Plastic Molding Part Design
A reasonable and optimal molded part structure design can not only simplify plastic injection mold manufacture, reduce the cost of the mold, but also to simplify its molding process to improve the good quality rate of products.
Before the design of plastic parts, a designer should learn about the function, environmental conditions and loading conditions （including dynamic and static load), understand relations between different parts and the effect of the assembly. Plastic parts function should be accurate and as detailed as possible, the more comprehensive the plastic parts design are, the better the design of the plastic parts can meet the requirements.
There are two main causes of plastic part failure: molded-in notches, and sharp internal corners. These are largely caused by designers failing to provide the part with a sufficient internal radius to avoid the issue. When radii are noted on drawings, they are often inadequately small, and their importance in high-stress areas is neglected.
In reality, inner corners require a carefully specified radius, to help cut down on stress concentrations at these essential areas. If the corner is too sharp, then the force factors at this point can be tripled or even quadrupled. For that reason, it is recommended that a fillet or radius of 0.5-0.6 times the wall thickness is incorporated at these points. Such parts will allow one to reduce stress on radiused corners compared to sharp, nonradiused sections.
It should be noted that materials with poor elongation qualities are highly susceptible to cracking under external and internal forces- for instance, a boss where a screen expands its outer diameter during assembly.
It is also necessary to specify radii on exterior corners of certain parts where a square-cornered shaft will need to fit into a plastic part. Examples of these parts include pulleys and gears. Putting radii on external corners has many benefits, from sink and stress reduction, to assisting with the material glow. Put simply, the larger the radius, the more impact load that section will be able to absorb.
For instance, some designs will require a sharp, 90-degree corner. If the part is subjected to impact, then radii are essential to absorb the energy at the corners, and reinforce the ribs of the part. However, radii can have a negative impact on the appearance of parts, as well as their ease of packaging. If the radii become too big for these factors, then the designer may wish to solve the problem by opting for an impact-modified material in the manufacturing process.
Under test conditions, when the radius is increased, parts are able to absorb more energy before failing, and in many cases, failure is averted entirely. Impact tests to external corners show that energy absorption can be increased up to five times by incorporating larger, more rounded corners- and at the top of the part, this increase can be as much as ten times. On the other hand, if ribs are added to make the part stiffer, they can have a negative impact on the part’s ability to absorb energy, since the walls cannot deflect.
If the rib section is properly designed, though, and an impact-modified resin is used, then even a part which was otherwise problematic may work well in practice. For instance, polycarbonate is extremely good at absorbing energy, and when coupled with a larger radius, means the part will be able to absorb a lot more energy before failing.
To gain a better understanding of why thermoplastic materials behave the way they do, and why some are naturally stronger than others, it is necessary to understand how they behave under standardized ASTM testing of physical properties. All plastics demonstrate a similar stress/strain curve, where the failure occurs at or close to the top of the curve (although this will vary depending on filler and reinforcements). Filled or reinforced resins come with a lower elongation- some 10% or under- meaning they are less able to absorb or deflect energy upon impact. This means they need a larger radii to cut down stress concentrations when they are faced with loads or impacts.
In the case of short or long glass-reinforced resins，this rule often comes under question when datasheet values are examined. In these cases, the data would seem to suggest that more highly reinforced resins have an increased impact value. In truth, though, this is down to the effect of the glass fiber/resin matrix strength at the notch; it does not reflect the absorbing value of the base resin itself. With a higher glass loading, a greater radius is needed to make the part tougher.
Nonuniform Part Thickness
After insufficient radii, the next leading cause of plastic part failure is nonuniform wall thickness. This can cause wider tolerances, warpage, voids and sinks, a poor fill due to pressure drops, and stresses which are molded-in. All of these issues are down to bad part design, which stops the resin from being able to pack out during the molding process fully. If uniform sections are used, then these can all be avoided.
Molded-in part stresses are often caused by a combination of thick and thin sections. When the part cools, this leads to different shrinkages. Since the thicker section holds more heat, this can lead to all manner of issues such as sinks, voids, and warpage.
It can also lead to flow problems around corners since it encourages pressure drops and dead pressure flow areas. If nonuniform section thickness is used, and thin part sections feed thicker sections, the pressure will drop in the thicker section. That may mean that the part is not sufficiently filled. On top of this, the thin section could become over-packed when trying to fill the thicker section.
Designers will, therefore, need to transition and core out thick sections, to make the part thickness as uniform as it can be. Not only does this reduce these internal sinks and voids, but it also makes the part stronger, and saves on wasted materials.
