Injection Molding Principles

Plastics Advantages Disadvantages Light Weight Complex Parts - Net Shape Variety of Colors (or Clear) Corrosion Resistant Electrical Insulation Thermal Insulation High Damping Coefficient Creep Thermally Unstable- Can’t withstand Extreme Heat U-V Light Sensitive Difficult to Repair/Rework Difficult to Sort/Recycle Plastics and plastic processes are widely used in manufacturing and becoming even more so. They have a number of excellent features, including... Light Weight-- Have exceptional strength to weight ratios in some cases (e.g. Kevlar-- although the strongest often have limited formability). However, despite the light weight they are all subject to creep Plastics are generally very good insulators-- electrical, thermal, and vibrational Plastics do not corrode as metals do, However they do degrade under UV light Moreover, they generally can not be reworked. If broken plastic can’t be welded and if misshapen, parts can’t be corrected, but must instead be melted and redone. Plastics are not magnetic, thus it is difficult to separate and recycle. Today there is an emphasis on design for manufacturability (Boothroyd and Dewhurst) which generally leads to a reduction in total number of parts. In general, if fewer parts are intended to to the same job, the parts must be more complex. Plastics (in particular processes such as injection molding) are well suited to the added part complexity.

Variety of Plastics & Plastics Processes Two basic types of plastics Thermoset- Heat hardening/ Undergoes chemical change Thermoplastic- Heat softening/ Undergoes physical change A wide variety of plastic manufacturing processes exist Extrusion Expansion Thermal Forming Lamination (Calendering) Foaming Spinning Casting Solid-Phase Forming Molding Two types of plastics Thermoset- NOT recyclable, permanent change w/ heat Thermoplastic- is recyclable, temporary change returns to its original state with removal of heat A wide variety of manufacturing processes exist- Extrusion- melt pellets, push through die Expansion (insul/packing mat’l) expandable polystyrene bead fill mold & bond (steam) Lamination- Stack layers onto one another Thermal Form. heat and pressure (vacuum & pressure forming) Foaming- liquid chemicals- combine&cure (isocyanate polyal) Molding- heat and pressure- 5 types Compression heat/pressurize large billets to fill mold (no inserts) Transfer- billet in separate tank, pushed into mold (inserts) Blow Mold- Extrude paracem- blow to fill mold Rotational Mold- Heat and spin to fill mold- start w/ powder (good fill) Injection (IM)- Force (plunger/plunger&screw) into mold NOTE: ITALICIZED TERMS are NOT covered in the VIDEO Reaction Injection Molding (RIM) (in same general category are Liquid Reaction Molding- LRM or Liquid Injection Molding- LIM)- involves thoroughly mixing two reactive liquids, then injecting into mold. Initial injection pressures typically much lower than traditional IM. Also notice the process known as calendering- basically sheets of plastics are laminated onto one another by rolling through heated rollers. Plastic Casting Processes work about the same as any other casting- Plastic is placed in the mold and then hardened into a rigid article. Spinning produces plastic fibers and is quite similar to extrusion. Solid phase forming involves forming the plastic below the glass temperature and bears many similarities to stamping, forging, etc. Thermoplastics- Injection Molding, Extrusion, Blow Molding, Calendering Thermosets- Compression Molding, High Pressure Lamination, Reaction Inj Molding Transfer Molding Blow Molding Injection Molding Compression Molding Rotational Molding Reaction Injection Molding

Injection Molding Basic Process-- Heat plastic to beyond Glass Transition Point Close mold, creating a fully enclosed cavity Pressurize plastic melt and inject into mold cavity Allow plastic melt to solidify, while keeping mold filled Open mold and eject part Key issues for design of part and mold 1) Filling Mold & Holding Pressure Mold layout designed to enhance fill (use good fluid flow principles) After part is filled, packing pressure is maintained, so part will not shrink away from walls as it solidifies 2) Ejecting Part Part should hang on moving side as it retracts (pulling free of fixed side) Ejector pins then push part out of moving side of mold Taper or draft required to ensure ejection Injection molding began as an industrial process with John and Isaiah Hyatt in 1872. They won a prize for $10,000 for the development of a material (cellulose nitrate) which could replace ivory in billiard balls. It is the most common of the plastic forming processes today, accounting for approximately 30% of all plastics produced. Basic working of injection molding is as follows- plastic material is heat to glass point (or beyond) and pressurized. Mold is closed. Pressurized plastic melt is forced into mold. Mold remains closed while part hardens. Mold opens, part is removed and process repeats. The key points to this are getting the plastic to fill the mold and keeping it there, and then ejecting part after it has solidified. To help ensure good fill, you must employ good fluid flow principles. This means we fill parts from thick to thin. In other words put the plastic in the thickest section and flow to the thinner sections. Don’t start in a thin section and move to the thick sections. Also try to avoid sharp corners in the runner system; use nice rounded curves. To allow for ejection one must include some taper or draft in the part. Typically between 1/2o to 3o is required for this.

