Ravago Manufacturing Americas 13001 Almeda Road Houston, Texas 77045 713-433-5604 Fax 713-433-9783 TECHNICAL NOTE Rotational Molding Guide Introduction: Rotational molding is a process where plastic materials are formed into useful articles by rotating a mold containing the plastic material in an oven sufficiently hot to melt the plastic. The mold is rotated in two directions to cause the plastic to coat all surfaces of the mold. There is little pressure inside the mold during the forming process. After the plastic is thoroughly melted to coat the mold surface the mold and plastic is cooled, the part is removed from the mold. The rotational molding process is ideally suited to fabrication of hollow parts. It also lends itself to production of rather large parts. The process can accommodate articles with complex geometry. Because the process is carried out at low pressure, the molds can be fabricated from relatively inexpensive materials. The relatively low cost of tooling allows rotational molding to be used to produce parts whose production volume would not amortize the high cost of expensive injection or blow molds. At first examination, the rotational molding process seems simple. Those who have had experience producing parts by rotational molding understand that the rotomolding is a complex manufacturing process, with many variables that can affect the quality articles produced. Polyethylene is a particularly good material for rotational molding. When used as a fine mesh powder, polyethylene s melting characteristics easily form a solid part. Polyethylene has adequate heat stability to withstand the relatively high temperature and long heating cycles of rotational molding. Parts formed from polyethylene have an excellent combination of strength, toughness and other mechanical properties. In order to optimize the properties, it is necessary to use proper molding conditions to form articles. Muehlstein rotational molding products are designed for optimized processability and properties in rotational molding. These products are compounded from prime quality materials by a process designed to yield consistent material, box-to-box and shipment-toshipment. This bulletin is designed to help the customer find the optimum molding conditions for their application. Because of the diversity of article made by the rotational molding process and the different types of rotational molding machines and molds used in the rotomold process, it is impossible to establish universal optimum molding condition that are optimum for all circumstances. We can offer recommendations for starting conditions and guidelines for adjustments to the process conditions to help optimize the process for the particular part being molded. Process Control: In order to achieve consistent quality rotationally molded articles, it is important that proper molding conditions are used and, as with any manufacturing process, that all variable in the manufacturing process be controlled as closely as possible. Each rotational molding cycle must be preformed under the same conditions as the previous cycle, as nearly as possible. Because different parts are often placed on the same arm or different arms of a multi-arm machine, often the molding cycle is a compromise to achieve acceptable quality for all parts being molded. Mold Design: Proper mold design is critical for production of quality parts. While the design of molds is outside the The technical information, suggested uses and applications presented are made without charge and are believed to be reliable; however Ravago Manufacturing Americas disclaims responsibility for results of use of this information. Ravago Manufacturing Americas makes no warranties, either expressed or implied, concerning our materials, including any warranties of merchantability or fitness for a particular use. All users should rely upon their own tests in determining suitability. 13001 Almeda Road, Houston, TX 77045 Tel: (713)433-5604
Rotational Molding Guide scope of this guide, there are a couple of areas of mold design that should be highlighted. Mold Venting: Mold venting is important to good part quality. Proper venting will allow the part to be heated and cooled at about atmospheric pressure. This will eliminate blowholes at the parting Proper venting will prolong the contact of the cooling part with the inner surface of the mold, resulting in a shorter cooling cycle and post-mold warpage. Lower pressure differential also results in longer mold life. In general, mold vents of sufficient size and number to allow equalization of the inner mold pressure. Vent tubes should extend to the center of the mold. The inside end should be loosely packed with fiberglass to prevent the unfused powder from exiting the mold early in the heating cycle. In order to be efficient vent tubes must be kept clean. If the fiberglass becomes clogged with plastic powder the vent tub will no longer function to equilibrate pressure. Drying: Polyethylene does not absorb moisture easily. Therefore, under normal conditions, pre-drying of the pulverized powder is not required. Under high humidity conditions, or when specialty grades, such as flame retardant, are used drying may be necessary. Measurement of Charge Production of consistent parts requires that each step of the rotational molding process be repeated precisely every cycle. This is especially important that the same weight of polymer be added to the mold for each part. The bulk density of pulverized polyethylene for rotational mold has some variability. Therefore, material should be measured on a weight rather than a volume