Silicone Rubber Prototyping

Introduction of Silicone Compression Molding & LSR Injection Molding And Their Processes

• 1. Silicone Compression Molding

  What is Silicone Compression Molding? 

  Compression molding of silicone is the process in which putting solid silicone rubber with vulcanizing agents into the cavity of each half of the mold, and then assemble the mold, and put the mold into a high temperature compression molding press to form the parts in cycle time. The hardness of molded silicone part is usually 30 -70 Shore A. Silicone parts can be made in almost all color which can be match with the Pantone color card number. The shape of the mold determines the shape of the silicone product. The compression molded silicone product is currently the most widely used in the silicone industry such as silicone industrial accessories, gifts, bracelets, watches, key bags, mobile phone sets, kitchen utensils, mats, ice trays, cake molds, etc.
  

• 2. LSR Molding 

  What is LSR Molding?

  Liquid Silicone Rubber (LSR) moulding, sometimes is also named as Liquid Injection Moulding (LIM), is the process in which the LSR feeder fully mixes the liquid raw material from two separate components (A+B) in a precise 1:1 ratio, and because some of the products are colored designs, they are equipped with a color pump and a color metering part. A+B components, additives, colors, etc. are fully mixed and then put into the plasticizing system. This plasticizing screw has the functions of homogenization and mixing. The mixture is injected into the hot mold through the screw. At the mold temperature of 170-200 °C, the silica gel undergoes a curing reaction and form the flexible silicone parts within seconds. 

Silicone Rubber Prototyping Details and Options

Process of silicone compression molding

• Firstly, Making compression mold

  The mold making starts with mold design as per the part 3D, then programming, cnc machining the rough shape, and then EDM spark and engraving machining, finally surface finish such as polish, texture, etc as per customer’s requirement.

• Secondly, according to the customer's requirement, feed the raw silicone through a the silicone mixing roller to mix in required colouring.

  The commonly used materials for processing are: silica gel, nitrile rubber, ethylene propylene rubber, fluorine rubber, etc. The color matching process should be done carefully, which needs constant pulling, cutting, flipping, etc. to make the color fusion more uniform. After the finished product comes out, the color remains the same, and the pass rate is high!

• Cutting the material

  After the silicone rubber compound is mixed, it is placed for 24 hours , and then according to the requirements of placing the rubber strips in the production and forming operation instructions, the rubber cutting and preparation processing are carried out, and the cut rubber materials are weighed and placed on the shelf for use.

• Compression molding
  

  After cutting the mixed silicone rubber into strips, and it needs to be weighed in the molding department to avoid loss and waste, and then put into the mold for compression molding at a high temperature of 200 °C.

  The molded silicone rubber parts usually have some flash, etc, it needs to use scissors and pliers to clean the flash.

• IPQC inspection and packing

  The quality staff inspect the molded rubber parts to check if meets requirement, such as appearance, size, function, etc. After IPQC, the products are packed and delivered to customers.

Advantages of silicone compression molding

• Silicone compression molded parts are tensile, tear-resistant, shock-resistant, stable performance.

• It resist low and high temperature (-30 degree C to +260 degree C)

• It can be Waterproof, fireproof, dustproof;

• The silicone compression molded parts are high precision, regular and beautiful shape.

• It allows adjusting any color matching the in

The Process of LSR Molding

The molding machine for LSR---Liquid Injection molding machine, Short name LIM machine

LIM uses a quantitative device (feeder) to control the ratio of two materials (component A and component B) at a ratio of 1:1. If the product has color requirements, add color paste (or additives, etc.), and then pass the mixer to fully After the mixing is pumped into the barrel of the injection molding machine, the screw accurately injects the liquid silicone into the mold cavity. At a certain mold temperature, the liquid silica gel undergoes a curing reaction.

Following are the major steps used in the LSR molding process:

• Uncured liquid silicone containing two containers, Compound A and Compound B, connected to the pump system. Drum contains the base forming material and Drum B contains the catalyst. A metering device automatically releases the two substances and the pigment additive in a fixed 1:1 ratio.

• LIM machine can program and customize the automatic molding machine and make injection settings to provide the correct injection volume for each job. Align the platen and piston to close the mold, and set the temperature, pressure, firing rate and cycle time according to customer requirements.

• After the shaping is completed, the molding machine will heat the mold to a suitable temperature and apply a suitable clamping force. The mold is closed together with the machine's syringes, pumps and supplies. The injection mechanism then pushes the material into the mold and cavity.

• Apply heat and pressure to the liquid silicone rubber to cure the material. After the cycle is complete, the mold is opened and the part and burrs are removed automatically or by an operator. Then close the mold and repeat the process.

• After the product is released from the mold, it will perform post-molding processes such as deburring, secondary curing, inspection, and packaging.

Advantages of LSR Injection molding

• Non-toxic, can be used for medical and food use.

• Non-toxic to human body, odorless and tasteless. LSR is more used in little baby’s products

• Environment friendly, it is degradable;

• No toxic and harmful substances are produced during combustion (the combustion products are mainly SiO2);

• Tear strength, resilience, water resistance and waterproofing;

• Weather resistance: Due to the high Si-O bond energy, higher energy than ultraviolet radiation, it is not easy to be decomposed by ultraviolet light and ozone, so it has good ultraviolet resistance, ozone resistance and aging resistance;

• Temperature resistance: good heat resistance and cold resistance, long-term use in the temperature range of -50℃~200℃, it still has good elasticity, no melting and embrittlement (special products can reach -110℃);

• Excellent Electrical insulation performance: its dielectric loss, withstand voltage, arc resistance, corona resistance, resistivity, etc. are among the best in insulating materials, and electrical properties are rarely  affected by temperature and frequency;

• Biological compatibility: Polysiloxanes are among the most inactive compounds known. They are very resistant to biological aging, have no rejection reaction with animals, and have good anticoagulant properties.

• Fast turn-around: LSR has low viscosity, fast vulcanization, shear thinning and high thermal expansion coefficient before curing, and can be produced by injection molding, rapid and repeatable mass production.