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Description of the Study
To engineer an injection mould construction project: Define tool configuration (number of cavities, number of tools, tool capacity), determine injection molding machine, calculate parts' cost and review tool for steel approval of a multi-cavity tool to produce the plastic parts as described below
Material: ABS
Annual Production : 1’200,000 Parts/Year
Part 1: Rectangular Plate
Part 2: Disk
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Step 1. Create the mold and enter technical information. (Demonstrates ProMax-One Mold Database™)
Create the mould record using ProMax-One Molds Database™. Select the Mold Database option from the main menu at www.injecneering.com and run the application
Create the Record.
Click on the "New" button.
Enter any available mold's technical information
Create the related parts. Place the mouse below the headings bar (under the RELATED PARTS tab) and right-click the mouse button. A menu like the one shown below will appear. Select the "Add New Part" option.
A part creation window appears on the screen. Enter the information for the first part
Repeat the same procedure to create the secont part. The parts will be displayed in the RELATED PARTS list shown below.
The tool information must be kept upto date as new information becomes available during the design, construction and trials of the tool.
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Step 2. Define Mould Configuration: Number of Cavities, Number of Tools, Cycle time, and work load (Demonstrates ProMax-One Mold Estimator™)
Open ProMax-One Mold Estimator™. Select the Mold Estimator option from the main menu at www.injecneering.com and run the program. This application helps to define best tool configuration to produce the projected volumes and to estimate the cycle time of the mould.
Retrieve the Mold Record. The mold was created in the previous step using the Mold Database program. To retrieve the tool (it may already be in your screen), click on the arrow button in the right side of the description box. A list of tools will be displayed. You can either find the description of the mold to be retrieved or start typing the name to filter down the list. For this case study, we can type "*disk" and only the records containing the word "disk" will be shown in the list. Click on the mould to be analyzed
Create a new tool configuration analysis.
• Click on the New Analysis button. Notice that some values will be automatically assigned. These values can be configured for your convenience by selecting the "Configuration" tab
• Type-in a description for the current analysis.
Calculate the Cycle time. (If unknown)
First, select the material generic and trade names: click on the arrow button in front of the trade name text box and select the desired material from the list as shown in the picture below.
Repeat the procedure to select the trade name.
Select the radio button of the Cooling Factor values to be used in the calculation Cooling time = Cool Factor • Wall thickness² (Ct = Cf • t²). Enter the maximum wall thickness among all the parts in the tool.
Enter maximum and minimum values for injection time and clamp open time (use your own criteria)
Check the results. Using the right side buttons, the maximum, minimum or average value can be assigned as the cycle time to use in the calculations. For this example, the value for the cycle time (50 sec) was manually entered which is close to the max cycle time calculated.
Run several scenarios.
Type-in the values in the text boxes. Only one box can be left empty. Several iterations will be analyzed to determine the number of cavities required to produce the projected volumes.
Once the variables are entered, position the sliding button in front of the value to be calculated. Try different scenarios and adjust the values until the results are satisfactory. The results window shows two values, the non-rounded and a properly rounded value. Since some values need to be of type integer, the application rounds the calculated value. If, for instance, the non-rounded value for the number of cavities is closer to the lower integer, consider changing other variables to determine if the lower integer can be used instead of the suggested upper number.
Three scenarios will be run for the case in study
Scenario 1: If building 1+1 Cavity tools, how many tools are required? (Notice that the values entered are 1 for the number of cavities and 1.2 mills for yearly production. 2 cav. and 2.4 mills could also have been used)
Results: 4 (3.48) tools are required. Since the non-rounded value is 3.48, some iterations modifying the working hours and other variables should be ran to determine whether three tools can produce the intended quantities.
Scenario 2: If building 2+2 Cavity tools, how many tools are required?
Results: 2 (1.73) Tools are required
Scenario 3: Can we produce the projected volumes building three 1+1 cavity tools (instead of the calculated 4) by increasing the working hours? Calculate and try different values until the results are satisfactory.
Results: By increasing the number of hours per day to 22 and the number of weeks per year to 50, 1,211,760 parts per year can be produced.
Conclusions. Three 1+1 cavity tools will be constructed. The tools will be run 22 hours x 6 days x 50 weeks.
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Step 3. Calculate clamp force, shot size and select injection molding machines (Demonstrates ProMax-One Machine Selector™)
Open ProMax-One Machine Selector™. Select the Machine Selector option from the main menu at www.injecneering.com and run the program. ProMax-One Machine Selector calculates the required clamping force and barrel size to specify the injection molding machine. Retrieve the mould record as explained in step 2.
Create a new machine selection study.
Click on the New Calculation button
Type in a description of the analysis
Create a new machine selection study.
Assign the Material’s Generic Name by pressing the arrow button in front of the corresponding text box and selecting it from the list. Do the same to select the Trade Name.
Click on the radio buttons to select the values to be used in the calculations. Use your experience and criteria to decide on the values. When a radio button is clicked, the corresponding textbox in the Used Values row is filled up with the chosen number. Alternatively, if based on your experience and knowledge none of the values satisfy your criteria, you can enter you own values manually.
