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 .

  • 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:

  1. Number of cavity
  2. Type of mold e.g., two-plate, three-plate, split line, cam,side core and  hot runner, etc.
  3. Type of runner system
  4. Type of gate
  5. ways of de-gating
  6. appication of robotics
  7. Estimated shot time


  1. Comprehend the customer’s requirements thoroughly; don’t take a stab in the dark.
  2. Adhere to the estimator’s original concepts & customer-approved drawings, ensuring the molding aligns with the estimate & quote. Should deviations be necessary, notify the estimator and customer for their consent.
  3. Cross-check mold drawing with part drawing to confirm proper finish, dimensional accuracy, and conformity with specifications.
  4. Consult the tool builder and molding operations foreman for insights to optimize construction & operation convenience.
  5. Ensure the mold possesses ample area to avert crushing in the designated press.
  6. Select the press to be employed, then tailor the mold’s mechanical parts to fit.
  7. Verify daylight & press stroke to guarantee sufficient press opening for molding removal.
  8. Procure mold material as soon as design determines size requirements.
  9. Upon mold design completion, design & order jigs, loading fixtures, cooling fixtures, etc., to prevent serious delays. Double-check operations required independently & reference the original estimate to include all fixtures needed for production.
  10. Avoid cramming excessive impressions into a multi-cavity mold, as it may lead to issues with flashing, uneven thickness, or operator oversight.
  11. Take into account operator convenience & safety during loading and unloading arrangements.
  12. Ensure the molding remains in the core half until ejector pins can push it off at the proper stage of the molding cycle.
  13. Construct mold’s mechanical operating parts for rigidity and unhindered action.
  14. Implement sectional construction of mold cavities & cores for simplified machining setups.
  15. Incorporate dowel pins to align mold’s retainer and backing plates.
  16. Design slender cores, prone to breaking in long production runs, as insertable parts for easy replacement.
  17. Steer clear of irregularly shaped inserts, e.g., hexagonal, which may complicate the molding operation.
  18. Utilize negative draft, zero draft, and positive draft in varying amounts for different molding scenarios.
  19. Ensure steam line outlets & inlets don’t obstruct mounting clamps.
  20. Strategically place drilled & tapped holes in mold parts for hardening.
  21. Allow ample clearance for steam lines passing by bolt holes or ejector pin holes to maximize drill run-out space.
  22. Opt for stock sizes of steel for cavities, cores, pins, etc., whenever feasible.
  23. Use standardized sizes of screws, guide pins, etc., to minimize small part inventory.
  24. Specify steel type for mold parts on detail drawings & keep this info updated for substitutions. Label each mold part with steel type for correct heat treating procedures.
  25. Position screws, guide pins, walls thickness, ribs, ejector pins, etc., symmetrically on mold plates to avoid layout confusion. Notify the layout man of necessary offsets.
  26. Provide mating contour notations at parting line for opposite mold halves, informing the toolmaker that contours must match.
  27. Dimension details for minimal arithmetic & additional dimensions for the toolmaker. Consider the best machining operations setup for mold parts and provide necessary dimensions.
  28. Offer toolmaker reasonable tolerances on mold dimensions.
  29. Don’t forget to account for shrinkage.
  30. Obtain shrinkage data from plastics material manufacturers.
  31. Maintain records of actual shrinkage with various cross-sections of different plastics materials, which can be invaluable for critical jobs.
  32. Document dimensional changes due to hobbing, carburizing, and heat treating various steels and block shapes/sizes.
  33. Keep records of miscellaneous computations for future reference.
  34. Short cuts in design & construction details often lead to regrets.
  35. In multiple cavity compression mold, avoid steam plates extending too far beyond the hydraulic press’s ram area, as it may cause difficulties in closing outer cavities due to press platen bending under load.
  36. Examine the molding’s projected area to ensure the press can handle the selected plastics material, and check mold area against press tonnage to prevent mold crushing.
  37. Refrain from using stripper plates for multiple cavity compression molds, as temperature inequalities may cause binding and damage the ejecting mechanism.
  38. Allocate sufficient loading space in compression molds, as some molding materials may have high bulk factors.
  39. Design steam lines for heating compression molds in series to prevent parallel lines from becoming waterlogged.
  40. Achieve uniform heating in compression molds for thermosetting materials with even steam line distribution.
  41. For thermoplastics materials, evenly distribute and closely space heating and cooling passages in compression molds to ensure uniform temperature distribution and rapid cooling/heating.
  42. Typically, fasten guide pins to the rear, or movable half of the injection mold, for operator convenience while working between open mold halves.
  43. Carefully consider venting provisions for injection molds, which may involve sectional assembly or orientation selection to prevent air trapping. Special venting passages may also be necessary.
  44. Install pillars in the ejector box to support cavity backing plates and prevent bending during material injection.
  45. Use stock of ample thickness for retainer and backing plates.
  46. Incorporate heating and cooling provisions in injection molds, such as drilled passages for steam or cold water.
  47. Avoid sharp turns in runners, as they significantly increase flow resistance.
  48. When a core extends beyond the mold half, ensure guide pins are longer than the core extension to support the mold half and prevent damage to polished cores.
  49. Avoid constraining toolmaker to specified thickness on retainer and backing plates or total shut height.
  50. Design stripper plates to ride on guide pins, not mold cores. Use stripper bolts to secure the stripper plate and ensure correct assembly by the molding press operator.

