Thermoplastic Elastomer Molding Guidelines:
TPE Design Considerations
- Press Tonnage
2.0 to 3.0 tons (1.8 MT-2.7 MT) of clamp per square inch projected total surface area. - Barrel Size
Individual shot size to be 40% – 80% of molding machine barrel capacity. - Maximum Residence Time
Maximum of 4 to 5 minutes (in the barrel). - Screw Selection
General purpose screws are acceptable. - Nozzle Diameter
Must be slightly smaller than the sprue base diameter in mold runner system being run.
TPE resins can be molded on reciprocating screw-type injection molding machines. A reciprocating screw machine is preferred as it produces a more homogeneous material and a more uniform melt temperature. It also permits processing at lower temperatures.
When running on reciprocating screw-type machines, utilization of 40% to 80% of the barrel capacity is desirable. Although shot weights smaller than 40% have been molded successfully, the material can degrade when the shot weight is too small and excessive heat builds up in the melt.
Following are important considerations in choosing a screw for injection molding TPE resins:
- A general-purpose screw with a length-to-diameter ratio (L/D) of at least 20:1 is satisfactory.
- A compression ratio of 2.0:1 to 3.0:1 is preferred. A 2.5:1 compression ratio is applicable for most situations.
- Rapid-transition (nylon-type) screws are not recommended because of the excessive melt temperature and consequent degradation of the resins that can occur with them.
- Exception is RTP 2900 Series which with certain grades will process generally better with a nylon-type design screw.
When designing a TPE part, there are a few general rules to follow:
- The part wall thickness should be as uniform as possible. Transitions from thick to thin areas should be gradual to prevent flow problems, back fills, and gas entrapments.
- Thick sections should be cored out to minimize shrinkage and reduce part weight (and cycle time).
- Radius/fillet all sharp corners to promote flow and minimize no-fill areas.
- Deep unvented blind pockets or ribs should be avoided.
- Avoid thin walls that cannot be blown off the cores by air-assist ejection.
- Long draws with minimum draft may affect ease of ejection.
The maximum achievable flow length is dependent on the specific material selected, the thickness of the part, and processing conditions. Generally, TPE compounds will flow much further in thinner walls. The one exception to this is the RTP 1200 series (TPE)The flow to thickness ratio should be 150 maximum. Longer flow lengths will require additional gates to keep the L/T below 150.
Whenever possible, use air poppets or plate ejection to avoid marking or deforming the TPE materials. Large diameter pins, stripper plates or rings, knock-out sleeves, or core retraction and/or ejection are suitable. All should act on the thickest sections of the part. The location of the ejection system should provide uniform stripping of the part. Avoid using small ejector pins that will deform or push through the part. A cold slug well, opposite the nozzle, should contain a means for pulling the sprue free from the nozzle tip and the sprue bushing.
The sprue should have sufficient draft, from 1° to 3° to minimize drag and sprue sticking. Longer sprues may require more taper (3° to 5°). Typically, the sprue diameter should be slightly larger than the nozzle diameter. Undercut or Z type sprue pullers are satisfactory for all grades. The undercut type is typically used for harder resins (higher than 90A) and the Z-type sprue pullers are more commonly used for softer resins. Permanent surface lubricant treatments have also been used successfully.
Non-return check valves prevent the molten polymer in the holding space in front of the screw from flowing back into the screw during the injection cycle. When processing TPE resins, use a free-flowing, sliding check-ring style non-return check valve made of fully hardened H-13 steel, preferably nitrided to retard wear. A fully channeled tip will minimize flow restrictions as TPE resins, like most thermoplastics, the material will degrade when subjected to excess shear at flow restrictions.
Most standard steel nozzle types used with other thermoplastics are satisfactory for molding TPE resins. A straight-through nozzle or a replaceable-tip nozzle with a reverse taper is recommended.
The nozzle should be as short as possible. If a long nozzle is necessary, it should have a large internal diameter proportional to its length. It is essential that the nozzle and sprue bushing mate properly. The nozzle orifice should be slightly smaller (about 20%) than the sprue bushing orifice.
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 TPE compounds is 0.0005 to 0.0010 in (0.012 to 0.025 mm) with a 0.040 to 0.060 in (10 to 15 mm) land. Past the land, the vent depth should be increased to 0.005 to 0.010 in (0.12 to 0.25 mm) to provide a clear passage for the air to exit the tool. Venting in areas below the parting line can be accomplished by allowing the ejector pin to be 0.001 in (0.0265 mm) loose on each side. Venting of ribs or pockets can be achieved by venting down an ejector pin. Ejector pin vents are self-cleaning, but should still be maintained and cleaned regularly to remove buildup.
A balanced H-pattern 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 over-packed and flashing may occur. As a result of over packing, parts may also develop high molded-in stresses, which lead to warpage.
| System Type | Advantages | Disadvantages |
|---|---|---|
| Cold Runner | • Lower tool cost • Easily modified • Enables use of robotics |
• Typically governs cycle time • Potential for cold slugs • Potential for sprue sticking • Scrap (though regrindable) |
| Hot Sprue or Extended Nozzle | • Faster cycle • Minimizes scrap • Easily maintained |
• Higher tool cost • Potential material degradation |
| Hot Runner | • No runner scrap • Faster cycle time • Precise temperature • Control |
• Highest tool cost • Purging • Material degradation • Maintenance |