Injection Molding of Polymers Lab and Mold Design Exercise By Daniel Walczyk, 7/22/2010 This lab is broken up into two parts: Part 1 involves determining the shrinkage rate of different polymers that are commonly injection molding into parts in the AML, and Part 2 involves performing a paper design of an injection molding tool set and process. First, general background information and manufacturing science dealing with the injection molding process will be presented. Injection Molding Injection molding uses high pressure to deliver a metered amount of heated and plasticized thermoplastic into a relatively cold mold, which solidifies the plastic material. The process is shown schematically in Figure 1. (a) (b) Figure 1 – (a) Cutaway view of an injection molding machine [www.xcentricmold.com/images/injection-molding-machine.gif] and (b) details of the injection molding cycle [www.milmour.com/milmour/injection_molding_images.asp]. Heat Required to Melt Plastic Pellets The raw plastic, usually in pellet form, is heated by a rotating single-stage screw until melted and then injected into the mold by axial movement of the screw. The heat required, Qmelt, to melt plastic pellets of mass m from their starting temperature, Tp (may not be ambient), to the melt temperature, Tm, can be estimated using , where: cp = specific heat capacity of the plastic. This relationship can be used to determine whether or not the heater capacity of an injection molding machine is sufficient. When a range of Tm values is given for a particular plastic, it is common to pick a melt temperature in the middle of that range. For example, if the suggested range of temperatures were 150-200°C, then Tm = 175°C would be a reasonable choice for the injection molding machine. Mold Clamping Force As the mold is packed with molten plastic, the machine must provide sufficient clamping force to prevent separation of the pressurized mold halves at the parting line, which if not prevented, would result in excessive part flash. The minimum clamping force, Fclamping, can be estimated by , where: pi = maximum injection pressure and Ap = projected area of the molded part at the parting line. Plastic Shrinkage and Mold Expansion Compensation In manufacturing processes, materials that undergo a phase change (i.e. from liquid to solid) involve a decrease in specific volume and a resulting shrinkage. A certain amount of shrinkage is inevitable in any process that involves cooling of plastic from elevated temperature and this is certainly the case with injection molded polymers. The plastic packed into the mold under high pressure shrinks from the time it solidifies at cooling to Tmelt (melting temperature of the plastic) in the mold until reaching room temperature. The shrink rate, S, is used to scale mold dimensions to compensate for shrinkage. Assuming perfectly isotropic shrinkage, the total volumetric shrinkage rate of a material, Sv, is related to the linear shrinkage rate S by the relationship 1 S = 1 − (1 − Sv ) 3 An often-used simplification of this relationship, based on neglecting higher order terms of the Sv binomial theorem expansion of (1 − S ) 3 , is S≈ 1 . 3 Hence, a material exhibiting 3% volumetric shrinkage should be undersize in all directions by 1.01% after cooling. Determining a single shrinkage compensation factor using either of the preceding equations for S is suitable for most applications you will encounter in AML, although this typically not needed because tables of S values are quite common. Even though linear shrinkage rates are available for a wide range of plastics, the reported values are usually given as a range. Shrinkage can vary based on material, processing, part geometry, etc. Furthermore, shrinkage may vary in the direction of flow versus transverse direction (anisotropic shrinkage). For example, Table 1 shows typical shrinkage rates for different polymers. A common practice is to pick the S value in the middle of its reported range. Table 1 – Common Linear Shrinkage Values (S) For Various Thermoplastics (from www.gepolymerland.com/research/tech/tip97dec.html) Material Shrinkage (S) in inches/inch (per ASTM D955) ABS- High Impact .005 - .007 ABS- Medium Impact .005 - .008 ABS-High Heat .004 - .006 ACETAL .020 - .035 ACRYLIC- General Purpose .002 - .009 ACRYLIC- High Flow .002 - .007 ACRYLIC- High Heat .003 - .010 ACRYLIC- Impact .004 - .008 NYLON- 6,6 .010 - .025 NYLON- 6 .007 - .015 NYLON- Glass Reinforced .005 - .010 POLYCARBONATE .005 - .007 POLYESTER .025 - .050 thick .006 - .0012 POLYESTER .050 - .100 thick .012 - .017 POLYESTER .100 - .180 thick .016 - .022 POLYETHERIMIDE .005 - .007 POLYETHYLENE- LDPE .015 - .035 POLYETHYLENE- HDPE .015 - .030 POLYPROPYLENE .010 - .030 PPO®/HIPS (NORYL®) .005 - .007 POLYSTYRENE- Crystal .002 - .008 POLYSTYRENE- Impact .003 - .006 POLYURETHANE .010 - .020 PVC-RIGID .002 - .004 PVC-FLEXIBLE .015 - .030 SAN .002 - .006 To compensate for shrinkage, the cavity dimensions of an injection mold must be made larger than the specified part dimension. The dimension of the cavity is then where: Dc = dimension of cavity and Dp = molded part dimension. For example, if the length of an injection molded polypropylene part is 5.0 inches, the corresponding cavity dimension using the preceding equation and assuming a linear shrinkage value of 1.0% is 5.05 inches. Since the mold is almost always heated, it experiences an expansion from room temperature. Hence, the room temperature dimensions (Dca) of the mold must also compensate for this expansion (i.e. be slightly undersized) using where: α = coefficient of thermal expansion of tool material, Tmold = mold temperature, and Tambient = ambient temperature. It is clear that a mold shape must be designed for each combination of polymer to be injected and the mold material. However, the mold dimensions calculated using S values from Table 1 and the Dc and Dca equations above represent a gross simplification of the part shrinkage issue. There are number of process factors that affect shrinkage, any of which change the amount of contraction experienced by a particular polymer. The most important factors and affect they have on part shrinkage are given below. • Injection pressure – higher pressure forces more material into the mold cavity and shrinkage is reduced. • Compaction time – assuming polymer in the gate does not freeze, more compaction time allows more material to be forced into the mold cavity while shrinkage is taking place, thereby reducing shrinkage. • Melt temperature – higher melt temperatures actually reduce shrinkage, since polymer melt viscosity is lowered allowing more material to be packed into the mold. • Part thickness – Thicker parts shrink more, since there is more molten plastic to contract inside the surface skin formed with thicker sections. Mold Cycle Time Mold cycle time can be estimated by considering the various actions that occur during injection molding, as shown in Figure 1(b), using , where: topen/close = time required to open or close the molds before and after part ejection, respectively; tf = mold fill time including pack and hold; tc = part cooling time after filling mold with plastic; and te = part ejection time. Both topen/close and te are set at the injection molding machine based on experience. However, minimum values for tf and tc can be estimated. For a given flow rate (if not limited by the fluid dynamics of mold filling), the mold fill time is where: Vt = total fill volume of the part and runner system and Qt = total volumetric flow rate from the injection molding machine (not to be confused with Q used for heat required). Estimating the actual mold fill time, especially for a complicated part shape, will require sophisticated simulation software such as Moldflow or SimTech, because of the complicated physics involved (see Figure 2 for behavior of the melt front, for example) and uncertainty about how long the mold must be packed at high pressure. Cooling Lines Melt Front Velocity Thickness Frozen Layers Profile Fountain Flow Figure 2 – Development of a solidified layer during injection mold filling Cooling time depends on the maximum wall thickness in the injection molded part. An estimate of the cooling time require for a part wall of thickness h is given by where: αd = thermal diffusivity of the melt, Tm = melt temperature (temperature of the molten plastic during injection) as before, Tmold = mold wall temperature (temperature that mold is maintained at), and Te = desired ejection temperature of the part. The cooling load, Qcool, for a heater/chiller unit to maintain the mold at a specific temperature, Tmold, can be estimated using . All variables in this equation have