if threads are shallow enough, or if the plastic is flexible enough, undercuts such as threads can be stripped from a core Just such a scenario was planned for the bottle cap shown in Fig 10.26 The desired thread profile (which was the profile cut in the mold core) is shown in Fig 10.27 (A) The rounded thread had a sharp corner where the thread contacted the wall of the cap During the stripping operation, the threads were distorted so that they looked something like the profile shown in Fig 10.27 (B) Figure 10.26 A bottle cap that is stripped from the core Ch 10: Case Studies 154 Figure 10.27 Bottle-cap thread profiles The bottle had buttress threads on the neck, with the flat side of the buttress thread toward the bottle The distortion of the threads in the cap caused interference between the bottle threads and the cap threads The cap was very difficult to thread onto the bottle and defied reliable installation by automated filling equipment No amount of tinkering with the molding machine conditions or mold temperature would resolve the problem, although adding a small radius, similar to that shown at Fig 10.27 (D), helped Ultimately, we had to accept that some distortion of the threads was inevitable Therefore, new cores were made with a profile that would have looked like the profile in Fig 10.27 (C) and (D) if the part could have been removed from the core without distortion We found that the radius shown in Fig 10.27 (D) was particularly important It apparently reduced the unit stress during ejection, made it easier to initiate the stripping action, and provided a smoother surface over which the threads slid After being stripped, the threads looked much more like the intended profile as shown in Fig 10.27 (A) stop-sign posts about 2.5 in in diameter to power-line poles that were 10 to 14 in in diameter performed superbly Several stop-sign posts were installed in a small Oklahoma town in the summer By the end of the summer, the signposts were leaning north by 20° to 45° The heat and the persistent south Oklahoma wind conspired to maintain a high enough load from the south to cause the plastic pipe to creep and unload the internal compressive forces and allow the pipe to “lean north with the wind.” But you should see some of the Oklahoma trees In some cases, the leaves and branches are all north of the trunk Maybe the plastic didn’t perform so badly after all 10.19 Excessive Shrinkage of GlassFilled Nylon A four-cavity, three-plate, center-gated mold for a cup-shaped part, shown in Fig 10.28, was built using published shrinkage data for 50% glass-filled nylon The gate design was a bit unusual in that it was a ring gate around a central core-pin that extended into the drop tube from the runner level of the three-plate mold This allowed the gate to break at the closed surface of the cup, leaving a center hole as required by the drawing At mold trial, the height of the part was just fine but the outside diameter was undersize While the cavity was drafted so that the open end of the part should have been larger than the closed end, the part was actually smaller at the open end than at the closed end The flow pattern of the mold oriented the fibers radially from the center gate and then parallel down the side walls 10.18 Plastic Post Creep It is politically and environmentally correct to divert used rubber and plastic products into secondary uses One such attempt was to make posts from groundup scrap rubber and plastic The particles were mixed with a bonding agent and packed into PVC or polyethylene pipe The theory was that the compressed rubber inside the pipe would keep the outside structural pipe in tension and thereby make it stiffer and stronger Initial tests yielded great results Everything from Ch 10: Case Studies Figure 10.28 Glass-reinforced nylon cup © Plastics Design Library 155 The radially oriented fibers in the closed end did not allow significant shrinkage on the diameter at the closed end However, there was nothing to inhibit shrinkage at the open end The parallel flow down the side walls inhibited shrinkage on the height of the cup, but the fibers oriented along the height of the cup did little to prevent circumferential shrink The fiber orientation caused the radial shrinkage to be at or below the published shrink rate, but the circumferential shrinkage at the open end of the cup was above the high-end published shrink rate The net result was that the differential shrinkage from the top to the bottom of the side of the cup was so great that the open end of the cup was below tolerance and the closed end of the cup was at or above the maximum tolerance Fortunately, the only critical dimension was the open end of the cup, and it was possible to correct the problem by increasing the diameter of the flange at the open end of the cup by grinding the mold cavity 10.20 Preventing Warpage in Thin Molded Lids Thin container-closure lids are often a very exacting and difficult molding operation Preventing warpage caused by differential shrinkage requires special attention The previous example discussed differential shrinkage caused mostly by fiber orientation In lid molding, the differential shrinkage is mostly due to differential pressure from the center gate to the periphery © Plastics Design Library of the molded lid The center of the lid is exposed to much higher pressure than the outside edges of the part Most of the differential pressure is caused by the increasing viscosity of the plastic as it flows away from the gate and is cooled It is well nigh impossible to control the temperature and pressure differential The next best alternative is to design the lid with a level offset to provide a flex ring to absorb the radial shrinkage variations Figure 3.13 shows a flex-ring design The figure is reproduced here as Fig 10.29 for the reader’s convenience The further from the gate, the greater the shrink rate Figure 10.29 shows that this lid has “toe in” that is caused by greater shrinkage at the open edge than at the closed edge of the cylindrical section The disk section attached to the closed end also restricts shrink Internal snap rings can be stripped from the core when molding polyethylene provided they are not too deep and are well-rounded See the molded, internally threaded part in Sec 10.17 Snap rings of 0.030 in are common If the depth of the ring exceeds 0.05 in., it may not be possible to strip it Figure 10.29 A molded lid with a flex ring and a stacking ring.[3] (Reprinted with permission of Voridian, Division of Eastman Chemical Company.) Ch 10: Case Studies