Plastic Mold Design

Since the introduction of injection moulding, plastic mouldings have been produced in a huge variety, ranging from the basic to extremely complex. High-complexity mouldings are now possible with advances in injection molding, toolmaking, and machining.

These technological advancements have led to the development of a wide range of standard components that meet the needs of consumers today by providing more and more high-quality products regularly. Utilizing these methods can make it easier to develop and construct tools and reduce both the design time and tool-making time.

Standard components are available which provide solutions to many different types of undercut or ejection problems. Many of the standard components are designed to provide solutions to these problems.

This suggests that wherever these can be successfully integrated into a design, it would be worthwhile to use them to their full potential. Components such as collapsible cores provide a self-contained solution of great value to designers and toolmakers.

When a new component has to be moulded, it is possible to divide mould designs depending upon whether they can be based on existing, proven mould designs or not. An existing design can be used and adapted to the mold design when possible since all bugs have been fixed and their behavior in production is well known.

The need for new solutions for specific problems arises inevitably. Some can be extended from existing solutions; others require a new approach. This makes previous design experience necessary to determine whether a new direction will succeed. Design should, however, try to provide the most simple solution possible to the problem.

In the case of a new mould tool, a sample moulding may often provide beneficial information, such as gate position and size, ejector pin size and location, etc. The mold maker should use an existing mould design with a new part unless it failed to mold it previously. For reliability and product quality, simple designs always beat complex ones. It is also advisable to use tested design and mould components instead of completely new ones.

To gain experience in mould design, one must gain these skills:

• Analyze other existing general arrangement diagrams.
• Toolmaking procedures to be familiarized.
• Understanding the working of mould tools during production.
• Identify amorphous vs. semicrystalline materials, and distinguish the differences.

How to?

Analyses of the predesign

In general, it is advisable to conduct some pre-design analysis before starting to design the construction of a building. It is an obvious statement, but you need to qualify it. Design as many moulds as you can first if you are not a mould designer. Develop at least three different approaches for splitting the line, ejection position, and gate position. This is the time to consider cavity construction, water cooling, etc.

Designers should emphasize solving problems, which can be difficult when they have little or no experience in the industry. Look at every moulding and try to detect its shape and how it was formed, its split lines and gates, how it was ejected, and any special features.

GA reading

As a result of this, you also gain a lot of valuable experience. Even experienced designers can’t read and understand most GAs in ten minutes. To fully understand how the tool design works in most cases, it takes hours rather than minutes.

This can be illustrated by the fact that a toolmaker will often contact the designer to clarify certain aspects of the drawing that he does not understand. If the drawing is unclear or even incorrect, it may be that the toolmaker is having difficulty understanding it; it may also be that the drawing is too complex for the toolmaker to comprehend.

It is not as important whether the designer has committed an error or whether the toolmaker does not understand the drawing. It is the willingness of both parties to resolve the problem. Such discussions frequently occur in practice.

Concepts related to toolmaking

Mold tool designers should ideally have some experience in toolmaking before designing a mold. There is no doubt that it is a prerequisite that you be certain that what you are designing can really be produced.

For those who have not had any experience in toolmaking, it is recommended to regularly visit a toolmaker to acquire an understanding of the various toolmaking techniques. In these circumstances, most toolmakers are reasonable and usually quite willing to conduct a short teaching session. Designing mould tools does not require a high degree of skill or expertise on the part of the toolmaker. Despite this, it is impossible to design mould tools without knowing whether what is being designed can be produced.

A mould design must also reflect the unique plant and equipment of different toolmakers. Others are equipped with relatively basic operations, while others have sophisticated computer-controlled machining centers. In addition, all tool makers have the following common equipment and machinery:

• A drilling machine
• Turning lathes
• Machines for milling
• Machines for grinding surfaces and cylindrical surfaces.
• Electro discharge machining

Examining Mold Tools

This is a noticeable shortcoming when you consider that many mould designers never see their production designs. For two reasons, all designers would be required to participate in the initial sampling trials:

1. It is common for molds to be complex and have many intricate details, which must be explained to the sampling technician. Several broken items have occurred due to incorrect understanding of complex unscrewing mechanisms and phased latch controls.
2. Mold designers can obtain valuable insight into their design and implement modifications to make the product more efficient.

