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Liquid Silicone Rubber Molding / LSR Mold Design Guide

Liquid Silicone Rubber Mold with Cold Runner (Needle Valve)

Liquid Silicone Rubber Mold, also called LSR Mould or Silicone Mold, which have some types according to liquid silicone rubber materials feeding systems, silicone materials state, mould runner systems and silicone molded machines types etc.
Actually, for mass production and precision liquid silicone molded parts or products, silicone mold manufacturers or customers mainly choose liquid silicone rubber injection molding with shut-off & needle valves, which can save lsr materials, have long mold lifetime, more precision and easy to automatic production etc.

1. LSR Injection Mold:
Following mainly talking about liquid silicone rubber injection mold with needle valves & silicone injection mold designs

Liquid Silicone Rubber Mold Design Guide
Silicone Injection Mold Design Guide

LSR Mold shut-off valve
Two different types of LSR injection machines and liquid silicon injection molded designs
1. Vertical LSR Injection Machines and Mold Designs:
This kinds of LSR injection mold & machines mainly designed for 2-shot & liquid silicone rubber overmolding, like liquid silicone rubber overmold with plastic, metal or glass, or two different LSR color, handness or silicone materials etc.

Liquid Silicone Injection MouldLiquid Silicone Rubber Injection Mould
Liquid Injection Mold

2. Horizontal LSR Injection Machines and Mold Designs
Horizontal Liquid Silicone Rubber Injection Machines, like plastic injection mold, which easier to achieve automated production.

LSR Mold
Liquid Silicone Injection Mould

Notes:  Common compression machines also can converted into LSR machines only change materials feeding systems, which only produce some LSR products & parts as not high tolerance demands of sizes or designs, like baby care products, packaging products or cosmetics etc. This kind LSR molding is cheap, similar to silicone rubber compression molding or rubber tranfer molding.
LSR Injection Molding Designs, is something like plastic injection molding with or without hot runner systems. LSR injection molding with shut-off valves, also called cold runner systems, which can save liquid silicone rubber materials and labor cost, also have good LSR product's surface (small injection gate). Liquid Silicone Rubber Injection molding without shut-off valves, like silicone injection molding, also called hot runner LSR & direct gate liquid silicone rubber injection molding, which can save some mold cost but bigger injection gate (not good lsr product's surface)

Liquid Silicone Rubber Mold with Hot Runner

Liquid Silicone Runner Injection Mold without shut-off valve & needle valve, also called hot runner systems LSR moulding, which designed for precision lsr molded parts or products and save lsr molding cost, like liquid silicone coating only for samples
Hot Runner LSR Injection Mold
Hot Runner Liquid Silicone Rubber Mold

Converted Compression LSR Mold & LSR Transfer Injection Mold

Common compression machines also can converted into LSR machines only change materials feeding systems, which only produce some LSR products & parts as not high tolerance demands of sizes or designs, like baby care products, packaging products or cosmetics etc. This kind LSR molding is cheap, similar to silicone rubber compression molding or rubber tranfer & injection molding. Notes: Those two kinds of LSR Mold, called false LSR mold or false silicone injection mold, as have many "sequela" like can't full vulcanization, short mold life and rough design & sizes tolerance.
Converted LSR Injection Mold which produced by Converted Compression Mold Machines, only change solid silicone materials into LSR materials (Manual Liquid Silicone Rubber), or change materials feeding systems (Silicone Tranfer or Injection Molding):

Notes: Those two kinds of LSR Mold, many suppliers would like called LSR injection mold or silicone injection mold. Actually, this called false LSR mold or silicone tranfer mold, as have many "sequela" like LSR can't full vulcanization, short mold life, rough design & sizes tolerance. 
1. Manual Liquid Silicone Rubber Mold Design
Manual Liquid Silicone Rubber Mold Design
Liquid Silicone Rubber Tranfer Molding & Silicone Injection Mold
Tranfer Injection Liquid Silicone Rubber Mold
Liquid Silicone Injection Mold

Liquid Silicone Runner Injection Mould

Liquid silicone rubber injection molding is the most precise, efficient, and safest and manufacturing process, the products by LSR injection molding with mini parting line and no and less silicone rubber burrs

 

Wall Thickness:

Because liquid silicone material doesn’t sink like plastic, so it’s  wall thickness does not have to be uniform. The wall thickness can be as thin as possible with silicone molding, can down to 0.127mm.

 LSR Injection Molding for Medical Gasket

Tolerance and Molded Part Shrinkage:

 Part shrinkage will effect tolerance of products directly. The shrinkage variations of liquid silicone can be little compared to plastic material, so the tolerance of LSR can be much closer. sizes tolerance can less +/- 0.02mm.

Needle Valves for LSR Injection Molding
Shrinkage:

It occurs during molding and post curing or unit fully cross-linked.

It depends on different materials. Typical shrinkage is between 1% and 5%.

Varies by the manufacture and product range from 0.25 per inch.

 

Optical Properties:

Liquid silicone is optically clear and have higher surface finishes compared to typical glass, and is widely used as optical lens. Some silicone refractive index can equal that of glass (1.53). what’s more, the silicone optical lens are UV aging, can operate at high temperatures, and have wider spectrum of transmission compared to acrylic and polycarbonate.
Liquid Silicone Rubber goggles

 

Radiu and Fillets:

Radiu and fillets not only can improve a product’s appearance but also increase it’s tear strength.

 

Surface Finish:

Surface finish can match the mold texture. Surface finish can be dull, shiny, or include part identification marks. Surface finish has a direct bearing on the coefficient of friction.

Usually, textured surfaces provide better part releases than polished surfaces when molding.

LSR molding design guide for silicone straw

Draft Angles:

Draft angels are not required for silicone parts but for plastic part.

 

Undercuts:

Undercuts is available on de-formable products. The products can be stretched to remove from cores without any deforms. Some times, we will add release agent to assist in expanding products to remove from cores. Note: usually, undercuts do not require expensive slides for the mold.

 

Parting Lines and Gate Location:

Liquid silicone are flexible, have small parting line, and can make the parting line at a suitable location compared to solid silicone and plastic parts.

LSR Products Surface Finishes

PM-F0: non-cosmetic, finish to protolab's discretion
PM-F1: low-cosmetic, most toolmarks removed
SPI-C1: 600 grit stone, 10-12 Ra
PM-T1: SPI-C1 + light bead blast
PM-T2: SPI-C1 + medium bead blast
SPI-A2: grade #2 diamond buff, 1-2 Ra

LSR Flow/Sulfur Analysis

1.The vulcanization chemistry of liquid silicone rubber requires a certain reaction time.
2.Ideal flow, within diameter 2mm, 170cm, and flow distance is more than 100cm.
3.Excessive mold temperatures will cause vulcanization, which cause the flow to be blocked.
 

LSR Common Problems and Solutions

Reasons:
Too low temperature and too short
Channel pollution
Solutions:
Increase temperature and prolong the time
Clean molding tubes and accelerate feed rate

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  2. Liquid Silicone Rubber Products & Parts Mass Production
  3. Moldability & Manufacturability Analysis and Designs Support 
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