General design guide for injection molded part

  • Design Process
  • Guidelines for injection molding[after Douglas M. Bryce, Plastic injection molding : manufacturing process fundamentals, Society of Manufacturing Engineers, 1996]
  • Parameter Change versus Property Effect

  • 1. Design Process [after Robert A. Malloy, Plastic Part Design for Injection Molding, Hanser Gardner, 1994]
    There are a number if different approaches that can be taken when developing a new product. Historically, new parts or products were developed using a Sequential Engineering approach to design, which begins with a new product idea generated by marketing groups and ends up with the manufacturing stage. The problem with this approach is that it may not result in an optimum design, and will no doubt take more time and money than would be required using a more Concurrent Engineering approach.
    The concept of concurrent engineering can be illustrated by the following figure. This "Parallel" or "Concurrent Engineering" approach to product design reduces development time, improves quality, and minimizes the potential for unanticipated production or performance problems.
    Concurrent Design Process for Plastic Injection Molded Parts (after Malloy)

    2. General Guidelines for injection molding[Douglas M. Bryce]

    (a). Determine required clamp force

    clamp force=projected area * injection pressure

    The projected area of the part can be found based on the geometry of the part. The thickness is important only if it is more than 1 inch.  For every inch of thickness over 1 inch, the total clamp force must be increased by 10 percent.

    The injection pressure must vary with the flow ability of the material. The typical value for injection pressure is between 2 to 8 tons per square inch (or 28,000~110,000 KPa). As a rule of thumb, 4 or 5 tons/inch^2 (55,000~69,000 KPa) can be used for most products. For example, if polycarbonate has been selected, then the injection pressure could be 5 tons per square inch.

    Too low a clamping force can lead to flash, or non-filled parts; too high a clamping force can lead to mold damage.
    (b). Determine the injection molding cost for a specific product
    For calculating actual manufacturing cost, the following information is needed:

    1. Material cost.

    The material cost can be determined using the following three-step formula:

    (1). Determine the total volume of the part (inch3).

    total volume = volume of the runner system + volume of the part.

    (2). Determine the weight per unit volume (lb/inch3).

    (3). Determine the cost per unit weight.

    cost= (cost / lb) * (lb / inch3) * (inch3)

    2. Machine cost.

    The total machine cost is determined by the machine hourly rate (the hourly cost of machine and operator) and overall cycle time (so-called gate-to-gate cycle time).

    The dominant effect in determining cycle time is the time it takes to cool the part, and cooling time depends on wall thickness. The cooling time for a plate-like part of thickness, h, can be estimated using this formula: [Tim .A. Osswald]

            Tm-melt temperature
            Tw-mold(wall) temperature
            Td-average part temperature at ejection
           -thermal diffusivity

    The following table provides some information about the wall thickness and corresponding overall cycle time.

    Wall thickness, in. (mm)
    Overall cycle time, seconds

    Wall thikness, the melt temperature, and mold wall temperature as well as the final part temperature when it is ejected all have effects on the cooling time. The melt temperature is usually available from the manufacturer. The suggested melt temperature and mold wall temperature are listed in the tables below.

    Material Melt Temperature(F)
    Acetal (copolymer)  400
    Acetal (homopolymer) 425
    Acrylic 425
    ABS (medium-impact) 400
    Cellulose acetate 385
    Nylon (Type 6) 500
    Polyallomer 485
    Polyamide-imide 650
    Polyarylate 700
    Polybutylene 475
    Polycarbonate 550
    Polyethylene (Low density) 325
    Polyethylene(High density) 400
    Polypropylene 350
    Polystyrene (general purpose) 350
    Polystyrene (medium-impact) 380
    Polystyrene (high-impact) 390
    PVC (rigid) 350
    PVC (flexible) 325

    Material Mold Temperature(F)
    Acetal (copolymer) 200
    Acetal (homopolymer) 210
    Acrylic 180
    ABS (medium-impact) 180
    Cellulose acetate 150
    Nylon (Type 6) 200
    Polyallomer 200
    Polyamide-imide 400
    Polyarylate 275
    Polybutylene 200
    Polycarbonate 220
    Polyethylene (Low density) 80
    Polyethylene(High density) 110
    Polypropylene 120
    Polystyrene (general purpose) 140
    Polystyrene (medium-impact) 160
    Polystyrene (high-impact) 180
    PVC (rigid) 140
    PVC (flexible) 80

    (c). Estimate the gate-to-gate cycle time

    The following table shows the typical time estimates. The actual cycle time is less than the sum of these values, because there are overlaps between some operations. As shown, the cooling time is the most important part.
    Average Value(second)
    Gate closing time
    Mold closing time
    Initial injection time
    Injection hold time
    Cooling time
    Screw return time
    Mold open time
    Ejection time
    Part removal time
    Mold inspection,clean,spray,etc.
    (d). Determine the wall thickness

    (1). Design guidelines:

    (2). The wall thickness is mainly determined by the flow ability of the plastic. The ability to flow also determines how far a plastic can be injected for a specific wall thickness of product.