A lot of instruction can also be obtained from watching other mould tools in operation to observe other people’s designs. It is also a good idea to inspect mould tools as they are being serviced or repaired. This can highlight areas for improvement in the future, such as seized-up parts, excessive flash, excessive wear, etc.

An Overview of Good Design Practices

1. Make your design as simple as possible.
2. Utilize standard components to the fullest.
3. Avoid new, unknown designs and use tested designs.
4. Analyze any existing samples that indicate gate positions, block locations, sinks, distortion, etc.
5. Make sure the toolmaker has the correct equipment.
6. Conduct sampling trials for feedback and to guide sample technicians.

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Mold design and construction

1. Ensure you understand the exact needs of the customer. Avoid guesswork.
2. To guarantee the molding will be produced as estimated and quoted, follow the estimator’s original design ideas and the customer’s approved drawings. The estimator and customer should be notified if the original idea should be altered.
3. Verify the mold drawing against the part drawing to ensure the mold produces the part accurately and in conformance with the specifications, including proper finish and dimensional accuracy.
4. If you want the mold construction to be as convenient as possible, check with the tool builder and the foreman of the molding operations.
5. If the mold is intended to be used in a press, it should have enough space to prevent crushing.
6. You must design the mold mechanically to fit the press you intend to use.
7. The press should be able to open far enough to allow the molding to be removed.
8. As soon as you have determined the size requirements for the mold, order the material for the mold.
9. As soon as the mold design is complete, design and order any jigs, loading fixtures, cooling fixtures, and the like that may be needed. Neglecting these items can lead to significant delays. Here, it is recommended to check the operations independently and then compare them with the original estimate to ensure all fixtures required are included and can produce the needed production.
10. Avoid employing too many impressions when making a multiple cavity mold. There may be flashing or uneven thickness of the pieces if the cavity area extends beyond the centre of the press clamping area. Additionally, the operator may find it difficult to see all of the pieces on the mold at once, which means he runs a lot of scraps before he notices a single cavity that may not be working. Too many cavities make it difficult to achieve uniform moldings because of differences in the size of the cavities, differences in runner resistance, and other factors.
11. Make loading and unloading arrangements that are convenient and safe for the operator.
12. Ensure that the molding stays in the core half until the ejector pins can remove it at the proper time during the molding process.
13. Make the mechanical parts of the mold rigid and free-moving.
14. Mold cavities and cores should be constructed sectionally to simplify machining set-ups.
15. Mold retainers and backing plates should be aligned with dowel pins.
16. To simplify the replacement of thin cores that are likely to break in long production runs, they should be designed as inserts if possible.
17. Do not use irregularly shaped _ inserts, e.g., hexagonal, which have to stick out of the surface of the mold, since the holes in the mold must be shaped accordingly to hold them in place during the molding process.
18. Molding practice takes advantage of both negative and zero drafts as well as positive drafts with varying amounts. A part can be forced to stay in a cavity by using a small negative draft. It is possible to mold deep holes with zero draft if they must be straight. Such cores should be highly polished and the surface should be wave-free. You may use positive drafts between a few thousandths of an inch and five to ten degrees so that the part can be easily released from the mold.
19. Mounting clamps should not interfere with steam line outlets or inlets.
20. If the steel is to be hardened, make sure there are holes drilled and tapped in the mold parts so the holes will all be in place prior to hardening.
21. To allow for maximum drilling run-out, steam lines should have plenty of clearance as they pass through bolt holes or ejector pinholes. It is recommended to allow for a clearance of 8″ for steam lines and 10″ for longer steam lines.
22. Utilize standard sizes of steel whenever possible for hollows, cores, pins, etc.
23. To reduce the need to carry large small parts stocks, use standardized screws, guide pins, and other components.
