golden rules of custom injection mold design
Before you start a new mold design, the particular plastic mold designer should be owning the next data (some components of that have been already stated).
An unambiguous totally comprehensive injection molded parts drawing
Specifications of the moulding material consisting of grade and color
The moulding machine technical specs
All the estimating details which includes any kind of blueprints
Tool features the following:
- Number of cavity
- Type of mold e.g., two-plate, three-plate, split line, cam，side core and hot runner, etc.
- Type of runner system
- Type of gate
- ways of de-gating
- appication of robotics
- Estimated shot time
PRACTICAL POINTS IN MOLD DESIGN AND CONSTRUCTION
- Be sure to understand just what the customer wants. Do not try to guess.
- 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.
- 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.
- Check the mold design with the tool builder and with the molding operations foreman to obtain information for maximum convenience in construction and operation.
- Make sure the mold has sufficient area to prevent crushing in the press for which it is designed.
- After selecting the press which is to be used, design the mechanical parts of the mold to fit it.
- Check daylight and press stroke to be sure the press will open far enough to allow removal of the molding.
- Order the material for the mold as soon as the design is far enough along to determine size requirements.
- 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.
- 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.
- Consider the convenience and safety of the operator in loading and unloading arrangements.
- 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.
- Design the mechanical operating parts of a mold for rigidity and free action.
- Employ sectional construction of mold cavities and cores to provide the simplest machining set-ups.
- Include dowel pins to align the various retainer and backing plates of a mold.
- 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.
- 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.
- 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.
- Be sure that steam line outlets and inlets do not interfere with the mounting clamps.
- 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.
- 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.
- Whenever possible, use stock sizes of steel for cavities, cores, pins, etc.
- Use standardized sizes of screws, guide pins, and other parts where possible to avoid carrying large stocks of small parts.
- 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.
- Locate screws, guide pins, walls thickness,ribs,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.
- Provide notations for mating contours at the parting line for opposite mold halves so the toolmaker will know the contours must match.
- 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.
- Give the toolmaker reasonable tolerances on mold dimensions.
- Do not forget to allow for shrinkage.
- Obtain shrinkage data from the plastics material manufacturer.
- 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.
- Keep a record of dimensional changes for hobbing, carburizing, and heat treating various steels and various shapes and sizes of blocks.
- Keep a record of miscellaneous computations and the data will be valuable on succeeding jobs.
- Short cuts in design and construction details are usually regretted.
- Avoid making the steam plates of a multiple cavity compression mold extend too far beyond the ram area f the hydraulic press in which it is to be used. Too much overhang may result in difficult closing of the outer cavities because of the bending of the press platens under the action of the load.
- Check the projected area of the molding to see if the press has sufficient capacity to mold the plastics material selected and then check the mold area against the tonnage of the press to be sure the mold will not be crushed.
- Avoid the use of stripper plates for multiple cavity compression molds. The inequalities in temperature of the stripper and the force plate may cause the stripper to bind so tightly on the cores that the first time the mold is opened the stripper will stick and ruin the ejecting mechanism.
- Provide ample loading space in compression molds. Some molding materials have high bulk factors and they may require an unusually large loading well.
- In laying out steam lines for heating compression molds they should run in series so the steam will have to pass all the way through the passages without danger of parallel lines becoming water logged by being by-passable.
- Compression molds for thermosetting materials require even distribution of steam lines for uniform heating.
- Compression molds for thermoplastics materials require even distribution and close spacing of heating and cooling passages for uniform temperature distribution and for rapid cooling and heating.
Injection Mold Design
- Generally, arrange the guide pins to be fastened to the rear, or movable half of the injection mold, so they will be moved out of the way of the operator when he is working between the open mold halves.
- Consider venting provisions carefully for injection molds. This may involve sectional assembly of the cavities, or selecting the orientation of the piece so that the material can fill the mold progressively without trapping air. Also it may be necessary to make special venting passages in the mold assembly.
- Provide pillars in the ejector box to support the cavity backing plates and to prevent their bending when the molding material is injected.
- Make retainer and backing plate of stock of ample thickness.
- Provide for heating and cooling injection molds. Drilled passages for steam or cold water are convenient.
