Injection Molding Mold Design
A wide variety of tool steels are available for injection mold construction. The table below lists the properties of common tool steels and the typical mold components for which they are used. Soft metals, such as aluminum and beryllium copper, can be used for prototype parts or short production runs up to 10,000 parts.
Some part designs may benefit from the use of higher thermal conductivity materials such as beryllium copper. This material is less durable than steel and may hob or wear faster than steel if used at the parting-line. Beryllium copper can be used for inserts, slides or cores to increase heat transfer rates and reduce cycle times. In cases where there is a long draw core, a fountain-type bubbler may be beneficial.
|Steel Type||Steel Properties||Mold Component|
|P-20||Pre-hardened, machines well, high carbon, general-purpose steel.Disadvantage: May rust if improperly stored.||Mold bases, ejector plates, and some cavities (if nickel or chrome plated to prevent rust).|
|H-13||Good general purpose tool steel. Can be polished or heat-treated. Better corrosion resistance.||Cavity plates and core plates.|
|S-7||Good high hardness, improved toughness, general-purpose tool steel. Machines well, shock resistant, polishes well.Disadvantage: Higher cost.||Cavity plates, core plates and laminates, as well as thin wall sections.|
|A-2||Good high toughness tool steel. Heat-treats and polishes well.||Ejector pins, ejector sleeves, and ejector blades.|
|D-2||Very hard, high wear characteristics, high vanadium content, somewhat brittle.Disadvantage: Difficult to machine.||Gate blocks, gib plates to prevent galling, gate blocks to prevent wear.|
|420 SS||Tough corrosion resistant material. Heat-treats and polishes well.Disadvantage: High cost.||Cavity blocks, ejector pins, sleeves, etc.|
Mold Surface Treatment
- In general, an EDM surface will produce a good texture and may improve release from the tool during part ejection. Matte or textured surfaces can also help to hide any flow marks or other surface defects.
- If a matte finish similar to that of a thermoset rubber is required, a rougher mold texture should be used (or GLS Versalloy TPV alloys, which naturally produce a matte surface).
- Vapor honing, sand or bead blasting and chemical etching are used to produce textured surfaces with varying degrees of gloss and appearance.
- To produce a glossy surface, a polished mold is required and an unfilled grade should be used.
- A highly polished tool and a transparent material are required to produce a part with good clarity.
- To aid in release, the cavity or core may be coated with a release coating such as PTFE impregnated nickel after it has been given a sandblast or EDM finish.
Sprue and Sprue Puller Design
The sprue should have sufficient draft, from 1° to 3° to minimize drag and sprue sticking. Longer sprues may require more taper (3° – 5°), as shown in Figure 1. Typically, the sprue diameter should be slightly larger than the nozzle diameter. An EDM finish is acceptable for most styrenic TPE materials. Permanent surface lubricant treatments have also been used successfully.
Sprue puller designs vary with the hardness of the material. The different sprue designs possible are shown in Figures 1 through 5.
The typical sprue designs for various TPE hardness values are shown in the table below:
|Typical TPE Hardness Range||Most Common Sprue Puller Types||Figure|
|> 50 Shore A||Tapered, Pin, Z-Type||1, 2 and 4|
|40-70 Shore A||Undercut||3|
|5-40 Shore A||Pine Tree||5|
Hot sprue bushings and extended nozzles may be used with GLS compounds. In many molds, the sprue is the thickest wall section in the mold and will control the minimum cooling time. The use of a hot sprue, which may be viewed as an extension of the machine nozzle, can sometimes reduce cycle time. Extended machine nozzles may also be used to reduce sprue length and size. When hot sprues are used, the machine nozzle tip should be a free-flow nozzle rather than a reverse tip.
*Spiral flow tests performed using 0.0625″ thick x 0.375″ wide channel at 400°F.
For spiral flow information about a specific grade or additional details about the spiral flow test procedure, please refer to the TPE Tip #7.
Conventional Runner Design
A balanced runner configuration is critical to achieve uniform part quality from cavity to cavity. In a balanced runner system, the melt flows into each cavity at equal times and pressure. The runner balance can be designed by using computer mold-flow analysis programs and verified by performing short-shot studies.
An unbalanced runner may result in inconsistent part weights and dimensional variability. The cavity closest to the sprue may be overpacked and flashing may occur. As a result of overpacking, parts may also develop high molded-in stresses, which lead to warpage. Examples of balanced runner systems are shown in the figures below:
Figure 8 shows different runner cross-sections and their associated efficiency. Full-round runners have the least resistance to flow and surface area, allowing the material to stay molten longer. The second most efficient runner cross-section is the modified trapezoid. This runner geometry most closely simulates a full round runner, but only requires machining in only one plate. Figure 9 shows typical ball cutter dimensions and the corresponding modified trapezoid runner sizes. Figure 10 illustrates typical runner dimensions.
Runner keepers or sucker pins provide undercuts to keep the runner on the desired plate but should not restrict material flow through the runner. Figures 8 and 9 show typical locations for runner keepers and sucker pins. Figure 11 illustrates an example design of a runner keeper.