24. Keep an updated list of the steel required for mold parts and specify if substitutions are required on detailed drawings. Make sure that each mold part is marked with the type of steel used so that the heat treater will use the correct procedure when annealing before reworking.
25. A symmetric layout on the mold plates will prevent the layout man from being confused by screws, guide pins, wall thickness, ribs, ejector pins, and the like. Please notify the layout man if you have to offset a hole so that he can see it.
26. Mark the contours of opposite mold halves’ mating surfaces at the parting line so that the toolmaker knows which contours must match.
27. Plan the details so the tool maker has as little arithmetic to do as possible. Ensure that the machining operations for the mold part are properly set up and that the set-up dimensions are correct.
28. Make sure mold dimensions are within reasonable tolerances for the toolmaker.
29. Take into account shrinkage when ordering. Ask the manufacturer of plastic material for shrinkage data.30. Plates should ride on the guide pins, not on the mold cores. In order to prevent the stripper plate from falling off and to prevent incorrect assembly by the molding press operator, it should be retained by stripper bolts.
31. material types. This will be highly beneficial to you when performing critical jobs.
32. Be sure to keep track of dimensional changes for hobbing, carburizing, and heat-treating various steels.
33. Keep a record of miscellaneous computations and the data will be valuable on succeeding jobs.34. It is usually regretted when shortcuts are taken in design and construction.
34. It is usually regretted when shortcuts are taken in design and construction.
35. Take care to keep steam plates for a multiple cavity compression mold from extending too far beyond the ram area of the hydraulic press where it is intended to be used. Overhanging the platens of the press too much may result in difficulty closing the outer cavities as a result of the platens bending under the force of the press load.
36. To ensure the mold will not be crushed, check the projected area of the molding for the amount of capacity of the press to mold the plastics material selected, and then check the mold area for the amount of pressure of the press to determine whether or not the press can mold the plastics material.
37. Verify that no stripper plates are used in a multiple cavity compression mold. When a temperature difference exists between a stripper and a force plate, the stripper can bind so tightly to the core that it sticks when the stripper is first opened, causing the ejection mechanism to fail.
38. In compression molds, leave ample space for loading. Molding materials may require strangely large loading wells due to their high bulk factors.
39. As steam lines are being laid out for heating compression molds, they should run in series so that the steam has to pass through the passages all the way without the possibility of parallel lines becoming waterlogged by being bypassable.
39. As steam lines are being laid out for heating compression molds, they should run in series so that the steam has to pass through the passages all the way without the possibility of parallel lines becoming waterlogged by being bypassable.40. A uniform distribution of steam lines is required for uniform heating of compression molds for thermosetting materials.
41. Molds for thermoplastic materials require even distribution and close spacing of heating and cooling passages for rapid cooling and heating and uniform temperature distribution.
42. As a general rule, arrange the guide pins so that they are fastened to the rear, or movable half of the injection mold, so they will not interfere with the operator as he work seen the open mold halves.
43. Injector molds should be vented carefully. Sectional assembly may be necessary in order to prevent air from accumulating within the mold cavities, or the piece may need to be oriented in a way that permits the material to fill the mold gradually. Additionally, special ventilation passages may have to be made in the mold assembly.
44. To prevent bending of the cavity backing plates when molding material is injected, provide pillars in the ejector box.
45. You will need to make a thick backer and retainer.
46. Allow injection molds to be heated and cooled. Water or steam can be drilled into the passages to make heating and cooling easier.
47. Be careful when turning a runner as sharp turns increase resistance.
48. The guide pins should be long enough to provide support for a mold half that extends beyond the mold half so that the pins will enter the bushings before the cores enter the cavity; also, so that the guide pins will prevent the mold half from resting on the polished cores.
49. Keep from requiring the toolmaker to assemble retainer and backing plates at a certain thickness.