basis. Oven Temperature: Proper cure of the rotationally molded part requires sufficient heat to melt the polyethylene for long enough time duration to completely fuse the polyethylene. The oven temperature must not e so high that the polyethylene is oxidized. On a multi arm machine, the heating and cooling cycles must be of equal time. On one hand, the molder would like the oven temperature to be as high as possible to shorten the heating time to a minimum. On the other hand, keeping the oven temperature as low as possible gives optimum part appearance, part toughness, reduces the cooling required and reduces thermal stress on the mold. The rotational molder must decide what combinations of these variable results in acceptable part quality and productivity. Starting Conditions: Our experience has shown that the following conditions are good starting points. The molders experience with existing molds and specific molding machines may suggest slightly different conditions. In general, Muehlstein polyethylene rotational molding compounds can be processed at 50 to 75 F lower oven temperatures than competitive polyethylenes, especially if a Muehlstein precolored polyethylene is replacing a dry-blended color. Oven temperature: 500 to 600 F (260 to 315 C) Oven time: 10 to 25 minutes You can reduce the trial and error involved in time/temperature parameters by measuring the PIAT (Peak Internal Air Temperature). The following table is a general guide for PIAT for RMA standard resins. These are suggested for a wall thickness of around 0.125 in and adjustments will need to be made for thinner or thicker walls: Table 1. GRADE PIAT C HMP-304 210 HMP-305 200 HMP-307 190 HMP-301 215 HMP-315 204 HMP-325 202 AQUATUF HMP-334 205 AQUATUF HMP-337 207 Cooling Like heating cycles, there is no universal best cooling cycle. For a multi-arm molding machine, the cooling time can be no longer than the heating time. On the other hand, high cooling rates increase the potential for warped parts. It is usually best to start the cooling with several minutes of air-cooling. This is followed by atomized water, air/water or water spray for the bulk of the cooling time. It is beneficial to finish the cooling cy- Page 2 Ravago Manufacturing Americas 20110701
Rotational Molding Guide cle with several minutes of air cooling, allowing the outside surface of the mold to dry. Polyethylene is semi-crystalline polymer. The degree of crystallinity is partly controlled by the cooling rate of the part. Rapid cooling results in lower crystallinity. Unfortunately, rapid cooling produces parts with low crystallinity at the mold side of the part and high crystallinity at the inner surface of the part. This disparity manifests itself as warpage. References: 1. Introduction to Rotational Molding Seminar, Dr. Glenn Beall, Association of Rotational Molders, Chicago IL. 2. Rotational Molding Troubleshooting Manual, (ARM-102-1089), Association of Rotational Molders, 1989, Chicago, IL It has been found that warpage is minimized with slow cooling. This must be accomplished with the constraints of the cooling time available. Slow cooling may adversely affect other physical properties, such as impact strength. AQUATUF Higher density, high performance resins in the 0.942 to 0.952 range tend to be more crystalline and have a higher shrink rate. These may need to be cooled more slowly to prevent warpage, and every attempt made to hold the part against the mold surface until the release point [This is best determined by a Rotolog type trace on the cooling cycle]. Common problems: Part Surface: Several factors should be considered when attempting to improve the quality of the part s surface. The surface of the part can be no better than the surface of the mold. Mold design is also of importance in the production of part especially in parts with recesses, high definition or inserts. Selection of the appropriate melt index polyethylene is another factor. The dry flow and particle size distribution of the molding power will also affect surface quality. The selection of the proper combination of heating and cooling conditions is also important factors. Rotation speed and rotation ratio effect part surface quality Consistent application and minimum use of mold release. Page 3 Ravago Manufacturing Americas 20110701
Rotational Molding Guide Typical Rotation Ratios Typical Speed (RPM) Ratio Shapes Major axis Minor axis 8 to 1 Oblongs (Horizontal) Straight tube (Horizontal) 8 9 5 to 1 Some defroster ducts 5 6 4.5 to 1 Balls & gloves 8 9.75 3.3. to 1 Any shape having overlapping lines of rotation at 4 to1 10 12 12.25 14.5 4 to 1 Cubes, balls or odd shapes Rectangular boxes, horses with bent legs 8 10 10 12.5 2 to 1 Rings, tires, balls Any rectangle that shows two or more thin sides when run at 4 to1 Picture frames, Round flat shapes Horses with straight legs, Auto crash pads (vertical) 1 to 2 Parts that should run 2 to 1 but show thin side walls. 1 to 3 Flat rectangles e.g. gas tanks, suitcases, tote bin covers, etc. 1 to 4 Tires, curved air ducts, Pipe angles, flat rectangles, Balls whose sides are thin at 4 to 1. 6 8 10 12 5 7 4 6 9.5 4 5 6 9 12 15 18 15 21 15 22.5 36 20 25 30 1 to 5 Cylinders, vertical 4 24 Vertical - Mounted parallel to major axis Horizontal - Mounted perpendicular to major axis. Page 4 Ravago Manufacturing Americas 20110701