- Avoid sharp turns in runners as they increase greatly the resistance to flow.
- When a core extends beyond the mold half, it is good practice to make the guide pins longer than the longest core extension so the pins will enter the bushings before the cores enter the cavity; also, so the guide pins will support the mold half face down on a bench without having it rest on the polished cores.
- Avoid holding the toolmaker to a specified thickness on retainer and backing plates or on total shut height.
- Stripper plates should be designed to ride on the guide pins, not on the mold cores. The stripper plate should be retained by stripper bolts to prevent its falling off and to insure that it will not be incorrectly assembled by the molding press operator.
For the Toolmaker
- Study a new job thoroughly before beginning to cut metal. Decide upon a sequence of operations and setups which will produce the finished tool parts as specified.
- Be sure to leave stock for grinding, or for polishing where polishing is likely to remove an appreciable amount of metal, as at the crests of threads.
- Mark the tool parts with the name of the steel used. Thus, if it has to be annealed and reworked for any reason, the heat treater will know what to do.
- Sink hobs deeper than the finished required depth so that the hobbed block can be squared true after hobbing.
- Plastics materials will flash into cracks or openings which are .00lwto .002″ wide. Make fits accordingly to avoid flashing.
- Do not fit delicate mold parts too tightly if they cannot be pressed apart without damaging a polished mold surface.
- Begin polishing by removing tool marks evenly to avoid losing the truly machined contour. Use a coarse grinding material and then remove its scratches with a finer one. Finish polish with a “flour” grinding material.
- Do not expect chromium plate to cover a poor polishing job. The steel itself must have a mirror finish to take a lustrous chrome plate. Chromium is extremely hard and it is much more difficult to polish to a luster than steel, if surface is rough.
- Steel parts may be built up with a deposit of chromium .0005″ to .005″ thick. It is often a convenient means to salvage a worn tool or a part which is slightly small.
- Dents in a mold surface may be repaired by upsetting the metal to bring up the depressed surface so it can be polished back to its original level.
- Pin holes or flaws in the steel mold cavity can be repaired by boring a hole and filling it with a plug, which is pressed or furnace brazed in place. It may leave a mark on the molded piece, so it is best to do this kind of repair job where the mark will not be critical.
- Damaged mold parts may be rebuilt or repaired by atomic welding a metal of the same composition as the parent metal. This results in an undetectable joint if properly done (preferably by a specialist).
- Chamfer the rear or entering edges of pin holes so that the pins can start into them readily.
- Relieve pin holes on their diameters so the pins need not be fitted for more than to of their length.
- Polish core pins lengthwise so the molding can slide off easily.
- Avoid burrs on cavity mouths, pin ends and other mold parts. Besides showing up poor workmanship, the burrs may scratch the molding as it is removed, or they may make it difficult to remove.
- Avoid undercuts, even if only a few thousandths of an inch, on core pins because they make it difficult to push the molding off freely.
- Check the length of core pins which enter cavities to be sure the pins are not too long before sending them to the press room for tryout.
- Fit ejector and knockout mechanisms to operate squarely and freely.
- Try out the assembling of the complete mold before sending parts to be hardened to make sure all screw holes and pin holes are in place and to make sure the parts go together.
- Have someone check your computations before you begin a cut.
- Use centering screws to mark drilled hole locations from tapped holes.
- Use templates for the parting line contours of the opposing mold halves to assure a good match. Dowel the templates through holes bored through both cavity blocks while they are clamped together. Use the same dowels to hold the cavity blocks in line when fastening them to their retaining plates.
- Work to the dimensions specified on the mold drawing.
- Report apparent errors on drawings to the designer before you go too far.
- Study the machine tools in your shop to know what backlash, angular errors and the like must be compensated.
For the Molding Operator
- Make sure the mold is properly set in the press before starting to run it.
- Make sure the proper molding materials and inserts, if any, are being used, and that gages are at hand.
- Find out the proper procedure for running the job.
- Have the first pieces checked by the inspector, and then rechecked as often as necessary.
- Keep the molding material clean. Dust or dirt may spoil a molding and small pieces of metal may damage the mold or other molding equipment.
- Keep the molding material containers off the floors as the floors may be flooded from time to time, and then the material may be damaged due to the moisture absorbed.