For the Toolmaker

  1. Scrutinize a novel occupation exhaustively prior to commencing metal cutting. Ascertain an operation & setup sequence, yielding the completed tool components per specifications.
  2. Remember to allocate stock for grinding or polishing, particularly when a substantial amount of metal is removed (e.g., thread crests).
  3. Label tool elements with the utilized steel type. This way, if annealing or reworking is necessary, heat treaters can get down to brass tacks.
  4. Submerge hobs deeper than the required finished depth, enabling the hobbed block to be squared true post-hobbing.
  5. Plastics materials may seep into cracks or gaps between .001″ and .002″ wide. Adjust fits accordingly to avert flashing.
  6. Refrain from fitting fragile mold components too snugly, lest they’re unable to be separated without impairing a polished mold surface.
  7. Initiate polishing by uniformly eradicating tool marks to maintain the true machined contour. Employ a coarse abrasive, followed by a finer one to eliminate scratches, and finally, a “flour” grinding substance for finishing.
  8. Don’t bank on chromium plating to mask shoddy polishing. The steel must possess a mirror finish for a radiant chrome plate. Chromium, being exceedingly hard, is challenging to polish to a luster compared to steel if the surface is rough.
  9. Steel parts can be augmented with a chromium deposit between .0005″ and .005″ thick, a handy method for salvaging worn tools or slightly undersized components.
  10. Mend dents in mold surfaces by upsetting the metal, raising the depressed region for polishing back to its original level.
  11. Repair pinholes or steel mold cavity flaws by drilling a hole, filling it with a plug, and pressing or furnace brazing it in place. This may leave a mark on the molded piece, so it’s best to bite the bullet and perform repairs in non-critical areas.
  12. Restore damaged mold components via atomic welding, employing a metal identical to the parent metal’s composition. This yields an imperceptible joint when executed adeptly (preferably by a specialist).
  13. Bevel the rear or entering edges of pinholes for effortless pin insertion.
  14. Provide pinhole relief on diameters so pins necessitate fitting for merely a portion of their length.
  15. Polish core pins lengthwise, enabling moldings to slide off with ease.
  16. Sidestep burrs on cavity mouths, pin ends, and other mold elements. Besides reflecting subpar workmanship, burrs may scratch moldings during removal or hinder extraction.
  17. Steer clear of undercuts, even if only a hair, on core pins as they impede the molding’s smooth release.
  18. Double-check core pin lengths entering cavities, ensuring they’re not overly lengthy before dispatching them to the press room for a trial run.
  19. Install ejector and knockout mechanisms to function squarely & unimpeded.
  20. Test the mold assembly in its entirety prior to hardening component dispatch, verifying screw hole & pinhole placement and proper part assembly.
  21. Have a second pair of eyes review your calculations before cutting.
  22. Utilize centering screws for marking drilled hole locations from tapped holes.
  23. Employ templates for opposing mold half parting line contours, guaranteeing a solid match. Dowel the templates via holes bored through clamped cavity blocks. Use identical dowels for aligning cavity blocks when fastening them to retaining plates.
  24. Adhere to mold drawing dimensions.
  25. Alert designers to apparent drawing errors before going off half-cocked.
  26. Familiarize yourself with shop machine tools, compensating for backlash, angular errors, and similar concerns.

