In recent years, liquid silicone rubber (LSR) injection molded products have become popular in medical, baby care, automotive and other application fields due to their unique properties. LSR is a thermosetting plastic. What are the differences between its injection molding processing and thermoplastic plastics? What should you pay attention to when processing? What are the types? What are the advantages of soft/hard overmolding of self-adhesive LSR with thermoplastics? What are the processing points for soft/hard overmolding? These issues must be understood and mastered by thermoplastic injection molding processors who want to get involved in LSR injection molding processing. To this end, this article answers these questions.
LSR is favored by medical, baby care and automotive applications for its unique properties (the upper left and right pictures include diving mask parts, automotive parts and baby pacifiers injection molded with LSR, and the lower right picture is a syringe injection molded with LSR sealing parts)
Although injection molded liquid silicone rubber (LSR) is not a new product, its applications in medical, automotive, baby care and general industrial fields have maintained a rapid growth momentum in recent years. As a thermosetting resin, LSR provides properties that currently cannot be provided by TPE. LSR’s impressive properties include heat resistance, excellent low-temperature flexibility, chemical resistance, biological inertness, and inherent lubricity. Due to the increasing popularity of soft/hard overmolding technology, many thermoplastic processors are looking to LSR injection molding as a way to expand their business. Fortunately, the processing of LSR has many similarities to the processing of thermoplastics. For all their differences, many thermoplastic processors have been able to successfully transition to processing LSR with appropriate attention and learning.
What is Liquid Silicone Rubber LSR?
The basic raw material of silicone rubber is sand or silica (SiO2). The sand is first processed into pure metallic silicon, which is then reacted with methyl chloride, through a series of steps that can form silicone in a variety of forms, from liquids and polymers to rubber-based compounds. Unlike most plastics, silicone has a backbone of alternating silicon and carbon atoms, with organic side chains that give the material cross-linkability and other properties. Because of this, LSR not only has some processing properties consistent with thermoplastic plastics, but also has some inherent characteristics of thermosetting plastics.
LSR uses a platinum catalyst for cross-linking with no by-products. Once cross-linked, LSR cannot be reprocessed or easily recycled and reused because of the high energy required to overcome the cross-links and break the backbone. LSR’s identical molecular structure enables its use over a wide temperature range. At the same time, the strong bonding force between silicon and oxygen atoms makes the degradation temperature of LSR much higher than other polymers. Some of LSR’s other attributes are chemical inertness and purity, which combined with its ability to withstand high-temperature sterilization make it an ideal material for many medical applications as well as baby care products.
LSR usually contains two components with different viscosities, which need to be added to the static mixer in a 1:1 ratio before being fed into the barrel of the liquid silicone injection molding machine.
Working principle of LSR injection molding process
After introducing the excellent characteristics of LSR, it is also necessary to introduce the similarities and differences between the processing of LSR and thermoplastics. First of all, LSR injection molding equipment looks very similar to thermoplastic injection molding machines. They both have a feeding system to fill the resin into the barrel of the injection molding machine, and then inject the resin in the barrel into the mold for molding. However, one of the biggest differences occurs inside the mold. Thermoplastics must be heated in the barrel to change from a solid to a molten state, and then cooled back to a solid state in the mold. LSR is just the opposite. Its initial state is fluid and is then heated in the mold for cross-linking and solidification. The barrel of the injection molding machine adopts a water cooling device to keep the temperature below the solidification point of LSR. The mold is usually heated to 180~200℃. Molds for thermoplastics can minimize nozzle and sprue materials by using hot runners, while LSR uses cold runners to keep the material cool until it enters the mold cavity. Since LSR is a high-value material that cannot be reused, waste reduction is even more important than with thermoplastics.
The method of adding LSR to the barrel is different from thermoplastics. LSR is a paste, similar to peanut butter, and usually consists of two components, of which component A contains a cross-linking agent and component B contains a catalyst. The two components can promote the curing reaction of LSR after being mixed. After the two components A and B are pumped into the barrel at a ratio of 1:1, they can be injected after being fully mixed. Usually this requires entering a water-cooled mixer to complete the mixing of the material before entering the injection unit. Most LSR injection molding machines also use an injection screw to achieve further mixing.
Compared with thermoplastics, LSR has a very low viscosity in hot molds, so moldmakers need to pay extra attention to the parting line and vents when making molds to prevent flashing of the product. This characteristic of LSR also has advantages, that is, it is easy to form very long and thin products.
The thermoplastic must fill the mold to ensure the integrity of the product’s shape. To do this, holding pressure must be applied so that additional material can be filled into the mold cavity as the product shrinks. However, this step is not required for molded LSR. Since LSR cross-linking itself will expand, pressure-maintaining and filling will definitely cause excessive expansion of the product during mold opening, resulting in uncontrollable size of the product. Silicone rubber also shrinks through cooling, but its shrinkage rate is uniform. For these reasons, when molding silicone rubber, only one pressure is required if the mold design has taken into account LSR expansion and contraction.
