Detailed Analysis of Hot Runner Technique - Optical Lens

Lens follows the rule of light reflection and uses transparent materials (glass, crystal, plastic) as its spherical surface.

Compared to other synthetic transparent materials, PMMA (Polymethyl methacrylate, abbr: Acrylic ) is cheaper and better qualified, so it is widely used for lamps, lighting equipment, optical glass, instruments and dials, adverting plate, etc.

The main topic of this discussion: Difficulties of Injecting PMMA in Hot Runner’s Mold.

In general, there are 3 plastic outlets in the sharp gate of hot runners, so some plastic products have welding mark (resembles the logo of Benz). Basically it can be removed by designing a lower PMMA flow gate. To avoid flow marks, the flow speed of PMMA can’t be too fast. If using sharp gate, this is hard to control since adjusting machine only adjust the speed of the screw, while the injection speed of plastic outlet depends on the size of outlets, space between outlets, mold temperature, etc, making it can’t be controlled. Thus, when it comes to the project using PMMA, basically a large outlet and needle-type valve would be chosen as the hot gate. It is not impossible to make a sharp gate for PMMA, but it’s difficult to control the accuracy.

It has been proofed that the temperature of 340℃/644℉ in PMMA processing won’t make it yellowed. It is caused by the cut of the hot runner gate instead. This will be very obvious in thin products such as light guide plate. For hot runner, it is not only to assure the temperature of the hot gate, but it also needs to decrease the temperature of cuts flowed by the plastic. For the black spot, it is mostly caused by a dead end of hot runner, spots contain un-injected plastics. These plastics remain in hot runner for a quite long time and caused the black spot. The runner must be polished to avoid black spot reappearing after several times. Thus, a harder material must be considered when making PMMA hot runner’s plate.

Problems such as welding traces，vinyl mark，flow mark，blister，shrinkage , air bubble often occur in transparent PMMA products, because of the high viscosity and poor fluidity of PMMA materials. Air resistance inside the mold causes fluidity of the plastic inside it becomes higher than plastic on the surface, resulting welding traces with unfixed positions when both plastics combined. Using higher temperature of sol and mold, increasing and maintaining pressure can solve these problems. However, if the design of hot runner gate is improper, cold plastic will be injected, bringing new problems.

A customer company of Shinzo produces LED lens with 64 cavity mold where plastic material flow conveys from 8 sprue hot runner to cold runner. The product is 2.5g x 96, 1-14mm in thickness, 2.5mm in diameter and its shaping period is 120s. After using our hot runner system, which features stable operation and precise temperature control, they successfully avoided producing defective products caused by filling imbalance.