Injection Molding - Polymer Materials - Lecture Slides, Slides of Engineering Chemistry

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ITEC 142

Polymer Processing

Chapter 12: Injection Molding

(Operations and Costs)

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Chapter 12: Injection Molding Operations

• Overview

• Costs
• Operations and Control
• Troubleshooting and Quality Improvement
• Part Design and Shapes
• Coinjection Molding
• Injection Compression

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Injection Molding Costs

• Part 2: Resins and Additives Costs
  • Type and grade of resin, e.g., pocket knife is nylon 6,6 (Dupont Zytel)
  • Cost of resin depending upon quantity, e.g., boxcar, gaylord, bag
  • Additives cost, e.g., colorants, fillers, stabilizers, etc. (black is $3/lb)
  • Total Material Cost = (resin cost)(resin fraction) + (additives cost) (additives fraction) - Example, Total Cost = $1.36 * 0.99 + $3.00 * 0.01 = $1.38 nylon and color

• Part 3: Part Costs

• Part costs = material costs plus factory costs
• Material cost is materials plus scrap from runners, sprues, and part rejects.
  • Example: Knife weighs 3.8 grams. The runners, sprues, and scrap is 5%. Total material uses us 3.8 + 0.05*3.8 = 4 grams.
  • Cost = $1.38 * 4/454 lbs = $0.0122 per part
• Factory costs represent convenient price figure and is a factor*material costs
  • Example: Factor = 1000. Then, factory costs = 1000 * $0.0122 = $12.

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Injection Molding Costs

• Part 4: Tooling Costs
  • Type of tooling material, e.g. steel, aluminum, kirksite.
  • Number of cavities in tool, e.g., single cavity has 1 part, dual has 2, multicavity tool can have 4, 6, 8 pieces, or more.
  • Number of slides or lifters in a tool to mold parts that have an internal flange or under cut.
  • Number of years the tool is amortized for tool life, e.g., payoff tool in 1 year, 3 years, or five years. (Every company has different accounting practices.
  • Internal (In-House) tool construction versus External (Outside) tool construction. Internal is usually less expensive per tool but has more overhead, thus need many jobs to reduce overhead costs.
  • Example, knife handle
    • $20,000 per tool (dual cavity, $5K internal, $15K external)
    • Cost per part is $20,000/ 4million parts * 1000 (pieces) = $5 per 1000 pieces

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Injection Molding Costs

• Part 6: Secondary Operations Costs
  • Many parts are subject to other operations costs after molding
    • placed or glued into an assembly, drilling of holes or attachments
  • Rates are determined from some rate and cycle time
    • Rate costs are dependent upon the type of machine used, $ per hour
    • Cycle time, parts per hour
    • Runner and sprue removal are not considered secondary operations since they are removed at the press after molding.
    • Example, Knife
      • Securing the blade to the handle with screws after injection molding
      • Cycle time is 10 seconds yields 60 parts per hour
      • Cost = $7 per hour / 60 parts per hour * 1000 = $19.44 per 1000 pieces
• Part 7: Purchase Items Costs
  • Many items are purchased and included in assembly
    • Example, Knife use Costs of blade and screw purchases
      • Cost of blades = $1250 per 1000 pieces
      • Cost of screw = $2.00 per 1000 pieces Docsity.com

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Injection Molding Costs

• Part 8: Packaging and Shipping Costs
  • Costs for shipping cartons, bags, blister packages, foam materials
  • Costs for transportation can be included
    • Example, Knife
      • Costs for blister packages = $50 per 1000 pieces
      • Cost of carboard box = $0.70 per box that holds 1000 pieces
• Total Factory Costs per 1000 pieces = $1443.
• General Administration Costs = 10% = $144.
• Marketing and Profit = 20% = $288.
• Total Cost per 1000 parts =$1,876.16,or $1.88 per knife

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Temperature Control

• Melt Temperature
  • Settings generally below melting temperature of resin to allow for shear heating of melt.
  • Overheating could cause material degradation and increase cycle time
  • Match heating zones with type of resin.
  • Temperature zones are adjusted to as production continues.
    • Example, Nylon will be run with a high temperature in the feed zone and decreasing temperature as the material is run through the barrel.
    • Caution: Too much heating in feed zone can cause bridging

• Mold Temperature

• Mold can be chilled for polyolefins or heated (140F) for engineering thermoplastics
• Cool molds yield faster cycle times but more stresses
• Important to have the gate be the first point to freeze
• Factors affect temperature of melt and mold
  • Shot size: larger shots take more heat
  • Injection rate: faster filling creates higher melt temperature due to shearing
  • Size of runner: longer runners require higher temperatures
  • Part thickness: thick parts require more cooling time and lower temperaturesDocsity.com

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Pressure Control and Injection Rate

• Injection and Hold Pressures
  • Injection should be with the lowest possible pressure without short shots or incomplete fill.
  • Pack and hold pressures are used to add more material as the part is cooling and shrinking.
  • Pack and hold pressures are much higher than injection pressures.
  • Holding time is kept until part cools below Tg.

• Injection Rate and Fill Time

• Injection rate is time to fill mold.
• Usually resin is injected as quickly as possible to reduce skin thickness. Exceptions for thicker skin exist.
• Long injection times can permit premature freezing.
• Thin parts have fast injection rates.
• Thick parts or small gates have somewhat lower rates.

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Trouble Shooting

• Table 11.2 outlines common defects and some suggestions

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Design and Product Considerations

• Shapes

• Injection molding can mold very complex shapes and can include
  • bosses, ribs, pins, inserts, undercuts, etc.
• These must be added to the part cavity or require slides or lifters.
• Hollow parts can be made with the use of slides.
• Threaded parts can be made with threaded insert.
• Hollow part with a hole on one side can be made with the use of core pins.

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Part Design

• Weld lines are caused by two or more flow fronts coming

together.

• Spring back is caused by internal stresses in part that are

relieved after the part is molded in minutes, hours, or days.