General Mold Design

General Mold Design

1  Be sure to understand just what the customer wants. Do not try to guess.

2  Follow the estimator’s original design ideas and approved customer drawings in order to be sure the molding will be produced as estimated and as quoted. If departure from the original idea is necessary, notify the estimator and the customer for their approvals.

3  Check the mold drawing against the part drawing to make sure the mold will produce the part correctly with proper finish, dimensional accuracy and otherwise in conformance with specifications.

4  Check the mold design with the tool builder and with the molding operations foreman to obtain information for maximum convenience in construction and operation.

5  Make sure the mold has sufficient area to prevent crushing in the press for which it is designed.

6  After selecting the press which is to be used, design the mechanical parts of the mold to fit it.

7  Check daylight and press stroke to be sure the press will open far enough to allow removal of the molding.

8  Order the material for the mold as soon as the design is far enough along to determine size requirements.

General Mold Design

9. Design and order jigs, loading fixtures, cooling fixtures, and the like, which may be required, as soon as the mold design is complete. Serious delays can be the result of neglect of this item. Here, it is good practice to check the operations required independently and then by reference to the original estimate so that all fixtures required may be included and be adequate to turn out the production called for in the estimate.

10.    Do not attempt to employ too many impressions in a multiple cavity mold. The cavity area may extend too far beyond the center of press clamping area and flashing or uneven thickness of the pieces may occur. Also, it may be too difficult for the operator to watch all of the pieces during the operation of the mold and he may run a lot of scrap before he discovers a single cavity which may not be functioning properly. It is difficult to make uniform moldings when too many cavities are used because of non-uniformity in size, differences in runner resistance and other factors.

11.    Consider the convenience and safety of the operator in loading and unloading arrangements.

12.    Make provisions so the molding will stay in the cavity half from which it is to be ejected until the ejector pins can push it off at the proper place in the molding cycle.

13.    Design the mechanical operating parts of a mold for rigidity and free action.

14.    Employ sectional construction of mold cavities and cores to provide the simplest machining set-ups.

15.    Include dowel pins to align the various retainer and backing plates of a mold.

16.    Slender cores which are likely to be broken in long production runs should be designed as inserted parts if possible to make their replacement simple.

17.    Avoid the use of irregularly shaped _ inserts, e.g., hexagonal, which have

to project from the surface of the molding because they must have a correspondingly shaped hole in the mold to hold them in place during the molding operation.

18.    Negative draft, zero draft, and positive draft of varying amount are all useful in molding practice. A part may be forced to stay on a cavity by employing a small negative draft. Deep holes may be molded with zero draft if they are required to be straight. In such cases the cores must be highly polished and without waves in the surface. Positive drafts from a few thousandths of an inch in several inches to five or ten degrees may be employed to secure easy release from the mold.

19.    Be sure that steam line outlets and inlets do not interfere with the mounting clamps.

20.    Be sure to locate drilled and tapped holes in mold parts which are to be hardened so the holes will all be in place before the steel is hardened.

21.    Be sure that steam lines have plenty of clearance as they pass by bolt holes or ejector pin holes so that the maximum space is available for drill run-out. An allowance of clearance for steam lines 8 long and to 10″ for longer lines is good practice.

22.    Whenever possible, use stock sizes of steel for cavities, cores, pins, etc.

23.    Use standardized sizes of screws, guide pins, and other parts where possible to avoid carrying large stocks of small parts.

24.    Be sure to specify on the detail drawings the kind of steel required for the mold parts and keep this information up to date in case of substitutions. Mark on each mold part the specific kind of steel used so that when annealing before reworking the heat treater will use the correct procedure.

25.    Locate screws, guide pins, ejector pins and the like symmetrically on the mold plates to avoid confusing the layout man. When it is necessary to offset a hole, notify the layout man so he will be sure to see it.

26.    Provide notations for mating contours at the parting line for opposite mold halves so the toolmaker will know the contours must match.

27.    Dimension the details so the tool maker will have a minimum amount of arithmetic and additional dimensions to figure. Find out how he can best set up the machining operations for the mold part and put on the dimensions required for this set-up.

28.    Give the toolmaker reasonable tolerances on mold dimensions.

29.    Do not forget to allow for shrinkage.

30.    Obtain shrinkage data from the plastics material manufacturer.

31.    Keep a record of actual shrinkage obtained with various cross sections of various plastics materials and it will be extremely helpful in “coming out on the nose” for critical jobs.

32. Keep a record of dimensional changes for hobbing, carburizing, and heat treating various steels and various shapes and sizes of blocks.

33. Keep a record of miscellaneous computations and the data will be valuable on succeeding jobs.

34.    Short cuts in design and construction details are usually regretted.

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