Precision Molded Optics: A Design Overview
Jim LeBlanc, Lara Brown
The use of plastics for precision optics is rapidly expanding in today's optical markets. Customers are opting for lighter, tougher, and more durable fracture resistant products.
New developments in CAE (Computer Aided Engineering) as well as CNC machining (Computer Numerically Controlled) and EDM (Electrical Discharge Machining) are allowing us to develop more complicated products, and quicker than ever before, without the costly trial and error process. Solid modeling software programs such as Pro Engineer makes it possible to get a 3 dimensional visualization of the product which can aid in identifying design flaws such as undercuts, that can easily be overlooked in a 2D drawing. With 3D drawing it is also possible to see how parts are going to fit together in an assembly, this allows you to spot any dimensional or structural problems prior to building prototypes or molding actual pieces. Once the part design has been optimized the 3 dimensional drawing can be used for flow analysis. Software programs such as Mold Flow and C-Mold can be utilized to analyze the flow behavior of the material through the cavity. Flow problems such as weld lines, gas traps, material stagnation, shear, stress, and hot spots can be detected prior to actual processing. With this information you can optimize variables such as part thickness, part shape, gate location, gate dimensions, and runner dimensions. Performing a preliminary analysis can identify any possible problems before any steel is cut, avoiding costly errors. This article discusses some of the important areas that need to be addressed prior to mold construction.
When choosing a polymeric material optical properties such as clarity, transmission, birefringence, yellow index, haze, and refractive index need to be considered. Other properties such as abrasion resistance, heat resistance, impact strength, chemical resistance, tensile strength, modulus, and dimensional stability are also important. It is also often necessary to choose a material that will allow the part to function in a wide variety of temperatures. A material that may function well in a cold environment may become soft and weak at warmer temperatures. In the same light, a material designed to be used at elevated temperatures may become brittle and fracture at lower temperatures. Heat and humidity can cause some materials to swell, while they may shrink and contract in a colder dry climate. The melt index of the material should also be considered, a less viscous material will flow easier which aids in faster, more uniform filling, and better knitting at the intersection of two flow fronts. When deciding on a material for an optically correct lens the Index of Refraction is of major concern. Since each plastic material has a different Index of Refraction the curvature of the cavity and core for the lens has to be adjusted according. The optimum lens material will yield a birefringence free molded part. One such newly developed material group is Birefringence-free acrylic copolymers. In these materials random copolymerization of the positve-birefringent monomers with the negative-birefringent monomers is used to produce a part that is birefringent free, even in the gate area. For those of whom are not familiar with the term birefringence it means that a single incoming monochromic wave is separated into two rays, each traveling through the material at different speeds. As a result the object behind the lens is viewed as a double image. Since the structure of these "birefringence-free" polymers contains cyclic hydrocarbons, the chemical resistance, heat resistance, and rigidity is increased, while the water absorption is decreased, allowing for greater dimensional stability.
All materials should be processed according to the manufacturer's specifications. Processing the material at temperatures in excess of what is recommended can cause material degradation, which is evident in discoloration, black specks, or reduced mechanical and optical properties. Processing the material too cold will fill the cavities at an increased viscosity, which will impede flow and prevent the relaxation of molecular orientation created during extended cooling of the part in the mold. A cold material is too viscous for proper knitting of opposing flow fronts so weld lines are formed. Not only are these weld lines visually undesirable, they have half the strength of the rest of the part. In addition to proper processing conditions some optical materials such as polycarbonate need to be dried. If the materials are not dried for the proper length of time and at the correct temperatures it will be evident as splay (sliver streaks) in the molded part.
The proper mold material is crucial for optical mold optimization. A high quality mold steel that has a high compression strength, excellent wear resistance, toughness, corrosion resistance, and hot hardness is desired. Mold steels come in both soft for machining and pre-heat treated condition. The prime advantage of pre-heat treating would be the elimination of secondary machining. Since during heat-treatment stresses are relieved which can distort the already machined cavities and cores, secondary machining is required to restore the proper dimensions. A pre-treated material eliminates this problem since the cavities and cores are machined after the heat treatment process. The disadvantage of pre-treatment is decreased hardness compared to post-treated steel.
