Troubleshooting: Injection Molding How to Solve Uneven Clamping
s PT Home PT Home Zones Materials Suppliers Materials DataCenter Machinery Database Resin Articles Resin Prices Subscribe Suppliers Research Zones Research Suppliers Materials DataCenter Machinery DataCenter Database Articles Prices Machinery Subscribe Database Troubleshooting: Injection Molding ng: Troubleshooting: Injection Injection MoldingMolding How to Solve Uneven Clamping Uneven How to Solve Uneven Clamping Clamping By John W. Bozzelli, Injection Molding Solutions tion By John W. Molding Bozzelli, Injection Molding Solutions Solutions Print This Article Print This Article Of of thevariables hundreds of variables involved in injection molding, clampingand mechanisms and Learn platens More often Learn More es Of the involved hundreds involved in injection in injection molding, clamping molding, mechanisms platens clamping often mechanisms and get than less attention than they Good clampis performance is critical to minimize ey get less deserve. attention they deserve. Good Gooddeserve. clamp clamp performance performance critical to minimize cycle times, cycle is times, critical to minimize c Visit the optimizeand part quality, process and maintain processAlignment consistency. Alignment of theplatens, mold halves,Visit platens, the of maintain optimize part quality, process maintain consistency. consistency. of the mold halves, Alignment the mold halves Injection and clamping system is therefore critical. Injection Molding ZoneMolding Zone erefore and clamping system critical. is therefore critical. Company Info Company Info When problem an alignment from theline, moldthe parting line, or the clamping em When stems an alignment from stemsproblem from the thestems moldmold parting parting platens, orthe theplatens, clamping line, the platens, or the clam Sensor Inc., damage, mechanism and tiebars, the can bemachine unnecessary wear, mold damage, Sensor broken Products Inc., Products he mechanism results and tiebars, the can results can be be results unnecessary unnecessary wear,machine mold damage, machine broken wear, mold tiebars, mold flash, and scrap parts. Madison, NJ Madison, NJ crap tiebars, mold parts. flash, and scrap parts. PARTING LINE ALIGNED? ED? PARTING LINE ALIGNED? One way to ensure proper mating of halves the tool parting halves at parting line is simply by having level molding machine and parallel One mating way to ensure proper of mating the of the tool tool halves at the linethe at is simply the by having parting a level moldinga machine line and parallel is simply by ha platens. Leveling a machine three machinist Start by getting all the adjustment screws on theofleveling feet of the the ad ne platens. requires Leveling a machine requires three three requires machinist machinist levels. Start bylevels. getting levels. all the adjustment Start screws on the by leveling getting feet the all press to the Place same torque. one level across the tiebars or platen support near the fixed platen, across or Place press to the same one torque. level one levelPlace across across the tiebars or the platen support tiebars near the fixed or platen, platen another acrossanother thesupport tiebars or the tiebars near the platen support near the back end of and the a machine, and a third level on alevel tiebar or platen support. Onetoperson goes toon a tie ack end the machine, and a third lengthwise platen support near of the back end of the machine, third level lengthwise on lengthwise a tiebar or platen support. One person goes the appropriate leveling foot and raises lowersit it as directed by another eyeing the levels. oot and raises or lowers as directed by another person eye the appropriate leveling foot and raises or lowers it as or directed by another person eyeing person the levels. Flash is a problem common part a host of possible answers. If you have shorts flash, you couldorhave a mold or clamping oblem Flash is a common with part awith host aproblem host of with possible of answers. possible If you have shorts answers. and flash, you and could have If a you mold clamping have shorts an alignment This may not be the case if you are are running a high-speed, thin-wallproblem product—the problem may alignment not problem. be This problem. may the not be the case case if you are if running you a high-speed, thin-wall running product—the a high-speed, may be simplymay not be simply notthin-wa clamp To help establish whether flash stems a mold or clamp alignment issue, check parting-line mating. In elp enoughestablish clamp enough force. To help force. establish whether whether flash stems flash from a mold stems orfrom clamp alignment from issue, checka parting-line mold mating. or In clamp align onefor typical check for uniform at theblueing parting line, applied blueing to agent applied to one mold half rm one typical clamp check uniform pressure clamp pressureclamp at thepressure at partingthe line, parting agent one line, mold half will blueing transfer towill thetransfer other agent if to the other if applied there is contact at the parting but it will not distinguish low force. or high touch force. information More detailed information be ting there is contact line, at the parting but line, it but itwill will line, not distinguish not between distinguish low orbetween high touch between More detailed low can be or can high touc obtained with pressure-indicating paper, which changes color to amount ofThe touch force. The to shade color change can dicating obtained with pressure-indicating paper, paper, which which changes color changes relative to the relative amount color of the touch force. relative shade of color changeof the can amount o be correlated e. be correlated with pressure.with pressure. Pressure-sensitive (suchfrom as Pressurex from Sensor Sensor Products) comes in a broad indicating range from 2 psi. psi to such as Pressurex from Products) comes in psi. a I broad Pressure-sensitive paper (such aspaper Pressurex Sensor Products) comes in a broad indicating range from 2 psi to 43,000 I 43,000 recommend a range of 7000 tofor 18,000 psi for parting-line troubleshooting. 