Injection Mold Layout The plastic melt flows from the injection nozzles and enters the mold at the sprue. From the sprue the plastic can flow into the runners and ultimately through the gates into the part. Gate and runner design is an important part of the mold design. To help ensure that the mold fills you want to balance the mold so that all cavities fill at the same time. When the cavities are the same, a symmetric layout is used. If the cavities are all markedly different, often the gates and runners must be sized/shaped differently in order to allow all cavities to fill in the same amount of time. Plastic flows from injection nozzle into sprue then into the runners and finally through the gate into the part Want to balance the mold so that all cavities fill at same time

Plunger Type Injection Molding Press Hopper Shooting Pot Torpedo Plunger Nozzle Band Heaters The first injection molding press was used to make blanks for dental plates. The original presses were plunger type presses. These presses are simple and relatively inexpensive In the RDPL we have a 20 ton Morgan Press which is a plunger type injection molding press. The Morgan Press costs about $15,000. Here the plastic is fed into the mold when a cylinder plunger extends and forces the plastic into the mold. After the plunger retracts more material can be fed from the hopper to the shooting pot. (Thus the stroke of the plunger determines the additional material fed in each time.) Of course the shooting pot is long enough to hold several shots, so the plastics stays in the pot for a while, giving the band heaters time to heat and melt the plastic. Notice the torpedo, which is basically an obstruction to the plastic flow in the shooting pot. As the plastic moves around the torpedo, it is better mixed. Material is stored (in pellet form) in the hopper Band heaters heat material as it moves through shooting pot Stroke of the plunger meters the shot size

Screw Type Injection Presses Hopper Screw meters plastic, plunger provides pressure Nozzle Band Heaters Reciprocating Screw Shooting Pot Virtually ALL industrial presses are screw type presses Added benefits of screw 1) Larger throughput 2) Obtain a more homogeneous melt (better mix) 3) More consistent from shot to shot 4) Added heat to melt- from action of screw The original plunger type has had one important modification. A reciprocating screw now forces material into the mold. This screw action ensures that the same amount of material is always metered in, and it is packed equally dense along the length of the screw. Additionally the material will be much better mixed by the screw action, all of which helps to maintain much more consistency from shot to shot. Since the screw action generally helps to pack the material in better, a given plunger travel will push more material into the cavity. Finally the action of the screw as it rotates and mixes, adds energy to the melt. (Band heaters are still needed to fully heat melt, however.) All of this results in a much better and more consistent part. This is why the screw press is essentially the only press found in industry. (Although small plunger presses are still made for prototype/lab purposes, such as the RDPL.)

Press Parameters 3 parameters commonly used to describe press capacity 1) Clamping force- Force available to hold platens together (tons) Can be from Hydraulic/Pneumatic Cylinder Mechanical Toggle Clamp 2) Shot size- Amount of material that can be transferred to mold in a shot (given in either cm3 or ozs) 3) Injection Pressure- Maximum pressure that can be developed at the sprue to force the plastic into the mold cavity Generally, when one hears the about the capacity of an injection molding (IM) press, one hears the maximum clamping force. Thus presses are talked about as being 20 ton, 50 ton, etc. Additionally, we are also concerned with the shot size and the injection pressure that the press can generate. Shot sizes are usually speced in cubic centimeters or ozs. (Not entirely sure why one set of units specs a volume and the other specs a mass, but English spec for shot size is typically given in ozs, where ozs represent 1/16 of a pound, not 1/16 of a pint). Finally one “specs” a press in terms of the maximum pressure it can generate at the sprue in order to force (inject) plastic into the mold.

