Principles of injection mold design for LSR.
With the low viscosity of LSRs, very rapid mold filling is possible, even at low injection pressures. To avoid air entrapment, good venting of the mold is critical. Overall, the fast cure of modern LSRs also allows very short cycle times (less than 20 seconds in some cases), but in order to take full advantage of this capability, the tooling, injection molding machine and part removal system must work together as a highly integrated unit.
Cold runner molds
Modern cold runner mold systems take the greatest advantage of the shear thinning and cure properties of LSRs, which permit true wasteless, flashless molding. Over the past three to five years, the use of cold runner molds has risen dramatically as competition among fabricators drives the need to increase production, reduce waste and lower the costs of labor.
LSRs do not shrink in the mold like a thermoplastic, but they do expand when hot and then shrink slightly as they cool, due to their relatively high CTE. As a result, parts do not always remain on the positive side of the mold; instead, they tend to be retained in the cavity with the greater surface area.
Similar to hot runner molds which improve material flow via heat in thermoplastics, cold runner tooling keeps the thermosetting LSR cool and flowable right up to the part gate, allowing production with virtually no wasted material. These tools demonstrate the greatest benefit in large-volume manufacturing of parts with similar sizes or configurations operating in near clean room environments. The ideal mode is to set up the tool and run around the clock with minimal operator intervention or adjustment for extended periods (days or even weeks).
Two basic types of cold runner design are used: positive shut-off and open runner. Both systems have advantages and disadvantages. Positive shut-off molds employ actuated pins or needle valves at each cavity to control the flow of LSR during the injection cycle. Open runner systems utilize constricted nozzles and gates to control the flow of material, based on the injection pressure applied.
Positive shut-off molds typically operate at lower injection pressures than open runner systems. The use of adjustable chokes in the runner system allows fine-tuning of the balance to overcome unequal runners or differences in material shear thinning properties. The drawback of positive shut-off systems is the added complexity and size of a mold for a given part size and mold cavitation.
Open runner systems rely on a fairly high shear rate through nozzles and gates to stop the flow when injection pressure is reduced in the cycle. In general, cavity fill times are slightly faster in open runner systems than in comparable positive shut-off designs. A high density of cavities can be achieved in open designs because of the small runner and nozzle sizes. However, the runner system must be naturally balanced and closely matched to the specific material rheology. Adjustable chokes are not normally used with open runner systems, as problems can result from the small runner sizes needed to effect good flow control and achieve the greatest pressure drop at the nozzle (instead of at the choke).
Determining the location of the parting line must be one of the first steps in designing an injection mold for liquid silicone rubber. Because venting is accomplished through channels placed within the parting line, vents need to be located in the area of the mold that the injected material reaches last. Selection of the parting line location with this in mind helps avoid trapped air and loss of strength along the weld line.
The parting line of the mold must be precision finished to avoid flash, due to an LSR's low viscosity. Even so, the parting line will usually be visible on a finished part. Demolding is influenced by the geometry of the part and location of the parting line. Slight undercuts in part design can help ensure a consistent affinity of the molded part to the desired cavity half.
Although liquid silicones do not exhibit in-mold shrinkage, they will often shrink 2-3% after demolding and cooling as a function of the high thermal expansion coefficient inherent to silicone rubber. The exact amount of shrinkage is somewhat dependent on the specific material formulation, but from a tooling standpoint, shrinkage will be influenced by several factors that can be manipulated to the designer's advantage with proper forethought. These include the temperature of the tool, material temperature at demolding, as well as the cavity pressure and subsequent material compression.
The location of the injection point is an added consideration, as shrinkage in the direction of material flow is usually somewhat greater than in the direction perpendicular to the flow. The physical dimensions of the part also have an effect, with thicker parts generally demonstrating less shrinkage than thinner ones. If the application requires post curing, an additional 0.5-0.7% shrinkage can be expected.
The air trapped in a mold cavity when it closes is first compressed as the LSR is injected, then expelled through the venting channels as the cavity fills. Because of an LSR's low viscosity, the cavity can be filled very quickly as shear is applied. If the air cannot escape entirely during this rapid filling, it will be trapped in the cured material (often recognized by a white edge along the part or small bubbles with a very smooth interior). Typical venting channels are 1-3 mm wide and 0.0040-0.005 mm deep, with many configurations used successfully.
Optimum removal of air from the cavity is achieved by pulling a vacuum in the mold as part of each cycle. A seal is included in the design of the parting line, and a vacuum pump quickly evacuates all cavities during a low clamp stage of mold closing. As soon as the vacuum has reached a pre-determined level, the mold closes completely and injection begins.
Another successful approach used by some injection molding equipment designs permits operation at variable clamping forces, which allows fabricators to clamp at low pressure until the cavities are 90-95% filled with LSR (to facilitate air escape), then switch to a higher clamping force to avoid flash from the expanding silicone.