- When molding material is stored in a cold room, dry it before using to remove any condensed water which may collect due to moving the material into a work room of higher humidity.
- Return unused molding material and inserts to the stock department at the end of a run.
- Keep scrap thermoplastics material clean. It can be ground and reused. Do not save overheated scrap as it may have suffered damage due to breakdown, loss of plasticizer, or color deterioration.
- Do not put scrap moldings which have metal inserts in them in the same container with sprue scrap from thermoplastics materials. Keep the moldings separate so the metal can be removed before the material goes to the grinder. Brass inserts require especially careful attention because the brass will not be picked out by a magnetic separator. The scrap grinder may grind small inserts without seriously damaging the grinder but the chips usually will plug the injection molding machine nozzle.
- Do not put scrap thermosetting material in the waste barrel if the moldings have metal inserts. The inserts can be salvaged.
- Before beginning another cycle, be sure the mold is cleaned after removing the molded piece.
- Do not use steel picks, bars, rods and the like to remove or pry out stuck moldings from a mold. Use brass or wood and very carefully.
- Watch the mold to detect bent or broken core pins or burrs which may cause defective moldings to be produced.
- Avoid distorting or marring the moldings when removing them from the mold.
- Learn the causes and cures of the common difficulties in molding practice.
- Do not cut the curing time or otherwise change the molding conditions without checking the results thoroughly. Thermosetting materials not cured long enough may lose the heat or water resistant qualities or they may not shrink to the dimensions required. Thermoplastics, injection molded, may develop air bubbles from too rapid loading, or they may shrink too much due to being removed from restraining cores too soon. Overcuring or overheating may cause color changes which may be cause for rejection.
- Good housekeeping is an essential factor in good molding practice.
- Lubricate ejector mechanisms occasionally to keep them operating freely. Do not lubricate excessively so the grease or oil works its way into the mold cavities where it can contaminate the molding material.
- See that the molding machine is lubricated properly and is kept in good working condition.
- Be sure the ejector pins are of the correct length and are adjusted correctly. Check the ejector space frequently to be sure there is no flash under the ejector or knockout plates to prevent their seating properly.
- Use a mirror to inspect the portions of a mold cavity which cannot be seen directly when the mold is in the press.
- Be sure to clean, polish and grease a mold after removing it from operation and before storing it. If repairs are required, they should be made so the mold will be ready to produce when it is taken out of storage again.
For the Estimator
- Study the available data thoroughly before beginning to lay out the estimating work. Ask for more if there is not sufficient data to make a good proposal.
- Study the part to select the best method of molding it and the best arrangement in the mold.
- Figure the number of cavity impressions required to allow production rates to be maintained on the equipment available.
- Make a sketch of the proposed mold, and figure roughly the space requirements.
- Calculate the steel requirements to determine the material costs.
- Estimate the cost of the mold. A thorough knowledge of mold sinking methods is required, and so is knowledge of the specific equipment available for building the mold.
- Compute the weight of the molded piece. For this operation it is well to assemble mathematical tables of the most convenient form to suit the individual estimator. Many times it will be possible to work out formulas for volumes and areas not commonly given in standard mathematical tables. These special formulas, if kept on file, will greatly assist in future jobs.
- In figurinp： the total material requirements for the molding, add a reasonable amount for rejects and waste material. Rejects may run from 5% to 30%, depending upon the complexity of the molding, the tolerances required, the number of finishing operations, and the kind of material to be used. Waste may run to 5% or more.
- Outline the molding, finishing and inspecting operations.
- Break down each operation into as many elements, as are required to provide a basis for accurately estimating the time involved. Accumulated experience will be sufficient to allow certain operations to be blocked off at so many pieces per hour, while some complex operations will have to be broken down completely even by the most experienced estimator. It is best to break down in case of doubt.
- Make allowance for personal time， set up time, repair time, and loss of production due to damaged cavities in determining net rates of production. One way to determine a factor for converting gross rate to net rate is to time out a few jobs while they are running and then check the actual production over an average 24-hour run and over the total run. The actual rate divided by the stopwatch rate will be a fairly accurate factor to apply.
- Set up an estimating form which will include all of the items which enter into the final cost. Lay it out conveniently so the items follow along in a logical manner, and so each succeeding item may be obtained from what has been computed before it.