00 recommend to a 18,000 range of 7000 to 18,000 psi psi for parting-line parting-line troubleshooting. troubleshooting. To check for good parting-line touch force pressure-sensitive with pressure-sensitive paper, cover parting linepaper, with thethe paper, the mold up the pa -line touch force with cover To check for good parting-line touch force with pressure-sensitive paper, cover the parting linethe with the paper, close moldclose up for 5 sec, then open anduniformity check for color uniformity onon the do paper. If you do not touch have uniform touch force on the partingnot line, the have u heck for 5 sec, then for open and color check for color uniformity on the paper. If you not the have uniform paper. force on If the parting you line, do the problem is either the mold orthe the clamp. Ifmachine machine level and the platens are the parallel, then the mold parting line is the d problem or the clamp. If the is level and the platens are pa is either the mold or the clamp. If machine isthe level and theisplatens are parallel, then mold parting line is the source of misalignment. Die-height adjustment will or only increase orincrease decrease itor cannot adjust clamp-pressure ie-height source of misalignment. Die-height adjustment adjustment will only increase will decrease only clamp force, itclamp cannotforce, adjust clamp-pressure decrease clamp fo uniformity, unless platens are bowing. ens uniformity,are unless thebowing. platens arethe bowing. CLAMPISSUES & TIEBAR ISSUES UES CLAMP & TIEBAR Molders find that clamp not lock up completely after The moldnormal closing. The normal to check the parting hat Molders the sometimes clamp findsometimes that the clamp will will the notnot lock upwill completely lock after up mold closing. completely response is after to response check theisparting mold closing. lineflash and look for flash or build-up that maybe be mold Iffrom Ifthe the will notmaking lock up after making closing uild-up there that may preventing mold from line and look for or build-up there that maythere be preventing thepreventing mold from the closing. the closing. clamp will not clamp lock up after parts for a relatively and there is nothing parting line preventing is line a hint more of something parts time for a relatively and short there time andshort theretime is is nothing nothing on the parting on linethe on preventing the lockup, parting this is lockup, a hint ofthis something preventing serious. more serious. loc One place to start is to note theoftemperature of of the mold and of the and of the te One place the to start temperature is to note the temperature the of mold and the the mold platen. If thethan mold is running hotter than 180 F, it could warm the platen. ing platen. Ifhotter the mold is running hotter than 180 180 F, it could F, warm it the could platen. warm the platen. Check for platen expansion at the top (while tiebars by (while warm) by measuring by measuring n Check at for platen the expansion top at the tiebars top tiebars (while warm) measuring warm) the distance between the top bars, then compare it to the distance top the distance bars, between the then top bars, then compare compare it to the distance it to the distance between the bottom tiebars. Take your measurement near the stationary the stationary rs. between Take the bottom tiebars. your Take your measurement measurement near the stationary near platen. Since the bottom of the fixed platen is secured to the frame the of the fixed platen is secured toof the frame of the platen. Since the bottom of the fixed platen is secured to the frame of the machine, it restricted tofrom some extent from thermal expansion. machine, some it is restricted extent toissome extent from thermal thermal expansion. expansion. However, top of the fixed platen has no such restriction and will xed However,platen the top of the the fixed has platen has no no such such restriction and restriction will and will as the platen warms. cana create a situation where the where the ms. expand as This the expand platen can warms. This create can createThis a situation situation where the Massive as they“wrap” are, platens “wrap” aroundMassive the mold Massive as they are, platens around the mold as distance between the topthe tiebars near the platen that is larger than that tiebars distance between the near top the near fixed fixed platen is fixed larger platen is larger than that between thetiebars bottom tiebars. As the moving platenthan is pushed toward the due to uneven application of clamp pressure. Toggles fixed platen to close the mold, the now pushed-out upper tiebars can apply pressure in the corners, hydraulic clamps do so restrict the movement of the moving platen. If this is occurring, a telltale in the center. sign is galling or scratches on the sides of the tiebars. The solution is to keep the platens relatively cool. Troubleshooting - Injection Molding: How to Solve Uneven Clamping http://www.ptonline.com/articles/200904ts1.html p://www.ptonline.com/articles/200904ts1.html /articles/200904ts1.html Page 2 of 3 4/7/2009 4/7/2009 Many processors feel that insulating the mold from the