For the Molding Operator

  1. Ascertain that the mold is aptly positioned in the press prior to operation commencement&.
  2. Confirm the utilization of appropriate molding materials, inserts (if applicable), and ensure the presence of gauges~.
  3. Determine the accurate modus operandi for executing the task.
  4. Request the inspector’s examination of initial pieces and subsequent reevaluation as required- no half measures.
  5. Uphold the cleanliness of molding materials; dust or grime may ruin a mold, while minute metal fragments may impair the mold or other apparatus.
  6. Elevate molding material containers above the floor, as occasional flooding may lead to moisture-induced damage.
  7. Preemptively dry cold room-stored molding materials to eliminate condensation-related issues caused by transitioning into a more humid workspace.
  8. Reallocate unused materials and inserts to the inventory division after concluding a run.
  9. Maintain the tidiness of scrap thermoplastics, as they can be reprocessed; however, refrain from preserving overheated scraps that may have undergone degradation, plasticizer loss, or color deterioration.
  10. Separate scrap moldings with metal inserts from sprue scrap of thermoplastics; this facilitates metal removal prior to the grinder phase~. Brass inserts demand extra vigilance, as magnetic separators won’t pick them out. Scrap grinders might handle small inserts without severe damage, but the chips could clog the injection molding machine nozzle.
  11. Salvage metal inserts from scrap thermosetting materials and avoid discarding them in waste barrels.
  12. Clean the mold after extracting the molded piece before initiating another cycle- cleanliness is next to godliness.
  13. Steer clear of steel picks, bars, or rods when dislodging stuck moldings; utilize brass or wood with utmost caution.
  14. Inspect the mold for bent or broken core pins, or burrs that may result in defective moldings.
  15. Exercise caution when extracting moldings from the mold to prevent distortion or marring.
  16. Familiarize yourself with common molding complications and their corresponding remedies.
  17. Refrain from altering curing time or molding conditions without meticulous assessment. Insufficiently cured thermosetting materials may lose heat or water resistance, or fail to shrink as needed. Injection-molded thermoplastics may exhibit air bubbles from rapid loading or excessive shrinkage due to premature core removal. Overcuring or overheating can induce color alterations, leading to rejection.
  18. Good housekeeping is the backbone of exemplary molding practice.
  19. Periodically lubricate ejector mechanisms to maintain smooth operation, but avoid excessive lubrication to prevent contamination of mold cavities.
  20. Ensure proper machine lubrication and maintenance to uphold optimal performance.
  21. Double-check that ejector pins are of adequate length and correctly adjusted. Regularly inspect ejector spaces for flash presence under ejector or knockout plates, which may impede proper seating.
  22. Employ a mirror for the examination of mold cavity sections that are obscured when the mold is in the press.
  23. Thoroughly clean, polish, and grease molds post-operation and prior to storage. If necessary, perform repairs to guarantee readiness for subsequent production cycles.

For the Estimator

Delve into the accessible data before commencing the estimation process; if the data is inadequate, request more to craft a solid proposal~.
Scrutinize the component to identify the optimal molding technique & arrangement within the mold.
Ascertain the number of cavity impressions necessary for sustaining production rates on the available apparatus.
Draft a rudimentary sketch of the intended mold & approximate the spatial requisites.
Compute steel requisites to establish material expenditures.
Gauge the mold’s cost – thorough comprehension of mold sinking methodologies & specific equipment for mold construction is imperative.
Determine the molded piece’s weight; assembling bespoke mathematical tables expedites this process, streamlining future endeavors.
To compute total material requirements, account for rejects & waste material (5-30% and 5%+, respectively), which depend on molding complexity, tolerances, finishing operations, and material type.
Delineate molding, finishing, and inspection procedures.
Decompose each operation into fundamental elements for precise time estimation; when in doubt, break it down further.
Account for personal time, setup time, repair time, and production loss due to damaged cavities when discerning net production rates; comparing actual and stopwatch rates offers a fairly accurate conversion factor.
Design an estimation form encompassing all cost-contributing factors, arranged logically to enable smooth calculation.
Government price control regulations necessitate consistency in determining selling prices.
Costs encompass direct labor, mold & tool amortization, factory expenses, supervision, machine operating amortization, maintenance, and sales/administrative expenses.
Don’t forget to include special tooling needed for finishing and inspection in the tool estimate; auxiliary items often add a substantial sum.
Transform the rough draft estimate into a written form, complete with a succinct yet comprehensive description of processes & operations; this prevents confusion and misunderstandings down the line.
To maintain efficiency, estimators should routinely review & follow up on previous estimates, comparing them with actual costs.
It’s good business to estimate as accurately as possible; being too conservative may cost you a pretty penny, while underestimating can lead to losses or delayed production.
Provide the customer with a detailed part drawing of the proposed molding for approval & signature.
Spend quality time with the mold design draftsman to ensure they’re on the same page as the original estimate.
Verify that the final tool designs align with the detailed part drawing, guaranteeing that the tools produce the promised component.

