Tips & Techniques for Fabricators
Suggested guidelines for milling, turning, cutting and bonding HDPE.
High-density polyethylene is a highly versatile polymer that exhibits excellent chemical resistance and is easily welded. Used extensively in the chemical industry, it has also been approved for use in food processing operations.
The end uses for the material are extensive. In addition to food cutting boards, the material is successfully used for: chemical tanks and tank linings, ventilation ducts, fume hoods and industrial work surfaces.
Carbide-tipped tools are long lasting and achieve high productivity rates on HDPE, although high-speed steel is adequate for cutting thermoplastic products. K-type hard metals are also recommended due to their low thermal expansion coefficients and high thermoconductivity.
A minimum cutting force, running in the direction of the cut, is required for machining plastics. The greater the clearance and rake angle, the less cutting force is needed. To avoid excessive weakening of the cut edge, there are limits to the clearance and rake angle that may be used. Care should be taken during machining to avoid deep scoring or sharp crossover marks which give a notch effect and can cause early breakage of the material under load.
Final polishing with a polishing wheel or edge polishing machine can improve the surface finish. Any edges left after sawing, drilling or milling can be reworked with blunt scrapers or special deburring equipment and chamfered.
What follows below is general information for machining this material.
Fast running band and circular saws are suitable for cutting HDPE. The cut surfaces can be smooth and a good finish can result when the teeth are lightly set. Saw blades with teeth no more than 15mm apart have been shown to produce a better quality cut. Other guidelines include:
Rake angle: 0-5°
Cutting speed: 3,000m/min
Surface and panel planers used in woodworking are also suitable for materials such as HDPE. The surface quality largely depends on the feed speed, cutting speed, clearance and rake angle as well as the state of the cutters. The following guidelines should be met:
Rake angle: 15-20°
Cutting speed: 3,000m/min
When milling, particular care should be taken to keep the machined cross-section as large as possible in order to reduce heat generation. The cutting depth and feed speed also need to be large, with a lower cutting speed producing better results.
Fast woodworking machines with fairly high feed rates and rpm, as well as universal milling machines, have been successfully used to mill this plastic material.
Spiral drill bits should be used to drill HDPE. For optimal results, the angle of fluting should be 20-30 degrees and the point angle should be at 110-120 degrees. A considerable level of heat is generated by drilling, which must be taken away with the swarf or by further cooling. For deeper drilling, it can be helpful to remove the drill from the hole to empty out the swarf.
HDPE and other thermoplastics can be turned on a lathe with tools ground for plastics, at speeds up to 550 m/min. For optimal results, the feed speeds should be kept low, with the maximum possible cutting depth per revolution of >0.5mm. If a tool with a slightly rounded tip is used, the cut surface will be clean. Carbide-tipped tools can be used to ensure accurate tolerances. HSS can also be a cost-effective solution.
Because of the high chemical resistance of this material, there is no bite on the surfaces at room temperature, making only adhesive bonding possible. Pre-treatment of the joint surfaces improves wettability.
According to Röchling, the following adhesives are suitable for bonding the material:
Pressure sensitive Solvents and dispersion binders
Contact adhesives Polyurethane
Two-package system Epoxy resin, polyurethane
Hotmelt adhesives Vinyl copolymers
The achievable strengths of the bond depends on the type of plastic material, quality of the surfaces to be joined and the type of adhesive used. In instances where the bond is critical, it is advisable to send samples of the material to the adhesive supplier in order to determine the best adhesive for the specific usage conditions.
The above information was supplied by Röchling Engineered Plastics. For more information, contact the company at (704) 922-7814 or visit www.roechling.com
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