platen solves this problem, but it just extends the time before thermal differences between upper and lower platen appear. The insulator actually still conducts heat to the platen, though at a slower rate. Molders should put the insulator board between the clamping plates and the mold, not between the clamping plates and the Troubleshooting - Injection Molding: How to Solve Uneven Clamping between the bottom tiebars. As the moving platen is pushed toward the fixed platen to close the mold, the now pushed-out upper tiebars can restrict the movement of the moving platen. If this is occurring, a telltale sign is galling or scratches on the sides of the tiebars. The solution is to keep the platens relatively cool. Page 2 of 3 due to uneven application of clamp pressure. Toggles apply pressure in the corners, hydraulic clamps do so in the center. Many processors feel that insulating the mold from the platen solves this problem, but it just extends the time before thermal differences between upper and lower platen appear. The insulator actually still conducts heat to the platen, though at a slower rate. Molders should put the insulator board between the clamping plates and the mold, not between the clamping plates and the platens. Another way to prevent this problem is to actively cool the platens. Molders can gun-drill the clamping plates for cooling lines and control them to a moderate temperature. It is rare to see a machine with cooling lines in the platens, but if cooled, the platen will not thermally expand as much, and process consistency will be improved as the platens will be thermally stabilized during startups or shutdowns. Gun-drill the clamping plate and use a fluid temperature-control unit (TCU) to keep the platens at or below about 130 F. Tiebars can also be thermally affected by hot air generated during processing. Since hot air rises, the top tiebars may get warmer than the bottom bars. Such a temperature difference may mean that the clamp won’t compress the parting line evenly. For example, if the tiebars are 10 ft long, and the upper tiebars are 15° F warmer than the bottom bars, the top bars would “grow” 0.012 in. longer than the bottom bars. This creates a difference in clamp force top to bottom. Mold cavities may flash due to the top tiebars being hotter and therefore longer than the bottom bars. HOW MUCH FORCE? It is important to know the clamp force required for a job and to use the smallest appropriate machine. Molders should find the minimum clamp force that also minimizes flash. Too much clamping force on a small mold causes the mold to bow in the middle. Less clamping force will provide more uniform pressure. An industry rule of thumb is 2 to 6 tons per square inch of projected area. Easy-flowing plastics should be in the lower range, while stiff-flowing resins like polycarbonate or acrylic are at the higher end of the range. If the nominal wall is thin or there is a long flow path, required tonnages go up dramatically. It may be hard to believe, but platens—as massive as they are—can “wrap” Pressure-sensitive paper shows much more than around a mold, especially if too small a mold is mounted on a large platen. blueing agent would about uneven clamp pressure at Molders should make sure that the mold occupies 70% or more of the the parting line that results in flash. (Pressurex paper distance between tiebars to avoid this. from Sensor Products was used.) Platen “wrapping” can occur regardless of whether the clamp force comes from a toggle or from hydraulic cylinders. It occurs also with tiebarless presses and with hydromechanical, two-platen types in which hydraulic cylinders pull, rather than push, the platens together. For toggle clamps, the moving platen “feels” the clamping force in the corners, while for a fully hydraulic clamp, the force is applied in the middle of the moving platen. You can see evidence of platen wrap when the mold is removed from the press and there is loose rust on the platen where the mold hung. This rust lies on the surface of the platen, not pounded into it as would be expected. If a mold has been in a press for more than a few hours, when it is removed you see only the outline of the mold. Platen bowing can lead to cracked cavities due to metal fatigue. The problem is so prevalent that special “negative-pressure” TCUs can be used that suck coolant through the mold rather than push coolant through it. This keeps water from getting into the cracked cavity and ruining the part. Another solution taken by some mold builders is to cut off the corners of a mold in an attempt to get the clamp force more evenly distributed across the mold face. This provides a bit better clamp pressure uniformity, as the corners are not the pressure points. Also, mold builders will “pre-stress” the mold with center support pillars that are longer or taller than side pillars by 0.002 to 0.004 in. The mold will bow slightly prior to clamp-up and injection. About The Author John W. Bozzelli has taught seminars on plastics design and processing for more than 30 years. He has extensive experience in polymer development and processing from more than 20 years with Dow Plastics. He is the founder of Injection Molding Solutions/Scientific Molding in Midland, Mich., a provider of in-plant training, troubleshooting, and consulting services. Tel: (998) 832-2424 or e-mail: [email protected] PT Home | Research Zones | Suppliers | Materials Database | Machinery Database | Articles | Resin Prices | News | Subscribe Plastics Technology - April 2009 http://www.ptonline.com/articles/200904ts1.html 4/7/2009
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