Vijay Kudchadkar, application engineer, Mold Hotrunner Solutions (MHS), Jörg Schmidt, director of business development, MHS, Derrick Jahr, micro moulding process engineer, Mold-Craft, Brian Matachun, national director medical sales, Westfall Technik
As the miniaturisation of devices creates exciting new possibilities for a healthier, more connected world, manufacturers require innovative and more reliable ways to produce small and micro-sized plastic components. Conventional approaches are costly, inefficient and unable to deliver the precision and speed required to meet industry demand.
Most micromoulders use cold runners to make micro-sized parts. These cold runners are ejected with the parts after each moulding cycle. The parts must be detached from the cold runners, which must then be either scrapped or converted back into granules and fed back to the moulding machine. There are significant drawbacks and costs associated with using cold runners; and unfortunately, the majority of micromoulders believe that cold runners are the only means of making micro-sized plastic parts.
Exorbitant costs of cold runner scrap
The weight of a cold runner system can be 2–50 times the weight of the part. It is an excessive amount of waste, especially when the cold runners cannot be recycled and the resin is expensive. To put this into perspective, it is similar to using 20 kgs of flour to make just four medium thin-crust pizzas. Even when regrind is permitted, it is not possible to reuse all of the cold runner scrap since the part volume is so small compared with the cold runner volume. The vast amount of cold runner material that cannot be reused must then be disposed of or sold, and in the worst-case scenario, ends up in landfills.
Cold runners require handling, which increases the number of steps in the process and production costs. Millions of parts have to be detached from millions of cold runners. Labour and energy are required to recycle cold runners, when regrind is permitted, as well as dispose of unused cold runners.
The costs of cold runner scrap can add up quickly. Consider an application where the production goal is five million PEEK microparts per year. In this application, the part weight is 0.0067 g and the material cost is US$198 per kg. The mould is a two-cavity cold runner mould with a runner-to-part weight ratio of 27.5. This would mean that in each cycle, two useful parts would be produced and material worth 55 parts would be scrapped. During the course of a year, at least 921.25 kg of resin worth $182,407.50 would be wasted. On the other hand, if one were to make these parts with direct gating, 100 percent of these costs would be avoided and almost a tonne of resin would be saved.
With bioabsorbable materials, the costs of cold runner scrap become astronomical. In bioabsorbable applications, the runner-to-part volume tends to be higher to reduce the shear stresses and shear rates that the material experiences. If the part weight is 0.027 g and the runner-to-part weight ratio is 18.52, at least 4,000 kg of resin would be wasted while making 8 million parts. If the cost of the resin is $3,760 per kg, the cost of the resin wasted would be a staggering $15 million. It is worth repeating that 100 percent of these costs would be avoided by using direct gating.
Long cycle times
Cold runners increase the cost of production by increasing the cycle time. In injection moulding, the required cooling time (for plates) is directly proportional to the second power of the thickness of the plastic. If the thickness of the cold runner is three times more than the thickness of the part, the required cooling time for the cold runner would be nine times more than the cooling time of the part. In other words, if the micro-sized part cools in 0.5 s, the moulder would be waiting an additional 4 s for the cold runner to cool down. If it took 43 days to produce 1.5 million microparts on a four-cavity cold runner tool, approximately 17 days would be wasted waiting for the cold runner to cool down.
Cold runners can increase production time by several months. Projects can be completed sooner, or production can be increased significantly in the same timeframe, if cold runners are eliminated from the process. Consider an application where the production goal is 16 million acetal parts per year. The part weight is 0.007 g, the cold runner-to-part ratio is 25:1 and the cycle time is 12 s. The part would reach ejection temperature in 1 s and the cold runner would need an additional 6 s to reach the ejection temperature. To make 16 million parts, one would need 556 days with a four-cavity cold runner tool and 278 days with an eight-cavity cold runner tool. If one were to eliminate cold runners and use an eight-cavity valve gated tool, the cycle time would reduce by 6 s and it would be possible to make the 16 million parts four months sooner. Furthermore, with a 32-cavity valve gated tool, one could save nine months and meet the production goal of 16 million parts in under 40 days.
Short shots and voids
A 76 percent increase in cold runner volume is required to avoid short shots and voids
In conventional micromoulding with cold runners, material must flow from the moulding machine through long cold runner channels before it reaches the micro-sized cavities. During this process, the plastic melt loses heat to the cold walls of the mould. The material that makes contact with the mould walls solidifies and forms a frozen layer. This frozen layer continues to grow until the runner completely solidifies. The core of the runner must be hot enough to be able to fill the part and then pack it sufficiently. If the temperature of the core drops too quickly, it may lead to short shots or voids.
The moulder then has two options to fix these problems: (1) increase the injection velocity and/or (2) increase the cold runner size. Increasing the injection velocity significantly increases the shear rates and shear stresses in the material and hence this solution cannot be used for shear sensitive materials. In many cases, the only option available to the moulder is increasing the cold runner size as much as 50–90 percent. Option 2 solves the quality problem, but then creates a production problem by increasing the costs of cold runner scrap and cycle times.
Inconsistent part weights
The weight of a plastic part depends on the cavity volume, material properties, melt temperature and pressure during the moulding cycle. For a repeatable process, the temperature of the melt delivered to each cavity during each cycle must be consistent. The temperature of the melt is not uniform inside the moulding machine; there will always be some variation.