been previously defined. Part Ejection Force Ejection pins force the part out of the mold after the part has cooled and solidified enough. As an injection molded part shrinks, it can literally form an interference fit around mold cores, especially if the part draft angle is not sufficient. The ejection force, Fe, can be estimated using , where: µ = frictional coefficient, p = pressure and A = area at the interference interface. PART 1 – Plastic Injection Mold Design and Analysis Design the tooling and injection molding process to make two identical ABS parts (one shown below in Figure 4 with all dimensions in centimeters) using a two-cavity mold. The mold will be machined out of aluminum (α = 23×10-6/°C).. Relevant properties for high impact ABS include: Injection molding temperature range = 180-260°C Melting temperature = 105°C S from Table 1 Recommended molding pressure pi = 100 MPa αd = thermal diffusivity = 9×10-8 m2/sec cp = 1470 J/(kgºC) (a) What’s a reasonable injection molding temperature to use? (b) Calculate and show the mold cavity dimensions (one cavity only at room temperature if the temperature that mold walls are maintained at during production is Tmold = 60°C. (c) Design a suitable mold set with two mold cavities including parting line, sprue, runner system, gates, vent holes and ejector pin details. Hand-draw your final design (dimensions not needed). (d) What is the minimum clamping force required for the injection molding machine? (e) Estimate how long (tc) it will take the part to cool down to an ejection temperature of 80°C. (f) Estimate the heat required to melt the enough plastic for the two plastic parts (neglect runner system for now) in the injection molding screw, and to cool the parts after injection molding to their ejection temperature. (g) Where do you expect the part to bind within the mold during ejection? (h) What major part defect do you anticipate with this part? If the part didn’t have to be solid, how could you minimize the defect? Write a memo responding to each design question above (a-h) including all assumptions and calculations. Figure 4 – Dimensions (in centimeters) for injection molded ABS part. PART 2 – Experimentally Determining Shrinkage Rate of Plastics Part 1 of your Injection Molding of Polymers lab involves determining the shrinkage rate of different polymers that are commonly injection molded into parts in the AML. To accurately predict shrinkage in injection molded parts, accurate shrinkage rates for the plastic your AML team will be using is required. This lab exercise allows you to experimentally determine shrinkage rates for five common polymers using a statistically significant number of test samples. Specifically, the length of injection molded tensile test specimens (ASTM D638), as shown in Figure 3, will be measured. The five polymers and the order in which they will be tested are listed in Table 2. Specimen Length Figure 3 – ASTM D638 tensile test specimen being used to measure polymer shrinkage values. Table 2 – Polymers available in AML to be tested. Polymer Relative Melting Temperature Polypropylene Low Polyethylene Low ABS Medium Acrylic Medium-High Polycarbonate High Specific tasks for this lab exercise are as follows: 1. Measure the length dimension L in the AML tensile test specimen injection mold. Note that this dimension will change as the mold is heated up due to the circulating water. Based on the previous equation, the actual mold cavity length Lc during injection molding will be Lc = L ⎡1 + α (Tmold − Tambient ) ⎤ ⎣ ⎦ where: α = coefficient of thermal expansion for aluminum mold = 12.8×10-6 1/ºF Tmold = mold temperature in ºF (probably the heating water temperature) Tambient = ambient temperature of the AML in ºF. 2. For a particular polymer, injection mold 30 standard tensile test specimens using the AML mold. Measure the lengths of all 30 specimens after they cool to room temperature using a Vernier Caliper, and calculate the average length Lave and standard deviation σL. 3. Calculate the shrinkage rate for the polymer S using the following equation: L − Lave S= c Lc 4. Repeat Steps 1-3 for each polymer. 5. As a group, submit a 2-3 page memo that describes the results of the shrinkage rate measurements, discusses the variability in the data as measured in Step 2, and includes all of the raw measurement data.