It is a common practice in optical molding to use inserts. This allows the moldmaker to use higher quality steel for the cavities and cores, while keeping mold material cost low. Special plating is also often desired to obtain an optimal surface for optical lenses. Plating can increase wearability, release qualities, and corrosion resistance. When plating a mold for an optical lens where exceptional polishability is required it is necessary to polish the mold prior to plating. Inadequate polishing of the cavity and cores can take the best-designed mold and turn it into worthless steel.
When machining the mold care must be taken to ensure the grain of the steel is going in the proper direction. It is more difficult for the melt to flow against the grain causing retardation of the flow and stress points. When machining it is best to practice "steel safe". An example of "steel safe" would machining a gate smaller, then increasing the size later if necessary. The reason for this is you can always take away more material, but it much more difficult, and often impossible to add it back.
When designing a mold some basic rules have to be followed such as rounded corners and thinner ribs than your wall thickness. An often overlooked issue is the sprue diameter. The diameter of the sprue should be larger than that of the nozzle orifice by approximately 1/32". The gate should not be placed in line with the sprue. The desired configuration is to have the runner pass the gate so the flow enters the cavity indirectly, preventing jetting. Inadequate mold venting is a common mistake made by the inexperienced mold designer. A general rule is to vent 50% of the parting line. Poor venting in a mold is evident by dieseling, shorts, and gas marks on the part. When calling out dimensions for the cavities and cores the proper shrinkage rates for the designated material must be factored in. Any miscalculation here will change the optics of the final part as well as it's dimensions. It is also important when designing a multi-cavity mold to balance the runner system. This means that ideally the melt arrives at each cavity at the same time, temperature, and pressure. Two ways to accomplish this are flow branching and pressure drop balancing. With flow branching the flow lengths as well as the runner diameters to each cavity are identical. Pressure balancing modifies the diameter of the runners to allow the melt to flow faster to the cavities that are the farthest away, and slower to cavities closest to the sprue. By controlling the runner diameters you can fill all the cavities at the same time. If you do not balance the flow you may end up with gross overpacking in one cavity and underpacking in another, both of which affect the optics of the lens.
In processing optical components we need to consider the surface area. Is it large or small? Is the gate positioned in an area that will minimize the flow length? Is the wall thickness constant? A longer flow length leads to more differential cooling and longer fill times. As the melt proceeds through the mold heat is removed from the melt by conduction through the cooler mold. As the heat is removed the viscosity of the melt increases until it begins to solidify on the cold mold wall. This is important when a part has varying thickness; you should always gate from thick to thin. When gating in a thinner area the melt can freeze off before the thicker section is filled, blocking any further melt from entering the cavity. As a result you will obtain a short shot. Optical components that have a constant wall thickness have the best chance of avoiding edge effects. Edge effects occur where the molten material gets sucked back along the edges of the cavity. Gate design and runner design are critical, they need to be sized for the best cycle, yet be large enough to properly pack the part. If your gate is too small it will freeze off prematurely, leaving a short shot and creating excess shear. A gate that is too large, won't be able to freeze off within a reasonable amount of time. As soon as the high pressures keeping the melt in the cavity are released, the melt can flow back out of the cavity. Not only does this create a poorly packed part, the back flow creates surface blemishes as well.
Alternate Processing Method
An alternate form of processing of plastic lenses is Injection-compression. In injection compression a pre-determined amount of material is injected into an open cavity, then compressed, squeezing the molten polymer into the shape of the final part. This process eliminates the stresses caused by polymer flow as the chains align throughout the cavity. It is also possible to reduce the clamp tonnage by up to 50% using this process. There are some drawbacks to this method, in some instances the material will start to orient prematurely, which prompted the creation of a new patented process called sequential molding. In sequential molding the clamp is simultaneously activated with the injection of material into the mold, allowing the compression of the material while it is still molten, and has not yet been subjected to the high injection pressures. The high injection pressure creates stresses which produce birefringence. In many applications where an optical reader passes threw a window these birefringence waves can cause error readings by deflecting the light beam.
A properly designed optical mold will give you years of service. Hopefully, this article will give some insight for your next optical molded product that will allow you to avoid costly mistakes, and mold an optimum optical product. It is crucial that the optics of the parts being molded are continually and consistently checked. Changes in process conditions as well as environmental conditions such as room temperature and humidity can cause changes in the optics of the part.