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Design of mould and choice of injection machine
Author:admin  From:Letian Plastics   Hits:2341  Update:2008-12-9

Choice and treatment of rolled steel for mould

For larger amount of reinforce filler such as glass fiber and inorganic additives are used, and the processing temperature is higher, the choice of rolled steel for mould should base on the temperature of 140-170. The alloyed steel should be chosen and hardened such as SK, SKS, SKD, and SKH.

Flow path and pouring head

The shape of flow path

The flow path should be circular and trapezoid shape, and the taper is 2-3º. The cold slug well is placed at the end of flow path.

The pouring head can elect one of needle, disk, flake, spoke, straightforward and side entrance shape. It is not suitable to use resting form, because the rigidity and molding shrinkage of PPS product. But in high cycles molding, pouring head of resting form could be used with bigger entrance angle and diameter. The pouring head should be placed at the thickest cross-section part of product, and be as short as possible.

When design the shape and place of pouring head for reinforced PPS product, it should be considered that the change of dimension and strength introduced by the flow direction of fluid (the vertical strength is about 50-60% as that of flow direction), in addition to the decrease of change introduced by the seamline. For the product that cannot avoid the existence of seamline, it is better to set overfall section at the seamline of mould and put the seamline in non-load condition as possible.

Air exhaust

The performance decrease if there is gas cavity in interior of PPS product. The bad state of air exhausting leads to the burned, color-changed and roughened surface of product. In order to obtain good PPS product, the vent should be open on the mould. Add the demould layer and stripper pin (pin-lift) can be used as air vent.

Stripper pin (pin-lift)

The diameter, number and position of stripper pin should be carefully considered. If local remained stresses are introduced in products, the problem of crack and inconsistent dimension precision are appeared.

Temperature control of mould

The heating of mould can used one type of electrical bar, belt and oil circulation to control temperature. The electrical bar is the most convenience and economical heating type. According to the size and space, the electrical bar is uniformly installed on the moving and fixed moulds under the principle of minor power and small interval. And the fixed mould is better temperature-controlled individually. To the precession of linear PPS, the hot flow path should be applied as required.

Choice of injection machine

For the characters of high process temperature, difference of fluid flow and the wear and corrosion of filler, several points should be pay attention to as shown in following:

The graded crew rod with wear and corrosion resistance for crystallization which has no-return ring and single head is chosen. The slenderness ratio is 18-20 and the compression ratio is 2-3.

The space between the screw rod and charging barrel should be controlled as about 0.05mm to prevent opposite flow;

The charging barrel made by wear and corrosion resisting materials should be chosen;

Open feed port can be used to injection molding. But in order to prevent overflowing, the closed and self-locking feed port with diameter 3-6mm can be elected. The heating equipment at feed port can be controlled individually to prevent the extension and condensation;

In assurance of PPS product with outstanding performance, inject machine with bigger mold-lock and Injection pressure should be applied.

 

Design of product

Design of R angle

PPS is a kind of crystallinity resin. The stress concentrations arose at the corner or by change of wall thickness in product, always lead to stress cracking, dimensional change and strength reduction. It should be noticed that R angle is added at the corner of product and the non-uniform wall thickness is avoided.

 

Ribs reinforcement

Reasonable rib reinforcement can improve the product stiffness, attain the effect that preventing the product distortion, remove the residual stress and increase the flow ability, and achieve the total equilibration of product. Good constitution of rib reinforcement is more effective than increasing the wall thickness. And increasing the number of ribs is better than increasing the height and thickness of ribs.

 

In general, the proper thickness of reinforced ribs is less than 1/2 of local wall thickness. The outer diameter of a boss should be less than twice the base diameter, and R is added at base.
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