While proper tool design can deliver a very small, consistent gate vestige, achieving a totally undetectable gate location is very difficult. Placing the gate in a non-critical area or inside surface will help avoid problems. As mentioned, the use of a cold runner system in molding LSRs exploits the materials to their greatest advantage and promotes the highest productivity. The objective is to fabricate parts in such a manner that no sprue needs to be removed, avoiding a labor-intensive process and the possibility of considerable material waste. In many cases, a sprue-less design will also reduce cycle times.
If a cold runner system is employed, it is important to create an effective temperature separation between the hot cavity and the cold runner. If the runner is too warm, the material may begin to cure before injection. Yet if the cooling is too aggressive, it will draw too much heat from the gate region in the mold and prevent complete cure. Gates in positive shut-off or pin shut-off cold runner systems are typically 0.5-0.8 mm to allow for pin movement and material flow around the pin. In open cold runner systems, the nozzles and gates are usually smaller (0.2-0.5 mm) to provide flow control.
For parts injected in a more conventional sprue such as a submarine gate or cone gate, small diameter feeds are typically preferred for the low-viscosity LSR materials. (Feed point diameters are usually between 0.2-0.5 mm.)
Unless specially formulated, cured LSRs tend to stick to metallic surfaces, and the flexible nature of the part can make demolding a challenge. Even so, the hot tear strength of current LSR formulations generally makes it possible to demold even large parts without damage. The most common demolding techniques include stripper plates, ejector pins and air ejection. Other popular methods include roller sweep, draw-off plate and robotic handling.
When ejector systems are used, they must be maintained within very close tolerances. If there is too much clearance between the ejector pin and bushing guide, or if the components have been worn over time, material flashing is the likely result. Reverse tapered or mushroom shaped ejectors have been very effective, as they allow the use of greater contact pressures to facilitate improved sealing.
Retainer plates are usually fabricated from unalloyed tool steel (no. 1.1730, DIN code C45W). Mold platens exposed to temperatures between 170[degrees]C and 210[degrees]C should be made from pre-tempered steel (no. 1.2312, DIN code 40 CrMnMoS 8 6) for impact resistance. For mold platens containing the cavities, tempered hot work steel is preferred for its temperature resistance.
For highly-filled LSRs such as oil-resistant grades, the use of even harder materials is recommended; flash chrome plated steel or powdered metals have been developed especially for this application (steel no. 1.2379, DIN code X 155 CrVMo 121). When building molds for highly abrasive materials, they should be designed using inserts or other replaceable tooling to allow the exchange of high-wear sections without having to replace the entire mold.
The surface of the mold cavity has a great influence on part finish. The most obvious is that the finished part will exactly duplicate the surface of the cavity. Polished steel is essential for transparent parts. Surface treated titanium/nickel steel has very high wear resistance, while PTFE/nickel facilitates easier demolding. Aluminum should be avoided, due to the somewhat abrasive nature of LSRs. Using the best material affordable (even for prototypes) will pay off in more consistent results and ease of transition from prototype to production tooling.
Electrical heating is typically preferred in LSR molding, usually in the form of strip heaters, cartridges or heating plates. An even temperature distribution throughout the mold is important to promote homogeneous cure of the LSR. On large molds, the most cost-effective heating method may be oil temperature control.
Encasing the mold with insulating plates will also help to reduce thermal losses. An insufficiently heated mold section may be exposed to large temperature fluctuations in between cycles or as a result of blow-off air. When surface temperatures fall too low, the material's cure slows down, frequently inhibiting part release and causing quality problems. The distance between heaters and parting line should be far enough to prevent any warping or bending of the plate, however, which would produce flash on molded parts.
If the mold is designed with a cold runner system, an exact separation between the hot and cold sides is imperative. Special titanium alloys, such as 3.7165 (Ti Al 6 V4), can be used for their much lower thermal conductivity vs. other types of steel. For whole-mold heating systems, insulation should be located between the mold and mold plates, to keep heat loss to a minimum.
As with thermoplastics molding, it is important to balance the layout of an LSR runner system in such a manner that all cavities are filled evenly. Using computational flow dynamics simulation software to design the runner gating and venting can greatly facilitate mold development and help avoid costly trial-and-error prototyping. The results can also be confirmed by fill studies, but proper simulation requires that engineers be in possession of the mechanical response properties specific to the LSR formulation being molded. Examination of the part design using Finite Element Analysis can also help to avoid highly stressed areas, which may in turn contribute to reduced demolding damage.
With proper design and planning, injection molding of LSRs can be a profitable and relatively trouble-free operation. The principles of mold- and process design are well understood, allowing fabricators to achieve very high efficiency while avoiding most of the potential obstacles. The excellent cavity fill and rapid cure times of these materials contribute to high output of quality parts and a positive impact on the fabricator's bottom line.
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|Date:||Dec 1, 2003|
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