Injection Molding Defects Short Shot- Insufficient material put in cavity/ material solidifies too soon Ejector Pin Marks- Ejector pins not flush/ undersized or plastic too soft at ejection Weld Lines- Occur wherever flow paths come together Sink Marks- Material shrinks away from walls of mold cavity Residual Stresses- Generated by constraining part while cooling Parting Lines- Plastic seeps through seam between mold sections Jetting- Melt moving rapidly, cools unevenly and traps flow lines Flashing- Material overflows cavity (too much material, not enough clamp force) These are some of the more common Injection Molding Defects. When designing a part and the associated mold you should keep these defects in mind as well as your fundamental objectives (fill mold & hold while solidifying, then remove part) in mind. (Note that many of the defects arise from not fulfilling the objectives.) Discuss in more detail short shot-- no extra slide but bring several examples Short shot occurs when there is insufficient material to fill the mold cavity. It has several causes, including insufficient injection pressure, or insufficient time allowed during the injection process. Sometimes the material will freeze in a given section before it can reach the edges of the mold. Also discuss flashing in more detail-- no extra slides but bring some examples. Flashing occurs when there is too much material and it pushes its way out of the die. This can be caused by too much injection pressure, too much injection time, or insufficient clamping force. It also can be caused by a poorly machined die that does not properly seal off the cavity. Final note that the cause of sink marks, warping, and residual stresses are related. As the part cools it tries to shrink away form mold walls, other features, etc. (It shrinks as it cools.) This can result in sink marks as the material is able to shrink away from other material or from walls of mold cavity, etc. It can also result in warping or in residual stresses as material is held by geometry and not able to freely alter its shape and size with the changes in temperature and state.

Short Shot Short shot occurs when there is insufficient material to fill the mold cavity. It has several causes, including insufficient injection pressure, or insufficient time allowed during the injection process. Sometimes the material will freeze in a given section before it can reach the edges of the mold.

Flash Part w/ Moderate-Heavy Flash Flash Flashing occurs when there is too much material and it pushes its way out of the die. This can be caused by too much injection pressure, too much injection time, or insufficient clamping force. It also can be caused by a poorly machined die that does not properly seal off the cavity. Flash

Weld Lines Weld lines are created when two flow fronts come together in the mold. Can result from poor gate placement Unavoidable w/ Solid Cores Gate Weld Line Gate Gate Weld lines are caused when flow fronts meet. If you have a part with a solid core weld lines are unavoidable. In some cases, however weld lines are avoidable. Generally they should be avoided if possible. Thus the solid square pictured above should only be gated in one place. In addition to being aesthetically unappealing, weld lines are also weaker than the rest of the part. The cooler the fronts are when the meet, the less the plastic will be able to “KNIT” together, and so the weaker the plastic will be (weld lines will be more pronounced). Amount the plastic cools is directly related to how far it must travel. For this reason multiple gates are sometimes used on parts with solid cores. There will be more weld lines, but each one will be stronger. Note that while weld lines are unavoidable when filling parts with solid cores, things can be done to maximize the knitting and thus minimize the weld line defect. (Keep sections hotter when they meet.) Weld Line Weld lines cause a decrease in the strength of the part The cooler the fronts are when they meet, the more pronounced the weld lines will be

Jetting Jetting occurs at high fill rates when there is a large open space between the gate and the opposite wall Material stream shoots to the opposite wall and freezes. Stream of fresh material then folds over this and freezes, trapping flow lines of material. Air can often be trapped between folds. Jetting To reduce the chance of jetting, gates should be located so that entering material flows into wall, not into open section. Alternate gating eliminates jetting

Ejector Pin Marks Marks are often left on the part in the area where the ejector pins were. These marks have three possible causes 1) Pin above flush 2) Pin below flush 3) Clearance around pin To some degree, these marks are unavoidable, thus one should try to place the ejector pins on hidden areas of the part. Another defect occurs when one tries to eject the part before it solidifies and pins push through the part

Sink Marks Sink marks occur at excessively thick wall sections, or where there are abrupt changes in thickness- thick sections solidify too late and shrink away from the wall Proper design eliminates unneeded material (ribs, core out sections) Thick sections cause sink marks Bad Design Improved Design When unavoidable, sink marks can be masked by surface texture

Runner Design 1) Keep runners as short as possible 2) Use a cold well at the end of each branch- collect solid chunks 3) Use cross sections that minimize perimeter for a given area Circles are best followed by rounded trapezoids and then trapezoids 4) Use good flow principles (round corners, etc) 5) Size the (effective) diameters of runners based on length Determine total flow length from sprue to gate Use table to determine needed diameter for this length- this value is minimum diameter at END of runner Increase the diameter of the upstream runner by 20% at each 90o turn When n multiple runners branch off a main feed runner, the diameter of the main runner should be increased by n*20%

Runner Sizing Table 2 “Secrets of gate, runner, and vent design,” Douglas M. Bryce. Plastics Design Forum. Sept 94

Gate Design 1) Gate so that material flow hits a nearby wall 2) Gate into the thick sections- so material flows thick to thin 3) Gate into an understressed region of the part (Gates introduce residual stresses as they freeze before other sections) 4) “Hide” gates whenever possible 5) Size gates based on (maximum) wall thickness of part Also depends on how well the material flows (viscosity) Determine relative material viscosities by comparing the minimum allowable wall thicknesses for different materials