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Mold design for Toolmaker

1. Before cutting metal, thoroughly study the new job. Choose the sequence of operations and setups that will result in a finished tool part according to the specification.
2. If polishing will remove a great deal of metal, such as at the threaded crests, leave stock for grinding or polishing.
3. Identify the steel used in the tool parts. The heat treater will know how to handle it if it needs to be annealed and reworked.
4. Hobs should be recessed deeper than the finished depth required to ensure the squared block can be squared properly after hobbing.
5. Flashing occurs in cracks or openings smaller than .00lw or .002″ in width. Take appropriate measures to avoid flashing.
6. Do not press a polished mold surface too hard when fitting delicate mold parts.
7. To prevent losing the true machined contour, begin by removing the tool marks evenly. Grinder coarsely and then use a finer material to smooth out the scratches. Finally, use a “flour” grinding agent to polish the surface.
8. An inadequate polishing job cannot be covered by a chromium plate. A lustrous chrome plate can only be applied to mirror-finish steel. Chromium is very hard, and if the surface is rough, it is hard to polish into a luster.
9. A layer of chromium .0005″ to .005″ thick may be deposited on steel components. A worn tool or part that is too small can often be salvaged this way.
10. The metal surface of a mold may be upended to raise the depressed area to be polished back to its original level if there is a dent.
11. Toolmakers can repair pinholes or flaws by boring a hole in the steel mold cavity and inserting a plug, which is pressed or furnace brazed into place. It is best to do such repair work where the marks will not be critical, so that there will be no visible marks on the molded piece.
12. Atomic welding can be used to rebuild or repair mold parts that are damaged due to deformation. If correctly done (ideally by a specialist), the joint is undetectable.
13. Ensure that the pins can easily enter pinholes by chamfering the edge of the rear or entry edge.
14. Pinholes should be large enough not to require fitting for more than two-thirds of the pin’s length.
15. Polish core pins lengthwise so the molding can slide off easily.
16. Mold surfaces such as cavity mouths, pin ends, and other parts should be free of burrs. The burrs may not only reveal poor craft, but they may also scratch or make it difficult to remove the molding.
17. It’s often difficult to push the molding off freely if there is an undercut on the pin, even if it’s a few thousandths of an inch.
18. Before sending them to the press room for a tryout, ensure that the pins that enter cavities are not too long.
19. Provide square and free operation of the ejector and knockout mechanisms.
20. Ensure all screw holes and pinholes are in place before sending the parts for hardening and that the parts fit together correctly.
21. Be sure your computations are accurate before you carry out a cut.
22. Mark tapped holes from drilled holes with centering screws.
23. To ensure a good match between opposing mold halves, use templates for the parting line contours. As the blocks are clamped together, insert the templates through the holes drilled through them. When fastening the cavity blocks to their retaining plates, use the same dowels to hold them in position.
24. The mold drawing specifies the dimensions to be used.
25. Contact the designer before going too far when you see an apparent error in a drawing.
26. Make sure you adjust your machine tools’ backlash and angular errors.

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Mold design For the Operator