- Government price control regulations require uniformity and consistency in the steps leading to selling prices.
- Costs include, besides direct labor, mold and tool amortization charges, factory expense, supervision expense, machine operating amortization, and maintenance cost, and sales and administrative costs.
- Be sure to include special tooling which may be required for finishing and inspecting in the tool estimate. Even though the mold is the largest part of the tooling cost, the auxiliary items often add a considerable amount.
- After the estimate is rough drafted it should be put into written form with brief but complete description of the process and operations outline. By following this procedure, much confusion and misunderstanding may be avoided, especially if a rather long time elapses between submitting the estimate and obtaining the order to go ahead with the job.
- In order to maintain efficiency in estimating, the estimator should review or follow up as many estimates as time will allow to keep a continuous check on actual costs for comparison purposes.
- It is good business to estimate as closely as possible. Too conservative an estimate may result in the loss of needed business, while underestimating a job may result in large losses or delayed production.
- Submit a detail part drawing of the molding as it is proposed to be furnished to the customer, and have him check and sign it.
- Spend enough time with the mold design draftsman to make sure he understands thoroughly how the job was planned when the estimate was made.
- Check the final tool designs against the detail part drawing to make sure the tools will produce the part as promised.
For the Finisher
- Study the drawing of the finished molding and consult the proper persons to determine exactly what finishing is required.
- Arrange for the proper tools to do each finishing operation.
- Provide gages which are required.
- Check the rates of production after a short time to make sure they come within the estimated rates. If not, study the operations which are falling behind to see what can be done to bring them up to standard. Not only is it financially important to do so, but it is necessary to keep up to estimated speed in order to meet delivery dates.
- Consider the safety of operators when setting up jobs for them.
- Keep aisles clear and material in process in order. Good housekeeping doesn’t cost anything.
- See that drilling operators are competent to grind their drills properly. There is a difference of 20% to 50% in production rates between properly sharpened tools and improperly sharpened ones.
- See that machining speeds are proper for the individual job. Production rates and finish are affected considerably by this item.
- Check with the molding department if moldings are coming to the finishing department with heavy flash or in a warped condition or are otherwise not of proper quality. Poor moldings cause loss of finishing production rates.
- It is often convenient to maintain a salvage department separately from the production finishing to rework rejected pieces and to save small lots of moldings which have heavy flash or are otherwise slow to handle under regular production procedures.
- Group production machinery for through-flow from molding department to inspection, to avoid needless backtracking of parts within the department.
- Do not store rejected or overrun parts in needed working space. Dispose of them promptly before they are forgotten.
For the Inspector
- In checking the first samples from a new mold, be sure the castings are properly molded before making changes in the mold to correct the samples.
- Check each dimension of the molding against the part print and keep the part print with the actual molding dimensions marked on it for a record.
- Keep sample moldings for reference in case of complaints or disputes.
- Arrange for storage of gages and templates not in use so they will not be lost or damaged.
- Handle master measuring instruments with care and do not use them for production gages.
- 138.Be sure that measurements are correct. Re-check before going to the next dimension.
- 139.Watch production operations as often as necessary to avoid long runs of scrap material.
- Check production gages before they are sent out for production runs and check them when they come back to see if repairs are needed. Order repairs to be ready for the next run.
- When a production run is finished see to it that all gages issued are returned, or accounted for.
142.Inspectors can do minor repairing jobs such as removing occasional burrs from finished parts.
143.Provide adequate lighting facilities for bench inspectors.
144.Check with the original process layout for a new job to see at what points inspections are planned.
- Be as liberal as possible with inspection standards in keeping with upholding the plant reputation and with allowances granted by customers.
- Make sure inspection rates are up to the standards set by the estimate. Too large a percentage of rejects lowers the inspection rate and results in wasting time looking at bad material.
- Keep inspection jobs in phase with production to avoid piling up finished material waiting for final inspection.
- It is usually necessary to include in the inspection system a method of placing responsibility for each operation in order to trace back to the source of rejected molding.
- The inspection department sees the whole operation of the molding plant, and is in a position to make worthwhile suggestions as to changes in procedure to improve the quality of the products.