For the Finisher

  1. Scrutinize the blueprint of the molded artifact & reach out to relevant parties to pin down the necessary post-production finishing.
  2. Secure the essential tools to execute each finishing step.
  3. Secure the gauges necessary for the job.
  4. Monitor production velocity after a brief period to guarantee it’s in line with expectations. If not, inspect the lagging operations to discover ways to bring them up to snuff. Keeping pace with estimates is crucial not only financially, but also to hit delivery deadlines.
  5. Prioritize operator safety while setting up tasks.
  6. Keep walkways uncluttered & keep in-progress materials organized. Good housekeeping don’t cost a thing.
  7. Ensure drill operators are skilled enough to sharpen their drills appropriately. There’s a 20-50% difference in production rates between well-sharpened and poorly-sharpened tools.
  8. Confirm machining speeds are suited for each task at hand. This significantly impacts production velocity & finish.
  9. Verify with the molding team if the moldings are of acceptable quality upon arrival at the finishing department (i.e. no heavy flash or warping). Poor moldings hurt finishing production rates.
  10. Maintaining a rework department separate from the production finishing is often a savvy move to salvage rejected pieces & to handle small lots of moldings that are otherwise slow to manage under standard production procedures.
  11. Streamline production machinery for a smooth flow from molding to inspection, avoiding redundant backtracking of parts within the department.
  12. Don’t stockpile rejected or surplus parts in necessary work areas. Get rid of them pronto before they get forgotten.

For the Inspector

  1. Before making any changes to a new mold, it is crucial to ensure that the castings are properly molded. Do not rush into corrections without first verifying the quality of the samples.
  2. To maintain a record of the molding dimensions, it is important to check each dimension of the molding against the part print. Keep the part print with the actual dimensions marked on it for future reference.
  3. It is advisable to keep sample moldings as a reference in case of complaints or disputes that may arise in the future.
  4. Arrange for safe storage of gages and templates not in use to prevent loss or damage to the equipment.
  5. Master measuring instruments are delicate and must be handled with care. Avoid using them for production gages to prevent damage.
  6. It is important to ensure that all measurements are accurate. Before moving on to the next dimension, double-check to verify the measurements.
  7. Keep a close eye on production operations to avoid long runs of scrap material, as this can be costly and inefficient.
  8. Before production runs, check production gages thoroughly. After the runs, check them again to see if repairs are required. Order any necessary repairs in advance to be ready for the next run.
  9. At the end of a production run, ensure that all gages issued are returned or accounted for.
  10. Inspectors can handle minor repairs, such as removing occasional burrs from finished parts.
  11. Adequate lighting facilities must be provided for bench inspectors to ensure accurate inspections.
  12. When planning a new job, check the original process layout to determine at which points inspections are planned. Inspection standards should be as liberal as possible, while upholding the plant’s reputation and meeting customer requirements.
  13. Ensure that inspection rates meet the standards set by the estimate. A high percentage of rejects results in wasted time looking at bad material and lower inspection rates.
  14. To prevent finished material from piling up waiting for final inspection, keep inspection jobs in sync with production.
  15. To trace back to the source of rejected moldings, it is necessary to include a method of placing responsibility for each operation in the inspection system.
  16. The inspection department oversees the entire operation of the molding plant and is well-positioned to make valuable suggestions for improving product quality by suggesting changes in procedures.