Gate Design and Location
Most conventional gating types are suitable for processing GLS styrenic TPE compounds. The type of gate and the location, relative to the part, may affect the following:
- Part packing
- Gate removal or vestige
- Part cosmetic appearance
- Part dimensions, including warpage.
Tab or edge gates (Figure 12) most commonly utilize a conventional sprue and cold runner system. Advantages of edge gates are ease of fabrication, modification and maintenance.
- The gate depth (D) should be 15% – 30% of the wall thickness at the gate entrance. Common practice is to start “steel safe”.
- A good starting point for the gate width should be 1.0 – 1.5 times the gate depth.
- The gate land should be equal to or slightly longer than gate depth.
Submarine / Tunnel Gates
Submarine or tunnel gates are self-degating. During part ejection, the tool steel separates the part and the runner. Figure 13 shows a typical design of a submarine gate. Cashew type submarine gates should not be used for medium to soft hardness compounds due to their high coefficient of friction and high elongation.
A fan gate is a streamlined variation of a tab gate (Figure 14). The fan gate distributes material into the cavity more evenly, thus it is normally used in parts that require a high degree of flatness and absence of flow lines. It also minimizes the possibility of gate pucker or part warpage.
Sprue or Direct Gate
The sprue or direct gate is often used on prototype parts because it is inexpensive. This type of gating is not recommended for GLS styrenic compounds because of their high elongation. In addition, the sprue will need to be trimmed thus appearance quality of the part is usually poor. If sprue gating is selected, care should be taken to keep both the sprue length and diameter as short and small as possible.
The diaphragm gate is used to maintain the concentricity of round parts. It allows even flow into the cavity and minimizes the potential for knit lines. Due to anisotropic shrinkage, flat round parts using center or diaphragm gating may not lay flat. A ring gate may also be used on the outside of a circular part.
|Pin gate (3-plate)||
|Valve gate (Hot runner systems)||
Styrenic TPE compounds are anisotropic, thus they have different physical properties in the flow direction versus the cross-flow direction. Depending on the product’s intended usage, these property differences could be critical to the performance of the final part. As a result, the anisotropic nature of the styrenic TPE needs to be taken into consideration when determining the gate location on the part.
The material flow may be estimated by eye or by using flow analysis programs. For higher shrinkage grades, the part may shrink near the gate, which causes “gate pucker” if there is a high molded-in stress at the gate. If filling problems exist in thin walled parts, adding flow channels or minor changes in wall thickness can alter the flow. In some cases, it may be necessary to add a second gate to properly fill the parts.
Recommended Gate Locations:
- At the heaviest cross section, to permit part packing and minimize voids and sink.
- To minimize obstructions (flowing around cores or pins) in the flow path.
- To minimize jetting.
- Where residual molded in stress around the gate will not affect part function or aesthetics.
- To minimize flow marks in cosmetic areas.
- To minimize potential knit lines.
- To allow easy manual or automatic degating.
- To minimize flow path length.
Mold venting is critical to the quality and consistency of the finished part. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. Potential air traps in the part design can be predicted by flow simulation software. Once the tool has been built, short-shot studies can be used to find the critical venting areas.
- Vents should be placed at the last place to fill and in areas where weld lines occur.
- The typical vent size for GLS compounds, is 0.0005″ – 0.0010″ (0.012 mm – 0.025 mm) with a 0.040″ – 0.060″ (10 mm – 15 mm) land.
- Past the land, the vent depth should be increased to 0.005″ – 0.010″ (0.12 mm – 0.25 mm) to provide a clear passage for the air to exit the tool (Figure 15).
- Venting in areas below the parting line can be accomplished by allowing the ejector pin to be 0,001 loose on each side (Figure 16).
- Venting of ribs or pockets can be achieved by venting down an ejector pin, or with the use of porous mold steels.
- Ejector pin vents are self-cleaning, but they should be wiped once a day to remove build-up. Porous plug vents need to be replaced or dismantled and cleaned when they become clogged.
Part ejection is more difficult in long draw areas. Ejector pins should be located at runner transitions and in areas of the part where they will not affect the aesthetics. Ejector blades, ejector sleeves and stripper rings can be used for part ejection.
Use the largest diameter pins possible to minimize pin push-through marks. Larger pins also allow the ejection of warmer parts, which can reduce cycle time. Use a 3Â° – 5Â° draft per side on all long draw areas.
Air ejection and the use of poppet can help strip large undercuts providing the material has room to deform when the air is applied. Mold surface texturing and special mold surface treatments can also help to pull the parts from the “A” half. Advancing cores are used usually when trying to strip large internal undercuts.
- The mold should have adequate cooling to optimize cycle time.
- The use of mold materials with high heat transfer, such as beryllium copper, can be used to cool slides or inserts.
- Commercially available fountain-type bubblers may also help to cool long cores.
- Separate chillers for the movable and stationary sides are suggested. This allows the processor to use differential cooling to help retain the parts on the movable (“B”) plate.
- Connecting cooling lines from the A to the B plate should be avoided.