    The approximate maximum flow-path-to-thickness ratio of some common thermoplastics are listed here:

    ABS 175:1
    Acetal 140:1
    Acrylic 130~150:1
    Nylon 150:1
    Polycarbonate 100:1
    Polyethylene(low density) 275~300:1
    Polyethylene(high density) 225~250:1
    Polypropylene 250~ 275:1
    Polystyrene 200~250:1
    Polyvinyl Chloride, rigid 100:1
    The following table is a listing of common materials and the wall thickness they can flow through.

    This table shows that an easy-flow material (such as crystalline nylon) allows thinner wall.

    Material Minimum (in./mm) Maximum (in./mm)
    ABS 0.030/0.762 0.125/3.175
    Acetal 0.015/0.381 0.125/3.175
    Acrylic 0.025/0.635 0.250/6.350
    Nylon(amorphous) 0.030/0.762 0.125/3.175
    Nylon(crystalline) 0.015/0.381 0.125/3.175
    Phenolic 0.045/1.143 1.000/25.400
    Polycarbonate 0.040/1.016 0.400/10.160
    Polyester(TP) 0.025/0.635 0.125/3.175
    Polyester(TS) 0.040/1.016 0.500/12.700
    Polyethylene(HD) 0.020/0.508 0.250/6.350
    Polyrthylene(LD) 0.030/0.762 0.250/6.350
    Polypropylene 0.025/0.635 0.300/7.620
    PPO 0.030/0.762 0.400/10.160
    Polystyrene 0.030/0.762 0.250/6.350
    PVC 0.040/1.016 0.400/10.160
    (e). Determine the runner system

    (1). General guidelines for runner and gate design:

    - The runner cross section diameter also depends upon the type of plastic being molded. High-viscosity(very stiff) materials require larger-diameter runners than low-viscosity materials.
    - The longer the flow path the plastic must travel along, the larger the runner diameter must be at the start.
    - Right-angled turn in a runner system requires an additional 20% increase in the diameter to compensate for pressure drops.
    - A part should be gated into its thickest section, from thick to thin, never the reverse.
    - Cavity sets should be located as close to the sprue as possible to minimize travel time and distance.
    (2). Runner diameters for some common materials
    Runner Diameter (in./mm)
    Runner Length(in./mm) 
    3 /76.2
    Runner Length(in./mm) 
    Runner Length(in./mm)
    ABS 0.093/2.4 0.109/2.8 0.156/3.9
    Acetal 0.062/1.6 0.093/2.4 0.125/3.1
    Acrylic 0.125/3.1 0.156/3.9 0.187/4.7
    Cellulose acetate 0.093/2.4 0.109/2.8 0.156/3.9
    Cellulose acetate butyrate 0.093/2.4 0.109/2.8 0.125/3.1
    Ionomer 0.062/1.6 0.093/2.4 0.125/3.1
    Nylon 66 0.062/1.6 0.078/1.9 0.093/2.4
    Polycarbonate 0.125/3.1 0.156/3.9 0.203/5.1
    Polyethylene 0.062/1.6 0.093/2.4 0.125/3.1
    Polypropylene 0.062/1.6 0.093/2.4 0.125/3.1
    Polyphenylene oxide 0.125/3.1 0.156/3.9 0.203/5.1
    Polyphenylene sulfide 0.125/3.1 0.156/3.9 0.203/5.1
    Polysulfone 0.156/3.9 0.187/4.7 0.218/5.5
    Polystyrene 0.093/2.4 0.109/2.8 0.125/3.1
    Rigid PVC 0.125/3.1 0.187/4.7 0.250/6.3
    (3).Hot runners
    The purpose for hot runner system is to reduce the overall cycle times. The advantage of the hot runner system is that the runner does not have to be included in the calculation of cycle times. The cooling portion of the molding cycle only applies to the molded part and the overall cycle can be much shorter than if runners were included.
    3. Parameter Change versus Property Effect [Douglas M. Bryce]
    What is the best setting for the injection pressure, back pressure, melt temperature and mold temperature, etc.?
    -It all depends on the material being molded and the type of mold being used, as well as the status of the injection machine and environmental conditions. Generally, the effect of parameters on the product properties would be:
    Property Effect
    Injection Pressure(+) Less shrinkage, higher gloss, less warp, harder to eject
    Injection Pressure(-) More shrinkage, less gloss, more warp, easier to eject
    Back Pressure(+) Higher density,more degradation, fewer voids
    Back Pressure(-) Lower density, less degradation, more voids 
    Melt Temperature(+) Faster flow, more degradation, more brittle, flashing
    Melt Temperature(-) Slower flow, less degradation, less brittle, less flashing
    Mold Temperature(+) Longer cycle, higher gloss, less warp, less shrinkage
    Mold Temperature(-) Faster cycle, lower gloss, greater warp, higher shrinkage