1. Clean the molding material regularly. The mold or other molding equipment can be damaged by dust or dirt, and small pieces of metal can ruin the mold.
2. The containers of molding material should not be placed on the floor, because the floors may flood from time to time, and the moisture absorbed by the floors can damage the molding material.
3. Molding material that has been stored in a cool room should be dried before using in order to remove any condensed water that may have collected after moving it into a room with a higher humidity level.
4. When a run is complete, the stock department should receive unused molding material and inserts.
5. Keep thermoplastic scraps clean. You can grind them and reuse them. You should not save overheated scrap since it may have been damaged due to heat breakdown, plasticizer loss, or color degradation.
6. Putting scrap molds with metal inserts in the same container as sprue scrap from thermoplastic materials is not recommended. Keeping the moldings separated will make it easier to remove the metal before they are ground. Magnetic separators cannot pick out brass inserts due to their magnetic properties. Injection molding machines will typically plug their nozzles once the chips from the scrap grinder get into the nozzle.
7. Scrap thermosetting material with metal inserts should not be disposed of in the waste barrel. You can salvage them.
8. Ensure that the mold has been cleaned properly after the molded piece has been removed before beginning another cycle.
9. It is not advisable to remove stuck moldings from a mold with steel picks, bars, or rods. Carefully remove them with brass or wood.
10. Be alert for bending or broken core pins or burrs on the mold which may result in defective moldings.
11. Remove the moldings from the mold without distorting or marring them.
12. Learn the causes and cures for the most common mold practice difficulties.
13. Change the molding conditions or the curing time without thoroughly reviewing the results. Insufficient curing of thermosetting materials may result in loss of heat or water resistance, or the material may not shrink to the desired size. Due to too rapid loading or being removed from restraining cores too early, thermoplastic injection molded parts may develop air bubbles or shrink too much. The color of the product may change due to overcuring or overheating.
14. A good mold practice depends heavily on good housekeeping.
15. Make sure the ejector mechanisms remain free-running by lubricating them periodically. Avoid excessive lubrication so that oil or grease gets into the mold cavities and contaminates it.
16. In order to keep the molding machine operating properly, it should be properly lubricated.
17. The length and adjustment of the ejector pins must be correct. Make sure that the ejector space and knockout plates are seated properly by regularly checking for flash under the ejector or knockout plates.
18. When the mold is in the press, use a mirror to inspect parts of the cavity you cannot see directly.
19. When removing a mold from service and before storing it, be sure to clean, polish, and grease it. The mold should be repaired if necessary so that it is ready for production once it is taken out of storage.
20. The inspector should check the first pieces and recheck them as necessary.
21. Before running the mold, make sure that it is properly seated in the press.
22. If inserts are being used, make sure that the correct materials are being used and that gages are available.
23. Find out how the job should be run.

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Mold design for the estimator

Before attempting any estimation work, thoroughly examine the available data. If you don’t have enough data to make an informed decision, ask for more information.

1. Select the best molding method and arrangement in the mold based on an analysis of the part.
2. Calculate the amount of impressions needed to maintain the production rate on the available equipment.
3. Sketch out the mold and figure out roughly how much space it will require.
4. Determine the material costs by calculating the steel requirements.
5. Determine how much the mold will cost. Mold sinking methods must be thoroughly understood, along with the specific equipment needed for making a mold.
6. Estimate the mass of the mold piece. When performing this operation, it is a good idea to prepare the mathematical tables in the form most convenient for each estimator. The formulas for volumes and areas that are often not found in standard mathematical tables can often be worked out by hand. It will be very helpful in the future if these special formulas are kept in a file.
7. Add a reasonable amount of rejects and waste material to the total material requirements for the molding. Depending on the complexity of the mold, the required tolerances, the number of finishing operations, and the type of material to use, rejects may range from 5% to 30%. Waste can amount to 5% or more.
8. Provide a brief description of the molding, finishing, and inspection operations.
9. Identify as many elements as are necessary to accurately estimate the time involved for each operation. Even the most experienced estimator will need to break down some complex operations completely even if they have accumulated enough experience to do so. The best thing to do when in doubt is to breakdown.
10. Calculate net rates of production after taking into account personal time, setup time, repair time, and production loss resulting from damaged cavities. During the running of some jobs, you can time them out and then check the actual production over a typical 24-hour run and over an extended period. Taking the actual pace divided by the stopwatch rate is a fairly accurate way to calculate the rate.
11. Create a form that includes all the items that contribute to the final cost. Arrange the items in a logical order so that they can be calculated from what has come before.
12. Regulations governing government price control call for uniformity and consistency in price setting procedures.
13. Other costs include direct labor, amortization charges for molds and tools, factory expenses, supervision costs, and maintenance costs, as well as sales and administrative fees.
14. If special tooling is needed for finishing and inspection, be sure to include it in the tool estimate. Molds are the largest part of the tooling cost, but auxiliary items often add significantly to the cost.
15. An outline of the process and operations should be written after the rough draft of the estimate is completed. This procedure will prevent a great deal of confusion and misunderstanding. This is especially true in the event that it takes a long time between submitting the estimate and getting the order to proceed with the work.
16. In order to keep track of actual costs for comparison purposes, the estimator should review or follow up on as many estimates as possible to keep an eye on efficiency.
17. Estimates should be as accurate as possible in business. Underestimating a job can lead to large losses or delayed production, while overestimating can lose needed business.
18. You should provide the customer with a detailed part drawing of the molding as it is proposed to be delivered, and have him review and sign it.
19. It’s important that you spend enough time with the mold designer to ensure that he understands how the job was planned when the estimate was prepared.
20. Make sure the final tool designs will produce parts according to the detail part drawing.