Rotational Molding Trouble-shooting Guide Warped parts Inadequate venting Non-uniform cooling of part caused by resin pulling away from mold. Provide adequate venting - 3/8-1/2 diameter vent per ft 3 of molded volume is suggested for thin walled parts. Rotate mold during cooling cycle. Provide adequate venting and make sure vents are not clogged. Use less mold release. Check for too effective mold release agent. Avoid large flat panels in part design if possible. Reduce cooling rate during initial part cooling cycle. Increase the cooling medium temperature, air cool, and then water cool. Apply air pressure through spine during cooling Poor impact strength Resin type Moisture on resin or pigment. Resin selection not correct Density increase during slow cooling Part design not appropriate Insufficient fusion of resin Use precolored, compounded resin. Use proper resin having adequate melt index and molecular weight distribution for application. Only use dry powder and/or pigment. Use lower density or lower melt index resin Increase cooling rate to maintain a lower density. Review and alter mold design if necessary, eliminate sharp corners and narrow passages. Increase oven time and/or temperature. Non-uniform cooling caused by uneven wall thickness in the part. See suggested remedies under problem heading Uneven wall thickness of molded parts. Improper coloring Select pigment and pigment level that does not effect impact. Use pre-colored, compounded resin. Non-uniform cooling caused by sections of the mold being shielded from heat and cooling medium. Uneven cooling caused by clogged water nozzles Mount mold to eliminate shielding problems, add baffles to direct heat and cooling into recessed or shielded areas. Check and clean nozzles on a periodic schedule. Parts sticks in the mold Over-curing of resin. Degradation of resin due to long-term high temperatures. Insufficient amount of mold release agent or the release agent has deteriorated with use. Decrease oven temperature or heating cycle. Reapply or use more release agent. Old release may have to be removed and a new one applied. Over-cured part. Degradation of the resin due to high temperature and/or excessively long heating cycle. Highly under-fused part. Some degree of under-fusion is advisable especially in the case of low melt-index resins to prevent degradation; however, highly underfused parts can cause significant loss of impact strength. Improper coloring Decrease oven temperature or heating time. Increase oven temperature or total heating time. Increase heattransfer coefficient, e.g. steel or aluminum. Select pigment and pigment loading that does not affect resin. Ineffective release agent or mold release does not withstand elevated temperatures. Mechanical interference during part removal Roughness and porosity of mold surface provide areas where resin may adhere. Presence of resin at parting line due to internal mold pressure forcing semi-molten Use suitable mold release agent that is effective for resin and temperature used. Apply according to supplier s instructions. Locate mold parting line at undercut or taper side walls of mold. Refinish damaged mold surfaces, plug, weld and sand smooth. Provide adequate venting, 3/8 to ½ diameter vent per cubic foot of mold Page 5 Ravago Manufacturing Americas 20110701
Rotational Molding Trouble-shooting Guide Blow holes through the part or ringworm effect under thin wall surface other than at the parting line Excessive flashing at mold parting Bubbles on the mold parting resin through parting Build-up of degraded resin in the mold may be caused by burning of thin walled sections. Shrinking onto large deep inserted areas. Undercuts in mold Low shrinkage value for resin. Porosity in the cast aluminum mold Pores or holes in welds. Internal mold pressure during heating cycle tends to force semimolten resin out through the parting During the first stages of cooling, there will be a rush of air into the part to fill the resultant partial vacuum. If there is inade- volume is suggested for thin walled parts. Clean mold periodically. Reduce oven temperature. Provide adequate taper to mold walls. Use effective mold release on insert areas. Remove part while warm. Provide adequate means for applying force to separate mold halves. Design mold to place undercuts at parting linen so that mold has draft angle for part removal. Use higher density polyethylene grade. Obtain better quality castings. Drill through void and drive pin or weld from inside. Relieve from outside by drilling into void. Remove parts from molds while warm to touch. This helps drive moisture out of pores. Use proper welding rod and procedure. Weld inside surface first to get good penetration. Provide adequate venting and make sure vents are not clogged. Remate mold parting line and adjust mold clamp pressure evenly. Clean mold flange to prevent gapping and apply new mold release on flange. Reduce internal air pressure if used. Use lower melt index pressure. Vent the mold to atmosphere pressure. Relocate vent to middle of mold. Use glass wool in vent. Use Teflon as vent tube. Discoloration of interior surface of part. Powder bridging or not filling narrow passages of mold. Poor part stiffness Lightning effect in quate venting, air will penetrate the molten resin, at parting line, becoming trapped as the part wall solidifies. Poor mold parting Degradation of resin due to high temperature and/or excessively long heating cycle... Mold design incorrect. Poor pourability (dry flow) of powder. Powder does not melt or flow properly. Cold spots on mold. Improper mold rotation. Part wall too thin Resin selection not correct. Part design not appropriate. Under fused parts. Moisture in pigment or resin. Make sure tube is adequate size. Remate molding parting line and adjust mold clamp pressure evenly. Clean mold flange to prevent gapping and apply new mold release on flange. Decrease