Uniform wall thickness should, therefore, a key part of design, and parts should be gated into the thickest section to ensure an even packout. Since material waste costs money, this should be the aim of all manufacturers, and it also cuts down on quality issues further down the line. Plastic parts will need to be cored out to the maximum to utilize material better, as well as minimizing the risk of part stress and warpage. If extra strength or rigidity are needed, then ribs are to be used.
Ribs for Strength and Quality
Where added part strength and rigidity are necessary, then thinner rib sections make for better designs. Multiple smaller ribs, or a single large yet thin rib, are more effective than one large, thick rib. This can be demonstrated by calculating the stiffening effects of each option. In the case of thinner rib sections, molding and part problems are cut, and the part gains additional impact resistance because molded-in stresses in these areas are lowered.
Designers should, therefore, use the moment of inertia calculations to come up with a rib design that reduces the risk of part problems whilst simultaneously strengthening the part. An effective design will use the plastic material in the optimum proportions to maximize the results. Proper rib design also sidesteps molding problems, and therefore gives a faster molding cycle with less quality control issues.
It is also important to note that color variation will occur in thick and thin sections. With a thicker section, the color will appear more vibrant and intense. This is especially noticeable when using translucent colors, and with certain types of resins depending on the section thickness.
Wherever the material is required to flow around an obstacle, hole, boss, or cutout, a weld-line will be formed, at the point that multiple melt flows meet. If this weld-line forms at a high stress point, then this may present a big problem to the integrity of the part.
Designers will need to make a note of high-stress areas on the part, allowing the mold designer to select the best gate location for the part carefully. The issue is often only noticed when part failure occurs, but through careful planning of cavity layout and gate location, it can usually be averted. So long as potential weld-line issues are known in advance, then there are several ways that those issues can be minimized.
Shifting the gate location, or working to minimize the weld-line effect at these obstructions are both valid options. Another option is multiple gating, which will naturally change the point at which the melt fronts meet, and the weld-line is formed.
One way to strengthen weld-lines at the very edge of a part is to use flow tabs, which will be removed later in the process. These tabs are typically used at the points where weld-lines form, to help the melt fronts to flow together smoothly. To use them, though, the mold cavity at this point needs to be suitably vented, or the air in the cavity will prevent a smooth meeting.
Excess air could slow down the melt fronts, and cause bubbles to form. Should the air not be vented fast enough, then it could well cool the melt fronts so that they don’t bond properly, creating a significantly weakened melt-front. To solve this issue, extra localized heating is often used on weld-lines to ensure the melt fronts come together nicely.
Weld-lines should be avoided in parts that require holes, and there are a range of methods to do this- especially if the lines occur at high-stress points. The holes may be marked for drilling later in a secondary operation. If countersunk bind holes are needed, lA of the section thickness should be retained. This will allow the material flow to minimize the weld-line formation opposite the hole. Where blind holes are concerned, the minimum thickness of the bottom should be at least ]/6 diameter. With reinforced materials, the part’s weld-line strength can be calculated by using the parent resin，s strength.
The reason for this is that reinforcements such as glass fibers will not flow over the weld-line to link with the opposing melt front. Even when using an external flow tab, reinforcement interflow is unlikely until the fronts are suitably far away from any obstruction. In the case of some part designs, the weld-lines may well be highly visible. For instance, using colors or reinforced materials means noticeable surface variations will occur as the knit line.
To increase weld-line strength, the melt temperature of the resin may be raised, or the mold itself heated. There are usually more issues with weld-line strength when using amorphous resins since they have a lower softening temperature and higher melt viscosity. Crystalline resins，which have a sharper melting point，may cool and freeze off faster, leading to a weak weld-line joint. Using flow tabs with sufficient venting，increased melt temperature, localized cavity heating, and hotter mold temperatures can limit these weld-line effects, and thereby improve joint strength.
Bosses act as the assembly point for mating parts. As such, they will need to be designed with a good strength of attachment in mind, but at the same time, not cause any problems with the part’s surface or appearance. To help prevent the latter, it’s a good idea to situate bosses behind the nonvisible surface of a part. Should bosses or ribs not be designed correctly, then this can lead to weld-line, void, and sink problems in the parts. Whether the boss is open or blind, they will still require radii, and need to have a uniform wall thickness throughout. To ensure this, without compromising on strength, the boss may be strengthened and supported through additional ribs.
The final dimensions of the boss will largely be determined by the type of fastener used within it. Material suppliers will be able to recommend the optimum boss design for their own respective material. This design is based around the insert or screw type required for the attachment, as well as its necessary holding strength.
