Wednesday, March 7, 2007

Hot runner construction and method

Hot runner is a method in plastic injection molding with a mold suitable for use insimultaneous molding of a large number of articles at a time by hot-runner molding technique. The mold has a hot-runner block in which disposed are a plurality of gates through which the molten resin is delivered to the molding cavities formed in a cavity block which is usually kept at a low temperature. The gate area of the hot-runner block is kept in contact with the cavity block, for the safe deliver of the resin to the molding cavities, in the injection step and, therefore, is cooled by the cavity block. Consequently, when the injection is completed, the gate area has been cooled to such a low temperature as to cause a solidification of the resin in the gate area. The hot-runner block is then separated from the cavity block and brought into contact with a hot gate-temperature recovering block, so that the gates are heated to remelt the solidified resin, before the next batch of injection is performed, so as to recover the fluidity of the resin

Types of hot runner systems
There are two types of hot runner systems:

Insulated runners
Insulated runner molds have oversized passages formed in the mold plate. The passages are of sufficient size that, under conditions of operation, the insulated effect of the plastic (frozen on the runner wall) combined with the heat applied with each shot maintains an open, molten flow path.

Heated runners
For heated runner systems, there are two designs: internally heated and externally heated. The first is characterized by internally heated, annulus flow passages, with the heat being furnished by a probe and torpedo located in the passages. This system takes advantage of the insulating effect of the plastic melt to reduce heat transfer (loss) to the rest of the mold. The second consists of a cartridge-heated manifold with interior flow passages. The manifold is designed with various insulating features to separate it from the rest of the mold, thus reducing heat transfer (loss).

hot runner system picture

One of the more important enhancements you can incorporate into your mold to improve molded part quality, reduce production times, and remain price competitive is to equip it with a quality hot runner system. A hot runner-equipped mold can :

• Materials cost savings - no runner to regrind or reprocess
Least expensive cost / piece
Reduction of energy costs
Shorter, faster cycle times - no runners to cool
• Smaller machines - reduced shot volume into runners
Automated processing – runners do not need to be separated from the parts
Gates at the best position for economical design
• No runners to remove or regrind

• Reduces the possibility of contamination
Lower injection pressures
Lower clamping pressure
• Shorter cooling time
Shot size reduced
Cleaner molding process
Eliminates nozzle freeze
Consistent heat within the cavity
There are, however, a few disadvantages to hot runner systems that need to be considered
• Hot runner molds are more complex and expensive to build than cold runner molds
• Higher initial start-up costs than for cold runner systems
• Complex initial setup prior to running the mold
• Risk of thermal damage to sensitive materials
• Elaborate temperature control required
• Higher maintenance costs – more susceptible to:
o Breakdowns
o Leakage
o Heating element failure
o Wear caused by filled materials

other type hot runner system

Lower Cycle Time, Increase Output
The cycle time of any mold is largely influenced by the cooling cycle—how fast the resin can be sufficiently cooled so that the part can be ejected without permanent deformation. In any given mold, the areas that take longest to cool are those with the thickest wall section.
njection time is another component that differs between comparable hot and cold runner equipped molds. The injection time difference will be the extra time required to fill the cold runner.
Close and open stroke of the press is extended with cold runner equipped molds. The travel must be increased to accommodate safe ejection of the cold runner.
Parts molded with hot runners better lend themselves to automated part removal. With no runner to interfere with part removal, secondary mold processing times involving manual labor, including part/runner separation, part trimming and packaging, are reduced or eliminated entirely.
Significantly Reduce Production Costs
Although a more expensive capital investment upfront, a hot runner system is a significantly more cost-efficient means to keep production costs to a minimum over the long run.

Resin Savings
Since there is no cold runner to discard or recycle, resin consumption is reduced. Depending upon the molding application (i.e., medical components or parts requiring FDA approval), the product may require 100 percent virgin material—increasing overall consumption.
Energy Savings
Energy is wasted plasticizing, cooling and regrinding each cold runner that is produced. Increased energy consumption also is a direct result of extended cycle times.
Labor Savings
Secondary operations—such as manual part de-gating and trimming—are eliminated entirely with a hot runner system.
Mold Cost Savings
A smaller cavitation hot runner equipped mold may be able to satisfy production quotas using a smaller number of cavities since it runs at a faster cycle. The smaller mold frame size may enable installation into a smaller press.
Injection Press Costs
Hot runners allow reduced injection pressures during packing, as the system does not have to deal with injecting resin through a cooled runner. Melt in the cold runner may lose heat en route to the gate, possibly requiring higher heats and/or pressures from the injection molding machine. By reducing the injection pressure and clamp tonnage required, it is often possible to run the same part in a smaller tonnage machine as the clamp tonnage required is not as great.

sample heater in hot runner

Benefits for Long-Term
Despite the higher initial cost, the long-term benefits of equipping a mold with a hot runner system can be easily justified. A hot runner-equipped mold can effectively reduce molding costs without significantly increasing the complexity of the mold design. Generally, mold build leadtimes are not impacted as the hot runner is designed and manufactured in parallel with the mold.
It is in the best interests of the moldmaker to continually suggest ways for his/her customer (the molder) to reduce mold operational costs and increase mold output. By demonstrating these initiatives, the moldmaker confirms to the molder that the moldmaker is taking an active role in increasing the molder's overall profitability, which in turn increases the likelihood of repeat business for the moldmaker.