Construction Materials
The appropriate mold steel should be selected based on application requirements. For prototyping, hardened mold steel is unnecessary. In most cases, pre-hardened steel or aluminum is used to reduce costs and facilitate mold adjustments during the prototyping stage. These soft metals can also mold sufficient test pieces and prefabricated parts. Pre-hardened mold steels (such as P-20 or NAKR-55) are often used for large-scale molds because hardening the mold steel is impractical for large applications.
For higher production volumes, hardened mold steel is required for the core and cavity. S-7, H-13, and stainless steel 420 are the most commonly used steels. S-7 is an excellent mold steel that can be used for extended production operations. If very high melt and mold temperatures are required, H-13 steel should be selected. H-13 can also be used to manufacture hot runner manifolds. H-13 has a very high tempering temperature and can withstand very high mold processing temperatures without affecting its hardness.
Stainless steel is the best choice for mold steels where high wear resistance is required or where significant condensation occurs in the environment. High-wear mold cavity inserts can be made using A2, ASP23, or D-2 type steels.
All mold steels can be protected against wear and corrosion by some type of coating. Only stainless steel can be repaired through welding and machining. For steels with existing coatings, repair is only possible after the coating is removed. After repair, the coating must be reapplied.
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Surface Finish
The surface finish of molded parts can vary depending on aesthetic requirements, customer needs, and functionality. From the mirror-like high-gloss finish of SPI #1 to textured surfaces achieved through etching, LNP composites can achieve virtually any type of molded part surface finish. It is important to understand that some materials exhibit improved performance with certain mold surface finishes. For example, polypropylene shows significantly better release properties in matte molds than in high-gloss molds. Achieving a high gloss finish is difficult with high-filler resins. Venting
Ventilation is a crucial part of the molding cycle to maximize mold performance. When thermoplastic material enters the mold cavity, air needs to be expelled. Vents are typically located at the last filled areas, near the seam line, and on the runner system. Additional vents distributed around the parting line can significantly improve overall venting performance. Residual gas in the mold appears as burn marks on the molded part. A basic rule of thumb is that gas in the mold must be able to escape at the same rate as the plastic entering the cavity.
Void depth varies depending on the material used – generally, amorphous thermoplastics require greater void depth due to their higher viscosity.
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Single-Cavity and Multi-Cavity
Three basic types of molds are most commonly used in industry, especially for molds using LNP composites.
The most common type is the two-plate cold runner configuration shown in Figure 1. Material is injected into the part through the runner bushing and the runner system. After cooling, all molded parts are ejected (including gates, runners, and sprues). The parts are then removed, and the remaining runners and sprues are discarded or recycled.
Figure 2 shows a three-plate mold with cold runners. With this design, material is injected into the part cavity through the runner and sprue system. After cooling, the mold is opened, separating the part from the gates and runners. The part falls off naturally from the runners. The runners, gates, and sprues still need to be recycled or discarded.
Figure 3 shows an electrically heated hot runner manifold mold. The manifold and gate probe channels are heated to or near the polymer's melt temperature, keeping the material molten. This eliminates the need to handle waste from the runners, gates, and sprues. These systems are more expensive than conventional two-plate or three-plate designs but offer better thermal control of the material and generate little waste.
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Hot Runner
Full hot runner molds, or "runnerless" molds, offer many advantages, but require careful design to ensure proper control of the melt flowing into the mold cavity. While runnerless molds generate virtually no waste during molding operations, they are prone to problems at startup, especially with semi-crystalline LNP composites, which are prone to rapid solidification. Principles to follow include:
• Use only well-balanced hot runner manifold systems specifically designed for the application.
• Depending on the material used, the hot runner can be designed to allow direct pouring onto the part using an open gate base plate, or a valve gate for residue-free molding, or the gate can be located on a small surface runner.
• The heated runner path and hot base plate should provide a smooth flow path into the mold cavity and be externally heated.
• When using long glass fiber reinforced resins, gate residue is unavoidable.
• Use a complete system from a single hot runner manufacturer, rather than using different components from different manufacturers.
• Gate size depends on the amount and type of material added, part size/ratio, and wall thickness.
• For more information on hot runner design, consult GE Plastics Group technical personnel.
2. Guide to Common Mold Surface Enhancement Methods in the Plastics Processing Industry
The table below provides molding operators and mold makers with a variety of common mold steel treatments to improve corrosion resistance or wear resistance. Other solutions may also exist.
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3. Runners, Sprues, and Gates
Sprue Bushings
When using conventional molds, hot material enters the mold through the sprue bushing. The runner system guides the material through the gate into the mold cavity. Proper design and sizing of the sprue bushing are crucial for proper melt distribution and correct ejection of the molded part. The sprue "O" diameter should be approximately 20% larger than the nozzle exit diameter to prevent sprue blockage when the mold opens. A schematic diagram of sprue dimensions is shown below. The "O" dimension is also closely related to the size of the parts and runners. Heated runner bushings can be used instead of conventional runner bushings. The initial cost of a heated runner (or "hot" bushing) may be higher than that of a "cold" bushing, but it significantly reduces waste because it eliminates the runner that runs along each part or runner system. With a heated bushing, the control of the melt temperature entering the runner system can be much more precise.
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Runner Design
Runners should be as short as possible to reduce unnecessary pressure drops. A circular cross-section is preferable. If the runner must be placed halfway into the mold, use a trapezoidal runner with a rounded bottom. Semi-circular and flat runners are inefficient and not recommended. Runners should allow material to flow evenly and unrestricted into one or more parts (if multiple cavities are used). Typically, a runner needs to be reduced in diameter by 20% for every 90 degrees of rotation. This needs to be considered when designing the runner system. Circular runner diameters are typically 0.125" – 0.375" (3.2 – 9.5mm). Image
Gate |
Gate design for LNP composite parts should provide ample machining margins without creating in-mold stress. Depending on the type of gate used (i.e., submarine, edge, lug, etc.), some basic gate dimensions control the fill rate, the amount of material flowing into the mold cavity, and the part's solidification rate.
Wall thickness determines the gate size, while the part geometry determines the gate's location on the part. There are three key gate dimensions: depth, width, and face. Specific designs for several gate types are shown on the next page.
Gate Size/Location
• Minimum gate size is 0.040" (1mm), 0.100" (2.5mm) for Verton long fiber composites - round or trapezoidal - 50–60% wall thickness (for crystalline composites) - 50–75% wall thickness (for amorphous composites)
• Ideally, the gate should be placed where the wall section is thickest - align the gate directly with the wall or bushing to prevent backfilling and "jetting" (except for Verton long fiber composites) - tunnel gates are not recommended for Verton composites.