There are, however, other methods of boss design. These require through holes in order to cut surface effects, while also increasing the strength of the boss and weld-line. This technique is quite similar to the one used for blind holes, allowing the material to flow around and through the bottom section of the boss. The base of the boss can then be drilled out in an additional operation, which will complete the part’s requirements. That being said, it is essential that part designers keep the molding, assembly function, and appearance in mind when they design these parts.
If blind holes are to be molded in a part, then the core pins which make up these holes require rigid support. This is particularly important with core pins where the length-to-diameter ratio is more than five. Core pins have to be tough and stiff in order to not shift or bend under the high injection pressure and speed at the point where polymer is injected. Should the given pin length-to-diameter be exceeded, then it may be necessary to use a stepped core pin design. Cooling will also need to be factored in, since these types of core pins become incredibly hot during the molding process. With holes in a side wall, retractable core pins may be used, or alternatively a split tool (but only if the side wall can be tapered or sloped to allow for this).
Either external or internal configurations may be used to create threaded plastic parts. Usually, the external thread will be created by finding the parting line on the center line of the thread. Where this is impossible, or if the threaded part is in the mold operation direction, then an unscrewing, thread-forming device, or cam-operated side cores, will need to be used for the mold.
On the other hand, internal threads will require an unscrewing device or collapsing core. They can be stripped from the mold, but only when the threads are well-rounded, and their depth and number of undercuts are kept to a minimum. Should one choose the unscrewing approach, then the individual part in the mold should be indexed, so that it doesn’t rotate within the cavity while the core is removed.
To strip threaded parts, the thread will need to be formed with adequate radii. Should sharp thread forms be used, then they could well split upon stripping or become impossible to remove from the core.
With parts that need to be stripped (such as in an internally threaded boss), then the cavity steel will first need to be removed from the external diameter of the boss.
That way, the boss can expand when the core is stripped. Both boss and part will need to be adequately supported throughout the stripping action, to prevent the part from warping. The boss must be stiff and strong enough to avoid collapsing or fracturing. Threaded parts which have a diameter-to-wall thickness ratio of at least 20:1 must be ejectable should the resin have the necessary elongation to be ejected off the core pin.
It is also possible to strip some glass reinforced resins from a mold, but only where the part temperature is high enough, and the per cent strain isn’t exceeded. During the ejection cycle, hotter parts have a greater amount of allowable material strain. For instance, a 33% glass reinforced nylon may be stripped from a 100 degree F mold only if the strain is below 1% or a 200 degree F mold below 2% strain. The material supplier should be consulted to ensure the right design is used for optimum ejection.
Molded-in threads must be terminated at a minimum of 0.1 inches from their ends. This is to cut down on material and part fretting due to repeated assembly and disassembly, and also stops compound sharp corners at the ends of threads.
Undercuts are a vital feature for plastic parts. They are mostly used to form attachment and assembly features, which will cut down on part costs and assembly hardware time. Parts which feature external and internal undercuts may be formed in three basic ways- either collapsible and pulling cores, split cavity molds, or being stripped from the mold in the same way as screw threads. Depending on the required degree of the undercut, as well as the return angle which forms beneath it, the majority of parts- including reinforced materials in many cases- may be stripped from a mold. It is only when the return angle nears 0.5 degrees that an undercut is no longer strippable. This sort of internal undercut can be created through the use of two separate core pins. Depending on the length-to-diameter ratio of the pins, one end can nest inside the mating pin to give additional stability, or they can slot tightly together to prevent any flashing at the pins’ meeting point.
There are other nonstrippable undercuts which use through-the-wall cores, or alternatively an offset ejector pin system. Due to the bending strength of the steel, this undercut depth is limited to the thickness of the plate or pin used to form it. What’s more, the undercut also needs to be assisted by additional knock-out ejector systems. A taper of at least 2 degrees needs to be present on the undercut to ensure proper release during operation, particularly when molding resins that are highly shrinkable. This will cut down on the bending stress on the sliding core, as well as on the wear to the core where it comes into contact with mating metal parts.
It is also possible to strip undercuts in glass-reinforced resins, but only if the design doesn’t exceed the material’s elongation. The allowable undercut percentage is about 1-2% for most materials. Using generous radii and release angles will aid in a smooth release, and cut stress concentrations during the ejection.
