1. ITEC 142 Polymer Processing Chapter 12: Injection Molding (Operations and Costs)
Professor Joe Greene
CSU, CHICO
Mfgt 142

2. Chapter 12: Injection Molding Operations
Overview
Costs
Operations and Control
Troubleshooting and Quality Improvement
Part Design and Shapes
Coinjection Molding
Injection Compression

3. Injection Molding Costs
Many methods are used to determine the cost of injection molded part
rough estimates based upon rules of thumb or experience
extremely detailed analysis based on costs for numerous plant functions
spreadsheet based analysis, IBIS and Associates
Pocket knife example
Table 11.3: Cost estimating form for injection molding
Appendix: Blank Form
Part 1: Introduction Section
Part name, customer, molder,
Tool source, date, estimator, approver

4. 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 * $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 = 4 grams.
Cost = $1.38 * 4/454 lbs = $ per part
Factory costs represent convenient price figure and is a factor*material costs
Example: Factor = Then, factory costs = 1000 * $ = $12.00

5. 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

6. Injection Molding Costs
Part 5: Machine Costs
Cost of machine is dependent upon the time the machine is in use to make parts and whether the machine has an operator or not.
Each machine will have two rates, e.g., automatic or manual.
Rates are determined from
Original cost of machine
Ongoing operations costs
Special equipment costs for particular jobs, e.g., special controllers or chillers
Cycle time
Example, Knife
Cycle time is 30 seconds yields 240 parts per hour (120 per cavity)
Hourly rate is $25 for manual with operator, and $15 on automatic
Knife example needs operator to cut runner and sprue off part
Cost = $25 per hour / 240 parts per hour * 1000 = $104.17

7. 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

8. 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 = $
General Administration Costs = 10% = $144.35
Marketing and Profit = 20% = $288.70
Total Cost per 1000 parts =$1,876.16,or $1.88 per knife

9. Operations and Control
Cycle Time
Key Parameters to control the injection molding process
temperature of the melt
temperature of the mold
pressure of injection and hold
injection time, dwell time, freeze time, and dead time

10. Temperature Control Mold Temperature 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 temperatures

11. 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.

12. Important Times Freeze Time Ejection (Dead) Time
Dwell Time (Hold Time)
The time that force or pressure is applied to the cavity after the cavity is full.
Freeze Time
The time interval after the pressure is relieved and before the mold opens. Time is governed by the time required for the part to harden so that it can be ejected.
Ejection (Dead) Time
Time to open the mold,
Eject the part,
Add inserts if needed, and
Close the mold.
Improved with the use of mold release.
Improved with pick robots or other automation

13. Trouble Shooting
Table 11.2 outlines common defects and some suggestions

14. 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.

15. Part Design Thermoplastic parts cool 400 F to 500 F in 10 seconds.
Shrinkage is a big concern. Differences in shrinkage in a part caused by different thickness is a leading cause of warpage, sinks, and voids.
The injection molding tool dimensions are increased to account for part shrinkage.
Mold shrinkage is the difference in the part dimensions and the dimensions of the mold.
Mold dimension = part dimension*(1 + shrinkage value)

16. 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.