The injection molding process of LSR should be viewed as consisting of 4 parts. The type and grade of LSR material is only one important component; the quality of the injection molding machine, pumping unit and mold also play an important role. For the injection molding production of LSR products, through the use of reasonable design and technology, automated production like the processing of thermoplastic plastics can also be achieved.
Diversity of Liquid Silicone Rubber LSR
Like thermoplastics, LSR comes in many varieties and grades. According to the touch and hardness, it can be divided into two types: “wet” (similar to rubber) and “dry” (smooth). General-purpose LSR is not filled with too much filler (typical filler is silica) and is suitable for general physical properties. Through the addition of additives and other fillers, LSR can withstand high temperatures or environments with oils and other liquids. The addition of phenyl group improves the low-temperature performance of LSR. Adding phenyl fluid can reduce the friction coefficient of the material and make the surface of the product smoother. There are also some LSRs that have a low coefficient of friction themselves and do not require the use of liquid to lubricate their surfaces.
For many processors and product designers, the use of self-adhesive grade LSR that enables soft/hard overmolding (also known as two-component or 2K molding) is of great significance. These new materials can be bonded to thermoplastics without the need for additional bonding steps.
This not only eliminates the bonding operation, but also enables in-mold bonding of LSR to thermoplastics because the curing time is close to the cooling time of the thermoplastic. This process is similar to soft/hard overmolding of thermoplastics. The difference is that the mold on the Liquid Silicone Rubber LSR side needs to be heated to cross-link and cure the LSR, so its heat resistance should be considered when selecting the thermoplastic substrate material (it will not be affected by the deformation due to the influence of curing temperature). In some applications, waste heat from solidification of the thermoplastic can be used to cure LSR.
The self-adhesive properties of LSR to a variety of thermoplastics can bring advantages to product design. LSR can be precisely placed in an area of a product that requires a special function without covering the entire surface of the product with functional material. Likewise, portions of the thermoplastic substrate material not clad with LSR do not need to be thickened to withstand LSR curing temperatures. For processors, this not only reduces material costs, but also allows products to become smaller to cater to the trend of product miniaturization.
It should be pointed out that the initial bonding strength of the current self-adhesive LSR technology is designed so that no damage occurs when the product is demoulded (no one wants LSR to adhere to the metal of the mold). Over two weeks, the bond strength will continue to increase and will generally reach its best level before being sent to the end user. If the processor wants to reach the maximum bonding strength faster, the product can be baked in a high-temperature oven at 100°C for 1 hour.
How to solve the problem about silicone injeciton molding mold sticking?
In liquid silicone injection molding, the mould adheres to the product, often known as the mould sticking, or the demoulding is particularly difficult and requires the use of a release agent and other physical means to help demoulding. This LSR silicone injection molding process will lead directly to low production efficiency of liquid silicone injection molding, damaging equipment and products. The silicone injection molding process quality also affects a lot on the final quality silicone products. Silicone products are stretched and deformed, and physical means are used to remove the silicone particles when they stick the mould, causing the mold to damage and damage, etc. The silicone injeciton mold lifespan will be shorten if it needs mold maintanence often.
The original traditional method:
cleaning and sticking the mold or removing it with potion, and using potion or brush to remove the flash on the parting surface.
However, for transparent products at that time, the mold was polished to a mirror surface, and the traditional function was to scrub it bit by bit with cotton pads. , and a professional polishing master is required, otherwise the mold will be directly disassembled, and the flash on the parting surface can only be picked out bit by bit with a toothpick. The efficiency can be imagined, even if it is ordinary silicone The efficiency of product cleaning with chemicals is also low, generally taking about 30 minutes.
The use of a new fluorine nano-coating process can effectively reduce the occurrence of mold sticking:
Coating features: anti-fingerprint, stain-resistant, easy to clean, smooth, release, wear-resistant
Uncoated mold: It is difficult to take off during molding, cannot be taken off, and is easy to get stuck. The product scrap rate is high due to mucous membrane, erosion, and corrosion during the die-casting process. Or during stamping and forming processing, the mold is often damaged due to wear. , sticking, bite, fatigue fracture and other factors lead to low production efficiency.
Coated mold: easy to clean, smooth, good release property, completely solves the problem of mucous membrane in the die-casting process in hydrophobic and oleophobic state, does not get stuck, and improves production yield rate.
For the touch: rubber mold, liquid silicone mold, self-adhesive silicone, the mold is made of stainless steel.
Processing process: The processing speed is fast and the efficiency is high. The full coating process can be completed within 1-3 days.