When stripping undercuts, most of the parts used are round. This is because rectangular and closed-wall shapes are unstrippable. Should a full-lipped undercut be used, then the container would likely bow, and hence lock up the part at its corners. This process would usually destroy the part, or at least stick in the cavity or on the core. For undercuts at the center wall sections to remain strippable, they will need to be kept short, and the supporting cavity steel removed first. This will allow the side wall of the part to deflect upon ejection. The undercut section of the stripped part will therefore need to be capable of deflection in order to be ejectable.
Assembling parts use threaded inserts. These will either be molded in to the part, or attached on in a later operation- either as a press fit, or inserted ultrasonically. Primarily, they are used to greaten the assembly holding force of parts, cut down on material creep at assembly points, and to enable access to areas that will need to be serviced.
Inserts often cost a lot of money, and depending on the part in question and how they can installed, they can be either helpful or harmful. As with bosses and ribs, they should therefore only be used when necessary. There are cheaper and easier alternatives, such as molded-in snap and press fits and screws, which may be a superior alternative. There are four main reasons why inserts are used:
- For threads which will be under a constant load or stress, or where parts will be frequently disassembled.
- To give a close tolerance on male or female threads.
- For permanently attaching two parts which are highly loaded- for instance, a gear shaft and a pulley.
- To allow an electrical current to flow between the two parts.
If it has been determined that inserts are required, then the next thing to think about is whether one will use inserts which are molded-in, or add them in a secondary operation. For parts made up of high-shrinkage material, where the exact dimensional location must be very precise, or if there is a risk of boss stress fractures, then ultrasonic insertion is the preferred approach. This is because it allows an exact insert location, and cuts down dramatically on the stress on the boss.
On the other hand, the reasons for using molded-in inserts are a bit more complicated. While they are cheaper and have the advantage of being permanent, there are still factors which need to be considered before they are chosen. The following problems may occur when using molded-in inserts:
- Loading the inserts by hand will necessarily disrupt the molding cycle. While the use of robots will prolong the cycle, it will at least even it out.
- Inserts can become unattached or float, which will damage the mold.
- Degreasing and preheating is required to cut down on boss stress.
- Salvaging rejected parts with inserts is much more expensive.
- In order to avoid flash in the threaded areas, a tight shutoff is needed at the insert mold face.
The weld-line strength around the inserts is also something that needs to be considered. Boss inserts for molding should be designed around the existing criteria for keeping weld-line formation to a minimum and avoiding lines that are too visible. When using reinforced resins, the weld-line strength might only be 60% of the unreinforced resin due to a weak bonding along the knit line. However, the boss strength at the mold line can be boosted by putting a rib at the weld-line junction. By using multiple ribs, one can add additional rigidity and strength. The following factors should also be taken into account when using inserts:
- Avoid sharp corners, and include an undercut to increase pullout strength.
- The insert should protrude at least 0.6 inches into the mold cavity to give a superior seal off and prevent resin contamination to threads.
- Below the insert, the thickness should be at least 1/6 of the insert diameter to add weld-line strength.
- Toughened grades of materials, with higher elongations, will lower the risk of boss stress cracking.
- Inserts must be clean and free of oil or grease.
- With high-shrinkage resins, the inserts should be preheated to lower part shrinkage and improve weld-line strength.
- A thorough end-use test should be carried out, it could test the part through temperature ranges and stress and vibration loading, to check for any problems with the selected assembly method.
A high degree of accuracy in molded parts is expensive to achieve. On small tolerances as close as -0.002 in. On large pieces, tolerances of about 土0.001 to 0.002 in. per in. are obtainable. Tolerances closer than actually mandatory should not be specified; as specified accuracy increases, cost increases disproportionately.
Though it is difficult to generalize about design factors for injection, compression and transfer moldings, the following design rules should be considered:
- Use sufficient draft on long thin shapes to permit their withdrawal from the mold.
- Minimize coring. When cores are used they should be easy to withdraw.
- Avoid internal and external undercuts; they make removal of parts difficult and require considerably more expensive molds.
- Provide ample fillets on inside comers, and avoid sharp external edges and comers except at the parting line of the die.
- Avoid large flat areas. Dappling or otherwise breaking up the surface is recommended.
- Keep tolerances as liberal as possible. Excessively close tolerances add to cost because of increased die costs and high rejection rate.
- Avoid abrupt changes in wall thickness.
- Locate parting lines so that flash can be removed easily without marring surrounding areas.
- Locate holes for easy coring.
- Use ribs to achieve desired strength and stiffness. Ribs permit materials savings by reducing section thickness.
- Use inserts for threaded holes where high stresses are anticipated or where considerable wear is to be encountered. Round inserts are preferred.