Energy Management in Rubber Processing - Part 4 - Moulding
The fourth in a series of energy efficiency worksheets by Dr. Robin Kent for
the Carbon Trust to help the
rubber industry reduce costs through efficient
use of energy.
UK Government Environment and Energy Helpline 0800 585 794
The theoretical SEC required to mould rubber is 0.1 kWh/kg but actual results range from 0.3 kWh/kg to 2 kWh/kg. These are efficiencies of only 5% and 33% respectively and most of this energy is used to heat the machinery and the surroundings.
Failing to control energy costs will affect your wallet
Process design based on energy efficiency can produce plant at a capital cost of not much more than for inefficient plant - if energy efficiency is included at the start of process. In addition, it can provide savings in maintenance, improved reliability, throughput and product quality.
Heat loss is a major source of inefficiency but it can be reduced by good insulation. Surfaces such as injection press barrels, autoclave doors and flanges can be easily insulated.
Tip: Insulating a press platen from the body is difficult because of the large pressures but strong insulating materials are available and they greatly reduce the heat loss from platen to back-plate. A two to three year payback period can be expected.
Insulation saves the most money in plant with large exposed areas operating long hours at high temperatures. Savings include lower maximum demands for electrically heated plant and smaller steam supply requirements for steam plant. Safety and comfort for operators are improved, heat-up times are shorter, and temperature distribution is more even, reducing ventilation or air-conditioning costs.
Tip: Insulating electrically heated presses saves electricity, which costs three to five times more per unit than gas, oil or coal.
The energy needed to heat a compound increases with the cure temperature (the heat losses also increase) but the cure rate acceleration more than compensates for the increased rate of heat loss.
Tip: Curing at the highest possible temperature reduces the time over which curing plant losses occur and minimises the curing SEC. High cure temperatures and rates also maximise the machine throughput.
Once the shortest cure cycle consistent with product quality has been found, the SEC can be reduced by:
- accurate temperature control
- accurate cure period timing
- accurate cure pressure control.
Cure temperatures, pressures or times above or below the optimum cause excess energy use, reduced throughput or increased scrap, all of which increase costs and the SEC.
Cure temperature control
In many electrically heated presses, simple thermostats do not provide accurate temperature control. This is best achieved using Proportional Integral and Derivative (PID) control systems that quickly reach set point with low overshoot and final offset to reduce the heat loss and energy cost and provide consistency for reduced cure times.
Cure time control
Curing plant can take a long time to warm up before the working shift can begin and may be left to run for longer than necessary. In small factories, simple timers can make dramatic savings at low cost but these must be reprogrammed when there is a change in plant usage. In larger factories, an Energy Management System (EMS) can produce savings and better control of plant.
Tip: Computerised cure prediction and machine control give quick determination of settings for curing plant and good cure cycle control.
Cure pressure control
The minimum moulding pressure consistent with product quality should be used to save pumping energy and to reduce flash and rubber wastage.
Tip: New curing plants using central computer control have achieved 50% energy savings.
Electrical heating methods convert 100% of the electricity supplied into heat, but transfer to the rubber is slow because of the very low thermal conductivity of rubber. The transfer of energy to rubber can be improved with appropriate conversion techniques such as electromagnetic radiation, conduction or convection in liquid or gas curing media.
Processes for low pressure curing are:
- microwave heating
- infrared heating
- induction heating
- high power density resistance heating for electric ovens.
Efficiently generated and well controlled steam is a far cheaper heat source than electricity, especially where good utilisation gives high boiler and distribution system efficiencies. As with compressed air it is best to ‘minimise the demand before optimising the supply’. This means:
- minimising the curing heat required
- minimising heat losses from the process plant
- maximising the heat transfer efficiency by prevention and removal of scale, corrosion, air ingress and condensate flooding of heat exchangers
- maximising the amount of condensate recovered and minimising temperature and pressure losses.
Tip: At sites where steam usage has decreased, the system is often too large for the demand. This gives very high standing losses, maintenance and manning costs. It can be economic to decentralise the steam supply using local gas or electric boilers.
Tip: Very low efficiency may indicate a need for system improvements or decentralisation of heat supply.
Hydraulic plant can consume as much as 50% of the energy used, especially when systems are operated at flows, pressures and loads significantly different from their design capacity.
Tip: Staggering of cure cycles will reduce the maximum fluid demand and the size or number of pumps required.
Tip: Use accumulators to smooth intermittent demand from presses and allow the use of smaller pumps.
Tip: A hydraulic network with oversized, undersized or redundant pipes will waste energy.
Tip: Use booster pumps if ‘spot’ pressures higher than the system pressure are needed. This is cheaper than running a high pressure system and using reducing valves.
Tip: Servo electric drives can be used as substitutes for hydraulic systems to reduce energy costs and plant complexity and improve performance.
Tip: Using throttling or by-pass valves to control pressure is very inefficient. In new installations, savings can be made by replacing control valves with pressure-controlled pump motors.
Tip: Heat transfer from the press to the hydraulic system often means that cooling systems are needed. Improved platen to back-plate insulation will reduce heat transfer and the cooling load.
The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.
Part 1 - Reducing energy costs - the first steps
Part 2 - The rewards
Part 3 - Compounding
Part 4 - Moulding (This Section)
Part 5 - Extrusion
Part 6 - Motors and drives
Part 7 - Compressed air
Part 8 - Buildings