Troubleshooting guide for Rotational Moulding Applications
Rotational Moulding Troubleshooting
Problem
Probable Cause
Suggested Remedy
Temperature Setting
Temperature setting are crucial parameter for proper plastic processing ,hence one should check temperature controller, thermocouple and solid state relay(SSR)
- when you cold start the machine ,wait for 2hours.
Warped parts
- Inadequate venting
- Non-uniform cooling of the mould caused by resin pulling away from mould wall
Over cured parts, degradation of resin due to high temperature and or excessively long heating cycle
- Highly under fused parts, 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 in impact strength
- Sagging of hot plastic, shrinkage of high density resin
- Provide adequate venting and make sure vents are not clogged ,0.5 inch inside diameter (ID) vent per cubic yard of mould volume is suggested for thin walled parts, Rotate mould during cooling cycle
Use less mould release agent, Avoid large flat panels in part design if possible, Reduce cooling rate during initial part of cooling cycle, Increase the cooling medium temperature (air or water),Apply air pressure through spindle during cooling
- Decrease oven temperature or heating cycle, purge part with inert gas (nitrogen)
- Increase oven temperature or total heating cycle, increase heat transfer rate by using thinner mould walls or make the mould from material with greater heat transfer coefficient, e.g. steel, aluminum, copper
- Reduce oven temperature, shorten cycle time, reduce amount of mould release agent, use lower MI resin
Parts sticks in mould
- Insufficient amount of release agent or release agent has deteriorated with use
- Insufficient mould release or release agent does not withstand elevated temperature
- Interference during part removal
- Roughness and porosity of mould surface provide areas where resin may adhere
- Presence of resin at parting line due to interna mould pressure that tend to force semi-molten resin through parting line
- Shrinking onto large deep inserted areas
- Undercuts in mould
- Degradation of plastic
- Incomplete shrinkage due to moulding being hot
- Apply or use more release agent, Old release agent may have to be removed and new one to be applied
- Use suitable mould release agent that is effective for resin and processing temperature used, apply according to supplier’s instruction
- Locate mould parting line at undercut or taper sides of mould
- Refinish damaged mould surface
- Provide adequate venting 0.5-inch ID vent per cubic yard of mould volume
- Provide adequate taper, use very effective mould release on insert area, remove part while warm, make adequate provision for applying force to separate mould valves
- Design mould to place undercuts at parting line so that mould has draft angle for part removal
- Reduce oven temperature or time slightly
- Increase cooling cycle
Poor impact
- Resin selection not correct
- Density increase during slow cooling
- Part not designed properly
- Insufficient fusion of resin
- Improper pigment shade
- Cooling too slow
- Use a lower density or lower melt index resin
- Increase the cooling rate to maintain lower density
- Review and alter mould design if necessary, eliminate sharp corners and narrow passage
- Increase temperature or heating time
- Select pigments and pigment loading that does not affect impact, use pre-coloured components
- Increase water pressure, use spray instead of fog nozzles, check venting to prevent pull way from mould
Long term part failure
- Part over cured during moulding
- Photodegradation of part caused by ultraviolet light from sun or internal lighting (fluorescent)
- Stress cracking due to multi-axial stresses imparted, may have been accelerated by chemical, environmental and temperature effects
- Decrease oven temperature or heating cycle
- Use UV stabilised resin in application, add suitable UV stabiliser and or pigment, fine well dispersed carbon black provides the best protection against UV
- Use stress crack resistant Polyethylene grade, do not store an environmental stress crack solution in a container moulded from a poor environmental stress crack grade of PE for a long period of time or at elevated temperature, modify design around area containing inserts, examine part under field conditions to determine adequacy of design and stress concentration points
Uneven wall thickness of moulded parts
- Improper mould relation
- Mould shielded
- Uneven mould wall thickness
- Optimise ratio and speed of the rotating mould to obtain even coverage and adequate number of powder trackings
- Mount mould to eliminate shielding
- Use care in designing moulds to prevent excessive variations in mould wall thickness(thin sorts attract more resin)
Bubbles on the parting line
- During first stage of cooling there will be a rush of air into the part to fill the resultant partial vacuum, if there is inadequate venting air will enter molten resin at the parting line and become trapped as the part wall solidifles
- Poor parting line, vacuum inside part
- Poor mould parting line
- Vent the mould to atmospheric pressure, relocate vent to the middle of mould, use glass wool in vent, use Teflon vent tube
- Clean parting line, check clamping pressure for sufficiency and uniformity, check vent for clogging
- Clean mould flange to prevent grapping, apply new mould release for flange
Long oven cycle
- Heat transfer rates not adequate to melt all the resin, excessively thick mould
- Heating not efficient
- Poor flow
- Oven heat capacity insufficient, mould shielded, oven temperature too low
- Increase heat transfer rate by using thinner mould walls or make the mould from material with greater heat transfer coefficient e.g. steel, aluminium, copper
- Increase air velocity around mould during heating cycle
- Use higher melt index resin
- Increase oven temperature, avoid any shielding of mould, increase air circulation in oven, use lower density powder, in case of polyethylene use higher melt index powder
Highly under-fused parts with many small bubbles in the wall or the inside surface is rough and powdery
- Oven temperature not high enough to drive air bubbles out of the part walls
- Heat transfer rate not adequate to melt all the resin
- Maximum temperature attained by plastic too low
- Increase oven temperature or total heating cycle
- Increase heat transfer rates by using thinner mould walls or make mould from material with greater heat transfer coefficient eg. steel, aluminium, copper
- Lengthen cycle time, increase oven temperature, use powder with lower density in case of polyethylene, reduce wall thickness of mouldings, dry mould before charging with powder, corrective welding of the moulds around such corners and joints
Discoloured moulding
Polymer degraded
Decrease oven temperature or heating cycle time
Surface pitting
Contaminated mould surface
Clean mould, avoid mould release agent
Excessive flash
Poor parting line
Remake parting line, check vent for clogging
Bridging of powder in mould, poor flow
Poor mould design, improper rotation
Rotate mould faster, use lower density powder in case of polyethylene, use higher melt index powder, provide more
