DHEC is a District Heating End Cap for sealing the pipe ends of pre-insulated pipes carrying fluids at various operating temperatures up to 120°C. The DHECs are a long-established product and have been manufactured by Thormac for the last fifteen years.
DHEC is a moulded part with different outlet diameters on each end to accommodate the diameters on both casing and service pipe. The installation is carried out directly on the prepared and preheated pipe surface, DHEC is slid over service and casing pipe and positioned in place. The DHEC is then heated with a regular gas torch. The high shrink ratio backing shrinks and forms a tight fit onto the pipe and casing. During recovery, the adhesive softens and flows to form a waterproof seal.
The Challenge
For the purposes of this case study, nested tooling is defined as the ability to make variations of the same basic part from a single mould tool, made up of a matrix of inserts. That is a part with the same basic geometry but with outlets or ports located on different parts of the final moulded part. This case study was devised to outline the advantages and disadvantages of nested tooling versus standard conventional tooling.
The Process
This case study describes the technical challenges and the engineering improvements that have been made to manufacture this complex product. Thormac’s team have made these improvements over time in order to manufacture this product consistently and to the highest possible quality.
The manufacturing process of the DHECs has four critical steps
- Mixing of the material and curing agent
The first stage is to mix the material and the curing agent. Added to the bespoke polyethylene material (which is used in the manufacture of our extensive range of heat shrinkable reticulation products), is a curing agent which consists of 0.7 % of the dry pellet weight. - Mould heating and control
The DHECs require a long cycle time for moulding, as the crosslinking process begins in the mould itself. Each DHEC mould is made up of multiple separately controlled heating zones. Each zone is tightly monitored as the parts are moulded in a narrow temperature window. - Expansion and finishing of Products
After the DHEC is moulded, it needs to be expanded. This is where the part is stretched and cooled, so that is can be reheated and allowed to shrink when it is slipped onto a pipe. - Coating
This the final stage of the process is a manual process enabling each individual part to be visibly inspected for quality, clarity of the marking and correctly applied sealing mastic before being packaged.
Mixing of the Material and Curing Agent
The first stage is to mix the material and the curing agent. Added to the bespoke polyethylene material used in the manufacture of our extensive range of heat shrinkable reticulation products is a curing agent which consists of 0.7 % of the dry pellet weight. This has been reduced over the years from 0.93% allowing an improvement in malleability of the material giving a reduction in curing agent usage and scrap. At this phase, particular attention needs to be paid to the length of time mixing. If the ingredients spend too long in the mixer, this will cause sheering and saturation of the finer particles, creating an uneven mix. This will manifest itself in a series of faults in the products such as:
- Inclusions
- Fault lines
- Uneven curing
- Uneven filling
- Shrinkage
Thormac has met its objective in this area to ensure an even and consistent mixture, which has led to a reduction of the curing agent and wastage created by sheared particles. This in turn has caused scrap from the moulding and expanding stages of the products. This is an important accomplishment for Thormac because once the material is mixed it cannot be reground, reworked or recycled.
In keeping with Thormac’s Continuous Improvement policy, a number of upgrades and improvements have been conceptualised. In the mixing process:
- Improved dosing with a metered dosing spray system
- Using a non-destructive mixer
As these would be large scale and need to be implemented together, this is in the planning stages of production development.
Mould heating and control
A recent improvement was the purchase of a new 350-ton moulding machine to replace the original and somewhat dated machine these products were produced in. This further demonstrated Thormac’s commitment to upgrade to more efficient and eco-friendly technologies. The DHECs require a long cycle time for moulding as the crosslinking process begins in the mould itself. With the current machine, the servomotor is only active for the injection part of the cycle, which represents 1% of the total cycle time.
Each DHEC mould is made up of multiple separately controlled heating zones. Each zone is tightly monitored as the parts are moulded in a narrow temperature window. The mould heating is critical for two reasons, curing and imbalance. If the curing is not right and the mould is too cold, then the parts will collapse on themselves – if they are too hot, the parts would become brittle and cannot be expanded. Imbalanced heat zones will cause deformation of the parts and hardness – which is why the higher heat and narrower process temperature window is needed. This window works to a tolerance of +/- 30 in the smaller moulds and +/- 60 in the larger ones.
As the DHECs are a long-established product, the original heating controllers used on an old analogue system which proved to react slowly. New technology allowed Thormac to achieve the greater accuracy required to ensure even curing and the Thormac team invested in converting the moulds to a more accurate digital system. This involved rewiring and replacing the heaters and heater connectors on each of the moulds, and purchasing a 48 zone controller to regulate the zones.
This solution not only improved the consistency of the cavity heating, but also had the added advantage of aiding with more precise fault finding. This led to a decrease in down time and uneven curing scrap by 27%, reducing our ECO footprint extensively. Even with the product development being well established and perfected, Thormac still endeavours to implement improvements to improve the product quality and competitiveness.
After the DHEC is moulded, it needs to be expanded. This is where the part is stretched and cooled, so that is can be reheated and allowed to shrink when it is slipped onto a pipe. This is also the stage where any hidden failings from the previous two stages come to light. Inclusions, fault lines, uneven filling all cause part splitting or in the case of uneven curing, part warpage.
Mould heating and control
To control any factors influenced by the expansion machines and – as stated previously, the DHECs are a long-established product, a number of projects have been undertaken to improve the process.
- New timers in the expanders
- Part counters installed to allow better control of production numbers
- Heating ovens refurbished with ceramic elements to reduce electrical usage
- Drives on expanders refurbished to direct drive instead of mechanical speed control
- Cooling system refurbished for expander 1
- Dry expander pneumatics upgraded
- Dry expander safety upgraded
These small but significant – and relatively inexpensive improvements have shown substantial results, according to the production unit head and operators.
It is also at this point that the product is printed to mark type and batch number to aid traceability.
Thormac has the capability of performing its on-site destructive tensile test should we encounter issues with a batch of product. The tensile tester is set 25% higher than the specified value for wall strength and elasticity, and this measured force is applied by an Instron tensile tester
Elastic modulus (E) is a measure of the stiffness of a material under compression or tension, although there is also an equivalent shear modulus. It is a property of the material and does not depend on the shape or size of the object
A small piece of rubber has the same elastic modulus as a large piece of rubber. Elastic modulus, also known as Young’s modulus, relates the force of squeezing or stretching an object to the resulting change in length.
Coating & packing
To control any factors influenced by the expansion machines and – as stated previously, the DHECs are a long-established product, a number of projects have been undertaken to improve the process.
- New timers in the expanders
- Part counters installed to allow better control of production numbers
- Heating ovens refurbished with ceramic elements to reduce electrical usage
- Drives on expanders refurbished to direct drive instead of mechanical speed control
- Cooling system refurbished for expander 1
- Dry expander pneumatics upgraded
- Dry expander safety upgraded
These small but significant – and relatively inexpensive improvements have shown substantial results, according to the production unit head and operators.
It is also at this point that the product is printed to mark type and batch number to aid traceability.
Thormac has the capability of performing its on-site destructive tensile test should we encounter issues with a batch of product. The tensile tester is set 25% higher than the specified value for wall strength and elasticity, and this measured force is applied by an Instron tensile tester
Elastic modulus (E) is a measure of the stiffness of a material under compression or tension, although there is also an equivalent shear modulus. It is a property of the material and does not depend on the shape or size of the object
A small piece of rubber has the same elastic modulus as a large piece of rubber. Elastic modulus, also known as Young’s modulus, relates the force of squeezing or stretching an object to the resulting change in length.
