Polycarbonate Fundamentals: Machining & Forming Tips Getting the most out of polycarbonates includes learning the best methods for sawing as well as forming and mold design.
There are few thermoplastics that have the outstanding engineering properties of polycarbonate. Developed commercially in 1957, it is one of the pioneering members of the family of engineering thermoplastics, created to compete with die-cast metals.
Polycarbonates are high-molecular weight, amorphous engineering thermoplastics that are characterized by a combination of outstanding impact strength, superior dimensional stability, glass-like transparency, remarkable thermal resistance and good electrical properties. In addition, they have a ductility that is normally associated with softer, lower-modulus thermoplastics. Polycarbonate is distinguishable from other engineering thermoplastics by its freedom of design; it allows engineers selecting a combination of these properties to meet specification requirements for a broad range of application needs.
Polycarbonate sheet is extruded in a number of standard widths and lengths and is available in gauges from 0.030 inch to 0.500 inch thick in monolithic sheet and 0.375 inch to 1.250 inches thick in laminated sheet. Grades include: general purpose, UV enhanced, abrasion resistant, aircraft , flame inhibiting, bullet resistant, forced entry resistant, FDA and sign grade. Polycarbonate is also manufactured in machine grade and is available in plate, rod and tube.
Polycarbonate has good thermoforming characteristics. When formed, fabricated, machined and finished according to recommended techniques and procedures for secondary operations, the final result will be a high quality product.
Fabricating Polycarbonate sheets
Circular sawing: A table saw or overhead panel saw is typically used for cutting polycarbonate sheet. A circular saw blade with carbide teeth, utilizing the “triple chip” tooth design, is the preferred method. Blades for cutting 3Ú32 inch and thicker material should have three to five teeth per inch and the hook or rake angle should be 10-degrees to 15-degrees. Circular saws should be run in the speed range of 6,000 to 8,000 feet per minute.
When sawing thin gauge polycarbonate, it is important to have a good supporting edge on the saw table with minimal gap between the saw blade and table-supporting edge. Care should be taken to ensure that tabletops are smooth and free from projections that might scratch or mar the sheet.
Band saws: Band saws are useful for trimming formed parts or irregular shapes. Bands saws should be run at 2,500 to 3,000 feet per minute, and have eight to 12 teeth per inch. Coarser (larger) blades perform better with thicker gauge polycarbonate sheet. Proper support of the part to be trimmed is important because vibration may induce cracking if the cut is not smooth.
Routing: Routing produces a smooth edge on polycarbonate sheet and may also be used to cut curved or irregular shapes. Routers with at least 1-horsepower motor and speeds of 20,000 to 25,000 rpm are preferred, used in conjunction with 1Ú4-inch to 1Ú2-inch diameter straight (fluted 2 or 3), carbide-tipped or high speed steel router bits.
The stock feed must be monitored closely as feeding polycarbonate sheet at excessive rates can cause vibration and cracking. It is important to feed the sheet against the rotation of the router bit and to provide a fence for sizing when making straight cuts.
Shearing: Die cutting polycarbonate sheet in gauges up to 0.080 inch thick is normally achieved by utilizing the following steps to calculate the required press tonnage:
F = (P x A)/2,000 where F= required force in tonnage of the press, P=10,000 psi (shear strength of polycarbonate sheet) and A= the sectional area to be cut.
Steel rule dies mounted in a press provide good results. Use 3PT-thick (0.042-inch) steel to fabricate steel rule die-flush, or center bevel ground provides a clean cut. Facet ground steel rule is used to cut thicker gauge sheets, above 0.60 inch. Be sure the platens are parallel and that the backup pad is in good condition. Backup pads can be made from a variety of materials such as nylon, HDPE, etc.
Drilling: Polycarbonate sheet is easily drilled using ordinary high speed steel drill bits. Fabricators are urged to regulate the pressure and speed until a continuous spiraling chip is observed. If needed, use air or water as a coolant; using oils may cause crazing. Be extremely careful if using taps or self-tapping screws; tapping creates notches which, in a notch-sensitive material like polycarbonate, can result in stress cracks. The recommended drill speed is 350 to 1,750 rpm.
Thermoforming: Polycarbonate sheet can be thermoformed on standard equipment, with vacuum forming, free-blown forming and line bending the most extensively used processes. While most standard forming techniques can be used, critical process modifications specific to polycarbonate sheet are necessary.
Polycarbonate sheet must be pre-dried prior to thermoforming and heating cycles need to be accurately controlled for uniform product quality. The thermoforming machine should be capable of generating and maintaining sufficient vacuum pressure throughout the thermoforming cycle. A minimum vacuum of 20 inch Hg throughout the entire vacuum cycle is necessary to retain part integrity.
