Phase One: Preparation Phase
Product and Mold Design

First of all, industrial designers will create 3D models of the pen barrel and cap, and determine their appearance, structure (such as pen clips, anti-roll structure, sealing clips), wall thickness, etc.

Mold engineers will manufacture precision steel molds (molds) based on product designs. A set of molds usually contains multiple cavities, and a single injection can simultaneously produce multiple identical parts, greatly enhancing efficiency.

Material preparation

Plastic particles: The commonly used plastic materials for marker pen components are polypropylene or ABS.

Polypropylene is inexpensive, has good toughness and excellent chemical resistance (it can resist ink erosion).

ABS has higher hardness and better surface gloss, and is often used to make pen caps and pen clips to achieve a better hand feel and appearance.

Masterbatch: According to the required color, mix the plastic particles with the corresponding color masterbatch in proportion to ensure uniform and consistent color.

Drying: Some materials (such as ABS) are prone to moisture absorption. Before injection molding, they need to be dried first to prevent bubbles or silver streaks from appearing on the surface of the product after molding.

Phase Two: Injection Molding cycle
This is the core link, which is carried out in a cycle on an injection molding machine, and each cycle usually lasts only a few dozen seconds. The loop steps are as follows:

Mold closing

The clamping system of the injection molding machine drives the two halves of the mold (the moving mold and the fixed mold) to close tightly at high speed and high pressure, preparing for injection.

Injection

The plastic particles in the barrel are heated to a molten state (turning into a viscous flow melt).

The screw of the injection molding machine advances forward, injecting a certain amount of plastic melt into the sealed mold cavity through the mold’s gating system (runner, runner, gate) at extremely high pressure.

Pressure holding

After the injection is completed, the screw continues to maintain a certain pressure (holding pressure) and replenishes a small amount of plastic forward to compensate for the volume reduction during the cooling and contraction of the melt, preventing defects such as shrinkage marks and depressions in the product.

Cooling

The melt begins to cool and solidify in the cold mold cavity, shaping into the form of the mold cavity.

The cooling time accounts for the majority of the entire molding cycle. The design of the cooling system (waterway) is of vital importance as it directly affects production efficiency and product quality.

Mold opening

The moving part of the mold moves backward, opening the mold.

“Top out

The Ejector plate inside the mold pushes the Ejector Pins forward, pushing the already cooled and formed plastic part out of the mold cavity.

After ejection, the mechanical arm or robot usually automatically takes out the product and places it in the collection box.

Reset

The ejection mechanism returns to its original position to prepare for the next mold closing action. Then, the next cycle begins.

Phase Three: Post-processing and assembly
Post-processing

Sprue trimming: Freshly injection-molded parts will have waste from the gate and runner system (referred to as “sprue”), which needs to be cut off manually or with special equipment.

Inspection: Conduct random checks on the products to see if there are any defects such as material shortage, flash, shrinkage, and uneven color.

Subsequent processing (for some components) :

Printing: The Logo and model text on the pen barrel are usually processed through screen printing or heat transfer printing.

Assembly: The injection-molded pen barrel and cap will be finally assembled with the pen tip, fiber core, ink and other components on the assembly line to form a complete marker pen.

Key Considerations in Injection Molding (for markers)
Uniform wall thickness: The pen barrel is a thin-walled long cylindrical product. When designing, it is necessary to ensure uniform wall thickness; otherwise, it is very likely to deform due to uneven cooling and contraction.

Demolding slope: The inner and outer walls of the pen barrel must have sufficient demolding slope (usually 1-2°), otherwise it will cause “pulling” or even bursting when ejected.

Mold venting: During injection, the air in the cavity must be able to be discharged smoothly; otherwise, defects such as burning and material shortage may occur. The sealing structure inside the pen cap particularly requires attention to the exhaust design.

Shrinkage rate: Different plastics have different shrinkage rates. The mold size needs to be precisely enlarged according to the shrinkage rate of the material to obtain a product with the correct size.