PP (Polypropylene Material Requirements For Mold Design


PP (polypropylene) is a crystalline polymer with characteristics of light weight, non-toxic, tasteless, corrosion resistance, high temperature resistance, and high mechanical strength. The polypropylene resin for injection is a white, waxy-like particle.


Features and benefits

1). Due to its good fluidity at melting temperature, wide forming process, and lower anisotropy than PE, it is particularly suitable for making various simple-shaped products, and its surface gloss, dyeing effect, and external scars are superior to those of PE materials.
2). Among general-purpose plastics, PP has the best heat resistance. Its products can be boiled at 100°C for disinfection, making it suitable for making tableware, kettles, and other medical devices that require high-temperature sterilization. The heat distortion temperature is 100°C~105°C, and it can be used for long periods above 100°C.
3). High yield strength and high bending fatigue life. The movable hinge made of PP can withstand 70 million times of folding and bending without significant damage under the appropriate thickness (such as 0.25~0.5mm).
4). It has a low density, making it one of the least dense plastics known to date.

The requirements for mold design are mainly reflected in the following aspects:

1.Liquidity considerations:

Due to the excellent fluidity of PP raw materials, attention should be paid to the depth of injection exhaust during mold design, preferably not greater than 0.03mm, to prevent the occurrence of overflow and flashing. Large orientation stresses are easily generated near the direct gate, resulting in warping and deformation of plastic parts, so direct gates should be used as little as possible, especially for flat plastic parts with large molding areas.

2.Mold Temperature Control:

Mold temperature plays an integral part in PP plastic part quality, especially during manufacture. Too little warmth means parts with significant orientation issues, poor welding strength and dull surfaces will suffer; too much warmth could result in warping and deformation issues; it is therefore vital that reasonable guidelines for setting mold temperatures be established – generally between 50-9o C for products with higher dimension requirements; higher temps may also be required depending on requirements.

3.The design of the gate and runner:

The design of the gate and runner is also an important part of mold design. The core temperature should be more than 5°C lower than the cavity temperature. The diameter of the runner is usually between 4-7mm, and the length of the needle-shaped gate is about 1-1.5mm, with a diameter as small as 0.7mm. For plastic products with a striped chain, the melt filling direction and pouring system should be reasonably designed. In addition, the mold must have good exhaust performance to avoid shrinkage marks.

injection mold design

4.Mold structure and other details:

mold design also needs to consider uniform wall thickness, sufficient demolding slope, gradual and smooth transition in transition parts, and prevention of sharp corners. The mold surface should be smooth and have low roughness. At the same time, the exhaust holes and grooves must be sufficient to allow timely discharge of air and gases in the melt.


In general, PP’s requirements for mold design cover material properties, temperature control, gate and runner design, mold structure, and many other aspects. During the mold design process, these factors need to be considered comprehensively to ensure the production of high-quality and stable PP plastic parts.

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