The amount of variation depends on the screw design, screw revolutions per minute (rpm), material viscosity, cycle time, back pressure and barrel temperature profile. When the melt flows from the machine to the cavities, the cold runners increase the melt temperature variation. Melt temperature variation also changes from cycle to cycle. When moulding with cold runners, it becomes difficult to deliver consistent melt temperature to each cavity during a cycle and also from cycle to cycle. This usually results in unacceptable parts and increases the scrap rates. Due to melt temperature variation, shot-to-shot part weight consistency and dimensional stability also reduces.
Cold runners must be detached from the parts. Degating may not be precise and while doing so, it is possible to leave gate vestige on some parts. Since the parts are micro-sized, the seemingly small gate marks become significant. Inconsistent degating and large gate vestiges create surface defects and microfractures, and can increase scrap rates.
Conventional hot runners and micromoulding
Micromoulders have explored the possibility of eliminating cold runners by using conventional moulding machines and hot runner technology. In conventional moulding with hot-runners, the melt is subjected to high processing temperatures in the machine barrel and the hot runner channels. Many have failed in this endeavour because the residence time for conventional hot runners is too high for many sensitive materials such as acetals and polyamides.
High cavitation is necessary for high-volume applications. However, with conventional hot runners, higher cavitation leads to longer residence times due to the increased hot runner volume. For a 50 mg part, an optimised 32-cavity hot runner would have a minimum volume of 40 cm3. For sensitive materials, it is important to remove the material from the hot runner system as quickly as possible. A small increase in cycle time or any stoppage in production can lead to material degradation. For these reasons, micromoulders have preferred cold runners to conventional hot runners.
A micromoulding process that eliminates cold runners
Harald Schmidt, founder and vice-president of Mold Hotrunner Solutions (MHS), and a team of MHS engineers have developed a machine that affords an injection micromoulding process named ISOKOR, designed to eliminate the use of cold runners. The fundamental idea behind ISOKOR is ‘to be kind to the polymer until it reaches the gates’. First, a low-shear screw gently melts the polymer. The temperature of the material is gradually increased inside a temperature-controlled runner that is engineered to provide high levels of temperature control and temperature uniformity. The material is only brought up to its processing temperature when it is near the valve gates, which significantly reduces the time it is exposed to high processing temperatures and shear heat, thus minimising melt residence time and maintaining the integrity of the resin. The gates are then opened and the material is injected directly into the cavity using plungers.
ISOKOR is significantly gentler on the polymer when compared with conventional moulding. A proprietary micromoulding technique ensures that the allowable residence time of the melt is increased. For example, the maximum allowable residence time for polyoxymethylene (POM) is 15 minutes using the conventional moulding process and over 55 minutes using ISOKOR. This allows the micromoulder to inject directly into the cavities without a cold runner system.
Increased production speeds
Cycle times are significantly reduced, and production speeds increased, through the elimination of cold runners in the ISOKOR process. The micromoulder does not have to wait the additional 4–10 s required for the cold runner to cool below its ejection temperature. This reduction in cooling times result in a 50–75 percent reduction in cycle time. In addition, the micromoulder does not have to wait for the cold runner to fill and pack during each cycle.
Improved part quality
In conventional moulding, the material experiences high shear stresses and shear rates before it reaches the cavities. The material temperature is also lower than the recommended processing temperature before it enters the cavities. In the ISOKOR process, the material is treated gently until it is injected and enters the cavities at the recommended processing temperature.
The ISOKOR process allows for better control of the injection process. The cavities are filled and pressurised in milliseconds, thereby improving shot-to-shot consistency, improving cavity replication, and reducing quality issues such as short shorts, sink marks and voids.
Simplified production scale-up
Clamp tonnage is directly proportional to the projected area and the melt injection pressure. In cold runner micromoulding, the projected area of the cold runner dictates the size of the moulding machine. Increasing the tool number of cavities results in an increased projected area of the cold runner. As moulding machines increase in size, the micromoulder loses the ability to control the filling of microparts. In the ISOKOR process, the clamp tonnage is dictated by the projected area of the micro-sized parts only, thereby making it easier to scale up production and still maintain a high degree of control over the moulding process.
Reduced carbon footprint
By using a process that does not require cold runners such as ISOKOR, micromoulders will help the environment and save money in the following ways:
- eliminating tonnes of cold runner scrap that would have ended up in landfills;
- saving labour and energy that would have been required to regrind or dispose of cold runners;
- saving energy consumed by the moulding machine, since the injection pressure required is lower; and
- using smaller machines for high-production volumes.
The table below shows the amount of scrap that would be eliminated during the production of an 11 mg polyaryletheretherketone (PEEK) part by using direct gating instead of cold runners.
Production volume 100,000 1 million 5 million 10 million
Amount of plastic saved (kg) 55 550 2,750 5,500
Summary
To conclude, the benefits of direct gating and the ISOKOR process are as follows:
- higher quality direct gated parts, without wasted cold runner material or secondary processes;
- cleaner gate vestiges;
- faster cycle times for small parts ranging from 0.001 to 0.400 g;
- extremely high production outputs;
- increased allowable melt residence times;
- high cavitation in a lower tonnage, and therefore smaller footprint, machine;
- suitability for cleanroom use; and
- energy savings.
Mold Hotrunner Solutions (MHS)