Part 7. Basic Mold & Part Design Guidelines
Plastic parts and injection molds require many design consider-ations to meet the expectations of the consumer. The following plastic part and mold design rules should be taken into consider-ation when designing either a part or mold.
7.1 Uniform Wall Thickness
Design the part with the most uniform nominal wall thickness as possible. This will minimize sinking, warping, residual stresses, material usage, and improve cycle times. Ribs should be placed in areas of stress instead of increasing the wall thickness. By minimiz-ing the wall thickness you will reduce the cycle time, shot weight, and have quicker cooling.
7.2 Corner Radii
All corners should have generous radii to avoid excessive stress concentrations. Such stress concentrations can lead to unexpected part failure and should be minimized during all parts of the plastic part production. All features such as bosses, ribs, and shut offs should have generous radii to reduce stress concentrations. To maintain uniform wall thickness and reduce stress concentrations, the use of an inside radius of ½ the nominal wall thickness and an outside radius of 1½ of the nominal wall thickness is recommended.
7.3 Ribs and Gussets
These design features should be placed anywhere the part needs in-creased stiffness. These features will reduce the material consump-tion, decrease cycle time, as well as increase the structural integrity of the part. Always avoid excessively tall, thick, thin, and sharp cornered features when possible.
7.4 Draft Angle
An appropriate draft angle will aide in the removal of the part from the mold. Without the proper draft angle parts may become hung up or stuck upon ejection and mold damage may occur. Be sure there are appropriate draft angles on all bosses, ribs, and nominal walls. The draft angle is most often between 1° and 2° depending on the material and desired surface finish.
7.5 Undercuts
Undercuts are mold features that prevent part ejection or mold opening. Undercuts such as clips are commonly added to the part design to simplify post molding operations such as assembly. Unfortunately, the features increase the cost and complexity of the mold because of the additional mechanisms needed to free the part from the mold.
7.6 Surface Finish
Most plastic part designs call out a specific surface finish or texture. These features may significantly increase the cost of the tool and should be taken into consideration based upon the application. Highly polished molds require many hours of work to achieve a mirror finish and can be exponentially more expensive than the same tool with a rougher finish. The highest finishes require all lower grades of finishing to be performed before they can be achieved. Textured molds can be very difficult and time consuming to make and may require expensive equipment or a higher degree of skill than polishing.
Figure 7c: Runner Shapes and Dimensions
Figure 7d: Suggested Circular Runner Dimensions
Figure 7e: Suggested Vent Depths
7.7 Tool Steels
Tool steels for a particular application are typically chosen based on their hardness, abrasion resistance, machining rate, resistance to deformation at elevated temperatures (red-hardness), and/or chemical resistance. Tool steel is generally heat-treated and an-nealed to produce the desired properties. The table below is a listing of common tool steels from DME Company LLC used for various structural and holder block applications.
Figure 7f: Steels for Structural and Holder Block Applications
Figure 7g: Common Mold Steels & Important Characteristics
Figure 7h: Common Non-Ferrous Injection Mold Metals & Important Characteristics
7.8 Basic Heat Treatment
Heat-treating is the act of heating and cooling the steel at predeter-mined temperatures and times to adjust its’ hardness and strength. Steels that consist of 0.4% carbon or more can be heat-treated.
During the heat-treating process, the chemical structure and dimensions of the steel are changed. As the steel is hardened, it typically increases in size and may cause distortion. The amount of dimensional change is case dependent. For example; an increase in hardness of 20 Rockwell C during heat treatment will result in a more severe change than an increase of 10.
When heated, the steel reaches a specific hardening temperature which transforms it to an austenite state. In the austenite state, the carbon is dispersed within the steel and then must be cooled.
When cooled slowly, or annealed, the steel is changed into a soft-ened state, called pearlite. Annealing involves placing the steel into a medium, such as hot oil, warm air, or salt. This process is used to soften the steel before machining, or to significantly reduce the stresses within the steel after machining.
When the steel is cooled quickly from the austenite state, it changes to a very hard, yet brittle, state called martensite. Hardening is performed in a medium, such as cold oil, air, or water, which quickly removes the heat from the steel.
Tempering involves heating the steel to a temperature slightly below the hardening temperature. In this process the steel slowly cooled to reduce hardness and to increase durability.
As with tempering, stress-relieving involves heating to a temperature slightly below the hardening temperature and then slowly cooling the steel. This reduces stresses within the steel before hardening to help reduce distortion. Stress-relieving is performed after a large amount of steel is removed, or after welding.
7.9 Common Surface Treatments
Surface hardening heats the surface of the steel to above the hard-ening temperature through the use of lasers or electron beams. The surface cools quickly and a hardened layer of steel is formed. As with heat-treating, this procedure can be performed on steels having a carbon content equal to or greater than 0.4%.
Steels that cannot be surface hardened are sometimes carburized. This process heats the steel in the presence of a high-carbon medium, such as leather or charcoal. The carbon-rich surface is then cooled quickly for hardening.
When processing corrosive materials such as PVC and acetals; mold components are often nitrided. This process involves heating the steel in an ammonia-rich environment to incorporate nitrogen into the surface.
Boriding involves surrounding the steel with boron-rich metal plates. The boron migrates to the surface of the steel being treated and creates a thin, yet hard layer of iron boride. This process cannot be used for components having complex geometry, since the steel must come in contact with the boron-rich plates.
7.10 Common Surface Coatings
To increase the surface hardness of steel, a layer of titanium carbide or titanium nitride is often added. Titanium carbide is among the hardest coatings available and is applied to prolong the life of high-production cores and cavities. Titanium carbide or titanium nitride coatings are commonly used with abrasive additives, such as glass fibers, talc, and mica.
Nickel or nickel-teflon plating is used to improve the chemical resistance and to reduce the surface friction of the steel. These are commonly used in applications to improve the speed of part removal due to sticking to either the core or cavity.
Adding a layer of chrome to the surface of the steel increases both its’ hardness and its’ chemical resistance. Chrome plating is usually reserved for treating highly polished core and cavity surfaces, or for use with polymers which create residue, such as ABS, PVC and polyethylene.
7.11 Wear Considerations
Galling occurs when mold materials of similar hardness wear together though contact. This results in the two mating surfaces balling up and creating large grooves on either side. To prevent gall-ing, mating materials must have a difference of at least 6 Rockwell C hardness.
To help reduce wear on actions, wear plates made of softer materi-als should be used whenever possible. These should be checked frequently and replaced whenever excessive wear is detected. Brass is commonly used for high wear components since it is easy to machine and will wear away before the harder steel components. Brass plated steel components are even more effective, having the wear characteristics of brass with the increased strength of steel.
Although expensive, graphite impregnated materials should be con-sidered whenever possible. These materials contain graphite inserts imbedded into the metal, which lubricate the surface and provide superior wear resistance.
7.12 Porous Metals
Porous metals allow gas to vent through the metal and are available in both porous steel and aluminum. To prevent volatile build up, pressurized air should be forced through the porous metal between each cycle.
Porous metals must be machined using either wire EDM or die sink-ing EDM machines. Other machining methods, such as milling and grinding, damage the surface by covering the pores. Since porous metals tend to be expensive, they are commonly reserved for small mold cavities, or used as inserts where localized venting is desired.