Gate Sizing w d l “Secrets of gate, runner, and vent design,” Douglas M. Bryce. Plastics Design Forum. Sept 94

Venting Vents allow air to escape ahead of the plastic melt d=depth w=width l=land Vents allow air to escape ahead of the plastic melt Vents help reduce-- Injection pressure and clamping force Cycle time Warping & shrinking Residual stresses

Vent Sizing and Frequency Note that runners should be vented if possible Vent 30% of runner length on both sides. Vent 20% of cavity perimeter if runners are vented. By vent 20% of the cavity means that if one sums the width of each vent and divide the result by the total perimeter of the cavity, the ratio should be around 20%. Vent 30% of cavity otherwise “Secrets of gate, runner, and vent design,” Douglas M. Bryce. Plastics Design Forum. Sept 94

Mold Design Example The part shown on the right is to be injection molded. 16 pieces should be made in each shot. (Material is polycarbonate). Design the mold. 1) Layout The configuration shown on the right keeps the runners small, with few bends, and shoots the melt into the mold wall. One weld line will be present on each part, but this is unavoidable.

Example- Runner Design 2) Runner Size Total Runner Length= 1.2”+7.5”/2+3.8”=8.75” From table EFFECTIVE diameter is 0.203.” For a flat trapezoid, with an angle of 15o, and height and width chosen to minimize the perimeter for a given area... h=0.871*(0.203”) = 0.176” w=1.135*(0.203”) = 0.230”

Example- Upstream Runners Recall that these dimensions (effective diameter = 0.203”) are for the runners nearest the gate Four of these cluster runners (n=4) branch off of each feeder. Effective diameter=(1+n*0.2)*dbranch dfeed = (1+4*0.2)*(0.203”) = 0.366” h=0.871*(0.366”) = 0.319” w=1.135*(0.366”) = 0.415” Two of these feeder runners (n=2) branch off of the main runners. dmain = (1+2*0.2)*(0.366”) = 0.512” h=0.871*(0.512”) = 0.446” w=1.135*(0.512”) = 0.581”

Example- Gates Wall thickness is given as 0.100”- t=0.100.” From tables, the minimum wall thickness when injection molding Polycarbonate is 0.040,” which is comparatively high. This indicates that Polycarbonate does not fill or flow as well as most plastics. Select d=0.7*t=0.7*(0.100”) = 0.070” Select w=2.75*d=2.75*(0.070”) = 0.193” Select l=0.030”

Example- Vents Mold cavity vent depth dcv=0.002” Runner vent depth drv=0.004” Choose wv=0.125.” Choose lv=0.045.” Spacing between vents on runners is 0.42” Spacing between vents in cavity is 0.500”

Estimating Clamping Force Clamping force is needed to overcome the force exerted by the pressure of the plastic melt against the mold cavity. A reasonable estimate of clamping force may be obtained by multiplying the pressure by the area of the mold in the parting plane. Assume that 1/2 of the pressure drops uniformly through the runner system. Assume that the remaining pressure holds constant in the mold cavity. (Note that these assumptions are very conservative.) Typical Injection Pressures (kpsi) Polycarbonate (Lexan) 16 Phenylene Oxides (Noryl) 14 Polyester (Valox) 10 ABS (Cycolac) 10 (From GE Product Guides)

Example- Clamping Force Force for previous mold 1) Mold Area Treat parts as perfect squares A=2”*2”-1.8”*1.8” A=0.76 in2 A = 16*0.76 in2 = 12.2 in2 2) Runner Area Main: (7.5”)*(0.581”)= 4.4 in2 Feed: 4*(3.8”)*(0.415”)= 6.3 in2 Branch: 16*(1.2”)*(0.230”)= 4.4 in2 A = (4.4+6.3+4.4)in2 = 15.1 in2 3) Typical Polycarbonate Injection Pressure: 16000 psi 4) Clamping force F=0.75*(16000 psi)*(15.1 in2 )+0.5*(16000psi)*(12.2 in2 ) F=280000 lbs = 140 tons

Project 3 To injection mold a Drexel keychain in Hess Lab. Select plastic material for the keychain (read Chapter 10-5 to 10-8) Select color for the keychain (red, blue and yellow) Make 50 good quality key chains by adjust pressure, temperature, time, etc. on the machine Logo design for the keychain Prepare 10 min. power Point presentation and detail report for this project

Available Plastic Materials in Hess Lab 1. Nylon Resin (Zytel) 2. Polyethylene (PE) 3. Acetal Resin (Delrin) 4. Polycarbonate Resin 5. Polyvinyl chloride, PVC 6. Polypropylene 7. Polyetherimide 8. Elastomers