2012 Issue 2: Efficiency and reliability—achieving Ideal Production

In-mold hold pressure slashes cycle times

 

In IMHP technology (In-Mold Hold Pressure) from DuPont, hold pressure is applied using a separate mechanism integrated within the mold during the screw-dosing phase. This parallel arrangement of two injection molding phases can help reduce cycle time. The actual time saving is largely determined by the degree of crystallinity of the processed material and the shot volume.
IMHP process phases with cold runner (top) and hot runner (bottom).
  

 

In fast-moving production environments, the reduction of cycle times is a regular and vital target for any organization. Even just fraction-of-a-second reductions can soon add up to huge savings in plants that work around the clock.

DuPont has developed a new technology, that can save between 20% and 30% in the cycle time when injection molding semi-crystalline thermoplastics such as DuPont™ Delrin® acetal resin.

 

By simply removing the mutual reliance on each other between the dosing and hold-pressure stages, the new In-Mold Hold Pressure (IMHP) technology allows the dosing phase in the barrel to take place simultaneously with the application of hold pressure, meaning that the cooling time can be factored out of the overall cycle time.

 

Semi-crystalline thermoplastics require relatively long holding times during which the volume shrinkage caused by the crystallization is compensated. In contrast to standard injection molding procedures, the required hold pressure in the IMHP approach is generated within the mold. This allows the plasticizing unit to dose the melt for the next shot at the same time. These two phases would traditionally follow one after another, and now they can be carried out simultaneously. When the required hold pressure time has elapsed, the mold is opened and the part ejected.

 

A consequence of this simultaneous operation is that the overall cycle time is reduced by an amount equal to the shorter of the two times (dosing/holding). The resulting gain in time and productivity increases with dosage volume. In the case of large-shot volumes or relatively short overall cycle times, the time savings can be up to 30%.

 

Two different methods of hold pressure generation have been evaluated—hot and cold runner. Both methods are currently being trialed and refined by DuPont, with a current emphasis on minimizing the additional space required.

 

DuPont has conducted numerous injection molding trials with different semi-crystalline thermoplastics grades at its European Technical Center in Meyrin, Switzerland. Conventional molds were modified and trials with Delrin® polyoxymethylene (POM), DuPont™ Zytel® polyamide (PA), DuPont™ Crastin® polybutylene terephthalate (PBT) and renewably sourced DuPont™ Sorona® polytrimethylene terephthalate (PTT) were conducted. The time savings achieved with IMHP technology proved to be considerable and were generally in the region of 20%-30%.

 

In general, the use of IMHP technology is confined to semi-crystalline engineering plastics such as POM, PA or PBT. At the end of the hold pressure phase, only these materials have sufficient stiffness and crystallization shrinkage to permit reliable ejection from the mold.

 

As well as significant time savings, DuPont was also able to demonstrate that there were no significant changes in terms of dimensional stability and mechanical properties between standard injection-molded samples and those produced using the IMHP technology.

 

From a molding perspective IMHP is also in contention for parts with high thickness; high shot volumes, such as large parts and crystallization time; and parts that use more than 50% of the screw capacity.

 

DuPont will further this work by collaborating with molders, hot runner and machinery producers to refine the engineering aspects of the process, ensuring market entry for this time- and cost-saving technology.