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Mold design for the Finisher

1. Consult the appropriate people to determine what specific finishing is needed after reviewing the drawing of the finished molding.
2. Ensure each finishing operation is performed with the correct tools.
3. Provide the appropriate gauges.
4. After a short period of time, check to see if the production rates are within the estimated range. If not, examine how you can bring up to standard those operations that are falling behind. Meeting delivery dates is not just economical important, but also necessary for keeping up with estimated speeds.
5. When setting up jobs for operators, take safety into account.
6. Make sure the aisles are clear and the materials in process are organized. Housekeeping doesn’t have to be expensive.
7. Have drilling operators grind their drills properly, so they are competent. Between properly sharpened and improperly sharpened tools, the production rate differs by 20% to 50%.
8. Verify that the machining speed is appropriate for the job. This will affect the speed and quality of the production.
9. If moldings are coming to the finishing department in poor condition or with heavy flash, contact the molding department. Bad moldings result in an inefficient finishing process.
10. Keeping a salvage department separate from production finishing is often advantageous to rework rejected pieces as well as to save small lots of moldings with heavy flash or that are otherwise difficult to process under normal production procedures.
11. To prevent unnecessary backtracking of parts within the department, group production machinery from the molding department to the inspection department.
12. Don’t store rejected or overrun parts in the working area where they are needed. Dispose of them as soon as possible to prevent them from being forgotten.

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Mold design for the Inspector

1. Before making changes to the mold to correct the samples, check the first samples from a new mold to make sure they are correctly molded.
2. The actual molding dimensions should be marked on the part print along with the dimensions of the molding. Keep the print with the actual molding dimensions marked on it for reference.
3. Keep samples of moldings as references in case of disputes or complaints.
4. Gages and templates not in use should be stored to prevent their loss or damage.
5. Do not use the master measurement instruments for production gages.
6. Measurements must be accurate. Verify before moving on.
7. Avoid long scrap runs by keeping a close watch on production operations.
8. Ensure that production gauges are inspected before and after production runs to determine whether any repairs are necessary. Order the necessary repairs in advance.
9. Make sure all gauges issued during a production run are returned.
10. A quality control inspector can make minor repairs to finished parts, such as removing burrs.
11. Ensure that bench inspectors have adequate lighting.
12. If you are initiating a new job, verify that inspections are planned at what points in the initial process. Be as generous with inspection standards as possible while maintaining the plant’s reputation and with allowances granted by customers.
13. Verify the inspection rates compared with the estimate. By having too many rejects, you lower the inspection rate and waste time examining poor quality materials.
14. You should keep inspection jobs in phase with production in order to prevent piles of finished material awaiting final inspection.
15. The inspection system requires a system to assign responsibility for each operation in order to trace back to the source of rejected moldings.
16. Inspecting the entire molding process allows the inspection department to suggest improvements in procedure to enhance the quality of the products.