oven temperature or heating cycle, or purge part with inert gas (nitrogen). Use resin with the proper amount and type of antioxidant. Check pigment for heat stability. Modify mold by increasing width to depth ratios across the mold opening. Design corners of mold with more generous radii. Avoid ribs with less than 4x wall thickness. Make sure powder has acceptable pourability and bulk density. Use finer mesh powder or resin with a higher melt index. Avoid any shielding mold areas. Check for mold wall thickness uniformity. Use correct ratio and rotation speed. Add more powder to initial charge. Use rein of higher density. Review and alter mold design if necessary. Increase oven temperature or total heating cycle. Increase heattransfer rate by using thinner mold walls, or make the mold from materials of greater heat-transfer coefficient, e.g. steel or aluminum. Try filling molds while hotter. If dry-blending, dry pigment or use pigment from unopened Page 6 Ravago Manufacturing Americas 20110701
Rotational Molding Trouble-shooting Guide colored parts. Blow holes through part around insert Speckled colors and lumps of pigment in dry blended colors Long oven cycles Static build-up Pigment not ground properly. Poor fit on inserts allowing moisture or vapors to be trapped around insert and expand, blowing a hole in the part. Bridging of resin because of close dimensions. Insufficient blending Heat-transfer rate not adequate to melt all resin, excessively thick mold. Heating not efficient. Low oven temperature. Resin powder too coarse Poor melt flow container. Use precompounded color resin powder. Dry resin completely or replace. Add small amount of mineral oil to resin or commercially available anti-stat. Make certain that all mixing and molding equipment is adequately grounded with high surface copper cable. Use 100-mesh pigment or pulverize pigment prior to mixing. Use precompounded color resin powder. Refit insets and relieve to allow trapping gases to escape to the outside of the mold. Drill a small hole through the insert bolt to relieve gas pressure. Change insert dimensions or location to allow powder to flow without bridging. Break up agglomerates of pigment before blending. Use high intensity mixer. If unable to achieve a desirable color balance, use a compounded color resin powder. Increase heat-transfer rate by using thinner mold walls, or make the mold from materials with greater heattransfer coefficient, e.g. steel or aluminum. Increase air velocity around mold during heating cycle. Check oven for air leaks. Increase oven temperature. Recalibrate instruments on regular schedule. Use finer mesh powder Use higher melt index resin. Reduce air-water cooling ratio Longterm part failure Uneven wall thickness of molded parts. Part over-cured during molding Photo-degradation of part caused by ultraviolet light from sun or internal lighting (florescent) Stress-cracking due to multi-axial stresses in part. Cracking may have been accelerated by chemical environment and/or temperature Inadequate resin additive system. Color change due to oxidation. Light colored parts may look yellow or pink. Improper colorants or blending. Improper mold rotation Mold shielding Uneven mold wall thickness Inadequate powder properties. Low bulk density, no powder pourability, large amount of fluff, particles have many tails that entangle into clumps during molding. Buffeting or air flow in deep dished areas. Decrease oven temperature or heating cycle Use UV stabilized resin in application. Add suitable UV stable pigment. Use polyethylene grade with good environmental stress crack resistance (ESCR). Modify design around the areas containing inserts. Examine parts in field use to determine adequacy of design around stress concentration points. Antioxidant type and level of concentration may be inadequate. Reduce level of internal mold release if used. Reduce oven temperature. Use colorants that disperse well in base resin. Use precompounded color resin powder for optimum dispersion of color and stabilizers. Vary ratio and speed of rotation of mold to obtain even coverage and adequate number of powder trackings Mount mold to eliminate shielding. Use care in designing molds to prevent excessive variations in mold wall thickness (thin spots attract more resin) Obtain an acceptable quality powder. F using bulk powder storage, empty stage silos before refilling to prevent accumulation of fine particles in storage silo. Extended cooling Avoid deep dished areas whenever possible. Reduce thickness of mold in dished Page 7 Ravago Manufacturing Americas 20110701
Rotational Molding Trouble-shooting Guide Highly underfused parts, with many small bubbles in wall or rough powdery inside surface. Poor flow-out into mold recesses Oven temperature not high enough to drive air bubbles out of part walls. Heat transfer rate not adequate to melt resin. Resin powder too coarse. Moisture in mold Poor mold design Improper mold rotation Melt index of resin too low areas. Open handles so air can flow through kiss-offs in mold. Increase oven temperature or total heating cycle. Increase heat transfer rate by using thinner mold walls or make mold from materials with greater heattransfer coefficient, e.g. steel or aluminum. Use finer mesh powder. Reduce moisture in mold by running with warm molds and dry mold before charging powder. Design shallow recesses with generous radii on edges. Preheat recessed areas with torch for 30- seconds before charging. Add heat deflectors or thermal pins. Change ratio and/or speed of rotation. Increase melt index of resin. Page 8 Ravago Manufacturing Americas 20110701