Most commonly used vacuum forming machines with infrared heating elements perform well for polycarbonate sheet forming. Rotary and shuttle designs with automatic or semi-
automatic controls are the most suitable because of their timer control accuracy, uniform heating sources and sufficient vacuum power. Single-sided heating has proven effective for polycarbonate sheet gauges up to 0.177 inches. For thicker gauges however, it is recommended that dual-sided heating ovens be used for effective radiation penetration.
Mold materials & mold design: Polycarbonate allows the use of a variety of mold materials such as wood, filled and unfilled polyesters, epoxiers and metals. Molds for vacuum forming need only 14 psi, so there is little wear on the tooling due to the low pressure of the material against the mold surface. Use of standard mold design practices and mold materials should be observed.
Molds: Male molds are normally more suitable for vacuum forming. However, other factors such as part size, finish and shape dictate the mold design. Molds are constructed of materials that relate to cost effectiveness for the length of the run of a given product.
Pre-drying: Polycarbonate sheet must be pre-dried before thermoforming because it absorbs moisture at a high rate. Trapped moisture forms vapor above 250F and the vapor expansion creates bubbles in the sheet. Sheets should be placed in a dehumidifying air circulating oven for pre-drying, with approximately 1-inch separation between sheets. Oven temperature should be 250F and monitored with controls.
Polycarbonate sheet begins absorbing moisture immediately upon removal from the pre-drying oven. The rate of absorption is dependent upon the ambient dew point. For this reason, it is crucial to transfer the sheet directly to the forming machine as quickly as possible.
Heating cycle: Heating polycarbonate sheet for vacuum forming requires a heat penetration range of 360F to 410F. The ideal forming temperature is 375F.
When polycarbonate sheet reaches forming temperature, uniform “sag” occurs. The amount of sag depends on the size and thickness of the sheet. Once the uniform temperature has been achieved, timers can accurately reproduce the condition, with part-to-part consistency maintained. Polycarbonate sheet “sets up” quickly compared to other thermoplastics, and can be removed from the mold in a short period of time.
Cold bending: Polycarbonate sheet can be cold formed into circular shapes for walkways, etc. The rule states that the radius of the curvature must be at least 100 times the material thickness.
Strip heating: Strip heating or line bending is commonly used for producing localized angular bends in polycarbonate. Generally, pre-drying is not required for a material thickness of 0.118 inch or less. On thicker gauges, pre-drying can be avoided by back-routing or V-grooving the sheet to 1Ú8 inch or less thickness.
Drape forming: Simple contours can be achieved by drape forming polycarbonate sheet. The sheet should be pre-dried, then brought to a forming temperature of 325F to 340F in the oven. Parts are then removed and placed immediately over a male mold covered with felt. This method is utilized to manufacture face shields or any other part requiring a simple radius curvature.
Brake forming: Brake forming polycarbonate sheet is not recommended. Brake forming imparts localized stresses, which can exceed the elastic limits of polycarbonate, resulting in stress cracking or crazing.
Mechanical fastening: Aluminum rivets and machine screws may be used to join polycarbonate sheet to other materials. To do so, drill oversized holes and use washers to distribute and cushion localized stress. Fabricators should be sure to consider the differentials in expansion factors for dissimilar materials. Also, be sure drilled holes are smooth and free from cracks.
Joint-planing: A standard woodworking jointer/planer is an excellent edge finishing machine for polycarbonate sheet. Blades must be made from carbide or high speed steel. Fabricators are advised to avoid removal of too much stock on each pass; trying to remove too much material may result in a rough edge or a cracked sheet. Typically, you will find that 1Ú64 inch or less stock yields the cleanest edge. If smoother edges are required, wet sanding with a fine grit #200 sandpaper is recommended.
Edge finishing: Normally, edges may be finished by planing and sanding, which provides a smooth, matte finish. Flame polishing polycarbonate is not normally an acceptable practice.
Polished edges may be achieved by solvent polishing. After sanding to remove all the tool marks, dip a cloth in a solvent such as methylene chloride and carefully wipe the sheet’s edges. Do not allow the solvent to drip on the sheet face as discoloration will occur.
Karl Wiecking is the marketing manager for DSM Sheffield. Located in Sheffield, MA, the 47-year-old company manufactures a wide range of polycarbonate sheets as well as other high performance sheet plastics. For more information, Wiecking can be reached at (413) 229-8711.
John Raynor is the marketing manager for Charlotte, NC-based Piedmont Plastics. Established in 1968, Piedmont distributes and fabricates polycarbonate as well as other types of materials. For more information, Raynor can be reached at (704) 597-8200.