As shown in the picture above, the chosen values for this study are 1.05, 1.8, 3 and [1.3-2]
2. Enter parts and tool information.
Enter the values for part number 1. Since the tool has not been designed yet, as the project is still in an early stage, some assumptions will need to be made. It is a good exercise to lay the parts’ views in a CAD program to better visualize how the mould may be laid out. Since the study will be saved in a database, it can be revisited and reviewed when a tool design is available.
Wall Thickness: If the part has different wall thicknesses, use an average value (use your judgement). Ignore wall thickness for ribs and bosses unless you think they can substantially affect the clamp force in which case, slightly reduce the value of main wall thickness. For our study, the parts’ wall thicknesses are uniform
Flow Length: This is the maximum distance the melt will travel inside the cavity. After the values are entered, the average pressure in the cavity is calculated
Projected area & Number of Cavities: Enter the projected area of one part and the number of “identical” cavities. The clamp force for part 1 will be displayed.
Slides projected Area & Back Angle: Sometimes the area in contact with the slide is large in proportion of the part’s projected area. The pressure of the melt on the slide is transferred to the contact surface between the slide and the heel block. By entering the slides area and the back angle, the program calculates the force component in the direction of the machine opening. This force is added to the previously calculated clamp force to calculate the total clamping force for the current part.
In this study, no slides are involved and hence no data is entered.
Runner Projected Area: this value can be entered either as a numeric number or as a percentage of the total projected area. Since the mould has not been designed yet, a percentile value will be entered. As previously mentioned, this information should be revisited when a tool design is available. A 5% of the total projected area will be assumed. The clamping force for the runner is now calculated. This value will change as new information is entered for the other parts.
Part Volume: Enter the part volume for part 1. The total volume is calculated.
Runner & Sprue Volume: As with the projected area for the runner system, the volume can also be enter either as a numeric or a percentile value. 10% will be used in this case.
To enter the information for the second part, click on the check box as shown below.
New text boxes be displayed. Enter the information for Part 2. Notice how the values for the Runner projected area and Runner & Sprue volume are recalculated.
The required clamp force and shot weight are displayed in the results area. Two approaches to calculate the clamping force are used; the first multiplies the projected area by the calculated average pressure. The second uses the rule of thumb where the total projected area is multiplied by a constant whose value is determined by the type of material (not our favourite approach as it does not factor the wall thickness and flow length)
Using the results.
The results suggest that (Using the P•A approach) 86 Tons of clamp force are required. Applying a safety factor of 15%, 100 Tons are needed. Using the generic machines database provided, the next standard machine is a 120 Ton machine. Our smallest generic 120 ?nject?on mach?ne has a barrel capac?ty of 148 cm3; about 2.4 shots in the barrel. Since the values are obtained through theoretical methods, it must be ensure that the tool also be buit to fit in a smaller machine (in our case an 80 Ton machine). That way, once the tool is build, the 80 Ton machine can be tried and evaluated. Save your analysis.
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Step 4. Calculate cost of the molding parts
Open ProMax-One Part Cost Estimator™. Select the Part Cost Estimator option from the main menu at injecneering.com and run the program. Retrieve the mould record as explained in step 2.
This program calculates the cost of the parts produced by a mold. This is a comprehensive tool that considers most variables that affect the cost to produce a molded plastic part.
Start a new cost analysis study. Click on the New Calculation button
Enter the materials information.
Enter values for all different parts. It is best to enter this information prior to any other information. To add a part, click on the check box below the part number
Since the specific gravity was already typed, when the part volume is entered, the weight is automatically calculated and vice versa. Each column represents a different part. For instance for a 2+3 cavity mold, only 2 columns must exist.
Use the Other Costs fields to include any costs incurred by secondary operations such as painting, chroming, de-gating, inspection etc.
Enter the remaining data
Enter only values that will affect the part cost in accordance to your quoting policies or terms negotiated. For instance, if only a fraction of the machinery cost is to be amortized in the piece price, enter only the fractional value. If the mould is provided by the customer, the Mold Cost value should not be entered. Yet the maintenance cost may be entered.
For the runner and sprue, you only need to enter one of three values: the weight as a percentage of the total shot size, its volume or its weight. For this case study, the percentile value is used as the mould has not been designed yet.
Analyze the results. Each part cost and a breakdown are shown in results area
A more in detail view of the values used in the calculations can be seen by clicking on the Show Calculations Data button.
The level of detail presented in the detailed view can be controlled by checking or un-checking the check boxes that appear in the upper area of the window. To quickly make visible one item only, right click on the corresponding check box. All other items will be hidden.
Note: All calculations are based on the fractional weight of a part to the complete shot weight. For example, the formula to calculate cost of labour is:
Mach Rate Per Part = Mach Rate Per Cycle * Fraction Of Total Shot
An not:
Mach Rate Per Part = Mach Rate Per Cycle / Number Of Cavities
Therefore, for this example:
Fract Of Total Shot = 31.50 g / ( 31.50 g + 27.94 g ) = 0.53
Mach Rate Part 1 = 1.389 $/Cycle * 0.53 = 0.74 $/part
And Not:
Mach Rate Part 1 = 1.389 $/Cycle / 2 Cav = 0.694 $/Part
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