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Purchasing Basics: Injection molding machines

 

Matching a molding machine to the jobs you will run is a key in the selection process.
Photo: Engel
 

Sometimes you go to purchase a molding machine for a specific job; sometimes you are looking for a machine to give you additional capacity for future parts, exact size and configuration unknown. Probably one of the most difficult aspects of purchasing a molding machine, particularly if it is not for a specific job, is ensuring that the quotes you solicit from different machinery manufacturers are comparable. Therefore, the more preparation done ahead of the request for quotation, the better the chance that the quotes will be comparable. Also, it is useful to explain the reasons behind certain unusual specifications so that you can be sure the machine you buy will meet your needs. 

One good place to start is looking at any deficiencies of your existing equipment, in terms of performance, maintenance costs, downtime, or supplier service. Let's assume you know the application that's going to run in the machine. The next critical step is to get accurate data on the tool and the part. The mold's outside dimensions will affect the size of the machine's platens, and the projected area of the part (in square inches) determines the injection pressure and the size of the injection unit required. If you're planning to run several molds in the machine, make a spreadsheet that includes all the molds, and then use the largest dimension for each mold, except for wall thickness. For that, use the part wall thickness with the smallest measurement as your guide. These are the dimensions you will need: 

When to add capacity
  • Using the rule-of-thumb method, a fairly standard annual capacity number is 70 percent. If you are looking to demonstrate the need for adding capacity, take the ideal number of available hours (hours per day available times number of days running per year) and reduce that ideal by multiplying by .70. This number takes into account a stable operation (one that is not increasing capacity by adding days or shifts), routine maintenance for tooling and machinery, and may leave a little room for the unplanned. This number is conservative if your shop has long runs of the same product. Conversely, a short-run shop with multiple material and tool changes may work hard to reach 70 percent.
  • The complete capacity or cost/benefit analysis approach uses as much information as you can gather about a specific project. Cost items to consider include the following: the initial purchase price; loan, buy, or lease costs; operating expenses (electricity, shop overhead for the specific area of the new equipment, lubricants, and fluids); spare parts inventory; shipping, rigging, and installation costs; new auxiliary equipment costs; and training expenses. Using these items instead of a standard machine rate requires more work but it separates costs into categories that can be studied for specific issues rather than setting press expense rates.—R. Caufman, RC Marketing Inc. 
  • Mold width. 
  • Mold length. 
  • Mold stack height. 
  • Mold opening distance. 
  • Ejector rod spacing. 
  • Center ring diameter. 
  • Part dimensions. 
  • Part wall thickness. 
  • Part flow length. 
  • Part weight. 
  • Runner weight. 
  • Number of parts. 
  • Total shot weight. 
  • Nozzle radius. 
  • Material (and its viscosity).

If it's a multicavity mold, multiply the individual part dimensions by the number of cavities, or total all parts if they are not the same. However, if you are using a stack mold, you need to multiply part surface area by only 1.4 to get the total projected area. Also, remember that if you are planning on robotic ejection, allow for space for the robot to enter the mold and retract the parts in figuring your total daylight. 

With these measurements, you or your supplier can analyze the injection pressure, both in the barrel and in the cavity, and the injection unit size required. The next step is to determine the clamp tonnage, based on defined injection pressure, projected area, and theoretical injection pressure. Once you've determined the clamping pressure, double-check that your mold will fit the tiebar spacing and that the opening stroke will allow your mold to open fully and get the part removed. 

Rules of Thumb
  • If machine cycles are less than 6 seconds, the shot volume should not be larger than 20 percent of the total shot volume (the material needs a minimum residence time to fully melt).
  • To calculate minimum opening stroke of a molding machine, figure the stack height of your mold plus two times the core length, plus any space for an automatic part removal device. 
The last step is to figure out which options are needed. One kind is required to produce and handle the parts. For example, what extra hydraulic functions will you need (such as core pulls, valve gates, inmold ejectors)? What air functions are required (valve gates, part blowoffs, core air, cavity air)? What about extra control functions? Will you need a robot or other auxiliary equipment interface, valve gate sequence control, mold heaters (electric or hot oil), nozzle shutoff, leveling pads, a special screw and barrel, and so forth? Other options are those that would be useful in improving machine operations, such as graphical operator interface, SPC/ SQC charts, quality control computer interface, or production monitoring interfaces (data logging, card readers, printers, cycle analysis, and so forth). 

If you are currently molding this part, pass the appropriate information (current machine specs and any deficiencies you want to overcome) along to your prospective suppliers. 

When quotes are returned, compare them on as equal a basis as possible. Evaluate each vendor's capabilities by looking at how the machines are built, if you can. Check out the machine's performance. Review the machine manuals, and ask how urgent spare parts orders are handled. Find out who provides customer training and telephone diagnostic assistance. Check supplier references. 

Machine cost calculations
At IMM, we get more questions about hourly machine rates than about any other single subject. We bring you this formula developed by Krauss-Maffei in order to help you develop what you should charge. To determine the basic machine cost, put together the information on finance charges, auxiliary equipment cost, maintenance costs, building location costs, electricity usage, water usage, and any other miscellaneous costs. All these costs divided by the operation time will provide you with an hourly rate. See this example for a 150-ton molding machine with a 50 mm screw. Add this figure to labor cost (average hourly rate plus burden), material cost, secondary services, tool cost, overhead, and profit, and you have a basis for quoting. 
 ExampleYour dataYour data
Production data   
Production days/year350____________________
No. of shiftsx 3____________________
Hours/shiftx 8____________________
Usagex 95%____________________
= Production hours/year7890____________________
Machine purchase cost   
Machine$138,100____________________
Options$15,000____________________
Auxiliaries$10,000____________________
Finance cost1%____________________
= Total cost$164,731____________________
Finance data   
Annual interest7%____________________
Depreciation (1/2/3 shift operation: 6/5/4 years)5%____________________
Maintenance cost   
Maintenance (%/year—1/2/3 shift operation: 3/4/5%)5%____________________
Building location costs   
Area required, sq ft82____________________
Factor2.5____________________
Cost (sq ft/month)$5.00____________________
Current consumption costs   
Institutional electrical power (kW)58____________________
Usage50%____________________
Power cost ($/kWh)$.04____________________
Water cost   
Water usage (cu ft/hour)30____________________
Water cost, cu ft$.02____________________
Miscellaneous cost ($/year)$1000____________________
Machine cost per hour   
Depreciation$4.13____________________
Interest.36____________________
Location cost1.54____________________
Maintenance1.02____________________
Power1.16____________________
Water.60____________________
Miscellaneous cost.13____________________
Total machine cost each hour$8.94____________________


Finally, don't forget to ask about rigging or setup costs. What will shipping charges be? Do you need CE certification on the machine? Any other engineering? If you communicate your objectives clearly to your potential supplier, you should end up with a comparable quotation. When the comparison of the less concrete attributes are completed, you should have your new supplier. 

With our thanks . . .
. . . to the following machine suppliers, who helped with material that contributed to this article: Husky, Krauss-Maffei, Niigata, and Wabash. 

Go to the IMM Almanac Online home page.

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